Di Renzo, F.; Cambon, H.; Dutarte, R. Micropor. Mater. 1997, 10 ...... Ladd, M. F. C.; Palmer, R. A. Structure determination by X-ray crystallography,. Plenum press ...
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SYNTHESIS, CHARACTERIZATION AND CATALYTIC APPLICATIONS OF REDOX AND ACID FUNCTIONALIZED MESOPOROUS SILICA THESIS SUBMITTED TO NATIONAL INSTITUTE OF TECHNOLOGY WARANGAL FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY IN CHEMISTRY
By
MURALASETTI NOOKARAJU
DEPARTMENT OF CHEMISTRY NATIONAL INSTITUTE OF TECHNOLOGY WARANGAL WARANGAL-506004, (A.P.), INDIA APRIL 2014
Dedicated to My Beloved Parents & My Grand Mother
DECLARATION I hereby declare that the research work presented in this thesis entitled “Synthesis, characterization and catalytic applications of redox and acid functionalized mesoporous silica” is based on the results of the investigations and research work carried out by me under the supervision of Prof. I. Ajit Kumar Reddy and Dr. Venkatathri Narayanan, Department of Chemistry, National Institute of Technology, Warangal. I declare that this work is original and has not been submitted in part or full, for any degree or diploma to this or any other university.
Date: Place:
(Muralasetti Nookaraju)
CERTIFICATE This is certify that the research work presented in this thesis entitled “Synthesis, characterization and catalytic applications of redox and acid functionalized mesoporous silica” submitted by Mr. Muralasetti Nookaraju for the award of the degree of Doctor of Philosophy in Chemistry, National Institute of Technology, Warangal (A.P.), is carried out under our guidance and supervision. This work has not been submitted earlier in part or full for any degree or diploma to this or any other university.
(Dr. I. Ajit Kumar Reddy)
(Dr. Venkatathri Narayanan)
Supervisor
Co-Supervisor
Professor of Chemistry
Assistant Professor of Chemistry
Department of Chemistry
Department of Chemistry
NIT Warangal
NIT Warangal
Date: Place:
Acknowledgements I wish to acknowledge my deep sense of gratitude to my research supervisor Prof. I. Ajit Kumar Reddy, Department of Chemistry, National Institute of Technology, Warangal for his invaluable guidance and pains taking efforts in the completion of this research work. He has been a great source of motivation and inspiration. The thesis would not have seen the light of the day without his unrelenting support and cooperation. I deem it a privilege to have worked under his amiable guidance. I am very glad to thank my co-supervisor Dr. Venkatathri Narayanan, Assistant Professor, Department of Chemistry, National Institute of Technology, Warangal for his guidance and encouragement throughout my thesis work. He has inspired me to stay focused in my research and complete the work. I express my sincere thanks and gratitude to Prof. T. Srinivasa Rao, Director, National Institute of Technology, Warangal for providing me an opportunity to carry out my research work. I thank him for his kind support and encouragement at every stage of this endeavor. I express my sincere thanks to Prof. B. Rajitha, Head, Department of Chemistry, National Institute of Technology, Warangal for her valuable help and support. I would like to express my gratitude to Prof. P. Nageswara Rao and Prof. K. Laxma Reddy former Heads, Department of Chemistry, National Institute of Technology, Warangal for their support and advice. In a special way I wish to thank Prof. B. V. Appa Rao, Professor, Department of Chemistry, NIT Warangal, and Doctoral Scrutiny Committee member for his cooperation throughout my research work. His valuable suggestions have enriched the quality of this work. I express my sincere thanks to Prof. M. K. Mohan, Professor, Department of Metallurgical and Materials Engg., NIT Warangal and Doctoral Scrutiny Committee member for his support and inspiring guidance throughout the course of my research work. I would like to express my special thanks to Prof. A. Ramachandraiah, Prof. G. V. P. Chandramouli, Prof. V. Rajeswar Rao, Dr. K. V. Gobi, Dr. P. V. Sreelaxmi, Dr. Vishnu Shanker, Dr. D. Kasinath and Dr. B. Srinivas, Department of Chemistry, National Institute of Technology, Warangal for their cooperation and encouragement. I am thankful to the Director, CSIR-NCL Pune for permitting me to carry out a part of my work at Catalysis Division of NCL Pune. I would like to express my deep sense of gratitude to Prof. N. C. Sarada, School of Advanced Sciences, VIT University, Vellore for allowing me to carry out a part of my research work in her laboratory.
I am thankful to Dr. K.V.V. Satyanarayana, Dr. Ch. V. Sreenivasa Rao, Dr. V. Krishna, Dr. Suresh Kaurm, Dr. D. Chakradhar, Dr. S. Vijaya Laxmi, Dr. Kannamba and Dr. K. Bhavani for their support. I am thankful to my fellow researchers A. Rajini, K. Chaitanya Kumar, V. Amarnath, S. Kanakaraju, K. Koteshwara Reddy, P. Kavitha, B. Janardhan, M. Narsihma Reddy, G. Srinivasa Rao, Santosh Kumar, Surendar T., Krishnaiah V., M. Satyanarayana, Vimal Kumar K., A. Ajay Kumar, Rajkumar R., P. Sreenu, K. Laxmi Narayana, P. Lakshmana Rao, G. Mallikarjun, Tewodros, Ramesh T., L. Suresh, K. Yugender Goud, Ashutosh Kumar Yadav, B. Mayuri and others for their help and encouragement. I take this opportunity to express my gratitude and regards to all my Teachers at ZPHS Thondangi, Lecturers at AMG Junior College, Bommuru and Faculty members at Ideal College of Arts & Sciences, Kakinada, whose inspiring messages motivated me to reach the stage of doctoral research. I have been blessed with incessant moral support and boundless inspiration from my parents, Sri. M. Rama Krishna and Smt. M. Vara Laxmi and Chakra Laxmi (Sister), Rambabu (Brother-in-law), M. S. V. Ganapathi (Brother), Veerababu (Nephew) and all other family members. Their love and affection has been motivating force behind what I am today. Above all, I fail to find apt words to thank my friends T. V. V. Satyanarayana, Vasu, Srikanth, fellow members of 10th Orientation progamme on Catalysis for Research Scholars, NCCR, IIT Madras with whose encouragement and support I could complete my doctoral work successfully. I greatly acknowledge Ministry of Human Resource Development, Govt. of India for the financial support in the form of Institute fellowship. I wish to express my sincere thanks to Dr. K. Madhavi, Assistant Professor, Department of Humanities and Social Sciences, NIT Warangal for her valuable suggestions in drafting the thesis. Last but not the least, it gives me immense pleasure to express my heartfelt thanks to all those who have been with me and lend me the necessary help in every possible way throughout the duration of my thesis work.
(Muralasetti Nookaraju)
CONTENTS CHAPTER – 1
Introduction
1-41
1.1
Catalysis
1
1.2
Porous materials and catalysis
2
1.3
Classification of porous materials
4
1.4
Ordered mesoporous materials
7
1.5
Mechanisms for the formation of mesoporous materials
9
1.6
Synthesis of mesoporous materials based on MCM-41: Literature review
12
1.7
Functionalization of MCM-41 materials
17
1.7.1
Methods of functionalization
18
1.8
Catalytic applications of mesoporous materials
21
1.9
Literature review on catalytic applications of MCM-41 and functionalized MCM-41 materials
21
1.9.1 Applications in adsorption and degradation
21
1.9.2 Applications in oxidation reactions
22
1.9.3 Applications in organic synthesis
24
1.10
Need of the present study
27
1.11
Objectives of the present work
28
1.12
Scope of the present work
28
1.13
Organization of the thesis
29
1.14
References
31
i
CHAPTER- 2
Experimental methods
42-65
2.1
Introduction
42
2.2
Materials
43
2.3
Synthesis of catalysts
2.3.1 Synthesis of functionalized MCM-41 materials
44
2.3.2 Synthesis of metal containing solid core mesoporous silica shell (MSCMS) materials
44
2.4
Catalyst notations
45
2.5
Characterization of the materials
2.5.1 Powder XRD
45
2.5.2 Nitrogen adsorption-desorption isotherms
49
2.5.3 Scanning electron microscopy
51
2.5.4 Transmission electron microscopy
53
2.5.5 Fourier transform infrared spectroscopy
54
2.5.6 UV-Visible spectroscopy
56
2.5.6.1 UV-Visible diffuse reflectance spectroscopy
58
2.5.7 Thermal analysis
59
2.5.8
X-ray photo electron spectroscopy
60
2.6
Catalytic studies
2.6.1 Catalytic studies on degradation of dyes over MMCM-41 materials
62
2.6.2 Catalytic studies using acid functionalized MCM-41 materials
62
2.6.3 Catalytic studies on TiSCMS and VSCMS materials
63
2.7
64
References ii
CHAPTER – 3
Synthesis, characterization of metal incorporated MCM-41 and their catalytic applications towards the degradation of Crystal violet and Rhodamine B
66-103
3.1
Introduction
66
3.2
Synthesis of MCM-41
68
3.2.1 Synthesis of MMCM-41 materials 3.3
69
Characterization of MMCM-41
3.3.1 Powder XRD studies
70
3.3.2 Nitrogen adsorption-desorption studies
71
3.3.3
SEM-EDX studies
73
3.3.4
FT-IR studies
75
3.3.5
XPS studies
77
3.4
Catalytic applications of MMCM-41 materials
3.4.1 Catalytic degradation of crystal violet
80
3.4.1.1 Effect of pH
81
3.4.1.2 Effect of H2O2 concentration
83
3.4.1.3 Effect of catalyst dosage
85
3.4.1.4 Effect of CV concentration
87
3.4.1.5 Studies on reusability of TiMCM-41 catalyst
89
3.4.2 Catalytic degradation of Rhodamine B over MMCM-41
89
3.4.2.1 Effect of pH
90
3.4.2.2 Effect of H2O2 concentration
92
3.4.2.3 Effect of catalyst dosage
94
3.4.2.4 Effect of RhB concentration
96
3.4.2.5 Studies on reusability of FeMCM-41catalyst
98
3.5
100
References iii
CHAPTER – 4
Synthesis, characterization of acid functionalized MCM-41 and their catalytic applications towards the synthesis of imines, 1-amidoalkyl 2-naphthols and benzylidine barbiturates
4.1
Introduction
104-129 104
4.1.1 Background to the synthesis of imines
105
4.1.2 Background to the synthesis of 1-amidoalkyl 2-naphthols
105
4.1.3 Background to the synthesis of benzylidine barbiturates
106
4.2
Synthesis of catalysts
4.2.1 Synthesis of MCM-41
107
4.2.2 Synthesis of sulfonic acid functionalized MCM-41
107
4.2.3 Synthesis of phosphotungstic acid functionalized MCM-41
108
4.3
Characterization of SO3HMCM-41 and PWMCM-41 catalysts
4.3.1 Powder XRD studies
108
4.3.2 Nitrogen adsorption–desorption studies
109
4.3.3 SEM – EDX studies
111
4.3.4 FT-IR studies
111
4.4
Catalytic applications of SO3HMCM-41 and PWMCM-41 in the synthesis of imines, 1-amidoalkyl 2-naphthols and benzylidine barbiturates
4.4.1 Synthesis of imines
113
4.4.2
One pot multi-component synthesis of 1-amidoalkyl 2-naphthols
116
4.4.3
Synthesis of benzylidine barbiturates
122
4.5
References
126
iv
CHAPTER- 5
Synthesis, characterization and catalytic applications of titanium and vanadium containing solid core mesoporous silica shell materials
5.1
Introduction
130-157 130
5.1.1 Background to the conversion of cyclohexanone to caprolactam
130
5.1.2 Background to the conversion of diphenylmethane to benzophenone
132
5.2
Synthesis of TiSCMS and VSCMS materials
134
5.3
Characterization of TiSCMS and VSCMS
5.3.1 Powder XRD studies
136
5.3.2 SEM and TEM studies
137
5.3.3 TG-DTA studies
139
5.3.4
141
Nitrogen adsorption-desorption studies
5.3.5 UV-Visible DRS studies
142
5.3.6 XPS studies
144
5.4
Catalytic applications of MSCMS materials
5.4.1 Conversion of cyclicketones to lactams
145
5.4.2 Catalytic oxidation of diphenylmethane to benzophenone
150
5.5
155
CHAPTER -6
References Summary and Conclusions
158-166
6.1
Summary
158
6.2
Conclusions
164
v
LIST OF FIGURES 1.
Fig. 1.1:
Scheme illustrating pore size distribution of porous materials
2.
Fig. 1.2:
4
The IUPAC classification of adsorption isotherms showing adsorption and desorption pathways
3.
Fig. 1.3:
5
Relationship between the pore shape and the adsorption-desorption isotherms
6
4.
Fig. 1.4:
Schematic representation of the M41S materials
8
5.
Fig. 1.5:
Schematic model of liquid crystal templating mechanism
10
6.
Fig. 1.6:
Schematic diagram of the mechanisms for the formation of MCM-41
7.
Fig. 1.7:
11
Schematic representation of amine functionalization of mesoporous material
18 47
8.
Fig. 2.1:
Braggs diffraction pattern from parallel crystal planes
9.
Fig. 2.2:
Quantachrome Nova 2000e surface area and pore size analyzer
51
10.
Fig. 2.3:
Schematic representation of scanning electron microscope 52
11.
Fig. 2.4:
(a) Specular reflection on mirror like surface and (b) Diffuse reflection from a matte surface
59 69
12.
Fig. 3.1:
Scheme representing formation of MMCM-41 materials
13.
Fig. 3.2:
Powder XRD patterns of TiMCM-41, VMCM-41, FeMCM-41 and CoMCM-41
70
14.
Fig. 3.3:
Nitrogen adsorption–desorption isotherms of MMCM-41
72
15.
Fig. 3.4:
SEM-EDX images of a) MCM-41 and b) TiMCM-41
73
vi
16.
Fig. 3.4:
SEM-EDX images of c) VMCM-41, d) FeMCM-41 and e) CoMCM-41
74 76
17.
Fig. 3.5a:
FT-IR spectrum of as-synthesized MCM-41
18.
Fig. 3.5b:
FT-IR spectra of TiMCM-41, VMCM-41, FeMCM-41 and CoMCM-41
19.
Fig. 3.6:
XPS Spectra of a) Carbon, b) Oxygen and c) Silicon in MMMC-41 materials
20.
Fig. 3.6:
Fig. 3.7:
78
XPS Spectra of d) Titanium, e) Vanadium f) Iron and g) Cobalt in MMCM-41 materials
21.
76
79
Variation of absorbance with time for the degradation of CV over MMCM-41 catalysts
80
22.
Fig. 3.8:
Effect of pH on the degradation of CV
82
23.
Fig. 3.9:
Effect of H2O2 on the degradation of CV
84
24.
Fig. 3.10:
Effect of catalyst dosage on the degradation of CV
86
25.
Fig. 3.11:
Effect of initial concentration of CV on its degradation using TiMCM-41
26.
Fig. 3.12:
88
Variation of absorbance with time for the degradation of RhB over MMCM-41 catalysts
90
27.
Fig. 3.13:
Effect of pH on the degradation of RhB
91
28.
Fig. 3.14:
Effect of H2O2 on the degradation of RhB
93
29.
Fig. 3.15:
Effect of catalyst dosage on the degradation of RhB
95
30.
Fig. 3.16:
Effect of initial concentration of RhB on its degradation using FeMCM-41
31.
Fig. 4.1:
97
Powder XRD patterns of a) MCM-41, b) SO3HMCM-41 and c) PWMCM-41
109 vii
32.
Fig. 4.2:
Nitrogen adsorption-desorption isotherms of a) MCM-41, b) SO3HMCM-41 and c) PWMCM-41
33.
Fig. 4.3:
SEM - EDX images of a) MCM-41, b) SO3HMCM-41 and c) PWMCM-41
34.
Fig. 4.4:
110
112
FT-IR spectra of a) MCM-41, b) SO3HMCM-41 and c) PWMCM-41
113
35.
Fig. 5.1:
Powder XRD pattern of TiSCMS
136
36.
Fig. 5.2:
Powder XRD pattern of VSCMS
137
37.
Fig. 5.3a:
SEM-EDX images of TiSCMS
137
38.
Fig. 5.3b:
SEM-EDX images of VSCMS
138
39.
Fig. 5.4a:
TEM image of TiSCMS
138
40.
Fig. 5.4b
TEM image of VSCMS
139
41.
Fig. 5.5:
TG-DTA thermograms of as-synthesized a) TiSCMS and b) VSCMS
140
42.
Fig. 5.6:
Nitrogen adsorption-desorption isotherm for TiSCMS
141
43.
Fig. 5.7:
Nitrogen adsorption-desorption isotherm for VSCMS
142
44.
Fig. 5.8:
UV-Visible DRS spectrum of TiSCMS
143
45.
Fig. 5.9:
UV-Visible DRS spectrum of VSCMS
143
46.
Fig. 5.10:
XPS spectrum of Titanium in TiSCMS
144
47.
Fig. 5.11:
XPS spectrum of Vanadium in VSCMS
145
48.
Fig. 5.12:
Effect of reaction temperature on the conversion and selectivity of oxidation of DPM to benzophenone in the presence of VSCMS
49.
Fig. 5.13:
151
Effect of reaction time on the conversion and selectivity of oxidation of DPM to benzophenone over VSCMS viii
152
LIST OF TABLES 1.
Table 3.1:
Textural characteristics of MMCM-41 materials
72
2.
Table 3.2:
Effect of pH on the degradation of CV
83
3.
Table 3.3:
Effect of H2O2 concentration on the degradation of CV
85
4.
Table 3.4:
Effect of catalyst dosage on the degradation of CV
87
5.
Table 3.5:
Effect of concentration of CV on its degradation
88
6.
Table 3.6:
Reusability of TiMCM-41 towards the degradation of CV
7.
Table 3.7:
Effect of pH on the degradation of RhB
92
8.
Table 3.8:
Effect of H2O2 on the degradation of RhB
94
9.
Table 3.9:
Effect of catalyst dosage on the degradation of RhB
96
10.
Table 3.10:
Effect of concentration of RhB on its degradation
97
11.
Table 3.11:
Reusability of FeMCM-41 towards the degradation of RhB
98
12.
Table 4.1:
Textural properties of acid functionalized MCM-41 materials 110
13.
Table 4.2:
Effect of acid functionalized MCM-41 catalysts on the synthesis of imines
14.
Table 4.3:
115
Synthesis of 1-amidoalkyl 2-naphthols in the presence of SO3HMCM-41 and PWMCM-41
15.
Table 4.4:
Table 4.5:
Table 4.6:
Table 4.7:
121
Effect of acid functionalized MCM-41 catalysts on the synthesis of benzylidine barbiturates
18.
120
Synthesis of 1-amidoalkyl 2-naphthols with substituted aromatic aldehydes in the presence of SO3HMCM-41
17.
118
Synthesis of 1-amidoalkyl 2-naphthols using different solvents in the presence of SO3HMCM-41
16.
89
124
Synthesis of benzylidine barbiturates using substituted aldehydes in the presence of PWMCM-41 ix
125
19.
Table 5.1:
Conversion of cyclohexanone and cyclododecanone over MSCMS materials
20.
Table 5.2:
148
Oxidation of DPM to benzophenone over MSCMS materials
153
x
LIST OF SCHEMES 1.
Scheme 1.1:
Functionalization of mesoporous silica by silylation reaction
2.
Scheme 4.1:
Synthesis of imines over acid functionalized MCM-41
3.
Scheme 4.2:
Plausible mechanism for the synthesis of imines over SO3HMCM-41
4.
Scheme 4.3:
Scheme 4.4:
Synthesis of 1-amidoalkyl 2-naphthols over acid
Scheme 4.5:
Scheme 4.6:
122
Plausible mechanism for the synthesis of benzylidine barbiturates over PWMCM-41 catalyst
123 135
8.
Scheme 5.1:
Schematic representation of the formation of MSCMS
9.
Scheme 5.2:
Conversion of cyclohexanone and cyclododecanone to their corresponding lactams over MSCMS
10.
Scheme 5.3:
Scheme 5.4:
Scheme 5.5:
149
Oxidation of diphenylmethane to benzophenone in air over MSCMS
12.
146
Plausible mechanism for the conversion of cyclohexanone to caprolactam over TiSCMS
11.
119
Synthesis of benzylidine barbiturates over acid functionalized MCM-41
7.
117
Plausible mechanism for the synthesis of 1-amidoalkyl 2-naphthols over SO3HMCM-41 catalyst
6.
114
116
functionalized MCM-41 5.
19
150
Plausible mechanism for the oxidation of DPM to benzophenone over VSCMS
xi
153
ABBREVIATIONS IUPAC
International Union for Pure and Applied Chemistry
nm
Nanometre
Å
Angstrom
θ
Theta
λ
Lambda
β
Beta
eV
Electron volt
ɛ
Epsilon
υ
Neu
mL
milli Litre
µL
micro Litre
M
Molarity
mM
milli Molar
h
Hour
mins
minutes
ºC
Degree centigrade
µm
Micro metre
mbar
milli bar
g
Grams
mg
milli Grams
mmol
milli Mole
RT
Room Temperature
xii
Chapter – 1 INTRODUCTION
Chapter - 1
1.1
Introduction
Catalysis Catalysis plays a significant role in the economic and sustainable growth of
chemical industries as it encompasses the production of more than 80% of all chemical products. Catalytic processes are involved in providing fuels, fertilizers, pharmaceuticals and fine chemicals. Catalysts provide sustainable and cost effective routes to transform raw materials to valuable chemicals. New catalytic materials and technologies are being continually developed to convert biomass, mitigate green house gas emissions, provide clean energy, reduce water pollution etc., to improve the quality of life. Catalysis is broadly classified into homogeneous and heterogeneous catalysis. Even though homogeneous catalysts are widely used in industry, they have the main disadvantage of difficulty in separation from the reaction mixture and often are required in stoichiometric amounts. Some of the drawbacks of homogeneous catalysts can be overcome by supporting the homogeneous catalytic species on solid materials and producing their corresponding heterogeneous catalysts. The advantages of using heterogeneous catalysts are their low cost, tolerance to a wide range of temperatures and pressures, easy removal from reaction mixture, requirement of small quantities, stability for long time and reusability. Synthesis of fine chemicals which are pharmaceutically important and having potential industrial applications is an intensely studied area of research. Development of new one-pot and multi component organic synthesis procedures has attracted the attention of many researchers owing to their applicability and easy workup. Newer catalytic systems with heterogeneous character are necessary for widening the scope of their applications. In this context, porous materials containing silica in their framework are believed to be versatile.
1
Chapter - 1
Introduction
Currently, separation and removal of organic contaminants is one of the most important and challenging areas of research in catalysis. Existing chemical treatment processes use either high energy ultraviolet light or strong oxidants of hazardous and undesirable nature. Therefore, water contaminated with toxic and biologically recalcitrant materials require new treatment technologies. The potential of new heterogeneous catalysts to degrade hazardous organic and inorganic compounds in wastewater is yet to be exploited to develop eco-friendly and economical treatment technologies. Investigations on degradation of harmful and non-biodegradable dyes in industrial waste waters would lead to the development of new technologies to mitigate water pollution. In recent years, porous materials have been the focus of material scientists due to their applications in various fields specifically in heterogeneous catalysis [1-5]. During the past two decades substantial progress has been made in the field of porous materials and several mesoporous materials have been introduced successfully [6, 7]. The design and development of porous materials for specific applications is a challenge to the chemists [8].
1.2
Porous materials and catalysis The term porous material is used for all materials that are full of pores, channels,
holes, or cavities which are deeper and permit the movement of fluids or gases. Their pore structure is usually formed in the stages of crystallization or subsequent treatment and consists of isolated or interconnected pores that may have similar or different shapes and sizes. Porous materials with small pore diameters in the range 0.3 nm to 10 µm are being studied for their molecular sieving properties [9]. Zeolites and porous silica are the important class of porous materials having wide ranging applications in catalysis. Zeolites are large family of crystalline aluminosilicates 2
Chapter - 1
Introduction
which were first discovered in 1756 by the Swedish scientist Cronstedt. Later, in the nineteenth century, studies on zeolite minerals have become the focus of researchers. The term molecular sieve was introduced by McBain in 1931 [10] when he found that chabazite, a mineral, had a property of selective adsorption of molecules smaller than 5 Å in diameter. In other words, molecular sieves retain the molecules that fit within the channels and block the larger ones from passing. Now, the phrase molecular sieve is used to describe a class of materials that exhibit selective sorption properties. However, a few years later Barrer et. al., [11] studied the sorption properties of chabazite and other porous minerals and reported that nitrogen and oxygen could be separated using zeolite that had been treated to provide the necessary shape selectivity. Breck et. al., have synthesized a number of zeolites in large scale and used them for the separation and purification of small molecules [12]. Since then, the nomenclature of this kind of porous material has become universal. The success of synthesizing crystalline aluminosilicates, aluminophosphates [13] and silicoaluminophosphates [14] made the concept of zeolites and molecular sieves more familiar. The small pore diameters in zeolites were attractive for commercial applications because they provide the opportunity for selective adsorption based on small differences in the size of molecules. There has been a continually growing interest in expanding the pore sizes of zeotype materials from the micropore region to mesopore region on account of their increasing demand in both industrial and fundamental studies [15]. In order to meet this demand, numerous experiments to create zeotype materials with pore diameters larger than those of the traditional zeolites were carried out [16]. As most of the organic templates used to synthesize zeolites affect the gel properties by filling the voids in the growing porous solid, many of these attempts used larger templates [17]. However, till
3
Chapter - 1
Introduction
1982, no success was achieved by changing the synthesis gel composition till the first ultra large pore molecular sieve, aluminophosphate was discovered [18-21]. Yanagisawa et. al., [22] have reported the synthesis of mesoporous materials with characteristics similar to that of presently well known MCM-41. Their preparation method is based on the intercalation of long chain alkyltrimethylammonium cations, into the layered silicate, followed by calcination to remove the organic species yielding a mesoporous material.
1.3
Classification of porous materials Depending on the predominant pore sizes, the porous solid materials are classified
[23] into the following three types by IUPAC (Fig. 1.1): 1. Microporous materials, having pore diameters up to 2.0 nm 2. Macroporous materials, having pore sizes exceeding 50.0 nm 3. Mesoporous materials, having pore sizes intermediate between 2.0 and 50.0 nm.
Fig. 1.1: Scheme illustrating pore size distribution of porous materials. 4
Chapter - 1
Introduction
Porous materials are also defined in terms of their adsorption properties. Adsorption of gas by a porous material is described quantitatively by an adsorption isotherm, which relates the amount of gas adsorbed by the material to the pressure of gas at a fixed temperature. Porous materials are characterized in terms of pore sizes derived from gas sorption data. IUPAC conventions have been proposed for classifying pore sizes and gas sorption isotherms that reflect the relationship between porosity and sorption [24]. The IUPAC classification of adsorption isotherms is illustrated in Fig. 1.2. The six types of isotherms are characteristic of adsorbents that are microporous (type I), nonporous or macroporous (types II, III, and VI), or mesoporous (types IV and V) [25].
Fig. 1.2: The IUPAC classification of adsorption isotherms showing adsorption and desorption pathways. The adsorption hysteresis in Fig. 1.2 (IV and V) is further classified and it is widely accepted that there is a correlation between the shape of the hysteresis loop and the texture of a mesoporous material. An empirical classification of hysteresis loops was given by the IUPAC (Fig. 1.3), which is based on an earlier classification of hysteresis by De Boer [26-28]. According to this, type H1 is often associated with porous materials
5
Chapter - 1
Introduction
consisting of well defined cylindrical like pore channels or agglomerates of approximately uniform spheres. Type H2 is ascribed to materials that are often disordered and the distribution of pore size and shape is not well defined. Materials that give rise to H3 hysteresis have slit shaped pores. The desorption curve of H3 hysteresis contains a steep associated with a force on the hysteresis loop, due to the tensile strength effect, a phenomenon that occurs for nitrogen at 77 K in the relative pressure range from 0.40 to 0.45. However, type H4 hysteresis is also often associated with narrow slit pores. The dashed curves in the hysteresis loops shown in Fig. 1.3 reflect low pressure hysteresis, which may be associated with the change in volume of the adsorbent [27].
Fig. 1.3: Relationship between the pore shape and the adsorption-desorption isotherms. Macroporous solids are not widely used as adsorbents and catalysts due to their low surface area and large non-uniform pores. Microporous and mesoporous solids, however, are widely used in adsorption, separation technology and catalysis. Owing to the need for higher accessible surface area and pore volume for carrying out chemical processes efficiently, there is a growing demand for new and highly stable mesoporous materials [26-28]. 6
Chapter - 1
1.4
Introduction
Ordered mesoporous materials Ordered mesoporous materials have received attention because of their potential
applications in catalysis, separation, selective adsorption and preparing novel functional materials [29]. These applications are possible due to the extremely high surface area combined with large and uniform pore size of mesoporous materials [30]. Furthermore, these materials have high thermal stability and adsorption capacity. Mesopores are present in aero gels and pillared layered clays which show disordered pore systems with broad pore size distributions [31]. Zeolites and zeolite like molecular sieves fulfil the requirements of ideal porous materials such as narrow pore size distribution and a readily tunable pore size. Until the late 1980s, most of the mesoporous materials were amorphous and had broad pore size distributions. In 1992, researchers at Mobil Corporation discovered the M41S family of silicate mesoporous molecular sieves with exceptionally large uniform pore structures prepared by templating silica species with surfactant molecules. This discovery has gained worldwide popularity [32-34]. Three different mesophases in this family have been identified i.e., lamellar (MCM-50), hexagonal (MCM-41) and cubic (MCM-48) phases (Fig. 1.4). The pores of this novel material are nearly as regular as zeolites, but are considerably larger than those present in crystalline materials such as zeolites, thus offering new opportunities for catalytic and other applications [35- 39]. MCM-41 has been investigated extensively because the other members in this family are either thermally unstable or difficult to synthesize [40]. In recent years, research in this area has been extended for many systems other than silica and also to the novel organic-inorganic hybrid mesoporous materials. Instead of using small organic molecules as the templating agent, as in the case of zeolites, Mobil scientists employed long chain surfactant molecules as structure directing agents during 7
Chapter - 1
Introduction
the synthesis of highly ordered materials. This supramolecular directing concept has led to a family of materials whose structure, composition, and pore size can be tailored during synthesis by variation of the reactant stoichiometry, nature of the surfactant molecule, auxiliary chemicals, reaction conditions, or by post-synthesis functionalization techniques [41, 42].
Fig. 1.4: Schematic representation of the M41S materials. MCM-50 (layered), MCM-41 (hexagonal) and MCM-48 (Cubic).
Following the initial announcement of MCM-41, there has been a surge in research activity in this area [43, 44]. Scientists have postulated that the formation of these molecular sieve materials is based on the concept of a structure directing agent called template. Templating has been defined as a process in which an organic species functions as a central structure about which oxide moieties organize into a crystalline lattice [45-47]. In other words, the template is usually an organic, around which a material, often inorganic, nucleates and grows in a skin tight manner. Upon the removal of the templating structure, its geometric and electronic characteristics are replicated in the inorganic materials [48]. MCM-41 is a mesoporous material endowed with structural simplicity and ease of preparation [41]. It possesses hexagonally packed uni-dimensional cylindrical channels
8
Chapter - 1
Introduction
whose diameters are in the range of 10-100 Å. The most prominent features of MCM-41 material are
1.5
Well defined pore shapes
Narrow distribution of pore sizes
High pore order over micro length scales
Amenable for tailoring and fine tuning of pore dimensions
Large pore volumes
Exceptionally high sorption capacity
Very high surface area
Easily modifiable surface properties
Mechanisms for the formation of mesoporous materials In M41S materials, a liquid crystal templating (LCT) mechanism has been
proposed by the Mobil scientists in which supramolecular assemblies of surfactant micelles act as structure directing agents for the formation of the mesophase (Fig. 1.5). This mechanism behind the composite mesophase formation is best understood for the synthesis under high pH conditions. Under these conditions, anionic silicate species, and cationic or neutral surfactant molecules, cooperatively organize to form hexagonal, lamellar, or cubic structures [41, 43]. The composite hexagonal mesophase is suggested to be formed by condensation of silicate species (formation of a sol-gel) around a preformed hexagonal surfactant array or by adsorption of silicate species onto the external surfaces of randomly ordered rod like micelles through columbic interactions. These randomly ordered composite species spontaneously get packed into a highly ordered mesoporous phase with an energetically favourable hexagonal arrangement, accompanied by silicate condensation. This process
9
Chapter - 1
Introduction
initiates the hexagonal ordering in both the surfactant molecules and the final product as shown in Fig. 1.5.
Fig. 1.5: Schematic model of liquid crystal templating mechanism.
Several research groups further revised this liquid crystal templating mechanism. Chen et. al., [49] studied this mechanism by carrying out in situ
14
NNMR spectroscopy.
They have found that the randomly ordered rod like organic micelles were interacting with silica species to form two or three mono layers of silica on the outer surface of the micelles. Then these composite species spontaneously self organize into a long range ordered structure to form the final hexagonal packing mesoporous MCM-41. In addition to the above mechanism, two other liquid crystal template mechanisms are proposed. The first mechanism was suggested by Monnier et. al. [32]. It is proposed that the surfactant is initially present in the lamellar phase regardless of the final product. This lamellar mesophase transforms to the hexagonal phase as the silicate network condenses and grows (Fig. 1.6 (i)). The second mechanism (Fig. 1.6 (ii)) proposed by Steel et. al., [50] suggests that, as silica source is introduced into the reaction gel, it dissolves into the aqueous regions 10
Chapter - 1
Introduction
around the surfactant molecules, and then promotes the organization of the hexagonal mesophase. The silicate first becomes ordered into layers between which the hexagonal mesophases of micelles are sandwiched.
Fig. 1.6: Schematic diagram of the mechanisms for the formation of MCM-41. i) by Monnier et. al. and ii) by Steel et. al.
11
Chapter - 1
1.6
Introduction
Synthesis of mesoporous materials based on MCM-41: Literature
review Mesoporous silica materials synthesized by the use of self assembled surfactants as templates have gained the attention of number of research groups in recent years. The family of highly ordered mesoporous siliceous materials, have the highest priority as new potential molecular sieves and supporting materials [41, 42]. Parallel to the development of M41S materials, many other ordered mesophases with similar properties have also been synthesized (e.g., HMS [51], FSM-16 [52], PCH [53], SBA [54], MSU [55], and KIT [56].) The usual procedure for the formation of meso-structured silica is to employ surfactants as structure directing agents and a simple organo silicate compound, such as tetramethyl orthosilicate (TMOS) or tetraethyl orthosilicate (TEOS) as the silica source. These can be regarded as tetraesters of silicic acid (Si(OH)4). Silicic acid polymerizes very easily and has never been isolated. Depending on the pH and the presence of salts, the condensation may lead to particle growth via processes that are relatively well understood [57]. The structure formed can be influenced by a number of parameters like choice of precursor, choice of surfactant, presence of specific ions, condensation rate, which are mainly governed by the pH and the temperature used [58]. It is likely that the control of crystal morphology that can be obtained at low temperature is due to the reaction proceeding under thermodynamically controlled conditions [59]. Kresge and co-workers [60] reported the first self assembly process to synthesize MCM-41 involving electrostatic interaction between positively charged quaternary ammonium micelles as surfactant (S+) and inorganic silica anions (I-) as framework precursors. They suggested a liquid crystal templating mechanism where surfactant molecules act as templates. The synthesis experiment was done under acidic conditions 12
Chapter - 1
Introduction
and at temperatures of about 100 ºC. The surfactant was not recovered, but simply burned off by calcination at elevated temperatures. The resulting MCM-41 showed hexagonal arrays of one dimensional pore with pore size ranging approximately from 1.5 to 10 nm and a surface area of nearly 1000 m2g-1. Huo et. al., [61] reported a generalized approach to the synthesis of periodic mesoporous materials using cationic and anionic surfactants under a range of pH < 7 and pH 10-13. They reported that, the cationic surfactants (S+) are useful for the structuring of anionic inorganic species (I-) (S+I- mesostructures). On the other hand, anionic surfactants (S-) such as alkylsulfonates, have been employed for structuring cationic inorganic species (I+) (S-I+ mesostructures). Organic-inorganic combinations with identically charged partners (i.e., S+ and I+ or S- and I-) are possible, but, the formation of the meso structures is mediated by the counter charged ions which must be present in stoichiometric amounts (S+X-I+ and S-M+I- mesostructures, where X- = Cl- or Br- and M+ = Na+ or K+) [62]. It was concluded that, in cases where the degree of condensation of the oligomeric ions, which form the walls is low, the removal of the surfactants leads to the collapse of the ordered mesostructure. The study also showed that similar surfactants in acidic medium produce mesoporous structures with similar, but slightly larger d-spacings than those obtained by basic medium synthesis [63]. Setoguchi et. al., [64] performed the synthesis of hexagonal mesoporous silica by adding water glass to a highly acidic solution of different cationic surfactants. Both the mean pore diameter and the d-spacing obtained from X-ray diffraction, increased with increasing alkyl chain length of the first series of surfactants. A cationic Gemini surfactant, (C12H25N+(CH3)2–(CH2)2–N+(CH3)2C12H25)2Br–, was used for making mesoporous silica with cubic geometry from sodium silicate [65].
13
Chapter - 1
Introduction
Gemini surfactants are known to self assemble at much lower concentration than their monomeric counterparts and hence are exciting structure directing agents. Tanev et. al., [51] have synthesized mesoporous MCM-41 using a neutral surfactant templating route (SoIo). In the neutral surfactant approach, a self assembly between neutral surfactants So (e.g., primary amine) and neutral inorganic precursors Io (e. g., tetraethyl orthosilicate) is based on hydrogen bonding. This approach affords mesostructures with larger wall thicknesses, small scattering domain sizes, and complementary textural mesoporosity. The thicker pore walls improve the thermal and hydrothermal stability of the mesopore framework, and the small crystallite domain size introduces textural mesoporosity, which facilitates accessing the framework-confined mesopores. A great advantage of this synthesis route with respect to that developed by Mobil researchers was that the neutral pathway allows for the facile recovery of the template from the mesopores by simple solvent extraction and the formation of frameworks with a much higher surface silanol concentration compared to those of their electrostatically assembled and calcined counterparts. Corma et. al., [66] studied the preparation of mesoporous materials, MCM-41, under highly acidic conditions. They reported that the formation of S+X-I+ type interactions occur and that the removal of the surfactant is easy by simply washing with water at room temperature. The facile removal of the surfactant indicates the weak interaction between the surfactant and the inorganic silica. They suggest that the mesostructures are formed by IoX-S+ interactions, giving a neutral structure instead of the positively charged framework that was proposed previously [61]. Sayari et. al., [67] reported the effect of the length of the hydrocarbon chain of the surfactant on the pore size and the final structure. They concluded that the pore size can be tailored by changing the length of the hydrocarbon chain. In other words, increase in 14
Chapter - 1
Introduction
the chain length will lead to an increase in the pore size. They used alkylhexadecyldimethylammonium derivatives, i.e., (C16H33)(CnH2n+1)(CH3)2N+ with n = 1-12, and concluded that, a hexagonal phase was obtained for n = 1, 3, 5, 7, while a lamellar phase was obtained for all other templates used. Beck et. al., [41] have studied the effect of the addition of an auxiliary organic, mesitylene to MCM-41 synthesis mixtures on the pore size. This study indicated a proportional increase in the amount of mesitylene added and the pore size as well as the d-spacing of the XRD d100 peak. However, above a certain concentration of the additive, the resulting MCM-41 had wide pore size distribution. Ulagappan et. al., [68] reported the synthesis of MCM-41 with different pore sizes utilizing alkanes of different chain lengths together with the surfactant as a template. The characterization results suggested that the surfactant molecules in the micelle are fully extended and the size of such micelles increased with the chain length of the n-alkane at least until the molecule has 15 carbon atoms. Namba et. al., [69] prepared silica MCM-41 materials hydrothermally by using 1,3,5-trimethylbenzene or 1,3,5-triisopropylbenzene as an auxiliary chemical. The BJH pore size of MCM-41 increased up to 12 nm with increased amounts of the former. However, MCM-41 materials prepared with 1,3,5-trimethylbenzene were found to display irregular pore arrangements and a half of these materials exhibit low thermal and hydrothermal
stabilities.
Whereas,
MCM-41
materials
prepared
with
1,3,5-triisopropylbenzene as an auxiliary chemical displayed regular pore arrangements and high thermal and hydrothermal stabilities, but their BJH pore sizes did not go over 4.0 nm. They also reported the synthesis of MCM-41 materials hydrothermally by employing
mixture
of
hexadecyltrimethylammonium
bromide
and
dodecyltrimethylammonium bromide with different molar ratios [70]. MCM-41 materials 15
Chapter - 1
Introduction
thus prepared were highly ordered and they were able to control the pore size by changing the molar ratio of the two surfactants. They found that very high temperature is needed for calcination, but this led to lowering of hydroxyl concentration. Coleman et. al., [71] studied the effects of surfactant concentration on the regularity and morphology of mesoporous silica prepared in the tetramethyl orthosilicate / Brij-56 / HCl system. For a fixed weight ratio of 1.8 TMOS: 1.0 Brij, they found that very low surfactant concentrations gave a disordered structure but by increasing the surfactant concentration, the order rapidly increased and resulted in a hexagonal mesopore structure. On increasing the surfactant concentration further, the hexagonal structure became progressively more disordered and wormhole like structure was obtained. In the synthesis of mesoporous silica from micellar solutions, the order was found to depend on the surfactant concentration. Here, the addition of silicate precursor resulted in phase separation in the form of small domains having a pseudo liquid crystal structure that acts as a model for the formation of silica network. However, for both routes it is only possible to obtain ordered mesostructures through the correct proportions of TMOS, surfactant and water. Ryoo et. al., [72] reported the synthesis of highly ordered MCM-41 by adding acetic acid to the reaction system to shift the equilibrium between the reactants and the mesophases formed towards the desired direction. They suggested that the MCM-41 phase was in dynamic equilibrium with the reactants. Therefore, the addition of acid to the reaction mixture was supposed to neutralize the hydroxyl and shift the equilibrium towards the positive direction. Cheng et. al., [73] described a simple method of controlling the channel diameter of the MCM-41 in the range of 26.1 - 36.5 Å and the wall thickness in the range 13.4 - 26.8 Å while using the same gel mixture. This is achieved by varying the synthesis 16
Chapter - 1
Introduction
temperature in the 70 - 200 °C range and reaction times in the range 0.5 - 96 h. MCM-41 with wider and thicker walled channels was prepared at high temperature and at longer reaction times. Thick-wall MCM-41 has higher thermal stability but lower surface area. This material with the thickest channel wall can withstand calcination at nearly 1000 °C with little structural damage. They also reported that highly crystalline MCM-41 with very narrow pore size distribution (1.5 Å), high surface area (1185 m2g-1), large grain size and thick channel walls (~ 17 Å) can be prepared in alkali free media by the use of tetramethylammonium hydroxide as the source of base [74]. Properties of these products depended on the source and concentration of the reactants, the gel aging time, temperature, and duration of the synthesis. Gruin et. al., [75] reported a new method for synthesis of MCM-41 at room temperature by co-precipitation method and investigated the influence of various surfactants on the pore characteristics and stability of the materials. They concluded that the
MCM-41
materials
synthesized
by
homogeneous
co-precipitation
using
cetyltrimethylammonium bromide (CTAB) as the surfactant have wide ranging applications and improved pore characteristics. These materials also have shown greater stability.
1.7
Functionalization of MCM-41 materials Besides the extension from silicate to non silica mesoporous materials, one other
important way of modifying the physical and chemical properties of mesoporous silica materials is by the incorporation of organic and inorganic components. The incorporation is either on the silicate surface, inside the silicate wall, or trapped within the channels [76]. Fig. 1.7 illustrates the functional groups in the internal pore surface. Introduction of organic groups in the mesoporous materials permit the tuning of surface properties like hydrophilicity, hydrophobicity, acidity, basicity, binding to guest 17
Chapter - 1
Introduction
molecules and surface reactivity. They also protect the surface from chemical attack, make the surface hydrophobic by silylation to preclude water attack, and modify bulk properties of the materials while at the same time stabilize the materials towards hydrolysis [77].
Fig. 1.7: Schematic representation of amine functionalization of mesoporous material.
Surface functionalized mesoporous materials are of great interest owing to their potential applications in areas like catalysis and adsorption. For instance, mesoporous silica having thiol groups on the pore surface showed high adsorption efficiency for heavy metals such as Hg, Ag, and Cd ions [78]. 1.7.1
Methods of functionalization Various literature reports describe methods for functionalizing the pore surfaces
of mesoporous solids such as MCM-41 [79-87]. These hybrid materials are generally synthesized via two methods [88]. The first one is the post synthesis grafting method in which pore wall surface of the synthesized inorganic mesoporous materials is modified with organosilane compounds after the surfactant removal. The mesoporous materials possess silanol (Si-OH) groups that facilitate the attachment of the organic functions to 18
Chapter - 1
Introduction
the surface. Silylation is the most commonly used reaction for the surface modification. Esterification is another reaction employed to carry out surface modification [89, 90]. The silylation reaction method is achieved by one of the following reactions, as shown in Scheme 1.1. R Si OH
Cl +
Si OSiR3 + HCl
Si R R
Si OH
R R1 O + Si R
1 Si OSiR3 + HOR
R
2
Si OH
2 Si OSiR3 + NH3
+ HN(SiR3)2
Scheme 1.1: Functionalization of mesoporous silica by silylation reaction.
The original structure of the mesoporous support is typically maintained after modification of the surface. Silylation occurs on all surface groups of the silica including the free or geminal silanols. However, hydrogen bonded silanol groups are less accessible to the modification because of the formation of hydrophilic networks [91]. In the post-synthesis grafting method, the host materials should be completely dried before adding the modification precursors in order to avoid the self-condensation of the precursors in the presence of water. The second method for modifying the internal surface of the mesoporous materials is direct synthesis. This method is based on the co-condensation of a tetraalkoxysilane (siloxane) and one or more organoalkoxy precursors through a sol-gel process. Siloxane precursors work as the main framework of the mesoporous materials while the organoalkoxy precursors contribute to the building of the framework and serve 19
Chapter - 1
Introduction
as functional groups on the surface [74-78]. The direct synthesis has advantage over the grafting method in terms of producing mesoporous materials with high loading of the functional groups. Grafting of the mesopore surface with both passive and reactive surface groups has been reported [92]. However, products obtained by post synthesis grafting are often structurally better defined and hydrolytically more stable. Although, pore sizes can be controlled to some extent by both the methods, it is more easily achieved by grafting. A recent development in functionalization of mesoporous materials has been the study of organic-inorganic species covalently bonded inside the mesoporous wall structure. The surfactant templated synthesis of these materials uses a precursor that has two trialkoxysilyl groups connected by an organic bridge [93]. This new technique allows stoichiometric incorporation of organic groups into silicate networks, resulting in higher loading of organic functional groups than by the grafting or direct synthesis method [94]. The major problem with this approach is the lack of availability of chemicals comprising of two trialkoxysilyl groups. Synthesis of MCM-41 containing copper and zinc with different metal contents at room temperature by direct insertion of metal ions in the initial stage of synthesis is reported to give materials with long range hexagonal ordering [95]. V, Fe and Cr incorporated MCM-41 materials have been synthesized by hydrothermal method. Subsequently, it was loaded with TiO2 following sol-gel technique. Characterization techniques revealed that the transition metal ions were well incorporated inside the framework of MCM-41 before the loading of titania [96]. However, some amount of transition metal ions was redistributed onto the surface upon the loading of titania. This redistribution phenomenon was strongly promoted by the loaded titania and accompanied
20
Chapter - 1
Introduction
by a loss of specific surface area, average pore diameter and dispersion of transition metal ions inside the framework of MCM-41.
1.8
Catalytic applications of mesoporous materials Mesoporous materials have found applications in diversified areas like adsorption,
separation, organic synthesis, energy storage, drug delivery and oxidative conversions owing to their stability and tuneable surface characteristics [41]. These materials possess large pore size, and hence allow variety of molecules to interact with their surface. The formation of porosity in two or three different length scales in an ordered manner with interconnectivity between the pores and with hierarchical structure would be advantageous for a variety of applications because the reactant molecules need to access readily inside of the pore structure [97 - 99]. Adequate diffusion of molecules through the catalyst pores allows direct interaction with acidic sites on the wall surface, promoting the reactions. In recent years highly active catalysts have been synthesized by introducing transition metals, noble metals and their oxides into ordered mesoporous silica materials for enhancing the scope of their applications [100 - 102].
1.9
Literature
review
on
catalytic
applications
of
MCM-41
and
functionalized MCM-41 materials 1.9.1 Applications in adsorption and degradation Mesoporous materials and functionalized mesoporous materials have been used as efficient catalysts towards adsorption of heavy metals, degradation of organic pollutants, hydro-desulfurization, hydro-denitrogenation, cracking of petrol, and supports for chromatographic separation [103 - 108]. Adsorption and separation of rare earth metals on to the surface of modified mesoporous materials has been investigated [109]. It is
21
Chapter - 1
Introduction
reported that the sorption capacity of these materials depend on the pore structure of the materials [110]. Studies on adsorption of phenol and o-chlorophenol over MCM-41 showed that its adsorption capacity is significant. Hydrophobicity created by surfactant in the MCM-41 is suggested to be the cause for this. The adsorption capacity is also found to be influenced by pH, temperature and other reaction parameters [111]. Li et. al., found that Co doped mesoporous titania with a crystalline framework (Co–MTiO2) and titania-loaded Co doped MCM-41 (TiO2/Co-MCM-41) are efficient for the degradation of gentian violet under visible light irradiation [112]. Monash et. al., reported the synthesis of sulphate modified MCM-41 at room temperature and investigated the adsorptive character of the synthesized materials towards the decolourization of crystal violet [113]. 1.9.2 Applications in oxidation reactions Oxidation reactions are among the most industrially important class of reactions [114 - 121]. Commonly used oxidation processes include hydroxylation of aromatics, epoxidation of olefins, sulphoxidation of organic sulphides [122 - 140]. MCM-41 based materials due to their effective catalytic behaviour, are ideal for developing environmental friendly methods for carrying out oxidation reactions. CoMCM-41 and NiMCM-41 have been found to be active for the oxidation of benzene to phenol and styrene to benzaldehyde with H2O2 as oxidant. CoMCM-41 with higher Co content showed higher activity and selectivity towards the formation of benzaldehyde from styrene and phenol from benzene. But, activity of these catalysts was found to be low in the oxidation of 1-hexene [141]. Oxidation of o-xylene to phthaleic anhydride in the presence of vanadium supported mesoporous materials is reported. The catalyst prepared by grafting showed 22
Chapter - 1
Introduction
good phthaleic anhydride selectivity [142]. Studies on the hydroxylation of 1-naphthol, phenol and epoxidation of norborenes in the presence of H2O2 or TBHP over SnMCM-41 showed that it is a good catalyst [143]. MnMCM-41 has shown significant catalytic activity for the oxidation of ethylbenzene by TBHP under mild reaction conditions [144]. Catalytic studies of anthracene oxidation in liquid phase at 348 K, using TBHP as oxidant showed that
the Ti-HMS
sample is
more effective
in
producing
9,10-anthraquinone than MCM-41 and AlMCM-41. It is reported that the performance of catalysts is directly related to the surface area, nature of the heteroatom introduced, Si/Ti molar ratio and strength and concentration of acid sites [145]. α-Pinene oxidation with hydrogen peroxide using hydrothermally prepared Ti-MCM-41 was reported. The major products in the reaction were observed as verbenone, verbenol and campholenic aldehydes [146]. Catalytic oxidation of eicosanol in the presence of TiMCM-41 and Cu or Cr supported TiMCM-41 catalysts has been investigated. Cr/TiMCM-41 has shown greater catalytic activity in the presence of oxygen than Cu/TiMCM-41 [147]. Qian et. al., have established that Bi containing MCM-41 catalyses the liquid phase oxidation of cyclohexane with oxygen as oxidant in solvent-free conditions [148]. Kilo et. al., have investigated oxidative dehydrogenation of propane in the gas phase and oxidation of cyclohexene with hydrogen peroxide in liquid phase over NbMCM-41 and vanadium containing NbMCM-41. It was found that, defects formed due to incorporation of niobium in MCM-41 are responsible for higher selectivity in gas and liquid phase oxidations. It was proposed that defect holes, generated by niobium permit the access of reagents to the active sites and make their diffusion easier [149]. Gomes et. al., investigated the catalytic activity of CrMCM-41, CuMCM-41 and VMCM-41 materials in the wet air oxidation of aniline aqueous solutions using a high 23
Chapter - 1
Introduction
pressure reactor at 200 °C with oxygen. They have observed that CuMCM-41 showed more significant catalytic activity with good conversion and selectivity after 2 h than other catalysts. They have also observed the good activity and stability of catalysts after re-use [150]. 1.9.3 Applications in organic synthesis Synthesis of organic compounds by using heterogeneous catalysts based on MCM-41 is one of the actively pursued areas of research. These attempts have gained importance because of their potential as green catalysts. AlMCM-41 and its supported catalysts were found to catalyze the solvent free synthesis of coumarin and coumarin derivatives [151]. Heteropoly acid supported MCM-41 and other supported MCM-41 were found to efficiently catalyse the synthesis of pyrimadines, tetrasubstitued imidazoles, 1,4-dihydropyridines under solvent free and environmentally viable conditions [152, 153]. Use of AlMCM-41 supported with phosphotungstic acid for symmetrical and unsymmetrical ring opening of succinic anhydride revealed that monoethyl succinate and diethyl succinate were obtained as products. Unsymmetrical alcoholysis of succinic anhydride with methanol and 1-butanol was also carried out successfully with the above catalyst [154]. Karthikeyan et. al., reported that heteropoly acid supported MCM-41efficiently catalyses the condensation of dimedone and aromatic aldehydes in ethanol and other solvents under liquid phase conditions to yield the corresponding xanthenedione derivatives [155]. MCM-41 and sulfonated MCM-41 materials have been employed as catalysts for the synthesis of dihydropyramidines and 14-aryl 14H-dibenzo (a, j) xanthenes under solvent free conditions [156]. These catalyst were also found to be suitable for the synthesis of polyhydroquinolines via Hantzch reaction in solvent free condition [157]. 24
Chapter - 1
Introduction
MCM-41 has been used as an efficient acid catalyst for Friedel–Crafts alkylation of 2,4-di-tert-butylphenol with cinnamyl alcohol and for the tetra-hydropyranylation of alcohol and phenol [158]. The catalytic activities of the HPW/MCM-41 catalysts for the Pechmann condensation, Friedel–Crafts acylation and esterification reactions are reported. The catalysts were found to be reusable without significant loss in activity [159]. Vanadium-incorporated MCM-41 synthesized by direct hydrothermal procedure, showed very high activity and selectivity in the production of ethylene from ethanol. Ethylene selectivity showed a significant increase with increase in temperature above 300 °C, while relatively high acetaldehyde selectivities were observed at lower temperatures [160]. Studies on vapor phase cyclization of cyclohexanone, formamide and ammonia at 350 ◦C over Co-modified Al–MCM-41 revealed that imine and amine intermediates are formed. These on further dehydrogenation and deamination gave octahydro acridylamine as a major product [161]. Investigations on vapour phase t-butylation of phenol over FeAlMCM-41 showed superior performance in acid catalyzed t-butylation of phenol employing t-butanol as the alkylation agent. They have found that a reaction temperature of 200 ºC is optimum for good phenol conversion with high selectivity for 4-tert-butylphenol [162]. Vapour phase reactions of t-butylbenzene and t-butylacetate in the temperature range 200–400 ◦C over AlMCM-41 have been studied. It was found that several dibutylbenzene derivatives are formed. The selectivity towards 1,3-di-tert-butylbenzene was found to increase with the increase in temperature. Conversely, the selectivity of 1,4-di-tert-butylbenzene decreased [163].
25
Chapter - 1
Introduction
The vapour phase reaction of phenol with ethyl acetate over (Al,Zn)MCM-41 has been studied. Phenol conversion is found to increase with increase in temperature up to 350 ◦C and then decreased at 400 ◦C. The decrease in conversion at higher temperature was attributed to blocking of active sites by coke. They have reported that low activity of (Al,Zn)MCM-41 is due to its high hydrophilicity, which reduces the chemisorption of ethyl acetate on its surface [164]. Studies on esterification of maleic anhydride with ethanol over AlMCM-41 and Hβ zeolite in liquid phase at 80-120 ºC have revealed that monoesterification of maleic anhydride to monoethyl maleate is very rapid [165]. Parida et. al., investigated base catalytic activity of hybrid MCM-41 materials such as amine and surfactant functionalized materials, for condensation reaction between benzaldehyde and diethyl malonate in solvent free, room temperature synthesis of cinnamic acid. They correlated the catalytic activity with the surface and textural properties of catalysts [166]. Ahn et. al., reported FeCl3-supported nanopore silica catalyzed, microwave assisted
synthesis
of
3,4-dihydro-pyrimidin-2-(1H)-ones
under
solvent
free
condition [167]. A convenient method for the synthesis of 2-arylbenzothiazoles in the presence of catalytic amounts of Cu(OAc)2/MCM-41 under ultrasonic irradiation is reported. Short reaction times, easy and quick isolation of the products, reusability of the catalyst and excellent yields were cited as the main advantages of this procedure. This catalyst has also been used for one-pot synthesis of 1,2,3-triazoles via the three component coupling reaction between benzyl or alkyl bromides, terminal alkynes, and sodium azide. The catalyst showed high catalytic activity and 1,4-regioselectivity for the [3 + 2] Huisgen cycloaddition [167, 168].
26
Chapter - 1
Introduction
This review of literature on the synthesis and applications of MCM-41 type of mesoporous materials revealed that, it is an area worthy of exploration both in terms of developing new methods of synthesizing the materials and also applying them to catalyze reactions of industrial and environmental importance.
1.10
Need of the present study Survey of the literature revealed that, mesoporous silica materials have
tremendous potential for applications in the areas of adsorption and catalysis. Nevertheless, most of the reports pertain to their synthesis using hydrothermal method at high temperatures which require long reaction time for crystallization of the material. There are very few reports on the synthesis of functionalized MCM-41 materials at room temperature by co-precipitation method and more so in basic medium. There is a need to evolve simple synthesis methods utilizing very less concentration of surfactant but yet yielding highly stable and functional mesoporous silica materials in quick time. Further, reports are scanty on the synthesis of core-shell mesoporous silica materials at room temperature using amines in the reaction medium. Use of a porogen like octadecyltrichloro silane in the synthesis process is expected to enhance the pore size by several folds as compared to commonly used surfactant templates. These materials with large pores can adsorb and catalyze the reactions of bulkier molecules. Degradation of dyes is important for environmental remediation to protect aquatic life and safe-guard human health. Degradation of dyes by conventional oxidation methods requires harsh reaction conditions or leave toxic wastes. There is a need to develop a suitable method for efficient degradation of dyes using heterogeneous catalysts. Currently, most of the industrially and pharmaceutically important organic compounds are produced by employing high temperatures, toxic reagents and expensive solvents. There is ample scope to improve these processes by using inexpensive and 27
Chapter - 1
Introduction
reusable heterogeneous catalysts. Functionalized mesoporous silica materials of the type synthesized in this work have the potential to be the catalysts for such green chemical approaches.
1.11
Objectives of the present work
To synthesize metal incorporated MCM-41, acid functionalized MCM-41 and solid core mesoporous silica shell materials by co-precipitation method at room temperature. To characterize the above synthesized materials by various physico-chemical techniques for establishing their structural and morphological properties. To study the catalytic degradation of dyes over metal incorporated MCM-41 materials at room temperature using hydrogen peroxide and to optimize the conditions for efficient degradation. To study the catalytic applications of acid functionalized MCM-41 materials in the synthesis of imines, 1-amidoalkyl 2-naphthols and benzylidine barbiturates and develop eco-friendly synthetic processes.
To investigate the catalytic behaviour of solid core mesoporous silica shell materials towards conversion of cyclicketones to lactams and diphenylmethane to benzophenone.
1.12
Scope of the present work The work presented in this thesis deals with synthesis of metal and acid
functionalized mesoporous materials. All the catalytic materials have been synthesized by co-precipitation method at room temperature. Ti, V, Co and Fe compounds were used for incorporation into the framework of MCM-41 while only Ti and V were used for metal
28
Chapter - 1
Introduction
incorporation into solid core mesoporous silica shell materials. Sulfonic acid and phosphotungstic acid were used for acid functionalization of MCM-41. Crystal violet and Rhodamine B dyes, which are very toxic and biologically non-degradable, are chosen for degradation studies using metal incorporated MCM-41 materials as catalysts. Studies on the synthesis of organic compounds using acid functionalized
MCM-41 were confined to the synthesis of imines, 1-amidoalkyl
2-naphthols and benzylidine barbiturates. Reactions chosen for catalysis by metal containing solid core mesoporous silica shell materials are conversion of cyclohexanone and cyclododecanone to their respective lactams and diphenylmethane to benzophenone. Techniques used for the characterization of synthesized materials included Powder XRD, SEM-EDX, TEM, TG-DTA, nitrogen adsorption-desorption, FT-IR, UV-Visible DRS and XPS.
1.13
Organization of the thesis The research work presented in this thesis is organized into six chapters. In
Chapter – 1, a brief introduction to catalysis, mesoporous materials and literature review on synthesis and applications of mesoporous silica is presented. Chapter – 2, deals with brief description of various experimental techniques employed, methods and materials used to synthesize, characterize and study catalytic applications of redox and acid functionalized mesoporous silica materials. Chapter – 3, comprises of the synthesis, characterization of metal incorporated MCM-41 materials and their catalytic applications towards the degradation of Crystal violet and Rhodamine B dyes. Chapter – 4, presents the synthesis and characterization of acid functionalized MCM-41. Catalytic applications of these materials towards one-pot synthesis of imines, amidoalkyl naphthols and benzylidine barbiturates are also discussed. 29
Chapter - 1
Introduction
Chapter – 5, deals with the synthesis and characterization of novel solid core mesoporous silica shell materials at room temperature. Catalytic applications of these materials towards the conversion of cyclohexanone and cyclododecanone to their lactams and diphenyl methane to benzophenone are discussed. In Chapter - 6, a summary of the results and important conclusions drawn from the work are presented.
30
Chapter - 1
1.14 1.
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V.
J. Mol. Catal. A 2006, 243, 183. 155.
Karthikeyan, G.; Pandurangan, A. J. Mol. Catal. A 2009, 311, 36.
156.
Bigdeli, M. A.; Heravi, M. M.; Mahdavinia, G. H. J. Mol. Catal. A 2007, 275, 25.
157.
Nagarapu, L.; Kumari, M. D.; Kumari, N. V.; Kantevari, S. Catal. Comm. 2007, 8, 1871.
158.
Kloetstra, K. R.; Bekkum, H. J. Chem. Soc., Chem. Commun. 1995, 1005.
159.
Khder, A. E. R. S.; Hassana, H. M. A.; Shall, M. S. E. Appl. Catal. A 2012, 411– 412, 77. 40
Chapter - 1
Introduction
160.
Dogu, G. T. Ind. Eng. Chem. Res. 2006, 45 (10), 3496.
161.
Ratnamala, A.; Lalitha, K.; Reddy, J. K.; Kumari, V. D.; Subrahmanyam, M. J. Mol. Catal. A 2008, 279, 112.
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Vinu, A.; Nandhini, K. U.; Murugesan, V.; Böhlmann, W.; Umamaheswari, V.; Poppl, A.; Hartmann, M. Appl. Catal. A 2004, 265, 1.
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Sudha, S.; Palanichamy, M.; Balasubramanian, V. V.; Arabindoo, B.; Murugesan, V. J. Mol. Catal. A 2006, 255, 220.
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Shanmugapriya, K.; Anuradha, R.; Palanichamy, M.; Arabindoo, B.; Murugesan, V. J. Mol. Catal A 2004, 221, 145.
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Bhagiyalakshmi, M.; Shanmugapriya, K.; Palanichamy, M.; Arabindoo, B.; Murugesan, V. Appl. Catal. A 2004, 267, 77.
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41
Chapter- 2 EXPERIMENTAL METHODS
Chapter –2
2.1
Experimental Methods
Introduction Characterization of catalysts is an integral part of study of catalysis. It enables us
to correlate the performance of catalysts with their properties. The physico-chemical characteristics of a catalyst largely depend on the experimental conditions used for its synthesis. During the characterization, the sample is subjected to suitable agents like photons or electrons to obtain data related to the surface and morphological properties of the catalyst. Such data provides information related to the sample properties like size, shape, phase, crystallinity, surface structure and chemical composition. A suitable technique is needed to assess the structure and properties of materials under investigation. Some techniques are qualitative in nature and offer information pertaining to the appearance and morphology. Whereas, some techniques are quantitative and provide information on chemical composition, size, concentration etc. Characterization also helps us to understand the mechanism of the processes taking place on the surface of the catalytic materials. Techniques such as powder X-ray diffraction, nitrogen adsorption-desorption, spectroscopy and electron microscopy are used to find the characteristics of the synthesized catalysts. Characterization techniques employed in the present studies, along with their applications are listed below: Powder X-ray diffraction (Powder XRD) method for pore structure, crystallinity and phase identification Nitrogen adsorption-desorption technique for surface area and pore characteristics Scanning electron microscopy (SEM) for morphological features Transmission electron microscopy (TEM) for particle size and shape analysis Energy dispersive X-ray absorption (EDX) for elemental composition of the catalyst
42
Chapter –2
Experimental Methods
Fourier transform infra red spectroscopy (FT-IR) for identification of functional groups in the framework of the material UV-Visible diffuse reflectance spectroscopy (UV-Visible DRS) for determining co-ordination of metal ions in functionalized materials Thermogravimetry / Differential thermal analysis (TG-DTA) for thermal behavior of as-synthesized samples X-ray photo electron spectroscopy (XPS) for surface elemental composition and binding energies of elements in the framework of the materials
2.2
Materials The following chemicals were used in the synthesis of catalysts that are
employed in the present studies.
Chemical
Formula
Company
Tetraethyl orthosilicate
(C2H5O)4Si
Aldrich, 98%
Titanium tetraisopropoxide
Ti[OCH(CH3)2]4
Aldrich, 98%
Vanadyl acetylacetonate
OV(C5H7O2)2
Aldrich, 98%
Vanadium Pentoxide
V2O5
Aldrich, 99%
Cobalt acetate
Co(CH3COO)2.2H2O
Aldrich, 97%
Ferric Nitrate
Fe(NO3)3.9H2O
SRL, 99%
Cetyltrimethylammonium bromide
CH3-(CH2)15-N(CH3)3Br
SDFCL, 98%
Ethyl alcohol
C2H5OH
Merck, 99%
Phosphotungstic acid
H3PW12O40
Merck, 97%
Sulphuric acid
H2SO4
Merck, 98%
Triethyl amine
N(C2H5)3
SRL, 95%
Octadecyltrichloro silane
CH3(CH2)17SiCl3
Aldrich, 98%
Ammonia
NH3
Merck, 25%
43
Chapter –2
2.3
Experimental Methods
Synthesis of catalysts Activity of a catalyst is strongly influenced by its method of preparation [1].
Co-precipitation method under room temperature conditions was followed for the synthesis of all the catalytic materials used in the studies presented in the thesis. 2.3.1 Synthesis of functionalized MCM-41 materials In a typical synthesis, template (cetyltrimethylammonium bromide) was dissolved in water and catalytic amount of ammonia was added to the solution. The resultant mixture was stirred for about 1 h. Required quantities of silica precursor followed by ethanol were added to the above mixture. An in situ method was adopted for the incorporation of metal ions into the framework of the material. Direct addition of metal precursor into the reaction mixture was done for this incorporation. Titanium tetraisopropoxide, vanadyl acetylacetonate, ferric nitrate, cobalt acetate are employed as precursors to obtain respective metal incorporation. In order to obtain acid functionalization, post grafting of the synthesized material with sulphuric acid or phosphotungstic acid was followed. Detailed procedure for synthesis of the functionalized catalytic materials is discussed in the respective chapters. 2.3.2 Synthesis of metal containing solid core mesoporous silica shell (MSCMS) materials Solid core mesoporous silica shell materials have been synthesized by employing modified Stober method. In this method, octadecyltrichloro silane is used as a porogen and promoter to get solid core mesoporous silica shell. Metal incorporation of the materials has been achieved by in situ method. Titanium tetraisopropoxide or vanadium pentoxide have been used as sources for respective metal incorporation. Detailed procedure for the synthesis of MSCMS is presented in chapter – V.
44
Chapter –2
2.4
Experimental Methods
Catalyst notations Material
Notation
MCM-41
MCM-41
Metal incorporated MCM-41
MMCM-41
Titanium incorporated MCM-41
TiMCM-41
Vanadium incorporated MCM-41
VMCM-41
Cobalt incorporated MCM-41
CoMCM-41
Iron incorporated MCM-41
FeMCM-41
Sulfonic acid functionalized MCM-41
SO3HMCM-41
Phosphotungstic acid functionalized MCM-41
PWMCM-41
Solid core mesoporous silica shell
SCMS
Metal containing solid core mesoporous silica shell
MSCMS
Titanium containing solid core mesoporous silica shell
TiSCMS
Vanadium containing solid core mesoporous silica shell
VSCMS
2.5
Characterization of the materials
2.5.1
Powder XRD XRD is one of the most versatile techniques for the qualitative and quantitative
analysis of solid phases and provides information on the particle size of specific components. It is also employed to identify the structure of the material, structural arrangement of pores, transition to different phases, lattice constants and presence of foreign atoms in the crystal lattice of an active component of the catalyst [2]. In this technique, a crystal diffracts an X-ray beam passing through it to produce beams at specific angles depending on the wavelength of the X-ray, the crystal orientation 45
Chapter –2
Experimental Methods
and the structure of the crystal. X-rays are predominantly diffracted by electron density and analysis of the diffraction angles produces an electron density map of the crystal. A beam of X-rays passing through a sample of randomly oriented micro crystals produces a pattern of rings on the distant screen [3, 4]. Modern powder X-ray diffractometers consist of an X-ray source, a movable sample platform, an X-ray detector and associated computer controlled electronics. The sample is either packed into a shallow cup-shaped holder or deposited as slurry on to a quartz substrate and the sample holder spins slowly during the experiment to reduce sample heating. X-ray tubes generate X-rays by bombarding a metal target with high energy (10-100 keV) electrons that knock out core electrons. An electron in an outer shell fills the hole in the inner shell and emits X-ray photon [5]. Two common targets are Mo and Cu, which have strong Kα X-ray emission at 0.7107 and 1.5418 Å respectively. X-rays can also be generated by decelerating electrons in a target on a synchrotron ring. These sources produce a continuous spectrum of X-rays and require a crystal monochromatic to select a single wavelength. The X-ray beam is fixed and the sample platform rotates with respect to the beam by an angle theta. The detector rotates at twice the rate of sample and is at an angle of 2 theta (2θ) with respect to incoming X-ray beam. Fig. 2.1 shows the principle of the XRD method, where monochromatic X-rays with wavelength λ are reflected from parallel crystal planes, with an incident angle θ between the beam direction and the planes.
46
Chapter –2
Experimental Methods
Fig. 2.1: Braggs diffraction pattern from parallel crystal planes.
From geometrical considerations, it can be shown that X-rays reflected from two adjacent parallel planes will be in the same phase and thus interfere constructively when the following condition is met, which is known as the Bragg equation (Eqn. 2.1): ……. (2.1) Where n is the reflection order and d is the interplanar spacing. The intensity of the diffracted X-ray beam is therefore dependent upon θ and d. Thus, by measuring the diffraction intensity at different values of 2θ, inter planar distances in the crystal can be elucidated. The steps generally followed to identify a single compound or a component of a mixture is as follows: 1. The proper Hanawalt group for d1 is located in the numerical index 2. The values of d2 and d3 are next sought within the group 3. Then the match of d-values is found for d1, d2 and d3 and relative intensities are compared. Agreement of the intensities as well as the d-values suggests the identity of phase and confirmation is obtained by reference to the compounds data from ICDD cards. The method is restricted to crystallinities larger than 40 Å, which is the limit of X-ray diffraction. 47
Chapter –2
Experimental Methods
Diffractograms consist of a plot of reflected intensities against the detector angle 2θ. In powder samples, all possible diffraction directions of the lattice should be attained due to the random orientation of the powdered material. Using β as the full width at half maximum of a broad diffraction peak, the average particle size (D) can be estimated by applying the Scherrer’s equation (Eqn. 2.2): ----------- (2.2) Where λ is the X-ray wavelength, K is the Scherrer constant whose value is 0.9, β is the integral width of a reflection (in radians) located at 2θ and θ is the Bragg’s angle. Crystals consist of planes of atoms with a certain distance (d) to each other and can be resolved into many groups of atomic planes, each with a different d-spacing. The atomic planes can be described by introduction of a suitable coordinate system. The reciprocal intercepts of the planes with the coordinate axes, called Miller indices (h, k, l), are used to distinguish the planes. The porous nature of the material is also obtained from the XRD technique. The wide angle spectra (peaks in the 2θ range of 10 - 70) are used to identify the micro porous nature whereas the low angle spectra (peaks in the 2θ range of 1.0 - 10) are employed to identify mesoporous nature of the materials [6]. One of the most important applications of X-ray diffraction method is the phase identification. Each crystalline powder gives a unique diffraction diagram called characteristic finger print, which enables the determination of phase purity and degree of crystallinity by comparing with a standard, taken from any X-ray powder data file catalogues, published by the American Society for Testing Materials (JCPDS). Powder X-ray diffraction patterns (XRD) of the samples used in the present studies were collected using Philips X’Pert X-ray diffractometer. X-ray radiation source was X-ray diffraction Cu anode tube type Empyrean of 2.2 KW. Angular measurements 48
Chapter –2
Experimental Methods
(θ - 2θ) were made with reproducibility of ± 0.0001 degree, applying steps of 0.05 degrees from 1 to 20 degrees. 2.5.2 Nitrogen adsorption-desorption isotherms Adsorption-desorption study of materials on surfaces of solid materials is a commonly used method for characterization of porous materials [7, 8]. Due to its inertness, N2 is often used as an adsorbate. Information on the porosity of the materials can be obtained from the shape of the adsorption-desorption isotherms, and the total surface area of the materials can be calculated. To estimate the total surface area and pore size distribution of porous materials BET (Brunauer, Emmet and Teller) [9, 10] and BJH (Barrett-Joyner-Halenda) [11] methods which involve physical adsorption of gases at their boiling temperatures are the most acceptable techniques. The significance of the BET method lies in its ability to determine the number of molecules required to form a monolayer of adsorbed gas on a solid surface despite the fact that a mono molecular layer alone is not formed. The basis for BET theory is multi-layer adsorption of an adsorbent. The basic equation for finding out the surface area by BET method is (Eqn. 2.3) =
+
------- (2.3)
Where, P = adsorption equilibrium pressure P0= saturated vapour pressure of adsorbate at its boiling point Va = volume of adsorbate corresponding to pressure P Vm = volume of adsorbate required for a monolayer coverage at STP C is a constant related to the heat of adsorption of adsorbate in 1st and subsequent layers. According to the BET method, a plot of P / Va (P0-P) against P/P0 yields a straight line (where P/P0 is in the range of 0.05 ≤ P / P0 ≤ 0.35). Slope and Intercept of the plot
49
Chapter –2
Experimental Methods
permits the calculation of Vm. Specific surface area of the catalyst is calculated from Eqn. 2.4.
-------- (2.4)
Where, N = Avogadro number W= Weight of the catalyst sample Am= Cross sectional area of adsorbate molecule (16.2 x 10-20 m2 for N2)
In this work, Quantachrome Nova 2000e surface area and pore size analyzer (Fig. 2.2) has been employed for the surface area measurements of the catalysts by nitrogen adsorption at liquid nitrogen (77 K) temperature. In a typical experiment, about 0.10 g of the sample has been taken into the sample chamber in a U-tube, which is attached to the high vaccum system through a stopcock. The sample is degassed at 423 K and 10-6 torr for about five hours to ensure that no pre-adsorbed gas is present on the surface. Dead space calibration has been carried out at 77 K using helium in successive steps by measuring the volume at different pressures. The adsorbed volume of nitrogen (Va) is obtained from the difference between the volume of N2 introduced and the volume of the helium introduced into the sample tube at the same equilibrium pressure. From the measured values of P0, P and Va, the specific surface area of the catalyst sample is calculated using the above equation.
50
Chapter –2
Experimental Methods
Fig. 2.2: Quantachrome Nova 2000e surface area and pore size analyzer. The pore volume and pore radius of the synthesized materials were evaluated by employing BJH method from desorption branch of nitrogen adsorption-desorption isotherms. 2.5.3 Scanning electron microscopy Electron microscopy techniques are extensively used in the characterization of nanomaterials and nanostructures [12]. The resolution of the SEM approaches a few nanometers, and the instruments can operate at magnifications that are easily adjusted from -10 to over 200000. In a typical SEM, a source of electrons is focused into a beam, with a very fine spot of nearly 5 nm size and having energy ranging from a few hundred eV to 50 KeV that is rastered over the surface of the specimen by deflection coils. As the electrons strike and penetrate the surface, a number of interactions occur leading to the emission of electrons and photons from the sample, and SEM images are produced by collecting the emitted electrons on a cathode ray tube. SEM techniques are differentiated on the basis of what is subsequently detected and imaged. Principle images produced in the SEM are of three types: secondary electron images, backscattered electron images and elemental X-ray maps (Fig. 2.3). When a high energy primary electron interacts with an atom, it undergoes either inelastic scattering 51
Chapter –2
Experimental Methods
with atomic electrons or elastic scattering with the atomic nucleus. In an inelastic collision with an electron, the primary electron transfers part of its energy to the other electron. When the energy transferred is large enough, the other electron will emit from the sample. If the emitted electron has energy of less than 50 eV, it is referred to as a secondary electron. Backscattered electrons are the high energy electrons that are elastically scattered and essentially possess the same energy as the incident or primary electrons. The probability of backscattering increases with the atomic number of the sample material. Although, backscattering images cannot be used for elemental identification, useful contrast can develop between regions of the specimen that differ widely in atomic number, Z. An additional electron interaction in the SEM is that, the primary electron collides with and ejects a core electron from an atom in the sample.
Fig. 2.3: Schematic representation of scanning electron microscope. a) Sample - electron beam interactions, b) Electron pathway from electron generation to the sample surface 52
Chapter –2
Experimental Methods
Combining with chemical analytical capabilities, SEM not only provides the image of the morphology and microstructures of bulk and nanostructured materials and devices, but also provides detailed information of chemical composition and distribution [13, 14]. SEM studies of the samples synthesized in the present studies have been carried out on JEOL JSM-6390LV/Leica Stereoscan - 440 scanning electron microscope. The samples were mounted on copper plates using an industrial glue (Bostik) followed by drying in an oven for 1 h. The samples were coated with Au on Au sputter prior to examination. This form of imaging is based upon the low energy (< 50 eV) secondary electrons emitted from the surface of the specimen. The beam can be concentrated to a small probe that may be deflected across the specimen using scanning coils. The secondary electrons can be detected above the specimen, and an image showing the intensity of secondary electrons emitted from different parts of the specimen is obtained. 2.5.4 Transmission electron microscopy Heterogeneous catalysts usually consist of highly divided solid phases that are closely interconnected and therefore difficult to characterize. TEM offers unique advantage of allowing the direct observation of catalyst morphology with a resolution tuneable in the range 10−4 to 10−10 m and obtaining structural information by lattice imaging and micro-diffraction techniques. The transmission electron microscopes are generally operated at voltages as high as 200 kV with a magnification of 300000 X. If the main objective is to resolve the finest possible details in specially prepared specimens, it is advantageous to use i) the shortest possible wavelength illumination, ii) an objective lens with very low aberrations and iii) a microscope with extremely high mechanical and electrical stabilities [15].
53
Chapter –2
Experimental Methods
A pin shaped cathode heated up by passing the current produces a ray of electrons. A high voltage under ultra high vacuum accelerates these electrons to the anode. The accelerated ray of electrons passes through a drill hole at the bottom of the anode. The lens systems consist of an arrangement of electromagnetic coils. A condenser first focuses the ray and then it allows the ray to pass through the object. The object consists of a thin (< 200 nm), electron transparent, evaporated carbon film on which the powder particles are dispersed. After passing through the object, the transmitted electrons are collected by an objective. Thereby an image is formed, which is subsequently enlarged by an additional lens system. The images formed thereby are visualized on a fluorescent screen or documented on a photographic material. Specimen preparation is a critical step in electron microscopy because image quality is highly dependent on how the different solid phases are dispersed on the microscope grid and on their thickness. The thickness of solid phases should be less than 50-100 nm to allow sufficient transmittance. Thinner the samples, better is the resolution and the contrast. Another important factor is the stability of the preparation. Specimens have to be deposited on 2/3 mm diameter copper grids (100-400 mesh) covered with a thin amorphous carbon film [16]. The easiest way is to ultrasonically disperse a few milligrams of the powder in a few milliliter of ethanol, then deposit a drop of the suspension on a carbon coated grid and let the liquid evaporate. Transmission electron microscopic images of selected catalysts in our studies were performed on a JEOL JSM-2000 EX electron microscope equipped with a slow scan CCD camera. 2.5.5 Fourier transform infrared spectroscopy The infrared region of the electromagnetic spectrum encompasses radiation with wavelengths ranging from 1 to 1000 microns. From the standpoint of both application and 54
Chapter –2
Experimental Methods
instrumentation, this range is divided into three regions, Near IR (12500 – 4000 cm-1), Mid IR (4000 – 200 cm-1) and Far IR (200 – 10 cm-1) [17]. The majority of analytical applications are confined to a portion of the middle region extending from 4000 to 400 cm-1. The absorption spectra in the infrared region originate from the transitions between vibrational (along with rotational) levels of a molecule present in its ground electronic state upon irradiation with infrared radiation. The atoms in a molecule are never stationary and a good approximation is to treat them as a combination of point masses held together by Hooke’s law of forces. By classical mechanics, it can be shown that the displacements of the masses from their mean positions are always the sum of the displacements due to a particular set of vibrations [18]. If in these set of vibrations the masses are in phase and the motion of all the nuclei involved are such that the centre of gravity of the molecule remains unaltered, then such vibrations are known as the fundamental modes of vibration of the molecule. Mostly, a normal mode is localized largely to a group within the molecule and hence corresponds to stretching or bending of one or few bonds only and hence associated with that particular functional group. Whether for the functional group or the entire molecule, the vibrations are universally classified either as stretching or as bending types. Stretching vibrations, which correspond to the oscillations leading to change in bond lengths, can be further sub-divided into symmetric or asymmetric stretching vibrations. Bending vibrations are characterized by continuously changing angle between the bonds and is further sub classified as wagging, rocking, twisting, or scissoring. Apart from fundamental modes, a large number of overtones (multiple of fundamental modes, 2ν or 3ν etc), combination tones (ν1+ν2, ν3+ν4 etc.) and difference tones (ν1-ν2, ν5–ν6 etc.) can also be observed in a typical infrared absorbance spectrum of a molecule [19, 20]. 55
Chapter –2
Experimental Methods
One of the primary requirements for vibrating molecules to interact with the oscillating electric field of the incident radiation and to undergo a transition between two vibrational energy levels is that the molecular dipole moment must change during the vibration. The intensity of the absorption is determined by the magnitude of this dipole moment change. Owing to symmetry, some of the vibrations in a molecule may not induce a change in dipole moment and hence are transparent to infrared radiations i.e. IR inactive. FT-IR spectroscopy provides information about (i) surface species identification (ii) dispersion of the reagent over the support surface and (iii) surface activity with the use of probe molecules. The most common ways of studying an insoluble solid are (i) as a mull (ii) as a disc and (iii) directly as a powder. FT-IR spectra of materials synthesized in this work were recorded on a Shimazdu FTIR 8201 / PerkinElmer instrument at room temperature using KBr pellets in the range of 4000 to 400 cm-1. The powdered samples were ground with KBr in 1: 10 ratio and pressed for making the pellets. The data were collected as average of 5 scans. 2.5.6 UV-Visible spectroscopy The UV-Visible spectroscopy is commonly used in chemical analysis due to its simplicity, versatility, accuracy and cost-effectiveness. Electronic transitions occur in the UV-Visible region of the electromagnetic spectrum. When energy of the incident photon matches with the difference in energy between an occupied orbital (ground state) and an empty (excited state) orbital, the photon is absorbed. Consequently, the electron is excited from its ground state to excited state. In general, this transition will occur between the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) [20].
56
Chapter –2
Experimental Methods
UV-Visible spectra tend to be broad in nature due to the fact that vibrational and rotational levels of the molecular orbitals are superimposed upon the electronic levels. This broad nature limits their use in identifying materials, although the technique is ideal for quantitative analysis of species in solution media. The electron excitation can occur from the ground state to different excited states. The possible transitions can involve different orbitals (i.e. σ, π, n, σ* and π*) arising in different electronic transitions namely σ → σ*, n → σ*, n → π* and π → π* [21]. The transition to the first excited state associated with the HOMO to LUMO excitation, is normally characterized by having low energy and high intensity. Molecular groups with conjugate in-saturations have significant influence on the λmax and the intensity of the peaks in the absorption spectrum. The presence of chromophores (color-bearing molecular features) which are functional groups not conjugated to other groups (i.e. nitro, azo, azo-amine, carbonyl, etc) and auxochromes, such as OH, NH2, CH3 and NO2 are responsible for changes in the absorption spectrum [19]. Other relevant factors that can affect the absorption properties of a UV-Visible spectrum are the presence of steric effects and the solvent used during the analysis. Beer-Lambert equation (Eqn. 2.5) correlates the absorbance of a substance (A) with its concentration (c). ------- (2.5) Where ε is the molar absorptivity and Ɩ is the length of sample tube. This equation is true only when monochromatic light is used and if the physical and chemical properties of the substance do not change with variations in concentration. UV-Visible spectroscopy is most suitable to study electronic transitions of the materials which are soluble in suitable solvents. But in the case of insoluble materials, UV-Visible reflectance or diffuse reflectance methods are used.
57
Chapter –2
Experimental Methods
Studies on catalytic degradation of dyes presented in this thesis were carried out by using Analytikjena Specord D 205 UV-Visible spectrophotometer. 2.5.6.1 UV-Visible diffuse reflectance spectroscopy UV-Visible diffuse reflectance spectroscopy is a non-destructive technique which provides the information about the structure and composition of materials [22]. In this method, when radiation interacts with samples, absorption and scattering takes place giving rise to a reflectance spectrum due to the electronic excitation. Electronic excitations are two types, d-d transitions and charge transfer transitions. The d-d transition gives information about the oxidation state and the co-ordination environment of the samples, whereas charge transfer transitions are intense and are sensitive to the nature of donor and acceptor atoms. In UV-Visible reflectance spectroscopy of solids, two types of reflections are encountered: (a) Specular or mirror like reflection, in which the angle of incidence and angle of reflection are identical (Fig. 2.4a) and (b) Diffuse reflection, in which reflection is from a matte structure (Fig. 2.4b). The second one serves as the basis of reflectance spectroscopy. It is an effective way of obtaining the UV – Visible spectra of powdered samples directly. This technique is ideal for studying optical and electronic properties of materials such as powders, films, pigments etc. Percentage of reflectance is given by the following equation: --------- (2.6) Where Is is the intensity of the reflected beam and Ir is the intensity of a reference standard (usually barium sulphate).
58
Chapter –2
Experimental Methods
Fig. 2.4: (a) Specular reflection on mirror like surface and (b) Diffuse reflection from a matte surface. UV-Visible diffuse reflectance spectra of our samples were recorded on Pye Unichem SP-8-100 UV-Visible spectrophotometer. 2.5.7 Thermal analysis Thermal analysis includes a group of methods by which the physical or chemical properties of a substance are determined as a function of temperature or time, while the sample is subjected to controlled temperature program. The program may involve heating or cooling or holding the temperature constant or any combination of these [23]. The gain or loss in weight of a sample as a function of temperature is measured by thermogravimetry (TG). It is a very useful technique for the study of solid-gas systems [24]. Thermogravimetric analysis is mainly directed at establishing optimum temperature ranges for drying or igniting precipitates. It has much wider potential in estimating the composition of moisture content, solvent content, additives, polymer content and filler content. It has also been used in the identification of characteristic decomposition temperatures, determination of thermal stability of the material, finding the rate of mass change or decomposition and phase transitions in various reactions like dehydration, decarboxylation, oxidation and decompositions. The instrument comprises of an ultra 59
Chapter –2
Experimental Methods
sensitive weighing device. Changes in the mass of the sample clearly imply evolution or uptake of matter by the sample. This method works well for compact samples. TG-DTA studies of materials presented in this thesis were recorded on Perkin Elmer, S11 Diamond TG/DTA analyzer by heating the samples from ambient temperature to 1273 K at a rate of 10 K min−1 under nitrogen flow at the rate of 20 mL min−1. 2.5.8
X-ray photo electron spectroscopy X-ray photo electron spectroscopy (XPS) is based on the photoelectric effect. An
atom absorbs a photon of energy hʋ and causes an ejection of a core or valence electron of binding energy Eb. Kinetic energy (Eqn. 2.7) of the electron (Ek) is given by ---------- (2.7) Routinely used X-ray sources are MgKα (hʋ = 1253.6eV) and AlKα (hʋ = 1486.3eV). XPS being a surface spectroscopic technique, is preferably used under high vaccum (pressures in the 10-13 bar range). In order to obtain meaningful results on the intrinsic properties of a clean surface, or of the surface with an adsorbed gas, contamination by residual gases in the measurement chamber should be prevented [25]. The XPS spectrum is a plot of intensity of photoelectrons, N (E) as a function of their kinetic energy Ek, or more often, binding energy Eb. Photoelectron spectral peaks are labeled according to the quantum numbers of the level from which the electron originates. An electron coming from an orbital with main quantum number n, orbital momentum Ɩ (0, 1, 2, 3,....... indicates as s, p, d, f,......) and spin momentum, s (+1/2 or -1/2) is indicated as nƖ Ɩ +s. For energy orbital momentum Ɩ > 0 there are two values of the total momentum: j = Ɩ + ½ and j = Ɩ – ½, each state filled with 2j+1 electron. Hence, most XPS peaks appear as doublets and the intensity ratio of the
60
Chapter –2 components is
Experimental Methods (Ɩ+1) / Ɩ. In case the doublet splitting is too small to be observed, + s term
in the subscript is omitted [26]. Because a set of binding energies is characteristic of an element, XPS can be used to analyze the surface composition of samples. Almost all photoelectrons used in laboratory XPS have kinetic energies in the range of 0.2 to 1.5 keV, and probe the outer layers of the catalyst. The mean free path of electrons in elemental solids depends on the kinetic energy. Optimum surface sensitivity is achieved with electrons at kinetic energies of 50 – 250 eV, where about 50% of the electrons come from the outermost layer. Binding energies are not only element specific but also contain chemical information. The energy levels of core electrons depend on the chemical state of the atom. Chemical shifts are typically in the range 0 - 3 eV. In general, the binding energy increases with increasing oxidation state. The binding energies measured by XPS are not necessarily equal to the energies of orbital from which the photoelectron is emitted. The difference is caused by the reorganization of the remaining electrons when an electron is removed from an inner shell. Thus, the binding energy of the photoelectron provides information on the state of the atom before photo-ionization and on the core-ionized atom left behind after the emission of an electron. Although XPS is predominantly used for studying surface compositions and oxidation states, the technique can also yield information on the dispersion of supported catalysts. The XPS spectra of the synthesized catalyst samples were measured on Bruker Axis 165 spectrometer equipped with Al-Kα and Mg-Kα dual source and hemispherical analyzer connected to a five-channel detector. During the measurement, the pressure of the system is maintained around 5 x 10-10 mbar. Spectra were recorded with constant pass energy of 20 eV. Binding energies were recorded by computer fitting of the measured 61
Chapter –2
Experimental Methods
data. Samples were pressed in indium foil. Binding energy correction is performed using C 1s peak at 284.8 eV as a reference.
2.6
Catalytic studies
2.6.1
Catalytic studies on degradation of dyes over MMCM-41 materials The degradation studies on Crystal violet and Rhodamine B using hydrogen
peroxide as oxidant in the presence of MCM-41 and MMCM-41 catalysts have been studied at room temperature. A stock solution of dye having concentration of 1 X 10-3 M was prepared and solutions of desired concentrations have been obtained by the successive dilutions of stock solution. The degradation studies have been carried out using a UV-Visible spectrophotometer by measuring the decrease in absorbance of the dye solution with respect to time at the respective λmax of dyes i.e., 580 nm in the case of Crystal violet and 642 nm in the case of Rhodamine B. Effect of pH, concentration of the dyes, concentration of oxidant H2O2 and amount of catalyst on the degradation has been studied. All the studies were carried out under room temperature conditions. The % degradation of the dye has been calculated by using the formula (Eqn. 2.8) -------- (2.8) Where A0 is the absorbance at initial time and At is the absorbance at time t. The details of experiments and results of dye degradation studies over MMCM-41 catalysts are presented in chapter – III. 2.6.2 Catalytic studies using acid functionalized MCM-41 materials Catalytic applications of Sulfonic acid and Phosphotungestic acid functionalized MCM-41 materials have been investigated towards the synthesis of imines, 1-amidoalkyl 2-naphthols and benzylidine barbiturates. Detailed synthesis procedures and results are discussed in chapter – IV. 62
Chapter –2
Experimental Methods
2.6.3 Catalytic studies on TiSCMS and VSCMS materials 2.6.3.1 Studies on conversion of cyclicketones to lactams Catalytic conversion of cyclohexanone to caprolactam and cyclododecanone to laurolactam were carried out over MSCMS using ammonia and H2O2. At regular intervals of time the reaction mixture was analyzed by a capillary gas chromatograph (Agilent 4890), HP-1 column with flame ionization detector). Products were identified by JEOL MS instrument, through GC retention times of the authentic samples and GC-MS splitting patterns. Details of the experimental aspects and results are presented in chapter-V. 2.6.3.2 Studies on oxidation of diphenymethane to benzophenone MSCMS materials were used as catalysts in the oxidation of diphenylmethane to benzophenone using air. The product mixture collected at regular intervals of time was filtered through 0.2 µm membrane filter. It was analyzed by HPLC (Shimadzu, CLASS VP model) with a UV detector using a column packed with octadecyl silane and a mobile phase of acetonitrile : methanol (60:40, v/v) at a flow rate of 1.0 mL min-1. An injection volume of 20 µL was used. Further details of experimental procedures followed are discussed in chapter - V.
63
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2.7
Experimental Methods
References
1.
Antonelli D. M.; Ying J. Y., Angew. Chem. Int. Ed. Engl. 1995, 34, 315.
2.
Cullity, B. D. Elements of X-ray diffraction, Addison-Wesley Publishing Comp. Inc., USA, 1978.
3.
Xu, R.; Pand, W.; Yu, J.; Huo, Q.; Chen, J. Chemistry of Zeolites and related porous materials: Synthesis and Structure, 2nd Edn., Wiley-Interscience Publication, New York, 2007.
4.
Ladd, M. F. C.; Palmer, R. A. Structure determination by X-ray crystallography, Plenum press, New York, 1994.
5.
Tanabe, K.; Misono, M.; Ono, Y.; Hattori, H. New Solids and Bases, Kodansha/Elsevier, Tokyo/New York, 1989.
6.
Bergeret, G.; Gallezot, P. Physical Techniques for Solid Materials, Plenum Press, New York, 1994.
7.
Skoog, D. A.; West, D. M.; Holler, F. J.; Crouch, S. R. Principles of Instrumental Analysis. 8th Edn.; Thomson Brooks/Cole, 2006.
8.
Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem. 1984, 57, 603.
9.
Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309.
10.
Langmuir, I. J. Am. Chem. Soc. 1916, 38, 2221.
11.
Barrett, E. P.; Joyner, L. G.; Halenda, P. P. J. Am. Chem. Soc. 1951, 73, 373.
12.
Goldstein J. I.; Newbury D. E.; Echlin P.; Joy D. C.; Fiori C.; Lifshin E. Scanning Microscopy and X-Ray Microanalysis, Springer, 3rd corrected edition, 2007.
13.
Charles N. S. Heterogeneous catalysis in Industrial Practice, 2nd edn., Krieger Publishing, 1996.
64
Chapter –2 14.
Experimental Methods
Cao, G.; Wang, Y. Nanostructures and Nanomaterials: Synthesis, properties and applications, 2nd Edn., World Scientific Publishing Co.pvt Ltd, 2011.
15.
Fultz, B. Transmission electron microscopy and diffraction of materials, Springer, 2001.
16.
Gai, P. L.; Boyes, E. D. Electron microscopy in heterogeneous catalysis, Institute of physics publishing, Bristol, 2003.
17.
Cahn, R. W.; Haasen P.; Kramer, E. J. Material Science and Engineering: Characterisation of Materials, VCH Publisher Inc., New York, 1991.
18.
David, H. Modern analytical chemistry, McGrawHill, USA, 2000.
19.
Catherin, E. H.; Alan, G. S. Inorganic chemistry, 2nd Edn., Pearson Education, England, 2005.
20.
Banwell, C. N.; McCash, E. M. Fundamentals of Molecular Spectroscopy, 4th Edn., Tata McGraw Hill Publishing Company Ltd. New Delhi, 1995.
21.
Lacombe, S.; Cardy, H.; Soggiu, N.; Blanc, S.; Jiwan, J. L. H.; Soumilloin, J.P. Micropor. Mesopor. Mater. 2001, 46, 311.
22.
Michel C.; Jacques C. V. Characterization of solid materials and Heterogeneous Catalysis: From structure to surface reactivity, Vol 1& 2, Wiley-VCH Verlag, GmbH & Co, KgaA, 2012.
23.
Kenkel, J. Analytical Chemistry for Technicians, 2nd Edn., CRC Press, 1994.
24.
Dodd J.W.; Tonge K. H. Thermal methods: analytical chemistry by open learning, Wiley, Chichester, 1987.
25.
Briggs, A.; Seah, M. P. Practical Surface Analysis, 2nd Edn.; Auger and X-Ray Photoelectron Spectroscopy, Vol. 1, Wiley, New York, 1990.
26.
Neimansstuedriet, J. W. Sppectroscopy in Catalysis. Wiley-VCH Verlag, GmbH & Co, KgaA, 2007. 65
Chapter – 3
SYNTHESIS,
CHARACTERIZATION
OF
METAL
INCORPORATED MCM-41 AND THEIR CATALYTIC APPLICATIONS TOWARDS THE DEGRADATION OF CRYSTAL VIOLET AND RHODAMINE B
Chapter –3
3.1
Metal Incorporated MCM-41
Introduction Today, entire world is facing environmental problems due to contaminated ground
water and hazardous industrial effluent [1, 2]. These effluents are highly colored and their disposal into water system causes damage to the environment as they affect photosynthetic activity in aquatic life owing to the reduced light penetration [3, 4]. The presence of dyes in very low concentrations (1 mg L-1) in the effluent imparts intense color and is highly hazardous to the environment [5, 6]. Thus, efficient color removal from waste waters involving physical or chemical methods [7] or biological methods or combination of methods has attracted the interest of researchers and technologists. Majority of the dyes consumed at industrial scale are azo, anthraquinone, indigoid, tryphenylmethane, xanthene, phthalocyanine etc. derivatives [8, 9]. These dyes are used extensively in textile industries owing to their favorable characteristics of bright color, water fastness and simple application techniques with low energy consumption. However, they are in general the most toxic, as they tend to pass through conventional treatment systems unaffected [10, 11]. Triphenylmethane dyes are widely used for dying wool, silk and paper [12]. These are used as staining agents in bacteriological and pathological applications. Crystal violet (CV) is an important member of triphenylmethane dyes and is a non-biodegradable [13] mutagen and mitotic poison [14]. It is therefore essential either to remove dyes like CV from waste water or to treat them to minimize their effects on the environment [15, 16]. Rhodamine B (RhB) is one of the most important and extensively used xanthene dyes and is known for its high stability [17]. It is a commonly employed dye in textile and paper industry [18]. It is also used as tracing agent, biological stain and dye laser. RhB is highly soluble in water and is toxic to aquatic organisms and is a potential carcinogenic.
66
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Treatment of effluents containing RhB dye is important for the protection of water and environment [19]
Structure of Crystal violet
Structure of Rhodamine B
Various treatment technologies such as adsorption [20-23], photo degradation [24–26], coagulation [27-29], flocculation [29], chemical oxidation [30], electrochemical oxidation [31], biological process [32, 33] are reported for the removal of dyes from wastewater. However, most of these methods have limitations with respect to the requirement of toxic chemicals in large quantities, disposal of sludge containing dye content [34, 35], recovery of the catalytic materials and longer reaction times etc. Advanced oxidation processes (AOPs) have been used to treat various organic pollutants during the last two decades [36-40]. AOP like Fenton’s process is based on oxidation with Fenton reagent which comprises of an oxidative mixture of hydrogen peroxide and ferrous ions as catalyst [41]. Major disadvantages of these methods are extensive use of harmful chemicals and non reusability of the catalyst [42]. Development of suitable processes involving heterogeneous catalysts would overcome most of the problems associated with removal of the dyes.
67
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By introducing suitable elements into porous materials, it is possible to obtain catalysts with desired chemical properties. Incorporation of metal ions into the framework of the porous material is expected to enhance their redox character due to surface heterogeneity [43-46]. Transition metal ions such as titanium, vanadium, cobalt and iron have good redox character due to their ability to exist in different oxidation states. In this direction, we have synthesized metal incorporated mesoporous materials based on MCM-41 and investigated their catalytic applications towards the degradation of dyes. Owing to the presence of large number of catalytically active sites on the surface, these materials are expected to have greater adsorption and degradation capability. This chapter comprises of the synthesis of metal incorporated MCM-41 (MMCM-41 where M = Ti, V, Fe or Co) materials by a simple room temperature method and their characterization by various physic-chemical techniques. Catalytic applications of synthesized MMCM-41 materials towards the degradation of Crystal violet and Rhodamine B at room temperature in the presence of hydrogen peroxide are discussed.
3.2
Synthesis of MCM-41 MCM-41 material was synthesized by a simple room temperature co-precipitation
method. In a typical synthesis, 2.40 g of surfactant cetyltrimethylammonium bromide was dissolved in 50.00 mL of distilled water and stirred continuously to form a clear homogeneous solution. 76.00 mL of ethyl alcohol followed by 13.00 mL of 25 wt. % aqueous ammonia was added to this homogeneous solution while stirring. 10.00 mL of tetraethyl orthosilicate (TEOS) was added drop wise to the above mixture. The solution turns milky and a gel is formed due to the hydrolysis of TEOS. The resultant mixture is stirred for about 2 h to completely hydrolyze TEOS. White precipitate thus formed was centrifuged and washed consecutively with distilled water and methanol. The product was
68
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Metal Incorporated MCM-41
dried overnight at 110 ºC. Solid product thus obtained was calcined at 550 ºC in air atmosphere for 5 h to remove the trapped surfactant. Advantages of this room temperature synthesis are simple reaction conditions and lesser crystallization time when compared to hydrothermal method [47-49]. 3.2.1
Synthesis of MMCM-41 materials In situ method was adopted for the synthesis of metal incorporated MCM-41.
These were synthesized by following the same method as described for MCM-41 except for the addition of suitable quantities (Si/M ion ratio of 100) of respective metal precursors, 20 minutes after the addition of TEOS. Ti, V, Fe, and Co incorporated MCM-41 materials were obtained by using titanium tetraisopropoxide, vanadyl acetylacetonate, ferric nitrate, and cobalt acetate as precursors for respective metal ions. Schematic representation for the synthesis of MMCM-41 is shown in Fig. 3.1.
Fig. 3.1: Scheme representing formation of MMCM-41 materials. 69
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Metal Incorporated MCM-41
3.3
Characterization of MMCM-41 materials
3.3.1
Powder XRD studies XRD patterns of MCM-41 and MMCM-41 (M = Ti, Fe, V, Co) samples are
presented in Fig. 3.2. The patterns show only one low-angle peak for d100 plane corresponding to the mesophase at 2θ value around 2.2o. This is characteristic of the long range hexagonal structure of MCM-41 [50].
Fig. 3.2: Powder XRD patterns of MCM-41, TiMCM-41, VMCM-41, FeMCM-41 and CoMCM-41.
Three peaks in 2θ range of 2-5º correspond to the presence of (100), (110) and (200) reflections of hexagonal array of MCM-41. The presence of three diffraction peaks indicates crystallographic ordering of the mesopores. Low value of 2θ is mainly due to
70
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Metal Incorporated MCM-41
the length of the carbon chain of template used for synthesizing MCM-41 [51]. Whereas, MMCM-41 materials show only one broad peak around 2θ of 2.5º. Other diffraction peaks corresponding to (110) (200) crystal planes were unresolved. In case of metal incorporated MCM-41, the intensity of peak becomes lower compared to MCM-41. This indicates that the presence of metal ions obstructs the structure directing action of template and alters the regular order of the materials.
3.3.2
Nitrogen adsorption-desorption studies
The nitrogen adsorption-desorption isotherms of MCM-41 and MMCM-41 materials are shown in Fig 3.3. It is observed that all the materials follow typical type-IV adsorption isotherm with no hysteresis. This indicates mesoporous nature of the materials [52]. The steep increase in the volume adsorbed in P/P0 range of 0.3 – 0.5 is due to capillary condensation of nitrogen in the pores of the materials [53]. The specific surface area of the materials is calculated from adsorption isotherms by applying BET method and it is found to be in the range of 650 m2g-1 to 1032 m2g-1. It can be seen that the surface area of the materials gets reduced by the incorporation of guest metal ions. This can be attributed to the occupancy of some of the pores by metal ions. The pore size and pore volume of the materials are evaluated by employing BJH method. The textural properties of MCM-41 and MMCM-41 materials are presented in Table 3.1. Pore size and pore volume of the MMCM-41 materials slightly decreased in comparison to MCM-41 indicating that the pore structure of the materials gets altered on metal incorporation. Powder XRD results also confirm this observation.
71
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Fig. 3.3: Nitrogen adsorption–desorption isotherms of MCM-41, TiMCM-41, VMCM-41, FeMCM-41 and CoMCM-41.
Table 3.1: Textural characteristics of MMCM-41 materials Material
SBET (m2g-1)
Pore Size (Å)
Pore Volume (cc g-1)
MCM-41
1023.50
17.20
0.28
TiMCM-41
942.30
16.60
0.24
VMCM-41
831.23
15.80
0.15
FeMCM-41
731.12
16.30
0.18
CoMCM-41
698.22
16.90
0.22
72
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Metal Incorporated MCM-41
3.2.3 SEM-EDX studies The SEM-EDX images of MCM-41 and MMCM-41 (M = Ti, V, Co or Fe) are shown in Fig. 3.4. SEM images of the materials reveal that all the materials possess spherical morphology similar to that of MCM-41. EDX analysis confirms the functionalization of materials with metal ions. It can be observed that morphology of the materials is not altered by incorporation of metal ions into the framework. (a)
(a)
(b)
(b)
Fig. 3.4: SEM-EDX images of a) MCM-41 and b) TiMCM-41.
73
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Metal Incorporated MCM-41
(c)
(c)
(d)
(d)
(e)
(e)
Fig. 3.4: SEM-EDX images of c) VMCM-41, d) FeMCM-41 and e) CoMCM-41.
74
Chapter –3 3.2.4
Metal Incorporated MCM-41
FT-IR studies The FT-IR spectrum of as synthesized MCM-41 (Fig. 3.5a) shows a very low
intense band at 2926 cm-1 corresponding to the -C-H stretching of the hydrocarbon chain of the surfactant [54]. In the hydroxyl region a broad band is seen around 3408 cm−1. This can be attributed to surface silanols and adsorbed water molecules. Whereas, the peak corresponding to the C-H stretching vibrations disappeared in FT-IR spectra of calcined MCM-41 and MMCM-41 materials, indicating the complete removal of surfactant from the frame work of the materials (Fig. 3.5b). The absorption band close to 1630 cm−1 is due to the bending vibration of adsorbed water molecules. The asymmetric stretching vibrations of Si-O-Si are observed as absorption bands at 1088 and 1235 cm−1. The band at 962 cm−1 can be attributed to Si-OH vibrations. The absorption peaks around 450 to 795 cm−1 are mainly due to bending vibrations of Si-O-Si bonds [55]. The FT-IR spectra of MMCM-41 materials closely resemble that of metal free MCM-41 (Fig. 3.5b). But band at 960 cm-1 corresponding to Si-O stretching in MCM-41 is found to broaden in MMCM-41 samples as a result of entrapment of metal ion into the framework of the material. Strong intensity of this band in pure MCM-41 is due to the large amount of silanol groups present in the material [56].
75
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Fig. 3.5a: FT-IR Spectrum of as-synthesized MCM-41.
Fig. 3.5b: FT-IR Spectra of MCM-41, TiMCM-41, VMCM-41, FeMCM-41 and CoMCM-41. 76
Chapter –3 3.2.5
Metal Incorporated MCM-41
XPS Studies The XPS spectra of silicon, oxygen, carbon and metal ions are shown in Fig. 3.6.
In the XPS spectra of TiMCM-41, the O 1s binding energy value around 534.0 eV is due to chemisorbed water and weakly adsorbed oxygen molecules on the surface. In the Ti 2p region, Ti 2p3/2 and Ti 2p1/2 were found at 461.9 eV and 467.6 eV respectively. According to the XPS standard spectrum, the Ti 2p3/2 peak of TiO2 should be at 458.8 eV, but it is found at 461.9 eV (Figure 3.6). The change in binding energy of Ti 2p3/2 in the TiMCM-41 sample confirms that Ti does not exist in the form of TiO2 in the frame work of silica matrix [57-59]. In VMCM-41 sample, the peak at 538.0 eV is due to the presence of V 2p species in the framework of the material confirming the presence of V5+ ions [60]. In the case of CoMCM-41, the peak at 775.0 eV corresponds to the presence of Co 2p in +3 state [61]. The XPS spectrum of the Fe 2p3/2 of FeMCM-41 sample indicates that the peak at 710.8 eV can be ascribed to Fe3+ ions. This binding energy lies within the reported range of the binding energy of the Fe3+ valence state (710.3 – 711.8 eV) [62]. The result confirms that Fe exists mainly in the +3 oxidation state in FeMCM-41. Thus, results of XPS studies confirm the presence of metal ions in the framework of corresponding metal incorporated MCM-41 materials.
Characterization results of MMCM-41 materials synthesized at room temperature revealed that they have large surface area and are mesoporous in nature. Incorporation of metal ion into the framework of the material leads to decrease in specific surface area of the parent material MCM-41. Metal ions in the framework predominantly exist in tetrahedral coordination.
77
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Metal Incorporated MCM-41
(C)
Fig. 3.6: XPS Spectra of a) Carbon, b) Oxygen and c) Silicon in MMCM-41 materials. 78
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Metal Incorporated MCM-41
Fig. 3.6: XPS Spectra of d) Titanium, e) Vanadium f) Iron and g) Cobalt in MMCM-41 materials.
79
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Metal Incorporated MCM-41
3.4
Catalytic applications of MMCM-41 materials
3.4.1
Catalytic degradation of crystal violet The catalytic degradation of crystal violet (CV) dye by H2O2 over MMCM-41
(M = Ti, V, Co and Fe) materials is investigated at room temperature. The % degradation of CV was evaluated by measuring the decrease in absorbance of CV solution with respect to time in the presence of H2O2. All the degradation studies were carried out at 580 nm, the visible absorption maximum of CV. Typical plots showing variation in absorbance of CV with time in the presence of MMCM-41 catalysts are given in Fig. 3.7. It is observed that absorbance of CV decreases exponentially and tends to reach a saturation value after about 120 minutes. Here, CV molecules get adsorbed on the surface of MMCM-41 catalysts and react with hydroxyl radicals generated from H2O2 and get converted to colorless benzenoid form of CV. This undergoes further degradation by repeated attack of the hydroxyl radicals [63, 64].
Fig. 3.7: Variation of absorbance with time for the degradation of CV over MMCM-41 catalysts. [CV] = 5.0 x 10-4 M, [H2O2] = 1.0 x 10-3 M, pH = 5.1, Catalyst dosage = 150 mg 3.4.1.1 Effect of pH 80
Chapter –3
Metal Incorporated MCM-41
pH of the medium plays a vital role in the degradation of dyes over heterogeneous catalysts because it affects both the surface charge of the catalyst and stability of dyes. Effect of pH on the degradation of CV has been studied by varying it in the range 2.1 to 5.1 in the presence of TiMCM-41, VMCM-41, FeMCM-41 and CoMCM-41 catalysts by taking H2O2 concentration as 1.0 x 10-3 M, CV concentration of 5.0 x 10-4 M and catalyst dosage of 150 mg. Variation of % degradation of CV with time at different pH for MMCM-41 materials are shown in Fig. 3.8. Increase in pH of the medium from 2.1 to 5.1 increased the % of degradation of CV over all the catalysts studied. Among the MMCM-41 catalysts used for CV degradation, TiMCM-41 is found to be the most efficient, at any pH studied. The % degradation increased from 43.1 at pH 2.1 to a maximum of 94.4 at pH 5.1 in 120 mins in the presence of TiMCM-41 (Table 3.2). In basic medium, hydrolysis of CV takes place and hence our studies are restricted to acidic pH. Increase in degradation of CV with increase in pH is due to higher adsorption of the cationic dye on the catalyst surface. As pH increases, interactions between dye molecules with surface hydroxyl groups of MMCM-41 increase due to changes in the electrostatic forces. Literature reports on zeta potential measurements on MCM-41 reveal that its surface charge is negative in the pH range studied and charge density decreases with increase in pH [65, 66]. In such a case, there will be decrease in the repulsive forces between the MMCM-41 catalysts and cationic dye which leads to increased adsorption of dye at higher pH. It has also been observed that the catalytic activity is about 1.8 to 2.5 times higher with metal incorporated MCM-41 materials as compared to MCM-41 at pH 5.1. Thus, MMCM-41 materials are more efficient than MCM-41 in degrading CV under the experimental conditions. 81
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Metal Incorporated MCM-41
Fig. 3.8: Effect of pH on the degradation of CV in the presence of a) TiMCM-41, b) VMCM-41, c) FeMCM-41 and d) CoMCM-41. [CV] = 5.0 x 10-4 M, [H2O2] = 1.0 x 10-3 M, Catalyst dosage = 150 mg
82
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Table 3.2: Effect of pH on the degradation of CV [CV] = 5.0 x 10-4 M, [H2O2] = 1.0 x 10-3 M, Catalyst dosage = 150 mg % Degradation pH MCM-41
TiMCM-41
VMCM-41
FeMCM-41
CoMCM-41
2.1
32.4
43.1
48.3
36.4
38.2
3.7
35.8
72.1
55.1
64.7
58.9
5.1
38.2
94.4
70.2
82.6
73.7
3.4.1.2 Effect of H2O2 concentration Effect of varying the concentration of oxidant H2O2 on the degradation of CV has been studied by varying the concentration of H2O2 in the range 1.0 x 10-4 to 2.0 x 10-3 M at pH 5.1 and catalyst dosage of 150 mg (Fig. 3.9). Extent of degradation increased with increase in H2O2 concentration till 1.0 x 10-3 M (Table 3.3). This can be ascribed to the .
increased number of OH radicals at higher concentrations of H2O2. Further increase in the concentration of H2O2 has little effect on the % degradation. This is due to the well known hydroxyl radical scavenging effect at higher concentrations of H2O2 [67]. Hence, 1.0 x 10-3 M of H2O2 is taken as optimum for the degradation of CV.
83
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Metal Incorporated MCM-41
Fig. 3.9: Effect of H2O2 on the degradation of CV in the presence of a) TiMCM-41, b) VMCM-41, c) FeMCM-41 and d) CoMCM-41. [CV] = 5.0 x 10-4 M, pH = 5.1, Catalyst dosage = 150 mg
84
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Table 3.3: Effect of H2O2 concentration on the degradation of CV [CV] = 5.0 x 10-4 M, pH = 5.1, Catalyst dosage = 150 mg % Degradation [H2O2] x 103 M MCM-41 TiMCM-41
VMCM-41
FeMCM-41
CoMCM-41
0.10
12.8
15.8
14.8
16.2
15.8
0.20
20.4
36.1
34.8
42.8
40.9
0.50
31.8
56.9
52.3
58.7
55.7
0.75
35.4
76.8
68.7
70.2
74.6
1.00
38.2
93.1
70.6
81.5
82.4
1.50
34.2
92.1
68.3
79.2
80.2
2.00
31.8
87.5
69.1
76.3
78.6
3.4.1.3 Effect of catalyst dosage Effect of catalyst dosage on the degradation of CV has been studied by varying the amount of catalyst in the range 25 mg to 200 mg at pH 5.1, by keeping concentration of H2O2 at 1.0 x 10-3 M and CV at 5.0 x 10-4 M. Plots showing % degradation of CV with time are depicted in Fig. 3.10. With increase in dosage of MMCM-41 from 25 mg to 150 mg, there is an increase in CV degradation from 32.8% to 94.4% in 120 mins in the case of TiMCM-41 (Table 3.4). Further, increase in the amount of catalyst did not have much influence on the degradation of CV. As the amount of catalyst increases, more dye molecules get adsorbed on to the surface of the catalyst due to the increased number of active sites. Once all the dye gets adsorbed, further increase in the amount of catalyst has
85
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negligible effect on the extent of degradation. Therefore, 150 mg of catalyst is chosen as the optimum amount for degradation of CV under the experimental conditions.
Fig. 3.10: Effect of catalyst dosage on the degradation of CV in the presence of a) TiMCM-41, b) VMCM-41, c) FeMCM-41 and d) CoMCM-41. [CV] = 5.0 x 10-4 M, [H2O2] = 1.0 x 10-3 M, pH = 5.1
86
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Table 3.4: Effect of catalyst dosage on the degradation of CV [CV] = 5.0 x 10-4 M, [H2O2] = 1.0 x 10-3 M, pH = 5.1 % Degradation
Weight of Catalyst MCM-41
TiMCM-41
FeMCM-41
CoMCM-41
25.0
17.4
32.8
31.2
36.2
28.7
50.0
22.4
43.6
48.1
52.1
42.7
100.0
31.7
68.1
63.1
68.4
62.5
150.0
38.2
94.4
70.6
80.2
83.7
175.0
36.3
92.3
69.1
80.1
82.8
200.0
33.4
89.5
68.3
78.4
79.9
(mg)
VMCM-41
Studies on the degradation of CV in the absence of MMCM-41 catalysts by keeping all other parameters identical showed that degradation is not appreciable in the same period. 3.4.1.4 Effect of CV concentration Effect of varying initial concentration of CV on its degradation in the presence of TiMCM-41 has been studied under optimum conditions of pH 5.1, [H2O2] = 1.0 x 10-3 M catalyst dosage of 150 mg. Variation of % degradation at different initial concentrations of CV is shown in Fig. 3.11. The results indicate that, increasing CV concentration in the range 2.0 x 10−4 M to 8.0 x 10−4 M has no significant effect on the % of its degradation (Table 3.5). This may be attributed to the saturation of catalyst surface with CV molecules at these concentrations. 5.0 x 10-4 M of CV has been chosen for all the studies on the degradation of CV such that the absorbance measurements are in suitable range for carrying out the experiments. 87
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Fig. 3.11: Effect of initial concentration of CV on its degradation using TiMCM-41. [H2O2] = 1.0 x 10-3 M, pH = 5.1 and TiMCM-41 dosage = 150 mg Table 3.5: Effect of concentration of CV on its degradation [H2O2] = 1.0 x 10-3 M, pH = 5.1, TiMCM-41 = 150 mg
% Degradation [CV] x 104 M
Time (mins) 2 .0
4 .0
6.0
8.0
10
21.2
22.9
22.3
18.8
20
37.4
39.8
40.9
37.4
30
44.1
43.3
45.4
43.2
40
54.5
57.5
58.8
57.7
50
60.0
60.9
63.4
64.6
60
67.4
68.6
69.5
73.4
90
75.0
76.7
78.9
79.4
100
81.9
83
85.1
82.8
120
94.2
93.7
94.1
93.3
150
94.1
93.6
94
93.1
88
Chapter –3
Metal Incorporated MCM-41
3.4.1.5 Studies on reusability of TiMCM-41 catalyst In order to explore the applicability of this method for CV degradation in industrial waste waters, recyclability of the catalyst has been investigated under optimum conditions of pH 5.1, catalyst dosage of 150 mg, [H2O2] = 1.0 x 10-3 M and [CV] = 5.0 x 10-4 M in the presence of TiMCM-41, the most efficient among the catalysts studied. Studies on reusability of catalyst were carried out after maximum degradation has been reached. The contents were centrifuged to recover the catalyst. It was washed with water followed by ethanol, dried and reused under optimum experimental conditions. It is observed that the % degradation of CV remains nearly same even after three successive runs (Table 3.6). This confirms that the activity of TiMCM-41 gets retained after repeated use. These studies also reveal that there is no leaching of metal ions from the framework of the catalytic material even after several washings and usage.
Table 3.6: Reusability of TiMCM-41 towards the degradation of CV [CV] = 5.0 x 10-4 M, [H2O2] = 1.0 x 10-3 M, pH = 5.1, TiMCM-41= 150 mg Catalyst
Fresh
Recycle 1
Recycle 2
Recycle 3
% Degradation
94.4
93.7
92.3
91.3
3.4.2 Catalytic degradation of Rhodamine B over MMCM-41 In order to test the generality of MMCM-41 materials as catalysts for dye degradation purposes, degradation of Rhodamine B (RhB) was also studied. The % degradation of RhB was evaluated by measuring the decrease in the absorbance of RhB solution with respect to time at 642 nm, the λmax of RhB. The studies were carried out in the presence of all the four metal containing MCM-41 catalysts by using H2O2 as 89
Chapter –3
Metal Incorporated MCM-41
oxidant. Variation of % degradation of RhB with time using 100 mg of MMCM-41 catalysts is presented in Fig. 3.12. Degradation of RhB is due to its adsorption on to the surface of MMCM-41 catalysts and its reaction with .OH radicals produced by H2O2. The decolorization of RhB is due to its conversion to lactones form. This may involve in further attacks by more .OH radicals to give multiple degradation products [68].
Fig. 3.12: Variation of absorbance with time for the degradation of RhB over MMCM-41 catalysts. [RhB] = 5.0 x 10-4 M, [H2O2] = 5.8 x 10-3 M, pH = 8, Catalyst dosage = 100 mg
3.4.2.1 Effect of pH Effect of pH on degradation of RhB by H2O2 in the presence of MMCM-41 catalysts has been studied at room temperature in the pH range of 2-10, keeping the concentrations of H2O2, RhB and catalyst dosage constant (Fig. 3.13). Extent of degradation of RhB was found to increase with the increase in pH till 8 and then decreased slightly till pH 10 (Table 3.7). Increase in the degradation efficiency with the increase in pH can be attributed to the same reasons as in the case of CV because RhB is
90
Chapter –3
Metal Incorporated MCM-41
also a cationic dye. Small decrease in the extent of degradation at pH >8 may be due to changes in hydroxyl radical formation at higher pH. Accordingly, pH 8 has been selected for carrying out further studies on the degradation of RhB.
Fig. 3.13: Effect of pH on the degradation of RhB in the presence of a) TiMCM-41, b) VMCM-41, c) FeMCM-41 and d) CoMCM-41. [RhB] = 5.0 x 10-4 M, [H2O2] = 5.8 x 10-3 M, Catalyst dosage = 100 mg
91
Chapter –3
Metal Incorporated MCM-41
It can be noted that MMCM-41 materials have greater effect on degradation compared to MCM-41 at any pH studied. FeMCM-41 is found to be the most efficient catalyst in degrading RhB as compared to other MMCM-41 catalysts studied. The catalytic activity of TiMCM-41, VMCM-41 and CoMCM-41 is observed to be nearly the same.
Table 3.7: Effect of pH on the degradation of RhB [RhB] = 5.0 x 10-4 M, [H2O2] = 5.8 x 10-3 M, Catalyst dosage = 100 mg % Degradation pH MCM-41
TiMCM-41 VMCM-41
FeMCM-41
CoMCM-41
2.0
38.2
55.6
58.2
65.5
63.9
3.0
41.0
63.2
64.7
71.2
67.8
4.0
42.9
72.3
68.3
86.9
70.2
5.0
43.7
74.2
70.1
90.3
73.8
6.0
44.8
76.5
72.3
91.5
76.5
7.0
46.2
78.6
74.2
91.6
77.4
8.0
47.1
81.2
77.8
94.2
82.0
9.0
46.4
80.3
77.2
89.1
81.4
10.0
44.5
78.7
76.1
87.3
80.3
3.4.2.2 Effect of H2O2 concentration Effect of varying concentration of H2O2 on the degradation of RhB has been investigated at room temperature in the range of 2.3 x 10-3 M to 7.2 x 10-3 M of H2O2 and results are represented in Fig. 3.14. The degradation of RhB increased with the increase in concentration of H2O2 till 5.8 x 10-3 M and further increase to 7.2 x 10-3 M did not 92
Chapter –3
Metal Incorporated MCM-41
have significant effect on the degradation (Table 3.8). Increased effectiveness of degradation with increase in H2O2 till 5.8 x 10-3 M can be understood on the basis of availability of more hydroxyl radicals for the attack on adsorbed RhB. Further increase in H2O2 concentration may not be contributing to the increase in .OH radicals due to scavenging effect [67].
Fig. 3.14: Effect of H2O2 on the degradation of RhB in the presence of a) TiMCM-41, b) VMCM-41, c) FeMCM-41 and d) CoMCM-41. [RhB] = 5.0 x 10-4 M, pH = 8, Catalyst dosage = 100 mg
93
Chapter –3
Metal Incorporated MCM-41
The % of degradation of RhB was found to be 94.2% at 5.8 x 10-3 M of H2O2 after a period of 120 mins. Consequently, all the other experiments on the degradation were carried out using 5.8 x 10-3 M of H2O2. Table 3.8: Effect of H2O2 on the degradation of RhB [RhB] = 5.0 x 10-4 M, pH = 8, Catalyst dosage = 100 mg % Degradation
Concentration of H2O2 x 103 M
MCM-41 TiMCM-41 VMCM-41
FeMCM-41
CoMCM-41
2.3
33.6
63.6
61.1
75.4
58.2
4.6
41.2
75.2
63.2
87.3
62.6
5.8
47.1
82.7
65.6
94.2
69.1
7.2
46.6
82.3
64.7
90.3
65.4
3.4.2.3 Effect of catalyst dosage Effect of amount of catalyst on the degradation of RhB has been studied by varying the amount of catalysts from 25 mg to 150 mg at pH 8, keeping the concentrations of H2O2 and RhB constant (Fig. 3.15). It is observed that the percentage degradation of RhB increases from 54.2% to 94.2% with increase in amount of FeMCM-41 catalyst from 25 mg to 100 mg (Table 3.9). Further increase in the amount of catalyst to 150 mg did not show much influence on % degradation. This is also found to be true in the case of TiMCM-41, VMCM-41 and CoMCM-41 catalysts. Hence, 100 mg of the catalyst has been chosen as optimum amount in all other investigations.
94
Chapter –3
Metal Incorporated MCM-41
Fig. 3.15: Effect of catalyst dosage on the degradation of RhB in the presence of a) TiMCM-41, b) VMCM-41, c) FeMCM-41 and d) CoMCM-41. [RhB] = 5.0 x 10-4 M, [H2O2] = 5.8 x 10-3 M, pH = 8
95
Chapter –3
Metal Incorporated MCM-41
Table 3.9: Effect of Catalyst dosage on the degradation of RhB [RhB] = 5.0 x 10-4 M, [H2O2] = 5.8 x 10-3 M, pH = 8 % Degradation Catalyst dosage (mg)
MCM-41
25.0
27.1
42.1
38.4
54.2
37.2
50.0
34.0
64.5
53.7
70.1
51.3
75.0
38.2
78.1
61.9
83.4
67.8
100.0
41.5
87.3
67.2
94.2
72.8
125.0
41.1
81.8
64.3
92.2
70.2
150.0
43.7
84.5
63.7
89.1
69.3
TiMCM-41 VMCM-41 FeMCM-41 CoMCM-41
3.4.2.4 Effect of RhB concentration Degradation of RhB was studied in the presence of FeMCM-41 by varying concentration of RhB in the range of 1.0 x 10-4 M to 7.0 x 10-4 M at pH 8 and H2O2 concentration of 5.8 x 10-3 M keeping FeMCM-41 dosage as 100 mg. The % degradation of RhB with time at different initial concentrations of RhB is given Fig. 3.16. The % of degradation is found to increase with increase in concentration of RhB from 1.0 x 10-4 to 5.0 x 10-4 M and further increase did not have appreciable change in the extent of degradation (Table 3.10). This may be attributed to increase in the amount of adsorbed dye with increase in RhB concentration till saturation of the catalyst takes place with it. Further increase in RhB concentration after saturation may only increase its concentration in the bulk where the rate of degradation is very low.
96
Chapter –3
Metal Incorporated MCM-41
Fig. 3.16: Effect of initial concentration of RhB on its degradation using FeMCM-41. [H2O2] = 5.8 x 10-3 M, pH = 8, FeMCM-41 dosage = 100 mg
Table 3.10: Effect of concentration of RhB on its degradation [H2O2] = 5.8 x 10-3 M, pH = 8, FeMCM-41 dosage = 100 mg % Degradation Time (mins)
[RhB] x 104 M 1. 0
2.0
3.0
4.0
5.0
6.0
7.0
10
2.3
2.6
3.4
3.9
9.2
3.9
3.2
20
7.5
11.3
18.3
19.4
16.4
17.4
17.2
30
13.0
17.2
24.0
25.3
34.9
21.3
22.5
40
16.2
22.8
36.4
33.0
41.5
23.0
34.2
50
22.0
34.5
48.3
43.2
50.4
37.2
52.1
60
35.1
47.5
54.7
56.2
61.8
46.2
61.3
90
39.6
51.7
63.4
63.8
68.4
53.8
69.8
100
45.2
54.0
71.4
72.0
81.6
75.0
78.1
120
52.1
65.5
76.9
90.3
94.2
87.3
86.1
150
52.1
66.1
76.9
91.2
94.1
92.8
85.9
97
Chapter –3
Metal Incorporated MCM-41
3.4.2.5 Studies on reusability of FeMCM-41 catalyst Studies on the reusability of catalyst on the degradation of RhB have been carried out by using FeMCM-41, the most efficient among the catalysts studied. The catalyst was separated by centrifugation after maximum % of dye gets degraded. It is thoroughly washed with water and then by ethanol. The recovered catalyst is reused in the next cycle under optimum reaction conditions. It is observed that FeMCM-41 effectively catalyzes the degradation of RhB without much loss of its activity for three successive runs (Table 3.11).
Table 3.11: Reusability of FeMCM-41 towards the degradation of RhB [RhB] = 5.0 x 10-4 M, [H2O2] = 5.8 x 10-3 M, pH = 8, FeMCM-41 dosage = 100 mg Catalyst % Degradation
Fresh
Recycle 1
Recycle 2
Recycle 3
94.2
90.1
89.2
88.7
Results presented in this chapter revealed that metal incorporated MCM-41 materials are mesoporous in character and they act as efficient catalysts for the degradation of Crystal violet and Rhodamine B dyes. TiMCM-41 has been found to be more efficient for the degradation of Crystal violet while FeMCM-41 is more effective in the case of Rhodamine B degradation, indicating the specificity of catalysts. The optimum conditions for the efficient degradation of Crystal violet have been established as pH = 5.1, [H2O2] = 1.0 x 10-3 M, [CV] = 5.0 x 10-4 M and TiMCM-41 catalyst dosage of 150 mg, while those for Rhodamine B have been found to be pH = 8, [H2O2] = 5.8 x 10-3 M, [RhB] = 5.0 x 10-4 M and FeMCM-41 catalyst dosage of 100 mg in the limits of investigations. These catalytic materials exhibited good reusability over 98
Chapter –3
Metal Incorporated MCM-41
three successive cycles. They have potential to be used as eco-friendly and economical catalysts for dye degradation of industrial waste waters in general and in the cases of CV and RhB in particular.
99
Chapter –3 3.5 1.
Metal Incorporated MCM-41
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103
Chapter – 4 SYNTHESIS,
CHARACTERIZATION
OF
ACID
FUNCTIONALIZED MCM-41 AND THEIR CATALYTIC APPLICATIONS TOWARDS THE SYNTHESIS OF IMINES, 1-AMIDOALKYL 2-NAPHTHOLS AND BENZYLIDINE BARBITURATES
Chapter – 4
4.1
Acid Functionalized MCM-41
Introduction One of the challenges in catalysis is the development of renewable energy based
processes, to diminish the detrimental environmental impact associated with chemical industries. Heterogeneous catalysts have gained a great deal of importance in recent years due to their economical and environmental friendly nature. These catalysts are generally more efficient, convenient to handle, easy to recover and result in reduced reaction times [1-4]. Multi component reactions (MCRs) have emerged as an attractive and powerful method in organic synthesis due to high selectivity, atom economy, simplicity and synthetic efficiency [5, 6]. Therefore, large number of academic and industrial research groups have increasingly focused on the use of MCRs to synthesize a wide range of products. Development of MCRs can lead to novel and efficient routes to synthesize variety of organic compounds. In recent years, single step MCRs have been the focus of researchers for the synthesis of many important heterocyclic compounds [7, 8]. Bigenelli [9], Ugi [10], Passerini [11] and Mannich [12] reactions are some of the examples of MCRs. Development of new MCRs, improvement and modifications of known MCRs are among the most intensely studied areas of research in modern organic chemistry. In view of the growing importance of single step MCRs to produce pharmaceutically important intermediates, we have studied the synthesis of (i) imines, (ii) 1-amidoalkyl 2-naphthols and (iii) benzylidine barbiturates in the presence of acid functionalized mesoporous materials as catalysts. Mesoporous MCM-41 has been extensively used as a heterogeneous solid catalyst in the synthesis of fine chemicals [13, 14]. It is possible to functionalize these materials by covalent anchoring of different organic moieties on the surfaces [15]. Incorporation of 104
Chapter – 4
Acid Functionalized MCM-41
compounds having Bronsted acid sites into the frame work of MCM-41 type materials enhances their surface acidic character [16]. Due to enhanced acidic character and high stability, these materials can be used as efficient and reusable heterogeneous catalysts for the synthesis of fine chemicals and pharmaceutically important compounds. Development of one-pot multi component synthesis using functionalized MCM-41 catalysts will be an eco-friendly approach [17]. Reactions carried out with this methodology are known to give excellent yields without isolation of any intermediates during the processes [18]. 4.1.1 Background to the synthesis of imines Imines have been found to have a wide range of biological activities such as lipoxygenase inhibition, anti-inflammatory [19] and anti-cancer behaviour [20]. They are also used as versatile components in the formation of optically active α-alkyl aldehyde [21], preparation of secondary amines by hydrogenation [22], nucleophilic addition with organometallic reagents [23] and in cyclo addition reactions [24]. Imines are usually prepared by a reversible condensation reaction between a primary amine and a carbonyl compound [25]. Some recent methods for the preparation of imines include use of different Lewis acids, like ZnCl2, P2O5/SiO2 [26, 27]. 4.1.2 Background to the synthesis of 1-amidoalkyl 2-napthols Variety of biologically important natural products, number of antibiotics and HIV protease inhibitors [28] contain 1,3-amino oxygenated functional groups. 1-amidoalkyl 2-naphthols are important class of organic compounds having 1,3-amino oxygenated moieties. 1-amidoalkyl 2-napthols and their derivatives have number of applications in biology and pharmacology [29]. These are very important intermediates in the synthesis of various pharmaceutical compounds [30]. 1-Amidoalkyl naphthols can be easily hydrolyzed to 1-aminoalkyl naphthols and produce biologically active compounds [31]. Some of these compounds exhibit spasmolytic, diuretic, anticoagulant, anticancer, and 105
Chapter – 4
Acid Functionalized MCM-41
antianaphylactic activity [32]. Development of facile and green routes for the synthesis of 1-amidoalkyl 2-naphthols is important for medicinal chemistry applications. Commonly, 1-amidoalkyl 2-naphthols have been synthesized in a three component reaction of aldehydes, amides and 2-naphthols in the presence of various catalysts. In recent years Iodine [33], Montrimilite – K10 [34], HClO4-SiO2 [35], FeCl3.SiO2 [36], Sulfamic acid [37] and ionic liquids [38], K5CoW12O40 .3H2O [39] H2NSO3H [40], cyanuric chloride [41], Yb(OTf)3 [42], Sr(OTf)2 [43] have been employed as catalysts for their synthesis. 4.1.3 Background to the synthesis of benzylidine barbiturates Barbituric acid derivatives are very sought after target compounds for organic and medicinal chemists due to their diverse biological activity [44]. Owing to the ready availability and multiple functionalization possibilities, the parent barbituric acid is a convenient starting compound for the preparation of different fused heterocycles and 5-substituted derivatives which are pharmacologically very important class of barbituric acid derivatives [45, 46]. Barbituric acid and 2-thiobarbituric acid undergo Knoevenagel condensation with aldehydes to give 5-arylidene derivatives that can be further subjected to various chemical transformations [47]. 5-arylidene barbituric acids contain a strongly polarized exocyclic double bond, with a positive partial charge on the methyne carbon atom. They can thus form Michael adducts with nucleophiles such as alkoxides, amines, thiols, and water [48]. 5-Arylidene barbituric acids can also react as dienes in a hetero Diels-Alder reaction to give different 5-aryl-pyrano[2,3-d] pyrimidine-2,4-diones which have received considerable attention due to their wide ranging biological effects, including antiviral [49], antibacterial, antifungal and prostate-protective activity [50-53]. Many researchers have focused on the synthesis of barbituric acid derivatives via Knoevenagel condensation. The synthesis of 5-arylidine barbituric acid derivatives has 106
Chapter – 4
Acid Functionalized MCM-41
been extensively investigated by conventional as well as microwave methods [54]. Several groups reported the synthesis of 5-arylidine barbiturates. However, these methods have certain limitations owing to the homogeneity and toxicity of the chemicals used [55, 56]. Hence, researchers are working on developing efficient catalysts for the synthesis of barbituric acid derivatives. Conventional methods for synthesis of imines, 1-amidoalkyl 2-naphthols and benzylidine barbiturates involve vigorous reaction conditions, tedious isolation processes and use of larger quantities of toxic chemicals that are harmful to the environment. Hence, there is a need to develop new catalytic methods for the synthesis of these compounds under environmental friendly conditions. Use of heterogeneous catalysts would pave way to carry out these reactions efficiently without formation of any side products. This chapter deals with synthesis, characterization of acid functionalized MCM-41 materials and their catalytic applications towards preparation of imines, 1-amido 2-naphthols and benzylidine barbiturates under solvent free and economically viable conditions.
4.2
Synthesis of catalysts
4.2.1 Synthesis of MCM-41 Mesoporous MCM-41 was synthesized by room temperature method as discussed earlier in Chapter-III (Section 3.2). Acid functionalization of synthesized material was adopted by post grafting route. 4.2.2 Synthesis of sulfonic acid functionalized MCM-41 (SO3HMCM-41) 1 g of calcined MCM-41 sample is treated with 30 mL of 0.5 N sulphuric acid and the mixture was stirred at room temperature for about 2 h. This mixture was evaporated
107
Chapter – 4
Acid Functionalized MCM-41
by heating the slurry at 70 ºC for 30 min. Resultant sample was dried at 110 ºC for 5 h and then calcined at 550 ºC for 5 h. 4.2.3 Synthesis of phosphotungstic acid functionalized MCM-41 (PWMCM-41) 1 g of calcined MCM-41 is treated with methanolic solution of 40 wt% phosphotungstic acid. The resultant suspension was stirred at room temperature for 22 h. The gel formed was evaporated and dried at 110 ºC for 30 min. The solid was calcined at 550 ºC for 2 h. 4.3
Characterization of SO3HMCM-41 and PWMCM-41 catalysts
4.3.1
Powder XRD studies The powder X-ray diffraction patterns of MCM-41, SO3HMCM-41 and
PWMCM-41 are shown in Fig. 4.1. These patterns feature distinct Bragg peaks in the 2θ range of 2.3–4.5º, which can be indexed as (100), (110) and (200) reflections of a two dimensional hexagonal structure of MCM-41 material [57, 58]. Presence of three diffraction peaks in XRD pattern corresponding to MCM-41, indicates the crystallographic ordering of the mesopores in it. Whereas, only one low angle peak for d100 plane at 2θ value of 2-3° was observed for acid functionalized MCM-41 materials [59]. Appreciable decrease in the intensity ratios of these reflections for SO3HMCM-41 and PWMCM-41 implies that the introduction of acid groups reduced the regular order of MCM-41 after grafting.
108
Chapter – 4
Acid Functionalized MCM-41
Fig. 4.1: Powder XRD patterns of a) MCM-41, b) SO3HMCM-41 and c) PWMCM-41.
4.3.2
Nitrogen adsorption–desorption studies The nitrogen adsorption-desorption isotherms of MCM-41, SO3HMCM-41 and
PWMCM-41 materials are shown in Fig. 4.2. Specific surface area of the synthesized MCM-41, SO3HMCM-41 and PWMCM-41 materials were calculated by applying BET method and were found to be in the range of 600 to 1000 m2g-1. The surface area of the functionalized MCM-41 materials is found to decrease with the loading of acid functional groups to the framework. It is observed that the adsorption follows the typical type-IV adsorption isotherm. This indicates the mesoporous character of the materials [60]. The pore volume and pore radius of the materials were calculated from desorption branch of multipoint BET isotherms by adopting BJH method. The textural properties of these 109
Chapter – 4
Acid Functionalized MCM-41
materials are listed in Table 4.1. The pore size and pore volume of SO3HMCM-41 and PWMCM-41 are found to be lesser than MCM-41. This could be attributed to the occupancy of some of the pores by acid groups.
Fig. 4.2: Nitrogen adsorption-desorption isotherms: a) MCM-41 b) SO3HMCM-41 and c) PWMCM-41.
Table 4.1: Textural properties of acid functionalized MCM-41materials SBET (m2g-1)
Pore Size (Å)
Pore Volume (cc g-1)
MCM-41
1023.5
17.2
0.28
SO3HMCM-41
862.3
16.6
0.16
PWMCM-41
631.1
15.3
0.15
Material
110
Chapter – 4
4.3.3
Acid Functionalized MCM-41
SEM–EDX studies The SEM and EDX images of MCM-41, SO3HMCM-41 and PWMCM-41 are
shown in Fig. 4.3. SEM images reveal spherical morphology of the materials. It can be observed that the morphology remains unaltered even after the functionalization with acid groups. The particle size in the respective materials is found to be uniform on the surface. The presence of P, W and S in the corresponding EDX spectra confirms the functionalization of the surface of MCM-41 materials with acid groups. 4.3.4 FT-IR studies The FT-IR spectra of MCM-41, SO3HMCM-41 and PWMCM-41 are shown in Fig. 4.4. The FT-IR spectrum of the MCM-41 sample reveals the bands close to 1643 cm−1 due to the bending vibration of adsorbed water molecules [61]. The asymmetric stretching vibrations of Si-O-Si are observed by the absorption bands at 1072 and 1228 cm−1. The band at 964 cm−1 can be attributed to Si-OH vibrations [62]. The absorption peaks around 450 to 795 cm−1 are mainly due to bending vibrations of Si-O-Si bonds and the band at 794 cm−1 corresponds to the presence of free silica [63]. SO3HMCM-41 exhibits additional peaks at 1169 cm-1 and 574 cm-1 corresponding to the sulfonic acid group. The peaks corresponding to the -S-O stretching vibrations of sulfonic acid are commonly observed in the range of 1000-1200 cm-1. However, these peaks are not resolved due to their overlap with the absorption of the -Si-O-Si stretch in the 1000-1130 cm-1 range and that of the -Si-CH2-R stretch in the 1200-1250 cm-1 range. In all the samples, the peaks associated with the non-condensed -Si-OH groups at 960 cm-1 are present. Strong peak around 1630 cm-1 is mainly due to the bending vibration of adsorbed H2O.
111
Chapter – 4
Acid Functionalized MCM-41
a
a
b b
c
c
Fig. 4.3: SEM - EDX images of a) MCM-41, b) SO3HMCM-41 and c) PWMCM -41. Four main absorption peaks observed for phosphotungstic acid at 1080 cm−1 (P–Oa), 983 cm−1 (W-Od), 893 cm−1 (W–Ob–W) and 800 cm−1 (W–Oc–W), can be ascribed to asymmetric bond stretching vibrations. Absence of any shift in W-Od band after functionalization on MCM-41 indicates that there is no fragmentation of phosphotungstic acid.
112
Chapter – 4
Acid Functionalized MCM-41
Fig. 4.4: FT-IR spectra of a) MCM-41, b) SO3HMCM-41 and c) PWMCM-41.
4.4
Catalytic applications of SO3HMCM-41 and PWMCM-41 in the
synthesis of imines, 1-amidoalkyl 2-naphthols and benzylidine barbiturates 4.4.1
Synthesis of imines SO3HMCM-41 and PWMCM-41 were used as catalysts for solvent free synthesis
of imines. A mixture of aromatic amine (10 mmol) and benzaldehyde (10 mmol) was taken in a mortar. 0.01 g of acid functionalized MCM-41 catalyst was added to this mixture. The reaction mixture was vigorously grinded at room temperature (Scheme 4.1). Progress of the reaction was monitored through thin layer chromatography using hexane:ethyl acetate as mobile phase. After completion of the reaction, the product was dissolved in hot ethanol and filtered. The filtrate was then evaporated under reduced pressure to yield crude product. The catalyst was recovered and washed with methanol for several times and dried at 100 ºC for reuse. Pure product was obtained by 113
Chapter – 4
Acid Functionalized MCM-41
re-crystallization using ethanol. The reactions were carried out with varying amounts of catalysts in the range 0.005 – 0.025 g to determine the optimum quantity of catalyst. It was found that, an amount as low as 0.01 g is able to catalyze the reactions efficiently. Accordingly, all the reactions were carried out by using 0.01 g of catalyst. The reactions were carried out with two amines i.e., aniline and phenyl hydrazine under identical conditions at room temperature without any solvent. Both the reactions proceeded with same ease and gave good yields of the desired products (Table 4.2). Well known products were authenticated by their melting points.
Catalyst
NH2
N
RT, Grinding O
+ Benzaldehyde
Aniline
Imine
Catalyst H2NHN
RT, Grinding
Phenyl hydrazine
N N H
Catalyst: MCM-41 or SO3HMCM-41 or PWMCM-41 Scheme 4.1: Synthesis of imines over acid functionalized MCM-41.
The time required for completion of the reaction is found to decrease significantly (Table 4.2) in the presence of acid functionalized MCM-41 catalysts when compared to the time taken for the reaction in the presence of conventional catalyst Mg(ClO4)2 [64]. While, it required only 15 mins in the case of SO3HMCM-41 and PWMCM-41 catalysts, it took 8 h in the case of Mg(ClO4)2. It is also observed that SO3HMCM-41 exhibits better catalytic activity than PWMCM-41 by way of giving higher yields. This may be probably due to the presence of relatively larger number of sulphonic acid groups on the surface compared to the bulkier phosphotungstic acid groups. Better efficiency of 114
Chapter – 4
Acid Functionalized MCM-41
SO3HMCM-41may also be due to its relatively larger surface area of 862.3 m2g-1 as compared to 631.1 m2g-1 of PWMCM-41. Details of % yield of the products along with the time taken for completion of reactions in the presence of SO3HMCM-41 and PWMCM-41 catalysts are presented in Table 4.2.
Table 4.2: Effect of acid functionalized MCM-41 catalysts on the synthesis of imines Reaction conditions: Benzaldehyde = 10 mmol, Aniline / Phenyl hydrazine = 10 mmol, Amount of catalyst = 0.01 g
Entry Benzaldehyde
Aniline
Catalyst
Solvent Time
Yield (%)
SO3HMCM-41
Nil
15 min
83.3
PWMCM-41
Nil
15 min
74.0
MCM-41
Nil
15 min
45.5
DCM
8h
80.3
SO3HMCM-41
Nil
15 min
95.5
PWMCM-41
Nil
15 min
80.3
MCM-41
Nil
15 min
37.5
DCM
8h
90.0
Mg(ClO4) Benzaldehyde
Phenyl hydrazine
[64]
Mg(ClO4)2
[64]
*
DCM = Dichloromethane
Plausible mechanism for the synthesis of imines over acid functionalized MCM-41 taking SO3HMCM-41 as example is depicted in Scheme 4.2. Aldehyde (I) gets adsorbed on SO3HMCM-41 and interacts with acidic hydrogen of SO3HMCM-41 catalyst. Aldehyde gets activated, and its carbonyl carbon becomes more electrophilic. Then the amine (III) undergoes nucelophilic addition on carbonyl carbon of II by donating its lone pair of electrons present on nitrogen and results in compound IV. This on elimination of water leads to the formation of imine (V). 115
Chapter – 4
Acid Functionalized MCM-41 O O
O
S
H
S O
O
O
S
O
H
O
O
H
MCM-41
O
MCM-41
O
MCM-41
H H
O
H2 N
H
+ (I)
(II)
(III)
-H
H O
O H
O
O
+
N H
H
S
H
N
N
-H2O
MCM-41
O
(V) (IV)
Scheme 4.2: Plausible mechanism for the synthesis of imines over SO3HMCM-41. 4.4.2
One pot multi-component synthesis of 1-amidoalkyl 2-naphthols One pot multi component synthesis of 1-amidoalkyl 2-naphthols was carried out
in solvent free conditions at 40 ºC in the presence of SO3HMCM-41 and PWMCM-41 catalysts. Equimolar mixture of 2-naphthol, aromatic aldehyde and acetamide were taken in a round bottom flask and 0.01 g of the catalyst was added and the contents of the flask were subjected to stirring (Scheme 4.3). Progress of the reaction was monitored through thin layer chromatography using hexane : ethyl acetate as mobile phase. After completion of the reaction, the mass was cooled and washed with water and dried at room temperature. The crude product obtained was re-crystallized with ethyl alcohol and authenticated by its melting point.
116
Chapter – 4
Acid Functionalized MCM-41
R O OH
NHCOCH3
O H
+
Catalyst
OH
+
NH2 Solvent free, 40 ºC R
2-naphthol
Aldehyde
Acetamide
1-amidoalkyl 2-naphthol
Catalyst: MCM-41 or SO3HMCM-41 or PWMCM-41 Scheme 4.3: Synthesis of 1-amidoalkyl 2-naphthols over acid functionalized MCM-41.
Results of the studies in the preparation of 1-amidoalkyl 2-naphthol using MCM41, SO3HMCM-41 and PWMCM-41 are summarized in Table 4.3. Effect of other catalysts on this reaction reported in the literature are also included in the Table for comparison. The data reveals that SO3HMCM-41 is a more efficient catalyst with respect to reaction time, temperature required and product yield than those reported earlier. In order to examine if the reaction proceeds more efficiently in the presence of solvents, the model reaction of 2-naphthol, benzaldehyde and acetamide was carried out in different solvents using SO3HMCM-41 as catalyst. However solvent free condition has been found to be more favourable (Table 4.4). While the reaction was completed in 1.5 h and the desired product was obtained in 92% yield under solvent free condition, yields in the presence of solvents were found to be very low even after 2 h of the reaction. Thus solvent free condition is employed for the synthesis of 1-amidoalkyl 2-naphthols.
117
Chapter – 4
Acid Functionalized MCM-41
Table 4.3: Synthesis of 1-amidoalkyl 2-naphthols in the presence of SO3HMCM-41 and PWMCM-41 Reaction conditions: Benzaldehyde = 10 mmol, Acetamide = 10 mmol, 2-Naphthol = 10 mmol, Temperature = 40 ºC, catalyst = 0.01 g Entry Catalyst
Solvent/ Temp. (ºC) Time (h)
Yield (%)
Ref.
1
SO3HMCM-41
Solvent free / 40
1.5
92
Present work
2
PWMCM-41
Solvent free / 40
1.5
77
Present work
3
MCM-41
Solvent free / 40
1.5
55
Present work
4
SO3HMCM-41 cycle 1
Solvent free / 40
1.5
88
Present work
5
SO3HMCM-41 cycle 2
Solvent free / 40
1.5
85
Present work
6
p-TSA
Solvent free / 125
6
86
[65]
7
Sr(OTf)2
CHCl3/ Reflux
9
83
[43]
8
K5CoW12O403H2O
Solvent free / 125
3
80
[39]
9
Montmorillonite K10
Solvent free / 125
1.5
72
[34]
10
Bi(NO3)3.5H2O
Solvent free / 80
1.2
80
[66]
11
HClO4-SiO2
Solvent free / 110
1.5
75
[35]
12
Amberlyst-15
Solvent / 60
3.5
35
[67]
Plausible mechanism involved in the synthesis of 1-amidoalkyl 2-naphthols over SO3HMCM-41 as model catalyst is presented in Scheme 4.4. When aldehyde (I) gets adsorbed on the surface of SO3HMCM-41 catalyst, its carbonyl carbon becomes more electrophilic (II). This results in the attack of
π – electrons of 2-naphthol (III) on the
carbonyl carbon of the aldehyde (II) and leads to the facile formation of o-quinone methide (IV). The generated o-quinone methide further reacts with amide (V) through conjugate addition to form 1-amidoalkyl 2-naphthol (VI). 118
Chapter – 4
Acid Functionalized MCM-41 O O O
S
MCM-41 S
O O
O
H
O
O
H
O
H
S
MCM-41
O
O
MCM-41
H
O
H
O
H
H
+ (I) (II)
(III)
H O HO
H H
-H
H O
O
-H2O
O O R
(V)
NH2
(IV)
R
O
R
O
H N
HN
H O
O
OH
+
O
H
S
MCM-41
O
(VI)
Scheme 4.4: Plausible mechanism for the synthesis of 1-amidoalkyl 2-naphthols over SO3HMCM-41 catalyst. 119
Chapter – 4
Acid Functionalized MCM-41
Table 4.4: Synthesis of 1-amidoalkyl 2-naphthols using different solvents in the presence of SO3HMCM-41 Reaction conditions: Benzaldehyde = 10 mmol, Acetamide = 10 mmol, 2-Naphthol = 10 mmol, Temperature = 40 ºC, SO3HMCM-41 = 0.01 g Entry
Solvent
Time (h)
Yield (%)
1
Dichloromethane
2.0
12
2
Ethyl acetate
2.0
23
3
Tetrahydrofuran
2.0
37
4
Ethanol
2.0
60
5
Acetonitrile
2.0
52
6
None
1.5
92
After establishing the optimum reaction conditions, the protocol was extended to involve substituted aromatic aldehydes. In all these cases, the corresponding 1-amidoalkyl 2-naphthols were the sole products. It was found that the time taken for the completion of reaction ranged between 1h to 3h depending upon the substituent present on the aromatic aldehyde. Results of the % yield of the products are presented in Table 4.5. All the reactions proceeded with same ease and gave good yields of the respective products irrespective of the nature of the substituent on the aromatic aldehyde. The reaction of benzaldehyde, 2-naphthol and acetamide to give 1-amidoalkyl 2-naphthol was chosen as a model reaction to test the reusability of SO3HMCM-41 catalyst. After completion of the reaction, ethanol was added to dissolve the product. The solid catalyst was filtered off, washed with methanol and dried under vacuum after each cycle, and then reused for the next cycle under the same reaction conditions. 120
Chapter – 4
Acid Functionalized MCM-41
Table 4.5: Synthesis of 1-amidoalkyl 2-naphthols with substituted aromatic aldehydes in the presence SO3HMCM-41 Reaction conditions: Aldehydes = 10 mmol, Acetamide = 10 mmol, 2-Naphthol = 10 mmol, Temperature = 40 ºC, SO3HMCM-41 dosage = 0.01 g S.No
Reactant
Product
CHO
1
Time (h)
Yield (%)
MP (ºC)
1.5
92
244-245
1.5
89
247-249
2.0
92
238-239
2.0
88
224-225
2.5
92
226-227
2.5
90
185-186
3.0
88
248-249
NHCOCH3 OH
2
CHO
O2N
NHCOCH3 OH
NO2
3
CHO
O2N NHCOCH3 OH
NO2
4
CHO
Cl NHCOCH3 OH
Cl
5
CHO
Br NHCOCH3 OH
Br
6
CHO
H3CO NHCOCH3 OH
OCH3
7
CHO
F NHCOCH3 OH
F
121
Chapter – 4
Acid Functionalized MCM-41
Yield of the product is found to be 88% in cycle I and 85% in cycle II as against 92% in the case of fresh catalyst (Table 4.3). These observations reveal that the activity of the catalyst is retained after three successive runs. Thus, solvent free condition, low reaction temperature, requirement of very low quantity of SO3MCM-41catalyst and its reusability constitute a very attractive protocol for the synthesis of 1-amidoalkyl 2-naphthols. 4.4.3
Synthesis of benzylidine barbiturates An equimolar mixture of aromatic aldehyde (1.0 mmol) and barbituric acid
(1.0 mmol) was taken in a reaction vessel. Small quantity of methanol (5 mL) as solvent was added to this mixture to get complete dissolution of the reactants. 0.01 g of acid functionalized MCM-41 catalyst was added and the reaction mixture was stirred at room temperature (Scheme 4.5). Progress of the reaction was monitored through thin layer chromatography. After completion of the reaction, the mixture was evaporated under reduced pressure to yield the crude product. It was dissolved in ethanol and filtered to separate the catalyst. The resultant product was re-crystallized by using ethanol:water in 1:1 ratio to yield pure product. Melting points of the products were found to be identical to their literature values. The recovered catalyst was washed with methanol for several times and dried at 150 ºC. O
O O HN
+ Ar
Aldehyde
H
O
NH
HN
Catalyst O
Methanol, RT
O
NH O
Ar
Barbituric acid
Benzylidine barbiturates
Catalyst: MCM-41 or SO3HMCM-41 or PWMCM-41 Scheme 4.5: Synthesis of benzylidine barbiturates over acid functionalized MCM-41.
122
Chapter – 4
Acid Functionalized MCM-41
The reaction occurred efficiently in short duration giving corresponding benzylidine barbiturates in good yield without formation of by-products. Among SO3HMCM-41 and PWMCM-41 catalysts used for the synthesis of barbituric acid derivatives, PWMCM-41 is found to exhibit higher catalytic activity by way of shorter reaction times and product yields (Table 4.6). Relatively higher yields in the presence of PWMCM-41 despite its lower surface area may be due to stronger and specific interactions between the catalyst and reactants. Plausible steps involved in the synthesis of banzylidine barbiturates taking PWMCM-41 catalyst as an example are illustrated in Scheme 4.6.
HO
HO O
O
OH O
P
w O
O
O
P
O H
O
w
O O
H H
MCM-41
OH
OH O
O
w
MCM-41
O
OH
P
O
H HO
OH
O
H
MCM-41
O
NH
H
OH
H
O
(I)
H
(II)
N H
(IV)
O
O
NH
-H
HO O
O O
N H
O
OH P
O
H
w
(III)
MCM-41
O OH
O OH
O
O
NH
NH
-H2O O
N H
H
O
H
O
(VI)
O
N H
NH
H
O
O
N H
O
H
(V)
Scheme 4.6: Plausible mechanism for the synthesis of benzylidine barbiturates over PWMCM-41 catalyst.
123
Chapter – 4
Acid Functionalized MCM-41
Here, aldehyde (I) gets adsorbed on PWMCM-41 catalyst and gets activated. The tautomeric form (IV) of barbituric acid (III) attacks the carbonyl carbon of II with its π - electrons to form an intermediate (V). This on dehydration gives the corresponding benzylidine barbiturate (VI). Table 4.6: Effect of acid functionalized MCM-41 catalysts on the synthesis of benzylidine barbiturates Reaction conditions: Benzaldehyde = 1.0 mmol, Barbituric acid = 1.0 mmol, Methanol = 5 mL, Amount of catalyst = 0.01 g
Catalyst
Time (h)
Yield (%)
MCM-41
1.5
55
SO3HMCM-41
1.5
65
PWMCM-41
1.5
75
PWMCM-41 cycle 1
1.5
66
PWMCM-41 cycle 2
1.5
64
In order to examine the general applicability of these catalysts, aldehydes with different substituents were used to react with barbituric acid in the presence of more efficient PWMCM-41. In all the cases, the reaction preceded smoothly giving desired benzylidine barbiturates in good yields in very short reaction times (Table 4.7). Reusability of PWMCM-41 catalyst has been studied on the model reaction between barbituric acid and benzaldehyde. It is observed that, the catalyst remains active over three cycles (Table 4.6). Results presented in this section on the synthesis of benzylidine barbiturates on acid functionalized MCM-41 materials as catalysts revealed that PWMCM-41 acts as an efficient catalyst at room temperature using very small quantity of methanol to dissolve the reactants. 124
Chapter – 4
Acid Functionalized MCM-41
Table 4.7: Synthesis of benzylidine barbiturates using substituted aldehydes in the presence of PWMCM-41 Reaction conditions: Aldehydes = 1.0 mmol, Barbituric acid = 1.0 mmol, Methanol = 5 mL, PWMCM-41 = 0.01 g
Ar
Time (min)
Yield (%)
Melting Point (ºC)
-C6H5
60
75
264-266
4-ClC6H4
75
70
235-238
4-MeOC6H4
50
73
268-271
4-NO2C6H4
70
72
255-257
4-OHC6H4
55
70
278-281
4-FC6H4
65
69
258-260
2-MeOC6H4
55
66
264-266
In summary, studies on SO3HMCM-41 and PWMCM-41 materials synthesized by simple room temperature post grafting technique revealed that the mesoporous character of MCM-41 is retained even after functionalization with sulfonic acid and phosphotungstic acid groups. The catalytic studies on these materials have shown that SO3HMCM-41 acts as an efficient catalyst towards the synthesis of imines and 1-amidoalkyl 2-naphthols under solvent free conditions, while PWMCM-41 is found to be efficient for the synthesis of benzylidine barbiturate in methanol. Possible mechanisms for the catalytic action have been proposed.
125
Chapter – 4
4.5
Acid Functionalized MCM-41
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129
Chapter- 5 SYNTHESIS, CHARACTERIZATION AND CATALYTIC APPLICATIONS
OF
TITANIUM
AND
VANADIUM
CONTAINING SOLID CORE MESOPOROUS SILICA SHELL MATERIALS
Chapter –5
5.1
Solid Core Mesoporous Silica Shell
Introduction Catalytic oxidation of hydrocarbons in general is employed in the manufacture of fine
chemicals. In particular, more than 60% of products synthesized by catalytic routes in the chemical industry are obtained by oxidation reactions [1-3]. These catalytic reactions have largely replaced the use of hazardous organic per acids, inorganic acids and other harmful oxidizing agents [4]. This chapter deals with synthesis and characterization of Ti or V containing solid core mesoporous silica shell (TiSCMS or VSCMS) materials and their catalytic applications towards (i) conversion of cyclicketones to their respective lactams using ammonia and H2O2 and (ii) oxidation of diphenylmethane to benzophenone using air. 5.1.1 Background to the conversion of cyclohexanone to caprolactam Caprolactam is an essential component of the versatile polymer nylon-6,6. The production of caprolactam involves ammoxidation of cyclohexanone to its oxime followed by Beckmann rearrangement of the oxime to caprolactam. The classical method of preparation of caprolactam from cyclohexanone oxime using fuming sulphuric acid as catalyst results in the formation of a large amount of by-products [5]. Consequently, researchers have been working on alternative methods to produce caprolactam from cyclohexanone using heterogeneous catalysts [6, 7]. There are reports on the use of alumina, heteropolyacids, boronphosphate, phosphoric acid, silica–alumina, boric acid on alumina and silica supported tantalum oxide as heterogeneous catalysts for the rearrangement of a variety of ketoximes to amides [8 - 12]. The Beckmann rearrangement is believed to take place at 100 ºC, but desorption of the
130
Chapter –5
Solid Core Mesoporous Silica Shell
product takes place only at temperatures higher than 300 ºC [13]. Above 360 ºC, there is a decrease in selectivity due to the decomposition of caprolactam on the catalyst surface [14]. However, the efficiency of the solid acid catalysts is low towards caprolactam formation, because of their rapid deactivation during the reaction [15, 16]. Sato et. al., [17] have reported that the Beckmann rearrangement occurs on the external surface of ZSM-5 type zeolites. Roseler et. al., [18] and Dahlhoff et. al., [19, 20] claim that only extremely weak acidic sites on zeolite like catalysts are needed for the Beckmann rearrangement. Yashima et. al., [21] studied the vapour phase Beckmann rearrangement of cyclohexanone oxime only on zeolites with different pore width and concluded that the selective rearrangement reaction occurred on the external surface of zeolite materials. Takahashi et. al., [22] studied the kinetics of Beckmann rearrangement on HZSM-5 and the effect of acid strength on the reaction. They concluded that, if the reaction had proceeded on the outer surface of the zeolite, the rate constant should be directly proportional to the outer surface area. However, the rate constant was not proportional to the outer surface area. Hence, the result indicated that the acid sites on the outer surface area were not necessarily effective for the reaction of cyclohexanone oxime. Silanol groups on SiMCM-41 are not sufficiently acidic [23, 24] to catalyse the rearrangement and neutral silanol groups are not active in Beckmann rearrangement [25, 26]. Ammoxidation of cyclohexanone to cyclohexanone oxime and its subsequent conversion through Beckman rearrangement to caprolactam has been studied using dilute aqueous
hydrogen
peroxide
and
liquid
ammonia
over
microporous
titanium
silicates (TS-1) [27]. However, restricted pore openings of these TS-1 materials (0.5 – 0.75 nm) underline the need to develop new materials with large pore dimensions of
131
Chapter –5
Solid Core Mesoporous Silica Shell
1 nm or higher where the bulky organic ketones can easily penetrate to the active sites located inside the channels. In this direction, titanium silicates possessing large pores have been synthesized [28]. However, these are found to be catalytically less active in ammoxidation of ketones because of the surface hydrophilicity associated with the presence of excessive surface Si-OH groups [29]. Reports on the conversion of bulkier cyclic ketones like cyclododecanone to their lactams are scanty. There is a need to develop suitable catalysts for such conversions. In this chapter, synthesis of metal containing mesoporous silica shell (MSCMS) materials with wide pores and study of their catalytic behaviour towards the conversion of cyclicketones to their respective lactams are discussed. 5.1.2 Background to the conversion of diphenylmethane to benzophenone Benzophenone is used as an intermediate in the synthesis of perfumes, photo-initiators, drugs and pharmaceuticals [30]. It is commonly prepared by Friedel-Crafts acylation of benzene with benzoyl chloride in the presence of Lewis acid catalysts [31-33]. In this process the catalysts are not recovered easily for reuse. Further, the catalyst is hazardous and also gets deactivated quickly. Manganese (III) Schiff base complex is found to be a good homogeneous catalyst for the oxidation of DPM with 30% H2O2 in acetonitrile under ambient conditions and it resulted in 69% conversion to benzophenone [34]. Since it is a homogeneous process, separation of the product from the reaction mixture is difficult. Therefore, it is important to develop a heterogeneous, easily separable, and reusable catalyst for the production of benzophenone from DPM. Survey of literature revealed several reports on the oxidation of DPM using different heterogeneous catalysts. Clark et. al., reported the first heterogeneously catalyzed oxidation of DPM to benzophenone over alumina supported chromium and manganese [35]. Shaabani et. al., reported solvent free
132
Chapter –5
Solid Core Mesoporous Silica Shell
oxidation of DPM over KMnO4 supported montmorillonite K10 with good yield of benzophenone [36]. The selective oxidation of DPM over KMnO4 impregnated on alumina under microwave irradiation in dry media was studied by Oussaid and Loupy [37]. Sodium chlorite (NaClO2) was employed as a selective catalyst for DPM oxidation either in combination with t-butylhydroperoxide (TBHP) or with N-hydroxyphthalimide [38]. But these processes are found to be hazardous. MnMCM-41 catalysts were also reported for DPM oxidation using air as the oxidant. However, the yield of benzophenone was found to be low [39]. Low DPM conversion (12-15%) and poor selectivity was observed over metal incorporated MMCM-41 (where M is Ti, V and Cr) using H2O2 as oxidant and acetonitrile as the solvent [40]. The direct oxidation of DPM to benzophenone was also studied over cobalt doped MCM-41 [41] and cobalt doped mesoporous TiO2 [42] employing H2O2 as the oxidant in acetic acid. It is reported that hydrotalcite like materials such as MnO4– exchanged MgAl showed high selectivity with very less conversion in the oxidation of DPM using molecular oxygen [43]. Liquid phase oxidation of DPM over ternary hydrotalcites with TBHP as oxidant giving good conversion and selectivity to benzophenone was reported [44]. However, TBHP is a sacrificial and hazardous oxidant and the process involved use of acetonitrile solvent. Thus, the literature revealed the use of either expensive solvents or hazardous materials for the oxidation of DPM. However, these studies indicated that incorporation of metal into framework of porous molecular sieves would lead to higher conversion of DPM and greater selectivity of benzophenone. Further, such framework incorporated catalysts can
133
Chapter –5
Solid Core Mesoporous Silica Shell
combine the high selective characteristics of homogeneous catalysts as well as the easy recovery and recyclability of heterogeneous catalysts. In this context, gas phase oxidation of DPM to benzophenone using air was studied employing newly synthesized large pore MSCMS materials as catalysts.
5.2
Synthesis of TiSCMS and VSCMS materials TiSCMS and VSCMS catalysts were synthesized by using sol-gel method at room
temperature. In a typical procedure, 74.00 mL of acetone was mixed with 11.00 mL of tetraethyl orthosilicate. 3.34 mL of triethylamine followed by 2.00 mL of octadecyltrichloro silane were added to the above solution. A calculated quantity of titanium tetraisopropoxide (Si/Ti = 100) or vanadium pentoxide (Si/V = 100) was added slowly to the above mixture. This was followed by addition of 10.00 mL of distilled water. The contents were subjected to constant stirring for 2 h. Solid product formed was separated by centrifugation. The product was washed with distilled water followed by methanol and dried at 110 ºC overnight. A portion of the sample was calcined at 550 ºC for 5 h. Synthesis of MSCMS materials was also attempted by using octyltrimethoxy silane or methyltrimethoxy silane or octadecyltrimethoxy silane in place of octadecyltrichloro silane. It was observed that these silane compounds did not lead to the formation of any solid product under the same reaction conditions. Poor hydrolyzing rate of these silanes under the reaction conditions could be the major cause for the formation of non solid product. Schematic representation of plausible steps involved in the synthesis of metal containing solid core mesoporous silica shell materials is shown in scheme 5.1. Here, tetraethyl orthosilicate reacts with triethylamine and forms a polymer (I). Simulataneously, octadecyltrichloro silane, metal precursor and tetraethyl orthosilicate react to give another 134
Chapter –5
Solid Core Mesoporous Silica Shell
condensed polymer (II). Polymer II acts as a porogen and gets further condensed on polymer I, to form a core shell silica structure.
O O O Si O O
O
O
+
M
O O Si O O
O
Tetraethyl orthosilicate
Tetraethyl orthosilicate
+
+
Cl Cl Cl Si
N
Octadecyltrichloro silane
Triethylamine
Cl
OH
O-
Cl HO
+
HO
Si
O
Si O M
Si O Si
OH
Cl
HO
OO-
(II)
Cl
(I)
Core
Shell MSCMS
Scheme 5.1: Schematic representation of the formation of MSCMS. M = Ti or V (originated from titanium tetraisopropoxide or vanadium pentoxide)
135
Chapter –5
5.3
Solid Core Mesoporous Silica Shell
Characterization of TiSCMS and VSCMS
5.3.1 Powder XRD studies Fig 5.1 shows powder XRD pattern of TiSCMS synthesized by a simple room temperature one step process. A high intensity low angle peak at 2.6 degrees 2 theta is observed. It is well known that the presence of high intensity peak at a 2 theta less than 5 degree indicates the mesoporous character of the materials [45]. Thus, mesoporous character of the synthesized TiSCMS material is clear from the PXRD pattern.
Fig. 5.1: Powder XRD pattern of TiSCMS.
Powder XRD pattern of VSCMS (Fig 5.2) shows a high intensity peak at 2 theta around 2.4 degrees, once again revealing mesoporous character of the materials. The XRD pattern is found to be similar to that of TiSCMS with a small decrease in the intensity of low angle peak. This infers relatively poor crystallization of VSCMS material under the experimental conditions 136
Chapter –5
Solid Core Mesoporous Silica Shell
Fig. 5.2: Powder XRD pattern of VSCMS. 5.3.2 SEM and TEM studies The SEM-EDX images of TiSCMS and VSCMS materials are presented in Fig. 5.3a and 5.3b. SEM images reveal spherical morphology of the materials with distinct particles. EDX analysis shows the presence of titanium and vanadium in the respective samples (5.3a and 5.3b). (a)
(a)
Fig. 5.3a: SEM-EDX images of TiSCMS.
137
Chapter –5
Solid Core Mesoporous Silica Shell (b)
(b)
Fig. 5.3b: SEM-EDX images of VSCMS. Transmission electron micrograph of TiSCMS and VSCMS are shown in Fig. 5.4a and Fig. 5.4b. TEM Images reveal that the particles are in spherical shape with distinct particle size of core and shell. The TiSCMS (Fig. 5.4a) shows a particle size of nearly 300 nm with shell size of 50 nm. While, transmission electron micrograph (Fig. 5.4b) of VSCMS shows a particle size of nearly 400 nm with shell size of 45 nm. (a)
Fig. 5.4a: TEM image of TiSCMS.
138
Chapter –5
Solid Core Mesoporous Silica Shell (b)
Fig. 5.4b TEM image of VSCMS. 5.3.3
TG-DTA studies The TG-DTA thermograms of as-synthesized TiSCMS and VSCMS samples are
shown in Fig. 5.5. They show 60% exothermic weight loss in the range of 25-625 ºC. Weight loss in thermograms of MSCMS materials is due to the decomposition of octadecyltrichloro silane and amine on heating at constant rate. DTA curves of these materials show two exothermic peaks at 255 ºC & 350 ºC corresponding to the decomposition of triethylamine and carbon chain length of octadecyltrichloro silane. Endothermic peak corresponding to the loss of physisorbed water is not observed in DTA curves indicating the hydrophobic character of as-synthesized materials.
139
Chapter –5
Solid Core Mesoporous Silica Shell (a)
(b)
Fig. 5.5: TG-DTA thermograms of as-synthesized a) TiSCMS and b) VSCMS.
140
Chapter –5 5.3.4
Solid Core Mesoporous Silica Shell
Nitrogen adsorption-desorption studies Nitrogen adsorption-desorption isotherms of TiSCMS and VSCMS materials
recorded at 77 K show a sharp uptake at relative pressure below 0.2 and typical type-IV curve with a capillary condensation between 0.3 - 0.5 (Fig. 5.6 and Fig. 5.7). A distinct hysteresis loop of type H2 is clearly observed. This reveals the mesoporous character associated with TiSCMS and VSCMS. Specific surface area of TiSCMS material by BET method is found to be 550.1 m2g-1. Pore size and pore volume of this material were estimated by BJH method and are found to be 210.2 Å and 1.02 ccg-1 respectively. Specific surface area of VSCMS is found to be 540.6 m2g-1. It is found to have an average pore size of 230.9 Å and pore volume of 1.29 ccg-1. Comparison of surface area of MSCMS materials with MMCM-41 materials reveal that, it decreases by about 40% in MSCMS. This can be understood on account of increased particle size. It is interesting to note that the pore size of MSCMS materials is much larger than the corresponding MMCM-41 materials (Table 3.1). This can be attributed to the use of octadecyltrichloro silane as porogen during the synthesis of MSCMS.
Fig. 5.6: Nitrogen adsorption-desorption isotherm for TiSCMS. 141
Chapter –5
Solid Core Mesoporous Silica Shell
Fig. 5.7: Nitrogen adsorption-desorption isotherm for VSCMS.
5.3.5
UV-Visible DRS studies The UV–Visible DRS spectrum of TiSCMS material is presented in Fig. 5.8. It
shows two peaks at 350 and 220 nm. The peak at 220 nm is attributed to the presence of -Si-O- linkages while peak at 350 nm indicates the presence of tetrahedrally coordinated Ti4+ species in the matrix of SCMS [46]. UV-Visible DRS spectrum of VSCMS material shows three distinct peaks at 350, 280 and 220 nm (Fig. 5.9). The peak at 220 nm is due to -Si-Ospecies, peak at 280 nm is due to tetrahedrally coordinated V5+ species and peak at 350 nm is due to blue shifted square pyramidal V4+ species [47]. Hence, it can be confirmed that two types of vanadium species (V4+ and V5+) are present in VSCMS.
142
Chapter –5
Solid Core Mesoporous Silica Shell
Fig. 5.8: UV-Visible DRS spectrum of TiSCMS.
Fig. 5.9: UV-Visible DRS spectrum of VSCMS.
143
Chapter –5
Solid Core Mesoporous Silica Shell
5.3.6 XPS studies XPS spectrum of TiSCMS is given in Fig. 5.10. Analysis of XPS intensity ratios indicates surface enrichment of titanium. The surface concentration of Ti slightly higher than the bulk values reported for the titanium silicate. The Ti 2p3/2 binding energy is found to be 461.2 eV. It is interesting to note that this value is higher than the acceptable octahedral Ti4+ binding energy of 457.8 - 458.0 eV. The enhancement of binding energy of Ti4+ in the framework is due to tetrahedrally coordinated titanium in the matrix [48]. Tetrahedral coordination of titanium in TiSCMS has also been confirmed by the presence of peak at 350 nm in UV-Visible DRS spectrum.
Fig. 5.10: XPS spectrum of titanium in TiSCMS. The X-ray photoelectron spectrum of VSCMS sample in V2p region (Fig. 5.11) shows two peaks corresponding to V4+ at 528 eV and V5+ at 539 eV [49]. The intensity of peak corresponding to V+5 is more than that of V4+, indicating the presence of relatively larger number V5+ ions in the framework of VSCMS.
144
Chapter –5
Solid Core Mesoporous Silica Shell
Fig. 5.11: XPS spectrum of vanadium in VSCMS.
In summary, the characterization studies of MSCMS materials revealed that they are mesoporous in nature with large pore size and pore volume. TEM analysis indicated the formation of core-shell structure with different particle sizes of core and shell. Results also confirm the presence of metal ions in the framework of solid core mesoporous silica.
5.4
Catalytic applications of MSCMS materials
5.4.1
Conversion of cyclicketones to lactams The conversion reactions of cyclohexanone and cyclododecanone to their
corresponding lactams over MSCMS using H2O2 and ammonia were studied at 80 °C (Scheme 5.2). The reactions were carried out in a two necked round bottomed flask fitted with a water condenser under N2 atmosphere. Cyclicketone and aqueous ammonia were taken in t-butanol in the flask and the catalyst (equivalent to 20 wt.% of the cyclicketones) was introduced into the mixture. The contents of the flask were homogenized through constant stirring. H2O2 was added drop wise for a period of 2 h through a slow feed pump. 145
Chapter –5
Solid Core Mesoporous Silica Shell
Reactants were taken in the molar ratio of Cyclicketone : Ammonia : H2O2 as 1 : 2 : 1, with t-butanol quantity equal to 10 times the number of moles of ketones. O O NH3, MSCMS, H2O2
HN
t-butanol, 80 0C
O
O HN NH3, MSCMS, H2O2 t-butanol, 80 0C
Scheme 5.2: Conversion of cyclohexanone and cyclododecanone to their corresponding lactams over MSCMS.
The reactions were carried out for a period of six hours. At regular intervals of time, the reaction mixture was collected in a glass container, catalyst was separated and contents were analyzed by a capillary gas chromatograph equipped with flame ionization detector to monitor the progress of the reaction. Results of catalytic activity of MSCMS materials giving % conversion and % selectivity are presented in Table 5.1. It can be noticed that TiSCMS is an efficient catalyst for the reaction of cyclohexanone with 92.3% conversion and 98.0% selectivity. This is comparable to that of well known TS-1 catalyst with Si/Ti ratio of 30. The catalytic activity of VSCMS on these conversion reactions has been found be lower than TiSCMS both in terms of conversion and selectivity (Table 5.1). This indicates that the nature of metal ion in the framework plays an important role in the catalytic process.
146
Chapter –5
Solid Core Mesoporous Silica Shell
In order to find the efficacy of TiSCMS as catalyst for oxidative conversion of bulkier cyclicketones, the reaction of cyclododecanone was carried out under same conditions as that of cyclohexanone and it was observed that 78.0% of cyclododecanone gets converted with 99.0% selectivity (Table 5.1). This conversion is very high when compared to that of TS-1 catalyst in the presence of which the conversion was found to be only 1%. It is evident that TiSCMS is an efficient catalyst for the conversion of cyclododecanone. This must be attributed to the larger pores present in TiSCMS which facilitate bulkier cyclododecanone molecules to penetrate and undergo the reaction. Relatively smaller pores in TS-1 restrict the passage of cyclododecanone molecules to the active sites and lead to poor conversion. To examine if the high catalytic activity of TiSCMS originated due to the leaching of Ti from the catalyst during this liquid phase reaction, controlled experiments were carried out in the case of cyclohexanone reaction. After 2 h of reaction, a part of the reaction mixture was removed with a syringe, the catalyst was separated and the contents were immediately transferred to another vessel maintained at the same temperature as that of the original reaction mixture. The composition of the reaction mixture was monitored periodically for another 4 h through GC. It was observed that, cyclohexanone concentration in the initial reaction mixture steadily decreased. On the other hand caprolactam concentration in the transferred solution did not increase in 2 - 6 h of reaction time. This indicates that the liquid phase conversion reaction over TiSCMS was purely due to catalytic action of Ti incorporated in solid core mesoporous silica shell.
147
Chapter –5
Solid Core Mesoporous Silica Shell
Table 5.1: Conversion of cyclohexanone and cyclododecanone over MSCMS materials Reaction conditions: Cyclicketone : NH3 : H2O2 = 1 : 2 : 1, Solvent = t-Butanol, Temperature : 80 ºC, Catalyst dosage : 20 wt.% of cyclicketone, Reaction time = 6 h S.No.
Catalyst
Reactant
Conversion (%)
Selectivity (%)
1.
TiSCMS
Cyclohexanone
92.3
98.0
2.
2nd cycle
Cyclohexanone
90.1
97.0
3.
3rd cycle
Cyclohexanone
90.0
97.0
4.
VSCMS
Cyclohexanone
75.0
81.0
5.
TS-1*
Cyclohexanone
97.0
95.0
6.
TiSCMS
Cyclododecanone
78.0
99.0
7.
2nd cycle
Cyclododecanone
75.0
98.0
8.
VSCMS
Cyclododecanone
62.0
75.0
9.
TS-1*
Cyclododecanone
1.0
99.0
*
TS-1 (Si/Ti = 30)
Reusability studies on TiSCMS catalyst in these conversion reactions reveal that there is no significant decrease in % conversion and selectivity, indicating that the activity of catalyst is retained (Table 5.1). This observation also leads to the conclusion that no leaching of titanium takes place during the reaction under the experimental conditions. Plausible mechanism involved in the conversion of cyclicketones to their lactams taking cyclohexanone as an example is shown in Scheme 5.3.
148
Chapter –5
Solid Core Mesoporous Silica Shell O
NH + NH3
- H2 O
(I)
(II)
Si Si O
O Si Ti O O Si
Si O
+ H2O2
Si O O Ti O O Si
Si
n n
(III)
Si
Si O
Si O O Ti O O Si
HO
NH Si
N Si O
+
+
O Si Ti O O Si
(IV) n
n
H H O
Si O Si Si O Ti O O Si
Ti n
Ti
N
O
N
N
- Ti OH
+
(III)
(III)
O
H O
H N
N
H O
-H
N H
O
N H
H (V)
Ti O
H
H O Ti H
+ H
Ti
+ H2O
Scheme 5.3: Plausible mechanism for the conversion of cyclohexanone to caprolactam over TiSCMS catalyst. 149
Chapter –5
Solid Core Mesoporous Silica Shell
Initially, cyclohexanone (I) reacts with ammonia to form cyclohexanimine (II). Thus formed cyclohexanimine gets oxidized in the presence of TiSCMS (III) and H2O2 to form cyclohexanone oxime (IV) through peroxy bond formation with the catalyst. Subsequently, cyclohexanone oxime undergoes Beckmann rearrangement in the presence of TiSCMS to form caprolactam (V). The advantage of using TiSCMS catalyst is that, it enables a two step reaction to proceed in a single step.
5.4.2 Catalytic oxidation of diphenylmethane to benzophenone In view of the catalytic potential of the MSCMS materials, oxidation of diphenylmethane to benzophenone, an industrially important reaction was studied in their presence (Scheme 5.4). This reaction is carried out in gas phase using a fixed bed down flow quartz reactor with CO2 free air as oxidant at atmospheric pressure in the temperature range 290 °C to
440 °C. Before commencing the reaction, the reactor was charged with 0.5 g of
the catalyst and preheated in a tubular furnace equipped with a thermocouple. Diphenylmethane was fed into the reactor through a syringe infusion pump at a rate of 2 mL h-1 for 2 h. Air flow rate was adjusted to maintain a molar ratio of DPM to air as 1:4. The reaction mixture was analyzed at regular intervals of time by HPLC. After every run, the catalyst was regenerated by passing a stream of dry air at a temperature of 550 °C for 6 h.
O MSCMS Air
Benzophenone
Diphenylmethane
Scheme 5.4: Oxidation of diphenylmethane to benzophenone in air over MSCMS
150
Chapter –5
Solid Core Mesoporous Silica Shell
The results of % conversion of DPM and selectivity of benzophenone at different temperatures over VSCMS catalyst are shown in Fig. 5.12. The conversion of DPM is found to increase gradually with the increase in temperature from 290 ºC to 440 °C. However, the selectivity of benzophenone decreased with the increase in temperature. At 320 °C highest selectivity towards benzophenone was observed. Therefore, 320 °C is taken as optimum temperature for studying the reaction. Decrease in selectivity of benzophenone at higher temperatures can be ascribed to the formation of byproducts.
Fig. 5.12: Effect of reaction temperature on the conversion and selectivity of oxidation of DPM to benzophenone in the presence of VSCMS. DPM : Air flow rate = 1 : 4, Feed rate = 2 mL h-1, VSCMS dosage = 0.5 g.
Effect of time on the oxidation reaction of DPM was studied over VSCMS at 320 °C keeping other reaction parameters identical. Results indicated that the conversion of DPM and selectivity of benzophenone are not influenced appreciably over a period of 12 h (Fig. 5.13). 151
Chapter –5
Solid Core Mesoporous Silica Shell
Fig. 5.15: Effect of reaction time on the conversion and selectivity of oxidation of DPM to benzophenone over VSCMS. DPM : Air flow rate = 1 : 4, Feed rate = 2 mL h-1, Reaction temperature = 320 ºC, VSCMS dosage = 0.5 g.
The oxidation of DPM to benzophenone was also carried out in the presence of TiSCMS. However, the % of conversion and selectivity were found to be much lower than in the case of VSCMS (Table 5.2). This observation once again indicates that, nature of the metal and its coordination in the framework have specific roles in the catalytic process. Studies on regenerated VSCMS catalyst revealed that conversion and selectivity decreased slightly over 3 cycles.
152
Chapter –5
Solid Core Mesoporous Silica Shell
Table 5.2: Oxidation of DPM to benzophenone over MSCMS Reaction conditions: DPM: Air flow rate = 1:4, Temperature = 323 ºC, Feed rate = 2 mL h-1. S.No.
Catalyst
Conversion (%)
Selectivity (%)
1.
VSCMS
53.5
82.0
2.
2nd cycle
48.0
78.2
3.
3rd cycle
50.0
76.5
4.
TiSCMS
32.3
48.0
Plausible mechanism for the oxidation of DPM to benzophenone in the presence of air and VSCMS catalyst is given in Scheme 5.5. O 2
O=O
V OSi
290 - 440 0C
+
2
OSi OSi
OSi
(I)
O
O
V
+
H
OSi OSi (III)
(II)
O
H O
H
O
H
O
V SiO
OSi OSi
OSi (VI)
H
O
.
+
V
+
O
H
OSi OSi
+
V OSi
(IV)
(V)
O
OSi OSi (II)
V OSi
OSi OSi (II) .
O
O
H
O
SiO
O +
V
+
(VII)
H O
V OSi
OSi OSi
(VIII)
+
H2O
OSi OSi (I)
(V)
Scheme 5.5: Plausible mechanism for the oxidation of DPM to benzophenone over VSCMS. 153
O
Chapter –5
Solid Core Mesoporous Silica Shell
Here, oxygen is chemisorbed on vanadium sites of VSCMS (I) catalyst to form peroxy radical (II). This peroxy radical abstracts hydrogen atom from the –CH2– group of DPM (III) to form diphenylmethyl radical (IV) and vanadium hydroperoxide (V). Now, framework-bound
vanadium
hydroperoxide
rapidly
transfers
its
.
OH
radical
to
diphenylmethyl radical to form diphenylmethanol (VI). Another vanadium peroxy radical abstracts hydrogen radical from diphenylmethanol to form diphenylmethanol radical (VII). Subsequently, diphenylmethanol radical rapidly transfers its hydrogen radical to form benzophenone (VIII) and vanadium hydroperoxide. The hydroxyl radical from V combines with the hydrogen radical from VII to give water and regenerate the catalyst. In conclusion, titanium and vanadium containing solid core mesoporous silica shell materials have been successfully synthesized by room temperature method using octadecyltrichloro silane as porogen. The catalytic conversion of cyclohexanone to caprolactam with high selectivity has been achieved in the presence of TiSCMS using ammonia and H2O2. The study revealed that bulkier cyclicketones like cyclododecanone can be efficiently converted to their lactams with good selectivity and conversion over TiSCMS. Studies on the oxidation of DPM to benzophenone with air revealed that VSCMS is a good catalyst for the reaction. Plausible mechanisms for the action of MSCMS catalysts for the conversion reactions have been proposed. More investigations are needed to develop this method of synthesizing the catalysts and also to explore their applications towards organic conversions.
154
Chapter –5
5.5
Solid Core Mesoporous Silica Shell
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Chapter -6 SUMMARY AND CONCLUSIONS
Chapter-6
6.1
Summary and Conclusions
Summary Catalysis plays a vital role in the production of fuels, fertilizers, pharmaceutical
intermediates and fine chemicals. Heterogeneous catalysts have the advantage of ease of recovery and recycling and are readily amenable to continuous processing. Catalysis by porous materials has emerged as one of the most active and promising fields of research owing to their industrial applications. The experience and understanding gained from these applications forms a sound foundation for their use in various catalytic processes such as degradation of dyes and organic conversions. The use of solid catalysts such as mesoporous materials in organic reactions has additional advantages in obtaining desired products with higher selectivity and conversion. Studies on the synthesis, characterization and applications of mesoporous materials have been the subject of intense research due to their wide ranging applications in adsorption, separation, energy storage, catalysis, sensors, drug delivery etc. New strategies and techniques are being developed for the synthesis and structure tailoring of mesoporous materials. Ordered mesoporous materials, based on MCM-41 are silicates obtained by hydrothermal synthesis and liquid templating mechanism. In 1992, the Mobil Oil Corporation introduced a novel type of silica-based molecular sieves designated as the M41S family. Main characteristics of this class of materials include long range order of mesopores formed by amorphous silica walls, high internal surface area of nearly 1000 m2 g-1, and pore diameters in the range of 3-5 nm. This class of materials was soon extended by the development of different strategies for their synthesis. Today, mesoporous silica materials with pore diameters in the range between 2-50 nm are accessible. Mesoporous materials are stable crystalline inorganic compounds characterized by the presence of three dimensional Si-O-Si linkages in the frame work. This structure leads 158
Chapter-6
Summary and Conclusions
to a regular network of uniform pores of specific molecular dimensions. The porous structure enables these materials to selectively admit some molecules while excluding those that are too large to fit into the pores. In the framework of MCM-41, silicon atoms are tetrahedrally coordinated with oxygen atoms in a continuous array. The framework charge deficiency, if any resulting from the tetrahedral coordination is balanced by hydrogen ions which are not an integral part of the framework. In order to manipulate and tune the silica surface suitable to specific applications, organic moieties that are covalently attached to the surface are employed. There are two different approaches to achieve such functionalization, (i) post-synthetic grafting of the surface and (ii) direct cocondensation during the synthesis. First chapter of the thesis deals with the introduction to catalysis, porous materials and literature review on mesoporous materials with focus on their synthesis, properties and catalytic applications. In view of the diversified applications of mesoporous materials, we have undertaken studies on the synthesis of functionalized mesoporous materials at room temperature by co-precipitation method and employed them as catalysts for the degradation of dyes, synthesis of organic compounds and organic conversions under solvent free or environmentally benign conditions. Accordingly, the work presented in this thesis deals with the synthesis, characterization and catalytic applications of redox and acid functionalized MCM-41 materials and redox functionalized solid core mesoporous silica shell materials. Chapter-2 embodies the experimental methods used in this work. General methods for the synthesis of functionalized MCM-41 and solid core mesoporous silica shell materials are also presented in this chapter. In this chapter, principles of characterization techniques employed are also briefly discussed.
159
Chapter-6
Summary and Conclusions
All the materials have been synthesized by co-precipitation method at room temperature. The materials were characterized by (i) Powder XRD for the determination of crystalline nature and phase identification, (ii) BET method to find the specific surface area and pore size, (iii) SEM-EDX for their morphology and elemental composition, (iv) FT-IR for the identification of functional groups in the framework of the material and (v) Diffused reflectance UV-Visible spectroscopy (UV-Visible DRS) for determining the co-ordination of metal ion in the framework. Some of the materials were characterized by XPS for surface analysis and TG-DTA for understanding the thermal behaviour of the materials. The catalytic degradation of dyes has been studied by measuring the decrease in absorbance of the reaction mixture with respect to time using UV-Visible spectrophotometer. Identification of products in the synthesis and organic conversions has been carried out by GC, GC-MS and TLC studies. All the products have been authenticated by their melting points. Chapter - 3 deals with the synthesis and characterization of metal incorporated MCM-41. Ti, V, Fe and Co incorporated MCM-41 (MMCM-41) materials have been synthesized by simple room temperature co-precipitation method by the addition of respective metal precursors to the reaction mixture. Synthesized materials have been characterized by Powder XRD, nitrogen adsorption-desorption, SEM-EDX, FT-IR and XPS studies to know their structural and morphological properties. The materials have been found to follow typical type-IV adsorption isotherm indicating the mesoporous character of the materials. With the incorporation of metal ion into the framework, surface area of the materials was found to decrease. As MCM-41 materials are formed by the Si-O linkage to one another, incorporation of metal into the framework to substitute Si atoms isomorphously leads to
160
Chapter-6
Summary and Conclusions
excess charge on the surface of the mesoporous materials. This charge is compensated by surface hydroxyl groups and results in imparting redox character to the materials. Application of MMCM-41 materials towards catalytic degradation of Crystal violet and Rhodamine B has been investigated. These dyes are extensively used in various industries such as textiles, paper and dyeing processes. Entry of these dye containing effluents into aquatic systems causes water pollution and harms aquatic organisms. The existing methods for degradation of dyes like Fenton’s process and other advanced oxidation processes have limitations due to their homogenous character. Hence, researchers are engaged in the development of new heterogeneous catalytic materials which can degrade the dyes easily under aerobic conditions at ambient temperatures. Mesoporous materials with their wide pore structure and large surface area are found to adsorb these dyes appreciably and hence, suitable methods for their degradation can be developed by utilizing these properties. Catalytic degradation of Crystal violet (CV) and Rhodamine B (RhB) on MMCM-41 materials in the presence of hydrogen peroxide has been investigated at room temperature. Effect of oxidant, pH and catalyst dosage on the degradation has been studied with a view to optimize the reaction conditions for the effective degradation of these dyes. Among the Ti, V, Co, Fe incorporated MCM-41 materials, TiMCM-41 is found to be the most effective catalyst for the degradation of Crystal violet whereas FeMCM-41 is found to be efficient catalyst for the degradation of Rhodamine B. The catalysts have been recycled and reused for 3 successive runs without appreciable loss of activity. Dye gets adsorbed on the surface of MMCM-41 catalysts and gets degraded in the presence of hydrogen peroxide. The optimum conditions for the degradation of crystal violet over MMCM-41 are found to be [CV] = 5.0 x 10-4 M, [H2O2] = 1.0x10-3 M, pH = 5.1 and amount of catalyst = 150 mg, whereas for
161
Chapter-6
Summary and Conclusions
Rhodamine B, the optimum conditions were found to be [RhB] = 5.0 x 10-4 M, [H2O2] = 5.8 x 10-3 M, pH = 8.0 and catalyst dosage of 100 mg. About 94 % of these dyes were found to degrade in 120 mins. Chapter-4 deals with the development of heterogeneous catalytic methods for the synthesis of imines, 1-amidoalkyl 2-naphthols and benzylidine barbiturates using acid functionalized MCM-41 materials. These compounds are important intermediates in pharmaceutical formulations. Development of one-pot multi component synthesis is one of the most studied areas of research in recent years on account of its environmental friendliness. This method has been found to give excellent yields, reduce reaction times without the need for isolation of any intermediate product during the process. The heterogeneous catalysts are ideal for these reactions because they are less expensive, highly reactive, eco-friendly, convenient to handle, simple to workup, recoverable and often lead to greater selectivity. It is possible to modify surfaces of MCM-41 materials by covalent anchoring of different organic moieties. Functionalization of MCM-41 with acidic groups enhances its surface acidic character due to the incorporation of these moieties onto the frame work. Due to the enhanced acidic character and high stability, these materials can be used as efficient and reusable catalysts for the synthesis of fine chemicals and pharmaceutically important intermediates. Moreover, they can be employed under solvent free conditions without loss of their acidic character. Acid functionalization of MCM-41 has been carried out by sulfonic acid (SO3H) and phosphotungstic acid (PW) by post grafting route. These materials are characterized by physico chemical techniques for their structural and morphological characteristics. The mesoporous character of the materials was found to be retained even after acidic
162
Chapter-6
Summary and Conclusions
functionalization. All the catalysts were found to follow type-IV adsorption isotherms and the surface area of the materials was found to be in the range of 600 – 1000 m2g-1. One-pot synthesis of imines at room temperature has been carried out using SO3HMCM-41and PWMCM-41 as catalysts in the absence of the solvent. SO3HMCM-41 has been found to catalyze the synthesis of imines more efficiently. 1-amidoalkyl 2-naphthols have been synthesized by one pot reaction of 2-naphthol, benzaldehyde and urea under solvent free conditions at 40°C over SO3HMCM-41 and PWMCM-41. SO3HMCM-41 has been found to catalyze the synthesis of titled compounds more efficiently. Benzylidine barbiturates were synthesized by reacting aromatic aldehydes and barbituric acid at room temperature in the presence of SO3HMCM-41 and PWMCM-41 using small quantity of methanol to dissolve the reactants. PWMCM-41 is found to catalyze the reactions more efficiently with good yields in shorter reaction period. Suitable mechanisms for all the above reactions have been proposed for the catalytic action of acid functionalized MCM-41 materials Chapter - 5 deals with the synthesis and characterization of Ti or V containing solid core mesoporous silica shell materials (MSCMS). A simple one-pot route has been adapted for the synthesis of these materials under room temperature condition, by using triethyl amine as a template and octadecyltrichloro silane as a porogen. Nitrogen adsorption-desorption studies of the materials showed type-IV adsorption isotherms, indicating the mesoporous character. The pore size and pore volume of the MSCMS materials were calculated by employing BJH method. It is observed that the pore size and pore volume of these materials are larger than those for MMCM-41 materials. MSCMS materials are found to have spherical morphology with distinct particle size of core and
163
Chapter-6
Summary and Conclusions
shell. The enhanced pore characteristics of MSCMS materials open new paths to selectively catalyse various organic reactions. The liquid phase conversion reactions of cyclohexanone and cyclododecanone to their respective lactams using ammonia and H2O2 at 80 ºC were carried out using MSCMS materials as catalysts. It is observed that TiSCMS shows greater conversion of these cyclicketones and higher selectivity towards their respective lactams. Oxidation of diphenylmethane to benzophenone in gas phase was carried out in the presence of MSCMS catalysts using air by varying the temperature in the range 290 °C to 440 °C at atmospheric pressure. VSCMS has been found to be active catalyst for the conversion of diphenylmethane to benzophenone with 84% selectivity towards benzophenone. Whereas the conversion and selectivity in the presence of TiSCMS has been found to be low. Plausible mechanisms for the conversion reactions in the presence of MSCMS catalysts have been proposed. All the synthesized catalysts have been examined for reusability. It is observed that catalysts remain active over 3 cycles.
6.2
Conclusions The following are the outcomes and conclusions of the studies presented in the thesis Ti, V, Fe and Co incorporation into MCM-41 has been achieved by in situ direct addition of metal precursors at room temperature by co-precipitation method. Acid functionalization of MCM-41 has been carried out by post-grafting method using sulphuric acid and phosphotungstic acid as sources to obtain SO3HMCM-41 and PWMCM-41 materials.
164
Chapter-6
Summary and Conclusions
Powder XRD analysis of all the synthesized materials confirmed that they are mesoporous in nature. The surface area of the metal incorporated and acid functionalized MCM-41 materials was found to be in the range of 700 m2 g-1. All the materials exhibited type-IV kind of adsorption isotherm, confirming the mesoporous character. The pore size of the materials was found to be in the nano range. The spherical morphology of the material has remained intact even after redox or acid functionalization. TiMCM-41 has been found to efficiently catalyze the degradation of crystal violet in the presence of hydrogen peroxide whereas FeMCM-41 efficiently catalyzed the degradation of Rhodamine B. About 94 % of these dyes have been degraded in 120 mins under ambient temperature conditions. SO3HMCM-41 efficiently catalyzed the synthesis of imines and 1-amidoalkyl 2-naphthols under solvent free conditions. Whereas, PWMCM-41 catalyst has been found to be more efficient for the synthesis of benzylidine barbiturates at room temperature. Ti or V containing solid core mesoporous silica shell materials (MSCMS) have been synthesized in one-pot at room temperature using octadecyltrichloro silane as the porogen. Characterization of MSCMS materials by various techniques confirmed that they are mesoporous in nature with large pore size and pore volume. These materials have been found to possess spherical morphology with distinct particle size of core and shell.
165
Chapter-6
Summary and Conclusions
Studies on the conversion of cyclicketones to their respective lactams by using ammonia and H2O2 in the presence of TiSCMS revealed that, it is an efficient catalyst for the conversions with 98 % selectivity towards lactams. Studies on oxidation of diphenylmethane to benzophenone using air revealed that, VSCMS is a good catalyst with 84% selectivity towards benzophenone at 320 °C. All the catalysts were found to be reusable for 3 cycles without significant loss of their activity.
166
List of Publications & Presentations in Conferences
LIST OF PUBLICATIONS 1. Catalytic degradation of Rhodamine B over FeMCM-41, M. Nookaraju, A. Rajini, I. Ajit Kumar Reddy and N. Venkatathri, Asian Journal of Chemistry, 24(12), 2012, 5817-5820. 2. Synthesis, characterization and catalytic application of acid functionalized mesoporous silica,
M. Nookaraju,
A. Rajini,
I. Ajit Kumar Reddy and
N. Venkatathri, Journal of Applicable Chemistry, 2(2), 2013, 122-128. 3. Titanium containing solid core mesoporous silica shell
: A novel efficient
catalyst for ammoxidation reactions, N. Venkatathri, M. Nookaraju, A. Rajini, I. A. K. Reddy, Bulletin of Korean Chemical Society, 34(1), 2013, 143-148. 4. Structural and catalytic properties of a novel vanadium containing solid core mesoporous
silica
shell
catalysts
for
gas
phase
oxidation
reaction,
N. Venkatathri, K. Vijayamohanan Pillai, A. Rajini, M. Nookaraju and I. A. K. Reddy, Journal of Chemical Sciences, 125(1), 2013, 63-69. 5. Degradation of crystal violet by hydrogen peroxide in the presence of Ti, V, Fe and Co incorporated MCM-41 materials, M. Nookaraju, I. Ajit Kumar Reddy, A. Rajini and N. Venkatathri (Manuscript under preparation). 6. One-pot solvent free synthesis of 1-amidoalkyl 2-naphthols over acid functionalized MCM-41 catalysts, M. Nookaraju, I. Ajit Kumar Reddy, A. Rajini and N. Venkatathri (Manuscript under preparation).
PAPERS PRESENTED IN INTERNATIONAL CONFERENCES 1. “Synthesis, characterization and catalytic application of acid functionalized mesoporous silica”, 2nd International Conference, Organized by Indian Council of Chemists, Kaula Lumpur, Malaysia, 6 - 8 June, 2012. 2. “Catalytic degradation of Rhodamine B using FeMCM-41”, International Conference on Global Trends in Chemical Sciences (ICGTCS-2012) organized by Asian Journal of Chemistry at Inder Residency, Udaipur, Rajasthan, 3-4 March, 2012. 3. “Synthesis, characterization and catalytic application of Ti incorporated nano structure material”, International Conference on Supramoelcualr and Nano Chemistry (ICSN – 2011), organized by University of Bombay, Mumbai, 13-15 February, 2011.
PAPERS PRESENTED IN NATIONAL CONFERENCES 1. “One pot synthesis of Amidoalkyl Naphthols over acid functionalized mesoporous silica”, 21st National Symposium on Catalysis (21NSC), organized by Catalysis Society of India at CSIR-IICT Hyderabad, 11-13 February, 2013. 2. “Phosphotungstic acid functionalized MCM-41: An Efficient catalyst for Knovenagel reaction” 31st National Conference of Indian Council of Chemists (XXXI ICC-2012), organized by Indian
Council of Chemists at Saurasthra
University, Gujarat, 26-28 December, 2012. 3. “Synthesis, characterization and catalytic applications of Ti containing solid core mesoporous silica”, National Conference on Chemistry for Sustainable Development
(SusCon-2012),
11-12 October, 2012.
GITAM
University,
Visakhapatnam,
CONFERENCES OR WORKSHOPS ATTENDED 1. Five Day National Workshop on “Modern Instrumental Methods of Inorganic Chemical Analysis of Engineering Materials”, NIT Warangal, Warangal, A.P., 22-24 October 2013. 2. Three Day National Workshop on “Innovations in Materials and Processes and Transfer of Technology to Industries”, NIT Warangal, Warangal, A.P., 17-19 October 2013. 3. 3rd National Training cum Workshop on “Preparing the best PhD thesis of an International Quality”, CSIR- CLRI, Chennai, Tamilnadu, 13-15 February, 2012. 4. National Conference on “Interface between Chemical Sciences and Technologies” (ICST-NITW), Department of Chemistry, National Institute of Technology Warangal, Warangal, 29-30 December, 2011. 5. Science Academies Lecture Workshop on “Current Trends in Nanoscience and Technology” Department of Chemistry, National Institute of Technology Warangal, Warangal, A.P, 23-24 December, 2011. 6. 15th National Workshop on Catalysis (NWC-15), National Centre for Catalysis Research, IIT Madras, Chennai, 11-13 December, 2011. 7. International Conference on “Frontiers in Naoscience and Technology (Cochin Nano-2011)”, Cochin, Kerala, 14-16 August, 2011. 8. 20th National symposium on “Catalysis for energy conversion and conservation of environment”, (20NSC10), National Centre for Catalysis Research, IIT Madras, Chennai, 19-22 December, 2010 9. International Conference on Recent Trends in Nano and Bio-Sciences (ICORTNBS-2010), Department of Physics, University College of Science, Osmania University, Hyderabad, 26-28 February, 2010
10. “10th Orientation Programme in Catalysis Research”, National Centre for Catalysis Research, IIT Madras, Chennai, 28 November –17 December, 2009. 11. National Symposium on “Recent trends in Organic and Medicinal Chemistry” NIT Warangal, Warangal, A.P., 16-17 January, 2009.
