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2012 IEEE Colloquium on Humanities, Science & Engineering Research (CHUSER 2012), December 3-4, 2012, Kota Kinabalu, Sabah, Malaysia

Synthesis and Characterization of Thiourea Complex Encapsulated Into Functionalized-MCM41 and Their Catalytic Activities for Oxidation Reaction Amirah Ahmad, Karimah Kassim, Hamizah Md Rasid Faculty of Applied Sciences Universiti Teknologi Mara 40000 Shah Alam, Selangor [email protected]

Sakthivel et al. (2003) reported Cr-MCM41 is catalytically active in allylic oxidation of cyclohexene with tertbutylhydroperoxide by giving high percentage of conversion and product selectivity [3].Thiourea are an important of classes ligand in coordination chemistry and have studied intensively in antimicrobial, antibacterial and adsorption. Thiourea ligands can be act as potential donor ligand for transition metal ion by the presence of nitrogen and sulphur [4]. Furthermore, incorporation of metal is important to generate active sites in catalyst. This is because the amorphous MCM-41 has a few catalytically actives sites due to presence of Si4+. Thus by introduction of metal ion other than Si4+ such as Mn, Cu, Cr, Ni, Zn, V, Sc, Ti, Fe, and Co can be generated a superior catalyst [5].

Abstract—Thiourea complex encapsulated into functionalizedMCM41 was carried out in four stages. First stage is synthesized the MCM41 framework by using Ludox as silica source and Cetyltrimethylammonium bromide as surfactant. Second stage is modification of surface MCM-41 with organic group which is 3aminoprophyltriethoxysilane. Next stage is encapsulation of 2thiouracil into functionalized-MCM41 and then was incorporated with copper acetate monohydrate to produce thiourea complex. The Cu(OAC)2-Thio-APS-MCM41 catalyst was characterized by X-ray diffraction (XRD), nitrogen desorption-adsorption isotherm, single-point BET, Fourier Transform infrared spectroscopy (FTIR) and field emission scanning electron microscopy (FESEM). The catalytic potential of Cu(OAC)2-ThioAPS-MCM41 catalyst was investigated in oxidation of cyclohexene with tert-butylhydroperoxide. The oxidation reaction was monitored by gas chromatography. It was observed that Cu(OAC)2-Thio-APS-MCM41 catalyst displayed good properties of catalyst for oxidation reaction by giving high percentage conversion of cyclohexene and selectivity of 2cyclohexen-1one. The catalytic activity was compared with the reaction without catalyst.

This paper aims to report the synthesis and characterization of thiourea complex encapsulated into functionalized-MCM41, as shown in Figure 1. This catalyst was tested for oxidation of cyclohexene with tert-butylhydroperoxide as oxidant in acetonitirile solvent.

Keywords-Thiourea complex, MCM41, Oxidation reaction

I.

II.

INTRODUCTION

A. Synthesis of MCM-41 The synthesis of MCM41 was carried out according to the molar composition below:

Porous material is a solid containing pores and widely used in catalysis, separation, and adsorption. It can be classified into 3 types based on their pore size. Among 3 types, mesoporous material have pore diameter in between 2 nm to 50 nm. MCM41 mesoporous materials are most attention to be used in applications due to high surface area, large pore volumes, stable in acidic medium and high thermal and hydrothermal stability [1]. However, previous researchers were reported that MCM-41 has hydrophilic properties due to high surface silanols in pores and framework of MCM-41. Hence, the structure of MCM-41 will be enhanced by immobilization with organic group or metal compelexes. This modification will increase the surface hydrophobicity and avoid leaching occurs [2].

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EXPERIMENTAL

6 SiO2 : CTABr : 1.5 NaOH : 0.15 (NH4)OH : 250 H2O Ludox was mixed with sodium hydroxide in doubly distilled water, as sodium silicate solution. The mixture solution was stirred for 2 hours. Cetyltrimethylammonium bromide was added in solution contained double distillated water and ammonium hydroxide aqueous solution, as template solution. The mixture solution was heated and stirred for 1 hour. Then, the sodium silicate solution was added into template solution. The solution was stirred vigorously for 15 minutes and aged overnight. The precipitate was filtered, washed and dried in oven. The materials were calcined at 550 °C in furnace.

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B. Modification of Surface MCM-41 2.0 gram of MCM-41 and 2.2 mL of 3aminopropyltriethoxysilane was added in n-Hexane solvent. The mixture was refluxed for 6 hours. Then, a solution was filtered, washed, dried at room temperature.

D. Incorporation of Metal into Functionalized-MCM41 Containing Thiourea Ligand For incorporation of metal, 2.5 gram of copper acetate was added to a solution of Thio-APS-MCM41 (2.5 gram) in nHexane solvent. The mixture was refluxed for 24 hours and then was filtered, washed and dried under vacuum overnight. E. Characterization of Catalyst The crystallinity of catalyst were determined using Rigaku D/max-2500 powder diffractometer with Cu-Kα source (λ = 1.5418 Å). IR spectra were obtained by using Perkin-Elmer Spectrum One FTIR Spectrometer. FTIR spectra were measured by using potassium bromide (KBr) pellet technique. The isotherm and surface area were obtained using Micromeritics ASAP 2010 Volumetric Adsorption Analyzer. The particle size and morphology were obtained using Zeiss Supra 35VP Field Emission Scanning Electron Microscopy. F. Oxidation of cyclohexene In round bottom flask equipped with magnetic stirrer, 3 mL of acetonitrile, 0.5mmol of cyclohexene, 1 mmol of tertbutylhydroperoxide and 0.01 gram of catalyst were added and heated at 70 °C. The reaction progress was monitored by gas chromatography.

III.

RESULTS AND DISCUSSION

A. X-ray Diffraction The three peaks at miller indices (100), (110) and (200) corresponding to the characteristics of hexagonal ordered MCM-41 structure [6]. Figure 2 (A) show the intensity for MCM-41 is high compared to Cu(OAC)2-Thio-APS-MCM41 catalyst. The intensity decreases due to structural rearrangement after encapsulation with thiourea complex. The lower structural ordering due to high of organic content inside the pores and may result in a partial collapse of mesostructure. However, the ordered mesoporous structure of catalyst was retained and can be used as catalyst. The lattice parameter also decreases after addition of thiourea complex. This suggested the contraction of the hexagonal unit cell. For high-angle XRD at 2θ in the range 20° to 100°, two peaks was observed at 2θ = 35.3° and 38.6° corresponding to monoclinic CuO, as shown in Figure 3 (b) [7]. Figure 1. Proposed reaction of Cu(OAC)2-Thio-APS-MCM41 catalyst

C. Encapsulation of thiourea ligand into FunctionalizedMCM41 The thiourea ligand was prepared by added 2.5 gram of 2thiouracil and 2.5 gram of functionalized-MCM41 in n-Hexane solvent. The mixture was refluxed for 24 hours. Then, the solution was filtered, washed and dried in vacuum oven overnight.

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monolayer adsorption of N2 on the surface MCM-41. The surface area of MCM-41 is high causes monolayer adsorption requires large amount of N2. Second region at relative pressure in between 0.2 to 0.4, indicates to capillary condensation of N2 inside the mesopores. Third region at relative pressure more than 0.4, indicates to multilayer adsorption of N2 at the external surface MCM-41. Fourth region at relative pressure more than 0.9, indicates to condensation of N2 within the interstitial void between MCM-41 and with high heteroatom content [8]. Figure 4 (B) shows the isotherm after encapsulation with thiourea complex. The result reveals the nitrogen uptake decrease due to addition of bulky group inside the pores causes collapse the structure of MCM-41. C. Single-point BET and Pore Size Distribution The result shows the surface area decrease after encapsulation with organic group and thiourea complex, as shown in Table 1. This suggested the pores were partial filled with organic groups and thiourea complex. Besides, may be the introduction of thiourea complex randomly and nondirectionally causes blocked the pores and collapsed some mesoporous framework. Figure 5 revealed the graph of pore size distribution. The narrow peak indicates prepared catalyst is in the mesopores range. After encapsulated with thiourea compelx, the broader peak appeared due to aggregation of particles as the content heteroatom increased [9]. This result was supported by FESEM micrograph that shows some structural rearrangement after encapsulate with thiourea complex.

Figure 2. Low-angle XRD: (A) MCM-41 and (B) Cu(OAC)2-Thio-APSMCM41

.

Figure 3.High-angle XRD: (a) MCM-41 and (b) Cu(OAC)2-Thio-APSMCM41

B. Nitrogen Adsorption-Desorption Isotherm The type IV of isotherm graph shows the typical mesoporous material, as shown in Figure 4 (A). It consist four regions: First region at relative pressure less than 0.2, indicates

Figure 4.Isotherm graph: (A) MCM-41 and (B) Cu(OAC)2-Thio-APSMCM41

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TABLE I.

BET SURFACE AREA

Samples

BET surface area (m2/g)

MCM-41

982.56

Functionalized-MCM41

286.91

Functionalized-MCM41 containing thiourea ligand

14.44

Cu(OAC)2-Thio-APS-MCM41

10.01

Figure 6. FTIR spectra: (a) Uncalcined-MCM41, (b) Calcined-MCM41 and (c) Cu(OAC)2-Thio-APS-MCM41

Figure 5. Pore size distribution: (A) MCM-41 and (B) Cu(OAC)2-ThioAPS-MCM41

D. FTIR FTIR spectra for uncalcined-MCM41 show the peak of surfactant around 2900 cm-1 to 2800 cm-1 as shown in Figure 6 (a). After calcination, the peak of surfactant was disappeared due to removal of surfactant. This can be proved that the surfactant was successfully removed from the framework [10]. The major peaks of MCM-41 structure was observed after calcination process such as Si-O-Si stretching in the range between 1200 cm-1 to 1000 cm-1, Si-O-Si bending at 460 cm-1 and Si-OH stretching vibration at 970 cm-1, as shown in Figure 6 (b) [11]. Figure 6 (c) shows the spectra of functionalizedMCM41. The peaks of C-N bond at 1639 cm-1 and -NH2 symmetric bending at 1553 cm-1 was observed after modification of surface MCM-41 with 3aminopropyltriethoxysilane. Besides, the peak at 2944 cm-1 corresponding to CH2 groups of the propyl chain of the sylilating agent. Figure 7 (a) shows the ftir spectra of starting material, 2-thiouracil. Most of the band of thiouracil was retained even after incorporation of metal (Figure 7 (b) and (c)). The peaks for C-N bond at 1706 cm-1 and C=S to N at 1220 cm-1 also appeared for functionalized-MCM41 containing thiourea ligand [12].

Figure 7. FTIR spectra: (a) 2-thiouracil, (b) Functionalized-MCM41 containing thiourea ligand and (c) Cu(OAC)2-Thio-APS-MCM41

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TABLE II.

OXIDATION OF CYCLOHEXENE WITH TERTBUTYLHYDROPEROXIDE AT 343 K FOR 6 HOURS

E. FESEM Figure 8 and 9 were displayed the FESEM micrograph for MCM-41 and Cu(OAC)2-Thio-APS-MCM41 catalyst, respectively. The micrograph shows that the prepared catalyst has agglomerated particles in between 1 to 3 µm. After encapsulation of thiourea complex, the structure of MCM-41 tends to agglomerate and coagulation causes the particles become larger.

Time

Conversion (%)

Product Selectivity (%)

6

89.6

78.1

G. Effect of Reaction Time The effects of reaction time were carried out for 2, 4, 8, 12, 24 and 26 hours, as shown in Figure 10 and Table 3. The results show that the percentage conversion of cyclohexene and product selectivity of 2-cyclohexen-1-one was increased almost linearly as the reaction time increased. The percentage conversion of cyclohexene was achieved 100 % after 26 hours. However, the percentage product selectivity of 2-cyclohexen1-one was decreased after 24 hours reaction due to the reactant and product further undergoes side reaction to produce side product.

Figure 8. FESEM micrograph for MCM41

Figure 10. Effect of reaction time for oxidation of cyclohexene catalyzed by Cu(OAC)2-Thio-APS-MCM41.

TABLE III

EFFECT OF REACTION TIME IN OXIDATION OF CYCLOHEXENE WITH TERT-BUTYLHYDROPEROXIDE AT 343 K

Figure 9. FESEM micrograph for Cu(OAC)2-Thio-APS-MCM41

F. Catalytic studies The performance of Cu(OAC)2-Thio-APS-MCM41 catalyst was tested in oxidation of cyclohexene with tertbutylhydroperoxide. The percentage of conversion and product selectivity to 2-cyclohexen-1-one for 6 hours was shown in Table 2. The others peak obtained in GC chromatogram was assumed as side product.

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Time (hour)

Conversion (%)

Product Selectivity (%)

2

66.6

73.3

4

81.5

75.6

8

93.3

78.2

12

96.8

80.7

24

98.2

82.8

26

100.0

75.7


H. Effect of Catalyst The effects of catalyst were observed by comparing the percentage conversion of cyclohexene and product selectivity of 2-cyclohexen-1-one for catalyzed and uncatalyzed reaction, as shown in Figure 11. The reactions were carried out at 343 K for 24 hours. The results clearly show that the catalyzed reaction gave higher percentage conversion and product selectivity than uncatalyzed reaction. It proved that that catalyst was increased the rate of reaction without affect the reaction.

ACKNOWLEDGMENT Special thanks to the Ministry of Science, Technology and Innovation Malaysia (MOSTI) for the research grant no: 600RMI/ST/FRGS5/3/Fst (41/2010), Faculty of Applied Sciences and Faculty of Pharmacy, Universiti Teknologi MARA and Ibnu Sina Institute, Universiti Teknologi Malaysia for facilities in completing this research. REFERENCES Hadi, N., “Heterogeneous chemocatalysis: Catalysis by chemical designA personal experience,” Ibnu sina institute for fundamental science study, Universiti Teknologi Malaysia, pp. 1-2, 2008. [2] Xianbin, L., Du, Y. Guo, Z., Gunasekaran, S., Ching, C. B., Chen, Y., Leong, S. J., and Yang, Y., “ Monodispersed MCM-41 Large Particles by Modified Pseudomorphic Transformation : Direct Diamine Functionalization and Application in Protein Bioseperation,” Microporous and Mesoporous Materials, 122, pp. 114-120, 2009. [3] Sakthivel, A., Dapurkar, S.E. and Selvam, P., “Allylic oxidation of cyclohexene over chromium containing mesoporous molecular sieves,” Applied Catalysis A: General, 246, pp. 283-293, 2003. [4] Griffin, TS., Woods, TS. and Klayman DL., In: Katritzky AR., Boulton AJ., (eds). Advances in heterocyclic chemistry. Vol. 18, Academic, New York,1975, pp. 99. [5] Selvakumar, S., Ravikumar, G.P, Joseph, K., Rajarajan, J., Madhavan, S.A., Rajasekar, P. and Segayaraj., (2007). Mater. Chem. Phys. 103, 153-157. [6] Occelli, M.L., Biz, S., and Auroux, A., “Effects of Isomorphous Substitution of Si with Ti nd Zr in Mesoporous Silicates with the MCM41 Structure,” Applied Catalysis A: General, 183, pp. 231-239, 1999. [7] Parida, K.M., Rath, D. and Dash, S.S. (2010). Synthesis, characterization and catalytic activity of copper incorporated and immobilized mesoporous MCM41 in the single step amination of benzene. J. Molec. Catal. A: Chem., 318, 85-93. [8] Chiola, V., Ritsko, J.E., and Vanderpool, C.D., (1971). Process For Producing Low Bulk Density Silica. U.S. Patent, Vol 3, pp. 556-725, 1971. [9] Bakar, A. F., “Synthesis of Al-MCM-41/ZSM-5 composite for oxidation of norbonene to norbonene oxide,” Master Thesis. Universiti Teknologi Malaysia, 2010. [10] Flanigen, E.M., Khatami, H., and Szymanski, H.A., “ Infrared Structural Studies of Zeolite Frameworks,” Journal of Advance in Chemistry Series, 101, pp. 290-227, 1971 [11] Hadi, N., Rahman, N.A., Endud, S., and Lim, K.W., “Thermal stability of conductivity of composite comprising polyaniline and MCM41,” Malaysian Polymer Journal (MPJ), Vol. 2, No. 2, pp. 12-21, 2007. [12] Vassileva, P., Tzvetkova, p., Lakov, L. and Peshev, O., “Thiouracil modified activated carbon as a sorbent for some precious heavy metal ions,” Journal of Porous Material, 15,pp. 593-599, 2008. [1]

Figure 11. Effect of catalyst in oxidation of cyclohexene with tertbutylhydroperoxide as oxidant

CONCLUSIONS Thiourea complex encapsulated into functionalizedMCM41 was successfully synthesized as confirmed by the characterization via XRD, nitrogen adsorption-desorption isotherm, FTIR spectra, single-pint BET and pore size distribution and FESEM micrograph. The Cu(OAC)2-ThioAPS-MCM41 catalyst showed a good catalyst for oxidation of cyclohexene with tert-butylhydroperoxide by giving high percentage conversion and product selectivity after 24 hours reaction.

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