Catalysis: Molybdenum-Based Hybrid Nanocatalysts

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trimethoxysilane to MCM-41, subsequent reaction with pypr Schiff base, and final complexation with MoO2(acac)2,. (acac=acetylacetonato ligand), as shown in ...
Catalysis: Molybdenum-Based Hybrid Nanocatalysts Majid Masteri-Farahani and Narjes Tayyebi CONTENTS Introduction and Definitions����������������������������������������������������������������������������������������������������������������������������������������������������� 100 Hybrid Materials and Their Classifications�������������������������������������������������������������������������������������������������������������������������� 100 Hybrid Catalysts������������������������������������������������������������������������������������������������������������������������������������������������������������������� 100 Molybdenum-Based Hybrid Nanocatalysts������������������������������������������������������������������������������������������������������������������������������ 100 MCM-41 as Solid Support��������������������������������������������������������������������������������������������������������������������������������������������������� 101 MCM-41-Supported Molybdenum–Schiff Base Complexes������������������������������������������������������������������������������������������ 101 Characterization of MCM-41-Supported Molybdenum Complexes with Schiff Base Ligands������������������������������������� 103 Magnetite Nanoparticles as Support������������������������������������������������������������������������������������������������������������������������������������ 104 SCMNP-Supported Molybdenum–Schiff Base Complexes�������������������������������������������������������������������������������������������� 105 Multiwalled Carbon Nanotubes as Support������������������������������������������������������������������������������������������������������������������������� 108 Multiwalled Carbon Nanotube–Supported Molybdenum Complexes���������������������������������������������������������������������������� 109 Summary����������������������������������������������������������������������������������������������������������������������������������������������������������������������������������� 112 References����������������������������������������������������������������������������������������������������������������������������������������������������������������������������������112

INTRODUCTION AND DEFINITIONS Hybrid Materials and Their Classifications Hybrid materials are defined as molecules or composites that at least one of the component domains has a dimension ranging from a few angstroms to several nanometers. These materials exhibit interesting properties, which might be a mixture of their initial components or even are completely different from them. For instance, hybrid solar cells combine organic and inorganic materials with the aim of utilizing the low-cost cell production of organic photovoltaics as well as obtaining other advantages, such as tunable absorption spectra, from the inorganic component.1–3 Hybrid materials are classified into two major classes according to the nature of the interaction between their components: Class I: Here, the components are linked together through weak bonds like van der Waals, ionic, or hydrogen bonds. The adsorption of organic dyes on TiO2 surface4 and embedment of aluminum oxide particles on polyvinylidene fluoride (PVDF)5 are examples of this class. Class II: In this class, the components are grafted together through strong covalent or iono-covalent chemical bonds, so a better stability than class I materials is expected. Pd (II)‐Schiff base complexes heterogenized on MCM‐41 are examples of this group.6

Hybrid Catalysts Today, catalytic-based reactions are of great interest, while over 60% of refining and petrochemical production and 90% of 100 © 2016 by Taylor & Francis Group, LLC

current processes rely upon them.7 There are homogeneous and heterogeneous catalysts that each one has its own advantages and disadvantages.8 Therefore, a new generation of hybrid catalysts was designed to combine the advantages of homogeneous and heterogeneous catalysts and bridge the gap between them. These heterogenized homogeneous catalysts can be synthesized by a number of methods, for instance, by grafting of soluble transition metal complexes to inorganic solid supports.9–11

MOLYBDENUM-BASED HYBRID NANOCATALYSTS Despite the great promise offered by homogeneous molybdenum-based catalysts over the last decades,12,13 a few of them have been commercialized. Mo-based hybrid materials have received extensive attention due to their wide applications in electrical conductivity, magnetism, medicine, and catalysis.14,15 Especially, epoxidation of olefins in the presence of high valent molybdenum compounds has attracted considerable academic and industrial interests. The immobilization or heterogenization of homogeneous catalysts on solid supports can be accomplished by various methods.10 Among them, covalent attachment of catalytically active parts to solid supports has been the most frequently used strategy due to good stabilities of the produced heterogeneous catalysts during the course of catalytic reactions. The stable covalent bonds experience no leaching from the support and thus provide an efficient approach to prepare site-isolated catalysts. This chapter focuses on molybdenum hybrid nanocatalysts synthesized by heterogenization of Mo complexes to different

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Catalysis: Molybdenum-Based Hybrid Nanocatalysts

nanoscale solid supports, including MCM-41, silica-coated magnetite nanoparticles (SCMNPs), and multiwall carbon nanotubes (MWCNTs).

have been reported.23–30 Preparation of such catalysts includes the covalent grafting of a spacer with special functional groups to MCM-41. These functional groups can undergo either a substitution reaction with presynthesized Schiff bases23–25 or in situ Schiff base formation.26–30 MCM-41-Supported Molybdenum Complexes with Presynthesized Schiff Base Ligands catalyst (pypr=[N,N′-bis(2MoO2pypr@MCM-41 pyrrolmethylidenaminopropyl)amine] Schiff base) (Scheme 2) has been prepared by covalent grafting of 3-chloropropyl trimethoxysilane to MCM-41, subsequent reaction with pypr Schiff base, and final complexation with MoO2(acac)2, (acac=acetylacetonato ligand), as shown in Scheme 1.23 The general procedure used for the preparation of two similar hybrid catalysts, that is, MoO2salpr@MCM-41 (a) [24] and MoO2acacdien@MCM-41 (b),25 is similar to that of MoO2pypr@MCM-41 catalyst (Scheme 2).

The Mobil composition of matter (MCM) family of mesoporous materials has channels with 1.5–10 nm diameter ordered in a hexagonal (MCM-41), cubic (MCM-48), and laminar (MCM-50) array.16 Since the discovery of MCM-41 by Mobil group in 1992,17 it has been extensively used as support in heterogenization of homogeneous catalysts due to its regular pore size, large surface areas, large number of surface silanol groups, and high chemical and thermal stability.18–30 In the following section, we present the syntheses of hybrid nanocatalysts with covalent attachment of molybdenum–Schiff base complexes to MCM-41. MCM-41-Supported Molybdenum–Schiff Base Complexes A major drawback encountered in the immobilization of molybdenum catalysts within different supports is the instability of the molybdenum species in the structure of the prepared catalysts due to leaching during reaction course. To overcome this problem, new hybrid catalysts of molybdenum using different Schiff base ligands supported on MCM-41

O O O O O O

Si

OH

MeO MeO Si MeO

Cl

Toluene/3 h reflux Si

OH

MCM-41-Supported Molybdenum Complexes with In Situ–Formed Schiff Base Ligands Various MCM-41-supported molybdenum complexes with in  situ–formed Schiff base ligands have been prepared as depicted in Scheme 3.26 First, the mesoporous molecular sieve MCM-41 was covalently grafted with 3-aminopropyl trimethoxysilane to give aminopropyl-modified MCM-41

pypr

MoO2 (acac)2

Benzene/Et3N 12 h reflux

EtOH/24 h reflux

O O O O O O

N

N

N Si

OH

O Mo

N

Si

O

N

SCHEME 1  Schematic illustration of MoO2pypr@ MCM-41 catalyst preparation. (From Masteri-Farahani, M. et al., Catal. Commun., 8(1), 6, 2007.)

N

(a)

O O O O O O

N

N Si

O

Mo

N

Si

O

O

O

O

O O MCM-41

MCM-41

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MCM-41 as Solid Support

OH

Mo

N

Si

O

O N

O O

O

Si

O

OH

O (b)

SCHEME 2  Structure of (a) MoO2salpr@MCM-41 (From Masteri-Farahani, M. et  al., J. Mol. Catal. A: Chem., 243, 170, 2006.) and (b) MoO2acacdien@MCM-41 catalysts. (From Masteri-Farahani, M. et al., J. Nanostruct., 1, 14, 2012.)

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Catalysis: Molybdenum-Based Hybrid Nanocatalysts

O O Si

MeO MeO MeO

OH OH OH

Si

NH2

MCM-41

MCM-41

OH OH OH

Toluene/24 h reflux

OH OH OH

O

MoO2 (acac)2

O

O

O

O O

(a)

Mo O

N Mo O

Absolute ethanol 24 h reflux

O

OMe O O Si OH O

HO

N

OMe

HO

O

(I) O O Si

O

N

Mo

N

O O Si

O

N

N

O

OMe

O O Si OH O O O Si OMe

O O

O O

O

(b)

O O Si

N Mo O

NH MCM-41

O

MCM-41

Mo

N O

O

NH2

H

MCM-41

O

O O Si

OH

O O Si

MCM-41

MCM-41

O

OMe

OMe

OH

N

O O Si

O

N Mo O

NH

OMe

O O Si OH O O O Si OMe

O

(c)

N O O

O

Mo O O O N

Mo O O O O

+

OMe

O

O O

(d)

O

Mo O

O

N

O

Mo O

O

O OMe O Si OH O O O Si OMe

O O

O O

(e)

N

N

Mo O

O

N

N

Mo O

O O Si MCM-41

O OMe O Si OH O O O Si

O

O O Si

acacH–

N

MCM-41

O O Si MCM-41

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O O Si

OMe O O Si OH O O O Si

O O Si MeO O O Si

OMe

Absolute ethanol 24 h reflux

OMe O O Si OH O

NH2

OMe

N Mo O O O O

OMe

O

OMe

O

O O Si OH O O O Si

O

O

(f)

N Mo O O O

(II)

SCHEME 3  (I) Typical preparation and (II) structures of the molybdenum hybrid nanocatalysts: (a) MoO2pycaAmpMCM-41, (b) MoO2amacAmpMCM-41, (c) MoO2salAmpMCM-41, (d) MoO2furAmpMCM-41, (e) MoO2acpyAmpMCM-41, and (f) MoO2acacAmpMCM-41. (From Masteri-Farahani, M. et al., J. Mol. Catal. A: Chem., 248(1), 53, 2006.)

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(AmpMCM-41). Reaction of this material with furfural, pyrrolecarboxaldehyde, 2-acetylpyrrole, 2-aminoacetophenone, salicylaldehyde, and acetylacetone afforded the corresponding supported Schiff base ligands. Subsequent reaction with MoO2(acac)2 leads to desired hybrid nanocatalysts. Other examples of molybdenum catalysts supported on functionalized MCM-41 containing N–N, N–P, and N–S chelating Schiff base ligands are shown in Scheme 4.

diffraction (XRD), scanning electron microscopy (SEM), and BET nitrogen adsorption–desorption analyses. For instance, FT-IR spectrum of MoO2pypr@MCM-41 (Figure 48) exhibits two points: (1) C=N band shift from 1632 cm−1 in free supported ligand to 1620 cm−1 in final catalyst that indicates the involvement of C=N group in complexation with molybdenum and (2) appearance of two adjacent bands at 901 and 945 cm−1 that confirms the presence of cisMoO2 group in the prepared catalyst.23 The effect of incorporation of molybdenum complex on the texture properties of the support has been investigated with XRD and nitrogen adsorption–desorption (BET) analyses. The XRD pattern of the final MoO2pypr@MCM-41 catalyst only shows the 100 reflection with lower intensity and the other reflections have been disappeared partly due to a

O O Si

Catalyst N

O O Si O

OH

O O Mo O O

R

–OOC

O O Si OMe

N

N

O

Mo

O

O O –

(a) O O Si

–CH3

MoO2 phenalaAmpMCM-41

–C6H5CH2

MoO2 leuAmpMCM-41

–CH2CH(CH3)2

MoO2 isoleuAmpMCM-41

–CH(CH3)CH2CH3

MoO2 valAmpMCM-41

–CH(CH3)2

MoO2 hisAmpMCM-41

–H2C

O

R

OOC

OMe

O

O

M O

N

O O Si PPh2 O

O

O

acac–

O O Si O O O Si

OMe

PPh2 O

N O Mo N O O O OH –OOC

OMe

N M

H N

N

N

OMe O O Si OH O O O Si

R

MoO2 alaAmpMCM-41

MCM-41

MCM-41

OMe

MCM-41

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Characterization of MCM-41-Supported Molybdenum Complexes with Schiff Base Ligands Characterization of the MCM-41-supported molybdenum– Schiff base complexes has been carried out with Fourier transform infrared (FT-IR) and inductively coupled plasma optical emission spectroscopies (ICP-OES), powder x-ray

acac–

N O O Mo N O O –OOC

(b)

H

H

(c) Si

OH

O Si MCM-41

O OH

O

Si

Si O

O

Si

OH

N (acac)O2 Mo S

(d)

SCHEME 4  Structures of the heterogeneous molybdenum catalysts: (a) MoO2(amino acid Schiff base)AmpMCM-41 (From ­Masteri-Farahani, M., J. Mol. Catal. A: Chem., 316, 45, 2010), (b) MoO2dppb@AmpMCM-41 (From Masteri-Farahani, M. et  al. J. Nanostruct., 2, 853, 2012.), (c) MoO2glyacacAmpMCM-41 (From Masteri-Farahani, M., J. Nanostruct., 2(1), 43, 2012.), (d) MCM-41SB-MoO2(acac). (From Tangestaninejad, S. et al., Catal. Commun., 10(6), 853, 2009.)

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Catalysis: Molybdenum-Based Hybrid Nanocatalysts

(a)

2000

1500

1000

Wavenumber (cm–1)

500

FIGURE 48  FT-IR spectra of (a) pypr@MCM-41 and (b) MoO2pypr@MCM-41. (From Masteri-Farahani, M. et  al., Catal. Commun., 8(1), 6, 2007.)

decrease in the mesoscopic order of the materials and mainly due to the contrast matching between the silicate framework and complex species located inside the MCM-41 channels.31 On the other hand, the increase in hexagonal unit cell or lattice parameter, which is a measure of the spacing between the hexagonal layers, indicates the unit cell expansion due to the incorporation of organic moieties and molybdenum complex within MCM-41. Nitrogen sorption analysis (Figure 49) shows that these materials exhibit the type IV isotherms in the IUPAC classification with the appearance of hysteresis loop resulted

from capillary condensation of nitrogen gas in mesopores. As expected, the specific surface area, total pore volume, and pore diameter (calculated with BJH method) decreased in MoO2pypr@MCM-4, which indicated the incorporation of MoO2pypr groups in MCM-41.

200

Catalytic Activities of MCM-41-Supported Molybdenum–Schiff Base Complexes The catalytic activities of MCM-41-supported molybdenum complexes are broadly examined in the epoxidation of olefins.23–30 The results of catalytic epoxidation of cyclooctene as a model substrate with TBHP in the presence of different MCM-41-supported molybdenum complexes are given in Table 5. As seen, the reported catalysts act as efficient selective catalysts for the epoxidation of olefins. The proposed mechanism of epoxidation of cyclooctene with TBHP in the presence of MoO2pyprMCM-41 has been shown in Scheme 5.23 The heterogeneous nature of the MoO2pyprMCM-41 catalytic system was inferred as the epoxidation of cyclooctene was allowed to proceed for 1.5 h, and then the reaction mixture was filtered. Continuation of the reaction on filtrate under similar conditions for 7 h showed just few increase in conversion, which showed the relative stability of the catalyst during the catalytic process.

100

Magnetite Nanoparticles as Support

600

(a)

500 Adsorbed volume (mL/g)

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Transmittance

(b)

400 (b) 300

0

(c)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Relative pressure (p/p0)

FIGURE 49  Nitrogen sorption isotherms of (a) MCM-41, (b) ClpMCM-41, and (c) MoO2pyprMCM-41. (From Masteri-Farahani, M. et al., Catal. Commun., 8(1), 6, 2007.)

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Heterogenization of homogeneous catalysts followed by decreasing their sizes into nanoscale is being viewed as a major advance in catalysis research. However, particles with diameters of less than 100  nm are difficult to separate by filtration techniques. In such cases, expensive ultracentrifugation is often the only way to separate the product and catalyst. Hence, easy recovery and reuse of expensive catalysts have recently attracted much research interest. MNPs, which

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Catalysis: Molybdenum-Based Hybrid Nanocatalysts

TABLE 5 Results of Catalytic Epoxidation of Cyclooctene with TBHP in the Presence of MCM-41-Supported Molybdenum Complexes Catalyst

Time (h)

Conversion (%)

Epoxide (%)

7 12 8 8 7 7 7 7 7 9 9 2

98 99 54 89 100 100 100 96 99 99 99 94

99 99 99 99 99 99 98 98 99 100 99 100

MoO2pypr@MCM-41 MoO2salpr@MCM-4124 MoO2acacdien@MCM-4125 MoO2furAmpMCM-4126 MoO2pycaAmpMCM-4126 MoO2acpyAmpMCM-4126 MoO2amacAmpMCM-4126 MoO2salAmpMCM-4126 MoO2acacAmpMCM-4126 MoO2dppbAmpMCM-4128 MoO2glyacacAmpMCM-4129 MCM-41-SB-MoO2(acac)30

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23

(CH3)3COOH

O O Si O O O Si O

N N OH

N Mo

N N

O

O O Si O O O Si O

O

N N

Mo N

OH

N OO

N

H O(CH3)3

O

O + (CH3)3COH

SCHEME 5  Mechanism of epoxidation of cyclooctene with TBHP in the presence of MoO2pypr@MCM-41. (From Masteri-Farahani, M. et al., Catal. Commun., 8(1), 6, 2007.)

display superparamagnetism, have been widely studied as support for heterogenized homogeneous catalysts.32–40 The specific advantages of magnetic nanoparticles in catalysis can be summarized as follows: • The nanoparticles possess high external surfaces. • The catalytically active sites can be distributed on the outer surface of the support; thus, pore diffusion constraints are avoided. • The small size of the particles (typically 99 98 99

SUMMARY The field of organic–inorganic hybrid materials is expanding because of their important role in the development of advanced functional materials. In this regard, design and construction of organic–inorganic hybrid nanocatalysts with tunable physical properties have attracted extensive research interest in catalytic chemistry. Attaching homogeneous catalysts to solid supports that are easy to recover and retain the activity of the homogeneous catalysts would have numerous benefits in terms of process design and environmental factors. Facile recovery and reuse while maintaining high catalytic activity of the supported homogeneous catalysts offer a particularly efficient catalyst for a wide variety of reactions. Thus, design of new hybrid nanomaterials, which have appropriate interactions between the support and active components, is of great importance for catalytic purposes. Great efforts have been made to immobilize homogeneous molybdenum catalysts on the surface of various supports. Our current interest in the immobilization of molybdenum complexes on different supports led us to investigate the preparation and characterization of new covalently attached molybdenum hybrid nanocatalysts on the surface of MCM-41 mesoporous material, SCMNPs, and MWCNTs. The obtained results showed that the prepared hybrid ­nanocatalysts are active and selective in the epoxidation of ­olefins. On the other hand, the hybrid nanocatalysts are easily recoverable and can be reused several times without significant loss of activity after recycling.

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