Highly porous organic–inorganic hybrid silica and its titanium silicate analogs as efficient liquid-phase oxidation catalysts

doi:10.1016/j.apcata.2007.12.041

Applied Catalysis A: General

Volume 342, Issues 1–2, 30 June 2008, Pages 29-34

https://doi.org/10.1016/j.apcata.2007.12.041 Get rights and content

Abstract

Vinyltrimethoxysilane was polymerized under non-aqueous conditions through radical initiation. The resulting polymer was used as precursor in the presence of self-assembly of cationic surfactant cetyltrimethylammonium bromide (CTAB) to obtain the mesoporous organic–inorganic hybrid polyvinylsiloxane (HPVS-1). An identical synthesis procedure was followed together with the addition of different amounts of titanium(IV) source in the synthesis gels to obtain its titanium silicate analogs (Ti-HPVS-1). XRD, TEM and N₂ sorption measurements suggested highly porous wormhole-like disordered framework structures with high BET surface areas for these materials. Solid-state ²⁹Si MAS NMR, UV–vis and FT IR spectroscopic tools and ICP-OES elemental analysis were used to characterize these materials. Spectroscopic results suggested the incorporation of isolated tetrahedral Ti(IV) sites in the organically modified hybrid silica frameworks. Ti-HPVS-1 showed excellent catalytic activity and high selectivity in the liquid-phase oxidation of R-(−)-carvone to the corresponding epoxide, using dilute aqueous H₂O₂ as oxidant.

Graphical abstract

Vinyltrialkoxysilane was polymerized under non-aqueous conditions. This polymeric precursor was used for the synthesis of organic–inorganic hybrid mesoporous silica and its titanium silicate analogs. Titanium containing hybrid materials showed good catalytic activity and epoxide selectivity in the liquid-phase oxidation of R-(−)-carvone using dilute aqueous H₂O₂ as oxidant.

Introduction

Microporous and mesoporous materials [1], [2], [3] have attracted widespread attention as adsorbents and catalysts because of their exceptional surface areas and well-defined pore sizes suitable for the diffusion of bulky adsorbate and substrate molecules. The versatile catalytic activity of microporous TS-1 in the presence of dilute aqueous H₂O₂ as oxidant in some liquid-phase partial oxidation reactions [4], [5], [6], [7] including epoxidation, C–H functionalization, hydroxylation, ammoximation, and cyclization, has motivated the use of mesoporous titanium silicates [8], [9] in some of these eco-friendly liquid-phase oxidation reactions. However, these materials were found to be hydrophilic due to the presence of excessive Si–OH groups on the surface, which is detrimental for liquid-phase oxidation reactions when water is present in the reaction medium. Post-synthesis silylation [10] or synthesis of mesoporous silica with a mixture of TEOS and organotrialkoxy silanes [11] are employed to improve the catalytic activity of these materials in the presence of dilute aqueous H₂O₂ as oxidant [12].

In this context, a new class of mesoporous hybrid silsesquioxane materials [13], [14], [15], [16], [17], [18], [19], [20], [21] having organic functionality as an integral part of the framework have been synthesized by using a single organosilane source. A transition metal-like Ti(IV) could be incorporated hydrothermally within the framework of these hybrid materials [17]. These organic–inorganic hybrid microporous and mesoporous materials showed hydrophobic surface properties including the high activity to oxidize bulky organics in the presence of dilute H₂O₂ as oxidant [22], [23]. However, for the synthesis of these organic–inorganic hybrid silica and their titanium silicate analogs, an expensive bridging organosilane precursor is required, otherwise they can only be synthesized on the laboratory scale using multi-step tedious organic synthesis routes. Vinyl-functionalized mesoporous silica materials are conventionally prepared by using a mixture of vinyltrialkoxysilane and tetraalkylorthosilicate as silica source. It is pertinent to mention that, due to the absence of highly cross-linked Q⁴ (Si*(OSi)₄) type species, alkyl-trialkoxysilanes alone cannot form silica-based molecular sieves. Thus there is no report of the synthesis of microporous and mesoporous silca framework by using vinyltrimethoxysilane alone as the silica source.

Herein, we report the syntheses of highly porous hybrid polyvinylsiloxane and its titanium silicate analog by using vinyltrimethoxysilane as a single silicon precursor with the aid of supramolecular assembly of CTAB under hydrothermal conditions via radical initiation of the vinyl monomer. Here, we have shown that ca. 2.0 wt% Ti(IV) could be incorporated while successfully preserving the highly porous organic–inorganic hybrid framework by using a simple hydrothermal method and avoiding the co-precipitation of titanium oxides during the synthesis. R-(−)-carvone epoxidation has also been studied extensively, as its epoxide product is an important intermediate [24] for the synthesis of biologically active organic compounds and other fine chemicals. We have used Ti-HPVS-1 in liquid-phase partial oxidation of R-(−)-carvone in the presence of H₂O₂ as oxidant.

Section snippets

Synthesis of polyvinyltrimethoxysilane (PVTMS)

Trimethoxyvinylsilane was used as received from Aldrich. A solution of trimethoxyvinylsilane (6.108 g, 0.04 mol) and azoisobutironitrile (AIBN, used as radical initiator, Aldrich) (0.123 g, 0.00075 mol) in dry THF (15 ml) were taken in a round bottom flask equipped with a condenser and rubber septum. Argon gas was purged for 30 min to remove the dissolved oxygen, followed by reflux at 333 K for 24 h. The as-synthesized polymeric organosilane was designated as PVTMS. PVTMS was purified and then used as

Results and discussion

The powder X-ray diffraction profiles of different as-synthesized and acid-extracted samples 1 and 3 exhibited a peak maxima at 2θ ≅ 2.45–3.1° (Fig. 1). This data suggested that the polyvinyl-bridged hybrid silica has a mesoscopically disordered structure. Sample 4 showed relatively lower values of 2θ. No high angle reflection was observed in any of these samples. The observed d spacing was reduced during the removal of surfactants, suggesting contraction of the framework as usually observed for

Conclusions

A new hybrid organosilane precursor has been prepared through radical polymerization of vinyltrialkoxysilane under non-aqueous condition. Polymeric precursor produced in situ has been used as the source for the synthesis of organic–inorganic hybrid silica and its titanium silicate analogs by using supramolecular assemblies of cationic surfactants. Moderately good amounts of Ti could be incorporated in these hybrid materials while retaining their mesostructures. Both HPVS-1 and Ti-HPVS-1

Acknowledgements

AB wishes to thank Department of Science and Technology, New Delhi, for providing Ramanna Fellowship and research grants. This work was partly funded by the Nanoscience and Technology Initiative, DST, Government of India.

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Oxidation of benzene to phenol with 30% H₂O₂ in liquid-phase was carried out over a series of Cu–Ce incorporated rice husk silica catalysts using acetonitrile as solvent at 343 K under atmospheric pressure. Cu and Ce incorporated rice husk silica catalysts were prepared by the sol–gel technique using cetyltrimethyl ammoniumbromide as the template. These catalysts were labeled as RHA–10Cu5Ce, RHA–10Cu20Ce, and RHA–10Cu50Ce. TG/DTG analysis of the catalysts confirmed complete removal of the template at 773 K and all catalysts were X-ray amorphous even after 50 wt% cerium incorporation. The rate for benzene oxidation increased linearly with cerium incorporation and depended directly on H₂O₂ decomposition. The high activity and phenol selectivity observed under mild reaction conditions (343 K, atmospheric pressure) could be correlated to the enhanced textural properties such as the BET surface area (403–279 m² g⁻¹) and large pore volume (0.90–0.54 m³ g⁻¹) of the catalysts. Conversion during catalytic performance followed the order RHA–10Cu50Ce > RHA–10Cu20Ce > RHA–10Cu5Ce while phenol selectivity followed the order RHA–10Cu20Ce > RHA–10Cu5Ce > RHA–10Cu50Ce. A redox mechanism between Cu²⁺/Cu⁺ and Ce⁴⁺/Ce³⁺ active centers on the silica surface was responsible for the high catalytic activity of 84% with 96% phenol selectivity given by RHA–10Cu20Ce.

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Highly porous organic–inorganic hybrid silica and its titanium silicate analogs as efficient liquid-phase oxidation catalysts

Abstract

Graphical abstract

Introduction

Section snippets

Synthesis of polyvinyltrimethoxysilane (PVTMS)

Results and discussion

Conclusions

Acknowledgements

Tetrahedron

Micropor. Mesopor. Mater.

Appl. Catal. A: Gen.

Micropor. Mesopor. Mater.

J. Mol. Catal. A: Chem.

Micropor. Mesopor. Mater.

Appl. Catal. A: Gen.

Hydrothermal Chemistry of Zeolites

Chem. Rev.

Chem. Commun.

Angew. Chem. Int. Ed.

Nature

J. Mater. Chem.

J. Phys. Chem. B