Mesoporous tin silicate: an efficient liquid phase oxidative dehydrogenation catalyst

https://doi.org/10.1016/j.apcata.2004.06.025Get rights and content

Abstract

Mesoporous tin silicate materials have been synthesized at room temperature using a self-assembly of cationic surfactant cetyltrimethylammonium bromide (CTAB) and tetraethylorthosilicate (TEOS) and SnCl4 as Si and Sn precursors, respectively, under mild acidic (pH 4.5–6.5) condition. Si/Sn mol ratios in the product for different mesoporous tin silicate samples were 12.4, 27.2, 48.5 and 74.7 vis-à-vis the ratios 10, 25, 50 and 75, respectively, in the hydrothermal synthesis gels. These mesoporous samples were characterized using powder X-ray diffraction (XRD), N2 sorption, UV–vis diffuse reflectance (UV DRS), FT-IR spectroscopy, and scanning electron microscopy (SEM) on an instrument with an energy dispersive X-ray (EDS) attachment. Characterization data demonstrated that the samples were moderately ordered, with pore dimension of 2.1–2.4 nm. UV DRS and IR spectral bands suggested the incorporation of tin in the tetrahedral lattice sites of the mesoporous silica network. After calcination, these tin silicate samples retained their mesophase and showed excellent catalytic activity in one-pot liquid phase selective oxidative dehydrogenation of cyclohexane to 1,4-cyclohexadiene using dilute aqueous hydrogen peroxide as oxidant. A very high selectivity for 1,4-cyclohexadiene (83–98%) was observed using different mesoporous tin-silicate samples and acetonitrile solvent. Very little to almost no further oxidation product of the parent and product olefins, i.e. epoxides or cyclohexanone/cyclohexanol was observed.

Introduction

Eco-friendly selective liquid phase oxidation catalysis over silica-based materials using dilute H2O2 as oxidant has attracted widespread attention for over a decade [1], [2], [3]. Already, these catalytic reactions have been largely replaced the use of hazardous organic peracids, inorganic acids and other harmful oxidizing agents in chemical plants and answered the needs of green chemistry [4]. The selective catalytic functionalization of organics remains one of the big challenges in the field of catalysis. Among these selective partial oxidation reactions, CH bond activation is of particular interest because oxygenated/dehydrogenated molecules are the building blocks of functionalized macromolecules of diverse interests. Dehydrogenation of cycloalkanes to the corresponding olefins is a reaction of special interest from its potential economic advantages. Cyclohexane oxidation reaction is one of the most studied reactions in heterogeneous catalysis [5] where cyclohexaneoxide, cyclohexanol and cyclohexanone and their further oxidation product adipic acid are predominantly obtained over most of the transition-metal-based catalysts [6] under different liquid phase reaction conditions. On the other hand, very few catalysts, such as Cu(II) in the presence of tert-butyl hydroperoxide (TBHP) [7] and cerium-exchanged zeolite Y [8] are known to yield cyclohexene predominantly upon oxidation of cyclohexane. However, yields of the olefin obtained over those catalysts are relatively poor [7], [8]. Moreover, to the best of our knowledge simultaneous dehydrogenation of cyclohexane at 1 and 4 positions have never been possible over mesoporous materials under liquid phase reaction conditions. Incorporation of Sn(IV) in the tetrahedral silica frameworks of mesoporous silica materials was expected to enhance surface acidity, because Sn(IV) salts are well-known strong Lewis acids [9] over the years which could be helpful in the dehydrogenation reaction. Moreover, incorporation of Sn(IV) in the silica network has the potential [10] to give an efficient oxidation catalyst using dilute H2O2 as oxidant. Tin-containing microporous molecular sieves of MFI, MEL, MTW, ZSM-48, BTA, etc. topologies have been reported [11]. Mesoporous tin silicates have also been synthesized using other synthesis methods [12], [13]. Very recently, Sn-Beta and Sn-MCM-41 have been successfully utilized in the liquid-phase Baeyer–Villiger oxidation reaction [14], [15] with a very high selectivity for the lactone and Meerwein–Ponndorf–Verley reduction of carbonyl compounds [16]. Hydrothermal synthesis under alkaline condition [15] was usually employed in the preparation of mesoporous tin silicates. However, under alkaline pH condition the maximum loading of Sn was limited to 2–2.5 mol% in silica. Here, we report the excellent catalytic activity of this Sn-rich mesoporous tin silicate synthesized under mild acidic pH in the selective oxidative dehydrogenation of cyclohexane to 1,4-cyclohexadiene, using dilute H2O2 oxidant and acetonitrile solvent.

Section snippets

Experimental

Unlike conventional syntheses of mesoporous silica-based materials [17] this mesoporous tin silicate material was synthesized under mild acidic pH, employing acid hydrolysis of silicon alkoxide source followed by addition of base for precipitation and condensation. SnCl4·5H2O (Loba Chemie) was used as the tin source in this study. Tetraethylorthosilicate (TEOS, E-Merck) was used as silica source and cationic surfactant cetyltrimethylammonium bromide (CTAB, Loba Chemie) was used for structure

Results and discussion

Chemical analysis of four mesoporous tin silicate samples revealed Si/Sn mole ratios of 74.7, 48.5, 27.2 and 12.4 (vis-à-vis their input mole ratios 75, 50, 25 and 10, respectively) in the final product after the removal of the structure-directing agent (SDA). These samples were labeled as samples 1, 2, 3 and 4, respectively. Low angle X-ray diffraction patterns of the surfactant-free mesoporous tin silicate samples are shown in Fig. 1(a). 100, 110 and 200 planes of the mesophases were observed

Conclusion

Mesoporous tin silicate samples with different Sn contents have been synthesized using cationic surfactant. Characterization data suggested the incorporation of Sn in mesoporous tin silicate samples. These tin silicate materials showed excellent catalytic activity and selectivity for direct oxidative dehydrogenation of cyclohexane to 1,4-cyclohexadiene under mild liquid phase condition using dilute H2O2 as oxidant and acetonitrile solvent. Although we can incorporate as high as 13.3 mol.% Sn in

Acknowledgements

AB wishes to thank DST and CSIR, Government of India, New Delhi, for their respective financial supports.

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