Mesoporous tin silicate: An efficient liquid phase oxidative dehydrogenation catalyst

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 2.1-2.4nm. 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. © 2004 Elsevier B.V. All rights reserved.