Synthesis and characterization of tin oxide-modified mesoporous SBA-15 molecular sieves and catalytic activity in trans-esterification reaction

Abstract

Mesoporous silica molecular sieve SBA-15 has been synthesized and incorporated with various amounts of tin via incipient-wetness impregnation using tin chloride in ethanol followed by calcination. Characterization was done by X-ray fluorescence spectrophotometry (XRF), powder X-ray diffraction (PXRD), nitrogen adsorption, diffuse reflectance ultraviolet (DRUV), scanning and transmission electron microscopy (SEM, TEM), FTIR, 29Si and 119Sn MAS NMR, and Sn-Mo¨ssbauer spectroscopic techniques to understand the chemical nature of incorporated tin. The silanol groups on the internal walls of SBA-15 are suggested to be the sites for tin incorporation. Bulk structural characterization techniques (X-ray diffraction and BET) are used to demonstrate that the hexagonal structure is maintained during impregnation. Tin oxide exists as a thin film anchored inside the mesopores of SBA-15. Complementarily, 119Sn NMR and Mo¨ssbauer spectroscopies are used to investigate the microstructure of the Sn centers grafted to the mesoporous walls. Sn-Mo¨ssbauer spectroscopic studies reveal that Sn2+ may form upon reductive treatments and can probably be stabilized atomically in the pore wall, whereas Sn4+ is stabilized as large entities. While SBA-15 itself shows some activity in transesterification of diethyl malonate with various alcohols, Sn-SBA- 15 samples are found to possess enhanced catalytic activity for the same.