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

摘要

Mesoporous silica molecular sieve SB A-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,~(29)Si and ~(119)Sn MAS NMR,and Sn-M6ssbauer 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,~(119)Sn NMR and Mossbauer spectroscopies are used to investigate the microstructure of the Sn centers grafted to the mesoporous walls.Sn-M6ssbauer spectroscopic studies reveal that Sn~(2+) may form upon reductive treatments and can probably be stabilized atomically in the pore wall,whereas Sn~(4+) 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.