Incorporation of tin into mesoporous silica SBA-15 molecular sieves

Mesoporous silica SBA-15 molecular sieves have been incorporated with various amounts of Sn by repeated post grafting technique using two different metal precursors viz., Sn acetate and Sn chloride. Physico chemical characterization was done by X-ray fluorescence spectrophotometer (XRF), powder X-ray diffraction (XRD), nitrogen adsorption, transmission electron microscopy (TEM), FTlR, Sn-Mössbauer and ¹¹⁹Sn MAS NMR spectroscopic methods to understand the chemical nature of incorporated Sn. Higher amounts of Sn could be incorporated into SBA-15 using Sn acetate. Adding dilute HCl to parent SBA-15 in the acetate precursor could have increased the number of silanol groups, thereby facilitating the incorporation of higher amounts of Sn. XRD data indicate a good mesoscopic order. The characteristic hexagonal features of SBA-15 were maintained in Sn-SBA-15 samples. Incorporation of Sn does not affect the original pore structure of the parent SBA-15 even at high Sn loading. The silanol groups on the internal walls of SBA-15 are suggested to be the sites for Sn incorporation. ¹¹⁹Sn NMR shows an isotropic chemical shift at δ = - 604 ppm which indicates the presence of Sn in essentially octahedral environment. Sn Mössbauer 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 size entities.