Direct hydrothermal synthesis of mesoporous Sn-SBA-15 materials under weak acidic conditions

Abstract

A direct synthetic route for the preparation of Sn-SBA-15 materials with nSi/nSn ratios ranging from 100 to 10 under milder acidic conditions than normally employed for the preparation of Si-SBA-15 is reported. The changes in the pH conditions of the gel were made through an adjustment of the molar ratio of nH2O to nHCl (<1 pH <2) during preparation. The samples prepared under three different acidic conditions have been characterized by various techniques. An expansion of the lattice (powder XRD) and an increase in mesopore area (low temperature N2 adsorption) indicate that the hexagonal structure of the SBA-15 is maintained with no loss of long range ordering. The UV–vis reflectance spectra of Sn-SBA-15 samples show the presence of Sn4+ ions both in tetrahedral and octahedral environment. 29Si MAS NMR spectra of samples prepared under an intermediate acid condition show the presence of Q3 and Q4 species. Their ratio increases with a decrease in tin content. The presence of Si in (2Si, 2Sn) i.e., Q2 environment may point to incorporation of considerable Sn4+ ions in tetrahedral positions. Sn-Mo¨ssbauer spectroscopic studies reveal that Sn2+ species form upon reductive treatments and can probably be stabilized in the pore wall upon reoxidation. This to some extent is an indication of the formation and stabilization of Si–O–Sn–O–Si linkages in Sn-SBA-15. A progressive increase in the pH of the medium (increasing the nH2O to nHCl ratio) results in the location of Sn4+ ions, (a) at the surface of the mesopores (surface of the corona region) as a thin film of SnO2 or small aggregates (loss in mesopore area) depending on the concentration of Sn; (b) at the walls of the mesopore structure, substituting Si4+ ions (some lattice expansion and tetrahedral Sn4+ ions); and/or (c) as a part of the corona region, neutralizing the resulting Si–OH groups (a loss of micropore area and octahedral Sn4+ ions). The studies reveal that the method of preparation, nH2O=nHCl ratio and the nSi/nSn ratio (concentration of SnCl4) of the gel significantly influence the type of tin species in the resulting Sn-SBA-15 samples.