Direct Preparation of Thermally Stable Sn-Incorporated SBA-15 Mesoporous Materials in the Self-Generated Acidic Environment

Abstract

Highly ordered Sn-incorporated SBA-15 mesoporous materials with Sn loading up to 10 mol % were prepared in one pot using P123 triblock copolymer as the template in a self-generated acidic environment with the aid of NaCl. The acidity of the synthesis solution generated by hydrolysis of Sn precursor was found suitable not only for the self-assembly of P123 micelles and tetraethyl othorsilicate (TEOS) but also to incorporate the Sn ions into the SBA-15 materials. Prehydrolysis of TEOS in the synthesis solution containing P123, NaCl, and H2O prior to adding Sn precursor was essential to reduce the strong interference from the Sn ions in the self-assembly process. A well-ordered 2D hexagonal p6mm pore arrangement was observed in the Sn-SBA-15 materials with Sn/Si ratios of 0.01−0.10, while tetragonal SnO2 nanocrystallites were observed when the Sn/Si ratio was higher than 0.07. In situ small-angle X-ray scattering showed that a worm-like pore arrangement appeared at the beginning of the self-assembly process. As the reaction period prolonged, further condensation of silica and rearrangement of pore structure resulted in a well-ordered 2D hexagonal p6mm mesostructure. For the Sn-SBA-15 materials with Sn/Si ratios of 0.07−0.1, it was noticeable that two sets of 2D hexagonal pore arrangements with slightly different d spacings were present. One set of larger d spacing formed by incorporation of very small tin oxide species at the interface of P123 micelles and silicate appeared faster than the other of smaller d spacing. When the Sn/Si ratio was smaller than 0.05, Sn was probably incorporated in the SBA-15 framework as isolated ions. Beyond this loading, tetragonal SnO2 nanocrystallites (∼4.7 nm) aggregated into rod- or tube-like morphologies of 5−10 nm in diameter and 20−50 nm in length were formed inside the mesochannels. The mesostructure of Sn-SBA-15 was thermally stable up to 1000 °C, which is superior to pure siliceous SBA-15. Moreover, the SnO2 nanocrystallites confined inside the mesochannels did not sinter into large SnO2 crystals even after calcination at 1000 °C.