Understanding the effect of postsynthesis ammonium treatment on the catalytic activity of Au/Ti-SBA-15 catalysts for the oxidation of propene

Article abstract of DOI:10.1016/j.jcat.2008.07.013

Postsynthesis ammonium treatment induces a substantial increase in the catalytic activity of Au/Ti-SBA-15 catalysts for the direct vapor-phase epoxidation of propylene using hydrogen and oxygen. The PO formation rate of a calcined Au/Ti-SBA-15 catalyst prepared by this method increased from 4.3 mg_PO h^-1 g^-1_cat to 37.2 mg_PO h^-1 g^-1_cat at 200 °C compared with a catalyst prepared in an identical manner without this treatment. The catalysts were characterized by XRD, N₂-sorption, UV-vis-NIR DRS, ²⁹Si MAS NMR, FT-IR spectroscopy, and TEM to gain insight into the relationship between ammonia treatment and catalyst activity. ²⁹Si MAS NMR measurements proved that the ammonium nitrate solution caused hydrolysis of {triple bond, long}Si{single bond}O{single bond}Si{triple bond, long} or {triple bond, long}Ti{single bond}O{single bond}Si{triple bond, long} bonds, resulting in a Ti-SBA-15 surface with a greater amount of surface hydroxyl groups. FT-IR measurements indicated the presence of amine species that favor the homogeneous deposition of Au nanoparticles. This was confirmed by TEM measurements showing greater metal dispersion for NH₄NO₃-treated Au/Ti-SBA-15 materials.