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

Abstract

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 mgPO h−1 g−1 cat to 37.2 mgPO 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 ≡Si–O–Si≡ or ≡Ti–O–Si≡ 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.