Solid-state NMR of silica—alumina prepared by chemical vapor deposition

Abstract

A supported type of silica—alumina catalyst prepared by depositing silica on an alumina (CVD SiO 2/Al 2O 3) was characterized by solid-state magic-angle spinning (MAS) NMR measurement. The 31P MAS NMR of trimethylphosphine chemisorbed on the catalyst elucidated a change in acid properties (Brönsted or Lewis type) with silica loading. Lewis acid sites on the alumina surface were converted into Brönsted acid sites by the deposition of silica. At lower silica loadings, both Brönsted and Lewis acid sites existed. Above 12 wt.% silica loading, Lewis acid sites disappeared and only Brönsted acid sites were observed. The catalytic activities of CVD SiO 2/Al 2O 3 for cumene cracking and 1-butene isomerization varied with silica loading, and the catalytic behaviors were correlated with the amount of Brönsted acid sites generated by silica deposition. The 29Si MAS NMR of silica deposited on the alumina surface revealed the microstructures of silica. The silica which was highly dispersed, forming a silica bilayer, exhibited high catalytic activities and strong Brönsted acidity. Further silica layering decreased both the catalytic activities and the acidity.