Creation of Brønsted Acidity by Grafting Aluminum Isopropoxide on Silica under Controlled Conditions: Determination of the Number of Brønsted Sites and their Turnover Frequency for <i>m</i>‐Xylene Isomerization

Abstract

Abstract There is not a unique Brønsted acid site for aluminosilicates (ASAs). IR spectroscopy following CO adsorption proves the creation of Brønsted acid sites on Al/SiO 2 ASAs, which are synthesized by the deposition of aluminum species on hydroxylated silica. These sites are active for ethanol dehydration and m ‐xylene isomerization. Controlled deposition under anhydrous conditions optimizes the number of sites, whereas the presence of water leads to alumina agglomerates with no Brønsted acidity. The turnover frequency for m ‐xylene isomerization (4.3×10 −4 s −1 site −1 at 350 °C, atmospheric pressure, and 0.6 cm 3 h −1 of m ‐xylene) is approximately 3 times lower than that of the Brønsted acid sites of Si/Al 2 O 3 and 75 times lower than that of an ultrastable Y‐type zeolite without extra‐framework aluminum.