Vapor-phase silylation of MCM-41 and Ti-MCM-41

MCM-41 mesoporous molecular sieves have been silylated with trimethylchlorosilane (TMCS) and trimethyleth-oxysilane (TMES) under anhydrous, vapor-phase reaction conditions. The silylation reactions were performed at 473 and 623 K following pre-treatment at either 523 or 973 K. At the pre-treatment temperature of 973 K, most hydrogen-bonded surface silanols in MCM-41 are dehydroxylated to form siloxane bridges. The reactivity of surface silanols and siloxane bridges with the silylation agents was monitored using in-situ infrared spectroscopy. More extensive silylation by TMCS and TMES is achieved at the higher reaction temperature. At 623 K, TMCS reacts with most lone and hydrogen-bonded silanols in non-dehydroxylated MCM-41 to yield a trimethylsilyl (TMS) surface coverage of approximately 1.2 TMS/nm(2). Silylation protocols incorporating the dehydroxylation pre-treatment step yield lower organic loadings, because the siloxane bridges do not completely react with the silylation agents or their co-products. Silylation of titanium-containing MCM-41 catalysts with TMCS enhances their hydrophobicity and improves their activity and selectivity for the epoxidation of cyclohexene with aqueous hydrogen peroxide. (C) 2003 Elsevier Inc. All rights reserved.