Unexpected Destructive Dealumination of Zeolite Beta by Silylation

Zeolite beta was silylated in a novel way, by chemical liquid deposition (CLD) of a cyclic organosiloxane, and characterized. Despite its large size, octamethyl-cyclotetrasiloxane (OMTS) did not provide a surface modification, as did the traditional tetra-alkoxysilanes (ethoxy, propoxy, and butoxy). OMTS treatment with minor Si deposition influenced the internal parts of the zeolite with destructive dealumination, as revealed by 27Al MAS NMR, IR, and XRD studies. Three types of AlO6 sites were found, (a) 17 ppm 27Al shift, (b) 0 ppm shift, 3782 cm−1 IR frequency, and (c) −20 ppm shift, exhibiting no Brønsted and Lewis acidity, respectively, toward pyridine. Apparently, OMTS's low reactivity toward condensation with the less acidic external hydroxyl groups promoted ring-opening at pore-mouth sites. This allowed the resulting linear siloxane tetramer to penetrate the pores and cause dealumination during calcination. Catalysts modified with OMTS had the lowest amounts of AlO4 sites, Lewis acidity, large void volume, and crystallinity.