Post-synthesis Sn beta: An exploration of synthesis parameters and catalysis

摘要

Sn beta is probably one of the best water tolerating heterogeneous Lewis acids for liquid phase catalysis. Instead of applying the usual lengthy hydrothermal synthesis to prepare Sn beta, this contribution uses a more hands-on two-step synthesis method, involving the grafting of Sn precursors in isopropanol under reflux conditions on a commercial beta zeolite that was dealuminated in acid. Among several reference synthesis procedures, this Sn introduction method resulted in active Sn catalytic sites. Taking advantage of this practical method, several synthesis parameters were explored and their impact on the catalytic activity in four different Lewis acid catalyzed reactions is discussed. The adsorption isotherm of Sn-IV in isopropanol over a broad range of Sn salt concentrations at reflux temperature is presented and discussed in relation with FTIR spectroscopy, UV-vis absorption characteristics and the porosity of the materials. The study reveals a selective Sn uptake, up to 2 wt% Sn loading, into silanol nests of the dealuminated precursor, forming a diversity of mononuclear Sn-IV. Higher Sn loadings result in less active Sn (hydrous) extraframework oxide phases, which also cause partial blockage of the zeolite micropores. Depending on the reaction type under study, space time yield may increase with increasing Sn loading, but the activity per Sn is always lower. Therefore it is concluded that a preferred synthesis should form high contents of isolated Sn active sites, especially for sugar isomerization and intermolecular Meerwein-Ponndorf-Verley, while the other reaction types like Baeyer-Villiger is also sufficiently catalyzed by the small Sn oxide clusters, albeit less actively. (C) 2015 Elsevier Inc. All rights reserved.