PNNL

Abstract

The acidic sites in dispersed tungsten oxide supported on SBA-15 mesoporous silica were investigated using a combination of pyridine titration, both fast-, and slow-MAS 15N NMR, static 2H NMR, and quantum chemistry calculations. It is found that the bridged acidic –OH groups in surface adsorbed tungsten dimers (i.e., W–OH–W) are the Brønsted acid sites. The unusually strong acidity of these Brønsted acid sites is confirmed by quantum chemistry calculations. In contrast, terminal W-OH sites are very stable and only weakly acidic as are terminal Si-OH sites. Furthermore, molecular interactions between pyridine molecules and the dimer Brønsted and terminal W-OH sites for dispersed tungsten oxide species is strong. This results in restricted molecular motion for the interacting pyridine molecules even at room temperature, i.e., a reorientation mainly about the molecular 2-fold axis. This restricted reorientation makes it possible to estimate the relative ratio of the Brønsted (tungsten dimer) to the weakly acidic terminal W-OH sites in the catalyst using the slow-MAS 1H-15N CP PASS method.