The active and reusable catalysts in the benzylation of anisole derived from a heteropoly acid

Benzylation of anisole with benzyl alcohol was carried out with H3PO4–WO3–Nb2O5-derived catalysts. The catalytic performance depended significantly on the calcination temperature. The catalyst calcined at 573–723 K exhibited low activity and poor reusability, and the formation of heteropoly acid (HPA), which is similar to 12-tungstophosphoric acid, was identified by infrared (IR) and X-ray diffraction (XRD) measurements. The catalysts calcined above 823 K were in essence inactive in the reaction due to the formation of crystalline WO3 that originated from HPA. In contrast, optimum activity was realized for the catalyst calcined at 773 K, which could be readily reused after filtration. The acid property of H3PO4–WO3–Nb2O5 was measured by means of NH3 TPD and the IR spectra of adsorbed pyridine. An enhancement of weak acid sites was observed for the catalyst calcined at 773 K, accompanied by the simultaneous reduction in the acidity due to HPA. The Brønsted acid sites were dominant in H3PO4–WO3–Nb2O5 calcined at 773 K. The partially decomposed HPA, which maintained the same local symmetry around W atoms as HPA, was observed with W-L3 edge EXAFS as well as W-L1 edge XANES. From these data, the active and insoluble species obtained by the calcination of H3PO4–WO3–Nb2O5 at 773 K was ascribed to the intermediate species of HPA formed in the decomposition process.