Effects of Heat Treatment on the Structure and Properties of the Tungstophosphoric Acid-Modified Zirconia Aerogel Solid Acids

Abstract

Based on detailed experimental results,the effects of heat treatment on the structure,acidity,and catalytic performance of the 12-tungstophosphoric acid(TPA) modified zirconia aerogel(ZA)(TPZA) solid acids were investigated.TPZA samples,with TPA mass fraction of 25% and heat-treated at 120-750 ℃,were characterized by N2 physical adsorption,X-ray diffraction(XRD),Fourier transform infrared(FTIR) spectroscopy,FTIR spectroscopy of adsorbed pyridine(Py-FTIR),and temperature programmed desorption of NH3(NH3-TPD).And they were used as catalysts in the polymerization of tetrahydrofuran(THF).The results show that,although the heat treatment might greatly influence both the surface density and type of tungsten species,the TPZA materials with the TPA loading of 25% could acquire suitable Brnsted and Lewis acidity and accordingly acceptable catalytic activities over the wide temperature ranges of 120-300 ℃ and 550-750 ℃.In the TPZA samples treated at 120-300 ℃,the presence of Brnsted acid sites can be associated with the presence of highly dispersed and somewhat distorted but intact Keggin-anions,whereas the Brnsted acid sites in the samples treated at 550-750 ℃ might originate from the zirconia-anchored,highly polymerized tungstate surface species.Moreover,all the samples show obvious Lewis acidity,which is associated with the presence of coordinatively unsaturated Zr4+ and phosphorous oxide produced by the decomposition of TPA.