Microstructure of Cesium Hydrogen Salts of 12-Tungstophosphoric Acid Relevant to Novel Acid Catalysis

Abstract

A comprehensive interpretation of the microstructure and mechanism of the formation of a versatile solid acid catalyst, Cs2.5H0.5PW12O40, has been attempted by combining the new results obtained with solid-state NMR, XRD, SEM, and N2 porosimetry with the data reported previously. The precipitates of Cs2.5H0.5PW12O40 just formed from aqueous solutions of H3PW12O40 and Cs2CO3 consist of ultrafine crystallites in which the acid form, H3PW12O40, is epitaxially deposited on the surface of Cs3PW12O40 crystallites. Calcination of the precipitates brings about the migration of H+ and Cs+ in the solid to form a nearly uniform solid solution in which protons distribute randomly through the entire bulk, as revealed by XRD and 31P solid-state NMR. Impregnation of Cs3PW12O40 with the aqueous solution of H3PW12O40 also gives the uniform salt after calcination. Pore-size distribution evaluated by the analysis of N2 desorption isotherm showed that Cs2.5H0.5PW12O40 has mesopores as well as micropores that are interparticle voids of the crystallites. The initial heat of NH3 sorption indicated the presence of very strong acid sites on Cs2.5H0.5PW12O40. High catalytic activity of Cs2.5H0.5PW12O40 reported for solid−liquid reaction systems is thus principally attributed to the strength and number of acid sites and the mesoporous structure appropriate for the rapid diffusion of molecules.