Microstructure of cesium hydrogen salts of 12-tungstophosphoric acid relevant to novel acid catalysis

A comprehensive interpretation of the microstructure and mechanism of the f ormation of a versatile solid acid catalyst, Cs2.5H0.5PW12O40, has been att empted by combining the new results obtained with solid-state NMR, XRD, SEM , and Nz porosimetry with the data reported previously. The precipitates of Cs2.5H0.5PW12O40 just formed from aqueous solutions of H3PW12O40 and Cs2CO 3 consist of ultrafine crystallites in which the acid form, H3PW12O40, is e pitaxially 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 randoml y through the entire bulk, as revealed by XRD and P-31 solid-state NMR. Imp regnation of Cs3PW12O40 With the aqueous solution of H3PW12O40 also gives t he uniform salt after calcination. Pore-size distribution evaluated by the analysis of Nz desorption isotherm showed that Cs2.5H0.5PW12O40 has mesopor es 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 rep orted for solid-liquid reaction systems is thus principally attributed to t he strength and number of acid sites and the mesoporous structure appropria te for the rapid diffusion of molecules.