Solid-State NMR Characterization of Brønsted Acid Sites of Cesium Salt of 12-Tungstophosphoric Acid

Abstract

Solid cesium salts of 12-tungstophosphoric acid show high surface area and strong acidity, which stimulate their applications as the catalysts for many reactions. Characterization of acid sites of the cesium salts is crucial since the information obtained could provide the details of mechanism of catalysis by these solid porous materials. By using 1H magic angle spinning nuclear magnetic resonance (1H MAS NMR), two types of acidic protons with the signals at 9.0 and 5.0 ppm have been identified for cesium salt of 12-tungstophosphoric acid, Cs2HPW12O40. A two-dimensional (2D) 1H MAS exchange NMR experiment has shown that the magnetization transfer occurs between different protonic sites at 293–473 K. Weak temperature dependence of the rate of magnetization transfer with apparent activation energy of 6.4 ± 0.3 kJ mol–1 is indicative of the spin-diffusion process rather than the chemical exchange between the acid sites. The 2D CP/MAS 1H–31P heteronuclear correlation NMR experiment reveals that the 1H NMR res...