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

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) H-1 MAS exchange NMR experiment has shown that the magnetization transfer occurs between different protonic sites at 293473 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 1H31P heteronuclear correlation NMR experiment reveals that the H-1 NMR resonance at 9.0 ppm correlates with 31P NMR signals of all subunit forms CsxH3xPW12O40 (x = 03) presented in the salt with a total formula of Cs2HPW12O40, whereas the protons with chemical shift of 5.0 ppm are associated only with phosphorus nuclei located in the subunit form of the cesium salt with x = 2 (Cs2H1 subunit). 1H MAS NMR studies of protiumdeuterium exchange between -OH groups of Cs2HPW12O40 and deuterated n-butane-d10 revealed that both types of acidic protons of this catalyst could be equally involved in the reactions with participation of hydrocarbons with molecular dimensions similar to the butane molecule.