Characterization of protonic sites in H3PW12O40 and Cs1.9H1.1PW12O40: a solid-state 1H,2H,31P MAS-NMR and inelastic neutron scattering study on samples prepared under standard reaction conditions Original Research Article

Spectroscopic techniques in controlled atmosphere, such as solid-state 1H, 2H, and 31P magic angle spinning nuclear magnetic resonance (MAS-NMR) and inelastic neutron scattering (INS) spectroscopies, have been used to investigate the effect of dehydration on structural modifications and acidic properties of solid 12-tungstophosphoric acid H3PW12O40 and its cesium salt Cs1.9H1.1PW12O40. Thermogravimetric analysis and XRD experiments gave complementary informations about proton/water contents and structure of the samples. 1H, 2H, and 31P MAS-NMR spectra were recorded as a function of the degree of dehydration/rehydration and allowed one to characterize the protonic species present in the samples, such as OH groups and protonated clusters H+(H2O)n. INS spectra, recorded at 4 K on samples dehydrated at 473 K, suggested the presence of hydroxonium ion H3O+ in bulk H3PW12O40 and of hydroxyl type species in the porous cesium salt Cs1.9H1.1PW12O40. After dehydration at a higher temperature, 573 K, the INS spectra showed the presence of hydroxyl groups in both samples. These four techniques provided a detailed description of the acidic features (nature, strength and number of the acid sites) of H3PW12O40 and Cs1.9H1.1PW12O40 samples in relation with their structure and hydration state.