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

Abstract Spectroscopic techniques in controlled atmosphere, such as solid-state 1 H , 2 H , and 31 P 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 H 3 PW 12 O 40 and its cesium salt Cs 1.9 H 1.1 PW 12 O 40 . Thermogravimetric analysis and XRD experiments gave complementary informations about proton/water contents and structure of the samples. 1 H , 2 H , and 31 P 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 + (H 2 O) n . INS spectra, recorded at 4 K on samples dehydrated at 473 K, suggested the presence of hydroxonium ion H 3 O + in bulk H 3 PW 12 O 40 and of hydroxyl type species in the porous cesium salt Cs 1.9 H 1.1 PW 12 O 40 . 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 H 3 PW 12 O 40 and Cs 1.9 H 1.1 PW 12 O 40 samples in relation with their structure and hydration state.