Characterization and Dynamics of the Different Protonic Species in Hydrated 12-Tungstophosphoric Acid Studied by2H NMR

Abstract

Solid heteropolyacids (HPAs) are promising nonpolluting superacids used as catalysts and solid proton conductors. The catalytic and conducting properties of HPAs are very sensitive to the amount of hydration water present in the system, as water molecules tend to interact with the HPA acid protons to form [H3O]+ and [H5O2]+ ions. These ions constitute active species that govern the catalytic reaction pathways and the proton migration mechanism. Establishing the structure and mobility of protonic species could yield important information concerning the functions of material based on HPA hydrates. In this work, we have performed the analysis of both 2H NMR line shape and the evolution of T1, T2 relaxation with temperature for the deuterated analogue of the solid 12-tungstophosphoric acid (TPA × nH2O) at different hydration levels (0 < n < 6) in the wide temperature range of 103–503 K. This allowed us to characterize in detail the mobility of different protonic species, including acidic OH groups, water molecules, and hydroxonium ions. Kinetic parameters of internal and diffusional motions for different protonic species at different hydration levels n of TPA × nH2O were derived.