Thermal transformations and proton species in 12-phosphotungstic acid hexahydrate studied by 1H and 31P solid-state nuclear magnetic resonance

The hydration environments around the heteropolyanion in 12-phosphotungstic acid (PW12O40H3, HPW) were analyzed as a function of temperature in the hexahydrated form (HPW·6H2O). Samples were obtained drying HPW·nH2O with n ~ 22 at 358 K for 2 h in a furnace with either uncontrolled air atmosphere or saturated with N2. The existence of a phase transition in HPW·6H2O and the change in the thermal balance of hydrated species were tested using a set of multinuclear NMR techniques: high-resolution 31P and 1H NMR, 31P{1H} spin-echo double resonance, and {1H}-31P cross-polarization. Differences in the proton dynamics were also probed measuring the spin-lattice relaxation time in the rotating frame (1H-T1ρ), using direct excitation of all 1H species or selecting only those in the neighborhood of the polyanion with cross-polarization techniques. Experiments were conducted between 223 and 358 K to induce appreciable variations in the dynamic equilibrium of hydrated species. Although XRD and TGA showed the same results for both samples, consistent with HPW·6H2O, structural differences were detected with NMR, indicating that the material obtained from the drying process under N2 is more compatible with the hexahydrate. 1H NMR confirmed the existence of an equilibrium balance between hydration species as a function of temperature: H5O2+ ↔ H3O+ + H2O ↔ H+ + 2H2O. Proton exchange was detected in H3O+ and also in hydrogen bonds in H5O2+ at a very fast rate, even at low temperatures. No evidence of a structural phase transition was found in this sample. For the sample dried in uncontrolled atmosphere, the presence of mobile water in clusters was detected. Heating from room temperature, the hydrated species undergo a transformation at 318 K, related to rearrangement/removal of interstitial H2O. A complex sequence of metastable states and hysteresis effects is observed when this sample is cooled down to room temperature. These results indicate that the transformation at 318 K is not an intrinsic property of the HPW hexahydrate structure, but it is related to the rearrangement of interstitial H2O.FAPESP (11/18271-5; 13/07793-6)CNPq (306202/2011-5)CAPE