Thermal Transformations and Proton Species in 12-Phosphotungstic Acid Hexahydrate Studied by ¹H and ³¹P Solid-State Nuclear Magnetic Resonance

Abstract

The hydration environments around the heteropolyanion in 12-phosphotungstic acid (PW 12 O 40 H 3 , HPW) were analyzed as a function of temperature in the hexahydrated form (HPW•6H 2 O). Samples were obtained drying HPW•nH 2 O with n ∼ 22 at 358 K for 2 h in a furnace with either uncontrolled air atmosphere or saturated with N 2 . The existence of a phase transition in HPW•6H 2 O and the change in the thermal balance of hydrated species were tested using a set of multinuclear NMR techniques: high-resolution 31 P and 1 H NMR, 31 P{ 1 H} spin−echo double resonance, and { 1 H}-31 P cross-polarization. Differences in the proton dynamics were also probed measuring the spin−lattice relaxation time in the rotating frame ( 1 H-T 1ρ ), using direct excitation of all 1 H 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•6H 2 O, structural differences were detected with NMR, indicating that the material obtained from the drying process under N 2 is more compatible with the hexahydrate. 1 H NMR confirmed the existence of an equilibrium balance between hydration species as a function of temperature: H 5 O 2 + ↔ H 3 O + + H 2 O ↔ H + + 2H 2 O. Proton exchange was detected in H 3 O + and also in hydrogen bonds in H 5 O 2 + 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 H 2 O. 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 H 2 O.

2‐Methyl‐1,3‐disulfoimidazolium polyoxometalate hybrid catalytic systems as equivalent safer alternatives to concentrated sulfuric acid in nitration of aromatic compounds

“…The characteristic sharp and symmetric resonance peaks of phosphorus in tetrahedral position of the Keggin‐structured anions in each of the heteropolyacids are slightly moved towards higher frequencies in the case of the hybrid compounds and thus indicate strong ionic interaction between the imidazolium cation and Keggin anion in the hybrid compounds. The up‐field resonance of 31 P signals signifies the retention of heteropolyanions with a small hydration sphere observed from dynamic dehydration equilibrium involving dioxonium group H 5 O 2 + (equation ) as proposed to explain the behaviour of hydration environment as a function of temperature close to room temperature using NMR study: …”

Qualitative and Semiquantitative Determination of Bromine in Hybrid Hydroxyurethanes-Poly(dimethylsiloxane) Films Containing Phosphotungstates ([PW<sub>12</sub>O<sub>40</sub>]<sup>3-</sup>)

“…The intermolecular protons transfer can be supported by the study Marcos de Oliveira, Jr. et al that mentioned that around the 12-phosphotungstic acid (H 3 PW 12 O 40 , PWA) there are hydration environments dependent of the temperature [53]. Also, 1 [53]. The effect of water on proton conductivity of PWA is one of the most key features in PEMFCs.…”

“…For the intermolecular pathway in PEM (between adjacent PWA molecules), due to the strong interactions between the silica surface and the PWA molecules (probably the most predominant), the corresponding distances between the latter will undergo small modifications during proton transport [15]. The intermolecular protons transfer can be supported by the study Marcos de Oliveira, Jr. et al that mentioned that around the 12-phosphotungstic acid (H 3 PW 12 O 40 , PWA) there are hydration environments dependent of the temperature [53]. Also, 1 [53].…”

The impact of preparation route on the performance of silver dodecatungstophosphate/β zeolite catalysts in the ethylene production

“…12-Tungstophosphoric acid, H 3 [PW 12 O 40 ]•6H 2 O, denoted as HPW, was synthesized in agreement with the literature (Rosenheim and Jaenicke 1917;De Oliveira et al 2014). The obtained hexahydrate sample was kept in a desiccator with silica gel before measuring mass.…”