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

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• 作者：Marcos de Oliveira ; Jr. ; Ubirajara Pereira Rodrigues-Filho ; J. Schneider
• 刊名：Journal of Physical Chemistry C
• 出版年：2014
• 出版时间：June 5, 2014
• 年：2014
• 卷：118
• 期：22
• 页码：11573-11583
• 全文大小：485K
• 年卷期：v.118,no.22(June 5, 2014)
• ISSN：1932-7455

文摘

The hydration environments around the heteropolyanion in 12-phosphotungstic acid (PW₁₂O₄₀H₃, HPW) were analyzed as a function of temperature in the hexahydrated form (HPW路6H₂O). Samples were obtained drying HPW路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路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鈥揺cho double resonance, and {¹H}-³¹P cross-polarization. Differences in the proton dynamics were also probed measuring the spin鈥搇attice relaxation time in the rotating frame (¹H-T_1蟻), 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路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⁺ 鈫?H₃O⁺ + H₂O 鈫?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.