The basic structure of hetropoly acid hydrates (HPAs∙nH2O) has already been determined by Keggin (1934), but the water and hydrated proton entities filing the channels (secondary structure) have not been studied much until the early 1990s. As heteropoly acids are superionic proton conductors at room temperature and good catalysts, it is clear that their activities will be the function of hydration degree and dynamics of proton species. Therefore, it was necessary to focus on the studies that identify protonic entities and follow their dynamics. The studies started on WPA-6, the most stable hydrate of WPA with well-known structure. Crystallographic results for WPA-6 show that Keggin’s anions in WPA-6 are stabilized by dioxonium ion H5O2+, which was defined as planar and rigid (Brown et al. 1970). On the other hand, IR and Raman spectra taken at room temperature have shown that different protonic species (H5O2+, H3O+, H2O) exist in dynamic equilibrium (Mioč et al. 1991, 2005; Colomban...
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Mioč, U.B. (2016). Free Protons in Solid Heteropoly Compounds. In: Drioli, E., Giorno, L. (eds) Encyclopedia of Membranes. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-662-44324-8_1734
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