Spectroscopic characterization of extra-framework hydrated proton complexes with the extremely strong hydrogen bonds in microporous silicate minerals

Abstract

Microporous materials containing hydrated silanol groups Si–OH as well as hydrated proton complexes, H_2n+1O_n⁺, including hydronium (n = 1), Zundel (n = 2), and Eigen (n = 4) cations, are of practical importance as potential ion exchangers and ion conductors. In this paper, we provide data on crystal-chemical features, hydrogen bonding and Raman spectra of alkaline microporous titano-, niobo-, zircono-, and aluminosilicate minerals belonging to the labuntsovite, lovozerite, eudialyte, and sodalite groups in which a part of sodium was substituted by hydrated proton complexes under low-temperature hydrothermal or supergene conditions. Most minerals studied in this work do not have synthetic analogues and are considered as possible natural prototypes of microporous materials with technologically important properties. The obtained experimental data and their comparison with the results of ab initio theoretical calculations published elsewhere show that Raman spectroscopy is an effective tool for the precise identification of hydrated proton complexes with extremely strong hydrogen bonds and estimation of corresponding O···O distances in the range of 2.37–2.68 Å. The presence of hydrated proton complexes in microporous silicates is a clear and sensitive geological indicator showing that a rock underwent the low-temperature alteration.