Proton transfer in hydrated ammonium zeolites: a 1H NMR study of NH4-chabazite and NH4-clinoptilolite

A wide-line 1H NMR approach was applied to study the rates of proton-transfer reactions between extra-framework species in hydrated ammonium-substituted zeolites chabazite and clinoptilolite. For both samples, a rather intense proton exchange between water molecules was registered with no sign of the cross-exchange with ammonium ions. The exchange observed is interpreted as a result of the proton-transfer reaction between H2O molecules and chemical defects represented by Brّnsted acid sites. From temperature measurements, the barriers for this reaction are found to be about of 40 and 60 kJ/mol in NH4-chabazite and NH4-clinoptilolite, respectively. The proton conductivity associated with this reaction is about a percent of the contribution produced by NH4+ migration.