Nanoconfined Water in Pillared Zeolites Probed by 1H Nuclear Magnetic Resonance

Here, we report the results of our H-1 nuclear magnetic resonance study of the dynamics of water molecules confined in zeolites (mordenite and ZSM-5 structures) with hierarchical porosity (micropores in zeolite lamella and mesopores formed by amorphous SiO2 in the inter-lamellar space). H-1 nuclear magnetic resonance (NMR) spectra show that water experiences complex behavior within the temperature range from 173 to 298 K. The temperature dependence of 1H spin-lattice relaxation evidences the presence of three processes with different activation energies: freezing (about 30 kJ/mol), fast rotation (about 10 kJ/mol), and translational motion of water molecules (23.6 and 26.0 kJ/mol for pillared mordenite and ZSM-5, respectively). For translational motion, the activation energy is markedly lower than for water in mesoporous silica or zeolites with similar mesopore size but with disordered secondary porosity. This indicates that the process of water diffusion in zeolites with hierarchical porosity is governed not only by the presence of mesopores, but also by the mutual arrangement of meso- and micropores. The translational motion of water molecules is determined mainly by zeolite micropores.

Automated summary

In this study, the dynamics of water molecules confined in zeolites were investigated using H-1 nuclear magnetic resonance. The results showed that water molecules exhibited complex behavior at different temperatures, including freezing, fast rotation, and translational motion. The activation energy for translational motion was significantly lower than in other materials, indicating that the hierarchical porosity of zeolites affects water diffusion not only through the presence of mesopores, but also the arrangement of meso- and micropores.