Dynamics of Water in Hierarchical Mesoporous H-ZSM-5 by Fast Field-Cycling NMR Relaxometry

Abstract

The dynamics of water molecules adsorbed in hierarchical mesoporous H-ZSM-5 was investigated by fast field-cycling NMR relaxometry. Three distinct sites in H-ZSM-5 that can host water molecules, designated a, b, and c, have been identified; no significant exchange of water molecules among these sites was observed. Investigations into the relative populations of molecules confined at sites c (Pc) and b (Pb) revealed that the population ratio (Pc/Pb) does not change significantly over the temperature and water-content ranges investigated. Furthermore, the motion of water at site b is very slow and shows almost no dependence on temperature. While H-ZSM-5 has pores of two different sizes (mesopores and micropores), the low population and observed slow motion of water at site b indicates a strong interaction between water molecules and the zeolite materials, which is possible only when water is adsorbed on the surface. The temperature of the phase transition of water provides insight into the properties of water adsorption. The transition temperature of water confined at site a varies with water content in the sample, indicating that it is located in a larger space; thus, site a represents mesopores. The transition temperature of water and the population of water molecules at site c are independent of water content, indicating that this site has been saturated; therefore, site c is the micropore, which is saturated because of capillary condensation.