Cation-Type Dependences of 1H and 29Si MAS NMR Chemical Shifts and Desorption Temperatures of Water Molecules in Faujasite-Type Zeolites

Abstract

In order to reveal adsorbed states of water molecules on faujasite-type zeolites (FAU) systematically, twenty five kinds of FAU were prepared in this study: Na ions in FAU with Si/Al ratios of 1.0, 1.2, 1.7, 2.4, and 3.5 were exchanged by each Li, K, Rb, and Cs ions. High-resolution solid-state 1H NMR spectra with a magic-angle-spinning (MAS) method showed a signal which can be assigned to H atoms of water molecules adsorbed on each FAU. Complex dependences of 1H chemical-shift (CS) value on cation types were recorded (the lowest CS value was detected in Na- or K-type FAU with each Si/Al ratio). The similar tendency was recorded on 29Si MAS NMR spectra. In addition, thermogravimetry (TG) and differential-thermal-analysis (DTA) lines showed that a desorption temperature of water molecules was increased with decreasing Si/Al ratios, conversely, the temperature showed complex dependences on cation properties (the minimum temperature was recorded in Na- or K-type FAU with each Si/Al ratio). In our simulations, the similar complex relations recorded on the 1H and 29Si MAS NMR spectra were obtained by use of a supercage model.