Solid-state nuclear magnetic resonance studies of cations and guest molecules in zeolites.

摘要

Solid-state nuclear magnetic resonance (NMR) is a powerful technique for the identification of the local structure in zeolites and molecular sieves. Zeolites are widely used in industrial processes including air separation, catalysts for fuel cracking, and as ion-exchangers. In order to understand the ability of a zeolite to perform a particular task, knowledge about the framework structure, type and location of the charge compensating cations and guest molecules is required. Advanced solid-state NMR techniques can identify the interactions of the zeolite framework and/or the charge compensating cations with the guest molecules present. Three specific applications of solid-state NMR to the understanding of zeolite structures, charge compensating cations and guest molecules are presented.; ZSM-2, CSZ-1, and ECR-35 are several FAU-EMT intergrowth materials. The exact structure of these materials is not well understood. The first project explores the possibility of charge compensating cesium cations acting as an indicator of zeolite cage sizes. 133Cs NMR spectra are collected for zeolites with known cage sizes and for the intergrowth materials.; The dehydrohalogenation of alkyl halides is an important reaction for the production of alkenes. When the reaction is performed in two different materials of the MFI structure type, drastic differences in reactivity, products and yields are observed. The second project utilizes solid-state NMR spectroscopy to probe the interaction of the organic guest molecules and the zeolite framework. An interesting side result demonstrates the ability of solid-state NMR to monitor the formation of the reaction products.; The separation of oxygen and nitrogen in air is of commercial importance. Pressure Swing Adsorption (PSA) is an efficient technique for the small- to medium-scale production of high purity oxygen gas and nitrogen gas. An effective adsorbent for the PSA technique is lithium-exchanged, low-silica zeolite X (LiLSX). The separation of oxygen gas and nitrogen gas is based on the high selectivity of LiLSX adsorbent for the nitrogen gas molecules. Solid-state NMR offers the ability to identify which lithium cation sites are involved in nitrogen gas adsorption and to what extent. The work is extended to a mixed gas system allowing for the semi-quantitative measure of oxygen gas adsorption on LiLSX.