Cation siting in low-silica zeolites with potential applications in pressure swing adsorption technology and structural studies of novel tectosilicates.

摘要

Adsorption and catalytic phenomena in zeolites are influenced by coulombic interactions within zeolite structure as well as by size restrictions imposed by zeolitic micropores. In the first part of this thesis, we investigate materials with potential applications in the selective adsorption of nitrogen. Studies have indicated the performance of LiX adsorbents correlates with the number of accessible lithium cations in the zeolite. However, in zeolite LiX only one-third of the lithium cations interact with adsorbed molecules. This prompted our investigation of other low-silica zeolites.; The first zeolite examined was a cancrinite prepared in the presence of 1,3-butanediol. Neutron diffraction studies on the lithium-exchanged cancrinite suggest most of the lithium cations are ideally positioned within the micropores. However, our experiments show that carbonate anions are occluded within the micropores of this cancrinite.; Another material examined in this study was zeolite (Ba,K)-GL. Neutron diffraction experiments show that $49$ of the extraframework charges are located in the micropores. Unfortunately, less than 20% of these cations are replaced after lithium-exchange. We also attempted to directly synthesize a (Ba,Li)-GL. However, diffraction experiments and chemical analyses show that most of the extraframework charge is compensated by barium.; Because of molecular size restrictions imposed by currently known zeolites, there is continued interest in the synthesis of materials that can be used for processing large molecules. This motivated our investigation of two tectosilicates MCM-61 and MCM-47. MCM-61 is an aluminosilicate made in the presence of the potassium-18-Crown-6 complex. Our structure solution shows MCM-61 is formed from novel [610412] polyhedral cage units that connect to form 18-membered-ring cages. The structure of MCM-61 suggests that crown ethers, azamacrocycles, and cryptands may be useful for the syntheses of extra-large pore zeolites.; MCM-47 is prepared in the presence of tetramethylene bis(N-methylpyrrolidinium) dibromide. The structure solution of MCM-47 shows it is composed of noncovalently bound layers. The structure refinement and 1H MAS NMR experiment indicate these layers are bound together by strong hydrogen bonds between siloxy and silanol groups. MCM-47 can be delaminated to yield a high surface area material with potential applications in the catalysis of large molecules.