Mordenite zeolites, framework structures related

Five zeolite minerals have been considered identical with mordenite with space group Cmcm. The possibility of a family of structures is considered four related ordered structures including Cmcm are proposed. Two of these (Cmcm and Imcm) have a one-dimensional system of large pores. The remaining pair (Cmmm and Immm) have a two-dimensional pore system with a second set of smaller channels. X-ray diffraction results show that synthetic and most mineral specimens have the Cmcm structure but also reveal mixtures of Cmmm, Immm, and Imcm with the Cmcm structure in three mineral specimens. Electron diffraction examination of a ptilolite sample reveals the Cmcm structure with an inter growth of the idealized structure Cmmm. Further tentative evidence for the existence of more than one ((mordenitef) framework structure, based on physical property characteristics, is discussed. 

Other related zeolites may also exist as families of framework structures. Models of modified framework structures of some of the other members of the mordenite family—i.e., ferrierite, dachiardite, and epistil-bite—have been constructed. Such structures have unit cell dimensions essentially equal to the reported structures, but of course they exhibit different symmetries. 

N2 02, neopentane) in the zeolites A, X, L, mordenite, omega, and a synthetic offretite type have been determined from isotherms. These have been compared with the void volumes calculated from the known crystal structures. For most adsorbates the measured and calculated void volumes are in good agreement. However, helium and nitrogen exhibit anomalous behavior. A void volume-framework density relation for zeolites is given. 

The size, location, and structure of platinum clusters in H-mordenite were modeled by molecular mechanics energy minimization and molecular dynamics simulation techniques [96G1]. It was suggested that the relative stability of monoatomic platinum sites in aluminosilicate mordenites is related to the specific aluminum insertion in T sites of the framework structure. The structural features of the platinum cluster confined to the 12-ring main channel are almost independent of the total Pt content and strongly dependent upon the surrounding zeolite structural field. 

Adsorption studies of methanol, water and ammonia, and binary mixtures of these compounds, have been used to explain the difference in the change in reaction behaviour of zeolites Rho and mordenite in the methanol amination reaction after hydrothermal treatment. It is found that the changes in the total adsorption capacities were related to the changing pore volumes and to the presence of extra-framework aluminium within the pores of the catalysts. The changes in the reaction performance of the catalysts for the methanol amination reaction were also correlated to the changes in acidity, structure and adsorption capacity of the catalysts. 

The only framework oxygen anions which can stabilize (by coordination) the dicarbonyl species and retard the formation of the tricarbonyl intermediates are those which have an enhanced and localized negative charge. The occurrence of such anions is governed by the density of TO4 (tetrahedral) entities related to the presence of structural A1 species. Thus, an increasing rate of CO exchange is expected for zeolites characterized by higher Si/Al ratios, which will displace the equilibria toward the formation of the tricarbonyl structure. This expectation has been confirmed for the series NaX < NaY < Na-mordenite... 

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