The Influence of Non-framework Sodium Cations on the Adsorption of Alkanes in MFI- and MOR-Type Zeolites

Abstract

Molecular simulations were performed for the adsorption of methane, ethane, and propane in MFI- and MORtype zeolites with various nonframework sodium and framework aluminum densities. The position of the nonframework sodium cations determined by Monte Carlo simulations is in agreement with positions determined by X-ray diffraction. The position and density of the sodium and aluminum atoms in the zeolite have a large influence on the adsorption of alkanes. The computed adsorption isotherms and Henry coefficients agree well with those obtained experimentally. Finally, we show that Configurational Bias Monte Carlo (CBMC) simulations are able to provide a better understanding of the effect of nonframework sodium on the selective adsorption of binary mixtures of isomers by these structures. Our results show that increasing the nonframework sodium density in MFI-type zeolites increasingly blocks the intersections and thereby increases the selectivity of MFI-type zeolites for adsorbing linear alkanes. By contrast, increasing the nonframework sodium density in MOR-type zeolites increases the number of sites favorable for adsorbing small linear alkanes. Crystalline silica can be synthesized with many different nanoporous structures. These silicas become catalytically active zeolites by substitution of trivalent aluminum for tetravalent silicon into the silica framework. Each substitution creates a net negative charge in the framework, which is compensated by a cation. The location of these cations influences the adsorption and the catalytic properties of the materials. 1 Although crystalline silica structures are well defined, the