Molecular Simulation of Adsorption of Simple Gases in Aluminophosphates and Pillared Clays
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MOLECULAR SIMULATION OF ADSORPTION OF SIMPLE GASES IN ALUMINOPHOSPHATES AND PILLARED CLAYS Roger CRACKNELL, Carolyn A KOH, Stephen M THOMPSON and Keith E GUBBINS* "*Cornell University School of Chemical Engineering, Ithaca NY 14853 ABSTRACT We report Grand Canonical Monte Carlo (GCMC) simulation studies of the adsorption and heat of adsorption of simple inert gases in two model microporous materials: aluminophosphates (AIPO 4-5, AIPO 4-8 and VPI-5) and alumina-pillared clays. The intermolecular potentials are spherical Lennard-Jones for both the fluid-fluid and fluid-solid interactions; both structured and structureless walls are considered. For argon in VPI-5 and AIP0 4-5 we find qualitative agreement with experiment, but the predicted maximum adsorption is about 20% higher than that obtained experimentally; possible reasons for this discrepancy are discussed. For the pillared clays we find a first order phase transition below some critical pillar density. This finding seems to be in qualitative agreement with existing experimental data. INTRODUCTION The aluminosilicate zeolites find widespread use in industry as catalysts for heterogeneous gas reactions, and in separation processes based on adsorption. Although they have the advantage of a regular structure and temperature stability, a limitation is their small pore size (below 10 A), so that they can only be used in processes involving relatively small molecules. There is a need in the oil and chemical industries for new inorganic materials that have larger pore sizes, while retaining the desirable properties of thermal stability and well-characterized pore structure. In this study we report molecular simulations of models of two classes of materials that provide larger pore sizes: aluminophosphates (ALPOs) and alumina-pillared clays (PILCs). The structure of ALPOs is one of tetrahedrally coordinated atoms (aluminum or phosphorous) which are linked by bridging oxygen atoms, with the stoichiometry AtPO.1 . An important feature of these materials is that they contain straight pores that are roughly cylindrical in shape and are not interconnected. Materials with various pore diameters have been produced, such as AIPO,1-5 [1], which has a ring of 10 tetrahedral atoms and A&PO.-8 [2], which has a ring of 14 tetrahedral atoms. These materials have pore sizes below 10 A. By contrast, the newer aluminophosphate, VPI-5 [3], has a ring of 18 tetrahedral atoms and a pore diameter of 12.1 A. The lack of pore connectivity and charge neutrality of the substrate make these materials relatively easy to model in a simulation. Moreover, for the larger pores present in VPI-5, it may be possible to study the transition from three- dimensional to one-dimensional behavior in pores by varying the diameter of the adsorbed gas molecule. In this paper we report the initial stages of such a study, in. which we investigate the adsorption behavior of the inert gases in VPI-5. By varying the adsorbed gas the reduced pore radius, R" = R/rr, where a is the diameter of the adsorbed gas molecule
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