MOLECULAR SIMULATION OF THE STEPPED ADSORPTION-ISOTHERM OF METHANE IN ALPO4-5

Grand ensemble simulations have been made for-two adsorbates, argon and methane, in the aluminophosphate, AlPO4-5, at several temperatures. Two approaches were used to 'model the adsorbate-adsorbent interactions: (i) a simple 12-6 potential between adsorbate and only the oxygens of the framework; (ii) a potential function which included two-body, three-body dispersion terms, induced interactions and exponential repulsions with each individual framework species. It is shown that only the second model can adequately describe the experimentally observed adsorption isotherms. The correspondence with experimental behaviour, assuming a rigid microporous crystal, suggests that the sub-step in the methane isotherm is due to a structural change of the adsorbed phase.