Molecular simulation study of the structural rearrangement of methane adsorbed in aluminophosphate AlPO4-5

Grand canonical Monte Carlo simulations have been performed for methane adsorbed in the aluminophosphate AlPO4-5. Several approaches were used in order to model the methane-methane as well as the methane-adsorbent interactions. It is shown that a potential function of the Pellenq and Nicholson (PN) type, which includes two-body, three-body dispersion terms, induced interactions and exponential repulsions with each individual framework species can adequately describe the experimentally observed substep in the adsorption isotherms. The correspondence between the calculated and experimental adsorption isotherms, assuming a rigid microporous crystal, confirms that the observed substep may be attributed to a structural rearrangement of the adsorbed phase, similar in nature to a phase transition. A good agreement was also found between the simulated and experimental isosteric heats of adsorption curves. The pressure at which the step is observed in the simulations depends crucially on the value of the repulsive Oxygen-methane parameter. Two structures of the adsorbed phase with maximum loading were found, using slightly different values of this latter parameter. Calculated neutron diffraction spectra for both structures agree fairly well with experiments. These two phases are presumably close to each other in energy and stability and further experiments are needed in order to conclude on the exact nature of the confined methane structure at high pressure.