Prediction of permeation properties of CO2 and N-2 through silicalite via molecular simulations

The sorption isotherms and self-diffusivities of CO2 and N-2 in silicalite have been calculated via grand canonical Monte Carlo and equilibrium molecu lar dynamics simulations over a wide range of occupancies, using various fo rce fields proposed in the literature. Predictions for the sorption thermod ynamics are in very favorable agreement with the experiment, especially whe n detailed point-charge models are used to represent the interaction of the quadrupole moments of the sorbate molecules with the lattice field and wit h each other. They indicate that the zeolite cannot be in its para (P2(1)2( 1)2(1)) form under the conditions of the measurements. Permeabilities corre sponding to a perfectly crystalline membrane have been estimated for CO2 an d N-2, as well as for methane, examined in past simulation work, from the p redicted sorption isotherms and low-occupancy self-diffusivities by invokin g the Darken equation. The ratios of pure component permeabilities obtained in this way agree very well with actual macroscopic values obtained from c arrying out permeation measurements for the different pure sorbates in the same silicalite membrane. Absolute magnitudes of the permeabilities, howeve r, exceed by more than 2 orders of magnitude the reported macroscopic value s, which themselves vary widely among different experimental investigations . The large, morphology-dependent nonuniformity in membrane thickness of ac tual supported silicalite membranes is proposed as a plausible reason for t his disparity.