Adsorption of n-alkanes in faujasite zeolites: molecular simulation study and experimental measurements

We report an application of a previously developed force field for adsorption of hydrocarbons in silicalite (Pascual, P., et al. in Phys. Chem. Chem. Phys. 5:3684-3693, 2003), to the case of the linear alkane-sodium faujasite systems. In order to extend this force field from siliceous to cationic zeolites, we propose to take into account the polarization part of the zeolite-molecule interaction energy. A first order polarization term is explicitly considered for this purpose, using standard molecular polarizabilities. Polarization appears to amount to 30-40% of the zeolite-alkane interaction energy, as a consequence of the strong electric field created by the sodium cation distribution and negatively charged framework. This approach is compared with experimental adsorption isotherms of ethane, propane, n-octane and n-decane in NaY from the literature and with original measurements of n-butane isotherms in NaY obtained by thermal gravimetry. Henry constants and heats of adsorption at zero coverage of n-alkanes (n=6-10) are also compared with experimental measurements. Although no specific parameter has been calibrated for extending the force field, the general agreement between simulation results and experiments is satisfactory. Cation redistribution upon alkane adsorption is not observed in these simulations.