Abstract
An application of a previously developed force field for the adsorption of hydrocarbons onto silicalite (Pascual et al. 2003) to linear alkane/sodium faujasite systems is reported. In order to extend this force field from siliceous to cationic zeolites, it is proposed that account be taken of the polarization component 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 the negatively charged framework.
This approach is compared with the experimental adsorption isotherms of ethane, propane, n-octane and n-decane in NaY taken from the literature and with original measurements of n-butane isotherms in NaY obtained by thermogravimetric methods. Henry constants and heats of adsorption at zero coverage of n-alkanes (n = 6–10) have also been compared with experimental measurements. Although no specific parameter was invoked for extending the force field, general agreement between the simulation results and experiment was satisfactory. Cation redistribution upon alkane adsorption was not observed in these simulations.