Understanding the role of sodium during adsorption: a force field for alkanes in sodium-exchanged faujasites.

We have developed a united atom force field able to accurately describe the adsorption properties of linear alkanes in the sodium form of FAU-type zeolites. This force field successfully reproduces experimental adsorption properties of n-alkanes over a wide range of sodium cation densities, temperatures, and pressures. The force field reproduces the sodium positions in dehydrated FAU-type zeolites known from crystallography, and it predicts how the sodium cations redistribute when n-alkanes adsorb. The cations in the sodalite cages are significantly more sensitive to the n-alkane loading than those in the supercages. We provide a simple expression that adequately describes the n-alkane Henry coefficient and adsorption enthalpy as a function of sodium density and temperature at low coverage. This expression affords an adequate substitute for complex configurational-bias Monte Carlo simulations. The applicability of the force field is by no means limited to low pressure and pure adsorbates, for it also successfully reproduces the adsorption from binary mixtures at high pressure.