Hierarchical atomistic/lattice simulation approach for the prediction of adsorption thermodynamics of benzene in silicalite

A lattice model is presented for predicting the adsorption thermodynamics of benzene in silicalite. The parameters for the lattice model are obtained from a detailed atomistic representation of benzene and the silicalite structure. The parameters describe the free energies in the various adsorption sites due to interactions between a benzene molecule and the zeolite, as well as interactions between neighboring benzene adsorbates. The development of the more coarse-grained lattice model from the full atomistic model is presented in detail, noting the approximations involved. The lattice model reproduces the adsorption isotherms calculated previously from atomistic grand canonical Monte Carlo simulations, which were in good agreement with experiment, but the lattice model is over an order of magnitude more efficient computationally. Calculations were performed for the silicalite structure with Pnma symmetry (the so-called ORTHO structure) and with P2₁2₁2₁ symmetry (PARA). The model accounts for the step seen in the experimental isotherms; this feature is ascribable to a transformation of the silicalite structure from the ORTHO form to the PARA form at a benzene occupancy of about four molecules per unit cell.