A HIERARCHICAL ATOMISTIC LATTICE SIMULATION APPROACH FOR THE PREDICTION OF ADSORPTION THERMODYNAMICS OF BENZENE IN SILICALITE

A lattice model is presented for predicting the adsorption thermodynam ics 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 molecu le and the zeolite, as well as interactions between neighboring benzen e adsorbates. The development of the more coarse-grained lattice model from the full atomistic model is presented in detail, noting the appr oximations involved. The lattice model reproduces the adsorption isoth erms calculated previously from atomistic grand canonical Monte Carlo simulations, which were in good agreement with experiment, but the lat tice model is over an order of magnitude more efficient computationall y. Calculations were performed for the silicalite structure with Pnma symmetry (the so-called ORTHO structure) and with P2(1)2(1)2(1) symmet ry (PARA). The model accounts for the step seen in the experimental is otherms; this feature is ascribable to a transformation of the silical ite structure from the ORTHO form to the PARA form at a benzene occupa ncy of about four molecules per unit cell.