THE SILICATE P-XYLENE SYSTEM .2. THE CORRECT CHOICE FOR STRUCTURAL AND THEORETICAL PARAMETERS IN COMPUTER-SIMULATIONS OF NONBONDED SORBENT SORBATE INTERACTIONS

Diffusion of p-xylene in silicalite is simulated by using the Buckingham exp-6 model. The best agreement with experimentally determined activation energies for the diffusion (25-30 kJ/mol) is obtained when the framework atoms are relaxed (24+/-2 kJ/mol). Study of the interaction energy versus the rotation angle of p-xylene along its long molecular axis (010 direction) shows the existence of a single minimum corresponding to a stable orientation of the molecule at the channel intersection (site I). The importance of including the Si-H interactions between the sorbent and the sorbate is emphasized : neglecting these interactions (the widely used Lennard-Jones model, where ''the crucial contact is the O...H distance'') yields overestimated sorption energies and two possible orientations for the p-xylene molecule at site I. A significant simplification of the computer simulations might be achieved by considering only the Si ... hydrocarbon interactions : in that case the calculated energies have to be multiplied by approximate to 2.5. Accordingly, as already shown by Si-29 CP-MAS nmr, the crucial contact seems rather to be Si...H.