BLUEMOON SIMULATIONS OF BENZENE IN SILICALITE-1 - PREDICTION OF FREE-ENERGIES AND DIFFUSION-COEFFICIENTS

Constrained reaction coordinate (Bluemoon) dynamics have been used to characterise the free energy profile of benzene in silicalite-1 at 300 K along the mean reaction path for diffusion. The reaction path was f ound empirically by fitting a parametric curve through the mean positi ons of the benzene centre of mass. Both rigid and flexible zeolite lat tices have been investigated. In both cases the primary adsorption sit e was located at the intersection of the straight and sinusoidal chann els. Two other distinct adsorption sites exist in the straight channel and four in the sinusoidal channel. Lattice flexibility was found to have a very strong influence on the relative free energies of the adso rption sites and on the free energies of the transition states connect ing them. The free energies, combined with estimates of the transmissi on coefficient, Ic, were used to obtain rate constants for diffusion b etween the main adsorption sites in the flexible lattice. Subsequent d iffusive Monte Carlo simulations, using these parameters, provided the self-diffusion coefficient, D, and its components, at 300 K. We obtai ned D = 3.36 x 10(-14) m(2) s(-1), in excellent accord with the best e xperimental value of 2.2 x 10(-14) m(2) s(-1). Diffusion was fastest i n the Y direction with the D-yy/D-xx = 1.49 and D-yy/D-xx = 5.65 being observed.