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 found empirically by fitting a parametric curve through the mean positions of the benzene centre of mass. Both rigid and flexible zeolite lattices have been investigated. In both cases the primary adsorption site was located at the intersection of the straight and sinusoidal channels. 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 adsorption sites and on the free energies of the transition states connecting them. The free energies, combined with estimates of the transmission coefficient, Ic, were used to obtain rate constants for diffusion between the main adsorption sites in the flexible lattice. Subsequent diffusive Monte Carlo simulations, using these parameters, provided the self-diffusion coefficient, D, and its components, at 300 K. We obtained D = 3.36 x 10(-14) m(2) s(-1), in excellent accord with the best experimental value of 2.2 x 10(-14) m(2) s(-1). Diffusion was fastest in the Y direction with the D-yy/D-xx = 1.49 and D-yy/D-xx = 5.65 being observed.