Diffusion and reaction in ZSM-5 studied by dynamic Monte Carlo

We have investigated the effects of occupancy and blocked sites on the diffusivity of penetrants in a three-dimensional lattice model of ZSM-5. Using our model, which is a realistic, coarse-grained description of the zeolite, we find that diffusivity decreases more strongly with occupancy and fraction of blocked sites than on a two-dimensional square or three-dimensional cubic lattice. The percolation transition for the ZSM-5 lattice occurs at an occupancy of 0.64, larger than that in a two-dimensional square lattice. We have also studied the effects of occupancy and of discrete, quenched reaction sites on the rates of the net reaction process in ZSM-5. The reaction studied is A ⇆ B, where A and B are assumed to have identical adsorption and diffusion properties. We show that there is an optimal fraction of reaction sites. This optimum is a result of the trade-off between increased ability for the reaction to proceed and decreased effectiveness of reaction sites as a consequence of reduced diffusive transport of species to and from the reaction sites. We have also found that lower occupancy of the zeolite enhances the rate of reaction by reducing the fraction of reaction sites occupied by adsorbed species.