INTRACRYSTALLINE DIFFUSIVITIES OF COKED HZSM-5 ZEOLITE

The high shape selectivity of HZSM-5 zeolite catalyst is affected strongly by the intracrystalline diffusivities of reactants and products. Carbonaceous materials, called coke, are deposited on the zeolite catalysts during reactions, resulting in the change in the magnitude of the diffusivity. The diffusivities of five aromatics and four paraffins within the HZSM-5 zeolite crystallite with different amounts of coke deposition were directly measured in the temperature range of 373-773 K. The diffusivity was found to decrease with the coke deposition (e.g. 70% at 2 wt% coke deposition). Furthermore, the network of the pores within the zeolite crystallite was assumed to be of lattice. Coke particles and hydrocarbon molecules were marked partially on lattice points, and the random movements of the molecules were simulated and the magnitude of the diffusivity was estimated. The obtained diffusivity was found to be in good agreement with the experimental data.