Rates of reaction and product-distributions for small and large pellets and for small and large crystals of the same zeolite (H-ZSM-5) were studied in order to evaluate the effect of diffusion concentration-gradients upon the conversion of (CH3)2O to hydrocarbons. The diffusivities of (CH3)2O and C6H6 in the zeolite crystals were deduced from sorption kinetics. The diffusivity of benzene at 293K was 2.30 x 10-10cm2/s. Macropore-diffusion affected activity and selectivity if pellets with a diameter above 2mm were used: intracrystalline mass-transfer did not seem to be important with respect to catalyst-activity and selectivity at temperatures below 600K. The conventional model for diffusion and reaction in porous catalysts could not be applied to the entire reaction-network in the zeolite-crystals, because the migration of olefins in the zeolite could not be treated as random-walk diffusion. Diffusion and Catalytic Reaction in Zeolite ZSM-5. J.Heering, M.Kotter, L.Reikert: Chemical Engineering Science, 1982, 37[4], 581–4

Table 61

Diffusion Parameters for Butanes in ZSM-5 Membranes