Measurement of the diffusivities within MFI- and Y-type zeolite catalysts in adsorption and desorption processes

Diffusivities of aromatics and paraffins within crystals of MFI- and Y-type zeolites in desorption and adsorption processes were measured in the temperature range from 423 to 673 K by the constant volume method, and by a newly developed method: the desorption under reduced pressure method. In the case of Y-type zeolites, the diffusivities in the adsorption and desorption processes were equal to each other because the pore size was larger than the molecular size. In the case of MFI-type zeolites, when the aromatics had the same minimum molecular size as a benzene ring, there were no differences in diffusivities between the adsorption process and the desorption process. In contrast, when the minimum molecular size of the aromatics was larger than a benzene ring, the diffusivity in the adsorption process was much smaller than in the desorption process. The diffusivities of paraffins of above six carbon atoms exhibited the same trend. The comparison of diffusivities between adsorption and desorption processes yielded diffusion rates within the pores and at the pore mouths. The diffusivities and activation energies were found to be expressed by similar equations to those defined in the field of metallurgy for the diffusion of impurities contained in a metal lattice. (C) 1998 Elsevier Science B.V. All rights reserved.