MEASUREMENT OF THE DIFFUSIVITIES WITHIN MFI-TYPE 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 pressu re method. In the case of Y-type zeolites, the diffusivities in the ad sorption 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 m inimum molecular size of the aromatics was larger than a benzene ring, the diffusivity in the adsorption process was much smaller than in th e desorption process. The diffusivities of paraffins of above six carb on atoms exhibited the same trend. The comparison of diffusivities bet ween adsorption and desorption processes yielded diffusion rates withi n the pores and at the pore mouths. The diffusivities and activation e nergies were found to be expressed by similar equations to those defin ed in the field of metallurgy for the diffusion of impurities containe d in a metal lattice. (C) 1998 Elsevier Science B.V. All rights reserv ed.