Measurement of Intracrystalline Diffusivities of Hzsm-5 Zeolite At Higher Temperatures and Predictions of Shape Selectivity

Abstract

Abstract A model is developed which describes the activity and the shape selectivity of methylation of toluene to produce xylenes over HZSM-5 zeolite catalysts by taking account of both intracrystalline diffusion and acid strength distributions inside and outside the crystallite. The shape selectivity is strongly affected by the relative rates of intracrystalline diffusions of hydrocarbon molecules. Uptake curves of amounts of benzene, toluene and xyleneisomers adsorbed on high-silica HZSM-5 zeolites, which have no catalytic activities, were measured in the range of temperature from 373–673 K. Effective intracrystalline diffusivities were calculated from the uptake curves over the temperature range. The ratio among the diffusivities of para-, meta-and ortho-xylene was about 10:1:1 in the above temperature range. Each acid strength distribution of acid sites inside and outside the crystallite was measured by combining the two methods developed by us. Rate constants of methylation and isomerizations of xylenes inside and outside the crystallite were estimated by performing these reactions over two kinds of the catalysts: one has acid sites only inside the crystallite, the other only outside the crystallite. Using these values, the proposed model was found to predict well the apparent shape selectivity. A chart based on the proposed model was also presented which provides a guide in preparing more highly selective HZSM-5 catalysts for the methylation of toluene.