HZSM-5ゼオライト触媒によるトルエンのメチル化反応の形状選択性の解析 : 結晶内および外表面の寄与

HZSM-5 Zeolites

A model is developed which describes the activity and shape selectivity for the methylation of toluene to produce xylene isomers over HZSM-5 zeolite catalysts by taking account of both intracrystalline diffusion and acid strength distributions inside and outside crystallite. Effective intracrystalline diffusivities were directly measured by use of high-silica HZSM-5 zeolite which had no catalytic activity for the methylation of toluene in a high temperature range (473 - 673 K). The ratio among the diffusivities of p-, m- and o-xylene was about 10:1:1 in the above temperature range. The acid sites inside and outside the crystallite were discriminated from each other, and each acid strength distribution function was measured by combining the two methods developed by ourselves. Rate constants of methylation and isomerizations of xylenes inside and outside the crystallite were estimated by performing these reactions over two kinds of catalysts : one has acid sites only inside the crysatllite, the other only outside the crystallite. Using these values, the proposed model was found to predict satisfactorily the apparent shape selectivity. A chart (Fig. 11) based on the proposed model is also presented which gives a guide in preparing highly selective HZSM-5 catalysts for the methylation of toluene.