Generation of shape-selectivity of p-xylene formation in the synthesized ZSM-5 zeolites

Quantitative study has been performed in order to reveal che generation of shape-selectivity of the zeolite ZSM-5 in the formation of p-xylene from the alkylation of toluene. More than 20 kinds of ZSM-5 species prepared at various hydrothermal conditions were characterized in views of crystal size, diffusion property, and the external surface acidity; then, the measured selectivity ac a constant conversion of toluene was correlated with them. The size of zeolite crystal was 0.5 to 6 mu m, and it was strongly correlated with the rate constant of adsorption of o-xylene. Distribution of acid sites in the zeolite crystal was estimated to be not homogeneous from measurements of acidity on the external surface. Selectivity in the p-xylene formation was clearly correlated with the rate constant of adsorption, except for only a species. It was thus concluded that the shape-selectivity strongly depended on the adsorption property on zeolites due to the size of crystal. Acidity on the external surface was not a prevailing factor, however, it was significant on the zeolite with the external surface acidity which was created by the nonframework aluminum atoms. (C) 1998 Academic Press.