Abstract
HZSM-5 samples synthesized by rapid crystallization method possessed smaller crystallite sizes, and Al distributions in the crystallite were more uniform than in sample prepared by the conventional method. In 2,7-dimethylnaphthalene (2,7-DMN) conversion, a high isomerization activity and nearly 70% selectivity to 2,6-DMN were obtained on an HZSM-5 having a small and uniform crystallite size. On the other hand, both the activity and shape selectivity on large crystallites were low. This is because small crystallites have a larger external surface, which offers a higher number of pore entrances and enhances the reaction occurring inside the pore channels compared with large crystallites. Furthermore, it is found that the selectivity to 2,6-DMN should be 100% if there is no acid site on the external surface; in other words, the shape-selective isomerization of 2,7-DMN to 2,6-DMN on ZSM-5 is based on the product selectivity inside the pore channels. From this evidence, it is concluded that in 2,7-DMN isomerization to 2,6-DMN, the reaction activity is related to the crystallite size, whereas the shape selectivity is determined by the ratio of surface acid sites to total acid sites; in other words, uniformity of Al distribution. Therefore, a reduction in crystallite size and decrease in acid sites on the external surface are significant ways to improve catalytic activity and shape selectivity, respectively.
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