Tailoring hierarchical zeolite composites with two distinct frameworks for fine-tuning product distribution in benzene alkylation with ethanol
The hierarchical zeolite composite with two distinct frameworks, the MOR@ZSM-5, has been successfully fabricated via a repeated crystallization of ZSM-5 nanocrystals on mordenite surfaces. To avoid their phase separation, the surface of mordenite was pretreated with tetra(n-butyl)ammonium hydroxide (TBAOH) to introduce the formation of ZSM-5 nuclei, and it was subsequently modified by a continuous growth of nanocrystalline ZSM-5 on the entire area of mordenite surfaces. Interestingly, the fully overgrown MOR@ZSM-5 composite exhibits the remarkable improvement in ethylbenzene selectivity (> 60%) obtained from alkylation of benzene with ethanol with respect to that of isolated zeolites, and their physical mixture due to the enhanced external surface area and hierarchical porosity as well as a reasonable acidity provided by fully dispersed ZSM-5 nanocrystals on mordenite surfaces. Moreover, coke species deposited on the designed composites are likely located at external surfaces and do not considerably deteriorate the catalytic performance, whereas they are deposited predominantly in micropores over the incompletely overgrown MOR@ZSM-5 composite. The present study illustrates the advantages of the overgrown zeolite composites of two incompatible frameworks in tailoring the hierarchical porosity, adjusting the acid properties, and eventually controlling the product selectivity in acid-catalyzed reactions such as alkylation of benzene with ethanol.