(735e) Synthesis of Hierarchical Mesoporous Zeolites and Their Catalytic Activity

Due to the intrinsic ordered micropore structures and unique catalytic activity, zeolites are extensively used as heterogeneous acid catalysts in crude oil refineries and petrochemical processes. However, the micropore structures and high intrinsic activities frequently lead these materials to be subject to diffusion limitations that restrict reactant accessibility to the active sites on the interior surfaces of zeolites and inhibit the full utilization of zeolite catalysts.

Synthesis of hierarchical zeolites with mesoporosity is a proven strategy to reduce the diffusion limitation in zeolite catalysts. The presence of mesopores in zeolite catalysts can enhance the mass transfer and reduce the residue time of molecules in the micropores by shortening their diffusion length. Both catalytic activity and catalytic life time of zeolite catalysts can be significantly improved by the strategy. Previous research on the field has revealed that in order to efficiently reduce the diffusion limitation such hierarchical zeolites should have the following features: (1) highly crystalline microporous structures for retaining shape selectivity and hydrothermal stability of zeolites; (2) interconnected and accessible mesopore structures for improving mass transport; (3) controllable acidity (e.g. concentration and strength of acid sites). 

In this study, we develop two different methods for the synthesis of hierarchical mesoporous zeolites with all three features (controllable mesoporosity, microporosity and acidity). Using confined space synthesis method, three dimensionally ordered mesoporous imprinted (3DOm-i) ZSM-5 with a wide range of Si/Al from 25 to 80 is synthesized. Hierarchical ZSM-5 with disordered mesopores is also synthesized under hydrothermal conditions without using any hard template. The effect of mesoporosity on the catalytic activity is examined by the etherification of benzene alcohol. It is found that the presence of mesoporosity can significantly improve the effectiveness factor of the zeolite catalysts for the etherification reaction by reducing the diffusion length in the catalysts. The improved performance of the hierarchical zeolite catalysts is not only determined by the mesopore sizes but also affected by interconnection of the mesopores.