A facile approach for construction of hierarchical zeolites via kinetics

It has long been a dream to construct hierarchical zeolites by a really simple and cost-effective method which is feasible in industry. Here we provide an alternative approach in which the crystallization kinetics is regulated by solely changing the added amount of NaOH in the initial gel of a traditional zeolite and then the adjustment of the morphology and structure of the zeolite products is realized. This approach overcomes most of the limitations associated with the popular methods, such as complex operations and costly secondary templates. As an example, hierarchical ZSM-5 zeolite is prepared with using the most common raw materials and synthesis technology which used in the large-scale production of traditional zeolites. The crystallization process of the hierarchical ZSM-5 zeolite is investigated in detail and compared with the microporous zeolite. The results indicate that the increase of NaOH concentration in the synthesis system can enhance the rate of nucleation but the hierarchical pore structure can be formed only under the moderate concentration. The crystallization of the hierarchical ZSM-5 zeolite takes place by a chain of processes: (i) formation of wormlike precursors, (ii) aggregation of the precursors, (iii) condensation of the precursors and formation of internal voids, (iv) formation of external shell layers. The performance studies show that the obtained hierarchical ZSM-5 zeolite possesses significantly enhanced diffusion property and superior ability for catalytic cracking bulky molecules.