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Catalysts 2017, 7, 367 24 of 31 Rimer et al. [38] designed ZSM-5 zeolite with passivated surface acidity by adding silicalite-1 shell on the external surface. Few acid sites exist on the external surface of ZSM-5 zeolite after adding silicalite-1 shell. Consequently, the carbon deposition on the external surface can be suppressed, resulting in high product selectivity. The external acid sites were deactivated by deposition of MgO on the external surface of ZSM-5 zeolite [125]. Magnesium acetate is selected as the precursor, which polymerizes to large-molecule coordination compounds in aqueous solution and cannot enter into the micropores. Then, MgO is deposited on the external surface after calcination and deactivated the external acid sites. The amount of carbon deposition decreased after MgO modification. Besides, a mechanochemical approach using powder composer was used to deactivate the surface acid sites. Shear force was imposed on the surface and selectively deactivated the acid sites on external surface. After mechanochemical treatment, ZSM-5 zeolite exhibited little activity on TIPB cracking, indicating few acid sites existed on the external surface of ZSM-5 zeolite [126]. 6. Conclusions and Outlook Many studies have been done to enhance the catalytic cracking performance of ZSM-5 zeolite including catalytic cracking activity, selectivity of light alkenes and carbon deposition in recent years. In this review, factors affecting the catalytic performance of ZSM-5 zeolite in hydrocarbon catalytic cracking were discussed from the aspects of ZSM-5 zeolite with special morphology, hierarchical ZSM-5 zeolite, nano-sized ZSM-5 zeolite and acid properties. They are all effective ways to enhance the catalytic cracking performance of ZSM-5 zeolite. The diffusion length decrease and diffusion efficiency of molecule can be enhanced by preparing ZSM-5 zeolite with special morphology. Hierarchical ZSM-5 zeolite can be obtained by template-assisted method or post-treatment method. The accessibility of acid sites in the micropores is improved by the additional mesopores or macropores. Especially for post-treatment method, it is convenient to obtain ZSM-5 zeolite with hierarchical pore structure. Nano-sized ZSM-5 zeolites with short diffusion length favor the cracking of hydrocarbon. Acid property is crucial for catalytic cracking reactions. By adjusting the acid properties, the catalytic activity and light alkene selectivity can be improved efficiently and the formation of carbon deposition can be suppressed. Each method has its own advantage and disadvantage, it would be better to integrate the advantages of each method and develop ZSM-5 zeolite with superior activity. By modifying nano-sized ZSM-5 zeolite with elements or compounds, ZSM-5 zeolite has the feature of short diffusion pathway and moderate acid property simultaneously, which would result in high catalytic conversion rate, high light alkene selectivity and less carbon deposition during hydrocarbon cracking. Preparing nano-sized ZSM-5 zeolite with hierarchical pore structure could decrease the diffusion length as well as increase the accessibility of acid sites in the micropores. Moreover, developing bifunctional catalysts by modification of MFI nanosheet zeolite with another catalyst is feasible to enhance the catalytic performance of ZSM-5 zeolite. Above all, most of these strategies to enhance the catalytic cracking performance are still in laboratory scale. Overall, elimination of compounds potentially harmful for the environment such as the hard/soft templates and zeolite structure-directing agent (SDA) remains a great challenge. It is difficult to separate the nano-sized ZSM-5 zeolite in large scale and it is costly to synthesize nanosheet zeolite hydrothermally with long time. Few studies have been reported to offer the possibility to scale-up in a cost-effective way. Therefore, developing a cheap and effective way to prepare ZSM-5 zeolite with excellent activity is significate for manufacture application. Finally, the above approaches are not only effective for catalytic cracking of hydrocarbon, but also suitable for other catalytic reactions. By synthesis of ZSM-5 zeolite with special morphology, hierarchical zeolite and nano-sized zeolite, the diffusion efficiency, diffusion length and accessibility of acid sites can be enhanced and facilitate the catalytic reaction efficiency.PDF Image | Strategies to Enhance the Catalytic Performance of ZSM-5
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