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catalysts Review Strategies to Enhance the Catalytic Performance of ZSM-5 Zeolite in Hydrocarbon Cracking: A Review Yajun Ji, Honghui Yang * and Wei Yan * The State Key Laboratory of Multiphase Flow in Power Engineering, Department of Environmental Science and Engineering, Xi’an Jiaotong University, Xi’an 710049, China; jiyajun928@stu.xjtu.edu.cn * Correspondence:yanghonghui@xjtu.edu.cn(H.Y.);yanwei@xjtu.edu.cn(W.Y.); Tel.: +86-29-8266-9033 (H.Y. & W.Y.) Received: 30 October 2017; Accepted: 21 November 2017; Published: 29 November 2017 Abstract: ZSM-5 zeolite is widely used in catalytic cracking of hydrocarbon, but the conventional ZSM-5 zeolite deactivates quickly due to its simple microporous and long diffusion pathway. Many studies have been done to overcome these disadvantages recently. In this review, four main approaches for enhancing the catalytic performance, namely synthesis of ZSM-5 zeolite with special morphology, hierarchical ZSM-5 zeolite, nano-sized ZSM-5 zeolite and optimization of acid properties, are discussed. ZSM-5 with special morphology such as hollow, composite and nanosheet structure can effectively increase the diffusion efficiency and accessibility of acid sites, giving high catalytic activity. The accessibility of acid sites and diffusion efficiency can also be enhanced by introducing additional mesopores or macropores. By decreasing the crystal size to nanoscale, the diffusion length can be shortened. The catalytic activity increases and the amount of carbon deposition decreases with the decrease of crystal size. By regulating the acid properties of ZSM-5 with element or compound modification, the overreaction of reactants and formation of carbon deposition could be suppressed, thus enhancing the catalytic activity and light alkene selectivity. Besides, some future needs and perspectives of ZSM-5 with excellent cracking activity are addressed for researchers’ consideration. Keywords: ZSM-5 zeolite; hydrocarbon; catalytic cracking; morphology; hierarchical zeolite; nano-sized zeolite; acid property 1. Introduction Zeolites are crystalline aluminosilicate fabricated by silica tetrahedron and alumina tetrahedron through oxygen bridges. They are widely applied in petrochemical chemistry [1,2], environmental protection [3,4] and adsorption [5] because of high BET (Brunauer-Emmett-Teller) surface area, special channel structure, abundant acid sites, and thermal and hydrothermal stability [6]. Catalytic cracking of hydrocarbon is significant for industrial manufacture, due to the advantages of high cracking conversion efficiency, high light alkene selectivity and less carbon deposition compared with thermal cracking [7]. Among the catalysts for hydrocarbon catalytic cracking, ZSM-5 zeolite is the most widely used due to its particular advantages of its acidity, special pore structure, and high thermal and hydrothermal stability [8,9]. When it was applied in n-hexane cracking under high pressure, it showed better catalytic activity and stability, higher light alkene selectivity and less sensitivity to deactivation by carbon deposition than H-MOR, H-BEA and USY zeolites [10]. The conventional ZSM-5 zeolite, however, is also easily deactivated by carbon deposition because of its simple microporous structure, long diffusion pathway and redundant strong acid sites. Although strong acid sites are crucial for catalytic reactions, especially for hydrocarbon cracking, the reactants would over-react on the acid sites and convert to aromatic hydrocarbons or carbon deposition if too many acid sites exist, resulting in the deactivation of catalyst. Moreover, as only microporous structure exists in conventional ZSM-5 zeolite, the diffusion efficiency of molecule is Catalysts 2017, 7, 367; doi:10.3390/catal7120367 www.mdpi.com/journal/catalystsPDF Image | Strategies to Enhance the Catalytic Performance of ZSM-5
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