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Catalysts 2017, 7, 367 4 of 3219 Another route to prepare hollow ZSM-5 zeolite is to selectively dissolve silicon components Another route to prepare hollow ZSM-5 zeolite is to selectively dissolve silicon components with with alkali solution. When ZSM-5 zeolite is prepared with the assistance of organic alkali solution. When ZSM-5 zeolite is prepared with the assistance of organic structure-directing structure-directing agent, the aluminum and silicon distribute heterogeneously, with aluminum rich agent, the aluminum and silicon distribute heterogeneously, with aluminum rich surface and silicon surface and silicon rich core. Thus, the silicon rich core is preferentially dissolved by alkali solution rich core. Thus, the silicon rich core is preferentially dissolved by alkali solution to generate hollow to generate hollow structure. Hollow structural ZSM-5 zeolite was obtained by sodium hydroxide structure. Hollow structural ZSM-5 zeolite was obtained by sodium hydroxide solution treatment by solution treatment by Groen et al. [33]. The shell of this hollow ZSM-5 zeolite was heterogeneous Groen et al. [33]. The shell of this hollow ZSM-5 zeolite was heterogeneous due to the non-uniformity due to the non-uniformity of the precursor and/or the severe dissolution of the zeolite crystals by the of the precursor and/or the severe dissolution of the zeolite crystals by the sodium hydroxide solution. sodium hydroxide solution. When the crystal size of ZSM-5 zeolite was below 100 nm, it was feasible When the crystal size of ZSM-5 zeolite was below 100 nm, it was feasible to obtain hollow zeolite with to obtain hollow zeolite with uniform shell thickness [21]. Xie et al. [20] prepared the regular ZSM-5 uniform shell thickness [21]. Xie et al. [20] prepared the regular ZSM-5 microboxes through sodium microboxes through sodium carbonate treatment, which was a milder method than sodium carbonate treatment, which was a milder method than sodium hydroxide treatment. Guo et al. [22] hydroxide treatment. Guo et al. [22] prepared hollow ZSM-5 single crystals by hydrothermal prepared hollow ZSM-5 single crystals by hydrothermal treatment in tetrapropylammonium hydroxide treatment in tetrapropylammonium hydroxide solution (TPAOH). The silicon in the interior of the solution (TPAOH). The silicon in the interior of the zeolite was selectively dissolved and recrystallized on zeolite was selectively dissolved and recrystallized on the exterior. They also proved that this the exterior. They also proved that this post-treatment method was only effective on zeolites with Si/Al post-treatment method was only effective on zeolites with Si/Al ratio ranged from 40 to 74 and the ratio ranged from 40 to 74 and the nanocrystal size ranged from 150 to 330 nm (Figure 2). It indicated that nanocrystal size ranged from 150 to 330 nm (Figure 2). It indicated that the selective dissolution of the selective dissolution of silicon components was limited to specific Si/Al ratio and nano-sized zeolites. silicon components was limited to specific Si/Al ratio and nano-sized zeolites. Besides, it was Besides, it was wasteful in industrial application, because much aluminosilicate was dissolved. wasteful in industrial application, because much aluminosilicate was dissolved. Figure 2. TEM images of ZSM-5 with different parent Si/Al ratios after TPAOH treatment at 170 °C Figure 2. TEM images of ZSM-5 with different parent Si/Al ratios after TPAOH treatment at 170 ◦C for for 72 h. (a,b) The parent Si/Al ratio is 40, and the concentrations of TPAOH are 0.1 and 0.2 M, 72 h. (a,b) The parent Si/Al ratio is 40, and the concentrations of TPAOH are 0.1 and 0.2 M, respectively. respectively. (c,d) The parent Si/Al ratio is 59, and the concentrations of TPAOH are 0.1 and 0.2 M, (c,d) The parent Si/Al ratio is 59, and the concentrations of TPAOH are 0.1 and 0.2 M, respectively. respectively. (e,f) The parent Si/Al ratio is 79, and the concentrations of TPAOH are 0.2 and 0.5 M, (e,f) The parent Si/Al ratio is 79, and the concentrations of TPAOH are 0.2 and 0.5 M, respectively [22]. respectively [22]. Copyright Wiley-VCH Verlag GmbH and Co., 2015. Copyright Wiley-VCH Verlag GmbH and Co., 2015. 2.2. Zeolitic Composites of ZSM-5 and Other Materials 2.2. Zeolitic Composites of ZSM-5 and Other Materials As only micropores exist and the diffusion pathway is too long for the conventional ZSM-5 As only micropores exist and the diffusion pathway is too long for the conventional ZSM-5 zeolite, many studies focused on synthesis of composite zeolites using ZSM-5 zeolite as the zeolite, many studies focused on synthesis of composite zeolites using ZSM-5 zeolite as the functional functional component to enhance the catalytic cracking activity, such as ZSM-5@MCM-41, component to enhance the catalytic cracking activity, such as ZSM-5@MCM-41, ZSM-5@SBA-15, ZSM-5@SBA-15, ZSM-5@SAPO-34, ZSM-5@KIT-1 and so on. The composite zeolites not only ZSM-5@SAPO-34, ZSM-5@KIT-1 and so on. The composite zeolites not only maintained the intrinsic maintained the intrinsic activity of ZSM-5 zeolite, but also showed synergistic effect with other activity of ZSM-5 zeolite, but also showed synergistic effect with other materials. For catalytic cracking materials. For catalytic cracking of hydrocarbon, a second zeolite was usually introduced to increasePDF Image | Strategies to Enhance the Catalytic Performance of ZSM-5
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