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Catalysts 2017, 7, 367 14 of 31 Although hard-template method is an effective method to prepare hierarchical ZSM-5 zeolite and adjust the texture properties by varying the number of hard templates, large number of hard templates is necessary to generate enough mesopores. Besides, much energy would cost to burn out the hard templates. Overall, the hard-template method is costly and it is still at the stage of laboratory study. 3.2. Double Templating with Soft-Template Method Usually, another relative large organic ammonium salt is used as the second template to synthesize hierarchical ZSM-5 zeolite. By using the amphiphilic organosilanes ([(CH3O)3SiC3H6N(CH3)2CnH2n+1]Cl, TPHAC) as mesopore-directing agent, mesoporous MFI zeolite was synthesized and showed better activity especially for large molecules. The mesopore diameters can be facilely adjusted from 2 nm to 20 nm by changing the chain length of amphiphilic organosilanes [85]. Bai et al. [72] prepared scroll-like ZSM-5 zeolite with micropores, mesopores and macropores by layer-by-layer wrapping of zeolite nanosheets with the assistance of n-hexyltrimethylammonium bromide (HTAB). A coulombic interaction existing between HTAB and the negatively charged aluminosilicate species stabilized the zeolite nanoparticles by decreasing their surface free energy, and facilitates the formation of hierarchical ZSM-5 zeolite at short crystallization time. Wu et al. [73] prepared hierarchical ZSM-5 zeolite using cetyltrimethylammonium bromide (CTAB) and 1, 6-diaminohexane (DAH) as structure-directing agents. The mesoporous silicoalumina species generated from the aluminosilicate precursor by the inducing of CTAB firstly. Then, it was transformed to hierarchical ZSM-5 zeolite with the assistance of DAH. The encapsulated CTAB could restrain the excessive growth and dissolution of the initially formed mesoporous particles, and preserving the large mesoporosity. Hensen et al. [86] prepared hierarchical ZSM-5 zeolite by combining an amphiphilic surfactant C16MP with 1, 6-diaminohexane (DAH) as the structure-directing agents. As there was suppression reaction between two different templates during the crystal growth, it was crucial and difficult to regulate the ratio of two templates to induce the crystal growth and mesopores generation. Cationic dimethyldiallyl ammonium chloride acrylamide copolymer (PDDA) solution can also be used as soft-template to prepare hierarchical ZSM-5 zeolite. The total volume, mesopore volume and external surface area increased with the increase of PDDA content [87]. Overall, it is feasible to synthesis hierarchical ZSM-5 by double templates method if two templates play synergistic effect other than inhibiting effect. However, the cost would also increase with the addition of another SDA. 3.3. Single Templating Method To avoid the suppression reaction between two different templates during the crystal growth and decrease the preparation cost, many studies have been conducted to prepare hierarchical ZSM-5 zeolite with single template. Wu et al. [74] prepared mesoporous ZSM-5 microspheres using n-hexylamine as the template. With the direction induction of n-hexylamine, small crystals generated and assembled to uniform cauliflower-like microspheres successively. Besides, the average diameter of the mesoporous zeolite could be effectively regulated by adjusting the alkalinity of the preparation solution. The mesoporous ZSM-5 zeolites with improved diffusion efficiency and acids accessibility showed better performance than the conventional ZSM-5 zeolite. TPAOH, a conventional microporous template, can be used as the single SDA to prepare hierarchical ZSM-5 zeolite [75,76]. Hua et al. [76] successfully prepared the hierarchical ZSM-5 zeolite using only TPAOH as the SDA through one-step hydrothermal process at a relatively low temperature of 100 ◦C, and they proposed a “nucleation/growth-demetallation/recrystallization” mechanism during the hydrothermal synthesis process. Initially, the precursor species nucleated and grew under the protection of aluminum complexes and TPA+ cations. Then basic etching became obvious with the consumption of aluminum complexes and TPA+ cations, resulting in the mesoporous structure. Finally, the dissolved species recrystallized and grew on the external surface of the zeolite crystals (Figure 11). During the catalytic cracking of 1, 3, 5-triisopropylbenzene (TIPB), the hierarchical ZSM-5 zeolite showed conversion ratePDF Image | Strategies to Enhance the Catalytic Performance of ZSM-5
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