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Catalysts 2017, 7, 367 12 of 31 Table 3. Textural properties of hierarchical ZSM-5 zeolite in the references. Sample MFI-PVA(3.0) SAC-20 HZ-32-170C-2d-13 MZ5-FS-0.1-60 ZM-3 Hi-ZSM-5 HSZs-120 DS-2 HM27_AcT_AT MFI27_6 SBET a Sext b VTot c Vmic d Vmes e f (m2·g−1) (m2·g−1) (cm3·g−1) (cm3·g−1) (cm3·g−1) R 345 395 682 426 377 433 427 391 450 363 487 140 0.33 -- [70] 132 0.35 - 0.84 112 0.46 119 0.24 133 0.30 158 0.53 135 0.33 136 0.42 - 0.38 - 0.80 0.12 0.23 [71] 0.17 0.67 [72] 0.14 0.32 [73] 0.12 0.12 [74] 0.12 0.18 [75] 0.12 0.41 [76] 0.11 0.22 [77] 0.13 0.29 [78] 0.08 0.3 [79] 0.12 0.68 [80] During the synthesis process of ZSM-5 zeolite, hard templates are used to be embedded within the zeolite crystals. After calcination, the hard templates were burnt out and large number of mesopores was generated. The ratio of mesopore volume to micropore volume can be adjusted by varying the number of hard templates. Viswanadham et al. [81] prepared hierarchical ZSM-5 zeolite using glucose as the second template through a steam-assisted crystallization process. Glucose was partially carbonized during the drying of synthesis gel at 170 ◦C in air and then acts as the hard template embedding in the zeolite crystals. The BET surface area, porosity and acidity are correlated with the concentration of glucose in the initial synthesis solution. Miyake et al. [70] selected poly vinyl alcohol (PVA) as hard template to synthesize hierarchical ZSM-5 zeolite with uniform mesopores (15 nm) by hydrothermal method. A proper amount of PVA was necessary during the synthesis process. At low content, the PVA molecules were not embedded in zeolite crystals to generate mesopores. On the contrary, large PVA would restrain the nucleation and growth of crystals. Recently, carbon nanotubes (CNTs) were applied to prepare hierarchical ZSM-5 zeolite. The mesopores were created not via occupying the crystals by CNTs, but preserved from nucleation and initial crystallization process. Besides, the CNTs can act as inhibitors in the steam-assisted crystallization process, preventing the excessive aggregation of zeolite crystals from generating large crystals. Large inter-crystalline mesopores were generated [82]. Liu et al. [71] studied the different roles of CNTs in hierarchical ZSM-5 zeolite preparation process by hydrothermal synthesis (HTS) and steam-assisted crystallization (SAC) methods. The crystallization rate in SAC method was lower than that in HTS method after adding CNTs, resulting in a lower Si/Al ratio and higher acid amounts in the final products in SAC method. It was explained that the growth of zeolite crystals during SAC method was via reorganization of hydrogel through solid–solid transformations. The reorganization process could be enhanced by decreasing the energy barrier with the assistance of CNTs. Moreover, more mesopores were generated using SAC method than that using HTS method, which led to higher and more stable catalytic activity in n-decane cracking. The conversion efficiency in n-decane cracking was over 90% after reaction for 11.4 h, but that of the conventional ZSM-5 zeolite was 35% (Figure 9). Jiang et al. [83] firstly prepared the hydrophilic carbon to enhance the dispersion in water by sodium hypochlorite solution treatment. Then the hydrophilic carbon was used as the hard template to prepared ZSM-5 zeolite with uniformed mesopores (Figure 10). Activated carbon (AC) was also applied in the preparation hierarchical ZSM-5 zeolite by embedding within the zeolite crystals followed by combustion [84]. AT 0.2-PI 0.01 a BET surface area; b External surface area; c Total volume; d Micropore volume; e Mesopore volume; f Reference. 3.1. Double Templating with Hard-Template MethodPDF Image | Strategies to Enhance the Catalytic Performance of ZSM-5
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