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Catalysts 2017, 7, 367 6 of 31 cracking reactions. It showed higher conversion of 80.4% and high selectivity toward C5–C11 of 58% for n-hexadecane cracking. ZSM-5@SAPO-34 zeolite with hierarchical pore sizes between 15 and 150 nm was prepared by partially dissolving the conventional ZSM-5 zeolite, and the acidity properties could be adjusted by varying the mass of ZSM-5 in the ZSM-5@SAPO-34, leading to higher catalytic performance than that of the parent ZSM-5 zeolite [28]. Li et al. [37] prepared the ZSM-5@KIT-1 composite zeolite with larger BET surface area and mesopore volume by adding ionic liquid as the mesoporous template during the conventional synthesis process. The precursor zeolite colloidal species were aged for different periods, followed by adding 1-hexadecyl-3- methylimidazolium bromide as the mesoporous template to form KIT-1 crystals around the surface of ZSM-5 crystals. Besides, the proportion of ZSM-5 zeolite and KIT-1 can be controlled facilely by varying the pre-aging time. ZSM-5@silicalite-1 zeolite was prepared by adding silicalite-1 as the shell. The surface acidity of ZSM-5 zeolite could be deactivated after adding silicalite-1 shell. It restrained the carbon depositing on the external surface, which avoided the decreasing accessibility of acid site in the pore channel. Overall, ZSM-5@silicalite-1 with passivated surface acid sites could enhance the catalytic stability and product selectivity without sacrificing its catalyst activity [38,39]. Vu et al. [40] prepared the nanosized-ZSM-5@SBA-15 composites through a two-step process. They firstly obtained the partially crystalline ZSM-5 crystals by pre-crystallization for certain time. Then the unreacted precursors are converted to ordered mesoporous SBA-15 by adding surfactant solution and followed by hydrothermal treatment at 200 ◦C for 24 h. The ZSM-5@SBA-15 zeolite exhibited superior catalytic activity due to its medium acid sites, improved molecular transport efficiency resulting from the hierarchical pore structure and good hydrothermal stability. This special morphology effectively suppressed the secondary reactions among primarily generated light alkenes, thus providing higher light alkene selectivity (93.2%) in the catalytic cracking of triglyceride-rich biomass [29,40]. 2.3. Nanosheet MFI Zeolite The nanosheet MFI zeolite has the feature of hierarchical pore structure and thin lamellar structure, which overcomes the defects of simple microporous and long diffusion pathway in conventional ZSM-5 zeolite simultaneously. In 2009, Ryoo et al. [12] successfully prepared the nanosheet MFI zeolite by using a long-chain diquaternary ammonium-type surfactant (C22H45-N+(CH3)2-C6H12-N+(CH3)2-C6H13, designated C22-6-6 hereafter) as the structure-directing agent (SDA). It showed higher and more stable catalytic cracking activity with less carbon deposition, even for large molecule. Thus, the nanosheet MFI zeolite has drawn much attention since then. Inspired by this conception, Hensen et al. [41] prepared highly acidic nanosheet ZSM-5 zeolite by replacing the hexyl end group with propyl of SDA. The crystallization rate increased because of the higher occupancy of the intersections of the MFI framework by propyl than that of hexyl. Its lifetime was nearly two times as high as the conventional ZSM-5 zeolite. Xu et al. [42] designed a single quaternary ammonium head amphiphilic template by introducing bi-quaternary ammonium head groups and biphenyl groups into the SDA ((C6H13-N+(CH3)2- C6H12-N+(CH3)2-(CH2)n-O-C6H4-C6H4-O-(CH2)n-N+(CH3)2-N+(CH3)2-C6H13(4Br−), designated BCPh-n-6-6 hereafter). It exhibited strong ordered self-assembling ability through π-π stacking in hydrophobic side, which induced the formation of single crystalline mesostructured nanosheet zeolite (Figure 4). Besides, the length of alkyl spacers between two charged nitrogen cations in SDA strongly affected the morphology of the final catalyst. Only Pr6-diquat-5 could induce the formation of sequentially inter-grown MFI zeolite due to its molecular dimension and stabilization energy, which resulted in the unusual fitting of Pr6-diquat-5 inside the channels of MFI zeolite [43].PDF Image | Strategies to Enhance the Catalytic Performance of ZSM-5
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