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Catalysts 2017, 7, 367 8 of 31 decreased with the increase of TPAOH concentration. Tsapatsis et al. [46] successfully prepared self-pillared MFI nanosheets by repetitive branching during one-step hydrothermal crystal growth with tetra(n-butyl)phosphonium hydroxide solution (TBPOH) as SDA. The nanosheets (2 nm thick) resembled and formed a house-of-cards construction. The short diffusion length and high external surface area promoted the reaction of large molecules. Srivastava et al. [47] prepared the nanosheet MFI zeolite by combining two structure directing agents of tetrapropylammonium bromide (TPABr) and C18-6-6 (Figure 5). TPABr induced the formation of microporous structure of MFI zeolite. The long hydrophobic chains of C18-6-6 facilitated the ordered stacking of microporous zeolite layers to produce nanosheet-like morphology. Besides, it restrained the excessive growth of zeolites and induced the formation of inter-crystalline mesopores. Figure 5. Proposed mechanism for the formation of ZSM-5 nanosheets in the presence of C18-6-6 and Figure 5. Proposed mechanism for the formation of ZSM-5 nanosheets in the presence of C18-6-6 and TPABr [47]. Copyright American Chemical Society, 2016. Besides, the nanosheet zeolite could act as an excellent supporter for loading metal Catalysts 2017, 7, 367 8 of 29 TPABr [47]. Copyright American Chemical Society, 2016. Besides, the nanosheet zeolite could act as an excellent supporter for loading metal nanoparticles nanoparticles to develop a multifunctional zeolite catalyst. It provides larger surface area to support to develop a multifunctional zeolite catalyst. It provides larger surface area to support the metal the metal nanoparticles, thus the nanoparticles could uniformly distribute within zeolite crystals and nanoparticles, thus the nanoparticles could uniformly distribute within zeolite crystals and prevent the prevent the aggregation among metal nanoparticles with the increase of loading amount. While for aggregation among metal nanoparticles with the increase of loading amount. While for conventional conventional ZSM-5 zeolite, they are only supported on the external surface, and it is easy to ZSM-5 zeolite, they are only supported on the external surface, and it is easy to aggregate resulting in aggregate resulting in the decrease of BET surface area or the blockage of pore channel [48]. the decrease of BET surface area or the blockage of pore channel [48]. Overall, to synthesize the nanosheet MFI zeolite, a suitable SDA is of great necessary. The tail of Overall, to synthesize the nanosheet MFI zeolite, a suitable SDA is of great necessary. The tail of SDA should be sufficiently hydrophobic to generate a bulky hydrophobic barrier in the micelle, so SDA should be sufficiently hydrophobic to generate a bulky hydrophobic barrier in the micelle, so that that the lamellar structure growth is confined around the hydrophilic tails and the nanosheet the lamellar structure growth is confined around the hydrophilic tails and the nanosheet thickness can thickness can be effectively controlled. It has been proven that the hydrophobic tail (CnH2n+1) should be effectively controlled. It has been proven that the hydrophobic tail (CnH2n+1) should be long enough be long enough (n = 10–22) to restrain the over growth of lamellar structure. When the hydrophobic (n = 10–22) to restrain the over growth of lamellar structure. When the hydrophobic tail is too short tail is too short (n < 8), the molecules of SDA are embedded inside the micropores, resulting in a (n < 8), the molecules of SDA are embedded inside the micropores, resulting in a blocky zeolite [49]. blocky zeolite [49]. The nanosheet MFI zeolite shows higher BET surface area, higher external surface area and larger The nanosheet MFI zeolite shows higher BET surface area, higher external surface area and mesopore volume than that of the conventional ZSM-5 zeolite, providing better textural properties for larger mesopore volume than that of the conventional ZSM-5 zeolite, providing better textural catalytic cracking reactions (Table 2). However, the duration of synthesis process is usually as long properties for catalytic cracking reactions (Table 2). However, the duration of synthesis process is as 5 days, which is time and energy-consuming in industrial manufacture. Therefore, developing a usually as long as 5 days, which is time and energy-consuming in industrial manufacture. Therefore, time-saving method to prepare the nanosheet MFI zeolite is of great significant. developing a time-saving method to prepare the nanosheet MFI zeolite is of great significant. Table 2. Textural properties of the conventional ZSM-5 and MFI nanosheet zeolites in the references. Sample SDA ZSM-5-bulk TPAOH Rh ZSM-5(6, 20, 423) C22-6-6(Br)2 SBET a (m2∙g−1) 373 685 Sext b (m2 g−1) 91 826 VTot c (cm3 g−1) 0.21 0.75 Vmic d (cm3∙g−1) 0.17 0.09 Vmes e (cm3∙g−1) 0.04 0.66 Tthic f (nm) - - T g (d) 3 [41] 5 [41] MFIPDF Image | Strategies to Enhance the Catalytic Performance of ZSM-5
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