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Catalysts 2017, 7, 367 18 of 31 Table 4. Textural properties of nano-sized ZSM-5 zeolite in the references. SBET b Sext c VTot d Vmic e Vmes f g (m2·g−1) (m2·g−1) (cm3·g−1) (cm3·g−1) (cm3·g−1) R a 253 4.1. Synthesis of Nano-Sized ZSM-5 Zeolite by Hydrothermal Method Hydrothermal method is the most widely used process to prepare nano-sized ZSM-5 zeolite. The amount of water, pre-aging time and pre-aging temperature are closely related to the crystal size. Usually, the zeolite crystal size decreases with the increase of pre-aging time and pre-aging temperature, which could facilitate nucleation of nanocrystals in the pre-aging stage. The nano-sized ZSM-5 zeolites showed a better catalytic activity, a longer lifetime and less carbon deposition compared with large-sized ZSM-5 zeolite during n-hexane cracking [98]. ZSM-5 zeolite with uniform size of 30 nm was prepared by pre-aging at low temperature (80 ◦C) and then high temperature hydrothermal treatment (175 ◦C) [109]. The concentration of initial synthesis gel facilitated the generation of nucleation centers. At pre-aging stage, the number of critical sized nuclei increased. Finally, the high temperature promoted the complete growth of crystals. Wang et al. [105] developed a facile route to prepare nano-sized ZSM-5 zeolite using silicate-1 as seeds with the assistance of CTAB. The seeds induced the aluminosilicates into ZSM-5 phase on their surface. While CTAB could restrain the intergrowth and secondary growth of crystal nucleus, thus large number of nanocrystals formed and assembled into zeolite aggregates. The prepared ZSM-5 zeolites showed a uniform morphology with 400–600 nm, aggregation with 20–50 nm crystals, a large external surface area (273 m2·g−1) and large mesopore volume (0.31 cm3·g−1) (Figure 14). Besides, if there is no metal cation in the synthesis gel, the proton formed ZSM-5 zeolite can be directly obtained simply by calcining the products after hydrothermal treatment without ion exchange with NH4NO3 [87]. It was a time-saving and energy-saving procedure. Catalysts 2017, 7, 367 18 of 29 Figure 14. The morphology and textural property of N-ZSM-5: (a–c) SEM images under different Figure14.Themmagonirfipcahtionlo;g(dy)Na2naddstoerpxttiounr/daelsoprprtoiopneirstoytheormfNan-dZBSJMH-p5o:re(asi–zec)diSstEriMbutioimn;a(eg)elsowunderdifferent magnification TEM image; and (f) high magnification TEM image [105]. Copyright Royal Society of magnification; (d) N2 adsorption/desorption isotherm and BJH pore size distribution; (e) low magnification TEM image; and (f) high magnification TEM image [105]. Copyright Royal Society Zeolite synthesized using SDA is uneconomical and environmental unfriendly due to vast Sample H2O/Si = 30 H2O/Si = 8.3 H-MFI(1) Z-0.02C-0.08S B-2 ZNA-373 Z60-AT P 106 40–45 20–50 - 20–40 30–500 400 403 - 416 424 536 430 40 0.28 0.17 49 0.56 0.17 - 0.30 0.12 271 0.30 0.07 42 0.21 0.13 403 0.89 0.05 0.11 [98] 0.39 [98] 0.18 [99] 0.13 [105] 0.08 [106] 0.84 [107] 79 - - - [108] a Particle size; b BET surface area; c External surface area; d Total volume; e Micropore volume; f Mesopore volume; g Reference. of Chemistry, 2016. Chemistry, 2016. exhaust gas generated by calcination, thus it is significant to prepare nano-sized ZSM-5 zeolite without using SDA. Larsen et al. [110] successfully synthesized nano-sized ZSM-5 zeolite with the assistance of seeds by regulating the synthesis parameters. A basic solution with pH between 9 and 11.5 was necessary to facilitate the nucleation of crystals. A proper amount of seeds (0.35 wt %) provides enough nucleation and growth sites. The optimum temperature and time is 165 °C and 14– 24 h to promote the complete growth of crystals. Nano-sized ZSM-5 zeolite prepared with seed-assisted method showed uniform crystal size (70–150 nm), high crystallinity and high hydrothermal stability. They showed a better activity and higher light alkene selectivity than the conventional ZSM-5 zeolite in catalytic cracking of naphtha (Figure 15) [106].PDF Image | Strategies to Enhance the Catalytic Performance of ZSM-5
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