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Catalysts 2017, 7, 367 5 of 31 Catalysts 2017, 7, 367 5 of 29 of hydrocarbon, a second zeolite was usually introduced to increase the diffusion of molecule and the the diffusion of molecule and the accessibility of acid sites in the micropores of ZSM-5 zeolite, thus accessibility of acid sites in the micropores of ZSM-5 zeolite, thus enhancing the catalytic activity of enhancing the catalytic activity of ZSM-5 zeolite [34]. ZSM-5 zeolite [34]. ZSM-5@MCM-41 composite zeolite was prepared by treating ZSM-5 zeolite with sodium ZSM-5@MCM-41 composite zeolite was prepared by treating ZSM-5 zeolite with sodium hydroxide solution, and then the dissolved silica and aluminum were recrystallized on the surface of hydroxide solution, and then the dissolved silica and aluminum were recrystallized on the surface ZSM-5 zeolites in the form of MCM-41 in the presence of cetyltrimethylammonium bromide (CTAB) of ZSM-5 zeolites in the form of MCM-41 in the presence of cetyltrimethylammonium bromide [23–25,35]. After addition of MCM-41, ZSM-5@MCM-41 composite exhibited more appropriate acid (CTAB) [23–25,35]. After addition of MCM-41, ZSM-5@MCM-41 composite exhibited more appropriate distribution and strength, shorter pathway length and higher external surface area. The conversion acid distribution and strength, shorter pathway length and higher external surface area. The conversion of n-decane cracking increased from 61.7 to 93.0% and it maintained at 89.3% after reaction for 170 of n-decane cracking increased from 61.7 to 93.0% and it maintained at 89.3% after reaction for 170 min, min, while that of the parent ZSM-5 was just 56.4% [24]. When it was applied in n-dodecane while that of the parent ZSM-5 was just 56.4% [24]. When it was applied in n-dodecane cracking, cracking, the conversion increased by 25% and it kept a stable activity within 30 min [35]. Wang et al. the conversion increased by 25% and it kept a stable activity within 30 min [35]. Wang et al. [25] studied [25] studied the influence of alkali concentration, dissolution temperature, dissolution time and the influence of alkali concentration, dissolution temperature, dissolution time and CTAB concentration CTAB concentration on the synthesis of ZSM-5-based mesostructures in laboratory scale. Above all, on the synthesis of ZSM-5-based mesostructures in laboratory scale. Above all, they expanded the scale they expanded the scale from 0.25 L to 5 and 50 L. The obtained products exhibited excellent textural from 0.25 L to 5 and 50 L. The obtained products exhibited excellent textural properties, which were of properties, which were of significant importance for industrial application. During the preparation significant importance for industrial application. During the preparation process, a proper amount process, a proper amount of CTAB was necessary to generate the MCM-41 shell. A high of CTAB was necessary to generate the MCM-41 shell. A high concentration of CTAB would cause concentration of CTAB would cause the recrystallization of ZSM-5 zeolite and restrain the formation the recrystallization of ZSM-5 zeolite and restrain the formation of hexagonal phase of mesoporous of hexagonal phase of mesoporous materials MCM-41 [36]. Liu et al. [26] prepared the materials MCM-41 [36]. Liu et al. [26] prepared the meso-HZSM-5@Al-MCM-41 zeolite by dissolving meso-HZSM-5@Al-MCM-41 zeolite by dissolving the parent ZSM-5 zeolite to generate the the parent ZSM-5 zeolite to generate the meso-ZSM-5 zeolite with tetramethylammonium hydroxide meso-ZSM-5 zeolite with tetramethylammonium hydroxide (TMAOH) or tetraethylammonium (TMAOH) or tetraethylammonium hydroxide (TEAOH) firstly. Then, the aluminosilicate fragments hydroxide (TEAOH) firstly. Then, the aluminosilicate fragments recrystallized with the assistance of recrystallized with the assistance of CTAB on the external surface of ZSM-5 zeolite to form this CTAB on the external surface of ZSM-5 zeolite to form this meso-HZSM-5@Al-MCM-41 zeolite. meso-HZSM-5@Al-MCM-41 zeolite. During the catalytic cracking of n-dodecane, the conversion rate During the catalytic cracking of n-dodecane, the conversion rate increased by 30% and it showed increased by 30% and it showed more stable catalytic activity than ZSM-5 zeolite. It was explained more stable catalytic activity than ZSM-5 zeolite. It was explained that the MCM-41 shell enhanced that the MCM-41 shell enhanced the accessibility of acid sites and diffusion efficiency of molecule due the accessibility of acid sites and diffusion efficiency of molecule due to the shorter pathway and to the shorter pathway and ordered mesoporous structure, as well as the pre-cracking of reactants ordered mesoporous structure, as well as the pre-cracking of reactants in the MCM-41 shell (Figure intheMCM-41shell(Figure3).Besides,thismethodavoidedtheionexchangewithNHNO and 43 3). Besides, this method avoided the ion exchange with NH4NO3 and calcining processes to obtain calcining processes to obtain protonated zeolite compared with NaOH treatment method. protonated zeolite compared with NaOH treatment method. Figure 3. A schematic diagram of catalytic cracking of hydrocarbon over Figure 3. A schematic diagram of catalytic cracking of hydrocarbon over meso-HZSM-5@Al-MCM-41 zeolite [26]. Copyright Elsevier, 2015. Similarly, Wu et al. [27] prepared a series of core–shell structured zeolitic composite with Similarly, Wu et al. [27] prepared a series of core–shell structured zeolitic composite with ZSM-5 as core and mesoporous aluminosilicate as shell by a dissolution and self-assembly method. meso-HZSM-5@Al-MCM-41 zeolite [26]. Copyright Elsevier, 2015. ZSM-5 as core and mesoporous aluminosilicate as shell by a dissolution and self-assembly method. The core–shell structured zeolite effectively reduced the diffusion length. Besides, the mesoporous The core–shell structured zeolite effectively reduced the diffusion length. Besides, the mesoporous shell could function as a supporter for Pt nano particles which led to a bifunctional catalyst for shell could function as a supporter for Pt nano particles which led to a bifunctional catalyst for catalytic catalytic 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 acidityPDF Image | Strategies to Enhance the Catalytic Performance of ZSM-5
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