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Catalysts 2017, 7, 367 22 of 31 Catalysts 2017, 7, 367 21 of 29 Figure 18. NH3-TPD profiles of the isomorphous MFI nanosheet zeolites (a) and light alkenes Figure 18. NH3-TPD profiles of the isomorphous MFI nanosheet zeolites (a) and light alkenes selectivity selectivity (b) of n-dodecane cracking at 550 °C under 4 MPa [44]. Copyright Elsevier, 2017. (b) of n-dodecane cracking at 550 ◦C under 4 MPa [44]. Copyright Elsevier, 2017. 5.2. Acid Amount 5.2. Acid Amount ZSM-5 zeolite with excess acid amount will result in the overreaction of reactants and ZSM-5 zeolite with excess acid amount will result in the overreaction of reactants and generating generating much carbon deposition. Therefore, the catalytic activity decreases quickly due to the much carbon deposition. Therefore, the catalytic activity decreases quickly due to the coverage of acid coverage of acid sites and blockage of pore channels by carbon deposition. sites and blockage of pore channels by carbon deposition. One method to change the acid amount is to prepare zeolite with different Si/Al ratio. The acid One method to change the acid amount is to prepare zeolite with different Si/Al ratio. The acid amount decreases with the increase of Si/Al ratio. ZSM-5 zeolite with low Si/Al ratio would facilitate amount decreases with the increase of Si/Al ratio. ZSM-5 zeolite with low Si/Al ratio would facilitate the catalytic conversion but promote the generation of carbon deposition simultaneously, leading to the catalytic conversion but promote the generation of carbon deposition simultaneously, leading to quick deactivation due to the coverage of acid sites and pore blockage by carbon deposition. A stable quick deactivation due to the coverage of acid sites and pore blockage by carbon deposition. A stable catalytic activity with less carbon deposition can be obtained by increasing the Si/Al ratio catalytic activity with less carbon deposition can be obtained by increasing the Si/Al ratio moderately. moderately. While increasing Si/Al ratio further, the catalytic activity decreased due to decrease of While increasing Si/Al ratio further, the catalytic activity decreased due to decrease of acid sites [115]. acid sites [115]. The other approach is to modify ZSM-5 zeolite with element or compound. ZSM-5 zeolite after The other approach is to modify ZSM-5 zeolite with element or compound. ZSM-5 zeolite after gold modification showed a reasonable number of strong acid sites, resulting in higher light alkene gold modification showed a reasonable number of strong acid sites, resulting in higher light alkene selectivity during catalytic cracking of light diesel [116]. During the catalytic cracking of hydrocarbon, selectivity during catalytic cracking of light diesel [116]. During the catalytic cracking of ZSM-5 zeolite with less strong acid site showed higher light alkene selectivity but lower conversion hydrocarbon, ZSM-5 zeolite with less strong acid site showed higher light alkene selectivity but rate, because decreasing strong acid sites would restrain the secondary reaction between the primary lower conversion rate, because decreasing strong acid sites would restrain the secondary reaction products [117]. Phosphorus modification of ZSM-5 zeolite is an effective method to enhance its catalytic between the primary products [117]. Phosphorus modification of ZSM-5 zeolite is an effective cracking performance. The acid amount decreases with the increase of phosphorus content, especially method to enhance its catalytic cracking performance. The acid amount decreases with the increase for strong acid amount. When the acid sites decrease to some extent after phosphorus modification, of phosphorus content, especially for strong acid amount. When the acid sites decrease to some it could avoid the overreaction of reactants on acid sites and generation of carbon deposition, resulting extent after phosphorus modification, it could avoid the overreaction of reactants on acid sites and in a stable catalytic activity with high light alkene selectivity in supercritical catalytic cracking of generation of carbon deposition, resulting in a stable catalytic activity with high light alkene n-dodecane (Figure 19). The gas generation rate reached to 68% at the phosphorus content of 0.5 wt %, selectivity in supercritical catalytic cracking of n-dodecane (Figure 19). The gas generation rate which was 21% higher than that of the parent ZSM-5 zeolite [118]. Furthermore, it has been found that reached to 68% at the phosphorus content of 0.5 wt %, which was 21% higher than that of the parent co-modification of ZSM-5 zeolite with phosphorus and metal cation is effective method to enhance ZSM-5 zeolite [118]. Furthermore, it has been found that co-modification of ZSM-5 zeolite with the catalytic activity and stability. Mg, Ca, La, and Zr were introduced into the phosphorus-modified phosphorus and metal cation is effective method to enhance the catalytic activity and stability. Mg, ZSM-5 zeolites, respectively and they showed better catalytic activity with less carbon deposition Ca, La, and Zr were introduced into the phosphorus-modified ZSM-5 zeolites, respectively and they compared with the parent ZSM-5 zeolite [119]. The acidity of lanthanum and phosphorus co-modified showed better catalytic activity with less carbon deposition compared with the parent ZSM-5 zeolite ZSM-5 zeolite decreased with the increase of lanthanum content, while the basicity increased with [119]. The acidity of lanthanum and phosphorus co-modified ZSM-5 zeolite decreased with the increasing lanthanum content [120,121]. The basicity could restrain the hydrogen transfer activity and increase of lanthanum content, while the basicity increased with increasing lanthanum content enhance the generation of light alkene. Therefore, ZSM-5 zeolites with moderate acidity and basicity [120,121]. The basicity could restrain the hydrogen transfer activity and enhance the generation of after lanthanum modification showed better catalytic activity and higher light alkene selectivity. light alkene. Therefore, ZSM-5 zeolites with moderate acidity and basicity after lanthanum Similarly, nickel and iron were introduced into phosphorus-modified ZSM-5 zeolite by impregnating modification showed better catalytic activity and higher light alkene selectivity. Similarly, nickel and method. They interacted with pre-introduced phosphorus, resulting in the recovery of partial Brønsted iron were introduced into phosphorus-modified ZSM-5 zeolite by impregnating method. They acid sites which were neutralized by phosphorus modification. The increased Brønsted acid sites interacted with pre-introduced phosphorus, resulting in the recovery of partial Brønsted acid sites enhanced the cracking of hydrocarbon and selectivity of light alkenes [122,123]. which were neutralized by phosphorus modification. The increased Brønsted acid sites enhanced the cracking of hydrocarbon and selectivity of light alkenes [122,123].PDF Image | Strategies to Enhance the Catalytic Performance of ZSM-5
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