The Modification Of Particle Size, Mesopores And Acidic Sites Distribution Of ZSM-5 And Its Catalytic Performance In Methanol To Gasoline Reaction

As a catalyst for MTG reaction, ZSM-5 possesses well-defined micropores and strong acidity, which catalyze the conversion of methanol to gasoline with high catalytic activity and selectivity. However, it is the intrinsic regular micropores that restrain the diffusion of reactant and products. It also accelerates the generation of coke and leads to the deactivation of catalyst. In order to prolong the catalytic lifetime, the diffusion in microporous ZSM-5 can be enhanced through two conventional strategies of reducing crystal size and introduction of mesopores into micropore network, which can promote the diffusion of bulky molecules and decrease coke generation. Nevertheless, there is not any contrastive study about these conventional strategies. While the contrastive study can help to find the advantages and disadvantages of each strategy, and the combination and utilization of these advantages guide to prepare ZSM-5 catalyst with prolonged lifetime. The nano-ZSM-5 exhibits better MTG catalytic performance. However, the effect of crystal size on the catalytic performance needs more specific research. Besides the effect of crystal size, the acidity of ZSM-5 also plays an important role in MTG reaction, which influences the catalyst lifetime and selectivity of products.Herein, the main work in this paper is the modification of diffusion property and Al distribution in ZSM-5. The advantages and disadvantages of the above mentioned strategies to enhance diffusion were contrastively studied. The effect of these strategies on the catalytic performance for MTH reaction was researched in terms of pore structure and acidity property. Then, based on the catalytic superiority of nano-ZSM-5, the size effect of ZSM-5 crystal on the catalytic performance was also studied. Finally, two similar-sized ZSM-5 crystals with different Al distribution were prepared by one-pot and seed surface direction methods, respectively. And the effect of Al distribution on acidity property and catalytic performance were further studied. Considering the above studies, in order to supply the theoretical evidence of designing catalyst, it is necessary to find the key factors of porosity and acidity that determine the catalytic performance by modifying the diffusion property and acidic sites distribution of ZSM-5. The conclusions of this paper are as follows:（1） The contrastive study of the effect of crystal size reduction and mesopore introduction in ZSM-5 on catalytic performance showed that these strategies enhanced the diffusion property in micropores, thereby resulting in the prolong of lifetime. And the catalyst modified by reduction of crystal size possessed the best catalytic performance. On one hand, alkaline treatment is an effective way to introduce the intra-mesopores in ZSM-5. However, there is only a slight improvement of catalytic performance due to its low mesoporosity degree. On the other hand, the sample with inter-mesopores prepared by salt-aided seed-induced method exhibis much larger external surface area than nano-ZSM-5. But the acidity is so strong that it limits the catalytic performance. By acid leaching, the amount and strength of acid were decreased, and thus the yield of liquid hydrocarbons and catalyst lifetime were improved. In conclusion, nano-ZSM-5 has a certain superiority in the MTG reaction, regardless of the practical problem in synthesis process, such as large template consumption, separation difficulties and low ZSM-5 yield. While due to its large external surface area, ZSM-5 catalysts with inter-mesopores prepared by salt-aided seed-induced method also has a potential application in MTG reaction if further modificated its surface acidity.（2） By controlling the H₂O/Si ratio in the gel during the synthesis process, some kinds of ZSM-5 with different crystal sizes were obtained. The final crystal size of ZSM-5 increases with the rise of H₂O/Si ratio. Combining the results of characterization, the external surface area decreased with the increase of crystal size, while the acid amount firstly increased and then kept at a stable level with the increase of crystal size. Due to the best diffusion property, the sample with crystal size of 70 nm exhibited the best catalytic performance. The product yield and catalyst lifetime reached maximum value of 30.8 % and 96 h, respectively（reaction condition: P=1.0 MPa, T=405 oC and WHSV=4.74 h-1）. In terms of ZSM-5 prepared with H2O/Si ratio of 75, it showed strong acidity and large external surface area due to the growth of small-sized crystals on the erternal surface, which improved its catalytic performance.（3） Two similar-sized ZSM-5 crystals with distinctly different Al distribution were prepared by one-pot and seed surface direction methods. For ZSM-5 prepared by the latter method, the acid sites were mainly distributed on the exteriors. By contrast, ZSM-5 synthesized by one-pot method has more uniform Al distribution in bulk phase, and both total and external acidity of ZSM-5 is weak, which can decrease the formation of coke. Under the same reaction condition（P=1.0 MPa, T=405 oC and WHSV=4.74 h-1）, the lifetime of ZSM-5 prepared by one-pot method can be as long as 237 h, much longer than the 54 h lifetime of ZSM-5 prepared by seed surface direction method...