Regulation Of The Pore Structure And Surface Acidity Of ZSM-5 Zeolites And Their Catalytic Performance Of MTG And Aromatization

Methanol-to-gasoline(MTG)reaction is an effective way to produce high-octane clean fuel from non-petroleum route,and olefin aromatization is the main method to convert olefins in fluid catalytic cracking(FCC)gasoline into aromatics to improve the fuel quality.The preparation of highly selective catalysts is the key to the efficient conversion of these two processes.ZSM-5 zeolite is proposed to be a suitable catalyst for MTG and olefin aromatization reactions due to its unique interconnected three-dimensional pore structure and shape-selective properties.However,ZSM-5 aluminosilicate with micrometer scale and solely micropore structure limits the formation and diffusion of the reaction transition state and products,and its strong acidity also leads to the rapid deactivation of the catalyst due to carbon deposition.To solve this problem,the morphology,pore structure and acidity of nanosized ZSM-5zeolites were adjusted and controlled by preparing nanosized ZSM-5 zeolites with a hollow structure and lamellar structure,and isomorphous substitution modification using in-situ and secondary synthesis.The structure-activity relationship between textural property,the acidity of the zeolites and MTG reaction and olefin aromatization were studied.A series of nanosized hollow ZSM-5 zeolites(Z5-HW)with a shell thickness of ca.50nm were prepared by electrostatic adsorption and in situ assembly,followed by calcination to remove the core polystyrene(PS)microspheres.Its acidity was adjusted by changing the Si/Al ratio and crystallization time of the initial gel.Due to the milder acidity of the Z5(50)-HW-24h catalyst,the gasoline selectivity of MTG reaction reached up to 69.0%.On the other hand,the Z5(20)-HW-36h catalyst with a smaller Si/Al ratio and a longer crystallization time showed higher stability.ZSM-5 zeolites with the lamellar structure were prepared using low-cost polyhexamethylene biguanide hydrochloride(PHMB)as a crystal growth inhibitor.The Al species in the framework of zeolite and its acidity was changed by extending the crystallization time.As the lamellar structure effectively shortens the diffusion of reaction intermediates and products in the zeolite channels,and the stronger Br?nsted acidity improves the hydrogen transfer activity of the catalyst in MTG reaction,the gasoline selectivity was higher.For the Z5-NS-3h catalyst,the gasoline selectivity reached up to 73.2%,among which aromatics accounted for 81.0%.Nanosized ZSM-5 zeolites were synthesized by the traditional hydrothermal method and steam-assisted dry-gel conversion method.The Ga-modified nanosized ZSM-5 zeolites were prepared by in-situ synthesis method under steam-assisted crystallization.The ZSM-5 zeolite prepared by dry gel conversion method showed the same activity and gasoline selectivity of MTG reaction compared to the sample prepared by traditional hydrothermal method.The insertion of Ga into the zeolite framework effectively reduced the strength of Br?nsted acid sites,suppressed the side reactions such as cracking,and active Ga O⁺species were formed with strong Lewis acidity.The dehydrogenation of cycloolefins acted as a rate-controlling step in the methanol conversion process was promoted and the gasoline selectivity was significantly improved.The(Ga,Al)ZSM-5 zeolites with Ga partially isomorphous substitution were prepared by seed-induced in-situ synthesis and secondary synthesis method.Compared with ZSM-5zeolites,the acid strength of(Ga,Al)ZSM-5 was weakened and the Ga O⁺active species were formed,the aromatic selectivity of 1-hexene aromatization and the stability of catalysts were higher.Moreover,compared with the in-situ isomorphous substitution method,intracrystalline mesopores were generated in zeolites prepared by the secondary synthesis method due to the framework dealuminization,and the Ga O⁺species with stronger dehydrogenation activity were promoted to locate in the zeolite channels and highly dispersed,thus the aromatization selectivity and stability of catalysts were significantly improved.The(Ga,Al)Z5-SS₂ catalyst prepared by the secondary synthesis method showed a 1-hexene conversion of more than99.0%with high aromatic selectivity of 60.0%after 100 h of reaction.In the process of preparing seed crystals and zeolites crystallization,microwave radiation heating(MW)and traditional electric heating(CE)were used to synthesize a series of nanosized ZSM-5 aluminosilicates and gallosilicates.The acid site distribution of the two catalysts had a significant effect on the BTX(benzene,toluene and xylene)selectivity in the products of 1-hexene aromatization.Theoretical calculations proved that the Br?nsted acid site of the catalyst has spatial proximity to the Ga O⁺species,which can achieve synergistic catalysis,thus significantly improving aromatic selectivity.For the Ga Z5-MW/MW prepared by two-step microwave radiation heating and crystallization for 3.5 h,the aromatic selectivity was as high as 61.7%.Therefore,the in-situ isomorphous substitution method using microwave radiation heating provided a faster,simpler and more effective strategy for adjusting the acidity of zeolite and improved its catalytic performance.