Study On The Property Regulation Of ZSM-5 Zeolite And Its Performance In Catalytic Cracking Of N-hexane

Ethylene and propylene are important organic chemical raw materials.Driven by the downstream products,their demand continues to increase.On the other hand,the rapid rise of new energy industry and the improvement of fuel efficiency significantly impact the oil product market.Production of ethylene and propylene from petroleum is the developing trend of petrochemical industry.Catalytic cracking is an important way to produce light olefins from petroleum.Because of the unique pore structure,good hydrothermal stability and high olefin selectivity,ZSM-5 is one of the most commonly used catalysts in catalytic cracking of petroleum components.The acidity and pore structure of the catalyst are closely related to their catalytic performance and distribution of cracking products.In this study,the acidity of the molecular sieve was regulated by metal modification,and the pore structure was controlled by preparation of molecular sieve with hierarchical pores,aiming to improve the activity and stability of the catalyst.The acid content,acid strength,pore structure and other properties of the prepared molecular sieves were analyzed,and the activity of the catalysts in the cracking of n-hexane was evaluated.The main research contents and results are as follows:（1）A new method to improve the catalytic cracking performance of zeolite by modifying with transition metal niobium（Nb）was proposed.Nb modified ZSM-5-USY composite zeolite catalyst was prepared.For comparison,the alkali metal potassium（K）,transition metals nickel（Ni）and tungsten（W）were also used to modify the composite molecular sieve.The acid strength,acid content,pore structure and other properties of the prepared catalyst were studied.It was found that the introduction of Nb could increase the acid content of the molecular sieve,which also exhibited suitable specific surface area and pore volume.The catalytic performance of the four metal modified composite molecular sieves in cracking of n-hexane,a model compound for petroleum component,was compared with that of ZSM-5 or USY.It was found that Nb/ZSM-5-USY had the best catalytic performance,which could be attributed to the more acid amount of this catalyst.Under the optimal reaction conditions(625℃,0.5 h^-1,the metal Nb loading of 1.5wt%),the conversion of n-hexane was 70.44%,and the yield of C₂-C₄ olefins reached 45.91%.The intrinsic kinetics of the cracking of n-hexane over 1.5%Nb/ZSM-5-USY was investigated,and the results indicated that the reaction was a first-order reaction,with the apparent activation energy of 115.16 k J/mol.（2）ZSM-5 nanosheets（Al-NZ）were synthesized using bisquaternary ammonium salt as the template,and Ga-containing ZSM-5 nanosheets（Ga-NZ）were also prepared by isomorphous substitution method.The BET and NH₃-TPD characterization showed that compared with conventional HZSM-5,nanosheet-shaped molecular sieves had much higher specific surface area and mesoporous volume,lower acid content,and most of the acid sites were weak ones.The catalytic activity and stability of nanosheets and conventional HZSM-5 molecular sieve were compared.Under the reaction conditions of 550℃ and 2 h^-1,the conventional HZSM-5deactivated rapidly,and the conversion dropped by 32%after 9 h.In the Ga-NZ system,aromatic compounds（with selectivity up to 30%）were generated along with the formation of light olefins.The Al-NZ catalyst had the best stability and olefins selectivity,which could be attributed to the excellent acid properties and pore structure of Al-NZ.The weaker acidity,the larger specific surface area and mesopore volume were more favorable for n-hexane cracking to generate light olefins.Furthermore,when the temperature was increased to 650℃,the conversion of n-hexane over Al-NZ could be as high as 90.90%,and the olefins yield could reach58.80%.Thus,high temperature was more favorable for the cracking reaction of alkanes to form light olefins.When the mass space velocity was increased from 2 h^-1to 10 h^-1,the amount of raw material needed to be processed per unit mass of catalyst increased,and the conversion of n-hexane and the yield of olefins decreased to 72.22%and 45.65%,respectively.