| Against the backdrop of refining-petrochemical integration,the conversion of fuel into chemical products is of great significance to achieve the goal of carbon neutrality and increase production of basic and important chemical raw materials.Therefore,the technology of efficiently converting C4-C8 olefins in fuel into light olefins through catalytic cracking has attracted extensive attention,such as industrialized OCC and Superflex.However,it still needs to be further optimized in terms of low light olefin selectivity and coking of the catalyst.How to design and develop an efficient catalytic cracking catalyst is a major scientific problem in the catalytic cracking process of olefins,and the key to solving this problem is to clarify the mechanism of olefin catalytic cracking and the influence of catalyst acid properties on the reaction.In order to assist the design of olefin catalytic cracking catalysts with high stability and high selectivity of light olefins,based on density functional theory(DFT),the catalytic cracking mechanism and microkinetics of olefins were deeply studied in this paper.First,the catalytic cracking mechanism and microkinetics of 1-butene at strong Br?nsted acid sites were investigated by DFT and microkinetic simulation methods.The migration of the double bond of 1-butene and the bimolecular cracking of 1-butene along Path I were found to be the optimal reaction paths for isomerization and bimolecular cracking,respectively.The mechanism study includes the main reaction bimolecular cracking,the main side reaction isomerization and aromatization,and proposes an aromatization mechanism with fewer reaction steps and lower reaction energy barriers.The microkinetics study found that with the increase of temperature,each reaction rate constant increased,and the reaction rate also increased first and then decreased,and 750 K was the optimal reaction temperature for the production of light olefins.However,the amount adsorbed of 1-butene decreased with the increase of temperature,which made the adsorption at high temperature into a rate-determining step of the reaction path.Therefore,it is suggested to increase the number of catalyst acid sites reasonably to increase the amount adsorbed of 1-butene at high temperature,which can effectively reduce the effect of adsorption on the conversion and improve the efficiency.Second,the reaction law of 1-butene at weak Br?nsted acid sites were studied by the same method,and the effects of strong and weak Br?nsted acid sites on the reaction rate were compared.Changes in the acid strength of the catalyst did not alter the relative size of the energy barriers for each reaction.Reactions that are relatively difficult to occur under strong acids are also more difficult under the weak acids.Compared with strong Br?nsted acid sites,weak Br?nsted acid sites have two characteristics.One is that weak Br?nsted acid sites have better charge distribution,and the Coulomb interaction between them and reactants is weak,which is not conducive to the stability of substances with uneven charge distribution,such as carbeniums.The other is the acidic hydroxyl groups in the weak Br?nsted acid sites have more degrees of freedom and can better adapt to the reactive species to reduce the repulsion between the alkoxide species and the zeolite wall.Therefore,it is recommended to appropriately increase the Br?nsted acid strength to improve the stability of carbeniums,thereby increasing the reaction rate.Finally,the catalytic cracking mechanism of 1-hexene at the strong Br?nsted acid site was investigated by the above method,and the optimal reaction paths for 1-hexene single molecular cracking and bimolecular cracking were proposed.The study found that single molecular pathway is the favorable reaction pathway for the production of light olefins.However,the cracking of hexene tends to be a bimolecular cracking pathway at the reaction temperature of current process.Therefore,it is suggested to increase the micropore ratio of catalyst to limit the bimolecular cracking to produce more light olefins. |
