Molecular Simulation Study On Catalytic Cracking Of Alkanes In Zeolites

With the rapid development of computational software and computer hardware,theoretical calculation has become an effective method to study the mechanism of alkane cracking reaction on zeolites.Theoretical calculation can provide the adsorption laws and reaction mechanisms so that it can provide theoretical basis for optimizing zeolite design.In this paper,density functional theory?DFT?was used to study the catalytic cracking reaction of alkanes and their reaction mechanisms.What's more,the influence of acid strength and pore-size of zeolites on alkanes cracking mechanism were discussed.This shows the interaction of alkanes with zeolite framework.The main achievements of this paper are as follows:Effect of pore confinement and acid strength on the adsorption of mono-branched alkanes were studied.It is found that confinement effect plays an essential role in stabilizing mono-branched alkanes.With the increase in the carbon number of alkanes,there is gradual increase of adsorption energy on both HZSM-5 and HY zeolites.Moreover,in the narrow channel of HZSM-5 zeolite,the change of adsorption energy?ethyl-alkane<methyl-alkane<n-alkane?is completely different from that in the pore of HY zeolite?ethyl-alkane>methyl-alkane>n-alkane?,which is mainly due to confinement effect rather than effect of Br?nsted acid.The influence of Br?nsted acid strength on the catalytic reactivity of carbenium ion was systematically studied.The calculational results revealed that the reaction barriers(E_act)of hydrogen transfer reaction decrease exponentially and the Eact of others reactions decrease linearly with the increase of Br?nsted acid strength.In weak-acid range,the different sensitivities of the reaction reactivities to acid strength are as follows:hydrogen transfer>isomerization>?scission>dehydrogenation,while hydrogen transfer reaction has the lowest sensitivity in the strong-acid range.The pore confinement effect on bimolecular and monomolecular cracking mechanisms of n-octane over HY and HZSM-5 zeolites were studied.It is found that the overall apparent reaction barrier for the monomolecular cracking reaction at 750 K in the HZSM-5 zeolite is5.30 kcal/mol,much lower than that?23.12 kcal/mol?for bimolecular cracking reaction,indicating that monomolecular mechanism is predominant in the HZSM-5 zeolite.In contrast,the bimolecular mechanism is predominant in the HY zeolite due to lower apparent reaction barrier energy barrier?6.95 kcal/mol?for bimolecular cracking reaction than that?24.34kcal/mol?for the monomolecular cracking reaction.Moreover,the intrinsic reason for the different mechanisms is further elucidated.The confinement effect can effectively decrease energy barrier when the size of transition state is comparable to pore-size of zeolite.