Molecular Simulation Of Adsorption And Diffusion Behavior Of Organic Molecules In Zeolites

Zeolites are silicate crystals with microporous structure.They have unique pore structure,selectivity and many excellent properties such as high specific surface area,adjustable pore size and good hydrothermal stability.Zeolites can be used as adsorbents,catalysts,separation media and ion exchangers,thus widely applied in chemical industry and other fields.In this work,the adsorption and diffusion behavior of small organic molecules in zeolites as separation materials and catalysts were analyzed by molecular simulation in order to meet the needs of diversified chemical processes.In Chapter 2,in view of the similar physicochemical properties of 1-chloropropane and allyl chloride,and the high energy consumption and low efficiency of distillation separation therefrom,the Monte Carlo method was used to calculate the adsorption isotherms of 1-chloropropane and allyl chloride in many zeolites.Four zeolites were screened out,which can only adsorb allyl chloride.Furthermore,the diffusion path of allyl chloride in these zeolites was analyzed by non-equilibrium molecular dynamics simulation,and the diffusion flux was calculated.The results show that the AHT zeolite has the best adsorption and diffusion performance.In this chapter,the mechanism of the ultra-high selectivity separation is discussed by thermodynamic analysis,combining with the data of dihedral angle-molecular potential energy change of 1-chloropropane and allyl chloride.A conformation recognition mechanism based on the pore geometry is proposed,which provides a new way to explore the separation of mixture of alkanes and olefins in the future.In Chapters 3 and 4,focused on the rapid deactivation of zeolite catalysts in isobutane/butene alkylation,the mechanism of deactivation and the way to slow down the deactivation of zeolite catalysts were studied.In Chapter 3,the adsorption isotherms and diffusion coefficients of alkylation components in Y zeolite were obtained,and the suitable feed ratios of isobutane and olefin and the silicon-aluminium ratio of zeolite were determined.On this basis,the influence of by-product C₁₂H₂₆ in zeolite pores was analyzed.Comparing the diffusion coefficients of components and the accessibility of active sites under different conditions,it is shown that the main reason of rapid deactivation of catalyst is that C₁₂H₂₆ and higher carbon number by-products mask the active sites and affect the diffusion properties of reactants.In Chapter 4,the influence of hierarchical Y zeolite on the adsorption and diffusion of mixture in alkylation reaction is discussed.The results show that the mesopores can effectively increase the concentrations of isobutane and 2-butene near the active site and the diffusion rates of C₁₂H₂₆.Thus,mesopores can slow down the blockage of pores and prolong the life span of zeolite catalyst.The application of hierarchical Y zeolite is possibly an effective way to solve the problem of rapid deactivation of zeolite catalysts in alkylation reaction.