Molecular Simulation Of Adsorption And Diffusion Of Polycyclic Aromatic Hydrocarbons On ?-Al₂O₃

Hydrogenation of polycyclic aromatic hydrocarbons(PAHs)is one of the most important reactions in the hydrotreating of petroleum fractions.In the reaction,PAHs,which have strong adsorption capacity,are easy to block the catalyst pores,thereby increasing the diffusion resistance of the reactant molecules.Therefore,the study on the adsorption and diffusion behavior of PAHs on the catalyst support plays an important guiding role in understanding the hydrogenation process.In this paper,molecular simulation method was applied to construct the alumina pore model,surface model and PAHs molecular models with different geometries,including naphthalene,anthracene,phenanthrene,methylnaphthalene and tetralin.The effects of molecular structure,including number and position of aromatic rings,and methyl side chain,reaction temperature and pore size on the adsorption and diffusion properties were comprehensively investigated.It was found that the five PAHs were distributed near the hydroxyl groups on the pore surface in a nearly parallel manner,and physical interaction is dominant between PAHs and surface.Under the same conditions(temperature and pore size),the order of adsorption heat(in alumina pore)and adsorption energy(on alumina surface)of five PAHs is anthracene >phenanthrene > methyl naphthalene > naphthalene > tetralin,and the order of diffusion coefficient is tetralin > naphthalene > methyl naphthalene > phenanthrene > anthracene.It shows that the diffusion resistance of PAHs in alumina pores increases with the increase of the number of aromatic rings.When the number of aromatic rings is same,the linear PAHs shows higher diffusion resistance.In addition,the introduction of methyl side chains also rises the diffusion resistance of PAHs.Besides,increasing temperature will enhances the mass transfer and reduces the adsorption capacity and adsorption heat of five PAHs in alumina pore.The adsorption performance of bicyclic aromatic hydrocarbons is greatly affected by temperature change,while the diffusion properties of tricyclic aromatic hydrocarbons are more sensitive to temperature variation.Meanwhile,increasing the pore size also decreases the adsorption heat of PAHs in alumina pores and increases their diffusion coefficients.Increasing the pore size is more significant for enhancing the diffusion properties of tetralin which contains fewer aromatic rings,while the diffusion coefficients of anthracene and phenanthrene which has more aromatic rings are less affected by the change of pore size.The simulation results illustrate the laws of adsorption and diffusion of PAHs with different geometries in alumina pores,provide theoretical guidance for characterizing the diffusion properties of PAHs under different conditions,and also shed light to the understanding of PAHs hydrogenation reaction.