Molecular Simulation Of Sulfur Resistance Mechanism Of Micro-Mesoporous Pd/ZSM-5/?-Al₂O₃ Composite Catalyst

The technology of coal tar hydrogenation for fuel oil has a very important significance in expanding application of coal tar in the energy field and solving the increasingly serious energy shortage,however,noble metal catalyst is prone to sulfur poisoning in hydrogenation of coal tar.Catalyst is the core part of hydrodesulfurization reaction,and sulfur poisoning of catalyst is the major hindrance to the application of this technology.Micro-mesoporous Pd/ZSM-5/?-Al₂O₃ catalyst integrates the advantages of strong acidity and high hydrothermal stability of microporous and rapid diffusion of large molecular in mesoporous has been synthesized in the preliminary work of our group,which showed high sulfur tolerance in the evaluation of coal tar hydrogenation reaction.However,the mechanism of sulfur tolerance was not clear.In this paper,tetralin and dibenzothiophene were used as the model compounds of coal tar,n-hexane and H2 S were used as the model compounds of coal tar hydrogenation desulfurization products.In this paper,the all atom model of Pd/ZSM-5/ ?-Al₂O₃ was constructed based on the structural characteristics of the synthesized mesoporous Pd/ZSM-5/?-Al₂O₃ catalyst by molecular simulation.The distribution of sulfide in the channels of Pd/ZSM-5/?-Al₂O₃ and the effect of active sites on adsorption were studied by Grand canonical Monte Carlo simulation;the diffusion of sulfide in the structure was studied by molecular dynamics method;the results of physical characterization of adsorbed sulfide samples were combined to discuss the mechanism of sulfur resistance under the action of pore confinement effect and the surface active site selectivity.First,all silicon type?Si/Al=50 type?Pd loading type models were constructed by molecular simulation method,respectively.The models were validated by comparing the experimental and simulated values of nitrogen adsorption isotherm?pore structure parameters and XRD,the result showed the models were designed reasonably.Secondly,adsorption characteristics of model compounds on the surface of catalyst was studied combining the heat of adsorption and space confinement effect.The result proved that the acid sites of the catalyst were the active adsorption center of dibenzothiophene,tetralin and H2 S while Pd metal active sites on the catalyst surface were the active adsorption centers of dissociative adsorption of H2,dibenzothiophene and tetralin.The order of adsorption heat was: dibenzothiophene>tetralin>H2S>n-hexane.Due to the confinement effect,dibenzothiophene and tetralin were mainly adsorbed in mesopores,some tetralin molecules could also be adsorbed in microporous.H2 S and tetralin could be adsorbed both in micropores and mesopores.N-hexane and tetralin adsorbed in microporous would inhibit the adsorption of dibenzothiophene in micropores.Thirdly,the mean square displacement curves and diffusion coefficients of coal tar model compounds in the catalyst channel were studied by molecular dynamics method.The results show that the mean square displacement and diffusion coefficient of all coal tar components increase with temperature,and the order of diffusion coefficient was H2S>tetralin>dibenzothiophene>n-hexane.The molecular diffusion rate of the molecules adsorbed active sites was low,as a consequence,the molecular diffusion coefficient first increased and then decreased with the increase of adsorption capacity.n-hexane was mainly diffused in micropores while dibenzothiophene was mainly diffused in mescropores,H2 S and tetralin could diffuse freely both micropore and mesopore.Finally,based on the simulation results above and the physical properties of sulfide samples,the sulfur resistance mechanism of micro-mesoporous Pd/ZSM-5/?-Al₂O₃ catalyst was inferred from two aspects of adsorption and diffusion.The results showed that the catalyst has high stability.The components of coal tar could be desorbed at the reaction temperature;the order of desorption temperature was as follows: dibenzothiophene >tetralin> H2 S >n-hexane.The noble metal Pd in the fresh catalyst existed in the form of a single substance,and the lack of electronic state of Pd?+ was formed after adsorption of sulfide,which is beneficial to inhibit the irreversible poisoning of the catalyst.The dual pore structure of micro-mesoporous catalyst Pd/ZSM-5/?-Al₂O₃ provides double reaction space for tetralin and the confinement effect of micropores prevented the adsorption of dibenzothiophene on microporous surface,which protected the active site distributed in micropores.In addition,the dissociated hydrogen in the micropores could be overflowed into mesoporous channels and reacted with dibenzothiophene adsorbed on the active site in mesoporous,which improve the sulfur resistance of the catalyst.