Adsorption And Diffusion Behavior Of Reaction Species In The Pore Structure Of Hydrofining Catalyst

The reaction performance mainly depends on the pore structure of hydrofining catalyst.Diffusion and adsorption are the precondition of reaction,so it is very important for the structure design of high-efficient catalyst to deepen understanding of the effects of pore structure on diffusion and adsorption.Therefore,it is of great significance to research the adsorption and diffusion behavior of reaction species in the pore structure of hydrofining catalyst.In this thesis,?-Al₂O₃ supports with different pore structures were prepared via cation-anion double hydrolysis method and MoO₃/?-Al₂O₃ catalysts with the same metal loading and different pore structures were synthesized by incipient-wetness impregnation method.By investigating the adsorption amount,adsorption mode,adsorption heat,adsorption interaction and diffusion rate of thiophene compounds on the surface of catalysts,the effects of pore structure on the accessibility of active sites and diffusion rate of thiophene compounds were explored.Finally,using 4,6-DMDBT as model compounds,the hydrodesulfurization activity of catalysts with different pore structures was evaluated.The pore sizes of supports can be tuned in the range of 2.5-8.7 nm by controlling synthesis methods and preparation conditions.The deposition of active component can lead to a decrease of pore sizes,pore volumes and specific surface areas of the catalysts,but the pore structure features of catalysts are basically similar to that of supports.Only Lewis acid sites exist on supports while catalysts possess both Lewis and Bronsted acid sites.The larger mesopores exhibit the more acid sites over the supports and catalysts.Moreover,this effect can be well embodied by collidine.The larger the pore size of catalyst,the lower the reduction temperature of Mo⁶⁺.The Acidity characteristics of supports and catalysts and reduction characteristic of active metal both show that there are pore confinement effects in the hydrofining catalysts.The pore structure of catalyst affects the accessibility of active sites.The adsorption amount of thiophene compounds?thiophene,dibenzothiophene,4,6-DMDBT?increases with the increase of pore size.When pore volume is almost the same,the specific surface area is not the main factor affecting adsorption but the samples with mono-large and bimodal mesopores are favor for thiophene compounds adsorption.There are many kinds of manners for thiophene adsorption.The adsorption heat of thiophene on catalyst with small pore size is bigger,but more chemisorbed thiophene is remained on catalyst with large pore size due to the existence of more physical adsorption in small pore.The steric effect of?-complexation is greater than that of?-bond during the process of thiophene adsorption,while the steric hindrance of?-bond is greater than that of?-complexation during the adsorption of 4,6-DMDBT.The pore structure of catalyst affects the diffusion rate of thiophene compounds.The smaller diffusion resistance in larger mesopores contributes the fast diffusion rate to the thiophene compounds.Compared with the specific surface area of catalyst,the influence of pore size on the removal efficiency of thiophene compounds is more significant.The larger the pore size,the higher the desulfurization rate and conversion rate of 4,6-DMDBT.In addition,bimodal mesopores have more advantages compared with mono-modal mesopores.