Preparation And Pore Size Optimization Of Diesel Hydrotreating Catalysts

More stringent environmental legislation imposes severe requirements to reduce the sulfur content in diesel to ultra-low levels with high-efficient catalysts.Mostly,γ-Al₂O₃is used as the support of traditional industrial catalysts.It can produce strong interactions between the support and the active metals,which will enhance the agglomeration of active phase and further affect the catalysis activity.Organic complexing agent has been applied in synthesis of catalysts for its outstanding metal complexing ability.The interactions between active metals and support can be weaken to enhance its redispersion through adding organic complexing agent.Besides,dibenzothiophenes in diesel encounter diffusion hindrance in traditional HDS catalyst because of its relatively large molecular diameter,which decreases the HDS activity.Therefore,it’s reasonable to design proper catalyst by alleviating the diffusion limitations of sulfur compounds.In the thesis,citric acid（CA）was used as organic complexing agent to modify the Ni Mo P/Al₂O₃ catalyst.By varying the impregnation sequence of CA,catalysts of pre-impregnation,co-impregnation and post-impregnation were synthesized.A series of technologies were used to characterize the support and catalysts,including physical and chemical properties,adsorption properties and catalytic properties.The performance of dibenzothiophene（DBT）and 4,6-dimethyl DBT in the catalysts was investigated by an autoclave.The results showed the interactions between active metals and the support were effectively reduced by post-impregnation of CA,which enhanced the redispersion of metal precursor on the surface of support and benefited the formation of active phase with high activity.The HDS results showed the conversion of two model compounds in the post-impregnated catalyst is higher than that in the catalysts synthesized by the other two methods.And the HDS performance of the post-impregnated catalyst is comparable to the commercial catalyst RS-2100.Furthermore,a series of post-impregnation catalysts were obtained by changing the CA/Ni molar ratio.Various characterization methods were used to explore the mechanism of CA and find the optimal impregnation metohd.At last,taking DBT as model compound to perform the HDS experiment on catalysts.It’s revealed that the reduction temperature of catalysts decreased by introducing CA,and the minimum value was reached when the CA/Ni ratio equals to 1.The sulfidation of the catalyst increased firstly and then decreased with the increase of the amount of CA,and it reached to the maximum value of 71.8%as the CA/Ni ratio equals to 1.The desulfurization rate of DBT follows the following sequence:CA1>CA2>CA0.5>CA3>Ni Mo P.Therefore,the CA/Ni ratio of 1 was regarded as the most suitable ratio for the catalyst synthesis in post-impregnation.At last,the catalysts with different pore sizes were prepared by post-impregnating solution with CA/Ni molar ratio of 1,and the HDS reaction performance was investigated by using an industrial diesel oil in a fixed-bed micro-reaction device.The hydrogenation results showed that the HDS conversion increased with the increase of the average pore size.And the optimal HDS activity was achieved by the catalyst with the largest average pore size.Further compositional transformation of the diesel oil was analyzed by GC-SCD.The results showed the removal of C2+-DBT sulfides increased with the increase of the temperature and catalyst pore size.