Study On Preparation And Hydrogenation Catalytic Performance Of Carbon-Based Support Materials

Hydrotreating technology,with the core of the design and preparation of high performance hydrogenation catalyst,is a necessary process for deep hydrodesulfurization of low quality diesel oil.During the preparation process,the microstructure of the active phase and the physical and chemical properties of the active sites are seriously affected by the crystal structure,electronic structure and surface acid-base properties of the support matrix materials.Compared with the conventional acidic alumina support,the active metals on the surface of basic magnesium oxide（Mg O）support have higher dispersion performance and hydrodesulfurization（HDS）activity.However,in the impregnation process,the aqueous solution will hydrate with Mg O and destroy the structure of the support,and the ion holes on the surface of Mg O produced by low coordination ions will interact strongly with the active metals,the reduction performance of the catalyst decreases.Therefore,in this dissertation,the method of carbon precursor predeposition is used to form subsurface carbon on the surface of Mg O support,which can change the surface properties of Mg O support directionally,and then regulate the interaction between active metals and support.Co Mo supported HDS catalysts are successfully prepared by the methanol-assisted slurry impregnation method,and different types of carbon-based composite oxide supports are successfully synthesized.The HDS reaction of aromatic heterocyclic sulfide dibenzothiophene（DBT）and 4,6-dimethyldibenzothiophene（4,6-DMDBT）with substituents are studied,and the structure-activity relationships among sulfides with different structure types,the physicochemical properties of support matrix materials and the microstructures of the active phase of catalysts are investigated.The idea and preparation method are provided for developing the support matrix material suitable for the efficient removal of sulfides with different structures.Firstly,the regular hexagonal lamellar MgO support materials with high specific surface area and single pore structure are prepared by hydrothermal treatment,and the reaction mechanism,reaction network,and hydrogenation kinetics of DBT and 4,6-DMDBT on Mg O-based catalysts are studied.It is found that the increase of hydrothermal treatment time is beneficial to the formation of small particle lamellar magnesia,and the particle size is inversely proportional to the number of defects on the surface of the support,the defect sites on the surface of the support are beneficial to the dispersion of the active metals,but not to the reduction performance of the catalysts.The results of the HDS reaction showed that both DBT and 4,6-DMDBT have direct desulfurization（DDS）and hydrodesulfurization（HYD）pathways on Co Mo/Mg O catalysts,but they are more efficient than conventional alumina-based catalysts and have higher DDS selectivity.Compared with Catalyst Co Mo/A,the conversion of sulfides on catalyst CoMo/PM2 is increased by 19.2%and 29.8%,DDS by 64.3%and 133.3%,and HYD by 25.0%and95.2%,respectively.The excellent HDS conversion and DDS selectivity of Mg O-based catalysts are mainly attributed to the good dispersion of active metals on the surface of Mg O and the avoidance of Co Al₂O₄ spinel formation.The basic sites on the surface of Mg O andπelectrons in aromatic heterocyclic sulfides repel each other,which is beneficial to improve the adsorption of sulfides on the active sites of catalysts by end-linked adsorption.Compared with lying adsorption,the C-S bond strength of sulfide in end-linked adsorption is lower,which is easy to occur direct hydrogen cleavage,and improves the DDS selectivity of sulfide.It is found that the reduction performance of active metals on Mg O-based catalysts is mainly affected by the number of strong basic sites.The porous carbon coated Mg O composite support is prepared by predeposition of carbon on the surface of the support with pitch powder as a carbon precursor.The results show that the introduction of the non-uniform porous carbon layer effectively reduces the alkalinity sites,especially the strong alkalinity sites produced by low-coordination O^2-ions.The H₂-TPR peak position of the oxidized Co Mo/M@AC-0.2 catalyst shifted about 40°C to the low temperature direction compared with the Co Mo/M catalyst.The sulfidation degree of Mo and Co increased by 17.0%and 10.0%,respectively,and the stacking number of Mo S₂ increased from 1～2 to 2～3 layers.The introduction of the porous carbon layer reduces the strong interaction between the active metal and the support.The analysis of hydrogenation route selectivity shows that the relative content of the HYD route product increases,indicating that the porous carbon layer promotes the diffusion and mass transfer of hydrogen species on the catalyst surface during the hydrogenation process the selectivity of sulfide to HYD pathway is improved.In the experiment of preparing carbon-based composite oxide support with pitch powder as carbon precursor,it is found that the coating of pitch powder has no directional selectivity,and there is a certain amount of amorphous carbon between Mg O particles,and the agglomeration of the active metals on the surface of the carbon layer is observed.To solve this problem,the chemical vapor deposition is used to adsorb the small carbon precursor methane at the defect site on the surface of Mg O,and then pyrolysis growth is carried out at high temperatures.With its unique two-dimensional spatial network,the graphene layer precisely regulates the strong interaction between the active metal and the support.It is found that the number of defects on the surface of graphene is proportional to the time of methane cracking due to the growth mechanism of graphene.It is easy to form amorphous carbon when methane is decomposed and grown,but the formation of amorphous carbon can be avoided by introducing defect sites on the surface of graphene layer through N-doping,therefore,the dispersion property of active metals can be effectively improved.Compared with Co Mo/CM catalyst,the HDS conversion of DBT and 4,6-DMDBT on the Co Mo/NCMG2 catalyst increased by 17.5%and 16.6%,respectively.Although the proportion of the“Type-II”CoMoS active phase is greatly increased,the existence of methyl group in 4,6-DMDBT still has a large steric hindrance effect on the adsorption process between the S atom and the active site.To promote methyl migration and reduce steric hindrance in 4,6-DMDBT,the idea of introducing Y-type zeolite into support and modifying Y-type zeolite with graphene by CVD method is proposed,the introduction of Y zeolite resulted in the transfer of methyl load on the beta site of 4,6-DMDBT,which improved the adsorption of S atom on the catalyst surface and then realized the deep hydrodesulfurization.The HDS activity of Co Mo/MYGx catalysts at 360°C,5 MPa and 1.5 h^-1 is evaluated by using inferior coker diesel(sulfur content4981.4μg·g^-1)as raw material,Co Mo/MYG2 catalyst has the highest HDS activity,and the sulfur content in the hydrogenation product is less than 10μg·g^-1,which meets the requirement of V/VI diesel standard for sulfur content,and the yield of liquid product is as high as 98.8%.