Study On The Preparation And Hydrogenation Activity Over Unsupported Ni-Mo-W Catalysts

The application of unsupported transition metal catalysts in hydrogenations have been addressed considerable attentions in recent years.Compared with traditional supported catalysts,unsupported catalysts could provide more amounts of active sites,and demonstrate higher hydrodesulfurization performance under mild conditions.Nevertheless,unsupported catalysts have some disadvantages such as low surface area and pore volume,poor metal active component dispersion and weak mechanical strength,which hinder their industrial applications.In academia,it is necessary to develop new methods for the preparation of unsupported metal catalysts to improve their performance.Thus,we carried out a series of research work,including the following sections:Unsupported Ni-Mo bimetallic catalysts were synthesized using hydrothermal and coprecipitation methods,which could be classified as precursor preparation routes.The respective synthetic conditions were studied carefully,and the performance of Ni-Mo catalysts prepared under optimal conditions based on two methods was comprehensively compared with each other.The results showed no differences with respect to the metal composition between Ni-Mo catalysts synthesized by different methods.The oxidation state structures were NiMoO₄,and the sulfide state structures were MoS₂.However,obvious differences in micro-structure,sulfidation morphology and HDS activities can be observed.Unsupported catalysts prepared by hydrothermal were crystalline,and SEM micrographs revealed that the active components were seriously aggregated and lack of surface defects.Comparing to the catalysts obtained by hydrothermal method,the catalysts prepared by coprecipitation were amorphous,and their particle sizes were also smaller and showed more surface defects.Therefore,unsupported catalysts prepared by coprecipitation showed better HDS activity.The synthetic conditions for unsupported Ni-Mo-W trimetallic catalysts using coprecipitation method were carefully studied.Comparing to unsupported Ni-Mo bimetallic catalysts prepared by the same method,Ni-Mo-W catalysts showed better hydrodesulfurization performance.It has been found that,a new structure of oxidized catalyst can be generated after introducing W.XRD and LRS analysis revealed the formation of a new kind of metal compound with different Mo or W coordination,and significant amounts of octrahedral Mo/W compounds in Ni-Mo-W catalysts,which could generate rich MoS₂/WS₂active phase during the transition from oxide to sulfide.Good synergistic effects among the active components of Ni-Mo-W catalysts could be induced under appropriate metal ratios,thus the active phases had suitable length and necessary curvature which could show higher HDS activity.Unsupported Ni-Mo-W catalysts were amorphous,therefore their surface area and pore volume were larger than that of unsupported Ni-Mo catalysts.The HDS rate of catalytic cracking diesel was 93.8%,which was 1.7%higher than that of Ni-Mo bimetallic catalysts.The new catalysts Ni-Mo-W-PEG600 and Ni-Mo-W-PEG600-DE,which combined the PEG600 and DE were used to improve the metal dispersion and mechanical properties of unsupported Ni-Mo-W catalysts.It has been found that the surface area and pore volume of Ni-Mo-W catalysts were significantly improved after adding PEG,which leads to higher HDS activity.The results indicated that the dispersion pathways and mechanisms of active components were related to molecular weight of PEG,and the maximum HDS rate was obtained when the molecular weight of PEG was 600.TPR results showed that the amounts of Mo/W components with octrahedral coordination could be significantly increased after adding PEG600,and the active component could be easily restored;SEM images showed that composite oxide particles of Ni-Mo-W-PEG600 were regular vesicles and orderly arranged,which was helpful for the exposure of surface defects;HRTEM images showed more stacking layers,bigger bending degree and richer active centers on Ni-Mo-W-PEG600.When using DE as dispersant,the HDS activity of Ni-Mo-W-PEG600 could be further improved,the rich pore structure of DE could make the pores of active components more prosperous,leading to a good inside-out diffusion of sulfides and products,and the good adsorption ability further dispersed active particles.In the mean time,the mechanical strength of Ni-Mo-W-PEG600-DE was 4 times higher than that of sample without DE,which could overcome the disadvantage of mechanical strength on unsupported catalysts and maintain their good chemical and physical stability under high temperature and bed pressure.In order to further investigate the hydrogenation performance of Ni-Mo-W-PEG600-DE,its activity was compared with FHUDS-2 using diesel as feed.It was found that FHUDS-2displayed lower MoS₂/WS₂ stacking layers and lower removal activity of DBTs as compared to Ni-Mo-W-PEG600-DE.On the contrary,the active phase of unsupported Ni-Mo-W-PEG600-DE showed higher stacking layers and distributed uniformly,thus the hydrodesulfurization activity of unsupported Ni-Mo-W-PEG600-DE was 2.4 times higher than that of FHUDS-2,and the HDS rate of catalytic cracking diesel on Ni-Mo-W-PEG600-DE was 99.8%.Furthermore,the cetane value of inferior diesel was increased 15.1.The hydrogenation mechanism of unsupported catalysts was proposed based on comparison and analysis of the products generated from hydrogenation reactions over Ni-Mo-W-PEG600-DE and traditional supported catalysts and the microstructures of these catalysts.It was found that the HDS process on FHUDS-2 followed direct desulfurization route?DDS?,S atoms in DBT adsorbed in active centers by means of?bonds,and fracture of C-S bonds was limited due to the larger steric hindrance,while the HDS process on Ni-Mo-W-PEG600-DE followed hydrodesulfurization route?HYD?,DBT absorbed in active centers by means of?bonds,and fracture of C-S bonds took place after hydrogenation,which can promote the removal activity of DBTs,especially for 4,6-DMDBT.Therefore,the HDS activity of Ni-Mo-W-PEG600-DE was much higher than that of FHUDS-2.