Catalyst Synthesis And Its Catalytic Mechanism For Hydrodenitrogenation And Hydrodesulfurization Of Liquids From Coal Pyrolysis/Direct Coal Liquefaction

As the second largest economy in the world,China's energy demand keeps growing at a high level.The foreign dependence ratio of oil has exceeded 70%.In the post-epidemic era,clean and efficient utilization of coal to obtain liquids from coal pyrolysis/direct coal liquefaction is expected to guarantee China's economic development and energy security.Due to the large amount of nitrogen-and sulfur-containing heterocyclic compounds in liquids from coal pyrolysis/direct coal liquefaction,they cannot be directly used as fuel oil or chemical raw materials before hydrotreating.Transition metal sulfide catalysts prepared by traditional sulfidation methods generally possess low hydrogenation activity,so that high temperature and high pressure are required to effectively remove nitrogen and sulfur compounds.Quinoline and dibenzothiophene with relatively high contents were chosen as research objects.In view of the low hydrogenation activity of transition metal sulfide catalysts,and the ambiguity of the interaction mechanism between hydrodenitrification and hydrodesulfurization on the surface of noble metal Ptcatalysts as well as the unclear reaction mechanisms,following works have been primarily conducted:1)Preparation of in situ presulfide catalysts with NiMoS active phase and hydrodenitrogenation and hydrodesulfurization activity evaluation;2)Promotional mechanism of TiO₂ addition to Pt/?-Al₂O₃ in hydrodenitrogenation and hydrodesulfurization;3)Combined in situ infrared experiments and theoretical calculations in the study of HDN reaction mechanisms over Ptcatalyst;4)Promotional mechanism of Fe to Pt/?-Al₂O₃ in hydrodesulfurization;5)Preliminary experiments in hydrotreatment of coal direct liquefaction oil.Main results and conclusions are summarized as follows:1.Presulfide catalysts were prepared by thermal decomposition of?NH₄?₂MoS₄.Appropriate slab length and stacking number were obtained by H₂ activation.H₂ facilitated the incorporation of Niinto the edges of MoS₂ slabs to create more coordination unsaturated sites?sulfur vacancy?,which favored the increase in the total number of active sites.The proportion of Ni-Mo-S active phase increased to 71%.The conversion of quinoline hydrodenitrogenation and dibenzothiophene hydrodesulfurization reached 23.8%and 93.3%at 340? and 3 MPa,respectively.Dibenzothiophene tended to adsorb on corner and S-edge sites,while quinoline preferred?b?rim and Mo-edge sites.2.The introduction of TiO₂ weakened the strong metal support interaction between Ptand?-Al₂O₃.Hydrogen spillover led to the partial reduction of TiO₂(TiO_2-x)which subsequently transferred electrons to Ptoxides,and facilitated the reduction of Pt,thus increasing the number of metal active sites and the dispersion of Pt.In addition,the electron transfer induced Ptparticles on the catalyst surface to present electron rich state,which was conducive to the most stable adsorption of quinoline.The hydrodenitrogenation rate of quinoline increased from 80.2%to 91.5%by adding TiO₂ as a promoter.Owing to the decreasing number of strong acid sites after the introduction of TiO₂,the irreversible strong adsorption of quinoline on acidic sites was reduced,and the formation of carbon deposition in the reaction process was mitigated.3.The mechanisms of hydrodenitrogenation over Ptcatalysts were investigated by combining in situ FTIR experiments and DFT calculations of pyridine hydrogenation.The results showed that hydrogenation was not a structure-sensitive reaction over Ptsurface,but the situation was quite different for C-N bond,which preferred to take place at the coordination unsaturated step sites on Pt.During hydrogenation,pyridine preferentially formed piperidine at the terrace sites,while it could feasibly conduct denitrogenation to yield n-pentane at the step sites.The real intermediate for C-N bond cleavage was found to be the hydrogen-deficient pentahydropyridine,rather than the generally proposed piperidine.The C-N bond cleavage of pentahydropyridine occurred at the step sites of Ptsurface through nucleophilic substitution mechanism,and the activation of?-C played a key role in C-N bond cleavage.4.The addition of Fe enhanced the reduction of PtO_x species,which led to the improvement of the dispersion of Pt.XPS characterization indicated that electron transfer occurred from Ptto Fe and induced Ptto present electron deficient state.HAADF-STEM images showed that Ptand Fe elements were uniformly distributed on the surface of the catalysts.These results demonstrated that Ptand Fe had formed Pt-Fe alloy.The hydrodesulfurization of dibenzothiophene could reach 97.7% at 300? and 3 MPa.DFT analysis showed that the addition of Fe changed the adsorption configuration of dibenzothiophene and caused a slight decrease in adsorption energy.Due to the strong affinity of Fe atom to S atom,the C-S bond was elongated and hydrogenolysis took place under the attack of hydrogen atom,which resulted in C-S bond cleavage was thermodynamically and kinetically more favorable to occur on the surface of Pt-Fe alloy.5.The prepared catalyst was used in the hydrogenation of dephenolized liquids from direct coal liquefaction oil,and showed good hydrogenation activity.The content of aromatics decreased,while that of hydrogenated aromatics and cycloalkanes increased.Oxygen and nitrogen containing compounds were also reduced.