Study On Catalysts For Lipid Hydrotreatment Into Second Generation Bio-Diesel And Their Catalytic Reaction Mechanism

Carbon-neutral bio-energy can replace or partially replace the non-renewable fossil fuel as well as solve the problems of diffcult disposal of agricultural and forestry waste and environmental pulution from them.The second generation bio-diesel,namely green diesel,from hydrotreatment of waste oil and inedible vegetable oil has similar chemical composition with the fossil diesel so it could be utilized directly in the existing diesel engine.At present,the study on second generation bio-diesel focuses on choosing catalyst,especially “Green Catalyst”,which has a low cost,high catalytic performance as well as no environmental pollution.The metal carbide and nickel catalysts are deemed as the most two promising “Green Catalysts” for lipid hydrotreatment.However,the agglomeration of molybdenum carbide and nickel upon the catalysts and their poor dispersion restrict the catalytic development of molybdenum carbide and nickel catalyst,respectively.In order to deal with these problems,herein the molybdenum carbide was supported on the nitrogen-doped activated carbon and nitrogen-rich carbon material,and the multifunctional supports were applied to support the nickel-palladium.Both catalysts were used in lipid hydrotreatment.Moreover,the effect of nitrogen in supports on regulating active sites of molybdenum carbide,the synergistic effect between nickel and palladium as well as the interation betwween Ni-Pd and supports were studied deeply.The main study results are as follows:1.The molybdenum carbide on activated carbon(Mo2C/AC)was prepared with the carbothermal hydrogen reduction method and its catalytic performance in fatty acid methyl ester hydrotreatment was compared with the traditional catalysts.It was shown that Mo2C/AC exhibited much higher activity than Mo O/Al2O3,Mo S/Al2O3,Mo/Al2O3,Ni P/Al2O3 and Ni/Al2O3.The high temperature and hydrogen pressure promoted the hydrotreatment reaction.Meanwhile the high temperature facilitated cracking reaction while the high hydrogen pressure promoted the HDO reaction pathway.2.The molybdenum carbide on activated carbon(Mo2C/AC)and tungsten carbide on activated carbon(W2C/AC)were synthesized with the carbothermal hydrogen reduction method,and the carburization temperature,flowing gas type during catalyst preparation as well as metal loading were studied.In the fatty acid hydrotreatment the activity of Mo2C/AC was higher than the W2C/AC catalyst.The carburization temperature played different roles in both catalysts: high temperature was beneficial for W2C/AC rather than Mo2C/AC because the support of Mo2C/AC was destroyed and Mo2 C particles agglomerated seriously at high temperature.The flowing H2 and N2 for carburization were viable to prepare W2C/AC while N2 was not suitable for Mo2C/AC preparation.For both catalysts when the metal loading was 20 wt%,the catalyst activity attained optimum.3.The nitrogen was doped into the activated carbon by in situ aniline polymerization,and the molybdenum carbide catalyst(Mo2C/NAC)was prepared using N-doped activated carbon(NAC)with carbothermal hydrogen reduction method.After nitrogen doping the Mo2 C particle size was reduced from 6.4 nm to 3.2 nm,and the Mo2 C dispersion was increased from 0.37% to1.19%.Much pyridine nitrogen and pyrrole nitrogen were detected in the N1.0AC-700.In the fatty acid hydrotreatment,the Mo2C/N1.0AC-700 achieved much higher activity than Mo2C/AC.The catalytic activity improvement of nitrogen-doped catalyst was attributed to the pyridine nitrogen and pyrrole nitrogen which can anchor the Mo2 C active sites and thus increase Mo2 C dispersion and reduce Mo2 C agglomeration.The kinetic study indicated that the apparent activation energy of stearic acid hydrotreatment over Mo2C/N1.0AC-700 was 49.7 k J/mol,lower than Mo2C/AC(73.9 k J/mol).In addition,the feedstock with saturated carbon-carbon bond and ester group was easier to convert into hydrocarbons.4.The nitrogen-rich carbon supported molybdenum carbide(MoC/CN)was prepared by carburizing the mixture of ammonium molybdate and dicyandiamide in flowing N2.The nitrogen content in the catalyst support was 59.9% and the summing percentage of pyridine nitrogen and pyrrole nitrogen in total nitrogen reached 95%.The MoC particle size was only 1.8 nm,much lower than 6.4 nm of molybdenum carbide on activated carbon(Mo2C/AC),and MoC/CN had more active sites.In the fatty acid hydrotreatment,catalytic activity of MoC/CN was extremely higher than the Mo2C/AC.The abundant pyridine nitrogen and pyrrole nitrogen in the MoC/CN decreased MoC particle size and improved MoC dispersion.Density Functional theory(DFT)calculations for Gibbs free energy of H2 dissociative adsorption on catalysts indicated that H2 dissociated more easily into H on MoC/CN than Mo2C/AC.5.25%Ni-Pd supported on various supports(SiO2-Al2O3,Al2O3,ZrO2,and Ce0.8Pr0.2O2)was prepared with two-step impregnation method,and their activity was tested in the tristearin hydrotreatment.The Pd addition greatly increased the activity of nickel catalyst.When the Pd addition was 0.75 wt%,the catalytic activity of 25%Ni-Pd/Al2O3 reached the optimum.After Pd addition,the Ni was reduced more easily and thus the number of Ni active sites was increased greatly.In addition,the generated CO upon Ni sites through decarboxylation/decarbonylation moved to the Pd sites,improving the catalytic efficiency of Ni sites.In the tristearin hydrotreatment the activity order of 25%Ni-0.75%Pd on different support was as follows: SiO2-Al2O3 > ZrO2 > Al2O3 > Ce0.8Pr0.2O2.The excellent catalytic performance of 25%Ni-0.75%Pd/SiO2-Al2O3 could be attributed to the following points: surface area and pore volum of25%Ni-0.75%Pd/SiO2-Al2O3 were large;the Ni was reduced easily over SiO2-Al2O3 support;the strong Br?nsted acidity of SiO2-Al2O3 changed the electronic character of Ni-Pd.Moreover,when the SiO2-Al2O3 was used as the support,the hydrodeoxygenation route of hydrotreatment pathways was enhanced,which was resulted from the dehydration of intermediate octadecanol into octadecene over Br?nsted acid sites.