Preparation And Catalytic Performance Of Ni-based Catalysts For Hydrodeoxygenation Of Fatty Acid Methyl Ester

In this paper,using?-Al₂O₃ as support,nickel and copper as active components,molybdenum as additive,series of Ni-based catalysts?including Ni/?-Al₂O₃,Ni-Cu/?-Al₂O₃,Ni-Cu-Mo/?-Al₂O₃?were prepared by extrusion,crushed and sieved,supersaturated impregnation processes.The physicochemical properties of the catalysts were characterized by XRD,BET,H₂-TPR,NH₃-TPD,XPS,SEM,EDS,TG.Using fatty acid methyl esters as model oil,the hydrodeoxygenation performances of catalysts were evaluated on a fixed-bed stainless steel reactor in trickle bed operation mode.The liquid products were identified and quantitative analysed by gas chromatograph-mass spectrometer and gas chromatograph.The effects of metal loading,impregnation sequence?sequential impregnation and co-impregnation?,operation parameters?reaction temperature;H₂ pressure;weight hourly space velocity and H₂/Oil?were investigated.The main contents and results are as follows:?1?The results of BET analysis show that the N₂ adsorption-desorption isotherm of catalysts exhibited the type IV isotherms,which were attributed to typical mesoporous materials.According to H₂-TPR data,the monometallic nickel catalyst only showed the reduction peak of the NiAl₂O₄ phase in the high temperature zone.The introduction of copper into the Ni-based catalyst can form the NiO-CuO phase to decreases the reduction temperature of the catalysts.When molybdenum was introduced by different impregnation sequence,the molybdenum oxide was reduced at different temperatures.Based on TG results,the addition of copper inhibit the carbon deposition on catalyst surface.NH₃-TPD analysis shows that the?-Al₂O₃ support were prepared by the extrusion method displayed two NH₃ desorption peaks in the low temperature and high temperature zones corresponded to weak acid sites and strong acid sites,the addition of nickel and copper active phase lead to the acid intensity of strong acid sites tended to weaken.When the loading of molybdenum reached 5wt%,one new desorption peak observed was assigned to the medium strength acid sites.As the loading of molybdenum increased,the peak intensity of medium strong acid sites became strong,while the peak intensity of strong acid sites tended to weaken,and the desorption peaks shifted towards low temperature zone.This suggested that the addition of molybdenum phase had significant impact on the surface acid sites of catalysts.Besides,the impregnation sequence may affect the generation of the medium strength acid sites.The results of XPS analysis show that two sharp peaks at852.7 and 870.3eVin Ni2p spectra of catalysts were assigned to Ni⁰.A peak at 933.6-934.6eV in Cu2p spectra of catalysts displayed was ascribed to Cu²⁺.As shown in Mo3d spectra of catalysts,the Mo3d_5/2peaks at 229.4-229.6eV and 232.9eV were related to Mo⁴⁺and Mo⁶⁺,respectively.Moreover,the catalysts prepared with the different impregnation sequence exhibited different element contents.According to SEM-EDS spectra of support and catalysts,the support?-Al₂O₃ displayed an irregular block structure,after impregnation process,the surface of the support became rough,but the structure and size of the support almost unchanged.The catalysts prepared by supersaturated impregnation showed little difference between the actual loading and theoretical of active components,confirming that the supersaturated impregnation method can ensure the loading required.?2?The catalyst performance evaluation confirmed that nickel-copper phase facilitated the decarboxylation/decarbonylation?deCO_X?reaction.The introduction of copper significantly improved the catalytic performance of nickel-based catalysts.Molybdenum-doped nickel copper catalysts ignificantly increased the catalytic deoxygenation activity.Catalyst prepared by the sequential impregnation method?first nickel and copperimpregnation then molybdenum impregnation?exhibited higher deoxygenation activity.20%Ni-6%Cu-5%Mo/?-Al₂O₃catalyst prepared by sequential impregnation?the former Ni and Cu impregnation,the latter Mo impregnation?showed better catalytic performances than others under the same evaluation operation mode(reduction conditions:temperature 400?,pressure 4.0 MPa,hydrogen flow rate60mL/min,reduction time 3h.operation conditions:temperature 350?,pressure 3.0 MPa,H₂/Oil ratio1250mL/min,WHSV2.0 h^-1),the conversion of fatty acid methyl esters retained 97.0%,yield of C_9-18 alkane attained 93.6%.?3?The single factor?temperature;H₂ pressure;weight hourly space velocity and H₂/Oil?experiments of 20%Ni-6%Cu-5%Mo/?-Al₂O₃catalyst were executed.At the optimized operation parameters of 350?,2.5MPa,WSHV=2.0h^–1,H₂/Oil ratio=1250mL/mL,the conversion of FAME and yield of alkane attained98.4%and 94.2%,respectively.?4?The stability of the 20%Ni-6%Cu-5%Mo/?-Al₂O₃ catalyst was tested under relatively appropriate operation conditions.The conversion of fatty acid methyl esters was 98.5%,yield of C_9-18 alkane was 94.8%in the beginning 5h.With the reaction running,the conversion of FAME and yield of C_9-18 alkane decreased gradually.After 45 h running,the conversion of FAME and yield of C_9-18-18 alkane decreased to 88.6%and82.0%.The results of BET and TG analysis of the fresh and spent 20%Ni-6%Cu-5%Mo/?-Al₂O₃ catalysts show that the carbon deposition during the reaction may be the cause of deactivation of the catalyst.