Study On Selectively Catalytic Hydrogenation Of Biomass Model Compounds With Nickel-Based Catalysts

Biomass is a renewable energy from a wide range of sources and can be converted into fuel or platform chemicals in a variety of ways.Among them,catalytic hydrogenation is one of the important ways of biomass utilization,which is of great significance in the stabilization treatment of biomass pyrolysis oil,the preparation of the biomass liquid fuel and the production of fatty alcohol.In this study,a variety of Ni based catalysts were designed and prepared for the hydrogenation of important biomass components and the selective production of target products to explore the catalytic mechanism and provide reference for the industrial utilization of biomass resources.Repolymerization is a huge problem in the storage and processing of biomass pyrolysis liquid（PL）.Herein,to solve this problem,mild catalytic hydrotreatment of PL was conducted to convert unstable PL model compounds（hydroxyacetone,furfural,and phenol）into stable alcohols with Ni/SiO₂ catalyst.The mild hydrotreatment of the single model compound was studied to determine the reaction conditions and pathways.More importantly,the mild hydrotreatment of mixed model compounds was evaluated to simulate the PL more factually.In addition,the effect of the interaction between hydroxyacetone,furfural,and phenol on the reaction results was also explored.In view of the phenomenon that phenol was inhibited in mixing mild hydrotreatment,competitive adsorption on Ni surface among model compounds was proposed based on DFT calculation and characterization results.And more step sites may help to reduce hydrogenation of phenolics.NiFe intermetallic compound（NiFe IMC）was prepared by introducing Fe into Ni/SiO₂ to increase the deoxygenation activity of Ni based catalysts on the hydrodeoxygenation of fatty acid.Compared with the monometallic Ni/SiO₂,appropriate introduction of Fe results in the complete conversion of lauric acid and nearly 100%yield of alkane as well as satisfactory stability on conversion.Further study on mechanism shows that for one thing,strong spin polarization enhances the interaction between Fe sites and aliphatic acid,and the subsequent dissociation of C（O）OH bond indicated by the DOS and transition state analysis.For another,dissociation of H2 on Ni site is promoted because of the higher charge density around Ni in the IMC according to the in-suit FTIR and Bader analysis.However,with the repeated use of the catalyst,the selectivity to alkane decreased gradually,which is ascribed to the oxidation of metal Ni-Fe bimetallic sites.The synergistic effect above significantly promoted the removal of the first O in the HDO of fatty acid with NiFe IMC.This demonstrates that the reduced Ni-Fe bimetallic sites rather than the oxidized ones are the active phases in the HDO of aliphatic acid to produce alkanes with the NiFe IMC.The oxidation treatment was performed on the NiFe IMC to prepare NiFe composite oxide catalyst（NiFe COC）based on surface reconstruction.Herein,the steric hindrance of lattice oxygen suppressed the adsorption of aldehyde,which is the crucial intermediate of objective product alcohol converting into a side product,to inhibit the further conversion of alcohol in the HDO of fatty acid.NiFe COC reaches 100%conversion of lauric acid with 90%selectivity to lauryl alcohol.Kinetic analysis indicated that the apparent activation energy of side reaction increases by 71.1 kJ/mol for NiFe COC relative to NiFe IMC,evidencing the inhibition for the conversion of objective product alcohol into alkane for NiFe COC.Furthermore,DFT calculation also suggests that the activation energy of the side reaction increases by 0.33 eV on NiFe COC compared to NiFe IMC.In addition,used NiFe COC can be totally regenerated via surface reconstruction during the reduction-reoxidation treatment.In order to improve the utilization rate of carbon atoms in the HDO of fatty acid to alkanes,a series of NiM/SiO₂（M=Ce,Co,Cu,Fe,Sn,Zr,Mo）catalysts were prepared to investigate the effects of different doped metal on the product distribution of the HDO of fatty acid.The results indicate that Mo doping can obviously improve the conversion of lauric acid and selectivity of dodecane,which possess the same number of C atoms with the reactant,in the product.Lauric acid can be completely converted into alkanes with Ni₅Mo₇/SiO₂,and the dodecane selectivity is more than 70%,which proves that hydrodehydration of fatty alcohols is promoted significantly,while decarbonylation is inhibited to some extent.Interaction between Ni and Mo promoted the reduction of Mo,and the low-priced Mo contributed to the dehydration reaction according to characterization.Meanwhile,adsorption of alcohol on Mo is more favorable than on Ni site,and the hydrogen bond between hydroxyl hydrogen and O atoms on the catalyst improves the adsorption stability of fatty alcohol.Further COHP analysis indicates that C-OH bond was activated when alcohol was adsorbed on Ni₅/MoO₂ surface,which promoted the hydrodehydration of fatty alcohols and improved carbon atom utilization.