Study On Cu-Based Catalysts For Liquid-Phase Hydrodeoxygenation Of Lignin-Derived Anisole To Arenes

Lignin is the only renewable carbon resource rich in aromatic structure in nature.Its utilization for fuels and high value-added chemicals is potential to release the pressure of environment and excessive dependence on fossils.The bio-oil obtained from lignin pyrolysis cannot be used as engine fuel directly due to its high content of oxygen.Thus,an upgrading to generate hydrocarbon fuel via selective hydrodeoxygenation（HDO）is needed.Frequently studied transition metal（such as Ru,Pt,Ni,etc.）catalysts showed high HDO activity in liquid-phase HDO of lignin-derived phenolic compounds,but the undesired saturation of benzene ring is dominant,resulting in low arenes selectivity.Metal Cu-based catalysts have shown high activity in HDO of fatty compounds containing oxygen,thus it is speculated to be potential in the conversion of lignin and its derivatives.Herein,we applied metal Cu-supported catalysts in liquid-phase HDO of lignin-derived anisole,and optimized the Cu active sites by metal oxides to promote the direct deoxygenation to arenes and inhibit the saturation of aromatic rings.Firstly,oxyphilic metal oxides,MO_x（M=Re,Mo and W）,were introduced into Cu/SiO₂ catalyst,and bifunctional sites were constructed between Cu sites and MO_x.By the synergistic effect of Cu site for H₂ activation and MO_x site for the activation of C-O bond in anisole,the HDO activity and arenes selectivity were both enhanced.While Cu-Mo O_x/SiO₂ and Cu-WO_x/SiO₂ owned higher selectivity to methyl-phenolics due to the existence of abundant acid sites,Cu/SiO₂ modified by ReO_x showed better HDO performance and deoxygenation capacity.When optimizing the structure between Cu and ReO_x sites,the Cu dispersion was enhanced with the addition of ReO_x,and more Cu-ReO_x synergistic sites were exposed on catalysts.Furthermore,the adsorption strength of oxygen atoms and aromatic rings in anisole was also optimized over Cu-ReO_x synergistic sites for a higher arenes selectivity.The highest arenes selectivity was obtained on 3Cu-ReO_x/SiO₂（58.2%）,and anisole conversion reached to 86.7%under liquid phase condition（320℃,2 MPaH₂）.Cu/MnAlO_x catalysts were prepared from hydrotalcite-like precursors,where MnO_x migrate to the surface of Cu particles.And abundant oxygen vacancies（O_v）were produced on MnO_x sites during H₂ reduction.The geometric effect of MnO_x on Cu surface and the enhancement effect of O_v sites for anisole HDO reaction were thoroughly studied.It was found that the Cu sites were blocked from direct interaction of aromatic rings in anisole over MnO_x modified Cu surface,thus the saturated hydrogenation activity on monometallic Cu/Al₂O₃ catalyst was efficiently inhibited.Besides,the synergistic sites between metal Cu and O_v on MnO_x enhanced the hydrodeoxygenation activity of anisole to arenes.Under the reaction conditions of 300℃and 2 MPaH₂,the highest arenes selectivity was obtained on 4Cu/8Mn4AlO_x（around65%）,which was more than 6 folds of that on 4Cu/4Al₂O₃,and the selectivity of saturated products was reduced to about 30%.We synthesized a novel catalyst（Cu@NS-SiO₂）with high anisole conversion（63.4%）and arenes selectivity（58.9%）,and then loaded alkali metal oxides M₂O（M=Na,K,Rb and Cs）on 10Cu@NS-SiO₂ to study the electron-donating effect of alkali metals on Cu sites in anisole HDO reaction（300℃,2 MPaH₂）.The electron density on Cu surface was increased due to electron transferring from M₂O,and thus the adsorption and saturated hydrogenation of aromatic rings was significantly inhibited.As a result,much higher arenes selectivity was obtained on M/Cu@NS-SiO₂,especially higher than 80%on Rb/10Cu@NS-SiO₂ and Cs/10Cu@NS-SiO₂.Based on Cs/10Cu@NS-SiO₂ catalyst,we conducted a further study on the geometric and electronic effects and HDO mechanism on Cu active sites.With Cs loading increasing,the electronic density over Cu particles surface gradually increased along with more intense geometric modification.Furthermore,the catalysts performance and DFT calculation of anisole adsorption revealed that the saturated hydrogenation was significantly inhibited due to both electronic and geometric effect of M₂O,and more efficient oxophilic sites for anisole deoxygenation was supplied at Cs atoms instead.When the loading of Cs is 2 wt.%,the selectivity of arenes products reached 83%.Besides,the arenes production activity based on Cu sites was 0.34mmol·m _Cu^-2·h^-1,achieving a high level over transition metal-based catalysts in liquid-phase HDO of lignin-derived phenolics.