Hydrodeoxygenation Of Lignin-derived Model Compounds On Fe-Ru And Co-Ru Bimetallic Catalysts

Lignin is abundant in nature,typically composing 15-30 wt%of lignocellulosic biomass,which is generally considered as an ideal source for the production of value-added chemicals in the future.However,the high oxygen content of lignin pyrolysis products leads to high viscosity,poor thermal stability and low energy density,all of which brings many problems to the transportation and potential utilization.Selective hydrodeoxygenation?HDO?is an effective method to improve the quality of such bio-oil,through removing oxygen atoms and preserving the aromatic ringsNoble metals,such as Ru,Pd,Rh,have high HDO activity,but tend to saturate the aromatic rings.Although transition metals such as Fe,Co with strong oxyphilicity favor the selective cleavage of aryl C-O bonds without hydrogenating the arene rings,their activity is much lower than that of noble metals.Such dilemma can be resolved by the synergistic effects between these two types of metals,resulting in both excelled activity and superior selectivity toward the desired aromatic products.Therefore,a series of bimetallic catalysts were prepared and applied to the conversion of lignin-derived phenolic compounds to aromatics by HDO in this work.We mainly focused on the conversion of the strongest C-O linkages?4-0-5 linkages?in lignin.Hence diphenyl ether was employed as a model compound,and the reaction was performed in decalin at 20 bar H₂ and 200 ?.Pd,Ru,Co-based monometallic catalysts and Pd-Fe,Ru-Fe,Ru-Co bimetallic catalysts were prepared and their catalytic performance was studied.Characterization methods,like XPS,H₂-TPR,etc.are employed to elucidate the physical structure and active sites of the catalysts.Results showed that a large number of cyclohexyl ethers and cyclohexyl phenyl ethers were produced in the HDO of diphenyl ether over Pd-based monometallic catalyst.Addition of Fe in the Pd-Fe bimetallic catalysts improved the ability of cleaving C-O bonds,but barely inhibited hydrogenation of aromatic rings.The selectivity of saturated hydrogenation products reached nearly 100%.The main products of Ru-based monometallic catalyst were found to be cyclohexanol and cyclohexane,indicating its enhanced hydrogenolysis than that of Pd.The highest yield of benzene reaches 27%over 0.5Ru1.5Fe/CeO2 catalyst.The addition of Fe not only improved the ability of cleaving C-O bonds,but also inhibited the over-hydrogenation of aromatic rings.Nevertheless,Fe monometallic catalyst itself was found to be inactive in this reaction under the conditions studied.Co monometallic catalyst showed excellent HDO performance,with benzene,cyclohexanol and cyclohexane being the main products.The highest selectivity of benzene is up to 48%,the highest reported in literature,indicating that the over-hydrogenation of benzene was almost completely suppressed.This striking result should be correlated with its strong oxyphilicity,and the improved hydrogenation ability compared to that of Fe counterpart.Ru-Co bimetallic catalyst with small amount of Ru significantly enhanced the catalytic activity,without detrimental effect of selectivity.The HDO mechanism of diphenyl ether was also explored,which was found to proceed via two pathways---hydrogenation and hydrogenolysis.A large number of ring-saturated products are produced by hydrogenation,while benzene and phenol are produced by the direct hydrogenolysis.The addition of Fe improved the selectivity of direct hydrogenolysis.The effects of reaction temperature and pressure of hydrogen on the selectivity and conversion were also investigated.Direct hydrolysis is preferred at low partial hydrogen pressure and high temperature.Considering saving energy and catalytic activity,we choose 200 ?,2 Mpa as the reaction condition.This work focused on Fe-Ru,Co-Ru catalysts and the insight of synergistic effect of bimetallic catalysts.The study sheds light on HDO of lignocellulose conversion that is critical to the valorization of biomass.