Selective Hydrogenolysis Of Lignin And Char Elimination In The Presence Of La-Ni/MgO

The extensive use of fossil resources aggravates the global energy crisis and environmental problems.Therefore,it is highly desired to explore new alternatives to non-renewable fossil resources,and the development of efficient and green energy sources have become the key to the transformation of the energy structure.Biomass is the fourth energy source and has become a typical representative of low-carbon energy.This natural biopolymer has also been regarded as the most promising substitute for non-renewable fossil energy because it is the unique fixed carbon-containing renewable resource on the globe.The efficient utilization of biomass also provides an important way for Chinese energy structure transformation,which is of great significance for extending the concept of green development.As one of the three components of biomass,lignin contains ample natural phenolic polymers and many special functional groups,such as methoxy,carboxyl,carbonyl,phenolic hydroxyl group,alcohol hydroxyl group and other multi-carbon side chains.Therefore,the lignin can be effectively converted into high value-added chemical through the depolymerization.However,many reported processes have challenged by considerable technical problems of low lignin conversion,low product yield and poor selectivity.In addition,the serious char formation resulted from the re-polymerization of the phenolic oligomer is also a big barrier for the efficient utilization of lignin.In this perspective,novel strategies for selective conversion of lignin to versatile petrochemicals with reduced char generation are important.Herein,we propose an efficient catalytic system for the selective cleavage of the specific linkage in lignin molecule and for the suppressing char formation using the La-Ni/Mg O catalysts.The results demonstrated that the ester linkage in the lignin molecule can be selectively cleaved,resulting in a versatile petrochemical of 4-ethylphenol.In addition,some key scientific problems were resolved,such as the relationship between catalyst structure and activity,the evolution of lignin structure,and the pathway for lignin selective hydrogenolysis.And thus,these findings provide a new insight for the valorization of the natural aromatic polymer of lignin.The main contents of this dissertation are summarized as follows:?1?La-Ni/Mg O catalysts,which had been prepared from nickel nitrate?Ni?NO₃?₂·6H₂O?,lanthanum nitrate?La?NO₃?₃·6H₂O?,and magnesium oxide?Mg O?,were extensively characterized by XRD,BET,SEM,TEM,XPS,CO₂-TPD and H₂-TPR.It was found that the doping of La substantially reduce the formation solid solution of Ni O and Mg O,and it results in an increase of the Ni particle and the base strength of catalyst.Subsequently,we measured the catalytic activity of these synthesized La-Ni/Mg O materials on the selectively hydrogenolysis of bagasse lignin.The experimental results showed that 6%La-20%Ni/Mg O catalyst has the highest catalytic activity under the optimized reaction conditions?270 ^oC,4 h,3.0 MPa H₂?.In which,84.44%of the lignin was converted,affording a monophenol yield of16.50%and a 4-ethylphenol selectivity of 43.45%?the yield of this chemical was 7.17%?.Importantly,the yield of THF-insoluble fraction?biochar?was only 11.04%,which was significantly lower than those with 20%Ni/Mg O.we also investigated the structural evolution of lignin before and after depolymerization using different characterization methods including GC-MS,GPC,FT-IR,EA,HSQC-NMR.The results reveal that all structural units of lignin?H,G,and S?have been broken in this process.Nevertheless,the p CA8 structure derived from the H-unit showed significantly higher reactive than others,which also explained well the impressive selectivity of 4-ethylphenol.?2?The hydrogenation activity of La-Ni/Mg O catalysts prepared by different methods?co-impregnation,step-impregnation,co-precipitation,citric acid complexation?and take further investigated.Under the optimized reaction conditions?270 ^oC,4h,3.0MPa H₂?,the yield of monophenol and 4-ethylphenol increased to 17.78%and 7.31%,respectively,whilst the solid residue was decreased to 10.57%.By through a series of catalyst characterization methods?XRD,BET,CO₂-TPD,XPS?,we found that the active component La introduced by the citric acid complex promoted the high dispersion of Ni on the carrier Mg O,which results in the weakening of the interaction between Ni O and Mg O,promoting the activity of catalyst and inhibiting the re-polymerization of phenolic oligomer.In addition,we proposed a plausible pathway for the selective hydrogenolysis of lignin to the main product of 4-ethylphenol through the hydrogenolysis experiments of lignin model compounds.The experimental results showed that 4-ethylphenol was obtained by cleaving the ester bond between the p CA structure and lignin fragments?such as H,G,and S units?and its following decarboxylation.