Hydrogen Peroxide Oxidative Depolymerization Coupled Pd/C Hydrogenolysis Of Lignin To Produce Aromatic Monomers

With the rapid development of society,the high-valued utilization of renewable biomass resources has become a hot research topic for the global sustainable development due to the shortage of fossil resources and environmental pollution problems.Lignin is an abundant natural phenolic polymer next to cellulose.But the complex structure and chemical inertness of lignin limited its high-valued utilization.It is of great significance to study the depolymerization of lignin to produce monophenolic compounds for the preparation of chemicals or prepare liquid fuels after hydrodeoxygenation.The catalytic oxidative degradation of lignin under alkaline condition using CuO/Fe₂?SO₄?₃ as catalyst and H₂O₂ as oxidant coupling with the hydrogenolysis of lignin under formic acid system with Pd/C as catalyst was investigated.The results showed that ethanol aqueous as the solvent was fourable for facilitating the oxidative-hydrogenolysis of lignin because ethanol hindered the repolymerization of small molecules depolymerized.In addition,ethanol also acted as hydrogen donors to promote hydrogenolysis.To a certain degree,increasing the concentration of alkali in the oxidative depolymerization process is beneficial to polarize ether bonds in the lignin molecule and further to depolymerize lignin.Compared with the hydrogenolysis of lignin,the hydrogen peroxide oxidative depolymerization coupled with Pd/C hydrogenolysis of lignin under the optimal conditions could obtain higher yield of monophenols,with the yield of monophenols increasing from7.50%into 12.87%.It was found from the analysis of depolymerized products and residues that compared with the hydrogenolysis of lignin,hydrogen peroxide oxidative depolymerization coupled Pd/C hydrogenolysis of lignin contributed to a higher cleavage rate of C?-C?bond,increasing from19.97%to 28.85%,and the content of F-type and S-type products,raising from 8.97%to22.35%and from 2.26%to 8.43%,resepectively.Generally,F-type products with active sites were capped or lack of active sites,so they have low reactivities and probabilities to repolymerize.Also the S-type products have better stability than the G-type or H-type product during the reaction.Therefore,the enhancement of the cleavage of the C?-C?bond in lignin and the better stability of the depolymerization products promoted the yield of monophenols.In addition,it was found that oxidative depolymerization facilitated the conversion of?-alcohol hydroxyl groups into carbonyl groups,and hence promoted the cleavage of?-O-4 bond.Consequently,the depolymerization efficiency of lignin and the yield of monophenols increased.