Research On Oxidation Of Lignin To High-value Phenolic Compounds In Alcohol-water System

Lignin is the only renewable source of aromatic structure on earth and the most potential alternative to fossil fuels.Lignin oxidative depolymerization in liquid phase can provide high value aromatic compounds with high such as aromatic aldehydes and aromatic ketones.In this paper,using corn straw lignin as raw material and oxygen as oxidant,we investigated the effects of solvent type,alcohol-water ratio,reaction temperature,reaction time and oxygen pressure on the oxidative depolymerization of lignin in alcohol/water mixed solvents,and prepared a series of Ce-based composite metal oxide catalysts which were applied to lignin oxidative depolymerization.All these experiments laid the experimental and theoretical foundation for the high value utilization of lignin.We used orthogonal test to investigate the effects of solvent type,alcohol-water ratio,reaction temperature,reaction time and oxygen pressure on lignin oxidative depolymerization under non-catalytic conditions.The products of lignin oxidative depolymerization in alcohol-water mixed solvents without catalysis were aromatic aldehydes,aromatic ketones,aromatic acids,dimers and oligomers.There were seven main monomer products:hydroxybenzaldehyde,vanillin,apocynin,methyl vanillate,vanillic acid,syringaldehyde and acetosyringone.The yield of liquid phase products and the absolute peak area of each monophenol product were used as test indexes respectively.Then we found that the effect of reaction temperature on lignin oxidative depolymerization was the greatest,followed by solvent,alcohol-water ratio and reaction time,and the effect of oxygen pressure on lignin oxidative depolymerization was the smallest.The optimum conditions were ethanol/water as solvent,alcohol/water ratio 1:1,reaction temperature 180?,reaction time 120 min,oxygen pressure 0.6 MPa.Under this condition,the liquid phase product obtained from lignin depolymerization was 61%.FTIR analysis showed that the characteristic structure groups of lignin oxidative degradation products of corn straw are similar to those of corn straw lignin,but the chemical structure of oxidative residue changed greatly,which is to degrade incomplete lignin.During the oxidative depolymerization of lignin,the demethoxylation of aromatic rings occured and the structure of benzene rings was destroyed.The composite metal oxides of rare-earth metal Ce and transition metal Co,Fe,Ni and Ce are prepared by sol-gel method and applied to the oxidative depolymerization of lignin.When lignin is oxidized and depolymerized in liquid phase catalyzed by CeCoO_?complex oxides,the optimum conditions are as follows:lignin 0.2g,catalyst0.02g,ethanol/water?v:v=1:1?,oxygen as oxidant?0.6MPa?,reaction temperature210?,reaction time 30 min.In this condition,the liquid product yield of lignin depolymerization is 61%,the vanillin yield of depolymerization is 4.39%,and the syringaldehyde yield is 2.81%.Among the four kinds of Ce-based composite metal oxides,CeCoO_?has the best catalytic effect.The existence of Co species in CeCoO_?in the redox process of Co³⁺?Co²⁺?Co³⁺,which may be the reason for its better catalytic effect.On the other hand,when Co enters into CeO₂ skeleton to form composite oxides,it promotes the redox ability of CeO₂ skeleton.CeCoO_?composite oxides were modified by rare earth metal La and transition metal Cu and Ni respectively.La doping gradually decreased the grain size of the samples and was beneficial to the formation of Co²⁺species,while the redox ability of CeO₂ phase decreased.The yield of vanillin and syringaldehyde reached the highest when the doping content of La was x=0.1,which were 5.15%and 3.3%respectively.The doping of copper also inhibits the redox ability of CeO₂ skeleton.The yield of vanillin and syringaldehyde decreases with the increase of Cu doping.The redox performance of composite oxides is greatly improved by Ni doping.The yield of vanillin and syringaldehyde decreases with the increase of Ni doping.