Oxidative Depolymerization Of Lignin Into Aromatic Compounds In Liquid Systems

Lignin is the only renewable aromatic resource in nature,its utilization has attracted much attention worldwide.However,the robust and complex structure makes lignin the most difficult ingredient to use in the three major components of lignocellulosic biomass.Among various high-value utilization approaches of lignin,catalytic oxidation is one of the most feasible conversion methods.With the designed catalyst,solvent and oxidant,lignin can be oriented converted into high value-added platform chemicals,which has a good industrial application prospect.But there are still many issues existing,such as most of the reported catalysts are non-reusable homogeneous catalysts,most of the employed solvent systems are the alkaline solutions or toxic organic solvents,most of the catalytic oxidation systems obtained from lignin models don't work on lignin macromolecules,and so on.Solving the above issues is significantly important for the full component utilization of biomass.In this paper,we developed a new catalytic oxidation system Sn/TiO₂-Me CN/H₂O-O₂and successfully applied it to the catalytic depolymerization of lignin and whole biomass.First,the applicability of Me CN/H₂O co-solvent and organovanadium catalysts on the catalytic oxidation of lignin dimers was studied,then the catalytic oxidation capacity of homogeneous transition metal chlorides was investigated in Me CN/H₂O co-solvent.The results showed that the proper volume ratio of acetonitrile to water for lignin dimer oxidation with organovanadium catalysts was 1:1.After oxidation with 24 h,vanadyl ^IVacetylacetonate can transform 95 mol%of lignin dimer,producing 43 mol%of phenol and 83 mol%of benzoic acid.During this process,the generated V^IV-O and V^V-O complexes resulted in the catalytic oxidation of lignin dimers.In addition,the oxidative coupling of phenolic products was caused by the produced phenolic radicals in oxidation atmosphere.The catalytic capacity of transition metal chlorides on lignin dimer oxidation was better than that of organovanadium,especially Sn Cl₄.It can convert 92 mol%of lignin dimer in 8 h,and produce 42 mol%of phenol and 91 mol%of benzoic acid.In addition,both Sn Cl₄and Zr Cl₄can effectively oxidize lignin macromolecules.Heterogeneous catalysts with excellent hydrothermal stable support and highly active metal elements were prepared,their catalytic oxidation properties on lignin dimers were investigated,and the oxidation mechanism of lignin dimer in heterogeneous catalytic system was discussed.The results showed that TiO₂had the best hydrothermal stability.Its structural properties basically remained after treated in Me CN/H₂O(1:1,v/v)co-solution with 8 h,whereas HZSM-5,?-Al₂O₃and Si O₂underwent element loss and structural changes.Among Sn/TiO₂,Zr/TiO₂,Cu/TiO₂and V/TiO₂catalysts,Sn/TiO₂exhibited the best catalytic performance.It can convert 98 mol%of lignin dimer to 51 mol%of benzoic acid,46 mol%of phenol,and 6 mol%of phenylglyoxylic acid at 140 ^oC in 8 h.In the heterogeneous oxidation system,the lignin dimer and O₂adsorbed on the catalyst surface first,then the oxygen atom attacked the C_?position,inducing the cleavage of C-O bond.The other oxygen atom would go on attacking the C_?or C_?position,producing benzoic acid and phenylglyoxylic acid respectively.Among the two competing paths,the dominant path was the one to produce benzoic acid.The Sn/TiO₂-Me CN/H₂O-O₂catalytic oxidation system was applied to the oxidative depolymerization of lignin,and propionaldehyde was used to cap the active?,?-diol in lignin,so as to inhibit condensation and promote depolymerization.The results showed that the oxidation of maple lignin mainly occurred on the side-chain linkages,which would not destroy the aromatic rings.The highest monomer yield of GVL lignin was 9.2 C%,while that of capped PA lignin was 17.4 C%,which was almost doubled.The dominant products of GVL lignin were aromatic acids whose highest selectivity was up to 73.9%.Whereas,the main products of PA lignin were aromatic aldehydes.Even under the worst conditions,the total yield of aromatic aldehydes still reached 41.2%.Furthermore,compared with GVL lignin,the depolymerization rate of PA lignin were significantly accelerated,the degree of depolymerization of PA lignin was much deeper,and the product homogeneity from PA lignin was also much better.The Sn/TiO₂-Me CN/H₂O-O₂catalytic oxidation system was further applied to the oxidative depolymerization of whole biomass.Based on the concept of lignin-first,the optimal conditions for maple wood depolymerization were obtained.Then,the applicability of this system to a serious of biomass and the structural evolution of their lignin component were investigated.The results showed that 19.7 wt%of aromatic monomers were produced under the optimal condition.Among them,the two major products were syringaldehyde and vanillin,with the yield of 4.3 wt%and 1.6 wt%,respectively.The total selectivity of aromatic aldehydes were as high as 81.1%,basically realizing the oriented catalytic depolymerization of the lignin component in maple.In addition,the species and yields of lignin products in different biomass varied a lot.The product species depended on the biomass type,whereas the product yields relied on the lignin properties.After catalytic oxidation in the Sn/TiO₂-Me CN/H₂O-O₂system,the side-chain linkages within lignin component were completely cleavaged,resulting in abundant aldehyde and ketone end groups.Furthermore,oxidative modification of the S-and G-type units occurred,together with the loss of the G-and G'-typed units.