Effect Of Solvent And Lignin Structure On The Depolymerization Of Lignin

The rapid development of modern industry is along with the rapid consumption of fossil resources, which are non-renewable and their overuse have a serious demage on the environment. Therefore, renewable biomass resources become the necessary requirement of the sustainalbe development. Lignocellulose is an abundant renewable resource. Currently, converting lignocellulose biomass into fuels and chemicals has been a research hotspot. The cellulose and hemicellulose can be effectively converted into fuel ethanol and carbohydrate plat compounds, whereas lignin cannot be converted into chemicals effectively due to its complicated structure, and hence becomes a bottleneck in the utilization of biomass. Therefore, it is of important to explore the depolymerization of lignin for the production of monophenolic compounds in order to promote high value utilization of lignin, and further promotes the utilization of lignocellulosic biomass.Hydrothermal oxidation depolymerization of lignin is one of the main methods for the production of monophenolic compounds from lignin. Hydrothermal oxidation depolymerized of lignin was carried out by using H2O2 as the oxidant and CuO/Fe2(SO4)3/NaOH as catalysts based on our previous work. The effect of the solvents, including water, methanol, 1,4-dioxane, tetrahydrofuran, ethanol, and their co-solvents, on the depolymerization of lignin was investigated. It was found that the suitable alkalinity of the reaction medium and a higher solubility of lignin are the two key factors affecting yields of monophonic compounds. The yield of monophenolic compounds reaches 17.92% by using 1:1 of methanol/water as the reaction medium. The suitable proportion of methanol to water not only effectively maintains the alkalinity of the reaction medium and the solubility of lignin, but also suppresses the demethoxylation during the process of depolymerization reaction, resulting in an increase of contents of syringyl style monophenolic compounds which prevents the re-polymerization. Consequently, the yield of monophenolic compounds increased.The structure and properties of lignin are varied by different sources, resulting in a various of the yield of monophenolic compounds even under the same depolymerization process. Two kinds of lignins with different content of S and G subunits were depolymerized in ethanol/formic acid system at different temperature, and the effect of methoxy content on lignin depolymerization was studied. Elemental analysis, FT-IR, 2D-NMR, Py-GC-MS were used for analysis the structure of two lignins. It was found that content of S monomer in wheat straw was higher than in Sigma lignin. Contributed to the more content of S subunit, wheat straw lignin was depolymerized easier, which means that the more content of S subunit facilitated lignin depolymerization. The degradation ratio and the yield of monophenols were 86.4% and 17.34% by using ethanol as medium at 300 °C, 2 h. Compared to feedstock, the S/G ratio in the products was significantly improved, indicating that the more content of S-type monomers in lignin facilitated lignin depolymerization.