Microwave Enhanced Liquefaction Of Lignin For The Production Of Bio-oil And The Digestion Of Residual Lignin

The conversion of renewable biomass for the production of bio-oil and chemicals has been a hot research area due to the increasing consumption of fossil resources. Lignin is a kind of abundant natural polymer, the content of which is second to cellulose in nature. More and more researches are focusing on the depolymerization of lignin to produce liquid fuel and chemicals, which is the bottleneck of the conversion of ligninin to liquid fuel and chemicals.The liquefaction of lignin using formic acid as an in situ hydrogen donator was studied. Effects of microwave heating, reaction temperature and time on the yield of monophenolics and the distribution of Syringyl(S), Guaiacyl(G) and para-hydrophenyl(H) type compounds were studied in the formic acid system. It was found that the addition of ethanol greatly enhanced the monophenolics yield of formic acid system. The maximum monophenolics yield in the formic acid-ethanol system reached 20.01%. Ethanol not only enhances the solubility of monophenolics but also prevents the repolymerization of the products, and hence improves the yield of monophenolics.Effects of liquefying agent, catalyst, reaction temperature and time on the yield of bio-oil in the microwave assisted liquefaction of wheat straw alkali lignin were investigated. Bio-oils were characterized by IR, GC-MS and NMR. It was found that microwave irradiation greatly shortened liquefaction time with methanol as liquefying agent and ferric sulfate as catalyst. The maximum bio-oil yield in methanol system is up to 55.22 % under the relatively low liquefaction temperature of 160 ℃ for 5 min. The structure of residual lignin exhibits very limited variation, indicating that the degraded lignin fragments has lower probability of the recondensation, and residual lignin can be reused to improve the utilization rate of lignin. The identified liquefaction products are mainly monophenolic compounds, in which S, G and H type compounds account for 57.72 %, 25.28 % and 8.98 %, respectively. The existence of the signals for β-O-4 and C-C bonds in the 1H-NMR spectra suggests that the bio-oils include a small amount of dimers and oligomers except for monophenolic compounds.The digestion of lignin with hydrogen peroxide for the removal of TOC and color was studied. Dilute lignin solution was successfully digested into colorless and clarified liquor under microwave-assisted oxidative digestion with hydrogen peroxide. High dosage of hydrogen peroxide is needed to effectively digest lignin, but excessive hydrogen peroxide may lead to re-condensation of formed fragments in digested lignin. Microwave irradiation greatly facilitates the oxidative digestion of lignin. Compared with conventional heating technique, microwave-assisted digestion achieves the same or higher digestion rate within a shorter time and/or at lower temperature. After digestion, total organic carbon(TOC) content of lignin solution decreases by 93.9 %, and a small amount of aliphatic alkane, alcohol, acid and ester are formed via the cleavage of aromatic rings as well as the deprivation of side chains in original lignin.