Microwave assisted liquefaction of wheat straw alkali lignin for the production of monophenolic compounds

The microwave assisted liquefaction process of wheat straw alkali lignin was investigated to obtain monophenolic compounds as the precursor of bio-fuel. It is found that the total yield of monophenolic compounds is significantly improved under microwave irradiation, reaching 15.77% under a relatively mild liquefaction condition of 10 wt% H2SO4 as the catalyst, 10 wt% phenol as the hydrogen-donor reagent at 120 ℃ for 40 rain. Compared with conventional thermal liquefaction process, microwave irradiation promotes the cleavage of C-C bonds, which gives an extra 29% of Caryl-Cα bond cleavage, and increases the yield of monophenolic compounds from 0.92% to 13.61% under the same conditions. The excessive temperature and prolonged time under microwave irradiation will promote the recondensation of degraded lignin fragments, so the key to obtain high yield of monophenolic compounds is to avoid the recondensation reaction. The selected solid catalyst promotes the dissociation of methoxy groups, and the addition of phenol into liquefaction can only slightly improve the yield of monophencolic compounds.

One-step fabrication of mesoporous sulfur-doped carbon nitride for highly selective photocatalytic transformation of native lignin to monophenolic compounds

Photocatalytic selective transform native lignin into valuable chemicals is an attractive but challenging task.Herein,we report a mesoporous sulfur-doped carbon nitride(MSCN-0.5)which is prepared by a facile one-step thermal condensation strategy.It is highly active and selective for the cleavage Cα-Cβbond inβ-O-4 lignin model compound under visible light radiation at room temperature,achieving 99%substrate conversion and 98%Cα-Cβbond cleavage selectivity.Mechanistic studies revealed that the Cβ-H bond of lignin model compounds activated by holes and generate key Cβradical intermediates,further induced the Cα-Cβbond cleavage by superoxide anion radicals(·O2-)to produce aromatic oxygenates.Waste Camellia oleifera shell(WCOS)was taken as a representative to further understand the reaction mechanisms on native lignin.33.2 mg of monophenolic compounds(Vanillin accounted for 22%and Syringaldehyde for 34%)can be obtained by each gram of WCOS lignin,which is 2.5 times as that of the pristine carbon nitride.The present work offers useful guidance for designing metal-free heterogeneous photocatalysts for Cα-Cβbond cleavage to harvest monophenolic compounds.

Solvent-enhanced Depolymerization of Lignin under Microwave Irradiation

Lignin is considered an ideal natural material for the production of sustainable monophenols.In this study,a microwave-assisted depolymerization(MAD)strategy was developed.The introduction of solvent vapors in the dynamic vapor flow reaction system was performed to enhance the lignin conversion efficiency.The results showed that no liquid products were generated from the MAD of lignin without solvent vapors.With the introduction of solvents(CH_(3)OH,HCHO,HCOOH,and CH_(2)Cl_(2)),liquid products appeared(especially with CH_(2)Cl_(2),which had the highest yield of 41.9 wt%).Results from gas chromatography/mass spectrometry of liquid products showed that seven kinds of compounds,including guaiacols,phenols,syringols,methoxyphenyls,heterocycles,esters,and aromatics were identified.CH_(2)Cl_(2) can significantly enhance the production of monophenols(guaiacols,phenols,and syringols).The introduction of these solvent vapors can also facilitate the generation of porous char with high Brunauer-Emmett-Teller specific surface areas.Some carbon nanospheres deposited on the surface of the char were obtained with the assistance of CH_(2)Cl_(2).This study provides a facile method for the utilization of lignin in the field of bio-based fine chemicals.