REDUCTIVE DEGRADATION OF LIGNIN IN SUPERCRITICAL SOLVENT AND APPLICATION IN PHENOLIC RESIN SYNTHESIS

The lignin was readily decomposed into phenol compounds via reductive liquefaction in supercritical ethanol media (formic acid decomposed into hydrogen at reaction temperatures), opening a promising opportunity to lignin-derived value-added aromatic compounds for highly possible application in phenol-formaldehyde resin production. The supercritical ethanol was the ideal reaction medium for lignin degradation, and the in situ generated hydrogen exactly promoted the decomposition reactions to smaller molecules via encapping the lignin radicals to terminate the further coupling and condensation. The liquefaction mechanism was radical thermal degradation. Some transition metal salts could catalyze the above lignin thermal decomposition, including nickel nitrate, cobalt nitrate and chloroplatinum acid. The yield of the lignin-degraded liquid increased with the increase of formic acid used in the ethanol solvent, while slightly deceased when the reaction temperature increased. The molecular weight of the lignin-derived liquid highly decreased with increase of the reaction temperature,e. g. at 350 degrees C the number average molecular weight was 143 while for the original lignin feedstock it was 588. The optimum reaction conditions are found to be ethanol-formic acid weight ratio at 1: 1; lignin load at 10 wt% ; reaction temperature at 350 degrees C for 4 h. The lignin-derived phenols were characterized by GC-MS technique, and the liquefied product was mainly phenol derivatives with simple structures. Compared with the lignin, the bio-phenols possessed highly improved solubility and reactivity in phenol-formaldehyde synthesis. As the ideal replacement of industrial phenol, lignin-degraded bio-phenol was utilized readily in resol type phenolic resin production with a replacement higher than 50 wt%. The thermal stability was slightly decreased with the increased amount of lignin-derived biophenol in the phenolic resin formula. The results showed the effective reductive degradation of lignin in supercritical ethanol would have a promising outlook in value-added phenol compounds and biomass-based renewable phenol-formaldehyde resins.