Depolymerization features of lignin under supercritical ethanol state in the presence of metal catalysts

Abstract

Catalytic depolymerization of lignin to monomeric phenols as an alternative to the petrochemical industry is the most promising approach to realize sustainable utilization of lignin. However, reaction pathways and chemistry behind this technology are very complex due to structural recalcitrance of lignin. This research focuses on an external effect on physicochemical properties of lignin depolymerized products to provide an insight into the lignin decomposition behavior. First, the effects of various parameters, such as temperature, reaction time, solvent amount, and initial hydrogen gas pressure on the distribution of lignin depolymerized products formed during solvolytical depolymerization of organosolv lignin (OL) was investigated. During this process, concurrent reactions involving depolymerization and recondensation as well as secondary decomposition were significantly accelerated with increasing temperature, leading to increase in both lignin derived phenols in the phenol-rich oil fraction (lignin-oil) and undesirable products (char and gas). Second, soda lignin (SL) was directly depolymerized over Pt/C, Pd/C, Ru/C, and Ni/C under supercritical alcohols (methanol, ethanol, 2-propanol, and t-butanol) to investigate the effect of catalyst and solvent types on the physicochemical properties of lignin-oil. 1H-/2D-HSQC-NMR and GPC analysis revealed that the Mw of lignin-oil remarkably lower than that of SL, which is clear evidence of β-O-4 and β-β bond cleavages. The top four main monomeric phenols in lignin-oil were 4-ethylphenol, guaiacol, 4-ethylguaiacol, and syringol, for which the sum was mostly produced in E-Pt (4.2 wt%). It was observed that excessive catalyst dosage caused side reactions, resulting in decreasing monomeric phenols yield. Third, SL was sequentially fractionated by organic solvents (ethyl acetate: F1, methanol: F2, acetone: F3, dioxane/water: F4, and insoluble fraction: F5) for homogeneous preparations of lignin. The Mw of SL, F1, F2, F3, and F4 were 2800, 1120, 2860, 5850, 7200 Da. 2D-HSQC-NMR analysis revealed that lignin fraction with lower Mw had highly condensed structure (poor β-O-4 linkage). Each fraction was efficiently depolymerized into lignin-oil under the combination of supercritical ethanol (350 °C) and 5 wt% Ru/C to compare depolymerization feature of lignin with different molecular distribution. The yield of lignin-oil, mixture of monomeric phenols as well as high molecular phenolics, ranged from 62.5 to 81.4 wt% and was inversely proportional with the Mw values of lignin fraction. The selectivity of monomeric phenols produced from lignin depolymerization process was clearly affected by the Mw value of lignin. Especially, relative highly condensed fraction yielded higher amount of non-alkylated phenols, methylated-, and ethylated phenols more than other lignin fractions. Lastly, phenol was selectively produced from SL via a RuNi/SBA-15-catalyzed supercritical treatment. The bimetallic catalysts used in this work were prepared by a wetness impregnation method with different molar ratios of Ru and Ni (RuxNi1-x, x = 0.2, 0.4, 0.6, 0.8, and 1.0). This study revealed that the chemical properties of lignin-oil were clearly affected by the physicochemical properties of bimetallic catalysts. By increasing the amount of desorbed H2 and the acidity of the catalyst, the yields of lignin-oil, total phenols, and phenol increased while that of char decreased. The yields of lignin-oil, total monomeric phenols, and phenol were largest with the Ru0.6Ni0.4 catalyst (77.5 wt%, 12.7 wt%, and 4.7 wt%, respectively).