Depolymerization of renewable resources—lignin by sodium hydroxide as a catalyst and its applications to epoxy resin

Abstract

Alkali lignin was successfully depolymerized into polyols with high hydroxyl number via direct hydrolysis using sodium hydroxide (NaOH) as a catalyst, without any organic solvent agent. Hydrolysis of lignin can produce a multitude of high-value products via alkali-catalyzed cleavage. This process usually gives good results with respect to the yield of phenols. Through this method, the numbers of the hydroxymethyl and phenolic hydroxyl groups of lignin had been dramatically increased, reaching 2.11%, nearly four times higher than that in the original one. Meanwhile, we added the same amounts (20 wt %) of different depolymerization of lignin (DL) into epoxy resin (EP), and the results showed that DL could not only increase the decomposition temperature of EP, but also remarkably improve its mechanical properties. The optimum reaction time was 1.5 h, the reaction temperature was 250 C, and the optimum sodium hydroxide concentration was 15 wt % for depolymerizing lignin. The mechanical and thermal properties of cured lignin-based epoxy resin (LEP) were compared with cured neat EP. The cured DL-based epoxy resin (DLEP) showed the highest adhesive shear strength up to 2.66 MPa, which displayed 122% of the adhesive shear strength of EP.

Recent Research Progress on Lignin-Derived Resins for Natural Fiber Composite Applications

“…Chen et al [ 65 ] prepared the polyols-like depolymerized lignin (DL) with high hydroxyl content by depolymerizing alkali lignin at the optimized NaOH concentration (15 wt%) and temperature (250 °C). They reported that this method increases the –OH group, about 2.11% in DL, and reduces environmental pollution.…”

Heterogeneously catalyzed lignin depolymerization

“…In base-catalyzed depolymerization, the choice of lignin influences the yield of bio-oil, but not the resulting composition [59]; however, the use of mineral bases may necessitate neutralisation of the subsequent bio-oil, in addition to reactor corrosion. Thring et al studied the effect of time and temperature on the depolymerization of Alcell lignin catalyzed by NaOH [57] obtaining guaiacol, syringol, and under certain conditions catechol and its derivatives [57,60,61]. Base-catalyzed depolymerization has proven an effective protocol in polar organic solvents, such as ethanol and methanol, suppressing char formation, and hence enhancing monomer and dimer production relative to solid acid catalysts.…”

Effect of Process Variables on the Solvolysis Depolymerization of Pine Kraft Lignin

“…The effects were detected using various advanced analysis techniques, including NMR and ultraviolet spectrophotometry, as well as size-exclusion chromatography. By depolymerizing lignin, a decrease in molar mass can be obtained, as well as an increase in the OH ph content [39], which could be favorable for the production of bio-based polymers.…”