Abstract
The present study demonstrated a sustainable and cost-effective approach to depolymerize/oxidize softwood (SW) and hardwood (HW) kraft lignins using concentrated hydrogen peroxide at temperatures ranging from 25 to 35 °C, in the absence of catalysts or organic solvents. The degree of lignin depolymerization could be simply controlled by reaction time, and no further separation process was needed at the completion of the treatment. The obtained depolymerized lignin products were comprehensively characterized by GPC–UV, FTIR, 31P-NMR, TGA, Py-GC/MS and elemental analysis. The weight-average molecular weights (Mw) of the depolymerized lignins obtained from SW or HW lignin at a lignin/H2O2 mass ratio of 1:1 after treatment for 120 h at room temperature (≈25 °C) were approximately 1420 Da. The contents of carboxylic acid groups in the obtained depolymerized lignins were found to significantly increase compared with those of the untreated raw lignins. Moreover, the depolymerized lignin products had lower thermal decomposition temperatures than those of the raw lignins, as expected, owing to the greatly reduced Mw. These findings represent a novel solution to lignin depolymerization for the production of chemicals that can be utilized as a bio-substitute for petroleum-based polyols in polyurethane production.
Highlights
The forestry and agricultural sectors worldwide are generating considerable amounts of lignocellulosic materials in the form of residues, an attractive carbon-neutral source for fuels and chemicals, which can be an economically important alternative to fossil fuels
Lignin was reacted with concentrated H2 O2 (50%) at ambient temperature
The obtained depolymerized kraft lignin (DKL) products were comprehensively characterized by GPC–UV, Fourier-transform infrared spectroscopy (FTIR), 31 P-NMR, thermogravimetric analysis (TGA), Py–GC/MS and elemental analysis
Summary
The forestry and agricultural sectors worldwide are generating considerable amounts of lignocellulosic materials in the form of residues, an attractive carbon-neutral source for fuels and chemicals, which can be an economically important alternative to fossil fuels. Significant advances have been made in the development of economically viable biorefineries involving the fractionation of lignocellulose into its three major constituents (i.e., cellulose, hemicellulose and lignin), and full valorization of these constituents [1]. Kraft pulping is the dominant chemical pulping process in the world. In this process, lignin and hemicellulose in wood chips are dissolved by sodium hydroxide (NaOH) and sodium sulfide (Na2 S) to form black liquor, which is subsequently concentrated and burned in the recovery boiler to recover chemicals and heat, both of which are crucial to the functioning of a kraft mill. Removing a portion of Molecules 2020, 25, 2329; doi:10.3390/molecules25102329 www.mdpi.com/journal/molecules
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