Valorization of Lignin via Oxidative Depolymerization with Hydrogen Peroxide: Towards Carboxyl-Rich Oligomeric Lignin Fragments

Abstract

The extraction and characterization of defined and carboxyl-rich oligomeric lignin fragments with narrow molecular weight distribution is presented herein. With regard to the well-known pulp bleaching process, oxidative lignin depolymerization was investigated using hydrogen peroxide in an aqueous alkaline solution (i.e., at T = 318 K, t = 1 h) and subsequent selective fractionation with a 10/90 (v/v) acetone/water mixture. While the weight average molecular weight (MW) of lignin in comparison to the starting material was reduced by 82% after oxidation (T = 318 K, t = 1 h, clignin = 40 g L−1, cH2O2 = 80 g L−1, cNaOH = 2 mol L−1) and subsequent solvent fractionation (T = 298 K, t = 18 h, ccleavage product = 20 g L−1), the carboxyl group (–COOH) content increased from 1.29 mmol g−1 up to 2.66 mmol g−1. Finally, the successful scale-up of this whole process to 3 L scale led to gram amounts (14% yield) of oligomeric lignin fragments with a MW of 1607 g mol−1, a number average molecular weight (MN) of 646 g mol−1, a narrow polydispersity index of 3.0, and a high –COOH content of 2.96 mmol g−1. Application of these oligomeric lignin fragments in epoxy resins or as adsorbents is conceivable without further functionalization.

A Green Cement Plasticizer from Softwood Kraft Lignin

“…It can also be observed that even though the M w decreased initially, the M n seemed to be increased with increasing temperatures in all OKLs when compared to KL ( M n of 1200 g/mol). The increased M n during lignin oxidation was also observed in an earlier study . The variation in the RI responses for all samples at 240 and 270 °C can be observed as a function of time in Figure f,g, and those for all the samples in all reaction parameters can be found in Figure S1 in the Supporting Information.…”

Mechanical Properties and Formaldehyde Release of Particleboard Made with Lignin-Based Adhesives

“…Structural changes (Figure 2) brought by oxidation (LO 20 compared to L20) are, as follows: the appearance of a shoulder at 1728 cm −1 assignable to unconjugated carbonyl groups; the shift of the aromatic skeletal vibration from 1590 cm −1 to 1601 cm −1 ; a shoulder at 1368 cm −1 specific to C-H deformation; a small shoulder at 1212 cm −1 , indicating C-O vibration in guaiacyl ring; a shift of absorption from 1160 cm −1 (assigned to -SO2-and C-OH) to 1150 cm −1 , and its significant decrease; a shift of C-O-C vibration from 1114 cm −1 to 1106 cm −1 , and its significant decrease. These may indicate depolymerization through the splitting of C-O-C bonds and oxidation leading to carbonyl groups, which is in accordance with the expected effects of mild oxidation [37].…”

“…at 1368 cm −1 specific to C-H deformation; a small shoulder at 1212 cm −1 , indicating C-O vibration in guaiacyl ring; a shift of absorption from 1160 cm −1 (assigned to -SO2-and C-OH) to 1150 cm −1 , and its significant decrease; a shift of C-O-C vibration from 1114 cm −1 to 1106 cm −1 , and its significant decrease. These may indicate depolymerization through the splitting of C-O-C bonds and oxidation leading to carbonyl groups, which is in accordance with the expected effects of mild oxidation [37]. Structural changes due to the addition of PMDI to oxidized magnesium lignosulfonate (LO 20 P1, LO 20 P3), as seen in Figure 2, have shown increased shoulder at 1730 cm −1 , indicating carbonyl CO groups in urethane structure; an increase of absorption at 1216 cm −1 (LO 20 P1), which is characteristic for transformation of -NCO groups into urethane structures; an increase of 1105 cm −1 , indicating C-O-C groups, (more visible for LO 20 P3).…”

“…The oxidation of lignin has been described as a way to weaken the lignin structure, making it more susceptible to depolymerization [36] and a good procedure to improve the properties of lignin [31]. An approach using mild reaction conditions is the base-catalyzed depolymerization of lignin and the addition of the hydrogen peroxide, considered to be an environmentally friendly oxidant [37].…”

Lignin conversion into C4 dicarboxylic acids by catalytic wet peroxide oxidation using titanium silicalite-1

“…However, this step is usually considered a bottleneck in pulp production (Ahmad et al, 2020;Mathias, 1993). Consequently, using part of this residual lignin as a raw material for conversion to added-value products would improve the sustainability of the pulping industry without affecting pulp production and the energy balance of the mill (Junghans et al, 2020;Rodrigues et al, 2018;Silva et al, 2009).…”