Oxidative Carboxylation of Lignin: Exploring Reactivity of Different Lignin Types

The increased interest in the utilization of lignin in biobased applications is evident from the rise in lignin valorization studies. The present study explores the responsiveness of lignin toward oxidative valorization using acetic acid and hydrogen peroxide. The pristine lignins and their oxidized equivalents were analyzed comprehensively using NMR and SEC. The study revealed ring opening of phenolic rings yielding muconic acid- and ester-end groups and side-chain oxidations of the benzylic hydroxyls. Syringyl units were more responsive to these reactions than guaiacyl units. The high selectivity of the reaction yielded oligomeric oxidation products with a narrower dispersity than pristine lignins. Mild alkaline hydrolysis of methyl esters enhanced the carboxylic acid content of oxidized lignin, presenting the potential to adjust the carboxylic acid content of lignin. While oxidation reactions in lignin valorization are well documented, this study showed the feasibility of employing optimized oxidation conditions to engineer tailored lignin-based material precursors.


Assignment of chemical shifts for quantification of lignin functional groups
a The yield of precipitated fraction was calculated based on the dry mass of precipitated fraction after workup step of alkaline hydrolysis treatment, compared to initial mass of EtOAc-soluble fraction of oxidized lignin before hydrolysis.

Figure S4 .
Figure S4.2D HSQC NMR spectra of reference eucalyptus kraft lignin, with f1 for 13 C and f2 for 1 H.

Figure S15 .
Figure S15. 31P NMR spectra of oxidized spruce kraft lignin of precipitated fraction

Figure S19 .
Figure S19. 31P NMR spectra of oxidized eucalyptus kraft lignin of precipitated fraction

b
Figure S34.2D HSQC NMR spectra of oxidized spruce kraft lignin of Ca 2+ ion-extracted watersoluble, with f1 for 13 C and f2 for 1 H.

Table S1 .
The chemical shifts of 31 P NMR, with water peak from Cl-TMDP at 132.2 ppm as reference.

Table S2 .
Quantification of lignin functional groups

Table S3 .
Yield of both precipitated and EtOAc-soluble fraction of oxidized lignin samples after alkaline hydrolysis treatment.