Valorization of Lignin under Mild Conditions: Biorefining Flavonoids and Lignin Nanoparticles

Abstract

Lignin is the largest store of renewable aromatics. Due to its refractory nature, however, its chemical potential is not fully realized. Here, we propose a biorefinery where the treatment of wastewater by a microbial electrolysis cell (MEC) produces “clean” water and a caustic catholyte that can depolymerize lignin under mild conditions into two high-value product streams. We determine the MEC operating conditions to produce a depolymerization solvent and quantify solution and colloidal phase products using an array of analytical techniques. In contrast to previous reports of limited high-value product yields, we obtain discrete aromatics (monomers and flavonoids) at 17% of initial lignin mass with bulk chemical analyses (11% for identified compounds using LC–MS n ). In part, our higher product yield is due to selective repolymerization to form flavonoids. We simultaneously produce lignin nanoparticles and close mass balance. Both flavonoids and lignin nanoparticles have potential applications in the pharmaceutical, nutraceutical, personal care, and agricultural industries.

The effects of lignin source and extraction on the composition and properties of biorefined depolymerization products

“…It is significant that herbaceous lignin is the only source that contains a detectable amount of the H monolignol which is likely responsible for some of the discrete aromatic products in the depolymerized corn stover that we identified in our previous work, including the monomer p -coumaric acid (making up to 0.5% of the depolymerized lignin mixture) and the flavonoid tricin (making up to 2.3% of the depolymerized lignin mixture). 33 These compounds were not detected by LCMS in depolymerized hardwood and softwood samples from our biorefinery.…”

“…In contrast to previous observations, neutralizing the samples fails to increase the SK LNP yield. 33 The low solubilization for neutral samples, as illustrated in Fig. 3a, indicates that there is not enough lignin in solution to promote LNP formation.…”

“…We have shown previously that LNPs formed in our biorefinery undergo a two-step process: (1) lignin is first depolymerized, which is catalyzed by high pH, then (2) the high salt facilitates micellization. 33 There are many specific interactions responsible for the mechanisms of LNP formation including: (1) hydrophilic–lipophilic interactions; (2) π–π interactions; (3) hydrogen bonding; (4) chain entanglement; (5) van der Waals forces; and (6) hydrophobic interactions. 59 The lower oxygen content in softwood hinders dissolution in a polar solvent and explains the low solubility in Fig.…”

“…Lignin depolymerization was carried out as described in Obrzut et al 33 Synthetic MEC catholyte was used for the experiments and prepared with a 227 mM phosphate solution (0.2 g L −1 KH 2 PO 4 ·H 2 O and 32 g L −1 Na 2 HPO 4 ) adjusted with sodium hydroxide to a final pH of 10.8. Lignin (250 mg) was combined with the phosphate solution (250 mL) in a beaker under magnetic stirring.…”

“…In this biorefinery we coupled a microbial electrolysis cell (MEC) that oxidizes the organic content of wastewater (as modeled by acetate) at the anode reducing BOD by 60% and producing a high pH, high salt effluent at the cathode which is then used to depolymerize an extracted lignin stream. 33 Under the ambient temperature and pressure conditions of our biorefinery, base catalyzed depolymerization of an Organosolv herbaceous (corn stover) lignin preserves the aromatic structure to yield discrete aromatics such as monomers ( e.g. , vanillin, p -coumaric acid) and flavonoids ( e.g.…”