Lignin alkaline oxidation using reversibly-soluble bases

Abstract

Lignin valorization approaches, which are critical to biorefining, often involve depolymerization to aromatic monomers. Alkaline oxidation has long held promise as a lignin depolymerization strategy, but requires high concentrations of base, typically NaOH, much of which must be neutralized to recover lignin-derived aromatic monomers. This consumption of base and associated waste generation incurs high cost and negative environmental impacts. In this work, we demonstrate that Sr(OH)₂ and Ba(OH)₂ perform comparably to NaOH in terms of total aromatic monomer yields in the aqueous aerobic alkaline depolymerization of corn stover lignin, and that up to 90% of these reversibly-soluble bases can be recovered via precipitation and filtration. Process modeling suggests that the use of Sr(OH)₂ could reduce the cost of alkaline oxidation by 20–60% compared to NaOH, depending on lignin loading. In contrast, the energy required to regenerate the Sr largely offsets potential improvements in sustainability over Na-promoted alkaline oxidation, though the sustainability comparison is likely sensitive to the lignin composition and could be improved by further optimization of the regeneration step.