Naphthols improve lignin separation and induce photoluminescent structural changes during glycolic acid/pentanol biphasic pretreatment of poplar

Abstract

Aromatic naphthol additives offer a scalable way to regulate biphasic pretreatment through phase-selective partitioning, enabling simultaneous lignin valorization into photofunctional lignin and efficient hemicellulose conversion within a single processing step. In a glycolic acid/pentanol system for poplar, partitioning analysis showed that 2-naphthol and 3-hydroxy-2-naphthoic acid were enriched in the pentanol phase. At 20 mM, both additives only mildly influenced aqueous-phase xylan hydrolysis, and xylan to xylooligosaccharides (XOS) selectivity was retained, with XOS yields remaining at 46.2–47.5%. This phase-selective partitioning directed lignin reactions toward arylation-type stabilization pathways, while delignification increased from 75.3% to 88.5–90.3%, surface lignin coverage on the solids decreased by 24%, and glucose yields after enzymatic hydrolysis rose from 79.1% to 90.3–92.6%. NMR spectroscopy showed near-complete loss of β-O-4 linkages (from 56.3% to 1.4–2.4%), attenuation of condensed phenolic structures, and clear naphthalene resonances indicative of naphthol incorporation. The resulting lignin fractions exhibited solvent-tunable autofluorescence, with a quantum yield of up to 14% in tetrahydrofuran, attributable to π-conjugation extension and rigidification that suppress nonradiative decay. This photofunctional lignin holds significant potential for applications in biobased fluorescent coatings. This approach establishes a novel route for the co-production of high-value photofunctional lignin and sugars from lignocellulose, transforming a traditional biorefinery waste stream into a structurally engineered optical material.