Graphene Oxide Membranes for Recovery of Catalytically Depolymerized Lignin Monomers and Oligomers

Efficient lignin valorization requires not only catalytic depolymerization but also a scalable, aqueous separation route that isolates bioavailable low-molecular-weight (LMW) products from recalcitrant oligomers/polymers. Here, oxidative depolymerization of poplar lignin was coupled with cross-flow nanofiltration using reduced-graphene-oxide (rGO) membranes (molecular-weight cut-off or MWCO ≈ 470 Da). Across six depolymerization liquors spanning pH 2.5-9.2 and distinct catalytic histories (of polyoxometalate (POM), NaOH, graphene oxide, and control), rGO membranes consistently rejected ≥90% of the >1.6 kDa fraction, while feed pH governed LMW partitioning. Acidic liquors favored a steric-sieving regime that enriched aliphatic and aromatic monomers in the permeate (separation factors ≈5-6 relative to the high-MW fraction), whereas alkaline conditions induced electrostatic exclusion, co-retaining anionic acids with oligomers and suppressing monomer enrichment. Mechanistic studies with model mixtures showed that dilution sharpened selectivity by mitigating concentration polarization and ionic-strength screening and that aromatics can modulate charge-based exclusion through competitive interactions within graphitic nanochannels. Continuous diafiltration of the monomer-rich liquor for 250 h sustained stable flux (~11 ± 1 L m^-2 h^-1) and selectivity, enabling complete recovery of aliphatic and aromatic monomers and LMW fractions while concentrating high-MW fractions. These results establish rGO membranes as a robust, water-based separation platform for MW-resolved lignin fractionation.