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.