Monitoring pilot-scale lignin depolymerization via nanoparticle size in water: A sustainable qualitative method

Lignin is the most abundant aromatic bioresource, but the complexity of its biopolymer structure hinders its use in many applications. Large-scale continuous systems for lignin upgrading via solvolysis or catalytic depolymerization are currently being developed to produce more defined and application-specific lignin oligomers. A key factor in the scale-up of the conversion process is monitoring the molecular weight of lignin fractions throughout the operation. However, traditional analytical methods such as gel permeation chromatography (GPC) are slow, while a fast response is essential to prevent significant product losses. In this study, we developed a simple, rapid, and qualitative method to assess the molecular weight range of the lignin-derived products during continuous depolymerization runs. This approach is based on establishing strong correlations between lignin molecular structure and nanoparticle size in aqueous dispersion. By optimizing lignin nanoparticle (LNP) fabrication for specific lignin fractions within a defined molecular weight range, we tested a series of lignin samples. The results obtained from GPC and LNP size analysis were compared to validate the accuracy of our method. Finally, the LNP-based qualitative method was applied to a pilot-scale depolymerization run to track potential deviations in molecular weight in the final product. Our findings demonstrate that LNP size can serve as a simple, reliable, and rapid technique for evaluating the molecular weight of depolymerized lignin. This method offers valuable potential for future industrial processes involving this abundant renewable resource.