Efficient conversion of lignin into a water-soluble polymer by a chelator-mediated Fenton reaction: optimization of H2O2 use and performance as a dispersant

Abstract

Room temperature Fenton (FEN) and chelator-mediated Fenton (CMF) reactions were examined for transforming lignin into a water-soluble polymer. Compared to depolymerization of lignin, this has the advantage of potentially yielding a product directly without requiring further upgrading. With the goal of optimizing the use of the expensive reagent H₂O₂, initial studies were performed with lignin from an organsolv process (OS) in thin films with a multi-well format that allows simultaneous assay of 76 reaction conditions. The results showed that H₂O₂ is more efficiently used in CMF compared with FEN, and that the greatest amount of lignin solubilized per mass of H₂O₂ consumed occurs at low initial concentrations of H₂O₂ (<1%). Further optimization of reaction conditions was performed with OS lignin in powder form. Results obtained upon optimizing reactant concentrations, pH and Fe-chelator, and with O₂ bubbling indicate that a yield of 1 g lignin solubilized per g of H₂O₂ consumed is achievable. Chemical and molecular weight analyses showed that the reaction results in extensive opening of the aromatic rings and generation of acid groups, yielding a water-soluble polymer with molecular weight distribution that is comparable to that of the starting material but with a small amount of low MW species. Similar yields and extents of ring opening resulted for three other distinctly different lignins. The effectiveness of the OS lignin-derived polymeric material in dispersing alumina particles was studied by zeta potential, light scattering, and Turbiscan stability measurements. These measurements showed that the lignin-derived material performs comparably to poly(acrylic acid) PAA of similar molecular weight. Considering that the selling price for bulk PAA is greater than twice that of H₂O₂ on a per mass basis, this approach holds promise for generating value from lignin.