Solvent-free catalyst-free microwave-assisted acylation of lignin

- Softwood lignin was esterified with acetic, propionic, butyric, methacrylic, or maleic anhydride.
- Esterification occurred under microwave irradiation, without solvent and without catalyst.
- Esterification was fast (10 min), efficient (>90% yield), and worked up with water or ethanol.
- All esterified lignins except maleated one showed higher thermal stability than the unmodified lignin.

Valorization of lignin into biomaterials requires its good compatibility with the polymeric matrix it is incorporated into. Softwood Kraft lignin was esterified with various anhydrides (acetic, propionic, butyric, methacrylic, maleic) using a solvent-free, catalyst-free, microwave-assisted method. The esterification was fast and efficient (degrees of substitution >90% after 10 min). The reaction was worked up using water or ethanol as purification agent. Esterified lignins were characterized by FT-IR, GPC, quantitative 31P NMR and TGA/DSC. NMR analyses indicated that non-cyclic anhydrides reacted with both aliphatic and phenolic OH groups, while maleic anhydride reacted exclusively with aliphatic OH groups. All acylated lignins except the maleated one showed higher thermal stability than the unmodified lignin. Lignins acylated with non-cyclic anhydrides can be applied as green fillers in apolar polyolefins while maleated lignin can serve as metal adsorbent or macroreagent for the synthesis of polyesters or polyamides. We showed in this study that lignin can be modified in a fast and easy way.

Lignin can be acylated without solvent and without catalyst using microwave irradiation.