Nitroxyl-mediated oxidation of lignin and polycarboxylated products
Methods of selectively modifying lignin, polycarboxylated products thereof, and methods of deriving aromatic compounds therefrom. The methods comprise electrochemically oxidizing lignin using stable nitroxyl radicals to selectively oxidize primary hydroxyls on β-O-4 phenylpropanoid units to corresponding carboxylic acids while leaving the secondary hydroxyls unchanged. The oxidation results in polycarboxylated lignin in the form of a polymeric β-hydroxy acid. The polymeric β-hydroxy acid has a high loading of carboxylic acid and can be isolated in acid form, deprotonated, and/or converted to a salt. The β-hydroxy acid, anion, or salt can also be subjected to acidolysis to generate various aromatic monomers or oligomers. The initial oxidation of lignin to the polycarboxylated form renders the lignin more susceptible to acidolysis and thereby enhances the yield of aromatic monomers and oligomers obtained through acidolysis.
1. A method of oxidizing lignin, the method comprising electrochemically oxidizing lignin in an oxidation process in the presence of a stable nitroxyl radical to form oxidized lignin, the oxidation process being performed at a pH that allows oxidation of primary hydroxyls on at least a portion of β-O-4 phenylpropanoid units comprising both primary and secondary hydroxyls to corresponding carboxylic acids while leaving the secondary hydroxyls on the portion of the β-O-4 phenylpropanoid units unchanged.
2. The method of claim 1 , wherein the oxidation process is conducted at a pH of from 7 to 11.
3. The method of claim 1 , wherein the oxidation process is conducted in a solvent comprising at least 45% water by volume.
4. The method of claim 1 , wherein the oxidation process forms oxidized β-O-4 phenylpropanoid units in the oxidized lignin from corresponding unoxidized β-O-4 phenylpropanoid units in the lignin, wherein at least 60% of the oxidized β-O-4 phenylpropanoid units comprise a carboxylic acid oxidized from a primary hydroxyl in the corresponding unoxidized β-O-4 phenylpropanoid unit and a secondary hydroxyl on the oxidized β-O-4 phenylpropanoid unit that is unchanged from the unoxidized β-O-4 phenylpropanoid unit.
5. The method of claim 1 , wherein the oxidation process forms oxidized β-O-4 phenylpropanoid units in the oxidized lignin from corresponding unoxidized β-O-4 phenylpropanoid units in the lignin, wherein at least 50% of the oxidized β-O-4 phenylpropanoid units comprise a secondary hydroxyl that remains unchanged from the unoxidized β-O-4 phenylpropanoid unit.
6. The method of claim 1 , wherein the oxidation process forms oxidized β-O-4 phenylpropanoid units in the oxidized lignin from corresponding unoxidized β-O-4 phenylpropanoid units in the lignin, wherein at least 60% of the oxidized β-O-4 phenylpropanoid units remain uncleaved.
7. The method of claim 1 , wherein the oxidized lignin comprises from 0.2 to 0.8 carboxylic acid or carboxylate functional groups per ppu.
8. The method of claim 1 , wherein the lignin comprises a total phenolic hydroxy content of less than 2.5 mmol/g.
9. The method of claim 1 , further comprising deprotonating the oxidized lignin.
10. The method of claim 1 , wherein the oxidized lignin comprises polycarboxylated lignin, and wherein the method further comprises isolating the polycarboxylated lignin from non-polycarboxylated components of the oxidized lignin.
11. The method of claim 1 , wherein the oxidized lignin has an average molecular weight of from 240 Da to 100,000 Da.
12. The method of claim 1 , further comprising separating oxidized lignin species less than 240 Da from oxidized lignin species greater than 240 Da.
13. The method of claim 1 , further comprising digesting the oxidized lignin with acid.
14. The method of claim 13 , further comprising isolating products having a size less than 240 Da from the acid-digested, oxidized lignin.