Polycarboxylated compounds and compositions containing same
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 polycarboxylated compound comprising phenylpropanoid units, wherein:
each phenylpropanoid unit comprises an α-carbon and a β-carbon;
the phenylpropanoid units are linked to each other via linkages selected from the group consisting of a β-O-4 linkage, a β-1 linkage, a β-5 linkage, a 4-O-5 linkage, a 5-5 linkage, and a β-β linkage;
the phenylpropanoid units comprise β-O-4 phenylpropanoid units;
each of the β-O-4 phenylpropanoid units is a single one of the phenylpropanoid units and is linked via its β-carbon with a β-O-4 linkage;
each β-O-4 phenylpropanoid unit in at least a portion of the β-O-4 phenylpropanoid units comprises a secondary hydroxyl moiety on the α-carbon of the β-O-4 phenylpropanoid unit and a carboxylic acid or carboxylate moiety on the β-carbon of the β-O-4 phenylpropanoid unit; and
the polycarboxylated compound further comprises a phenylpropanoid unit linked via a β-1 linkage, a phenylpropanoid unit linked via a 4-O-5 linkage, or a phenylpropanoid unit linked via a β-1 linkage and a phenylpropanoid unit linked via a 4-O-5 linkage.
2. The polycarboxylated compound of claim 1 , wherein the polycarboxylated compound comprises a molecular weight between 7,500 Da and 100,000 Da.
3. The polycarboxylated compound of claim 1 , comprising from 0.05 to 0.7 carboxylic acid or carboxylate functional groups per phenylpropanoid unit.
4. The polycarboxylated compound of claim 1 , wherein the polycarboxylated compound is in a purified form.
5. A polycarboxylated compound comprising phenylpropanoid units, wherein:
each phenylpropanoid unit comprises an α-carbon and a β-carbon;
the phenylpropanoid units are linked to each other via linkages selected from the group consisting of a β-O-4 linkage, a β-1 linkage, a β-5 linkage, a 4-O-5 linkage, a 5-5 linkage, and a β-β linkage;
the phenylpropanoid units comprise β-O-4 phenylpropanoid units;
each of the β-O-4 phenylpropanoid units is a single one of the phenylpropanoid units and is linked via its β-carbon with a β-O-4 linkage;
each β-O-4 phenylpropanoid unit in at least a portion of the β-O-4 phenylpropanoid units comprises a secondary hydroxyl moiety on the α-carbon of the β-O-4 phenylpropanoid unit and a carboxylic acid or carboxylate moiety on the β-carbon of the β-O-4 phenylpropanoid unit; and
the polycarboxylated compound comprises a molecular weight greater than 7,500 Da.
6. The polycarboxylated compound of claim 5 , wherein the polycarboxylated compound comprises a molecular weight between 7,500 Da and 100,000 Da.
7. The polycarboxylated compound of claim 5 , comprising from 0.05 to 0.7 carboxylic acid or carboxylate functional groups per phenylpropanoid unit.
8. The polycarboxylated compound of claim 5 , wherein the polycarboxylated compound is in a purified form.
9. A polycarboxylated compound comprising phenylpropanoid units, wherein:
each phenylpropanoid unit comprises an α-carbon and a β-carbon;
the phenylpropanoid units are linked to each other via linkages selected from the group consisting of a β-O-4 linkage, a β-1 linkage, a β-5 linkage, a 4-O-5 linkage, a 5-5 linkage, and a β-β linkage;
the phenylpropanoid units comprise β-O-4 phenylpropanoid units;
each of the β-O-4 phenylpropanoid units is a single one of the phenylpropanoid units and is linked via its β-carbon with a β-O-4 linkage;
each β-O-4 phenylpropanoid unit in at least a portion of the β-O-4 phenylpropanoid units comprises a secondary hydroxyl moiety on the α-carbon of the β-O-4 phenylpropanoid unit and a carboxylic acid or carboxylate moiety on the β-carbon of the β-O-4 phenylpropanoid unit; and
the polycarboxylated compound comprises from 0.05 to 0.7 carboxylic acid or carboxylate functional groups per phenylpropanoid unit.
10. The polycarboxylated compound of claim 9 , wherein the polycarboxylated compound comprises a molecular weight greater than 600 Da.
11. A polycarboxylated compound comprising phenylpropanoid units, wherein:
each phenylpropanoid unit comprises an α-carbon and a β-carbon;
the phenylpropanoid units are linked to each other via linkages selected from the group consisting of a β-O-4 linkage, a β-1 linkage, a β-5 linkage, a 4-O-5 linkage, a 5-5 linkage, and a β-β linkage;
the phenylpropanoid units comprise β-O-4 phenylpropanoid units;
each of the β-O-4 phenylpropanoid units is a single one of the phenylpropanoid units and is linked via its β-carbon with a β-O-4 linkage;
each β-O-4 phenylpropanoid unit in at least a portion of the β-O-4 phenylpropanoid units comprises a secondary hydroxyl moiety on the α-carbon of the β-O-4 phenylpropanoid unit and a carboxylic acid or carboxylate moiety on the β-carbon of the β-O-4 phenylpropanoid unit;
the polycarboxylated compound comprises from 0.2 to 0.8 carboxylic acid or carboxylate functional groups per phenylpropanoid unit.
12. The polycarboxylated compound of claim 11 , wherein a majority of the β-O-4 phenylpropanoid units in the portion of the β-O-4 phenylpropanoid units comprise a carboxylate moiety.
13. The polycarboxylated compound of claim 11 , wherein the polycarboxylated compound is in a deprotonated form.
14. The polycarboxylated compound of claim 11 , wherein the polycarboxylated compound is in a salt form.
15. The polycarboxylated compound of claim 11 , wherein the polycarboxylated compound is in an acid form.
16. The polycarboxylated compound of claim 11 , wherein the β-O-4 phenylpropanoid units in the portion of the β-O-4 phenylpropanoid units are substantially devoid of phenolic hydroxyl groups.
17. The polycarboxylated compound of claim 11 , wherein the polycarboxylated compound has a solubility in water of at least 5 g/L at pH 9 and 23° C.
18. The polycarboxylated compound of claim 11 , comprising a molecular weight of from 700 Da to 100,000 Da.
19. The polycarboxylated compound of claim 11 , wherein the polycarboxylated compound comprises a molecular weight greater than 600 Da.
20. The polycarboxylated compound of claim 11 , wherein the polycarboxylated compound is in a purified form.
21. The polycarboxylated compound of claim 11 , wherein at least 60% of the β-O-4 phenylpropanoid units comprising a carboxylic acid or carboxylate moiety also comprise a secondary hydroxyl moiety.
22. The polycarboxylated compound of claim 11 , wherein the phenylpropanoid units further comprise β-5 phenylpropanoid units, wherein each of the β-5 phenylpropanoid units is a single one of the phenylpropanoid units and is linked via a β-5 linkage, and each β-5 phenylpropanoid unit in at least a portion of the β-5 phenylpropanoid units comprises a carboxylic acid or carboxylate moiety.