Depolymerization and Demethylation of Kraft Lignin in Molten Salt Hydrate and Applications as an Antioxidant and Metal Ion Scavenger

To improve the reactivity and enrich the functionality of lignin for valorization, kraft lignin was depolymerized and demethylated via cleaving aryl and alkyl ether bonds in acidic lithium bromide trihydrate (similar to 60% LiBr aqueous solution). It was found that the cleavage of the ether bonds followed the order of beta-O-4 ether > aryl alkyl ether in phenylcoumaran > dialkyl ether in resinol > methoxyl (MeO). The depolymerization via beta-O-4 cleavage occurred under mild conditions (e.g., <0.5 M HCl at 110 degrees C), while sufficient demethylation of the lignin needed harsher conditions (>1.5 M HCl). Both depolymerization and demethylation generated new aromatic hydroxyl (ArOH). With 2.4 M HCl, MeO content dropped from 4.85 to 0.95 mmol/g lignin, and ArOH content increased from 2.78 to 5.09 mmol/g lignin. The depolymerized and demethylated kraft lignin showed excellent antioxidant activity and Cr(VI)-scavenging capacity, compared with original kraft lignin and tannins.

Automated summary

Under acidic lithium bromide trihydrate conditions, kraft lignin can be successfully depolymerized and demethylated, generating new aromatic hydroxyl groups and demonstrating excellent antioxidant activity and Cr(VI)-scavenging capacity.