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Depolymerization and Demethylation of Kraft Lignin in Molten Salt Hydrate and Applications as an Antioxidant and
Xiaohui Yang1,2,3, Zheng Li2, Long Li1
1Jiangsu Province Key Laboratory of Biomass Energy and Material; Jiangsu Province Co-Innovation Center of Efficient Processing and Utilization of Forest Resources; Key Lab. of Chemical Engineering of Forest Products, National Forestry and Grassland Administration; National Engineering Lab. for Biomass Chemical Utilization, Institute of Chemical Industry of Forestry Products, Chinese Academy of Forestry, Nanjing 210042, China.
Abstract:
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 (∼60% LiBr aqueous solution). It was found that the cleavage of the ether bonds followed the order of β-O-4 ether > aryl alkyl ether in phenylcoumaran > dialkyl ether in resinol > methoxyl (MeO). The depolymerization via β-O-4 cleavage occurred under mild conditions (e.g., <0.5 M HCl at 110 °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.
