Microwave-enhanced extraction of lignin from birch in formic acid: Structural characterization and antioxidant activity study

Abstract

A rapid and mild extraction protocol for the preparation of lignin was achieved by microwave-assisted heating in formic acid at 101°C under atmospheric pressure. In this case, birch lignin was extracted with microwave heating process (ML) in formic acid and characterized by elemental analysis, FTIR, GPC, 1H NMR and 13C–1H HSQC. In addition, the antioxidant activity of the samples was investigated. For comparative study, milled wood lignin (MWL) and lignin extracted with oil bath heating process (OL) were prepared. The results showed that the lignin yield under microwave heating was much higher than that under oil bath heating. A maximal delignification degree (89.77%) was achieved when microwave heating time was 30min. When double time (60min) was used under oil bath heating, the delignification degree was 66.11%. The structural characterization showed that the lignin structure of ML did not change dramatically, which is a mixture of GS-type with β-O-4′ ether bond as the major inter-unit linkage. As for antioxidant activity against DPPH, the radical scavenging index (RSI) of ML was 1.20, which was higher than that of MWL (0.53), suggesting that ML exhibited much higher antioxidant activity than MWL.