Oxidative depolymerization of lignin using nitric acid under ambient conditions

Abstract

In this work, we demonstrated an innovative and cost-effective strategy to produce high- value depolymerized kraft lignin (DKL) from softwood kraft lignin (SKL) and hardwood kraft lignin (HKL). The depolymerization of kraft lignin was successfully achieved by an oxidative approach using nitric acid under ambient conditions (room temperature and pressure). This novel approach has the following features: ambient process conditions, no heat required, absence of catalysts or organic solvents, and no further purification process being required. The obtained DKL products were characterized extensively by gel permeation chromatography with ultraviolet detector (GPC-UV), Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), pyrolysis–gas chromatography–mass spectrometry (Py-GC/MS), potentiometric titration and elemental analysis. Treatment of SKL or HKL at a KL/nitric acid (HNO3) mass ratio of 1:4 (w/w) at room temperature for 84 h resulted in DKL products with a weight-average molecular weight (Mw) of 1350 Da, much lower than that of the original lignins. In addition, the DKL products displayed a significant increase in carboxylic acid group content, a high solubility in water at low pH and, significantly lower decomposition temperatures compared to the original lignins. It was confirmed that nitration of the lignin structure occurred during nitric acid-oxidative depolymerization, as evidenced by the presence of 5% nitrogen (N) content in the DKL products.