Oxidative depolymerization of lignin using nitric acid under ambient conditions

• Published in: Industrial crops and products 2021-10, Vol.170, p.113757, Article 113757
• Main Authors: Ahmad, Zaid, Paleologou, Michael, Xu, Chunbao Charles
• Format: Article
• Language: English
• Subjects: Ambient temperature; Depolymerized kraft lignin; Hardwood; Kraft lignin; Nitric acid; Softwood
• ISSN: 0926-6690; 1872-633X
• DOI: 10.1016/j.indcrop.2021.113757

[Display omitted] •Pulping waste: kraft lignin was converted to valuable products via depolymerization.•Cost-effective strategy proved to produce high value depolymerized kraft lignin.•Kraft lignin nitration and oxidation at room temperature was achieved.•No heat/catalyst/solvent and no product purification process are required.•This approach can easily be scaled-up to the commercial level. 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.