A novel environmentally friendly process for depolymerization of hydrolysis lignin using Kraft cooking liquor: chemicals recoverable by the Kraft recovery cycle

Abstract

AbstractHydrolysis lignin (HL) refers to a lignin‐rich residue obtained after the enzymatic hydrolysis of biomass. It is recalcitrant, heterogeneous, insoluble in most common solvents, and less reactive than other lignins. To enhance the reactivity of HL, a novel environmentally friendly depolymerization approach was demonstrated to produce depolymerized hydrolysis lignin (DHL) using Kraft cooking liquor, white liquor (WL) – recoverable by the Kraft recovery cycle. The effects of various process parameters such as reaction time, WL/HL ratio, and reaction temperature on lignin depolymerization were investigated using a 2 L Parr reactor under N2. The DHLs obtained were then characterized by Gel permeation chromatography (GPC)‐Ultraviolet Detector (GPC‐UV), 31P Nuclear Magnetic Resonance (NMR) spectroscopy, and ultraviolet visible (UV–visible) spectroscopy, while the filtrates were characterized by high‐performance liquid chromatography (HPLC) for saccharinic acids. The DHL yield reached 45–70% from the treatments at 150–190 °C for 1 h at a WL/HL mass ratio of 1:4 ~ 2:1. The weight‐average molecular weight (Mw) value of the DHL obtained at 190 °C after treatment for 1 h at a WL/HL ratio of 2:1 (w/w) was 2600 Da. Moreover, a significant increase in non‐condensed phenolic hydroxyl and carboxylic acid group content was observed with decreasing Mw. Compared with various existing lignin modification approaches, the approach reported here is less expensive and more environmentally friendly if integrated into Kraft pulp mill operations with the residual WL from the lignin depolymerization process being recycled to the mill chemical recovery cycle. Process scale‐up was also demonstrated using a 20 L circulating reactor. In this case, the Mw of the DHL produced after treatment at 170 °C for 2 h was 2400 Da. © 2020 Society of Chemical Industry and John Wiley & Sons, Ltd.