(534c) Effects of pH and Multistage ALPHA Process on the Phase Behavior of Kraft Lignin

As a key component of lignocellulosic biomass, lignin is an abundant biopolymer, notable for its relatively high amount of carbon, aromatic moieties, and functional hydroxyl groups. Compared to polysaccharides, lignin can achieve higher char yields, benefiting applications such as activated carbon or carbon fibers. Furthermore, the aromatic hydroxyl groups, and other specific chemistry not commonly found in other biomolecules open pathways for lignin-based polyurethanes, phenol-formaldehyde resins, or polyesters. However, as a consequence of its biomass origins, lignin applications must also contend with the impure and polydisperse nature of this resource. Extraction methods such as the Kraft pulping process, or the alkaline pulping used by prospective bioethanol refineries, do not target lignin as the primary output. Instead, lignin is often treated as a co-product at best, and a by-product at worst. This results in lignin streams regularly containing as much as 5% ash, 5% sugar, and varying extents of chemical modification (sulfur addition in the case of Kraft lignin).

With renewability and potential for low-cost underpinning all things lignin, these paradigms must be upheld in all downstream fractionation and purification. To meet this end, a new process of fractionating lignin was developed in 2016 by Thies and coworkers called Aqueous Lignin Purification with Hot Agents (ALPHA). This process makes use of a mixture containing an organic solvent such (acetic acid, ethanol, etc.) and water to form two liquid phases. These phases consist of a lignin-rich phase with high molecular weight and high purity, along with a solvent-rich phase with low molecular weight and lower purity. This method of purifying lignin has proven to be successful with previous studies showing an ultrapure fraction with <25ppm of sodium, and controlled high molecular weight of >10,000 Da.

To optimize the ALPHA process and achieve control of lignin purity and molecular weight, the underlying phase behavior must be understood. There are several operating conditions to consider: organic solvent concentration, temperature, solvent-to-feed ratio, and pH. While the effects of manipulating the temperature, organic solvent, and solvent-to-feed ratio have been previously studied, the effect of system pH has not. It has since been found that pH has a significant impact on phase behavior – suggesting that (de)protonation of lignin is likely to impact its chemical properties. When dealing with lignins coming from different processes, with different levels of acidic/basic impurities, understanding the role of pH in the system is critical for successful fractionation.

In this work, the role of pH and molecular weight on the phase behavior of selected lignins are investigated. This knowledge enables a multi-stage process that can successfully control the molecular weight and purity of lignin fractions in order to enable down-stream applications research.