Adam S. Klett

Predicting lignin depolymerization yields from quantifiable properties using fractionated biorefinery lignins

Lignin depolymerization to aromatic monomers with high yields and selectivity is essential for the economic feasibility of many lignin-valorization strategies within integrated biorefining processes. Importantly, the quality and properties of the lignin source play an essential role in impacting the conversion chemistry, yet this relationship between lignin properties and lignin susceptibility to depolymerization is not well established. In this study, we quantitatively demonstrate how the detrimental effect of a pretreatment process on the properties of lignins, particularly β-O-4 content, limit high yields of aromatic monomers using three lignin depolymerization approaches: thioacidolysis, hydrogenolysis, and oxidation. Through pH-based fractionation of alkali-solubilized lignin from hybrid poplar, this study demonstrates that the properties of lignin, namely β-O-4 linkages, phenolic hydroxyl groups, molecular weight, and S/G ratios exhibit strong correlations with each other even after pretreatment. Furthermore, the differences in these properties lead to discernible trends in aromatic monomer yields using the three depolymerization techniques. Based on the interdependency of alkali lignin properties and its susceptibility to depolymerization, a model for the prediction of monomer yields was developed and validated for depolymerization by quantitative thioacidolysis. These results highlight the importance of the lignin properties for their suitability for an ether-cleaving depolymerization process, since the theoretical monomer yields grows as a second order function of the β-O-4 content. Therefore, this research encourages and provides a reference tool for future studies to identify new methods for lignin-first biomass pretreatment and lignin valorization that emphasize preservation of lignin qualities, apart from focusing on optimization of reaction conditions and catalyst selection.

Identifying thermal phase transitions of lignin–solvent mixtures using electrochemical impedance spectroscopy

Lignin is unique among renewable biopolymers in having significant aromatic character, making it potentially attractive for a wide range of uses from coatings to carbon fibers. Recent research has shown that hot acetic acid (AcOH)–water mixtures can be used to recover “ultraclean” lignins of controlled molecular weight from Kraft lignins. A key feature of this discovery is the existence of a region of liquid–liquid equilibrium (LLE), with one phase being rich in the purified lignin and the other rich in solvent. Although visual methods can be used to determine the temperature at which solid lignin melts in the presence of AcOH–water mixtures to form LLE, the phase transition can be seen only at lower AcOH concentrations due to solvent opacity. Thus, an electrochemical impedance spectroscopy (EIS) technique was developed for measuring the phase-transition temperature of a softwood Kraft lignin in AcOH–water mixtures. In electrochemical cells, the resistance to double-layer charging (i.e., polarization resistance Rp) is related to the concentration and mobility of free ions in the electrolyte, both of which are affected by the phases present. When the lignin–AcOH–water mixture was heated through the phase transition, RP was found to be a strong function of temperature, with the maximum in RP corresponding to the transition temperature obtained from visual observation. As the system is heated, acetate ions associate with the solid lignin, forming a liquefied, lignin-rich phase. This association increases the overall impedance of the system, as mobile acetate ions are stripped from the solvent phase and thus are no longer available to adsorb on the polarizing electrode surfaces. The maximum in RP occurs once the new lignin-rich phase has completely formed, and no further association of the lignin polymer with AcOH is possible. Except at sub-ambient temperatures, the phase-transition temperature was a strong function of solvent composition, increasing linearly from 18 °C at 70/30 AcOH/water to 97 °C at 10/90 wt% AcOH/water.

Recovery of Low-Ash and Ultrapure Lignins from Alkaline Liquor By-Product Streams

Although a number of treatment methods can be used to separate cellulose and lignin components from biomass, aqueous alkaline treatment methods are dominant. Typically, the cellulose is precipitated from solution, and the lignin ends up in the highly alkaline liquor by-product stream. Today the vast majority of this lignin is burned in situ as a fuel; however, it potentially has far more value as a renewable biopolymer. Here, two new technologies are presented for recovering lignin from alkaline liquor streams generated either from a pulp-and-paper mill or a lignocellulosic biofuels refinery. With SLRPTM technology, the lignin precipitation step is carried out at above-ambient conditions such that a liquid (and not a solid) lignin phase is formed. Thus, the SLRP process for producing low-ash (1–2 %) lignin is continuous, not batch. Furthermore, the “liquid-lignin” phase can be readily fractionated by pH into fractions having different bulk and molecular properties. The ALPHA process was developed with the express purpose of taking low-ash lignins produced from alkaline-liquor by-product streams to the “ultrapure” state. The technology uses adjustable mixtures of biorenewable acetic acid and water to create a unique liquid–liquid solvent system that can be used to simultaneously fractionate, purify, and solvate lignins. Metals contents in ALPHA lignins well below 100 ppm are readily achieved in fractions of both low and high molecular weight.