Optimization of colloidal lignin particle production through process integration in an organosolv biorefinery

Abstract

The utilization of lignocellulosic materials in biorefineries is a highly relevant part of thetransition from fossil resources into a sustainable, renewable-based economy. Despite itsabundance, lignin is currently still mostly used for energy production. However, due to theirhigh specific surface area, colloidal lignin particles (CLPs) have the potential to facilitate ligninin material applications.Compared to commercial pulping processes, organosolv extraction produces high-quality ligninsuitable for material uses, but has some economic drawbacks. The production of CLPs fromorganosolv extracts by precipitation could potentially offset these obstacles and make theprocess viable by unlocking high value applications. To find a balance between high degreesof biomass delignification in the extraction and lignin of a suitable quality, the influence of theprocess conditions in the extraction on the resulting CLPs needs to be studied.In this work, four extraction parameters (type of raw material, extraction temperature, catalystconcentration, and liquor separation temperature), and five parameters of the CLPs productionby precipitation (type of raw material, extraction temperature, catalyst concentration, ligninconcentration, and precipitation temperature) were studied on their impact on the ligninextraction and the properties of the resulting CLPs. The results showed that extracts fromdifferent raw materials can be precipitated into CLPs with comparable properties. Highertemperatures and catalyst concentrations increased the lignin content in the extract, but abovetemperatures of 200 °C or catalyst concentrations of 0.6 wt% the precipitated CLPs hadsignificantly lower quality in terms of hydrodynamic diameter and color. Regarding theprecipitation temperature, higher mixing temperatures lead to smaller hydrodynamic diametersand slightly reduced precipitation yields. The experiments on the impact of the liquor separationrevealed that more lignin is solubilized at higher separation temperatures, which indicated thepotential for a simple and technologically reasonable option for process optimization.Based on the studies on extraction and precipitation, parameters for a final study on processintegration of the two steps were chosen, where the liquor was separated at differenttemperatures between 20 and 80 °C and directly precipitated. The resulting suspensions hadcomparable properties, demonstrating that the production of CLPs was still possible whenintegrating extraction and precipitation. Due to the increase in lignin concentration with theliquor separation temperature, the overall yield of CLPs also increased with the separationAbstractVIItemperature, as well as the hydrodynamic diameter of the CLPs. However, analysis of theprimary particle size with electron microscopy showed no significant trend in the size of theprimary lignin particles. Likewise, no correlation between liquor separation temperature andantioxidant activity or molecular weight of the CLPs was found. These findings indicate thathigher liquor separation temperatures increase the yield without a negative impact on the CLPproperties.