Chemical characterization of heat-treated kraft lignin after fractionation with organic solvents and pH effect

Lignin, the main source of renewable aromatics, has a complex structure, high polydispersity, and low reactivity, which hinders its large-scale use. This study aims to improve lignin reactivity through heat treatment combined with fractionation by organic solvents or the pH effect. Heat treatment of commercial hardwood black liquor was performed at 225 °C for 150 min. The samples were then fractionated either by using organic solvents (ethyl acetate, ethanol, methanol, and acetone) or by lowering pH using hydrochloric acid (HCl) (pH values 1, 3, 5, 7, and 9). The fractionation was carried out in one step and in sequential mode. The fractionated samples were characterized chemically by the content of acid-soluble and acid-insoluble lignin, carbohydrates, ash, inorganic compounds (metals), elemental analysis, and by pyrolysis-gas chromatography–mass spectrometry (Py-GC–MS). The highest mass yields were obtained using the one-step mode for both fractionations, with yields as high as 53.3% for organic solvents and 47.8% for the pH effect. Solvent fractionation reduced ash content by up to 75.4% and increased calorific value, carbon content, and total lignin content. Fractionation by pH effect showed an ash reduction of up to 55.9% and an increase in calorific value and carbon content of up to 8.7 MJ kg⁻¹ and 49.7%, respectively. The Hansen solubility parameters were also calculated to address lignin's solubility in each of the tested solvents. These findings indicate that combining heat treatment and fractionation techniques improves lignin's chemical properties significantly, making it a more viable raw material for industrial use. This approach supports the circular economy by transforming lignin into a high-value product, thereby promoting sustainable and efficient resource utilization across industries.