Fractionation methods of eucalyptus Kraft lignin for application in biorefinery

Abstract

Lignin is the second most abundant biopolymer on Earth, being the main organic compound present in residual liquors from chemical processes to obtain cellulose. Although almost all of this lignin is currently burned in the recovery cycle of the kraft process, applications in biorefinery have been increasingly studied, due to the richness of functional groups present in this polymer. However, a large industrial application still requires overcoming its difficult workability, since it has high polydispersity and low reactivity. Thus, this study aimed to obtain more homogeneous and pure lignin fractions from the application of one-step and sequential fractionation methods using organic solvents and acid precipitation. The organic solvents used were ethyl acetate, ethanol, methanol and acetone. The pHs tested were 9, 7, 5, 3 and 1, by adding hydrochloric acid. The fractions were characterized in terms of acid- soluble and insoluble lignin, carbohydrates and ashes, as well as by elemental analysis. S/G ratios were determined by Py-GC-MS. All fractions obtained in both fractionation methods showed higher carbon contents, higher purity and lower S/G ratio than the corresponding initial materials (eucalypt kraft lignin and eucalypt kraft black liquor), characteristics that are very favorable for application in biorefinery. Acetone-soluble (sequential) and pH 1 (one- step) precipitated fractions are the most suitable for carbon fiber production. Fractions soluble in ethyl acetate (one-step) and insoluble at pH 3 and 1 (sequential) appear to be the most appropriate for applications that require good oxidative properties. The fractions soluble in ethanol (one-step), methanol (one-step), acetone (one-step) and precipitated at pH 9 (one-step) and pH 5 (sequential) are the ones that allow better chemical substitution in obtaining bioproducts. Fractions soluble in ethanol (sequential) and precipitated at pHs 5 and 1 (sequential) are not of commercial interest due to their low yield. Keywords: Biorefinery. Lignin. Organic Solvents. pH Effect. Eucalyptus.