Fractional Precipitation of Softwood Kraft Lignin: Isolation of Narrow Fractions Common to a Variety of Lignins

Abstract

In a manner similar to crude oil, technical lignins need refining if their potential as reactive polyphenols of well-defined molecular weight polymers and oligomers is to be actualized. In this paper, we demonstrate that a continuum of narrow fractions can be isolated by the incremental addition of a nonpolar solvent (hexanes) in a polar (acetone) solution of softwood wood kraft lignin. Three distinct commercial samples of softwood kraft lignin were used to examine the validity of the developed protocol using detailed chromatographic and quantitative functional group analytical methods. It was shown that all samples contain a common relatively monodisperse fraction of a polyphenolic material that can be isolated from the different lignins in yields ranging between 10 and 20% w/w. The versatility of the developed fractional precipitation protocol was further validated by creating artificial physical mixtures of the examined lignins in different proportions and isolating from them precisely calculated fractions of nearly identical molecular weight distributions and composition. Overall, the fractional precipitation approach described here offers the possibility that representative specific narrow fractions, common to a variety of softwood kraft lignins, can be isolated irrespective of the manufacturing details of the pulping process. As such, the otherwise known heterogeneous kraft lignin material, whose composition is relatively unpredictable due to manufacturing variations in making pulp, may now offer consistently homogeneous lignin streams with significant commercial ramifications.

Lignin Prepared by Ultrafiltration of Black Liquor: Investigation of Solubility, Viscosity, and Ash Content

“…Thus, ultrafiltration can be a technically interesting way of obtaining technical lignin as it is or in combination with ways of fractionating lignin based on selective acid precipitation or extraction with organic solvents (Cui et al 2014;Lourencon et al 2015;Dodd et al 2015).…”

COST Action FP1105: effect of raw materials and pulping conditions on the characteristics of dissolved kraft lignins

“…Selective precipitation and ultrafiltration are potentially useful techniques, depending on the intended lignin application (Brodin et al 2009;Toledano et al 2010;Ponomarenko et al 2014;Sevastyanova et al 2014;Duval et al 2015). It has been demonstrated that an increased content of OH aliph and OH phen groups improves the acetone solubility of lignin, which would be beneficial for fractionation based on acetone and hexane (Cui et al 2014). As hardwood lignins have a higher OH phen content, they would be more suitable for such fractionation.…”

Valorization of Side-Streams from a SSF Biorefinery Plant: Wheat Straw Lignin Purification Study

“…In this regard this application has the pivotal role in obtaining lignin with high purity (Cui et al 2014;Frigerio et al 2014). A deep chemical characterization was achieved by the GPC, FTIR, 2D-HSQC, and 31 P-NMR analyses.…”

“…The major issue that precludes the complete utilization of lignin is that the SE process is not optimized to produce pure lignin (Cui et al 2014;Frigerio et al 2014), and the array of aromatic polymers obtained is contaminated by impurities such as ash and residual carbohydrates, which are often still covalently connected to lignin (Shevchenko et al 1999). While lignin extraction from black liquor, for example in the pulp industry, has already been extensively explored (e.g.…”