Holzforschung 2015; aop *Corresponding author: Herbert Sixta, Department of Forest Products Technology, Aalto University, P.O. Box 16300, Vuorimiehentie 1, FI-00076 Espoo, Finland, e-mail: herbert.sixta@aalto.fi Wenwen Fang, Marina Alekhina and Olga Ershova: Department of Forest Products Technology, Aalto University, Espoo, Finland Sami Heikkinen: Laboratory of Organic Chemistry, Helsinki University, Helsinki, Finland Wenwen Fang, Marina Alekhina, Olga Ershova, Sami Heikkinen and Herbert Sixta* Purification and characterization of kraft lignin Abstract: To upgrade the utilization of kraft lignin (KL) for high-performance lignin-based materials (e.g., carbon fiber), the purity, molecular mass distribution (MMD), and thermal properties need to be improved and adjusted to target values. Therefore, different methods, such as ultra- sonic extraction (UE), solvent extraction, dialysis, and hot water treatment (HWT), were applied for the purification of KL. The chemical and thermal properties of purified lignin have been characterized by nuclear magnetic reso- nance, Fourier transform infrared, gel permeation chro- matography, elemental analysis, differential scanning calorimetry, and thermogravimetric analysis. The lignin fractions obtained by UE with ethanol/acetone (E/A) mix- ture (9:1) revealed a very narrow MMD and were nearly free of inorganic compounds and carbohydrates. Further, the E/A-extracted lignin showed a lower glass transition temperature ( T g ) and a clearly detectable melting tempera- ture ( T m ). Dialysis followed by HWT at 220 °C is an efficient method for the removal of inorganics and carbohydrates; however, lignin was partly forming condensed structures during the treatment. Keywords: chemical analysis, hot water treatment, kraft lignin, thermal analysis, ultrasonic extraction DOI 10.1515/hf-2014-0200 Received July 6, 2014; accepted November 26, 2014; previously published online xx Introduction Kraft lignin (KL) as a byproduct of sulfate (kraft) cooking process amounts to 45 × 10 6 t year -1 , which is approximately 85% of the global lignin production (Tejado et al. 2007). However, only 1–2% of this lignin is isolated from black liquor (BL) and used as material (Gosselink et al. 2004; Vishtal and Kraslawski 2011). Many efforts have been done to develop high value-added products from KL, such as carbon fibers (Kadkla et al. 2002; Baker and Rials 2013), composites (Kharade and Kale 1999; Thielemans et al. 2001), binders and resins (Cavdar et al. 2008), activated carbons (Carrott and Carrott 2007), and some low molecu- lar weight chemicals (vanillin, hydroxylated aromatics, and quinones) (Borges da Silva et al. 2009). The problem, in this context, is the inhomogeneity of KL, such as its wide molecular weight distribution and high content of impurities. The inorganic contaminants of KL originate from the process chemicals (e.g., NaOH and Na 2 S) (Chakar and Ragauskas 2004). The content of inorganics in BL is approximately 30% (Mansouri and Salvadó 2006). However, the sodium content of the precipitated KL can be reduced to approximately 0.5% by acid washing, whereas the remaining sulfur content in KL is 1%–3% (Öhman 2006). Sulfur compounds are toxic and can cause odor problems during thermal treatment. This is because the majority of sulfur is chemically linked to KL (Svensson 2008). KL also contains a certain amount of carbohydrates originating from lignin-carbohydrate complexes (Iversen and Wännström 1986; Vishtal and Kraslawski 2011). KL purification is rewarding. A hardwood KL puri- fied by organic solvent extraction showed excellent spin- nability in contrast to the poor spinnability of unpurified lignin due to the presence of infusible inorganics (Baker et al. 2012). KL can be fractionated and purified by means of various organic solvent systems, such as methanol, acetone, and diethyl ether (Mörck et al. 1986; Thring et al. 1996; Ropponen et al. 2011; Saito et al. 2013). Brodin et al. (2009) improved the homogeneity and purity of lignin significantly by membrane fractionation of industrial BLs followed by acidification according to the LignoBoost process (Öhman et al. 2006) and further purification by ion exchange. Enzymatic hydrolysis followed by acid hydrolysis is able to remove almost all carbohydrates from lignin but introduces new protein contaminants (Argyro- poulos et al. 2002). Sulfur removal is a priori difficult, as it is organically bonded with KL. Raney nickel reduction is efficient for this purpose (Svensson 2008) but is expen- sive for industrial realization, and sulfur may inactivate the catalyst. B r o o o u g h t t o y o u b y | A a l t o U n i v e r s s i t y A u t h e n t i c a t e d D o w n l o a d D a t e | 2 / 1 0 / 1 5 9 : 2 0 A M