Holzforschung, Vol. 66, pp. 935–943, 2012 • Copyright © by Walter de Gruyter • Berlin • Boston. DOI 10.1515/hf-2011-0180 Pretreatment of softwood dissolving pulp with ionic liquids Dongfang Li, Olena Sevastyanova and Monica Ek* Division of Wood Chemistry and Pulp Technology, Department of Fiber and Polymer Technology, KTH Royal Institute of Technology, Teknikringen 56-58, SE 100 44, Stockholm, Sweden * Corresponding author. E-mail: monicaek@kth.se Abstract Few Scandinavian pulp mills produce dissolving pulps; how- ever, the demand on textile fibers is increasing. This study investigates the chemical interaction of dissolving pulp with ionic liquids (ILs), where softwood and hardwood industrial dissolving pulps were pretreated with ILs 1-butyl-3-methyl- imidazolium acetate ([C 4 mim + ]CH 3 COO - ) and 1-butyl-3- methyl-imdazolium chloride ([C 4 mim + ]Cl - ). Time and tem- perature dependence of the dissolution process as well as the impact of the pretreatment on the molecular weight proper- ties, thermal stability, morphology, and crystallinity of the cellulose were evaluated by means of size exclusion chro- matography (SEC), thermogravimetric analysis (TGA), scan- ning electron microscopy (SEM), X-ray diffraction (XRD), and solid state nuclear magnetic resonance (NMR). It was shown that the dissolution of cellulosic material in ILs is a temperature-dependent process; however, the viscosity of ILs affected the efficiency of dissolution at a given temperature. Molecular weight properties were affected negatively by increased dissolution temperature, while the type of antisol- vent for the regeneration had no major impact on the degree of polymerization of cellulose. Water was more efficient than ethanol for the regeneration of cellulose when performed at an elevated temperature. The pretreatment decreased the crystal- linity of cellulosic material. This might lead to the increased accessibility and reactivity of cellulose. Keywords: 1-butyl-3-methyl-imidazolium acetate; 1-butyl-3- methyl-imdazolium chloride; cellulose; crystallinity; dissolu- tion; dissolving pulp; ionic liquids (ILs); molecular weight distribution; morphology; regeneration; softwood; thermal stability. Introduction Cellulose and its derivatives are the most abundant biological and renewable materials, widely used in various industries. As such, wood and cotton can be directly utilized without modification. However, for many other industrial applica- tions – for papermaking and pharmacy as well as for textile, polymer, and paint production – cellulose has to be isolated, modified, and derivatized. In principle, the cellulose is first isolated either chemically or mechanically from lignocellu- losic materials and then treated chemically or transformed into its derivatives. Dissolving pulp is the major raw material for the produc- tion of regenerated cellulose. Its high cellulose content (90– 99%), brightness, and uniform molecular weight distribution (MWD) are important for the production of various cellulose derivatives. The price of dissolving pulp increased drastically on world markets because of rising demand in the areas of tex- tile, health, and hygiene (Patrick 2011). Dissolving pulps can be produced from softwoods (SWs) and hardwoods (HWs); however, in Sweden, the dominating SW species spruce and pine are preferred (Henriksson et al. 2009). The utilization of cellulosic materials is still challenging. The solution of cellulose is difficult due to its fibrillar struc- ture stabilized by inter- and intramolecular hydrogen bonds, dipole and Van der Waals interactions. Mainly the less- ordered (paracrystalline) regions of cellulose are accessible to chemicals (Krässig 1993). The cellulose solvents can be broadly classified as deriva- tizing and non-derivatizing solvents, including aqueous and non-aqueous media. However, the traditional dissolution methods such as cuprammonium and xanthate processes are cumbersome and expensive. The solvents with high ionic strength and the relatively harsh processing conditions cause serious problems (Treiber 1985; Chanzy et al. 1990; Swatloski et al. 2004; Heinze and Koschella 2005). Many solvents are also problematic from the environmental point of view, and their recycling is difficult. Ionic liquids (ILs) containing cations as well as anions are a class of non-volatile solvents with unique solvating properties. The organic molten salts with imidazolium cations are inert and promising cellulose solvents (Swatloski et al. 2004). Owing to the low melting temperature below 100°C, ILs are often liquid at room temperature. ILs are also referred to as “green” solvents as they are non-flammable, thermally and chemically stable, recyclable, and have low vapor pressures. ILs have recently received increased attention (Swatloski et al. 2002; Li et al. 2005; Pernak et al. 2005a,b; Zhang et al. 2005; Xie and Shi 2006; Zhu et al. 2006; Kilpeläinen et al. 2007; Heinze et al. 2008; Stasiewicz et al. 2008; Nakamura et al. 2009; Gayet et al. 2010; Jia et al. 2010; Liebner et al. 2010; Mikkola et al. 2010; Nakamura et al. 2010; Li et al. 2011; Polyakova et al. 2011; Ramalingam and Banerjee 2011; Schrems et al. 2011; Viell and Marquardt 2011; Ignatyev et al. 2012; Qu et al. 2012). ILs as molten inorganic salt hydrates are effective for cellulose dissolution and derivatization (Fischer 2004). The cellulose dissolved in ILs can be regenerated with water, ethanol, or acetone. After precipitation, the degree of polymer- ization (DP) and polydispersity of cellulose is left unchanged, Brought to you by | Royal Institute of Technology Authenticated | 130.237.78.187 Download Date | 1/9/13 11:56 AM