PEER-REVIEWED ARTICLE bioresources.com Jablonský et al. (2015). “Deep eutectic solvents,” BioResources 10(4), 8039-8047. 8039 Deep Eutectic Solvents: Fractionation of Wheat Straw Michal Jablonský, a, * Andrea Škulcová, a Lucia Kamenská, a Milan Vrška, a and Jozef Šima b Deep eutectic solvents (DESs) are a relatively new topic in science. Their usage is not yet clearly defined, and the areas in which DESs may be applied are constantly growing. A simple and clean fractionation of the main components of biomass represents a very important step in creating a clean, renewable carbon economy. A major challenge is the use of DESs for fractionation of biomass components at lower temperatures, without the use of expensive raw materials. In this work, wheat straw was pretreated with six different DES systems composed of choline chloride with urea (1:2), malonic acid (1:1), lactic (1:9; 1:10), malic (1:1), and oxalic acid (1:1). The pretreated biomass was characterized in terms of lignin content, ash, and holocellulose. A deep eutectic solvent, composed of choline chloride and oxalic acid, was found to produce the best delignification results. The solvents are not selective in the process of delignification. Keywords: Deep eutectic solvents; Fractionation; Lignin; Biomass pre-treatment; Delignification Contact information: a: Slovak University of Technology, Faculty of Chemical and Food Technology, Institute of Natural and Synthetic Polymers, Department of Wood, Pulp and Paper, Radlinského 9, Bratislava, 812 37, Slovak Republic; b: Institute of Inorganic Chemistry, Technology and Materials, Department of Inorganic Chemistry, Radlinského 9, Bratislava, 812 37, Slovak Republic; * Corresponding author: michal.jablonsky@stuba.sk INTRODUCTION Biomass can be converted into diversified materials, biofuels, and biochemicals, especially through the modernization of existing biorefinery technologies, accompanied by the development of new procedures. Several methods have been developed for the pretreatment of lignocellulosic biomass (Zavrel et al. 2009; Surina et al. 2015) in order to obtain a high yield of the desired product. The methods must be adapted to the specific source of lignocellulosic biomass (Mäki-Arvela et al. 2010). The unit operations may cause significant degradation of various lignocellulosic components. In the course of degradation, the original material undergoes modification, the proportions of its individual components are changed, and it becomes thus more suitable for subsequent processing. The pretreated material may be utilized mainly in hydrolytic or fermentation processes. Moreover, the process of delignification and the substrate ability to interact with other chemicals used in these processes may be improved. Some pretreatment methods require extreme conditions such as high temperatures and pressures, or strong acids and bases, and special facilities are frequently required. One of the most promising technologies for fractionation of components is the use of deep eutectic solvents. A deep eutectic solvent (DES) is a type of ionic solvent with special properties. It is composed of a mixture which forms a eutectic with a melting point much lower than either of its individual components (Abbott et al. 2004). Frequently, DESs are formed by mixing choline chloride with hydrogen bond donors,