Fractionation of Lignocellulosic Materials Using Ionic Liquids: Part 2. Effect of Particle Size on the Mechanisms of Fractionation Timo Leskinen, ‡ Alistair W. T. King, † Ilkka Kilpela ̈ inen, † and Dimitris S. Argyropoulos* ,†,‡,§ † Department of Chemistry, University of Helsinki, PO Box 55, 00014 Helsinki, Finland ‡ Departments of Chemistry and Forest Biomaterials, North Carolina State University, Raleigh, North Carolina 27695-8005, United States § Center of Excellence for Advanced Materials Research (CEAMR), King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia * S Supporting Information ABSTRACT: In part 1 of this effort (Ind. Eng. Chem. Res. 2011, 50, 12349-12357), we studied how wood dissolved in ionic liquid (IL) is precipitated into different molecular weight ranges upon the addition of a cosolvent. In this article, we further analyze the chemical compositions of these fractions and elucidate the mechanisms of fractionation. Specifically, we fractionated Norway spruce wood solvated with 1-allyl-3-methylimidazolium chloride ([amim]Cl) and analyzed the resulting fractions by Klason lignin analysis and FT-IR and NMR spectroscopies. We found that separation of the different components can be tuned by the variable dissolution of wood based on particle size, resulting from preparatory milling. It is possible to obtain cellulose-rich material with a relatively low (6.2%) lignin content, from spruce sawdust. This can achieved by extracting the cellulose from the insoluble lignin-carbohydrate complex (LCC) matrix. Extensive milling of wood afforded a soluble LCC matrix, and its precipitation was based on molecular weight and not on chemical composition. Indications of the presence of LCCs in the hemicellulose fraction were obtained by utilizing multidimensional NMR spectroscopy. ■ INTRODUCTION Lignocellulosic materials are a reasonable carbon-neutral option for the production of energy and materials in the future. Agricultural and forest residues and dedicated energy crops could be utilized in the production of biofuels and industrial chemicals, 1-4 in addition to the manufacturing of novel polymeric 5-7 and composite 8 materials. In this article, we focus on wood, which is an abundant lignocellulosic feedstock. Wood is composed of three main polymeric components: cellulose, lignin, and hemicelluloses. Cellulose, the most abundant, is a linear polymer consisting of glucose units joined together by 1,4-glycosidic linkages. Because of the intra- molecular hydrogen bonding between hydroxyl groups in the 2, 3, and 6 positions of the glucose units, these molecules are rigid and tend to arrange in layered structures. Intermolecular O3-O6 hydrogen bonds fasten the cellulose chains together into microfibrils that show both crystalline and amorphous domains. 9 Lignin is a highly branched heterogeneous polymer that is built of phenylpropanoid units, linked with various types of ether and carbon-carbon bonds. 10 Hemicelluloses are branched heteropolysaccharides. Two of the main hemi- celluloses in softwood, on which we focus in our work, are galactoglucomannan and arabino-4-O-methylglucuronoxylan. Their main structural units are mannose and xylose, respectively. Some of the backbone saccharide hydroxyl groups are functionalized as acetyls and glucuronic acid esters and with monosaccharide units. 9 Together with lignin, they are thought to form a network structure in which lignin and hemicelluloses are bonded by benzyl ether, benzyl ester, and phenyl glycoside linkages. 11 These networks are clustered into parallel regions between the cellulose fibrils acting as a composite matrix. 12 Ionic liquids (ILs) have been recognized as a promising way to fulfill goals in the utilization of woody biomass. 13-15 In recent years, several groups have attempted the separation of wood components with ILs. Regardless of the different approaches, their efficient separation has turned out to be a demanding task. The main approaches include the selective precipitation of materials dissolved in an IL by nonsolvent addition 16 or the selective extraction of one the main components with the IL and precipitation of the extracted fraction. 17-19 A combination of these techniques seems promising, based on the fact that Sun et al. 20 reported a more selective separation of carbohydrates when the whole wood starting material was not completely dissolved but was removed from the IL prior to the solvent precipitation step. One potential limiting factor hindering the efficient separation of polysaccharides from lignin is the occurrence of lignin-carbohydrate complexes (LCCs). Recent studies carried out simultaneously with our work strongly support such a contention. Conditions able to physically alter or depolymerize lignin, such as high temperatures above its glass transition, 21 oxidizing agents, 22 and acid formation through autocatalytic processes, combined with an IL treatment have been found to enhance the cellulose enrichment in the regenerated materials. Therefore, a sound scientific understanding of the orientation of the wood components within the cell-wall structures is needed. In addition, knowledge related to the actual type and Received: October 22, 2012 Revised: February 13, 2013 Accepted: February 27, 2013 Article pubs.acs.org/IECR © XXXX American Chemical Society A dx.doi.org/10.1021/ie302896n | Ind. Eng. Chem. Res. XXXX, XXX, XXX-XXX