Timo Leskinen, Alistair W. T. King, and Dimitris S. Argyropoulos

Abstract Ionic liquids (ILs) have been recognized as a promising way to fractionate lignocellulosic biomass. During recent years, a number of publications have intro­duced a variety of technical developments and solvent systems based on several types of ILs to fractionate lignocellulose into individual polymeric components, after full or partial dissolution. In this chapter we briefly review the latest developments and knowledge in this field of study and introduce an alternative fractionation method based on the controlled regeneration of components from 1-allyl-3-methyl — imidazolium chloride ([amim]Cl). Norway spruce (Picea abies) and Eucalyptus grandis woods were dissolved in their fibrous state or by utilizing ball milling to improve solubility. The resulting wood solutions were precipitated gradually into fractions by addition of non-solvents, such as acetonitrile and water. Further water extraction of the crude fractions resulted in better separations. By analyzing molec­ular weight distributions of the fractions, together with their chemical composition, we have obtained fundamental information concerning the mechanisms of wood fractionation with ILs. Fractionation efficiency is found to be highly dependent on the modification of the wood cell wall ultrastructure and the degree of reduction of the molecular weights of the main components, arising from mechanical degradation.

T. Leskinen

Departments of Forest Biomaterials and Chemistry, North Carolina State University, Raleigh, NC 27695-8005, USA

A. W.T. King

Department of Chemistry, University of Helsinki, Helsinki 00014, Finland D. S. Argyropoulos (*)

Departments of Forest Biomaterials and Chemistry, North Carolina State University, Raleigh, NC 27695-8005, USA

Center of Excellence for Advanced Materials Research (CEAMR), King Abdulaziz University, P. O. Box 80203, Jeddah 21589, Saudi Arabia e-mail: dsargyro@ncsu. edu

Z. Fang et al. (eds.), Production of Biofuels and Chemicals with Ionic Liquids, Biofuels and Biorefineries 1, DOI 10.1007/978-94-007-7711-8_6,

© Springer Science+Business Media Dordrecht 2014

Isolation of cellulose enriched fractions was archived with Spruce sawdust and ball milled Eucalyptus, evidently following from distinct dissolution mechanisms.

Keywords Wood • Cellulose • Lignin • LCC • Ionic liquids • 1-allyl-3-methylimi — dazolium chloride • Fractionation • Extraction • Separation • Molecular weight • Pulp • Biofuels

6.1 Introduction

In a relatively short time, the research area of ionic liquid-mediated fractionation and pretreatment of wood has emerged from the interest of a small group of scientists into a noteworthy and diverse field of study. Global interest in lignocellulosic biomass is experiencing a renaissance, not only because of the growing financial potential in lignocellulose-based liquid biofuels, but also because it represents a source of bio-based materials and chemicals. Ionic liquids (ILs) have been recognized to have potential in many applications that can be categorized under the advanced utilization of grassy and woody biomass. The ability to dissolve various biopolymers and a general status as a green alternative to organic solvents makes IL platform technologies attractive to industry. This is mostly in areas pertaining to the manu­facture of novel polymeric materials, by derivatization or blending of cellulose. Alternatively, in biomass pretreatments, including structural or compositional alter­ation of plant cell walls and acid catalysed hydrolysis of plant polysaccharides, for the purposes of biofuel production [1-3]. Aside from the use of ILs as a media for modification, fractionation of lignocellulosic biomass can be integrated into a variety of applications as it can also be used as a method to obtain purified or specified polymeric raw materials, for further use [4]. IL-mediated fractionation is suitable for the general concept of a biorefinery, serving the demand of component separation for subsequent multiple product streams. Ideally this method should be tunable. How­ever, the selectivity of fractionation of native woods using ILs is still poorly devel­oped or understood, from a mechanistic point of view.

Understanding the fundamentals of the separation of polymeric cell wall com­ponents has improved after initial publications concerning cellulose and whole wood dissolution into ILs [5—7]. Ideally, there are two ways to fractionate ligno- cellulose: (1) complete pre-dissolution of biomass followed by selective precipita­tion of the sought components as purified fractions, by addition of a non-solvent, or

(2) selective extraction of components from the biomass. The first efforts to isolate purified fractions using ILs can be roughly categorized under either of the afore­mentioned approaches [6, 8—10]. However, the complex recalcitrant structure of wood greatly hinders a complete dissolution and efficient fractionation. During the last few years, a variety of new methods have emerged resulting in enhanced fractionation. In addition to introducing our work on wood fractionation, in this effort we will also present a brief overview of the latest technical advances in the IL-based fractionation systems and of our findings related to the mechanisms controlling the dissolution and fractionation of the complex materials of the wood cell wall.

Our work concerning wood fractionation has focused on dissolution of Norway spruce (softwood) and Eucalyptus grandis (hardwood) woods, as completely as possible under mild conditions, followed by a stepwise regeneration of wood com­ponents, with the addition of non-solvents. From initial screening, non-solvents were chosen in an attempt to enhance the selectivity of component precipitation. In this selection we considered the optical brightness of the precipitated samples, the ease of recovery (defined precipitate vs. emulsions) and the ability to fractionally precipitate the dissolved material. In our overall work we have selected the IL 1-allyl-3- methylimidazolium chloride ([amim]Cl) for the fractionation experiments, which has been demonstrated to have a good dissolution capability for cellulose and wood materials [7,11-13]. The starting materials were either coarse TMP softwood pulp or sawdust and fine ball-milled powders from soft — or hardwood. In contrast to the usual approach, in component regeneration from IL solutions, we did not use excess of non-solvent causing rapid precipitation, but gradually increased the amount of a single polar non-solvent to control the amount of precipitated material. By this method, only a fraction of the dissolved material was precipitated, while the rest of the material remained in solution. Careful gel-permeation chromatographic analyses of the fractions offered a visualization of the molecular weight and the distribution of species within the dissolved components. Ball-milled wood dissolved completely in the IL, but the coarse sawdust or TMP pulp preparations were not fully soluble on a microscopic level. It has been noticed earlier that the solubility of softwood in [amim] Cl and subsequent phosphitylation of all hydroxyl functionalities is greatly dependent on the preliminary mechanical treatment [14]. Surprisingly, this partial insolubility of sawdust has enabled a more efficient component separation, by selective extraction of components, compared to the soluble fine powder preparations, which separate according to molecular weight distributions. From the coarse material, a cellulose — rich fraction was extracted and the rest of the lignin-hemicellulose matrix could be isolated as an insoluble material. In the case of increased pulverization, the observed better solubility was rationalized by the fragmentation of the matrix formed by lignin — carbohydrate complexes (LCC). An increase in the amount of water extractable lignin from crude fractions of milled wood, after the IL treatment, points to the presence of soluble fragments, originating from an LCC matrix.