Processing of lignin and the removal of detrimental with deep eutectic solvents

Lignocellulosic biomass, the primary building block of plant cell walls, consists of three main components, i.e. cellulose, hemicellulose and lignin. A rather important, but in current processes inefficiently used substance of this biomass is lignin. Lignin is a natural occurring complex biopolymer found in the cell walls of higher plants and woody tissue. It consists of a complex network of phenylpropanoids connected mainly by ether bonds and condensed C-C linkages. In the paper industry, lignin prohibits the production of high quality cellulose pulp directly from biomass and efficient delignification is required. It is favored for this lignin to be removed without altering its structure for further valorization. For this development Deep Eutectic Solvents (DESs) show promising characteristics. DESs are low transition temperature mixtures (LTTMs) consisting of at least one hydrogen bond donor (HBD) and one hydrogen bond acceptor (HBA) that result in a liquid mixture showing an unusual low melting point.¹ Due to the high hydrogen bonding interaction, some of the promising characteristics of ionic liquids are shared by DESs.² For example, DESs show a wide liquid range and good solvation properties. Additionally, they can easily be prepared upon mixing at moderate temperatures without the need of purification. Generally, DESs are considered as environmentally benign solvents.³ Currently in industry delignification takes place by cooking with chemicals at high temperatures and pressure followed by multiple bleaching steps. A ‘greener’ and more efficient method for wood would be highly beneficial for both environment and business. Previously, ionic liquids (ILs) have shown their ability to selectively extract lignin from biomass without disrupting the cellulose.