Héctor RODRÍGUEZ1, Carlos A. PENA1, María C. CASTRO1, Ana SOTO1
1CRETUS, Department of Chemical Engineering, Universidade de Santiago de Compostela, Santiago de Compostela, Spain
Lignocellulosic biomass is abundant, well geodistributed, and dynamically biorenewed at a relatively high pace. Moreover, its three major biopolymers (cellulose, hemicellulose, and lignin) do already embed naturally a very interesting chemical richness. Due to these characteristics, it holds the potential to be a key resource in the development of more sustainable technological platform for the production of chemicals and materials. For effective and efficient exploitation of the valuable biopolymers involved in its recalcitrant lignocellulosic three-dimensional matrix, this biomass is typically subjected to a pretreatment stage, improving accessibility to the different components. The currently most common pretreatment processes are based on the use of strong acids or bases, or of volatile organic solvents, and involving harsh processing conditions. To avoid this negative contribution of the pretreatment stage to the sustainability credentials of biorefinery-based schemes for the valorisation of biorenewable lignocelluloses, new pretreatment processes with a more sustainable character must be sought.
Ionic liquids, with the unique set of properties that they can exhibit (combining for example a negligible vapour pressure with a great solvation ability, a reasonably good thermal stability, and non-flammability), have the potential to contribute to the development of milder and more sustainable pretreatment processes. The capacity of some ionic liquids to dissolve lignocelluloses directly without derivatisation has suggested a dissolving approach for biomass pretreatment, with fractionated recovery of the biopolymers from the ionic liquid solution by means of a suitable strategy of addition of antisolvents. However, the subsequent and necessary recovery of the ionic liquid from its mixtures with the antisolvent(s) is likely to represent an excessively high energetic penalty at an industrial scale. The possibility of disruption of the lignocellulosic matrix for improved accessibility of the biopolymers by means of a non-dissolving approach, which can be also carried out by other ionic liquids or by the same ionic liquids at other conditions, would be a potentially attractive alternative.
In this work, the pretreatment of milled Eucalyptus globulus wood with the archetypal biomass-processing ionic liquid 1-ethyl-3-methylimidazolium acetate ([C2mim][OAc]) was investigated at non-dissolving conditions; namely atmospheric pressure and temperatures of 45-65 °C. Both the pure ionic liquid and its 95:5 wt/wt mixture with ethanol were explored in this simple non-dissolving approach involving a direct solid-liquid contact. The thus-pretreated wood samples, recovered by filtration, were subjected to hydrolysis by means of an enzymatic cocktail, and a remarkable improvement in the hydrolysis kinetics and the extent of saccharification, as compared to the untreated equivalent sample, was observed. A correlation between the crystallinity reduction of the pretreated wood and the improvement of the hydrolysis kinetics could be identified.
This work was supported by Xunta de Galicia through project ED431B 2020/021, co-funded by the European Regional Development Fund.