(420e) Beyond the Scientific Curiosity: Smart Use of Ionic Liquids in Integrated Biorefinery Concept

The low carbon economy is one of the main long-term targets to be attained by the European Union by 2050.¹ The sustainable use of lignocellulosic biomass as a renewable feedstock is expected to display a key role to reach that objective. The lignocellulosic biomass is an abundant and renewable source of carbon, but up to now it has been scarcely explored as a raw material in the industry except wood-based materials, pulp and paper factories, biomass-derived fibres or 1G biofuels.²Hence, a more extensive valorization of lignocellulosic biomass, a low-price feedstock, is required to accomplish the biorefinery concept and achieve low carbon economy objectives.

The efficient lignocellulosic biomass processing requires a selection of suitable technologies and strategies that underpin maximal separation of main fractions of biomass, namely cellulose, hemicellulose and lignin. However, a very complex inter- and intramolecular network between cellulose, hemicellulose and lignin makes the efficient fractionation of lignocellulosic biomass hindered and by this very challenging.³ To overcome the biomass recalcitrance, chemical and thermal technologies, performed at elevated temperature and pressure with hazardous chemicals, have been widely applied in biomass processing.⁴ The conventional acid and alkaline treatments are some of examples of chemical processes in which partial biomass fractionation takes place and a recovery of the catalysts is still intricate. On the other hand, water-based processes such as steam explosion, liquid hot water and autohydrolysis do not require the addition of extra chemical, but the removal of hemicellulose (often oligosaccharide form) takes place at elevated temperature.⁵

To solve those bottlenecks and to maximize the diversification of potentially available commodities from biomass, alternative, greener and more sustainable technologies of lignocellulosic biomass conversion should be developed. In this context, ionic liquids,⁶ carbon dioxide^7,8 and others^9,10are often a first choice, because they allow achieving a selective biomass processing at less sever conditions contributing to CAPEX and OPEX reduction.

Ionic liquids (ILs) are organic salts with low melting point, composed of ions among which, at least one, the most often cation, is organic and large-size with a low charge density and weak electrostatic interactions with counter-ion. This makes ILs to possess negligible vapor pressure, high thermal stability, wide electrochemical window and a great solvent power.¹¹Due to these intriguing characteristics, ILs have been recognized to be excellent alternatives to VOCs in many applications however in most of the cases the processes are still in the lab scale.

A partial or total dissolution of lignocellulosic biomass in ILs is associated to the anion (e.g. acetate or chloride) properties due to an alkaline character of this ion. Thus, a strong hydrogen bond network with biomass polymers can be formed¹² and by this improved biomass fractionation into main constituents can be achieved.¹³ Although efficient biomass fractionation using ILs has been frequently reported, the produced solid polysaccharide-rich fractions still require the hydrolysis (e.g. enzymatic) step to produce fermentable sugars, such as hexoses (mostly glucose from cellulose) and pentoses (hemicellulose derived sugars such as xylose and arabinose). Enzymatic hydrolysis is one of the major challenges in 2G biofuels’ production due to high enzyme charge needed to achieve satisfactory hydrolysis yield similar to 1G biofuels.¹⁴ Hence, the process intensification and a sole use of acidic ILs, able to integrate pre-treatment and hydrolysis of biomass polysaccharide polymers in a single step, is an important advance in the bioenergy production context.

All works^15-19 on this subject published up to date have one main challenge that is low or in the best case scenario moderate yields of reducing sugars. Additionally, acidic ILs have shown that besides the capability to hydrolyse carbohydrate polymers into reducing sugars, in some cases further conversion (dehydration) into furans may take place.⁶ Actually, sugar dehydration into furfural or 5-hydroxymethylfurfural (HMF) seems to be favored in detriment of reducing sugars, due to the high acidic potential of tested ILs.^20-22 This can be a limitation factor when the obtained liquor are foreseen to be used in biotechnological processes, once furfural and HMF are recognized as inhibitor of fermentation processes.²³ Taking in account the dehydration reaction of monosaccharides to furans, the presence of H₂O shall have an impact on this equilibrium as it was observed previously in other works.¹⁶ Thus, the present work demonstrates the sole use of both [emim][HSO₄] and H₂O to produce pentoses from lignocellulosic biomass with very high yield (above 80 mol %) and by this avoid energy and cost-demanding pre-treatments and enzymatic hydrolysis steps as well as efficient downstream processing of pentoses (above 85 mol %) and ionic liquid (above 90 mol %) and recovery of cellulose and lignin and their valorization in the context of cascade approach of biomass processing in the biorefinery concept.

Acknowledgments

This work was done in the frame of AMBITION - Advanced biofuel production with energy system integration project (Grant Agreement number: 731263 – H2020-LCE-2016-2017/H2020-LCE-2016-ERA) financed by the European Commission.

Acknowledgment is also addressed to the Fundação para a Ciência e Tecnologia (FCT/MEC, Portugal) for funding SFRH/BD/90282/2012, IF/00471/2015 grants and by BBRI - Biomass and Bioenergy Research Infrastructure (ROTEIRO/0189/2013) and the Associated Laboratory for Sustainable Chemistry - Clean Processes and Technologies - LAQV which is financed by national funds from FCT/MEC (UID/QUI/50006/2013) and co-financed by the ERDF under the PT2020 Partnership Agreement (POCI-01-0145-FEDER - 007265). Acknowledgment is also given to FCT and CAPES for financing Bilateral Cooperation Portugal (FCT)/Brazil (CAPES) project (FCT/1909/27/2/2014/S and CAPES 371/14, AUXPE Nr 0005/2015, project 23038.002463/2014-98) and CAPES for INCT-Catálise (Brazil).

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