Abstract
Abstract Lignocellulosic biomass composes a diversity of feedstock raw materials representing an abundant and renewable carbon source. In majority lignocellulose is constituted by carbohydrate macromolecules, namely cellulose and hemicellulose, and by lignin, a polyphenilpropanoid macromolecule. Between these biomacromolecules, there are several covalent and non-covalent interactions defining an intricate, complex and rigid structure of lignocellulose. The deconstruction of the lignocellulosic biomass makes these fractions susceptible for easier transformation to large number of commodities including energy, chemicals and material within the concept of biorefinery. Generally, the biomass pre-treatment depends on the final goal in the biomass processing. The recalcitrance of lignocellulose materials is the main limitation of its processing once the inherent costs are excessively high for the conventional pre-treatments. Furthermore, none of the currently known processes is highly selective and efficient for the satisfactory and versatile use, thus, new methodologies are still studied broadly. The ionic liquid technology on biomass processing is relatively recent and first studies were focused on the lignocellulosic biomass dissolution in different ionic liquids (ILs). The dissolution in IL drives to the structural changes in the regenerated biomass by reduction of cellulose crystallinity and lignin content contrasting to the original biomass. These findings provided ILs as tools to perform biomass pre-treatment and the advantageous use of their specific properties over the conventional pre-treatment processes. This review shows the critical outlook on the study of biomass dissolution and changes occurred in the biomass during this process as well as on the influence of several crucial parameters that govern the dissolution and further pre-treatment process. The review of currently known methods of biomass fractionation in IL and aqueous-IL mixtures is also discussed here and perspectives regarding these topics are given as well.
Highlights
Lignocelluloses such as hardwood, softwood and agriculture residues are the main raw material from renewable feedstock representing an abundant carbon source [1]
A higher solid loading can result in problems with agitation of the mixture and cause heat and mass transfer limitations in the mixture as well as a significantly decrease in the ionic liquids (ILs) accessibility to the biomass [71,86]. These results show that pre-treatment in ILs could have limitations with respect to the wood load capacity necessary to reach an efficient process
A 20% water content in the IL provided the best dissolution of legume straw. These results enabled an optimised pre-treatment at 150°C for 2 hours under ambient pressure, with a maximum of a 29.1 wt% legume straw dissolved in the aqueous-[bmim][Cl] mixture with water content of 20 wt%
Summary
Lignocelluloses such as hardwood, softwood and agriculture residues are the main raw material from renewable feedstock representing an abundant carbon source [1]. Morphological changes were observed for pre-treated wood flour with [emim][CH3COO] and [bmim][CH3COO] [58] This is a great advantage of biomass pre-treatment with ILs, since cellulose surface area decreases and it provides good accessibility to cellulase and enhances the catalytic efficiency of such enzyme [43,69]. On the other hand, [emim][Cl] works more efficiently with smaller biomass particle sizes in respect to glucose yield This opposite trend is explained by the different dissolution rates of the tested ILs. The [emim][CH3COO] showed a faster dissolution rate for the smaller particle size allowing more extensive depolymerisation to lower molecular weight water soluble compounds. The increase in temperature from 60°C to 120°C for 2hour pre-treatments of sugarcane bagasse with [emim] [CH3COO] resulted in a water-soluble product that revealed a 10-fold magnitude extraction yield from the original biomass and a 2-fold glucose yield in the enzymatic hydrolysis of regenerated bagasse [69]. The vast majority of papers presenting work about the pre-treatment of biomass with ILs do not challenge these issues, thereby resulting in the incomplete demonstration of the pros and cons of biomass processing with ILs
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