application of ionic liquids is still a relatively new field and the physicochemical properties are not properly defined, in the end causing safety concerns [67]. The major issue related to the use of ionic liquids, however, is the hindered separation of oxygenated products such as furanics, given the higher stability of these molecules in charged media [68].
Switchable solvents are attractive for coupling desirable solvent properties that would otherwise be on the opposite ends of the spectrum. Switchable solvents are able to quickly undergo reversible property switches that are activated by an external change. The external trigger is typically a change in pH, temperature or concentration [65]. Switchable solvents are like biphasic solvents in that they both can possess two different solvent properties in just one system. One highly desirable solvent property for manipulation is hydrophilic character [69]. A switchable hydrophilic solvent (SHS) has been used for extracting different fractions such as proteins, lipids, and carbohydrates from microalgal biomass [69] and also for extracting phenols from lignin pyrolysis oil [70]. The SHS employed in both of these studies was N,N‐dimethylcyclohexylamine [69,70]. However, limitations are present with switchable solvents as it is a newer field lacking in research.
Other solvents worth noting are fatty acid/glycerol‐based solvents, advantageous for chemical inertness, and liquid/supercritical CO2, advantageous for wide availability, good solubility, and low toxicity [65]. The high boiling point of the glycerol mixtures (higher than DMSO) may be used as an advantage for separating volatile molecules but hinders their recyclability if nonvolatile molecules are coproduced (e.g. humins). Alternatively, employing liquid/supercritical CO2 as a solvent indicates costly high‐pressure systems/vessels and additional safety requirements.
Another solvent approach is the use of biphasic systems. With two different phases, reactions take place in the aqueous phase but extraction and separation in the organic phase [5971–74]. Hydrophobic extracting phases (e.g. cyclopentyl methyl ether, CPME) in the presence of a chloride salt (e.g. NaCl) can enhance the partitioning coefficient of the organic solvent, favoring extraction [71]. Otherwise, polar solvents with low water solubility (e.g. methyl isobutyl ketone, MIBK) can be used in the sugar conversions. Even so, the addition of multiple solvents increases the production costs even if recycled (small loss of solvents, specialty molecules) and reduces the greenness of the overall process.
By using these (mixtures of) solvents, one‐pot transformation of lignocelluloses via different methodologies has been attempted [50]. However, one‐step procedures are difficult to achieve with biomass processes because of the intricacies associated with solvent selection, catalyst, and other operating conditions. With this in mind, biomass processes that focus on individual bio‐components as opposed to entire systems could be more effective.
1.6 Pretreatment of Lignocelluloses
Pretreatment is a necessary measure for handling biomass on an individual component basis. One of the main functions for pretreatment is to facilitate separation and allow for improved access of the different biomass fractions, particularly from the rigid components that make up the plant wall [75]. In lignocellulose, these rigid components that significantly hinder solubilization are lignin and cellulose. Peculiarly, the separation of each bio‐component without further decomposition (e.g. to by‐products) could greatly contribute to the development of efficient conversion strategies, improving the competitiveness of a bio‐economy.
Biomass pretreatments can be classified as physical, chemical, physicochemical, or biological [75–77]. Physical pretreatment is reserved mainly for less complex applications where only an increased surface area is mostly desired. Some common physical pretreatment methods are sheer mixing, milling, and grinding that physically break apart the plant wall components [76]. Chemical pretreatment is widely used for its ability to greatly improve solubilization in order to make subsequent biomass processes possible. Chemical methods include acid/alkali treatment, ozonolysis, and organosolv [77]. Many researchers have been utilizing acid treatment as a simple chemical transformation route that is particularly useful for releasing some of the bio‐sugars that are locked behind the more rigid components. Furthermore, if the targeted reaction is acid‐catalyzed dehydration of the sugars to furanics or levulinics, the plausible residual presence of acids may only enhance the rate of said reaction. On the other hand, most biological treatments are safe and green processes that utilize fungi or other microorganisms. The enzymes break down hemicellulose and lignin rather well, while leaving intact cellulose. However, biological processes proceed at rather slow rates and the microorganisms typically only thrive in a fine‐tuned aqueous environment. A majority of the innovative pretreatment methods fall under physicochemical, as many benefits from the combinatory approach. These combination treatments include steam explosion, ammonia fiber expansion, carbon dioxide explosion, and wet oxidation with steam explosion being one of the most used. Steam explosion uses high‐pressure steam that creates a self‐reacting autohydrolysis environment for transforming biomass mechanically and chemically [25].
With current progress, pretreatment is a necessary measure for processing lignocellulosic biomass. The lignin content is largely responsible for complicating the heterogeneous nature of lignocelluloses and for contributing recalcitrant properties that make it difficult to handle. Without pretreatment, most valorization approaches are not cost effective [78].
Ideally, a pretreatment step would efficiently separate lignocellulose into its single components. If this is achieved, biomass processes could be standardized (based on each component) to greatly alleviate the issues of biomass inconsistency. It would also contribute to optimized processes that maximize process conditions such as yields and overall costs. When contemplating biocatalytic conversion of biomass (e.g. yeast fermentation), typical yields are low (<20%) without pretreatment. This phenomenon is given by the barrier effect of lignin to enzyme, physically hindering the hydrolysis of the digestible components (i.e. sugars) [79,80]. Improvements of product yields have been obtained with either biological [81], physical [82], and chemical steps [78,83] or physicochemical [84,85] pretreatments, thus giving higher resource efficiency. Cost‐effective solutions