catalysts Review Deep Eutectic Solvents as Catalysts for Upgrading Biomass Payam Kalhor 1 and Khashayar Ghandi 2,* 1 MOE Key Laboratory of Bioorganic Phosphorous Chemistry and Chemical Biology, Department of Chemistry, Tsinghua University, Beijing 100084, China; [email protected] 2 Department of Chemistry, University of Guelph, Guelph, ON N1G 2W1, Canada * Correspondence: [email protected]; Tel.: +1-519-993-7312 Abstract: Deep eutectic solvents (DESs) have emerged as promising green solvents, due to their versatility and properties such as high biodegradability, inexpensiveness, ease of preparation and negligible vapor pressure. Thus, DESs have been used as sustainable media and green catalysts in many chemical processes. On the other hand, lignocellulosic biomass as an abundant source of renewable carbon has received ample interest for the production of biobased chemicals. In this review, the state of the art of the catalytic use of DESs in upgrading the biomass-related substances towards biofuels and value-added chemicals is presented, and the gap in the knowledge is indicated to direct the future research. Keywords: catalysis; deep eutectic solvent; biomass; biofuel; valorization 1. Introduction In 2003, Abbott et al. proposed the concept of deep eutectic solvents (DESs) as a new generation of ionic liquid (IL) analogs, and probably the most novel class of solvents, Citation: Kalhor, P.; Ghandi, K. Deep composed of two or three components with a relatively large depression of melting points Eutectic Solvents as Catalysts for relative to those of the ideal liquid mixtures [1]. These components, performing the role of Upgrading Biomass. Catalysts 2021, either hydrogen bond donors (HBDs) or hydrogen bond acceptors (HBAs) can be Lewis 11, 178. https://doi.org/10.3390/ or Brønsted acids and bases involving a variety of neutral and ionic species [2]. The catal11020178 environmentally friendly properties of DESs such as low vapor pressure, recyclability and low toxicity have motivated research on the applications of DESs as alternative novel Academic Editors: Javier solvents to common organic ones [2,3]. DESs have found applications in several fields such Bilbao Elorriaga, Idoia Hita Del Olmo as biofuel production [4,5], bio-oil production [6,7], catalysis [8–12], extraction [13–15] and and Peter J. Deuss separation processes [16,17]. Received: 31 December 2020 They are easily prepared with usually no need for purification [3,18]. A remarkable Accepted: 25 January 2021 Published: 28 January 2021 property of DESs is the possibility to tailor the solvent in a task-specific way [2] and to a greater extent compared to ILs [19] that are already very good in this regard. This Publisher’s Note: MDPI stays neutral can be done by varying the molar ratio of the components, substituting a component with regard to jurisdictional claims in with the one that brings the favorable property or by simply adding a specific amount of published maps and institutional affil- a cosolvent [20–22] such as water [23–26]. The DES–cosolvent preparation is especially iations. important as most DESs have high viscosities [2] due to the extensive hydrogen bonding (H- bonding) networks [27,28] and van der Waals and electrostatic forces between species [29]. The high viscosity of the DESs can be mitigated via mixing with water [23–26]. However, care should be taken when diluting a DES with a cosolvent as the fundamental and desired properties of the DES may be affected by the cosolvent [25]. Most of the distinct properties Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. of DESs are because of some specific underlying intermolecular interactions, with the most This article is an open access article important one being the H-bonding. For instance, the significant decrease in the melting distributed under the terms and point of a typical DES is assumed to arise from the charge transfer between components, conditions of the Creative Commons usually from the halide anion of an HBA to the HBD through H-bonds [22,26,30]. It has Attribution (CC BY) license (https:// been found that as the H-bonds between HBD and HBA strengthen, the melting points creativecommons.org/licenses/by/ depress more [1]. Depending on the DES constituents and their molar ratio, DESs may 4.0/). have reduced thermal stability [31–33]. Catalysts 2021, 11, 178. https://doi.org/10.3390/catal11020178 https://www.mdpi.com/journal/catalysts Catalysts 2021, 11, x FOR PEER REVIEW 2 of 29 Catalysts 2021, 11, 178 2 of 32 points depress more [1]. Depending on the DES constituents and their molar ratio, DESs may have reduced thermal stability [31–33]. The diminishing reserves of easily available fossil fuels and growing concerns about the global pollution as well as the upward demanddemand for energy have significantlysignificantly affected the researchresearch directionsdirections towards towards developing developing sustainable sustainable energy energy resources resources [34 –[34–44].44]. Therefore, There- inventivefore, inventive and appropriate and appropriate uses of uses the of naturally the naturally abundant abundant supplies supplies are important are important towards to- a sustainablewards a sustainable future. As future. a very As promising a very promising alternative alternative to fossil fuels,to fossil biomass fuels, isbiomass increasingly is in- drawingcreasingly attention. drawing Biomassattention. is Biomass widely abundant,is widely abundant, distributed distributed worldwide worldwide and relatively and inexpensiverelatively inexpensive and has been and used has tobeen produce used variousto produce value-added various value-added chemicals [45 chemicals–51]. Biomass [45– transformation51]. Biomass transformation into valuable into chemicals valuable not chemicals only has revived not only the has green revived chemistry the green principles chem- butistry has principles also paved but thehasway also topaved alleviate the way the currentto alleviate high the reliance current on high fossil reliance fuels [48 on]. How-fossil ever,fuels the[48]. optimization However, the of chemicaloptimization transformations of chemical towardstransformations a sustainable towards methodology a sustainable and methodologyinvestigation ofand pivotal investigation factors affecting of pivotal process factors efficiency affecting remain process as challenges. efficiency Amongremain allas challenges.the factors affectingAmong all the the conversion factors affecting efficiency the and conversion the selectivity efficiency of the and obtained the selectivity products, of the obtained development products, and design the development of catalysts and play design a crucial of catalysts role [52 –play59]. a Catalysts crucial role have [52–59]. been Catalystsregarded ashave important been regarded tools to as accomplish important atools more to sustainableaccomplish chemicala more sustainable industry [ 56chem-]. In icalthis industry context, various[56]. In this catalytic context, systems various have ca beentalytic proposed systems tohave produce been proposed upgraded to chemicals produce upgradedand value-added chemicals products, and value-added with the purpose products of drawing, with the the purpose full chemical of drawing potential the of full the chemicalbiomass [potential11,53,55,60 of– 62the]. biomass In recent [11,53,55,60– years, DESs62]. have In beenrecent extensively years, DESs used have as been active exten- cata- sivelylysts in used valorization as active and catalysts upgrading in valorization of biomass and in variousupgrading types of ofbiomass reactions in various [10,63] where types ofthey reactions integrate [10,63] the advantageswhere they ofintegrate both homogenous the advantages and of heterogeneous both homogenous catalysts and hetero- [64,65]. Thegeneous catalytic catalysts DESs [64,65]. can dissolve The catalytic a wide DESs range can of dissolve reactants a andwide consequently range of reactants change and a consequentlyheterogeneous change catalytic a heterogeneous mechanism into cataly a homogeneoustic mechanism one into [66 a]. homogeneous one [66]. The catalytic DESs can be categorized as ei eitherther Lewis acid-type or Brønsted acid-type DESs [[10].10]. The Lewis acid-type DESs consist of a few DESs, usually chlorides of transition metals suchsuch asas ZnZn [[67],67], FeFe [ 68[68]] and and Cr Cr [ 69[69]] combined combined with with mainly mainly choline choline chloride chloride (ChCl) (ChCl) in indifferent different molar molar ratios. ratios. On theOn otherthe other hand, hand, there there are diverse are diverse Brønsted Brønsted acid-type acid-type DESs mostly DESs mostlycontaining containing ChCl combined ChCl combined with organic with organic acids suchacids assuch oxalic as oxalic acid [acid70], citric[70], citric acid [acid71], [71],acetic acetic acid acid [72], [72], malonic malonic acid acid [8], formic[8], formic acid acid [73], [73],p-toluene p-toluene sulfonic sulfonic acid acid (p-TSA) (p-TSA) [74 [74]] or oralcohols alcohols such such as as ethylene ethylene glycol glycol [75 [75]] or or glycerol glycerol [76 [76]] or or amides amides such such as as urea urea [77 [77].]. Figure Figure1 1shows shows the the general general scheme scheme for for the the conversion conversion of of biomass biomass toto value-addedvalue-added chemicalschemicals usingusing catalytic biomass. catalytic biomass. Figure 1. Conversion of biomass to value-added chemicals using catalytic deep eutecticeutectic solventssolvents (DESs).(DESs). This