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Ionic Liquids for Sustainable Chemical Processes
Ionic Liquids for Sustainable Chemical Processes Paula Berton, McGill University, Montreal, QC, Canada Julia L Shamshina, McGill University, Montreal, QC, Canada; and 525 Solutions, Inc., Tuscaloosa, AL, United States Robin D Rogers, McGill University, Montreal, QC, Canada Ó 2017 Elsevier Inc. All rights reserved.
Ionic Liquids: Green Solvents for Sustainable Technologies? Ionic liquids (ILs) are most commonly defined as salts with melting points below 100 C (Holbrey and Rogers, 2002). It was initially conceived that ILs are molten salts and, as such, these are composed entirely by ions (two at a minimum). There is now a large structural diversity of ions used to form ILs; a few examples of five different well-known classes of ILs are shown in Fig. 1. Ions vary from simple to complex, from ionized acid or base to charged bridging ligands, from metallate coordination polymers to organic polymeric metal ions. Because of their versatility and possibility of tailoring their physicochemical properties, ILs dissolve and are able to immobilize both organic substrates and a wide range of inorganic and organometallic catalysts. The structural diversity of ILs led to an extensive variety of applications, as shown from the evolution of ILs from solvent replacements to energetic materials to biologically active compounds, where ILs have specialized, high-value applications such as novel forms for active pharmaceutical ingredients (API). ILs are used or being considered in many chemical and industrial processes, including biotechnology, power generation, biorefineries, pharmaceutical and medicinal applications, electrochemistry, biocatalysis, nanotechnology, and even as fuels for rockets engines (Hardacre and Parvulescu, 2014). A deeper understanding of these systems suggests that not only fully ionic systems but “systems with intermediate ionicity” fall under the IL category (Kelley et al., 2013). Here, multicomponent systems may be formed in these low-melting liquids and present a combination of properties, leading to unusual behaviors which are intermediate between purely ionic or purely neutral compounds and thus not easily described. Examples of these systems are ILs made by reacting Brønsted acids and bases, typically referred to as “protic ILs,” which could be in equilibrium with their neutral conjugated acids and bases (Greaves and Drummond, 2015). In another example, low-melting multicomponent systems of APIs can be formed by adding excess of the acid or base to the API, generating oligomeric ions, for example, lidocaine salicylate-salicylic acid (Bica and Rogers, 2010). Such multicomponent systems could also be a result of hydrogen-bonding interactions between APIs and partially ionized mixtures of acid and bases (Bica et al., 2011). It is important to note then that ILs are chemicals, and, as such, their compositions and ionicity can be quite variable, and thus their classification based on ionicity can interfere with their consideration in important applications. Interest in ILs blossomed in the mid-1990s, when earlier efforts by the United States Air Force in applying ILs as electrolytes developed into a collective perception toward these fluids as reusable, nonvolatile, and therefore green alternatives that could replace organic solvents. At the same time, the green chemistry field, directed to the design of environmentally benign processes, flowered in 1998. From that moment, green chemistry became (and to a certain degree still is) the driver of the field’s development and ILs were considered as one of the tools toward sustainable product development, which got misinterpreted as “ILs are green.” This “green solvent” concept has driven the view of ILs as a single thing, yet, as described earlier, these liquids are much more complex compared to molecular solvents. This launched a counterargument “ILs are never green,” even though it was never a matter of saying ILs were healthy chemicals, but rather what was envisaged was the concept that sustainability and IL chemistry should
Fig. 1 Several types of “classic” ionic liquid ions. The substituents on the cations, the “R” groups, are alkyl chains or other functional groups such as halogen-, amino-, alkoxy-, alkenyl-, and methoxy- group.
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reinforce each other. Considering such a common fallacy, it is logical and almost inevitable that the initial views on ILs prevalent from the general public (and often from scientists) were often hyped and mutually exclusive. From this standpoint, ILs were found to be suitable for a diverse range of applications and this conveyed a false impression to the scientific community that “ILs are magic”; for example that ILs are universal solvents that dissolve almost everything. However, these overgeneralizations resulted in many stereotypes in regards to ILs and are based on a simplification when reality is far more complex. The way the term “ionic liquid” is used leads people to group them into a single class without scientific basis. The complexity of the ILs and variability of ions is exacerbated by the inability to attribute a single common characteristic to all ILs except melting point, and there is no single model that could be generalized to describe the entire family of ILs. The perceptual problems were exacerbated when researchers used the wrong IL set for the wrong applications, such as imidazolium or even phosphonium salts in “green” applications and then were criticized for not conducting life-cycle analysis. The misconceptions arise both from the ILs chosen, often ill-suited for the purpose for which they were studied, and from the fact that many think that because the compound is an IL, it is a representative of ILs as a class, and therefore is “green.” Similar misconceptions relate to all ILs being “renewable” (or “sustainable”). Even for biodegradable ILs, the environmental impact often seems to be high when their entire life-cycle is considered, because most of these ILs are derived from petroleum (Zhu et al., 2009). As alternatives, biobased-derived ILs have been designed using amino acids, sugars, and fatty acids obtained from proteins, biopolymers and vegetable oils, respectively. Yet, obtaining the amino acids, sugars, and fatty acids themselves involves nonsustainable separations and chemical transformations (Hulsbosch et al., 2016). Just because something is renewable and just because it can be made into an “ionic liquid,” cannot be enough to forget it is still a chemical. The antihype cycle is equally as bad. The same overgeneralizations are found when the toxicity of ILs is addressed. Originally, due to IL’s negligible vapor pressure, it was assumed that their evaporation and thus environmental pollution would be low. Yet, even though vaporization is minimized in case of nonvolatile ILs, many of the commonly used ILs are toxic in nature, and the level of their toxicity depends on the organisms and trophic levels (Amde et al., 2015). This led to the idea that “ionic liquids are toxic,” or as it was recently published “Ionic liquidsda threat to the health and the environmentdthe unknown side of ionic liquids” (Kud1ak et al., 2015). On the other hand, ILs prepared from “generally regarded as safe” ions such as choline and amino acids did not exhibit marked toxicity (Gouveia et al., 2014). Similarly, many of the protic ILs, composed of substituted amines and organic acids are not toxic (Oliveira et al., 2016). Here again, generalizations can lead to incorrect conclusions. From an economic point of view, the overgeneralization that “ionic liquids are expensive” leads to misconceptions and the wrong starting points when designing sustainable technologies. This sweeping conclusion is a direct consequence of thinking and evaluating only “traditional” ILs mostly used for lignocellulosic biomass pretreatment, that is, dialkylimidazolium cations (Kuzmina, 2016). However, this statement is not true for other ILs; the production costs of the two protic ILs triethylammonium hydrogen sulfate and 1-methylimidazolium hydrogen sulfate were estimated to be relatively low ($1.24 kg 1 and $2.96 kg 1, respectively). Another point to have in mind when performing the techno-economic analysis of an IL-based process is that ILs can often be recovered and reused although this will depend on the process and on the IL (Kuzmina, 2016). From the above discussion, it is clear that many different chemicals fit into the class of “ILs,” and no one characteristic can be applied to them all, except perhaps melting point. A single member of the class “ILs” won’t do everything and won’t be suitable for all applications. A better knowledge of ILs combined with an understanding of the needs of specific applications would allow the design of ILs for the task at hand. Indeed, ILs will best contribute to various fields when their unique properties are accepted and used to enable new approaches that are fundamental improvements in sustainable technology, not merely incremental advancements. The places where ILs have been successfully introduced in industrial processes are evidence of this. In 2003, the first large-scale industrial application was announced by BASF, who developed a multiton process called the BASIL (biphasic and scavenging using ILs) process, which generates the IL in situ to remove waste acidic hydrogen chloride. Recently, Chevron has licensed a process that uses a chloroaluminate IL as a liquid alkylation catalyst and replaces the 75-year old traditional technologies that use hydrofluoric or sulfuric acids (McCoy, 2016). When one considers ILs for entirely new industrial technologies, even more radical possibilities for sustainability arise. Here we present one such “case study,” a clear example of an IL-based technology that can help us move toward a Sustainable Society: the replacement of nondegradable synthetic polymers with Nature’s biopolymers directly from biomass sources.
Biopolymer Recovery From Biomass Using ILsdA Case Study Toward Sustainable Processes Synthetic plastics pose one of the most critical environmental challenges today. The world is covered in oil in the form of nonbiodegradable synthetic plastics which mar our landscapes and threaten our oceans. Such escalation of pollution due to the use of nondegradable synthetic plastics is clearly an urgent Societal problem. Even the so-called “biodegradable plastics” require chemical processing steps to make them. While efforts being made toward elimination of plastics, for example, recently France set a ban on the use of plastics in plates, cups, and cutlery by 2020 (McAuley, 2016), synthetic polymers represent a $654 billion market worldwide and are ubiquitous in modern life. Efforts in banning synthetic plastics with no viable alternatives would be almost impossible to implement, and catastrophic from both a social and economic point of view. As a response, there is an increased interest in finding alternative resources; particularly from renewable sources including biomass which is not surprising, since humanity has always relied on Nature for survival. However, biological and chemical approaches being taken to utilize biomass are still limited, mainly due to the difficulty in processing biomass feedstock, the
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Structures of the biopolymers cellulose (A), hemicellulose (B), lignin (C), and chitin (D).
high energy needed for separation of the components, and market factors involved such as low supply, low production numbers, and high production costs. In order to use Nature’s biopolymers one must overcome the complex structures which have evolved to prevent biomass’ environmental and biological degradation, making its dissolution not easily accessible. For example, in vascular plants, cellulose (Fig. 2A), the most abundant biopolymer, is synthesized as crystalline elementary fibrils consisting of 36 cellulose chains; the fibrils are subsequently assembled into microfibrils. The microfibrils are then cross-linked to macrofibrils, which are then incorporated into a matrix of hemicelluloses and lignin (Fig. 2B and C, respectively), and pectins (Thakur and Thakur, 2016). Another example is chitin (Fig. 2D), the second most widespread biopolymer in nature, present in the supporting tissues of crustaceans, fungi, and insects. This linear polysaccharide is ordered into helicoidal, microfibrillar structures that are embedded into a protein matrix and closely associated with minerals, lipids, and pigments. For both lignocellulosic and chitinous biomasses, it is this combination of covalent cross linking, structural hierarchy, and composite matrices that make biopolymers almost impossible to process using simple solution- or melt-based chemistry; pulping processes are used instead. In the case of lignocellulosic biomass, Kraft pulping is the most predominant way of obtaining cellulose. The method involves lignin removal from cellulose and hemicellulose matrix using hazardous chemicals, for example, sodium hydroxide and sodium sulfide solutions, and heavy metals used in the regeneration baths. The process also requires harsh conditions (elevated temperatures and pressures) and produces large amounts of environmentally threatening compounds, for example, sulfur dioxide, methylmercaptan, dimethyl sulfide, dimethyl disulfide, and total reduced sulfur gases, mainly consisting of hydrogen sulfide (Thakur and Thakur, 2016). The use of these chemicals during the process requires treatment of the air and waste before disposal to meet environmental regulations. Furthermore, several additional purification steps are required after pulping to develop cellulose products of high purity. This technique also results in significant degradation of the biopolymer. Chemical pulping is also used to obtain chitin. The current industrial chitin pulping process (predominantly from shrimp shells) uses hydrochloric acid for a demineralization step and sodium hydroxide for a deproteinization step. The process requires temperatures up to 160 C, lasts a few days, and generates c. 1.4 tons of aqueous waste per each kilogram of chitin (Barber et al., 2013a). The use of harsh acid and caustic causes depolymerization of the product and affects the inherent properties of chitin, decreasing its molecular weight and degree of acetylation (Barber et al., 2013a). A final bleaching step to obtain a purified polymer damages chitin structure, even at brief exposures. For lignocellulosic biomass, approaches prevailing in academia, industry, and research centers are now based on maximizing the value derived from the biomass feedstock. Thus, in addition to obtaining pure biopolymers, the “biorefinery concept” is actively pursued, in which the sustainability and value of the process is increased by focusing not on the recovery of the pure high molecular weight polymers, but rather on obtaining valuable chemicals from conventional waste streams. Contrarily to lignocellulosic biomass, furthering of chitinous biomass utilization is yet in its infancy (Yan and Chen, 2015). Yet, there still seems to be relatively little emphasis on using Nature’s biopolymers as polymers rather than as feedstocks for producing molecular chemical entities. A facilitated and more efficient access to natural biopolymers, and subsequent enabling of a materials and products technology platform based on renewable sources would be a major step toward sustainability. ILs have two roles to play in this area, the extraction of biopolymers themselves as an alternative to pulping, and the manipulation of biopolymers in solution to make advanced materials.
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ILs-Assisted Dissolution and Regeneration of Biopolymers In 2002, the dissolution of pure cellulose using the IL 1-butyl-3-methylimidazolium chloride ([C4mim]Cl) was reported (Swatloski et al., 2002). The ability of ILs to dissolve natural cellulose and wood (lignocellulosic biomass) was demonstrated seven years later (Sun et al., 2009). Here, selective cleavage of the lignin-carbohydrate linkages leads to the ready fractionation of the biopolymers from natural sources, with far fewer chemical steps and little to no degradation (Wang et al., 2014). The mechanism of the IL-based dissolution involves unraveling its natural polymeric constituents through a disruption of the inter- and intramolecular hydrogen bonding and the formation of new hydrogen bonds between the carbohydrate hydroxyl protons and the anions of the IL. While unraveled, these polymers are separated into pure streams without any damage to the polymer chain. In the case of pure chitin polymer, its dissolution was initially conducted using the [C4mim]Cl IL (Xie et al., 2006), and later using 1-butyl-3-methylimidazolium acetate ([C4mim][OAc]) IL (Wu et al., 2008). However, it was not until 2010 that the extraction of chitin from biomass was demonstrated. The extraction of chitin from shrimp shells was conducted using 1-ethyl-3methylimidazolium acetate ([C2mim][OAc]) IL, in a microwave-assisted dissolution process (Qin et al., 2010). Once dissolved, chitin was coagulated using an antisolvent (water or alcohol), which solubilizes the IL and precipitates purified biopolymer. Another strategy proposed to recover the biopolymer from its IL solution is supercritical CO2 (or simply CO2)-assisted coagulation (Barber et al., 2013b). The ILs can be recovered and reused, as in cellulose processing. The strategy of isolating chitin by ILs has been shown on different biomass sources and new strategies for using ILs are being developed to access chitin from many unexploited sources, including crab, squid pen, lobster shell, crayfish, and krill (Setoguchi et al., 2012). In addition to increased safety due to the nonvolatile nature of the IL, the advantages of the [C2mim][OAc]-based chitin recovery include high polymer purity, retention of both its high molecular weight and degree of acetylation, and the ability to manufacture new materials (described later). However, chitin dissolution in ILs has implications that go beyond its purification. Nowadays, chitin is commercially produced from marine crustaceans, for example, crab, shrimp, lobster, and prawn, due to a large amount of fishery/food processing waste that creates a disposal problem (Ravi Kumar, 2000). Chitin, together with proteins, minerals, and other chemicals, for example, pigments, flavors, and nutrients, present in the biomass add value to these abundant waste streams. However, due to the deficiencies of the industrial process described above, chitin production has been moved outside of North America. Since the discovery of IL processing of wood and chitinous biomass, the number of ILs to recover other biopolymers from biomass has been greatly expanded (Fig. 3). The process is amenable to scale up and agnostic to biomass type or source, which is exemplified by the development of IL-based methods for the isolation of a large number of biomacromolecules other than cellulose or chitin, such as silk fibroin, lignin, starch and zein protein, wool, keratin, etc., with high efficiency (Wang et al., 2014). For rational design of ILs for efficient, fast, and nondegrading dissolution of biopolymers, priority should be given to those with strongly basic anions, able to break the hydrogen bonds between biopolymeric chains. On the other hand, the role of cations on the dissolution of biopolymers is still to be systematically studied, although imidazolium cations seem to currently work the best. It is worth noting that the main disadvantage of the IL-based biopolymer recovery is the requirement of large amounts of currently expensive ILs that must be recycled. An alternative was recently published by Shamshina et al. (2016), who proposed an IL-based pulping method in which the ILs were basic enough to remove the proteins and at the same time acidic enough to remove inorganic minerals, leaving the chitin undissolved. Using the IL hydroxylammonium acetate ([NH3OH][OAc]), the chitin was isolated with high purity and high degree of acetylation and crystallinity, and it allowed 10 times the biomass loading compared to aforementioned extraction process. The big disadvantage of the method is that the recovered chitin had lower molecular weight. Yet, this work presents the option to use less expensive ILs that do not have to be recovered or recycled. A large body of work developed since has focused on the emerging context of the biorefinery, chemicals, and biofuels, because it offers the opportunity to use the biopolymers as a consistent feedstock for producing molecular chemical entities (Bogel-Lukasik,
Fig. 3
Cations and anions mostly used in ILs proposed for biomass dissolution.
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2016). For biopolymer degradation, both chemical and enzymatic methods are used (Sun et al., 2011; Li et al., 2013; Frederix et al., 2016). Such widespread implementation of an IL-based platform is nowadays applied to many types of feedstocks available (mostly lignocellulosic), and dramatically decreases the costs associated with biorefineries. This strategy also increases biorefineries’ profit and adds new products to their lines of goods (Brandt et al., 2013). However, one wonders about the wisdom of taking Nature’s abundant biopolymers such as cellulose, hemicellulose, lignin, and chitin and chopping them up to create monomers or fuels while burning the rest. The IL technology gives the opportunity of not only recovering these biopolymers in a pure state and in a simple process but also their transformation into products that would replace those made with synthetic polymers (Rogers, 2015).
Biopolymer Materials Made by IL Solution Processing The clean separation of the major components of biomass using ILs has been explicitly recognized as a viable and sustainable platform for product development. This is mostly due to the fact that after biopolymers’ recovery, their dissolution for further manipulation into materials is challenging, and there are just a few technologically convenient solvents which can process their normally intractable structure. Often, concentrated mineral and organic acids or nonaqueous solutions of inorganic salts, and fluorinated organic solvents are used (Aida et al., 2014; Wang et al., 2014). These solvents are corrosive, toxic, often volatile, and are usually difficult to recycle. Additionally, the molecular weight of the polymers tends to be reduced, especially when strong acids are employed. Because of the difficulty in controlling polymer degradation, there is also a low control of material quality. An alternative is to convert biopolymers into their soluble form, for example, chitosan from chitin by deacetylation, however, such conversion modifies the inherent properties of the biopolymer. Besides, concentrated aqueous solutions of acids and alkalis cause wastewater and environmental contamination (Bochek, 2008). By avoiding many of these reactive chemicals and facilitating a physical dissolution process, ILs overcome the disadvantages of these conventional practices. The ability to dissolve biopolymers in ILs and handle the resulting solutions has opened a wide range of approaches for forming new materials which were previously restricted to only soluble forms. From the major components of lignocellulosic biomass (cellulose and hemicellulose), paper, fibers, membranes, hydrogels, and other materials are directly produced that have wide applications in textiles, packaging, biomedicine, water treatment, optical/electrical devices, agriculture, food, etc. Lately, research has also turned toward lignin as a source of chemicals, for example, binders, dispersants, and emulsifiers, composite materials, and carbon fibers (Ragauskas et al., 2006). IL-based dissolution of lignocellulosic biopolymers not only permits their processing and manipulation, allowing the preparation of various materials, including blends, composites, fibers, and ion gels (Isik et al., 2014), but also presents significant improvements in material quality and reproducibility, through a sustainable process. However, even though there is widespread research into IL-based processing of lignocellulosic biomass, these processes are in competition with well known, inexpensive chemical processing used by industry today, and thus adoption of any IL technology may be slow. From chitin, different products have been made, including hydrogels for drug delivery (Shen et al., 2016), chitin-calcium alginate composite fibers for wound care (Shamshina et al., 2014) and films with tunable strength and morphology (King et al., 2017). Electrospun chitin nanomaterials with specific function such as catalysts, sorbents, or filters, have also been developed (Barber et al., 2013c; Shamshina et al., 2017). Furthermore, ILs are capable of dissolving several bio- or synthetic polymers simultaneously with ionic or molecular additives, suspending and stabilizing nanoparticles, etc., which makes IL-based processing to be a platform for preparing composite materials.
Future Remarks Pioneering work in ILs has continued to lead to new opportunities and resulted in development of many sustainable technologies, including a way to extract Nature’s biopolymers directly from any biomass source, and form them into any desired shape and function, just like synthetic plastics. However, in order to be able to fully substitute synthetic plastics with bioplastics, the same consistent supply, economies of scale, and market acceptance enjoyed by synthetic plastics are needed. The focus should be on the use of biopolymers in high-value, market ready products where synthetic plastics control the markets. A big difference between those IL applications which have been adopted by industry and IL-based biomass processing is that the former could be integrated into existing processes and infrastructure without the need for dramatic change. Contrarily, IL-based biomass processing will require significant changes to both the process and to the physical plant needed to operate the process. Thus, initial investment is needed to demonstrate scaled-up processes before the true techno-economic feasibility of the technology is known. The toxicological and environmental assessments for the scaling up of IL-biomass dissolution are still needed and are very important. There are numerous examples, such as nanoparticles or polyhalogenated flame retardants, where promising technologies were scaled up and marketed before any toxicological and environmental impact studies were done. Researchers in the IL field have a chance to learn from previous experiences and understand the environmental impact that our technology would have. Presently, toxicological studies have generally focused on the weaknesses of ILs commonly used in academic research. These studies have been helpful in directing research away from potential “dead ends,” but there is a need for toxicological and economic research that goes beyond recognizing imperfect systems and instead leads to better design choices.
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IL-based technologies, especially those in biomass processing, could have profound effects on the availability of renewable resources such as biomass feedstocks for further chemical processing, leading to additional utilization of waste sources and renewable chemicals in advanced applications. Ultimately, sustainability will require society to look to such unconventional resources using technologies which may not even be imaginable at present. Perhaps, ILs represent one such technology whereby pure biopolymer streams can be taken directly from Nature and used by Society in the form of advanced materials and thus eliminate the chemical industry as we know it.
Acknowledgment This work was undertaken thanks to funding from the Canada Excellence Research Chairs Program.
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Swatloski, R.P., Spear, S.K., Holbrey, J.D., Rogers, R.D., 2002. Dissolution of cellulose with ionic liquids. Journal of the American Chemical Society 124, 4974–4975. Thakur, V.K., Thakur, M.K. (Eds.), 2016. Handbook of sustainable polymersdStructure and chemistry. CRC Press, Boca Raton. Wang, H., Gurau, G., Rogers, R.D., 2014. Dissolution of biomass using ionic liquids. In: Zhang, S., Wang, J., Lu, X., Zhou, Q. (Eds.), Structures and interactions of ionic liquids, 1st edn. Springer, Berlin/Heidelberg, pp. 79–105. Wu, Y., Sasaki, T., Irie, S., Sakurai, K., 2008. A novel biomass-ionic liquid platform for the utilization of native chitin. Polymer 49, 2321–2327. Xie, H., Zhang, S., Li, S., 2006. Chitin and chitosan dissolved in ionic liquids as reversible sorbents of CO2. Green Chemistry 8, 630–633. Yan, N., Chen, X., 2015. Don’t waste seafood waste. Nature 524, 155–157. Zhu, S., Chen, R., Wu, Y., et al., 2009. A mini-review on greenness of ionic liquids. Chemical and Biochemical Engineering Quarterly 23, 207–211.
Further Reading World Commission on Environment and Development, 1987. Our Common Future. Oxford University Press, Oxford.
Ionic Liquids: Green Solvents for Sustainable Technologies? Ionic liquids (ILs) are most commonly defined as salts with melting points below 100 C (Holbrey and Rogers, 2002). It was initially conceived that ILs are molten salts and, as such, these are composed entirely by ions (two at a minimum). There is now a large structural diversity of ions used to form ILs; a few examples of five different well-known classes of ILs are shown in Fig. 1. Ions vary from simple to complex, from ionized acid or base to charged bridging ligands, from metallate coordination polymers to organic polymeric metal ions. Because of their versatility and possibility of tailoring their physicochemical properties, ILs dissolve and are able to immobilize both organic substrates and a wide range of inorganic and organometallic catalysts. The structural diversity of ILs led to an extensive variety of applications, as shown from the evolution of ILs from solvent replacements to energetic materials to biologically active compounds, where ILs have specialized, high-value applications such as novel forms for active pharmaceutical ingredients (API). ILs are used or being considered in many chemical and industrial processes, including biotechnology, power generation, biorefineries, pharmaceutical and medicinal applications, electrochemistry, biocatalysis, nanotechnology, and even as fuels for rockets engines (Hardacre and Parvulescu, 2014). A deeper understanding of these systems suggests that not only fully ionic systems but “systems with intermediate ionicity” fall under the IL category (Kelley et al., 2013). Here, multicomponent systems may be formed in these low-melting liquids and present a combination of properties, leading to unusual behaviors which are intermediate between purely ionic or purely neutral compounds and thus not easily described. Examples of these systems are ILs made by reacting Brønsted acids and bases, typically referred to as “protic ILs,” which could be in equilibrium with their neutral conjugated acids and bases (Greaves and Drummond, 2015). In another example, low-melting multicomponent systems of APIs can be formed by adding excess of the acid or base to the API, generating oligomeric ions, for example, lidocaine salicylate-salicylic acid (Bica and Rogers, 2010). Such multicomponent systems could also be a result of hydrogen-bonding interactions between APIs and partially ionized mixtures of acid and bases (Bica et al., 2011). It is important to note then that ILs are chemicals, and, as such, their compositions and ionicity can be quite variable, and thus their classification based on ionicity can interfere with their consideration in important applications. Interest in ILs blossomed in the mid-1990s, when earlier efforts by the United States Air Force in applying ILs as electrolytes developed into a collective perception toward these fluids as reusable, nonvolatile, and therefore green alternatives that could replace organic solvents. At the same time, the green chemistry field, directed to the design of environmentally benign processes, flowered in 1998. From that moment, green chemistry became (and to a certain degree still is) the driver of the field’s development and ILs were considered as one of the tools toward sustainable product development, which got misinterpreted as “ILs are green.” This “green solvent” concept has driven the view of ILs as a single thing, yet, as described earlier, these liquids are much more complex compared to molecular solvents. This launched a counterargument “ILs are never green,” even though it was never a matter of saying ILs were healthy chemicals, but rather what was envisaged was the concept that sustainability and IL chemistry should
Fig. 1 Several types of “classic” ionic liquid ions. The substituents on the cations, the “R” groups, are alkyl chains or other functional groups such as halogen-, amino-, alkoxy-, alkenyl-, and methoxy- group.
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reinforce each other. Considering such a common fallacy, it is logical and almost inevitable that the initial views on ILs prevalent from the general public (and often from scientists) were often hyped and mutually exclusive. From this standpoint, ILs were found to be suitable for a diverse range of applications and this conveyed a false impression to the scientific community that “ILs are magic”; for example that ILs are universal solvents that dissolve almost everything. However, these overgeneralizations resulted in many stereotypes in regards to ILs and are based on a simplification when reality is far more complex. The way the term “ionic liquid” is used leads people to group them into a single class without scientific basis. The complexity of the ILs and variability of ions is exacerbated by the inability to attribute a single common characteristic to all ILs except melting point, and there is no single model that could be generalized to describe the entire family of ILs. The perceptual problems were exacerbated when researchers used the wrong IL set for the wrong applications, such as imidazolium or even phosphonium salts in “green” applications and then were criticized for not conducting life-cycle analysis. The misconceptions arise both from the ILs chosen, often ill-suited for the purpose for which they were studied, and from the fact that many think that because the compound is an IL, it is a representative of ILs as a class, and therefore is “green.” Similar misconceptions relate to all ILs being “renewable” (or “sustainable”). Even for biodegradable ILs, the environmental impact often seems to be high when their entire life-cycle is considered, because most of these ILs are derived from petroleum (Zhu et al., 2009). As alternatives, biobased-derived ILs have been designed using amino acids, sugars, and fatty acids obtained from proteins, biopolymers and vegetable oils, respectively. Yet, obtaining the amino acids, sugars, and fatty acids themselves involves nonsustainable separations and chemical transformations (Hulsbosch et al., 2016). Just because something is renewable and just because it can be made into an “ionic liquid,” cannot be enough to forget it is still a chemical. The antihype cycle is equally as bad. The same overgeneralizations are found when the toxicity of ILs is addressed. Originally, due to IL’s negligible vapor pressure, it was assumed that their evaporation and thus environmental pollution would be low. Yet, even though vaporization is minimized in case of nonvolatile ILs, many of the commonly used ILs are toxic in nature, and the level of their toxicity depends on the organisms and trophic levels (Amde et al., 2015). This led to the idea that “ionic liquids are toxic,” or as it was recently published “Ionic liquidsda threat to the health and the environmentdthe unknown side of ionic liquids” (Kud1ak et al., 2015). On the other hand, ILs prepared from “generally regarded as safe” ions such as choline and amino acids did not exhibit marked toxicity (Gouveia et al., 2014). Similarly, many of the protic ILs, composed of substituted amines and organic acids are not toxic (Oliveira et al., 2016). Here again, generalizations can lead to incorrect conclusions. From an economic point of view, the overgeneralization that “ionic liquids are expensive” leads to misconceptions and the wrong starting points when designing sustainable technologies. This sweeping conclusion is a direct consequence of thinking and evaluating only “traditional” ILs mostly used for lignocellulosic biomass pretreatment, that is, dialkylimidazolium cations (Kuzmina, 2016). However, this statement is not true for other ILs; the production costs of the two protic ILs triethylammonium hydrogen sulfate and 1-methylimidazolium hydrogen sulfate were estimated to be relatively low ($1.24 kg 1 and $2.96 kg 1, respectively). Another point to have in mind when performing the techno-economic analysis of an IL-based process is that ILs can often be recovered and reused although this will depend on the process and on the IL (Kuzmina, 2016). From the above discussion, it is clear that many different chemicals fit into the class of “ILs,” and no one characteristic can be applied to them all, except perhaps melting point. A single member of the class “ILs” won’t do everything and won’t be suitable for all applications. A better knowledge of ILs combined with an understanding of the needs of specific applications would allow the design of ILs for the task at hand. Indeed, ILs will best contribute to various fields when their unique properties are accepted and used to enable new approaches that are fundamental improvements in sustainable technology, not merely incremental advancements. The places where ILs have been successfully introduced in industrial processes are evidence of this. In 2003, the first large-scale industrial application was announced by BASF, who developed a multiton process called the BASIL (biphasic and scavenging using ILs) process, which generates the IL in situ to remove waste acidic hydrogen chloride. Recently, Chevron has licensed a process that uses a chloroaluminate IL as a liquid alkylation catalyst and replaces the 75-year old traditional technologies that use hydrofluoric or sulfuric acids (McCoy, 2016). When one considers ILs for entirely new industrial technologies, even more radical possibilities for sustainability arise. Here we present one such “case study,” a clear example of an IL-based technology that can help us move toward a Sustainable Society: the replacement of nondegradable synthetic polymers with Nature’s biopolymers directly from biomass sources.
Biopolymer Recovery From Biomass Using ILsdA Case Study Toward Sustainable Processes Synthetic plastics pose one of the most critical environmental challenges today. The world is covered in oil in the form of nonbiodegradable synthetic plastics which mar our landscapes and threaten our oceans. Such escalation of pollution due to the use of nondegradable synthetic plastics is clearly an urgent Societal problem. Even the so-called “biodegradable plastics” require chemical processing steps to make them. While efforts being made toward elimination of plastics, for example, recently France set a ban on the use of plastics in plates, cups, and cutlery by 2020 (McAuley, 2016), synthetic polymers represent a $654 billion market worldwide and are ubiquitous in modern life. Efforts in banning synthetic plastics with no viable alternatives would be almost impossible to implement, and catastrophic from both a social and economic point of view. As a response, there is an increased interest in finding alternative resources; particularly from renewable sources including biomass which is not surprising, since humanity has always relied on Nature for survival. However, biological and chemical approaches being taken to utilize biomass are still limited, mainly due to the difficulty in processing biomass feedstock, the
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Structures of the biopolymers cellulose (A), hemicellulose (B), lignin (C), and chitin (D).
high energy needed for separation of the components, and market factors involved such as low supply, low production numbers, and high production costs. In order to use Nature’s biopolymers one must overcome the complex structures which have evolved to prevent biomass’ environmental and biological degradation, making its dissolution not easily accessible. For example, in vascular plants, cellulose (Fig. 2A), the most abundant biopolymer, is synthesized as crystalline elementary fibrils consisting of 36 cellulose chains; the fibrils are subsequently assembled into microfibrils. The microfibrils are then cross-linked to macrofibrils, which are then incorporated into a matrix of hemicelluloses and lignin (Fig. 2B and C, respectively), and pectins (Thakur and Thakur, 2016). Another example is chitin (Fig. 2D), the second most widespread biopolymer in nature, present in the supporting tissues of crustaceans, fungi, and insects. This linear polysaccharide is ordered into helicoidal, microfibrillar structures that are embedded into a protein matrix and closely associated with minerals, lipids, and pigments. For both lignocellulosic and chitinous biomasses, it is this combination of covalent cross linking, structural hierarchy, and composite matrices that make biopolymers almost impossible to process using simple solution- or melt-based chemistry; pulping processes are used instead. In the case of lignocellulosic biomass, Kraft pulping is the most predominant way of obtaining cellulose. The method involves lignin removal from cellulose and hemicellulose matrix using hazardous chemicals, for example, sodium hydroxide and sodium sulfide solutions, and heavy metals used in the regeneration baths. The process also requires harsh conditions (elevated temperatures and pressures) and produces large amounts of environmentally threatening compounds, for example, sulfur dioxide, methylmercaptan, dimethyl sulfide, dimethyl disulfide, and total reduced sulfur gases, mainly consisting of hydrogen sulfide (Thakur and Thakur, 2016). The use of these chemicals during the process requires treatment of the air and waste before disposal to meet environmental regulations. Furthermore, several additional purification steps are required after pulping to develop cellulose products of high purity. This technique also results in significant degradation of the biopolymer. Chemical pulping is also used to obtain chitin. The current industrial chitin pulping process (predominantly from shrimp shells) uses hydrochloric acid for a demineralization step and sodium hydroxide for a deproteinization step. The process requires temperatures up to 160 C, lasts a few days, and generates c. 1.4 tons of aqueous waste per each kilogram of chitin (Barber et al., 2013a). The use of harsh acid and caustic causes depolymerization of the product and affects the inherent properties of chitin, decreasing its molecular weight and degree of acetylation (Barber et al., 2013a). A final bleaching step to obtain a purified polymer damages chitin structure, even at brief exposures. For lignocellulosic biomass, approaches prevailing in academia, industry, and research centers are now based on maximizing the value derived from the biomass feedstock. Thus, in addition to obtaining pure biopolymers, the “biorefinery concept” is actively pursued, in which the sustainability and value of the process is increased by focusing not on the recovery of the pure high molecular weight polymers, but rather on obtaining valuable chemicals from conventional waste streams. Contrarily to lignocellulosic biomass, furthering of chitinous biomass utilization is yet in its infancy (Yan and Chen, 2015). Yet, there still seems to be relatively little emphasis on using Nature’s biopolymers as polymers rather than as feedstocks for producing molecular chemical entities. A facilitated and more efficient access to natural biopolymers, and subsequent enabling of a materials and products technology platform based on renewable sources would be a major step toward sustainability. ILs have two roles to play in this area, the extraction of biopolymers themselves as an alternative to pulping, and the manipulation of biopolymers in solution to make advanced materials.
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ILs-Assisted Dissolution and Regeneration of Biopolymers In 2002, the dissolution of pure cellulose using the IL 1-butyl-3-methylimidazolium chloride ([C4mim]Cl) was reported (Swatloski et al., 2002). The ability of ILs to dissolve natural cellulose and wood (lignocellulosic biomass) was demonstrated seven years later (Sun et al., 2009). Here, selective cleavage of the lignin-carbohydrate linkages leads to the ready fractionation of the biopolymers from natural sources, with far fewer chemical steps and little to no degradation (Wang et al., 2014). The mechanism of the IL-based dissolution involves unraveling its natural polymeric constituents through a disruption of the inter- and intramolecular hydrogen bonding and the formation of new hydrogen bonds between the carbohydrate hydroxyl protons and the anions of the IL. While unraveled, these polymers are separated into pure streams without any damage to the polymer chain. In the case of pure chitin polymer, its dissolution was initially conducted using the [C4mim]Cl IL (Xie et al., 2006), and later using 1-butyl-3-methylimidazolium acetate ([C4mim][OAc]) IL (Wu et al., 2008). However, it was not until 2010 that the extraction of chitin from biomass was demonstrated. The extraction of chitin from shrimp shells was conducted using 1-ethyl-3methylimidazolium acetate ([C2mim][OAc]) IL, in a microwave-assisted dissolution process (Qin et al., 2010). Once dissolved, chitin was coagulated using an antisolvent (water or alcohol), which solubilizes the IL and precipitates purified biopolymer. Another strategy proposed to recover the biopolymer from its IL solution is supercritical CO2 (or simply CO2)-assisted coagulation (Barber et al., 2013b). The ILs can be recovered and reused, as in cellulose processing. The strategy of isolating chitin by ILs has been shown on different biomass sources and new strategies for using ILs are being developed to access chitin from many unexploited sources, including crab, squid pen, lobster shell, crayfish, and krill (Setoguchi et al., 2012). In addition to increased safety due to the nonvolatile nature of the IL, the advantages of the [C2mim][OAc]-based chitin recovery include high polymer purity, retention of both its high molecular weight and degree of acetylation, and the ability to manufacture new materials (described later). However, chitin dissolution in ILs has implications that go beyond its purification. Nowadays, chitin is commercially produced from marine crustaceans, for example, crab, shrimp, lobster, and prawn, due to a large amount of fishery/food processing waste that creates a disposal problem (Ravi Kumar, 2000). Chitin, together with proteins, minerals, and other chemicals, for example, pigments, flavors, and nutrients, present in the biomass add value to these abundant waste streams. However, due to the deficiencies of the industrial process described above, chitin production has been moved outside of North America. Since the discovery of IL processing of wood and chitinous biomass, the number of ILs to recover other biopolymers from biomass has been greatly expanded (Fig. 3). The process is amenable to scale up and agnostic to biomass type or source, which is exemplified by the development of IL-based methods for the isolation of a large number of biomacromolecules other than cellulose or chitin, such as silk fibroin, lignin, starch and zein protein, wool, keratin, etc., with high efficiency (Wang et al., 2014). For rational design of ILs for efficient, fast, and nondegrading dissolution of biopolymers, priority should be given to those with strongly basic anions, able to break the hydrogen bonds between biopolymeric chains. On the other hand, the role of cations on the dissolution of biopolymers is still to be systematically studied, although imidazolium cations seem to currently work the best. It is worth noting that the main disadvantage of the IL-based biopolymer recovery is the requirement of large amounts of currently expensive ILs that must be recycled. An alternative was recently published by Shamshina et al. (2016), who proposed an IL-based pulping method in which the ILs were basic enough to remove the proteins and at the same time acidic enough to remove inorganic minerals, leaving the chitin undissolved. Using the IL hydroxylammonium acetate ([NH3OH][OAc]), the chitin was isolated with high purity and high degree of acetylation and crystallinity, and it allowed 10 times the biomass loading compared to aforementioned extraction process. The big disadvantage of the method is that the recovered chitin had lower molecular weight. Yet, this work presents the option to use less expensive ILs that do not have to be recovered or recycled. A large body of work developed since has focused on the emerging context of the biorefinery, chemicals, and biofuels, because it offers the opportunity to use the biopolymers as a consistent feedstock for producing molecular chemical entities (Bogel-Lukasik,
Fig. 3
Cations and anions mostly used in ILs proposed for biomass dissolution.
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2016). For biopolymer degradation, both chemical and enzymatic methods are used (Sun et al., 2011; Li et al., 2013; Frederix et al., 2016). Such widespread implementation of an IL-based platform is nowadays applied to many types of feedstocks available (mostly lignocellulosic), and dramatically decreases the costs associated with biorefineries. This strategy also increases biorefineries’ profit and adds new products to their lines of goods (Brandt et al., 2013). However, one wonders about the wisdom of taking Nature’s abundant biopolymers such as cellulose, hemicellulose, lignin, and chitin and chopping them up to create monomers or fuels while burning the rest. The IL technology gives the opportunity of not only recovering these biopolymers in a pure state and in a simple process but also their transformation into products that would replace those made with synthetic polymers (Rogers, 2015).
Biopolymer Materials Made by IL Solution Processing The clean separation of the major components of biomass using ILs has been explicitly recognized as a viable and sustainable platform for product development. This is mostly due to the fact that after biopolymers’ recovery, their dissolution for further manipulation into materials is challenging, and there are just a few technologically convenient solvents which can process their normally intractable structure. Often, concentrated mineral and organic acids or nonaqueous solutions of inorganic salts, and fluorinated organic solvents are used (Aida et al., 2014; Wang et al., 2014). These solvents are corrosive, toxic, often volatile, and are usually difficult to recycle. Additionally, the molecular weight of the polymers tends to be reduced, especially when strong acids are employed. Because of the difficulty in controlling polymer degradation, there is also a low control of material quality. An alternative is to convert biopolymers into their soluble form, for example, chitosan from chitin by deacetylation, however, such conversion modifies the inherent properties of the biopolymer. Besides, concentrated aqueous solutions of acids and alkalis cause wastewater and environmental contamination (Bochek, 2008). By avoiding many of these reactive chemicals and facilitating a physical dissolution process, ILs overcome the disadvantages of these conventional practices. The ability to dissolve biopolymers in ILs and handle the resulting solutions has opened a wide range of approaches for forming new materials which were previously restricted to only soluble forms. From the major components of lignocellulosic biomass (cellulose and hemicellulose), paper, fibers, membranes, hydrogels, and other materials are directly produced that have wide applications in textiles, packaging, biomedicine, water treatment, optical/electrical devices, agriculture, food, etc. Lately, research has also turned toward lignin as a source of chemicals, for example, binders, dispersants, and emulsifiers, composite materials, and carbon fibers (Ragauskas et al., 2006). IL-based dissolution of lignocellulosic biopolymers not only permits their processing and manipulation, allowing the preparation of various materials, including blends, composites, fibers, and ion gels (Isik et al., 2014), but also presents significant improvements in material quality and reproducibility, through a sustainable process. However, even though there is widespread research into IL-based processing of lignocellulosic biomass, these processes are in competition with well known, inexpensive chemical processing used by industry today, and thus adoption of any IL technology may be slow. From chitin, different products have been made, including hydrogels for drug delivery (Shen et al., 2016), chitin-calcium alginate composite fibers for wound care (Shamshina et al., 2014) and films with tunable strength and morphology (King et al., 2017). Electrospun chitin nanomaterials with specific function such as catalysts, sorbents, or filters, have also been developed (Barber et al., 2013c; Shamshina et al., 2017). Furthermore, ILs are capable of dissolving several bio- or synthetic polymers simultaneously with ionic or molecular additives, suspending and stabilizing nanoparticles, etc., which makes IL-based processing to be a platform for preparing composite materials.
Future Remarks Pioneering work in ILs has continued to lead to new opportunities and resulted in development of many sustainable technologies, including a way to extract Nature’s biopolymers directly from any biomass source, and form them into any desired shape and function, just like synthetic plastics. However, in order to be able to fully substitute synthetic plastics with bioplastics, the same consistent supply, economies of scale, and market acceptance enjoyed by synthetic plastics are needed. The focus should be on the use of biopolymers in high-value, market ready products where synthetic plastics control the markets. A big difference between those IL applications which have been adopted by industry and IL-based biomass processing is that the former could be integrated into existing processes and infrastructure without the need for dramatic change. Contrarily, IL-based biomass processing will require significant changes to both the process and to the physical plant needed to operate the process. Thus, initial investment is needed to demonstrate scaled-up processes before the true techno-economic feasibility of the technology is known. The toxicological and environmental assessments for the scaling up of IL-biomass dissolution are still needed and are very important. There are numerous examples, such as nanoparticles or polyhalogenated flame retardants, where promising technologies were scaled up and marketed before any toxicological and environmental impact studies were done. Researchers in the IL field have a chance to learn from previous experiences and understand the environmental impact that our technology would have. Presently, toxicological studies have generally focused on the weaknesses of ILs commonly used in academic research. These studies have been helpful in directing research away from potential “dead ends,” but there is a need for toxicological and economic research that goes beyond recognizing imperfect systems and instead leads to better design choices.
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IL-based technologies, especially those in biomass processing, could have profound effects on the availability of renewable resources such as biomass feedstocks for further chemical processing, leading to additional utilization of waste sources and renewable chemicals in advanced applications. Ultimately, sustainability will require society to look to such unconventional resources using technologies which may not even be imaginable at present. Perhaps, ILs represent one such technology whereby pure biopolymer streams can be taken directly from Nature and used by Society in the form of advanced materials and thus eliminate the chemical industry as we know it.
Acknowledgment This work was undertaken thanks to funding from the Canada Excellence Research Chairs Program.
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Further Reading World Commission on Environment and Development, 1987. Our Common Future. Oxford University Press, Oxford.