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Keywords:

  • ionic liquids;
  • lignocellulose;
  • pretreatment;
  • switchable;
  • bio-based

Abstract

  1. Top of page
  2. Abstract
  3. (LIGNO)CELLULOSE AND BIO-BASED ECONOMY
  4. SWITCHABLE IONIC LIQUIDS
  5. DISTILLABLE IONIC LIQUIDS
  6. CHOLINE-BASED IONIC LIQUIDS AND DEEP-EUTECTIC-SOLVENTS
  7. CONCLUDING REMARKS
  8. REFERENCES

Recent years have witnessed the use of different ionic liquids for biomass processing, either at the level of lignocellulose pre-treatment, to fractionate biomass in its main components, separating hemicellulose and lignin from cellulose, or directly in cellulose decrystallization by dissolving it in the ionic liquid and subsequent precipitation by adding anti-solvents. Yet, most of the ILs employed in these strategies (e.g. imidazolium-based solvents) are (still) expensive for such applications, and provide discussable ecological footprints. In an attempt to combine the highly useful generated knowledge with novel neoteric solvents with improved properties, economics, availability and ecology, several new trends have appeared in these areas during recent years. They comprise the use of switchable ILs, based on strong organic bases and CO2, the application of distillable ILs, as well as the use of bio-based and low-cost ILs and deep-eutectic-solvents (DES), e.g. choline chloride-based derivatives. Apart from other emerging uses, for all these solvents some preliminary applications in biomass processing involving pretreatments, cellulose dissolution and other applications have been successfully reported. This Minireview contextualizes these recent trends and discusses them with emphasis on future use of them in biorefineries and biomass valorization. © 2013 Society of Chemical Industry


(LIGNO)CELLULOSE AND BIO-BASED ECONOMY

  1. Top of page
  2. Abstract
  3. (LIGNO)CELLULOSE AND BIO-BASED ECONOMY
  4. SWITCHABLE IONIC LIQUIDS
  5. DISTILLABLE IONIC LIQUIDS
  6. CHOLINE-BASED IONIC LIQUIDS AND DEEP-EUTECTIC-SOLVENTS
  7. CONCLUDING REMARKS
  8. REFERENCES

The depletion of fossil resources, combined with environmental challenges and unstable geopolitical energy-dependence, is stimulating the research on biomass as the future feedstock for chemical industries, to provide an array of platform chemicals and biofuels.[1-4] Many (bio)catalytic strategies are being put forth for holistic integration within the so-defined biorefineries. The ultimate goal of these technologies is the entire valorization of lignocellulosic biomass to compensate processing costs by providing a palette of high-added to low-added value products with diminished waste formation.

In this scenario cellulose has a core position as it is the most abundant biopolymer over the earth, encompassing a substantial proportion of the lignocellulosic materials (up to c. 50–60 wt%).[1-7] Typically, pretreatment methods applied to lignocellulose aim at separating their main components, namely, lignin, hemicellulose, and cellulose, to subsequently manufacture all these fractions separately for value generation. In this respect, lignin may represent a source of aromatics, plastics, as well as other derivatives;[1, 8] and from hemicellulose, xylose, furfural and its further chemistry may be applied.[1] Cellulose may deliver a remarkably broad product range, from paper production and other cellulose-based materials,[1, 7, 9-11] to a source of sugars for their use either in fermentations or in the production of platform chemicals, such as 5-hydroxymethylfurfural (HMF), among other relevant and promising examples (Scheme 1).[1] From that perspective, it may be easily inferred that a key step in biorefineries is the set-up of efficient, sustainable and economic methods for cellulose depolymerization to afford glucose of high purity and low cost.[6]

image

Scheme 1. From lignocellulose to three main components, lignin, hemicellulose and cellulose. Brief schematic overview with emphasis on cellulose and further derivatization.

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Once pretreatment methods have been applied, the focus is on cellulose and related polysaccharides. Cellulose is a crystalline and highly packed polymeric material, which confers high resistance and stability. This converts cellulose into a challenging substrate for its further processing, whatever biological or chemical methods are considered for its depolymerization.[6] When the necessary provision of low-cost glucose for biorefineries is envisaged, either for fermentations or for other (bio)chemical derivatizations (Scheme 1), considerable research efforts have been undertaken in developing efficient and clean cellulose hydrolytic strategies. One approach for the depolymerization of cellulose is the use of cellulases, a group of different enzymes specifically designed by nature for such purposes. Tremendous developments have been reached in this area, representing a promising strategy, especially if further fermentative processes are considered. The mild hydrolytic conditions needed for efficient catalysis with glycosidases, aqueous solutions, room temperature, ambient pressure, mild pHs, etc., prevent the formation of degradation productions (e.g. HMF), which may inhibit the further microbial growth.[1-5] Likewise, another widely used strategy is the acid-catalyzed depolymerization of cellulose. A broad number of processes based on different acid catalysts have been set up for the depolymerization of cellulose and analogous polysaccharides.[6] Typically mineral acids like H2SO4 or HCl are used, either in concentrated form at mild temperatures (25–50°C), or in diluted form at higher temperatures (170–240°C). Furthermore, organic carboxylic acids have also been suggested as biogenic catalysts for biorefineries. As a common pattern for all these processes, a hydrolytic proton-catalyzed performance is supposed to proceed for all acidic depolymerizations, whereas a proton donor–acceptor cooperative mechanism is noted for enzymes.[12]

For all options envisaged for cellulose depolymerization, the high crystallinity of cellulose often decreases the efficiency of hydrolysis, and thus attempts to decrease cellulose crystallinity have been proposed. Herein, one remarkable option is the use of ionic liquids as solvents able to dissolve cellulose to a significant extent. Albeit analogous strategies for polysaccharide dissolutions were actually reported many decades ago using pyridinium molten salts,[13] the field of ionic liquids and (ligno)cellulose dissolution has undergone tremendous development in recent years.[14-16] Imidazolium-based ILs have been the most typically used, and nowadays novel imidazolium-based ILs are still being designed and reported for such purposes (e.g. IL containing nitriles to extract lignin from bamboo biomass).[17] The motivation behind these studies was that after dissolution of cellulose and further recovery upon precipitation with an anti-solvent (e.g. acetone, water or methanol), the recovered cellulose had lost part of its crystallinity, and therefore it was more accessible to (bio)catalysts under milder conditions, providing fermentable glucose with less significant by-product formation.[16] Many cation–anion combinations have been reported in this respect, comprising integrated concepts of IL-pretreated (ligno)cellulose followed by either solid-acid hydrolysis,[18] or by cellulases.[19, 20] Likewise, some ILs also trigger the transformation of cellulose I to cellulose II, more accessible to enzymatic hydrolysis, after regeneration of the polysaccharide.[21-23]

Yet, despite the development and promising success that the (pre)treatment of biomass with imidazolium-based ILs has brought, the potential implementation of such ILs at practical scale has not gone commercially further. One reason is the actual environmental impact of such imidazolium-based ILs, typically regarded as ‘green solvents’, which has been assessed by several research groups.[16, 24] Likewise, the economics of these derivatives represent an important barrier for its envisaged commercialization, albeit outstanding very promising examples of IL recycling of up to 99.7% have been recently published[25], especially in areas like (ligno)cellulose processing, with the aim of providing cheap fermentable sugars and low-value bulk-based platform chemicals. However, despite these barriers, ionic liquids are largely tuneable, as cations and anions can be mixed in thousands of combinations, and therefore, based on the acquired knowledge with (among others) imidazolium-based ILs applied to cellulose dissolution, in recent years the field has started to evolve to the design of novel ionic liquids with improved features for biomass processing purposes. Thus, there are examples of bio-based ILs enabling (ligno)cellulose and biomass dissolution with promising economics for low-added value bulk applications. Furthermore, switchable and distillable ILs represent emerging creative and elegant approaches that have recently been assessed for biomass processing as well. Envisaging the potential practical interest that these alternatives may deliver in the coming years, herein these recent developments are contextualized and discussed.

SWITCHABLE IONIC LIQUIDS

  1. Top of page
  2. Abstract
  3. (LIGNO)CELLULOSE AND BIO-BASED ECONOMY
  4. SWITCHABLE IONIC LIQUIDS
  5. DISTILLABLE IONIC LIQUIDS
  6. CHOLINE-BASED IONIC LIQUIDS AND DEEP-EUTECTIC-SOLVENTS
  7. CONCLUDING REMARKS
  8. REFERENCES

The so-called switchable ILs were described for the first time some years ago by the Jessop group.[26, 27] In a broad sense, switchable ILs are formed through the mixing of equimolar mixtures of an alcohol with a strong organic base (e.g. amidine, or 1,8-diazabicyclo[5.4.0]undec-7-ene, DBU), both forming a simple organic solution, with gaseous CO2 under ambient pressure and room temperature. The combination of those three components leads to an exothermic transformation, converting the mixture into an actual ionic liquid by in situ formation of the alkyl-carbonate, between CO2 and the alcohol, to subsequently form the amine salt (Scheme 2). Quite remarkably, upon addition of N2 or other gases to shift the (non-hazardous) CO2, the system is shown to be fully reversible, rendering again the starting materials.[26] Importantly, solvent properties of the mixture amine–alcohol compared with the switchable IL are obviously quite different. Therefore, a broad range of non-polar solvents (e.g. alkanes) may typically be dissolved in an amine–alcohol mixture, but they tend to form a clear second phase with a switchable IL. Likewise, the addition of anti-solvents, such as methanol, may trigger the precipitation of dissolved polymers. Thus, an ample range of innovative options for process development can be envisaged with this elegant approach. So far, the topic gives the impression of being largely underdeveloped, albeit several interesting applications have been suggested.26b

image

Scheme 2. Concept of the switchable ILs reported by the Jessop group.[26]

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Stimulated by the straightforward formation and recovery of these neoteric solvents, some of these switchable ILs have started to be used in biomass processing as well, with very promising prognoses reported so far.[28, 29] As stated in the introduction, two of the main key steps to set-up efficient biorefineries are the lignocellulose fractionation (pretreatment), to afford hemicellulose, pulp, and lignin, together with the (crystalline) cellulose depolymerization to render fermentable sugars of high purity and low-cost. With these considerations in mind, very recently butanol- and hexanol-based switchable ILs, formed using CO2 as gas, were used for the highly selective extraction of hemicellulose from spruce wood.[28, 29] After some days of treatment, lignin and cellulose remained suspended and were easily recovered as non-degraded material (for their further valorization). Likewise, the xylans from hemicellulose were successfully isolated after degassing of CO2 to reverse the switchable IL to a conventional amine–alcohol solution (Scheme 2), or by adding anti-solvents (e.g. methanol) to trigger the precipitation of the dissolved polysaccharides. Clearly this out-of-the-box approach may represent a promising way to make use of the properties of IL, and the knowledge generated thereof with imidazolium-based derivatives, at the same time decreasing costs related to IL synthesis, recovery and reuse, together with providing better ecological prospects than other analogous derivatives.[24] Switchable ILs are also tuneable like other ILs, and therefore different alcohols or gases can be applied to form them. In this vein, the use of SO2 or glycerol as components to form DBU-based switchable ILs has been reported,[30] and their successful use for hemicellulose extraction shown for different biomasses as well.[31] In Scheme 3 an overview of the switchable ILs reported for biomass processing are depicted.

image

Scheme 3. Overview of switchable DBU-based ILs reported for biomass processing, specifically for the selective hemicellulose dissolution.[27-30]

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The above discussed options were assessed for the direct processing of biomass (lignocellulose), aiming at conducting a pretreatment-like approach by selectively dissolving the more amorphous parts of polysaccharide, namely xylans and hemicelluloses. In an analogous but different approach, very recently the first concept of using switchable ILs for the direct treatment of microcrystalline cellulose was reported by the Jerome group.[32] Herein, the combination of DMSO and several non-ionic bases led, upon addition of CO2, to the efficient dissolution of microcrystalline cellulose (up to 15 wt%) at very mild temperatures (25–40°C). Interestingly, in this case it was postulated that hydroxyl groups present in cellulose were the actual alcohols where the carbonate was formed (Scheme 4). These results clearly show that the concept of switchable ILs may become very powerful for cellulose treatment as well, providing an extremely mild and selective operational framework for novel applications.

image

Scheme 4. Several non-ionic bases to form switchable ILs, employed in the dissolution of microcrystalline cellulose, in combination with DMSO as co-solvent.[31]

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Importantly, the process of dissolution of microcrystalline cellulose proved to be reversible upon pertinent removal of CO2. To achieve cellulose dissolution, the addition of DMSO as co-solvent was needed. Yet, remarkably, without adding DMSO the formation of gel-like cellulose was observed, leading to a less crystalline material after the innovative treatment. Hence, an envisaged approximation for more practical applications would be to avoid the use of DMSO and the straightforward and direct formation of a less crystalline cellulose gel that could be then more effectively depolymerized by (bio)catalysts in a subsequent step. Likewise, beyond the provision of fermentable sugars at low cost for biofuel purposes, this proof-of-concept may lead to highly innovative applications in the area of cellulose as well, where the generation of novel tailored cellulosic materials can be reached. Moreover, it must be noted that many alcohols with promising properties may be derived from natural resources and thus, a broad range of bio-solvents might be envisaged to form switchable ILs with specific properties. Taking these considerations together, the formation of sugar- or polysaccharide-based (e.g. glucose, chitosan, starch, etc.) switchable ILs should be feasible as well (Scheme 4), with a broad array of novel structures for innovative purposes and the development of smart tailored materials. In this vein, other analogous DBU-based ILs have been suggested recently for biomass processing as well.[33]

DISTILLABLE IONIC LIQUIDS

  1. Top of page
  2. Abstract
  3. (LIGNO)CELLULOSE AND BIO-BASED ECONOMY
  4. SWITCHABLE IONIC LIQUIDS
  5. DISTILLABLE IONIC LIQUIDS
  6. CHOLINE-BASED IONIC LIQUIDS AND DEEP-EUTECTIC-SOLVENTS
  7. CONCLUDING REMARKS
  8. REFERENCES

Another emerging and highly promising group of neoteric solvents that might be used in different areas of lignocellulose processing and biorefineries is the so-called ‘distillable ionic liquids’.[34-41] Actually, most of the reported ILs exert a negligible vapour pressure. However, it was recently realized that novel distillable ILs could be formed by combining dimethylamine and CO2 (2:1 eq/eq), triggering the formation of dimethylammonium ion and dimethylcarbamate ion (Scheme 5).[34-41] Notably, such ILs can be efficiently distilled at 45°C, resulting in straightforward formation, handling and recovery. Different applications have been envisaged for these novel derivatives in catalysis.[33-35] Moreover, as a relevant example applied to biomass processing, distillable ILs may be applied for the extraction of tannins from plant materials, representing a promising case of neoteric solvents gathering simplicity, efficiency and economics due to their straightforward recovery and reusability.[35]

image

Scheme 5. Distillable IL formed by combining dimethylamine and CO2.[34]

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With analogous focus on biomass, Kipeläinen and co-workers[39] reported on the development of a set of several guanidine-based distillable ILs by combining guanidinium base with different aliphatic carboxylic acids. Some of them, increasing the size of the carboxylic acid (up to propyl-based ILs), were able to dissolve microcrystalline cellulose to a significant extent (up to 5% w/w) when combined at 100°C for 18 h (Scheme 6).

image

Scheme 6. Guanidinium-based distillable ILs (m.p. 60–100°C) able to dissolve up to 5% w/w of microcrystalline cellulose at 100°C.[38]

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A slight cellulose depolymerization was observed when dissolved and recovered upon distillation of the IL and regeneration of the microcrystalline cellulose, presumably due to the acidic effect of the anionic counterion of the solvent. These interesting results of using distillable ILs for cellulose treatment remain still at the level of proof-of-principle, and no data related to the further acid-catalyzed hydrolysis of the regenerated cellulose are available in the literature. Yet, based on the obvious potentiality of that approach, a novel direction on the use of ILs in cellulose chemistry can be considered by means of these derivatives. Importantly, the proposed strategy would certainly combine efficiency with adequate costs and ecological footprints. Likewise, the development of neoteric solvents that may selectively dissolve other parts of lignocellulose (e.g. lignin or hemicellulose), whereas cellulose remains intact, may represent another approach to benefit from these fields, to implement pretreatment approaches. Further knowledge and characterization of these novel solvents is needed to convert it in innovation. In this sense, first academic studies on the properties of such ILs, compared with other classic ILs, have started to appear.[40]

CHOLINE-BASED IONIC LIQUIDS AND DEEP-EUTECTIC-SOLVENTS

  1. Top of page
  2. Abstract
  3. (LIGNO)CELLULOSE AND BIO-BASED ECONOMY
  4. SWITCHABLE IONIC LIQUIDS
  5. DISTILLABLE IONIC LIQUIDS
  6. CHOLINE-BASED IONIC LIQUIDS AND DEEP-EUTECTIC-SOLVENTS
  7. CONCLUDING REMARKS
  8. REFERENCES

Apart from the above-reported innovative type of ionic liquids starting to be assessed for applications in biorefineries, other different options are being investigated. Thus, the design of (cheaper) PEG-functionalized ionic liquids using alkylammonium or piperidinium cations as solvents for efficient cellulose dissolution have been put forth,[41] together with DMSO-amide-based solutions adding small amounts of ILs to rapidly dissolve cellulose.[42] Likewise, the design of bio-based ionic solvents with potential use in (ligno)cellulose processing is being considered. In this area, the motivation may be found in the expected reduced costs of the solvents produced, especially when compared with imidazolium-based ILs, together with the improved ecological footprints that may be assumed from their bio-based origin. Moreover, another incentive to undertake research in this area lies is the option of providing petroleum-free solvents, which may then be broadly available once petroleum resources are exhausted. In this area, a relevant example is choline chloride, typically used as cation for such bio-based ILs. Several cholinium alkanoates have been reported as useful solvents for the dissolution of cork biopolymers, aiming at separating suberin from cork.[43] In the same area, a number of bio-based ionic liquids containing choline and natural amino acids (as anions) have been identified as good solubilizers of hemicellulose (xylan) and lignin, whereas cellulose remains suspended or with little dissolution profiles (Scheme 7). Many of them can dissolve lignin in loadings of up to ∼200 mg g-1, and xylans in the range 60–70 mg g-1.[44, 45]

image

Scheme 7. Bio-based choline-based ionic liquids enabling the dissolution of lignin and xylan.[43, 44]

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Thus, the strategy in this case can be applied for selective delignification and separation of hemicellulose, leading to a fraction of cellulose that is then easily hydrolyzed by cellulases, compared with the starting lignocellulosic material.[44, 45] In the same area, other choline-based ionic liquids, formed with acetic acid as well as other alkanoates, are able to decrystallize microcrystalline cellulose, thus representing again a promising area for its future development and application in biorefineries.[46] Overall, all these technologies are still incipient, but represent an interesting synergy between the know-how derived from imidazolium-based ILs, and attempts to circumvent the economic and ecologic weaknesses that those solvents may have. Many creative applications can be expected for biorefineries. Once pretreament of cellulose is performed with these ILs, acid-catalyzed processes are expected to proceed in milder conditions than those reported for direct hydrolysis of crystalline polysaccharides in aqueous solutions.

In an analogous area, another important group of emerging neoteric solvents for (ligno)cellulose dissolution and processing is represented by the so-called deep-eutectic-solvents (DES) or low-transition-temperature mixtures (LTTMs).[47-49] Albeit as such eutectic mixtures have been known for many years,[49-51] they started to receive more attention when Abbott et al. put them in the context of neoteric solvents,[52, 53] boosting the development and use of DES significantly[47-49] with promising applications in areas like (bio)catalysis, metal electrodeposition, as well as for biomass processing and biorefineries as well.

DES are typically formed by complexion of quaternary ammonium salts, e.g. choline chloride and derivatives, with hydrogen bond donors (HBD) upon a gentle mixing for several hours at moderate temperatures (normally up to 100°C). The mixture creates a charge delocalization through hydrogen bonding between the halide anion and the hydrogen donor compound, decreasing the freezing point of the mixture, and generating the eutectic. The first case reported by Abbott et al. comprised the mixing of choline chloride (m.p. 302°C) with urea (m.p. 133°C) in different molar ratios, to afford a liquid DES with the deepest freezing point of 12°C (at 1:2 eq:eq proportion).[52] Assuming the composition, DES share many interesting properties with other conventional ILs, such as their non-reactivity with water, non-volatility and broad tuneabiliy. Moreover, DES are often formed with low-cost bio-based components, being fully biodegradable and accessible at large scale. Remarkably, a broad range of bio-based structures can be used for the formation of DES, some of them even being chiral structures (e.g. isosorbide),[54] and enabling the design of chiral solvents with promising cost prognosis and reduced ecological impact. Within this bio-based perspective, it can even be envisaged that as future options biorefineries might be able to produce their own solvents to be used in different processes within these processing plants. Apart from this, the formation of DES can also be triggered by means of other non-bio-based components (e.g. other quaternary ammonium salts) assuring a broad tuneability for diverse applications.

Taking the approach of using ILs for biomass dissolution and (pre)treatments, an envisaged application for DES was their utilization as solvents for challenging lignocellulosic materials and other polysaccharides.[55] In this sense, applications of DES as starch modifiers (e.g. by decreasing its crystallinity) have already been proposed.[56, 57] Interestingly, it has been considered whether DES would be the natural solvents for biochemical reactions, considering the very low solubility of many natural compounds in aqueous solutions.[58] Thus, the in situ and natural generation of DES within living organisms, e.g. combination of malic acid and sucrose to form a DES, might afford the dissolution of highly hydrophobic structures, facilitating in vivo biochemical reactions in such non-aqueous (micro)environments. DES have recently been utilized as extractive agents of natural products such as those flavonoids,[59] providing useful sustainable and economic options as novel solvents for such purposes.

Yet, most of the reported DES have failed so far in the attempt of dissolving cellulose within a realistic concentration range. Yet, this property has been taken as an advantage, and due to the already mentioned huge tuneability and versatility of DES, virtually analogous to that of ionic liquids, it was reported that some DES are actually able to dissolve significant amounts of lignin, whereas cellulosic regions remain suspended on it.[60] Up to 26 different DES were assessed for the dissolution of different lignins, from several origins, starch and cellulose, showing different results depending on the DES mixture applied (Scheme 8). Some quaternary ammonium-based DES, as well as amino acid-based derivatives enabled the dissolution of significant amounts of lignin. Likewise, promising results were observed from the dissolution of wheat straw in some DES, affording again the dissolution of lignin, what should allow the use of the delignified cellulose fraction for further valorization.[60]

image

Scheme 8. Selected novel DES—quaternary ammonium-based and amino acid-based ones—enabling the dissolution of different amounts of lignin.[59]

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CONCLUDING REMARKS

  1. Top of page
  2. Abstract
  3. (LIGNO)CELLULOSE AND BIO-BASED ECONOMY
  4. SWITCHABLE IONIC LIQUIDS
  5. DISTILLABLE IONIC LIQUIDS
  6. CHOLINE-BASED IONIC LIQUIDS AND DEEP-EUTECTIC-SOLVENTS
  7. CONCLUDING REMARKS
  8. REFERENCES

Although the proof-of-principle of the dissolution of polysaccharides by ionic liquids-based solvents has been known for decades, it is only in recent years that a tremendous interest in developing novel solvent systems to be used in (ligno)cellulose processing has emerged. Along many successful proof-of-concepts and in-depth assessments, it was realized that most of the ILs employed still had some challenging issues like high costs and discussable benign ecological footprints. Taking all that know-how, several novel approaches in using neoteric solvents for biomass treatments have started to appear. This paper has contextualized and discussed some emerging approaches in this area. Switchable ILs based on strong bases and CO2, as well as distillable ILs, represent highly innovative and elegant approaches to process and modify polysaccharides under mild, controllable and economic conditions, upon the option of recovering and recycling the solvents. Moreover, a novel array of ILs based on low-cost bio-based resources is starting to emerge, together with the deep-eutectic-solvents (DES). Once again, the broad tuneability of ILs enables the development of smart solvents with tailored applications, which might be also economically envisaged for biomass-based processes as long as raw material costs, ecology and recyclability can be aligned. In this sense, the examples reported in this paper display very promising prognoses for that.

REFERENCES

  1. Top of page
  2. Abstract
  3. (LIGNO)CELLULOSE AND BIO-BASED ECONOMY
  4. SWITCHABLE IONIC LIQUIDS
  5. DISTILLABLE IONIC LIQUIDS
  6. CHOLINE-BASED IONIC LIQUIDS AND DEEP-EUTECTIC-SOLVENTS
  7. CONCLUDING REMARKS
  8. REFERENCES
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