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Current Opinion in
Green and Sustainable Chemistry
Green solvents for sustainable separation processes
Boelo Schuur, Thomas Brouwer, Dion Smink and
Lisette M. J. Sprakel
Solvent-based separation processes can reduce the required
energy input for separation, improve biocompatibility, and allow
for mild responsive separation systems that are applicable
when distillation is technically not feasible because of the
delicate nature of (bio)molecules to be separated. Owing to the
increasing awareness of the need for a green and sustainable
industry, the interest in green solvents for separation pro-
cesses is growing. Being able to tailor solvent properties and
solvent biocompatibility are key properties for making pro-
cesses sustainable and allowing flexibility regarding feed and
product composition of the separation processes involved.
This work aims to give an overview of solvent developments
toward more sustainable and green separation processes. For
all solvent systems, it is key that not only the primary sepa-
ration operation is considered, but the entire process including
solvent recovery, because that is typically where the energy
should be invested.
Addresses
Sustainable Process Technology Group, Process and Catalysis Engi-
neering Cluster, Faculty of Science and Technology, University of
Twente, The Netherlands
Corresponding author: Schuur, Boelo (b.schuur@utwente.nl)
Current Opinion in Green and Sustainable Chemistry 2019,
18:57–65
This review comes from a themed issue on Green Solvents
Edited by Mara G. Freire and João A. P. Coutinho
Available online 4 January 2019
https://doi.org/10.1016/j.cogsc.2018.12.009
2452-2236/© 2019 Elsevier B.V. All rights reserved.
Introduction
The societal awareness of the need to manufacture
goods in a sustainable fashion with preferably no to little
CO2 emissions is a strong driver to develop new pro-
cesses and improve existing ones. This also holds for a
range of industries where separations are key and CO2
emissions are significant, that is, the oil industry, the
chemical industry, the mining industry, the food in-
dustry, and the pharmaceutical industry. In (bio)chem-
ical industries, typically around 50% of the energy costs
are due to separations [1,2], and with improving these
separations, a major contribution to a smaller CO2-
footprint can be achieved. With the rise of the
petroleum-based economy in the twentieth century,
distillation towers have been erected as the working
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horses for separations. Although effective, distillations
are often highly energy intensive [1] and solvent-based
separations might replace them. Another reason why
solvent-based separations become more important is the
transition from the oil refinery to the biorefinery. For the
highly complex biomass streams, distillation is often not
technically feasible and solvent-based separation stra-
tegies may be the only option or the most appropriate
option. Not seldom, solvent-based biomass fractionation
includes reactive fractionation, for example, depoly-
merization of lignocellulose [3]. Next to the CO2-foot-
print, also other aspects of solvents should be
considered such as (eco)toxicity [4]. Another very
important solvent-based separation technique is liquide
liquid extraction (LLX). LLX is often operated in
countercurrent mode in columns, as displayed in
Figure 1. Biphasic systems are typically schematically
represented as in Figure 1a, whereas the multistage
countercurrent operation in the column is displayed in
Figure 1c. Stage performance is nowadays computed
with thermodynamic models, but ternary diagrams are
still common for illustration purposes Figure 1d. An
important trend in research on LLX is the use of
switchable solvents of which properties such as polarity
can be manipulated with external stimuli Figure 1b.
We have identified ‘aqueous solvent systems’, ‘ionic
liquids (ILs)’, ‘deep eutectic solvents (DESs)’, ‘bio-
based solvents’, and ‘switchable solvent systems’ as
main solvent classes in which key developments have
taken place over the years 2016e2018 that can help the
industry become greener and leaner. In the following
subsections, each of these classes is discussed in detail
and we conclude with an outlook ‘how green, how
sustainable?dtoward more sustainable separations’.
Aqueous solvent systems
A solvent that is generally considered safe, nontoxic, and
environmentally friendly is water. Especially for
biochemical separations for nutraceutical and pharma-
ceutical applications, aqueous systems are of interest,
because organic solvents often are not safe enough [5].
However, for many systems, the properties of water are
not appropriate for the intended separation, and there-
fore, many types of composite solvents based on water
have been developed such as aqueous two-phase systems
(ATPSs) [6], also used to make cloud point extraction
systems [7], hydrotrope forming solutions [5], and
aqueous solutions of CO2-switchable solvents [8].
urrent Opinion in Green and Sustainable Chemistry 2019, 18:57–65
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Figure 1
Current Opinion in Green and Sustainable Chemistry
Schematic overview of countercurrent liquid– liquid extraction process (c), where two liquid phases are in equilibrium (a). Dimensioning of such pro-
cesses requires proper equilibrium description, for which ternary diagrams are illustrative (d). Stimuli responsive solvents may be applied to facilitate
mild solvent regeneration (b).
58 Green solvents
Cloud point extraction systems are aqueous surfactant
solutions with temperature responsive critical aggrega-
tion behavior. Below the critical temperature, micelles
are formed that can solubilize hydrophobic species, and
above the critical temperature, the micelles break up,
inducing a macroscopic phase split which allows for
product recovery. Recently, cloud point extraction sys-
tems have been reported for extraction of lipids from
microalgae [9], which due to the low toxicity appears to
be a solution that may be applied for microalgae bio-
refineries with food applications [7].
Next to solubilization in surfactant systems (Figure 2a),
it is also possible to use amphiphilic molecules or ions
containing a hydrophobic moiety not large enough to
form micelles on their own but can form hydrotropes
together with hydrophobic species in solution (see
Figure 2b).
De Faria et al. [5]** showed that extremely hydrophobic
species (triterpenes) may be solubilized in aqueous so-
lutions of surface active ILs, enhancing their solubility
up to eight orders of magnitude. This remarkable solu-
bility enhancement may be applied in other fields such
as pharmaceutical engineering where aqueous solubil-
ities of steroids may be boosted similarly to enable much
more sustainable processing than with traditional
(halogenated) organic solvents. Another recent advance
Current Opinion in Green and Sustainable Chemistry 2019, 18:57–65
involving hydrotropic solutions was the microwave-
assisted phytoextraction of geraniol [10], where due to
the combination of microwave action and hydrotropes, a
much shorter extraction time could be achieved.
ATPSs are solvent systems consisting of two aqueous
phases that are at least partially immiscible. The degree
of immiscibility is expressed by the tie-line length, and
systems with long tie-line length exhibit a limited
mutual miscibility. Immiscibility of aqueous phases is
due to the repulsive behavior of its main constituents,
for example, a salt and a polymer or two polymers, or an
ionic liquid and a polymer or a salt [6]. Because both
phases are water-based, very mild separations can be
performed without disrupting biomolecules of interest,
for example, monoclonal antibodies, enzymes, and vi-
tamins. Recentwork in this field is strongly emphasized
on the use of ILs, for increased polarity and specific
interactions [11], biocompatibility [12], induced salting
out effects for selective separations [13], and induced
response to external stimuli such as temperature [12]
and pH [14].
Ionic liquids
ILs are salts that are liquid at T < 100 �C, and by
changing the structure of either the organic cation and/
or the organic or inorganic anion, the ion properties can
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Figure 2
Current Opinion in Green and Sustainable Chemistry
The schematic representation of (a) micellization by sodium dodecylsulphate and (b) the formation of a hydrotrope by 1-butyl-3-alkylimidazolium,
surrounding ursolic acid. The blue and red areas on the solvent molecules indicate a polar and apolar functional group, respectively.
Green solvents for sustainable separation processes Schuur et al. 59
be tailored. An often praised property is their negligible
vapor pressure that prevents solvent losses through
evaporation. ILs form the solvent class that received by
far the most attention in the past decade [3,6,15]. ILs
have been applied in a wide range of processes,
including many separations such as analytical separa-
tions [16,17], aromatic-aliphatic separation [18,19],
carbon capture [15], metal extractions [20,21]*, acid
extractions [22], and biomass fractionations [3]. In our
opinion, the biggest hype around ILs is over now, and
critical articles are also welcomed that bring the use of
ILs in perspective and compare with traditional sol-
vents, for example, for aromatic-aliphatic separation
[18]. Indeed, we are convinced that, for many applica-
tions, ILs can be very beneficial but they are not the
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perfect solution to everything and too enthusiastic
claims should be considered with caution. For example,
ample literature is available on CO2-capture using ILs,
and recently, several claims (myths) about physical sol-
ubility of CO2 in ILs were dispelled in an excellent
article by Carvalho et al [23]**. Next to the physical
solubility, in our opinion, the fact that the total ab-
sorption capacity including both physical and chemical
absorption for state-of-the-art IL-based absorption sys-
tems does not exceed 1 mol of CO2 per mol of ion pair
[24] should be regarded critically, as traditional amine
solvents reach similar values [25] and typically exhibit a
lower molar mass. Nevertheless, in comparison to
aqueous solutions of volatile amines, regeneration of ILs
may be much greener and cheaper by avoiding volatile
urrent Opinion in Green and Sustainable Chemistry 2019, 18:57–65
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60 Green solvents
amine emissions and reducing water evaporation.
However, viscosity may severely affect heat transfer and
mass transfer, which were identified as key challenges
for use of ILs as CO2 capture solvent because they can
result in absurd requirements for column heights as
shown in Figure 3 [26]**.
Also, for separation of aliphatics and aromatics, huge
numbers of articles have appeared just reporting distri-
bution coefficients. It is key that also other important
aspects are considered, such as overall process
efficiency and toxicity, as was performed in an excellent
combination by Dı́az et al. [27]*.
Metal extractions with classical molecular solvents have
been studied and industrially applied for decades, often
using kerosene as a diluent, intrinsically posing a risk
due to the flammability. This field may benefit from the
development of IL-based separation systems for metal
extractions in terms of safety. In the most recent years, a
new dimension has been added to metal extractions,
that is, urban mining, meaning that recycling of metals
from end-of-life electronics is aimed [21]*. Given that
the average lifetime of electronic devices such as cell
phones is nowadays only a few years and these devices
contain many elements, fractionations of (rare) metals
Figure 3
Required column heights for CO2 capture with several ILs in perspective. Repro
The Royal Society of Chemistry.
Current Opinion in Green and Sustainable Chemistry 2019, 18:57–65
from electronic waste are of key importance for our so-
ciety. The article by Li et al [21]* offers an excellent
approach for metal fractionation, whereas another key
article authored by Gras et al [20]** shows excellent
extraction yields for several metals using IL-based
ATPS, intrinsically safe and a potential breakthrough
not only for urban mining but also for the traditional
mining industries.
In the field of biorefinery applications of ILs, the most
important recent developments include lignin depoly-
merization [28], nanocellulose production [29], and
fractionations from aquatic biomass such as microalgae
[30]**. The article by Desai et al. [30]** describes a
microalgae biorefinery in which both pigments and
proteins are fractionated from the algae biomass in a
single stage using microgel particleestabilized IL
emulsions. In the absence of the microgel particles, the
protein was less stable. It is envisioned that this
approach may be much wider applicable in biorefineries
to prevent denaturation of sensitive molecules during
fractionation. Next to hydrophobic ILs stabilized by
microgel particles, IL-based ATPS also might be applied
to obtain mild systems able to fractionate proteins, for
example, from carbohydrates [31].
Current Opinion in Green and Sustainable Chemistry
duced from Ref. [26]**—Published under Creative Commons license by
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Figure 4
Current Opinion in Green and Sustainable Chemistry
Examples of molecules that are frequently applied in DESs. Often they
are designated as hdyrogen bond acceptor (HBA) or as hydrogen bond
donor (HBD), but many can be both donating and accepting hydrogen
bonds.
Green solvents for sustainable separation processes Schuur et al. 61
Deep eutectic solvents
DESs are composite solvents that exhibit deep eutectic
behavior, that is, upon mixing the constituents of these
solvents, the mixtures’ melting points reduce consid-
erably more (>50 �C) than would be the case for ideal
mixtures. Deep eutectic behavior is typically observed
by mixing hydrogen bonding constituents, and by vari-
ation of hydrogen bond acceptor and/or hydrogen bond
donor, these solvents are tunable like ILs are tunable by
variation of ions. DESs, and in particular, natural DESs
(NADESs) [32,33] have been claimed to be less toxic,
more biodegradable, and significantly cheaper than ILs
[34]. Although in our opinion caution should be applied
upon making strongly generalized statements with
regard to toxicity and biodegradability, in many exam-
ples these properties do apply. Furthermore, owing to
their tremendous combination possibilities, this solvent
class can also be considered a designer solvent class and
numerous applications have already been explored. Ex-
amples are pretreatment of lignocellulosic materials
without cellulase deactivation [35], enhanced fraction-
ation of lignocellulosic materials [36], and urban mining
of nickel and cobalt [37]. In the field of mild separation,
DESs have been applied for proteins separation, both
with DES-coated magnetic nanoparticles [38] and in
DES-based ATPS systems [39]. Because the properties
of DES are not only dependent on their molecular
structures but also strongly on the ratio in which they
are present in a fluid phase, the stability of ATPS from
DESs might be limited to certain combinations [40].
More conventional processes, such as sulfur removal
[41], aliphatic/aromatic separations [42], and CO2 cap-
ture [43] have also been evaluated. For CO2-capture
high equilibrium capacity was found to compromise with
viscosity [44], an aspect that should be considered with
care, because, for ILs, viscosity was found to be a very
critical parameter [26]**. Using hydrophobic DESs
impregnated in a membrane, furans could be separated
from aqueoussolutions [45]*. Similar to ILs, also most
(NA)DESs reported in literature are hydrophilic, but
these hydrophobic examples (see Figure 4) offer in our
opinion highly interesting options for future research on
recovery of bio-based chemicals from dilute aqueous
streams.
From the diversity of already reported applications and
the outlook on future possible applications, it may be
concluded that (NA)DESs are comparably versatile as
ILs while also highly compatible with a circular econ-
omy, as displayed in Figure 4 and are expected to receive
ample attention in the coming years.
Bio-based solvents
Besides NADESs, other highly interesting biodegrad-
able and nontoxic bio-based solvents have recently been
evaluated for various separations where they exhibited
often comparable properties to conventional solvents.
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Examples of bio-based solvents include Cyrene [46,47],
2-methyltetrahydrofuran [47], g-valerolactone [47], and
methyl(2,2-dimethyl-1,3-dioxolan-4-yl)methyl carbon-
ate [48]. Recently, access to a rich variety of Cyrene
derivatives was reported, and a new solvent class called
Cygnet x.y appears as a potential renewable replace-
ment for N-methyl pyrrolidone (NMP) [49]**. The
ability to vary the side groups on x and y positions
through synthetic modifications (see Figure 5 for
structure of Cygnet x.y) [49]** fosters versatility in
applicability.
Sustainable ILs have also been reported, although they
still require conventional solvents in their production
process [50]. The use of bio-based solvents is essential
in closing the carbon balance for a circular economy, see
Figure 5.
Switchable solvent systems
Solvents that switch their nature reversibly upon an
external stimulus can be applied in affinity separation
systems where the nature of the solvent during the
primary separation is different from the nature in the
regeneration stage. Such a switch can aid the regener-
ability of the solvent. Temperature-responsive solvents
(surfactant based or polymers) are best known [9], but
recently also other stimuli get more attention. For
example, solvents that switch polarity upon exposure to
urrent Opinion in Green and Sustainable Chemistry 2019, 18:57–65
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Figure 5
Current Opinion in Green and Sustainable Chemistry
Examples and circularity of sustainable, nature-derived solvent systems. ATPS, aqueous two-phase system; NADES, natural deep eutectic solvent;
MMC, methyl(2,2-dimethyl-1,3-dioxolan-4-yl)methyl carbonate.
62 Green solvents
CO2 are considered as very promising for application in
several separation and synthesis processes. Pioneered by
Jessop et al [51], CO2-switchable solvents (CO2-SSs)
have been used in extracting lipids from microalgae [52]
and as an entrainer in extractive distillation of heptanee
toluene mixtures [53]. Very recently another highly
interesting approach was shown by Cicci et al [54]**,
who developed the circular extraction concept, in which
both states of the solvent were used to extract polar
compounds with the ionic form and apolar compounds
with the neutral form. When applied in an aqueous so-
lution (requiring the neutral state also to be water sol-
uble), switching a CO2-SS between its uncharged state
and an ionic state switches the ionic strength [12],
which allows for reversible salting out of organic com-
pounds from water [8]. CO2-SSs are typically based on
amines that are not always environmentally benign,
making the full solvent life cycle including consider-
ations on solvent losses important [12]. To aid the sol-
vent screening of CO2 switchable hydrophilicity
solvents, a technique using a microfluidic device was
Current Opinion in Green and Sustainable Chemistry 2019, 18:57–65
developed by Lestari et al [55] to study the performance
and recovery of the solvent, as well as the losses in the
process [55].
In addition, redox potential may be applied to switch
solvents, and on the basis of work from the Hatton group
on redox responsive gels [56], solvents [57], and elec-
trodes [58]*, solvents with these specific redox
responsive groups may be switched between different
states reversibly, allowing electrochemical tuning of
their functional group affinity with great potential in the
field of carbon capture [57], water purification, and se-
lective electrochemical separations [58]*.
Other stimuli that may aid separations are magnetism,
for example, to collect droplets of magnetic fluids
effectively [59] and light. Van Dijken et al [60]**
demonstrated that aggregation of photoresponsive am-
phiphiles can be photo-controlled and anion binding can
be affected with light when photo switching receptors
are applied [61,62]**. Such photoresponsive
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Green solvents for sustainable separation processes Schuur et al. 63
aggregation and binding enables very mild recovery by
photo switch.
How green, how
sustainable?—toward more sustainable
separations
Following the twelve principles of green chemistry [63],
a conscious solvent choice is essential. This requires a
balance of solvent function and minimal toxicity and
biodegradability (principle 4 and 10) and solvent pro-
duction from renewable sources (principle 7). The
current trend from conventional solvents and ILs to bio-
based solvents, NADESs, and ATPS is usually in
accordance with these principles. A key aspect of a
sustainable solvent-based separation process develop-
ment should be the ease of recovery. When solvents are
applied with extremely good distribution coefficients,
the energy level of the extract phase may be very low,
compromising on the regenerability and requiring large
energy input to achieve the ‘separated components’
state. A similar statement was recently formulated for
acid extractions as ‘thinking beyond the partition ratio’
[64], and we suggest to always check if sustainability
measures conform the green chemistry principles when
developing a new solvent-based separation system.
Sustainability analysis of solvent-based separation pro-
cesses should always include the full process concept,
including (bio)compatibility with the chemistry that the
solvent is applied to and solvent regeneration. It is key
not to consider a solvent that is green and sustainable
because it has been identified as such for another
application, because a solvent that is sustainable in one
process is not necessarily sustainable in another process
in terms of (bio)compatibility and regenerability. For the
highly innovative switching systems, this means also
thinking about the overall energy efficiency of the
switching system, including light sources, which may
seriously affect the overall process evaluation. Never-
theless, such switching systems open pathways to use
green electricity as energy source for the regeneration,
which can seriously reduce CO2 emission, and may offer
a unique mild route that allows product recovery that
would otherwise not be possible.
Conflict of interest statement
Nothing declared..
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www.sciencedirect.com/science/journal/24522236
	Green solvents for sustainable separation processes
	Introduction
	Aqueous solvent systems
	Ionic liquids
	Deep eutectic solvents
	Bio-based solvents
	Switchable solvent systems
	How green, how sustainable?—toward more sustainable separations
	Conflict of interest statement
	References