2. Fuel 293 (2021) 120297
2
desulfurization units are the most needed in refineries around the world.
According to the Organization of the Petroleum Exporting Countries
(OPEC) [4], until 2040, an additional global desulfurization capacity of
22.5 million barrel/day is needed, against to 19.6 million barrel/day of
total crude distillation capacity [4,6].
Hydrodesulfurization (HDS) is the current industrial technology used
for the removal of organic sulfur compounds, depicted in Fig. 2, from
refined products, such as, gasoline, diesel, jet fuel, fuel oil, and naphtha.
In this technology, the sulfur-containing compounds are catalytically
converted into hydrogen sulfide at high temperatures (>300 ◦
C), high
pressure (20–60 bar) and requiring a huge amount of hydrogen [7–9].
Hence, HDS is an expensive technology due to its high operational cost.
In addition, although the hydrogenation process is efficient for the
removal of thiols, sulfides and dissulfides, its low effectiveness in the
removal of heterocyclic sulfur compounds, such as dibenzothiophenes
(DBT) and derivatives is major drawback. Fig. 3 [10,11] depicts the
reactivity in HDS of representative chemical structures of sulfur com
pounds typically present in different types of fossil fuels. It can be seen
that HDS is not very effective in handling larger sulfur compounds
derived from DBT, typically present in diesel fuels. This is due to the very
small pores of common HDS catalysts, which hinder the diffusion of
large molecules to the active sites present in the pores. Therefore,
reducing sulfur contents to levels that comply with modern regulations
requires increased severity, energy and the use of expensive catalysts,
which makes this a highly energy and hydrogen-intensive process [12].
Furthermore, these extreme operating conditions are often not
compatible with other fuel requirements such as oxygen content, vapor
pressure, benzene content, overall aromatics content, boiling range and
olefin content for gasoline, and cetane number, density, polynuclear
aromatics content, among others [13].
The development of innovative complementary technologies for
deep desulfurization of fuels has been the focus of many research groups
and the target of many industries. Although HDS-based technologies
have been greatly investigated [14–17] and widely reviewed [7,18–22],
other alternative processes, such as extractive desulfurization (EDS)
[23,24], adsorptive desulfurization [25,26] (ADS), oxidative desulfur
ization (ODS) [27,28], biodesulfurization (BDS) [29,30] among others
[31,32], have also been researched. Among these, EDS and ODS are two
of the most promising desulfurization processes due to their simple
operation, low cost and benign effects on the final fuel quality (in the
case of EDS), no need of hydrogen and high compatibility with other
refining technologies [33,34]. However, the organic solvents typically
used as extractants raise environmental, public health, safety and cost
concerns due to their volatility and flammability. In the last two de
cades, the search for alternative solvents that can overcome the draw
backs of the organic solvents used has been intensified. Ionic Liquids
(ILs) and, more recently, Deep Eutectic Solvents (DES) have shown great
potential as possible substitutes of several hazardous organic solvents
commonly used in industrial operations, in particular in fuels
desulfurization.
ILs are organic salts with melting points below the boiling point of
water (100 ◦
C) [35,36]. They are composed of organic cations and either
organic or inorganic anions. The first reported IL was introduced around
a century ago, but only in 1982 with the publication of Wilkes et al. [37]
on dialkylimidazolium chloroaluminate melts, this class of solvents
started to have scientific impact. Nowadays, those ILs are called the first
generation, since they are water sensitive [38]. The second generation of
ILs, which contain air and moisture stable ILs, was reported by the same
research group, a decade later [38]. They are the iconic 1-ethyl-3-meth
ylimidazolium-based ILs, which today have a widespread use. Since
then, ILs have been widely investigated, and their first industrial
application occurred in 2003, by BASF, for biphasic acid scavenging
[39]. In the last two decades, research in this field focused on finding
greener, task-specific ILs, and finding alternative cheaper and simpler
synthetic routes. Typically, IL cations contain nitrogen (ammonium,
imidazolium, pyridinium or pyrrolidinium cations) and alternately
phosphorous (phosphonium cations) or sulfur (sulfonium cations).
Regarding IL anions, the most used are halides, for example, chloride
(Cl-
) and bromide (Br-
), fluorinated anions such as tetrafluoroborate
([BF4]-
), hexafluorophosphate ([PF6]-
), bis[trifluoromethylsulfonyl]
imide ([NTf2]-
) and trifluoromethanesulfonate ([TfO]-
), cyano-type an
ions such as dicyanamide ([N(CN)2]-
)- or thiocyanate ([SCN]-
) and ni
trate ([NO3]-
), among others [36].
ILs are an environmentally attractive alternative to traditional
organic solvents, once they have unique physicochemical properties,
namely, high thermal stability, non-flammability, non-volatility and
adjustable miscibility and polarity, since they can be adequately tailored
by changing the cation and/or the anion [38,40–42]. This makes ILs
extremely versatile solvents, having the capacity to dissolve a wide
range of organic and inorganic compounds, as well as polymeric mate
rials [43,44]. Therefore, these solvents have been proposed for a wide
range of different applications [45]. For example, they have been
Fig. 1. Evolution of the maximum sulfur allowed in highway fuels (in EU) overlapping with deep desulfurization research using ILs and DES, over time.
F. Lima et al.
3. Fuel 293 (2021) 120297
3
extensively studied as alternative solvents to replace traditional organic
solvents in a wide range of processes, such as organic reactions, catal
ysis, bio-catalysis, separation, extraction reactions. ILs have also been
applied as safe electrolytes in electrochemistry applications, notably in
batteries and fuel-cells [36,38,44,46], and more recently, they emerged
as versatile building blocks for advanced functional materials [44].
Although these compounds have many advantageous properties, they
also present major drawbacks like, the high cost of production and, in
some cases, toxicity problems [46,47], which have, so far, hindered their
industrialization.
Meanwhile, a new class of alternative solvents has been gaining
ground – the DES. Abbott et al. [48], introduced DES in 2003. The first
DES reported was a mixture of choline chloride and urea, in a molar
proportion of 1:2, resulting in a mixture with melting point of 12 ◦
C (a
deep deviation from the melting point of choline chloride and urea,
which are 302 ◦
C and 133 ◦
C, respectively). DES are sometimes
considered as a new class of IL analogues, sharing many characteristics
and properties, since they are partially constituted by ions [49–51].
Fig. 2. Chemical structures of the main families of sulfur compounds present in fuel oil.
Fig. 3. Reactivity in HDS of representative chemical structures of sulfur compounds, present in different fractions of crude oil distillation, regarding their sizes and
positions of alkyl groups on the ring ().
reproduced from [10]
F. Lima et al.
4. Fuel 293 (2021) 120297
4
Nevertheless, it is necessary to point out that, in fact, ILs and DES are
two different classes of alternative solvents [49].
In general, DES are systems formed from a eutectic mixture of Lewis
or Brønsted acids and bases. Due to the large number of possible
different combinations, in 2007, Abbot et al. [52] suggested a simple
classification system which divided DES into four main groups - type I to
type IV. Type I are analogous to the well-studied metal halide/imida
zolium salt systems. The type II differs from the previous by using hy
drated metal halides combined with choline chloride or other organic
salts. The type III is based on the combination of choline chloride or
similar salts and hydrogen bond donors (HBD), such as amides, car
boxylic acids, and alcohols, which makes this group of DES particularly
adjustable since they can be easily tailored for specific applications, due
to the wide variety of HBD available [49,52]. Finally, type IV, which was
introduced in 2007 by Abbot, refers to eutectic mixtures of zinc chloride
(metal halides) and HBD molecules such as urea and acetamide [52].
Regarding the DES preparation process, it simply consists of mixing
at least two compounds, in a certain proportion, at specific pressure and
temperature conditions (generally moderate heating and atmospheric
pressure) [48,49,53]. This is one of the main advantages of DES - low
production cost - comparatively to ILs [50]. Consequently, DES pro
duction process is carried out with 100% atom economy which means
that, contrarily to ILs, no further purification is needed [54].
Although DES share the adjustable properties of ILs, they also offer
other advantages, such as high diversity, low-cost and easily obtained
raw materials and a green and simple synthetic process [50,53]. More
over, DES can be easily prepared from natural compounds, such as,
amino acids, sugars, organic acids among others, and this class is
designated as natural deep eutectic solvents (NADES) [55]. This almost
unlimited number of DES has boosted their application in various fields,
including catalytic reactions, organic synthesis, electrochemistry, ma
terials preparation and separation process [50,53]. Recently, the use of
DES in the biotechnological field [56,57] has also been explored, leading
to an intensification of DES toxicity studies [56–58]. Nevertheless, there
is still a long way to go into the full understanding of DES properties,
mainly in what concerns their limitations - viscosity and chemical sta
bility - and also their characteristics – corrosivity, physicochemical
features, interaction with water, structural organization, etc.
In recent years, EDS and ODS using DES have been intensively
studied. The research shows a bright future for deep desulfurization
using DES, either as a potential alternative or as a complement to HDS.
The purpose of this review is to provide a comprehensive and critical
compilation of the recent advances in deep desulfurization of fossil fuels
using DES in detriment of ILs, in order to achieve ultra-light sulfur fossil
fuels. From our point of view, this comparison is of high interest, since
DES research and ILs research in the desulfurization field, can comple
ment each other (Fig. 1). The knowledge acquired using ILs over the last
two decades can be transposed into the DES field and boost DES appli
cation. In addition, the drawbacks of DES and ILs use will be highlighted
and discussed, as well as the diverse factors that can influence the
desulfurization processes. Finally, environmental and economic con
siderations will be presented.
2. From ILs to DES for desulfurization
In 2001, Bösmann et al. [59] suggested a new approach for deep
desulfurization of diesel fuels using ILs as extractant solvents. Since
then, more than 350 studies have been published addressing the use of
(task specific) ILs in different technologies to remove sulfur from fossil
fuels. During these nearly 2 decades, large efforts have been undertaken
to implement ILs as key players in desulfurization processes. If at
beginning the studied ILs were essentially neutral ILs and Lewis acidic
ILs, with imidazolium cation being the most used [59,60], in the
following years other cations, such as pyridinium-based, aroused the
interest of the research community [61–65]. These preliminary works
allowed reaching some important conclusions: desulfurization is equally
affected by the structure and the size of the cation and the anion; the
longer the alkyl side chain in the imidazolium cation, the higher the
extraction yield; and the mass ratio of IL to fuel influences the extraction
yield [66]. However, some problems were also pointed out: for example,
in the case of the alkyl side chain length of the ILs cations, if on the one
hand larger chain lengths favoured desulfurization rates, on the other
hand, they increased the solubility of ILs in the fuel phase [64]. Thus, in
order to overcome this and other problems, new ILs and approaches
were introduced. In 2009, Kuhlmann et al. [67] studied and compared,
for the first time, two different desulfurization processes based on the
same IL: a simple liquid–liquid extraction (LLE) and a solid–liquid
extraction using supported IL phases (SILP). After the screening of ILs
through liquid–liquid extraction, the SILPs were prepared by dispersing
the IL as a thin film on highly porous commercial silica. The supported
systems exhibited a significantly higher extraction performance than the
conventional LLE due to their large surface area, reducing the sulfur
content from 500 ppm to less than 100 ppm in one stage, against 200
ppm in the conventional method. Even though this technology allows to
overcome problems associated to the high viscosity of some ILs, the mass
ratio of SILPs to fuel used was very high, since equal amounts of sorbent
and fuel were used. Moreover, leaching of the IL to the fuel was also
detected, hindering its industrial application. In 2010, Asumana et al.
[68] performed EDS with low-viscous ILs based on the dicyanamide
anion, combined with different cations – two imidazoliums, a cyclic
thiophenium and a tetrahedral trialkylsulfonium - concluding that the
aromatic ILs were more efficient in extracting thiophene (Th) and DBT.
Negligible sulfur content was achieved in model fuel phases after 4–5
extraction cycles, independently of the temperature and the initial sulfur
content [68–70].
Regarding Lewis acidic-based ILs, most of them are imidazolium
based [59,71–74]. For example, Ko et al. [73] and Ban et al. [71] reached
extraction efficiencies of DBT, benzothiophene (BT) and 4,6-dimethyldi
benzothiophene (4,6-DMDBT) higher than 96%, employing a series of
imidazolium-based ILs combined with metal salts (e.g. FeCl3). This good
performance was attributed to both the high Lewis acidity and the
fluidity of the extractants. It is important to note that these ILs con
taining metal ions (e.g. [C1C8Im]Cl.xFeCl3) are more effective in
removing sulfur compounds compared those bearing common anions (e.
g. [BF4], [PF6]), a fact that has been justified by π-complexation between
metal anions and the sulfur atom in aromatic rings [75]. In 2012, Li et al.
[76], motivated by the importance of developing new ILs with cost
effective materials, easy synthesis, low viscosities and sustainable
recycling methods, introduced amine-based protic ILs in EDS. These
aspects - high cost, high viscosity, toxicity - are exactly what prevented
the overall ILs industrialization, leading to a general slowdown in the
interest of the scientific community in these solvents. That fact is evident
0
10
20
30
40
50
60
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
Number
of
publications
per
year
Year
IL DES
Fig. 4. Comparison of the number of publications per year addressing ILs
materials (blue) and DES (orange) in sulfur removal (data collected from Web of
science on March 20th, 2020). (For interpretation of the references to colour in
this figure legend, the reader is referred to the web version of this article.)
F. Lima et al.
5. Fuel 293 (2021) 120297
5
in Fig. 4, where it can be seen that since 2014, the number of publica
tions addressing the use of ILs in the desulfurization area has dropped,
which has been offset by the increased interest in DES. Nevertheless, the
search for new ILs to be used in desulfurization continues, and there are
many families still being tested, such as the case of phosphonium-based
ILs [77–81]. Since there are excellent reviews on the use of ILs in this
area [36,46,47,65,66], this review will mainly make use of ILs results for
comparative purpose only.
Nevertheless, there are some important conclusions from the ILs
studies that must be highlighted since they can be of use to DES com
munity. The mechanisms of desulfurization is one of them, as it has a
great impact on the design of the most suitable solvents for the process.
In early works, π–π interactions between the aromatic rings of ILs and
the unsaturated bonds of aromatic sulfur species were proposed to be
responsible for the high extraction efficiencies [82–84]. This explains
the early wide use of conventional ILs, characterized by their aromatic
cationic cores, such as the case of imidiazolium- and pyridinium-based
ILs. However, when other families of ILs began to be tested, such as
tributylmethylammonium methylcarbonate [85] or 1,1,3,3-tetramethyl
guanidinium lactate [86], which had no aromatic cores and also yielded
good desulfurization efficiencies, it became obvious that other kind of
interactions and/or factors contribute in a decisive way to the extraction
ability of ILs. For example, Zhou et al. [87] showed that the interactions
between Th and imidazolium-based ILs with [BF4] or [PF6] anions were
mainly electrostatic attractions. Regarding non-aromatic ILs, Lu et al.
[86], studied several piperazinium-based ILs in the removal of various
aromatic model sulfur compounds and concluded that these, halogen-,
aromatic- and metal-free ILs were able to extract efficiently aromatic
model sulfur compounds due to their polarity. These authors showed
that DBT and its derivatives have delocalized π electrons that can be
polarized by these ILs. In that work, DBT was removed more effectively
than BT and Th, which is in accordance with their electron density on
the sulfur atoms, which follows the order: DBT > BT > Th [78,86]. In
2012, Li group. [76], through 1
H NMR studies, proposed the hydrogen
bond formation between the active hydrogen atoms of amine-based ILs
and the sulfur atoms of DBT as the extraction mechanism. In conclusion,
it is not possible to identify one single mechanism for the extraction of
sulfur compounds from fuels using ILs.
In 2013, Li and co-workers [50], motivated by their finding - the
main driving force of the EDS process could be the hydrogen bonding -
and also by the possibility to design and synthesize cost effective sol
vents, introduced DES into desulfurization. They studied several com
binations of hydrogen bond acceptors (HBA) and HBD, as extractants of
BT from n-octane, using tetrabutylammonium chloride and poly
ethylene glycol - TBAC:PEG (1:2) – reaching up to 83% of extraction
efficiency in one cycle and 99.48% after five cycles. These authors also
showed that both HBA and HBD play a relevant role in the extraction
efficiency. Since then, more than 60 publications came out addressing
the use of DES in sulfur removal from fossil fuels (Fig. 4). The DES
commonly used for desulfurization typically belong to type III [52], thus
being formed by a salt (HBA) and a HBD. These DES are typically
composed by a quaternary ammonium or phosphonium salt as HBA, and
carboxylic acids [24,88], or glycols [89,90] as HBDs. More recently,
binary- and ternary-DES based on metal ions have also been widely
studied [53,91–95].
Until now, and according to Fig. 5, DES have been applied in sulfur
removal according to two main non-HDS approaches: EDS and ODS. In
general, the chemistry of the DES depends on the approach, being acidic
DES more frequently used in ODS. In Tables 1–3 a summary of the DES
used in EDS and ODS is presented. In both technologies, several aspects
regarding the operational process have been deeply studied, namely of
temperature, ratio between fuel and DES, stirring speed and time of
contact. The effect of DES composition on the extraction capacity has
also been widely explored.
3. DES in EDS
Extractive desulfurization is one of the most attractive non-HDS
technologies. Given the distinct properties of sulfur compounds and
hydrocarbons, it is possible to select solvents that preferentially solu
bilize the sulfur compounds, with a minimum solubility in fuels [13].
This technology represents an alternative to HDS, since it can be oper
ated at mild conditions, it does not interfere with final quality of the
fuels and the extracted sulfur compounds can be reused as raw materials
[6]. The requisites that a suitable extractant must fulfil are: i) high
solubility and selectivity towards sulfur compounds over hydrocarbons;
ii) different boiling point and different density from fuel, so that an
efficient regeneration and separation can be attained; iii) high thermal
stability; iv) null solubility in fuel and negligible toxicity [75]. The
major drawback is the need of large amounts of solvent, typically
organic solvents. The molecular nature of commonly used organic sol
vents brought many important health and environmental concerns
which, together with their low efficiency and the difficult regeneration,
limited so far the final application of EDS in industries [112]. In order to
overcome these limitations, ILs were proposed as alternative solvents for
extractive desulfurization of fuels [59] but, despite the good results, the
industrial use of ILs in desulfurization is still not feasible. Although some
toxic issues have been pointed out, it is mainly their high production cost
which has not yet allowed ILs industrial exploitation.
DES were proposed as solvents in EDS for the first time in 2013. Li
et al. [50] pioneer work studied several DES, mainly neutral, to remove
BT from n-octane. These authors tested ChCl-, TMAC-, and TBAC-based
DES, and found out that the TBAC-based DES were the most successful in
terms of BT extraction, reaching up to 82.83% extraction efficiency in
one cycle and 99.48% in five cycles, under optimal conditions. In
addition, through FTIR and 1
H NMR studies, they pointed out that the
hydrogen bond formed between DES and BT were responsible for the
high desulfurization efficiencies.
In 2014, Gano et al. [91] suggested the application of DES based on
ferric chloride (FeCl3) and zinc chloride (ZnCl2), and study the ability of
30 DES to solubilize Th and DBT in a range of temperatures. They
combined anhydrous metal salts with ammonium and phosphonium
salts as HBA and also with ethylene glycol as HBD, concluding that
FeCl3-based DES exhibited much higher solubilities for DBT than the
ZnCl2-based DES. Motivated by this study, in 2015, the same group
evaluated the extractive desulfurization ability of DES formed by TBPB
salt and FeCl3 [53]. Using a model fuel, extraction efficiencies of 64%
and 44% for DBT and Th, respectively, were obtained, achieving deep
desulfurization after 5 extraction cycles. In the case of a real fuel, an
extraction efficiency of 32% of total sulfur was achieved. In agreement
EDS
ODS
Others
Fig. 5. Application of DES according different desulfurization technologies
(data collected from Web of science on March 20th, 2020).
F. Lima et al.
6. Fuel 293 (2021) 120297
6
with several studies using ILs containing metal ions [71,73], these au
thors concluded that sulfur compounds removal is driven by the for
mation of π-complexation between metal halide and the aromatic sulfur
compounds. Since then, the use of metal ions to produce both binary
(Table 1) and ternary (Table 2) DES for desulfurization has been broadly
explored, with several metal salts being used, such as AlCl3, SnCl2, NiCl2
Table 1
List of binary DES used in extractive desulfurization (EDS).
DES Ref.
HBA HBD/CA Abbreviation Molar ratio
Choline Chloride Malonic acid ChCl:MA (1:1) [50]
Glycerol ChCl:Gly (1:2) [50]
Ethylene glycol ChCl:EG (1:2) [50]
Propionic acid ChCl:PrA (1:2) [50]
Tetramethylammonium chloride Glycerol TMAC:Gly (1:2) [50]
Ethylene glycol TMAC:EG (1:2) [50]
Phenylacetic acid TMAC:PAA (1:2) [50]
Malonic acid TBAC:MA (1:2) [50]
Tetrabutylammonium chloride Glycerol TBAC:Gly (1:1) (1:2) (1:3) [50] and [96]
Tetraethylene glycol TBAC:TtEG (1:2) [50]
Ethylene glycol TBAC:EG (1:1) (1:2) (1:3) [50] and [96]
Phenylacetic acid TBAC:PAA (1:2) [50]
Caproic acid TBAC:CaA (1:2) [50]
Acetic acid TBAC:AcA (1:2) [50]
Propionic acid TBAC:PrA (1:2) [50]
Polyethylene glycol TBAC:PEG (1:2) [50]
Malonic acid TBAC:MA (1:1), (1:2) (1:3) [96]
Tetrabutylphosphonium bromide Iron chloride TBPB:FeCl3 (1:1.5) (2:1) [91] and [53]
Stannous chloride dihydrate TBPB:SnCl2⋅2H2O (1:1) [92]
Butyltriphenylphosphonium bromide Iron chloride BTPPB:FeCl3 (1:1)
(1.5:1)
[91]
Zinc chloride Ethylene glycol ZnCl2:EG (1:3), (1:4), (1:5), (1:6) [91]
Triethylamine Formic acid TEA:FoA (1:2), (1:3), (1:5) [24] and [97]
Acetic acid TEA:AcA (1:2), (1:3), (1:5) [24]
Propionic acid TEA:PrA (1:3), (1:5) [24]
Tetrabutylammonium bromide Acetic acid TBAB:AcA (1:1) [97]
Propionic acid TBAB:PrA (1:1) [97]
Formic acid TBAB:FoA (1:1) [97]
Oxalic acid TBAB:OA (1:1) [97]
Malonic acid TBAB:MA (1:1) [97]
Adipic acid TBAB:AA (1:1) [97]
Sulfolane TBAB:Sul (1:7) [98]
Triethylene glycol TBAB:TEG (1:4) [98]
Ethylene glycol TBAB:EG (1:4) [98]
Imidazole TBAB:Im (1:1) [99]
Ethylene glycol TBAB:EG (1:2) [100]
Glycerol TBAB:Gly (1:2) [100]
Polyethylene glycol 200 TBAB:PEG200 (1:2) [101]
Polyethylene glycol 600 TBAB:PEG600 (1:2) [101]
Methyltriphenylphosphonium bromide Ethylene glycol MTPPB:EG (1:4) [98]
1-methylimidazole Propionic acid MIM:PrA (1:1) [102]
Nitric acid MIM:NiA (1:1) [102]
Diethanolamine Propionic acid DEA:PrA (1:1) [102]
Butylpyridinium bromide Malonic acid BPyB:MA (1:0.5) – (1:3) [88]
Succinic acid BPyB:SA (1:1) [88]
Glutaric acid BPyB:GA (1:1) [88]
Adipic acid BPyB:AA (1:1) [88]
Table 2
List of ternary DES used in extractive desulfurization.
DES Ref.
HBA HBD CA Abbreviation Molar ratio
Tetrabutylammonium chloride Polyethylene glycol Iron Chloride TBAC:PEG:FeCl3 (1:2:0.1), (1:2:0.25), (4:1:0.05) [93]
Zinc Chloride TBAC:PEG:ZnCl2 (1:2:0.1), (1:2:0.25), (4:1:0.05) [93]
Nickel Chloride TBAC:PEG:NiCl2 (1:2:0.25) [93]
Cobalt Chloride TBAC:PEG:CoCl2 (1:2:0.25) [93]
Cooper Chloride TBAC:PEG:CuCl2 (1:2:0.25) [93]
Benzoic acid Iron chloride TBAC:BzA:FeCl3 (1:2:0.1) [93]
Propionic acid TBAC:PrA:FeCl3 (1:2:0.1) [93]
Tetraethylammonium chloride Polyethylene glycol Iron Chloride TEAC:PEG:FeCl3 (1:2:0.1) [93]
Choline chloride Formic acid Iron Chloride ChCl:FoA:FeCl3 (1:2:0.1) [93]
Ethylene glycol Zinc chloride ChCl:EG:ZnCl2 (1:2:0.1) [93]
Iron chloride ChCl:EG:FeCl3 (1:4:1) [94]
Acetic acid Iron chloride ChCl:AcA:FeCl3 (1:2:0.1) [93]
Phenol Iron chloride ChCl:Phenol:FeCl3 (1:2:1) [94]
F. Lima et al.
7. Fuel 293 (2021) 120297
7
and CoCl2 [92–94,113,114]. Nevertheless, DES with FeCl3 are still the
most efficient in the removal of sulfur compounds, as described in the
first screening [91], which is also in line with the studies on metal-based
ILs for desulfurization [115]. In Table 4, a comparison of EDS sulfur
compounds extraction efficiency using metal-based ILs and metal-based
DES is presented. In the case of ILs, metal salts have been mostly com
bined with imidazolium ILs, with different alkyl chains lengths and
anions. It is possible to conclude that, in all cases, the presence of the
metal salt improves the desulfurization efficiency according to the
following order: FeCl3 > MoCl2 > ZnCl2 > CuCl2 > MnCl2 > CoCl2.
Nevertheless, that beneficial effect seems to be dependent on the metal
salt quantity. From the study of Ren et al. [115] is possible to conclude
that there is an optimal proportion of FeCl3, since increasing the molar
ratio of FeCl3 to [BMIM]HSO4 to values higher than 1, reduces again the
extraction efficiency of DBT. This can possibly be explained by the fact
that increasing the amount of FeCl3 allows increasing the number of
complexes formed between the anion and the metal salt, up to a point
that there are not enough anions to form complexes, with the increase in
viscosity also hindering the mass transfer rate. Regarding metal-based
DES, the metal salts tested are essentially those tested in ILs, with
similar beneficial effects on desulfurization (FeCl3 > ZnCl2 > CoCl2 >
NiCl2). However, the effect of the metal in DES is not as pronounced as in
the ILs. In fact, more studies are needed, since it is difficult to understand
the role of the metal salts, especially in ternary DES.
In another vein, and taking advantage of FeCl3 magnetic features,
Khezeli et al. [94] prepared magnetic ternary DES to extract Th, through
an ultrasound assisted liquid–liquid microextraction, as shown in Fig. 6.
After the dispersion of the magnetic DES in the model fuel, ultrasound
irradiation was used to enhance surface area and thus the extraction
capacity and, in the end, the microdroplets were simply collected with a
magnet. With this methodology, almost 100% of extraction efficiency of
Th was reached, using a small amount of DES, thus presenting a cheap,
easy and effective alternative to metal ILs.
Besides all advantages, the use of FeCl3 brings a lot of concerns
regarding the operational process, since it is a strong Lewis acid and thus
very corrosive. Other kind of DES, such as carboxylic acid-based DES
[24,88,97], have also been tested. For instance, Li et al. [97], prepared a
series of DES by the combination of an ammonium salt (TBAB) and
several carboxylic acids. In that work, it was concluded that TBAB:FoA
(1:1) was the best extractant for Th, BT and DBT, and FTIR and NMR
studies showed that hydrogen bonding between the sulfur compound
and the DES was the main driving force of the EDS process. More
recently, the same group used DES composed of dicarboxylic acids and
butylpyridinium bromide (BPyB), and pointed out that the combination
with malonic acid in a molar ratio of 1:1 was the most effective system
[88].
Nevertheless, DES composed of neutral compounds seem to be more
attractive to EDS, as they do not present any corrosion problems and, in
general, allow good or even better results than the acidic-based DES. In
Table 5, it is possible to compare the EDS capacity of neutral and acidic
DES. For instance, Li et al. [50] extracted BT with several neutral (PEG-
based) and acidic (propionic acid-based) DES, reaching very similar
results. Shu et al. [96] also performed EDS with several DES composed
by TBAC with Gly/ EG/ MA, which presented different EDS capacity,
following the order EG > Gly > MA. Moreover, Wang et al. [24]
extracted several model sulfur compounds with acidic DES, but extrac
tion efficiencies lower than 52.3% were achieved. In 2017, Rahma et al.
[101] explored the use of DES composed by TBAB salt and two different
light grades of polyethylene glycol, reaching to 100% and 95.15% of
extraction efficiency of DBT and Th, respectively, after three extraction
cycles. Regarding the use of glycol-based DES in EDS, they have been
mainly combined with ammonium and phosphonium salts
Table 3
List of DES used in desulfurization of fuels according an oxidative approach (ODS).
DES Ref.
HBA HBD Abbreviation Molar ratio
Choline chloride Formic acid ChCl:FoA (1:2) [103]
Acetic acid ChCl:AcA (1:2) [103]
Propionic acid ChCl:PrA (1:2) [103]
Butyric acid ChCl:BuA (1:2) [103]
Valeric acid ChCl:VaA (1:2) [103]
Glycerol ChCl:Gly (1:2) [104]
Choline chloride 4-aminosalicylic acid ChCl:PAS (1:2) [51]
p-toluenesulfonic acid ChCl:p-TsOH (1:1), (1:2) [51]
Tetrabutylammonium bromide TBAB:p-TsOH (1:2) [51]
Tetraethylammonium bromide TEAB:p-TsOH (1:2) [51]
Tetrabutylammonium chloride TBAC:p-TsOH (1:2) [51]
Tetraethylammonium chloride TEAC:p-TsOH (1:2) [51]
5-sulfosalicylic acid TEAC:SSA (1:2) [51]
Tetramethylammonium chloride Oxalic acid TMAC:OA (1:2) [105]
Tetrabutylammonium chloride TBAC:OA (1:1), (1:2), (1:3) [105]
Triflic acid TBAC:TfOH (1:1) [106]
Tetraethylammonium chloride TEAC:TfOH (1:1) [106]
Butyltrimethylammonium chloride Urea BTAC:Urea (1:2) [107]
Choline chloride Oxalic acid ChCl:OA (1:2) [105]
Polyethylene glycol 200 ChCl:PEG200 (1:2) [90] and [107]
Butylene glycol ChCl:BG (1:2) [90] and [107]
Ethylene glycol ChCl:EG (1:2) [90] and [107]
Glycerol ChCl:Gly (1:2) [90], [104], [107] and [108]
Propionic acid ChCl:PrA (1:2) [90] and [107]
Malonic acid ChCl:MA (1:1) [90]
Urea ChCl:Urea (1:2) [90] and [107]
Triflic acid ChCl:TfOH (1:1), (1:1.5), (1:2) [106]
Acetic acid ChCl:AcA (1:2) [107]
Formic acid ChCl: FoA (1:2) [107]
Zinc chloride Phenylpropanoic acid ZnCl2:PPA (1:2) [109]
Propionic acid ZnCl2:PrA (0.1–0.6:1) [110]
L-Proline Oxalic acid L-Pro:OA (1:2) [111]
Glutaric acid L-Pro:GA (1:2) [111]
Malonic acid L-Pro:MA (1:2) [111]
p-toluenesulfonic acid L-Pro:p-TsOH (1:2) [111]
F. Lima et al.
8. Fuel 293 (2021) 120297
8
[100,116–118].
More recently, new DES were proposed. For example, Kucan et al.
[119] tested DES composed of betaine with 3 different glycols as solvent
for EDS. However, the extraction efficiency of Th, from a simulated
gasoline, did not even reached 50%. Moreover, when a test with a real
gasoline was performed, none of the studied betaine-based DES showed
potential to remove sulfur compounds.
Apart all of these experimental studies, some theoretical work has
been carried out regarding EDS with DES. For instance, quantum
chemistry [120,121] and molecular dynamic simulations [122,123]
have been applied to better understand the mechanism of interaction
between the sulfur compounds and the different DES. In 2018, Gutiérrez
et al. [123] selected a common DES and studied its interaction with Th,
BT and DBT, as well as the interfacial properties between the DES and
the model oil, for different amounts of sulfur. The authors found that
sulfur compounds interact preferentially with the HBA cation, and that
the sulfur compounds have large affinities for the specific DES. In
another vein, other computational tools, such as the case of COSMO-RS
[124] have also been explored for instance to perform a pre-screening of
the DES. Despite the importance of these works, it would be extremely
important, just as in the case of experimental works, that real fuel
matrices would be considered, since until now they are based only on
very simplified model systems.
Table 4
Comparison of metal-based ILs versus metal-based DES as solvents for EDS of model fuels.
IL/DES EE
(%)
Sulfur
compound
Sulfur amount
(ppm)
Model fuel Conditions Ref
IL a
[BMIM]HSO4 48 DBT 500 n-octane 1 h, 27 ◦
C, 1:1 (vIL/vFuel); [115]
a
[BMIM]HSO4 : 0.5FeCl3 65.9
a
[BMIM]HSO4 : 1FeCl3 100
a
[BMIM]HSO4 : 1.5FeCl3 46.1
a
[BMIM]HSO4 : 1CuCl2 79
a
[BMIM]HSO4 : 1CoCl2 60.9
a
[BMIM]HSO4 : 1ZnCl2 86.8
a
[BMIM]HSO4 : 1MoCl2 96.1
a
[BMIM]HSO4 : 1MnCl2 63.5
b
[BMIM]Cl 17.2 DBT 5000 n-heptane [73]
b
[BMIM]Cl : 1FeCl3 42.2
b
[BMIM]Cl : 2FeCl3 100
a
[BMIM]Cl : 2FeCl3 23.4 Th 680 n-heptane + toluene 0.5 h, room T, 1:5 (wIL/wFuel); [74]
a
[BMIM]Cl : 2AlCl3 16
c
[Omim]Cl 22.9 DBT 200 n-octane 0.5 h, 25 ◦
C, 1:5 (wIL/wFuel); [71]
c
[Omim]Cl : 0.5FeCl3 24.4
c
[Omim]Cl : 1.5FeCl3 80.2
c
[Omim]Cl : 2.5FeCl3 99.5
DES TBPB:SnCl2⋅2H2O (1:1) 69.6 Th 500 n-decane + cyclohexane + iso-octane +
toluene
100 min, 30 ◦
C, 1:2 (wDES/
wFuel);
[92]
47.3 DBT 500
TBPB:FeCl3 (2:1) 44 Th 500 n-decane + cyclohexane + iso-octane +
toluene
163 min, 30 ◦
C, 1:2 (wDES/
wFuel);
[53]
64 DBT 500
TBAC:PEG:FeCl3
(1:2:0.1)
81.15 DBT 1600 s n-octane 30 min, 25 ◦
C, 1:1 (wDES/
wFuel);
[93]
TBAC:PEG:FeCl3
(4:1:0.25)
89.53
TBAC:PEG:FeCl3
(1:2:0.25)
~ 80.7
TBAC:PEG:ZnCl2
(1:2:0.25)
~ 80.5
TBAC:PEG:CuCl2
(1:2:0.25)
~ 80.2
TBAC:PEG:CoCl2
(1:2:0.25)
~ 79.3
TBAC:PEG:NiCl2
(1:2:0.25)
~ 78.1
ChCl:Phenol:FeCl3
(1:2:1)
100 Th 1000 n-heptane 5 min, 25 ◦
C, 0.02:1.5 (vDES/
vFuel);
[94]
ChCl:EG:FeCl3 (1:4:1) 14
a
1-butyl-3-methylimidazole
b
3-butyl-1-methylimidazole
c
1-methyl-3-octylimidazole
Fig. 6. Schematic representation of an ultrasound assisted liquid–liquid
extraction of Th using magnetic DES ().
reproduced from [94]
F. Lima et al.
9. Fuel 293 (2021) 120297
9
4. DES in ODS
Oxidative desulfurization (ODS) is also a promising alternative
among the non-HDS technologies, since it enables the oxidation of sulfur
in fuels, at low temperature and atmospheric pressure, to their corre
sponding sulfones or sulfoxides, which are more polar and thus easily
removed by adsorption or a simple liquid–liquid extraction [125,126].
Regarding the process operation, ODS can be carried out in as single
stage process, where oxidation and extraction may occur simultaneously
or in a two steps process, where the oxidation step is followed by the
extraction step. Concerning the oxidation step, both catalytic oxidation
(ECODS) or simple chemical oxidation (EODS) have been explored.
While in ECODS, a catalyst, an oxidant (usually hydrogen peroxide) and
an extractant are required, in EODS, only the last two are required since
DES and ILs can advantageously act, simultaneously, as catalyst,
extractant and reaction media. In addition, within non-hydrogen
peroxide systems, different oxidation methods such as radiation assis
ted oxidation, ultrasound assisted oxidation, photo-oxidation, electro
chemical catalytic oxidation and plasma oxidation have been tested
[28,36,127]. Nevertheless, the extracting agent always needs to be
present. As expected, the extraction efficiency of the oxidized sulfur
compounds depends on the solvent’s polarity. However, polarity is not
the only criteria for the selection of a suitable solvent for ODS, since
other properties such as density, boiling point, freezing point, and sur
face tension also need to be taken into account [125]. Just like any LLE
process, the correct choice of a solvent will allow good separation and
recovery of the solvent for recycling and reuse [28].
Although ODS as an alternative to HDS has been widely explored in
the last decades, only recently DES have become the focus of the
attention as the new alternative solvents for ODS. Already in 2015, Zhu
et al. [103] applied DES in an ODS approach to remove sulfur com
pounds for the first time. These DES, acidic in nature, were used as
extractant media, being the system also composed by air as oxidant, an
aldehyde and UV light irradiation. In this way, a metal-free photo
chemical oxidation system was developed using DES. Regarding ECODS,
several common catalysts (e.g. phosphotungstic acid) [90] have been
tested, as well as new catalysts, as is the case of the polyoxometalates
(POMs) [107,128,129]. Jiang et al. [107] synthesized a series of SO3H-
functionalized acidic POMs and compared their catalytic activity with
neutral POMs, using H2O2 as oxidant and DES as extractants. The acidic
POMs exhibited higher catalytic performance, reaching 100% of DBT
removal using the ChCl:AcA (1:2) DES as extractant. Using other organic
solvents, the sulfur removal did not reach 10%, highlighting the crucial
role of DES in this extraction process. In addition, these authors
Table 5
Sulfur extraction efficiencies, according an extractive desulfurization approach, by different DES, mainly based on acidic and neutral HBD.
DES EE /
%
Sulfur compound [Sulfur compound] /
ppm
Model fuel Time
/min
Temp
/◦
C
Ratio (DES:
fuel)
Ref.
TBAC:PrA (1:2) ≈70 BT 1600 n-octane 30 25 1:1 w/w [50]
≈ 52 0.5:1
≈77 1.5:1
≈82 2:1
TBAC:PEG (1:2) ≈71 BT 1600 n-octane 30 25 1:1
≈ 53 0.5:1
≈79 1.5:1
≈83 2:1
TBAC:EG (1:2) 64 2-MT + BT 1000 n-octane 20 30 1:1 w/w [96]
TBAC:Gly (1:2) ≈56
TBAC:MA (1:2) ≈49
TEA:FoA (1:2) 48.1 DBT + BT + 4,6-DMDBT
+ RSH
500 + 250 + 250 + 250 n-octane 10 30 1:2 w/w [24]
TEA:FoA (1:3) 37.5
TEA:AcA (1:2) 48.9
TEA:AcA (1:3) 49.4
TEA:PrA (1:2) 52.3
TEA:PrA (1:3) 51.6
TBAB:FoA (1:1) 55.8 Th 500 30 30 1:1 w/w [97]
63.8 BT 500
60 DBT 500
TBAB:Sul (1:7) 35.07 Th 100,000 n-heptane 6*60 25 1:1 w/w [98]
TBAB:TEG (1:4) 20.67
MTPPB:EG
(1:4)
21.42
TBAB:EG (1:4) 16.08
MIM:PrA (1:1) 53.6 DBT n-octane 10 30 1:2 w/w [102]
DEA:PrA (1:1) 18 20 30
TBAB:PEG200
(1:2)
53.06 Th 500 Cyclohexane + isso-octane + n-
decane + toluene
30 25 1:1 v/v [101]
75.47 DBT 500
TBAB:PEG600
(1:2)
62.16 Th 500
82.40 DBT 500
BPyB:MA (1:1) 95.86 Th 500 n-octane 30 40 1:2 w/w [88]
88.37 2-MT
77.44 DBT
72.69 BT
70.66 Diethyl sulfide
63.43 1-Heptanethiol
TBAB:Im (1:1) 47 Th 1000 n-decane + cyclohexane + iso-octane
+ toluene
95 30 1:2 w/w [99]
70 DBT
F. Lima et al.
10. Fuel 293 (2021) 120297
10
evaluated the effect of the nature of DES on the desulfurization effi
ciency, and concluded that acidic and basic DES worked better than
neutral ones, reaching extraction efficiencies from 44.6% with ChCl:BG
(1:2) DES to 90.8% with ChCl:U (1:2) and even higher for DES composed
of acids. A possible explanation for these results might be the fact that
the acidic and basic DES allowed the dissolution of the catalyst and thus
a homogenous system is attained, increasing the performance of the
oxidative step. In Table 6, a summary of literature on ECODS capacity of
several DES combined with additional catalysts is presented.
Since some DES can act simultaneously as extractants and catalysts,
the oxidation and extraction occur simultaneously, as illustrated in
Fig. 7. Consequently, a much simpler, effective and integrated system
can be designed, in order to effectively oxidize the sulfur compounds
and separate the two phases. As the oxidant is in the DES phase, the
sulfur compounds are first extracted into the DES phase and then
oxidized, which allows the diffusion of more sulfur compounds from the
diesel phase to the DES phase. Taking into account the state of art, a lot
of attention is being payed to this integrated approach, essentially for
economical purposes.
Table 7 presents a literature overview of the use of DES in EODS
approach. In 2015, Lu et al. [105] reported high activity of several
oxalate-based DES, in the oxidative desulfurization of DBT, under mild
reaction conditions without any additional catalyst. Their results on DBT
removal were very dependent on the HBA and on the molar ratio of the
DES, varying from 41% to 91%, according to the order: TMAC < ChCl <
TBAC. The best result was obtained using TBAC:OA DES in a proportion
of 1:2 (Table 7). Moreover, from electron paramagnetic resonance (EPR)
characterization, a dual activation model mechanism was proposed,
meaning that DBT is first extracted to DES, interacting then with the
system in the DES phase. Due to the strong interaction between DBT and
DES, the aromaticity of DBT is weakened which allows its oxidation by
the peracids formed from the oxidation of carboxylic acids in DES by
H2O2. In the same year, Yin et al. [51] reported a similar approach to
extract BT from n-octane, and pointed out again the importance of acidic
DES, establishing a correlation between the DES acidity and the desul
furization efficiency. More recently, several works were published using
different compositions of DES, trying to ascertain the role of DES acidity
in the desulfurization efficiency. For instance, the group of Rong-xiang
Zhao [106,109,110] worked with DES composed of zinc chloride and
several acids, while Hao et al. [111] reported the use of L-proline-based
DES (combined with organic acids). However, in this last case, the au
thors could not find a correlation between the DES’s acididy and
desulfurization ability. Therefore, the acidic character of DES containing
metals does not seem to be the main factor accounting for deep oxidative
desulfurization. A more recent work, showed that the combination of
ternary DES composed by CoCl2, ChCl and PEG with peroxymonosulfate
as a solid oxidant allowed nearly 100% of DBT removal [130]. Other
authors [131], applied caprolactam-based acidic DES in EODS, pro
posing also a dual activation mechanism. However, the focus of that
study was the relationship between the ODS and hydrogen bonding
strength, by investigating the effect of DES composition (molar ratio).
The authors were able to correlate the highest ODS capacity with the
DES maximum hydrogen-bonding capacity and the lowest Tg, suggest
ing hydrogen bonding plays an important role in desulfurization ca
pacity of these DES.
5. EDS versus ODS - influence of operational parameters
5.1. Effect of the DES acidity/nature
As reported above, inspired by previous works which advanta
geously introduced acidic ILs in the desulfurization field [72,126],
different groups have been using several DES with considerable acidity
to sulfur removal, either through extractive [113] or oxidative ap
proaches [110,132].
Table 6
Sulfur removal efficiencies in catalytic oxidative desulfurization (ECODS) using DES.
DES
(extractant)
EE
(%)
Oxidant Catalyst Model
fuel
Sulfur
compound
Sulfur amount
(ppm)
Time
(min)
Temp.
(◦
C)
DES:fuel
ratio
Ref.
ChCl:AcA (1:2) 96.3 Air Isobutylaldehyde (IBA) n-octane BT, DBT, 4,6-
DMDBT
500* 180 30 1:5 v/v [103]
ChCl:PEG200
(1:2)
99.1 H2O2 Phosphotungstic acid
(HPW)
n-octane DBT 500* 60 50 1:10 v/v [90]
ChCl:Gly (1:2) 97.06 H2O2 Cu-Fe/TiO2 Dodecane DBT 100 30 + 60 25 1:5 v/v [104]
ChCl:AcA (1:2) 100 H2O2 [PSTEtA]3PW12O40 n-octane DBT – 150 40 1:5 v/v [107]
ChCl:PEG200
(1:2)
68.9
ChCl:EG (1:2) 81.5
ChCl:BG (1:2) 44.6
ChCl:Gly (1:2) 47.2
ChCl:FoA (1:2) 85.0
ChCl:PrA(1:2) 98.8
ChCl:Urea (1:2) 90.8
BTAC:AcA (1:2) 95.6
*concentration of elemental sulfur.
Fig. 7. Suggested reaction mechanism of the ODS of DBT, using ChCl:PEG200
(1:2) DES ().
adapted from [90]
F. Lima et al.
11. Fuel 293 (2021) 120297
11
In 2015, Yin et al. [51] prepared acidic DES based on several strong
acids, such as p-toluenesulfonic acid (p-TsOH), and quaternary ammo
nium salts and showed that DES acidity was the main driving force of
their desulfurization capacities using EODS approach (where the DES
acted as extractant media and catalyst). Moreover, the stronger the
acidity of DES, the higher the desulphurization efficiency. Following this
study, the Rong-xiang Zhao’s group published three studies, where the
effect of the DES acidity in the EODS approach was adressed
[106,109,110]. In one of those studies, acidic DES based on quaternary
ammonium salts were explored, but combined with a different acid -
triflic acid [106]. In the other two works, a different kind of acidic DES
were studied - Lewis-acidic DES - based on a combination of zinc chlo
ride (ZnCl2) and phenylpropanoic [109] and propionic [110] acids.
From these studies, a similar conclusion was reached about the direct
correlation between the increase of DES acidity with the increase of the
extraction/oxidation efficiency of the sulfur compounds. However, this
increase seems to be limited, since very strong acidities can lead to the
rapid decomposition of the oxidant (H2O2), which explains the decrease
of the oxidation/extraction capacity of the several systems with high
molar proportion of the acidic component. Regarding the optimal
desulfurization efficiency depicted in Fig. 8, it can be observed that, by
adjusting the molar ratio of the DES components, most of systems
reached sulfur extraction efficiencies from model fuels close to 100%.
However, these results were obtained for simple simulated fuels, where
the commonly used sulfur model compounds varied between BT or DBT.
Interestingly, when the extraction of Th was tested, the highest extrac
tion efficiency values in the literature was only around 40% [106,109].
This fact was explained by the low electron density of thiophene [109].
On the other hand, Hao et al. [111] studied a series of DES based on L-
proline and 4 different organic acids for the DBT removal using EODS
approach, measuring their Hammett acidity functions. However, no
correlation between the acidity of each DES with the corresponding
extraction capacity was found. For example, the DES with the strongest
acidity – L-Pro:OA (1:2) – only reached up to 10% of DBT removal
(Fig. 8).
Regarding the simple extractive approach (EDS), in 2015, Tang et al.
[113] studied the effect of DES acidity in the desulfurization efficiency.
However, no correlation could be establish, since all studied DES had
very similar acidity [113].
In conclusion, in ODS approach most authors report a positive cor
relation between the DES acidity and the extraction efficiency of sulfur
compounds. The acidity in the DES phase can have direct or indirect
(acidic DES allows a better dissolution of several catalyst) positive ef
fects. However, in what concerns acidity of DES in EDS, no direct in
fluence of this parameter on the removal efficiency of sulfur compounds
was observed.
5.2. Effect of the temperature
The influence of the operation temperature on EDS/ODS has been
studied in the process optimization perspective. Regarding the use of ILs,
it is well known that very viscous ILs need an increase in operating
temperature to improve the sulfur removal [112]. In Table 8, a literature
overview on the influence of temperature on the removal of sulfur
compounds using DES is presented. In general, a small increase in
temperature, relative to room temperature, is beneficial in ODS
approach. However, in several cases, when the operation temperature
approaches to 50 ◦
C / 60 ◦
C, the extraction efficiency decreases. For
example, Liu et al. [90] used ChCl:PEG (1:2) DES for the removal of DBT
in ECODS approach and reached an extraction efficiency of 53.7% and
99.1% at 40 ◦
C and 50 ◦
C, respectively, which decreased to 74.3% at
60 ◦
C. The explanation for this fact is the decomposition of the oxidant,
which is usually hydrogen peroxide. Thus, in oxidative approaches, two
reactions are occurring simultaneously: the oxidation of the sulfur
compound and the thermal decomposition of the chemical oxidant
(H2O2). Thois fact explains why at high temperatures the degradation of
the H2O2 leads to a decrease in the oxidation rate [90,107].
Regarding the effect of temperature in EDS, no significant influence
of this parameter was observed. Nevertheless, it is possible to state that
Table 7
Sulfur removal efficiencies according an EODS approach, where the DES acts as reaction media, extractant and catalyst.
DES (extractant and catalyst) EE (%) Oxidant Model fuel Sulfur compound Sulfur amount (ppm) Time (min) Temp (◦
C) DES:fuel ratio Ref.
ChCl:OA (1:2) 71 H2O2 n-octane DBT 500* 180 50 1:5 v/v [105]
TMAC:OA (1:2) 41
TBAC:OXA (1:2) 91
TBAC:p-TsOH (1:2) 99.99 H2O2 n-octane BT 1600* 60 25 1:2 w/w [51]
ChCl:p-TsOH (1:2) 99.99
ZnCl2:PPA (1:2) 99.23 H2O2 n-octane DBT 500 80 50 1:4 v/v [109]
96.12 BT 500
98.4 4,6-DMDBT 500
L-Proline:p-TsOH (1:2) 99 H2O2 n-octane DBT 500* 120 60 1:5 v/v [111]
ChCl:TfOH (1:1.5) 24.35 H2O2 n-octane Th 500 180 40 1:5 v/v [106]
39.53 BT
98.65 DBT
96.8 4,6-DMDBT
ZnCl2:PrA (1:0.5) 99.42 H2O2 n-octane DBT 500 180 30 1:6.7 v/v [110]
98.8 4,6-DMDBT
CoCl2:ChCl:PEG (1:1:1) ~99.7
~80.1
~60.1
PMS n-octane DBT
4.6-DMDBT
BT
500*
500*
500*
60 20 1:3 w/w [130]
*concentration of elemental sulfur.
00 20 40 60 80 100
ZnCl2:PrA (1:10)
ZnCl2:PrA (1:5)
ZnCl2:PrA (1:3.3)
ZnCl2:PrA (1:2.5)
ZnCl2:PrA (1:2)
ZnCl2:PrA (1:1.7)
ZnCl2:PPA (1:2)
ZnCl2:PPA (1:1)
ZnCl2:PPA (2:1)
ZnCl2:PPA (3:1)
ChCl:TfOH (1:1)
ChCl:TfOH (1:1.5)
ChCl:TfOH (1:2)
L-Pro:GA (1:2)
L-Pro:MA (1:2)
L-Pro:p-TsOH (1:2)
L-Pro:OA (1:2)
DBT removal /%
DES
Fig. 8. ODS performance of different acidic DES, for DBT (DES:fuel volume
ratio of 1:5; temperature of oxidation/extraction between 40 and
60 ◦
C) [106,109–111].
F. Lima et al.
12. Fuel 293 (2021) 120297
12
temperatures above 40 ◦
C might have a negative effect in the extraction
of sulfur compounds. Li et al. [93] suggested that the interaction be
tween DES and sulfur compounds is exothermic, which might explain
the slight decrease on the removal efficiency when the temperature is
increased (Table 8).
In general, it can be concluded that while ODS is very much
dependent on the operational temperature, depending on the reactive
system (type of catalyst, DES constitution and oxidant) used, EDS is less
dependent on this variable, enabling this process to be carried out at
room temperature (25 ◦
C), which is an advantage from an economic and
operational point of view.
5.3. Effect of the initial S-content
The initial sulfur content is a very important parameter to be studied,
since the sulfur content of industrial fuel oils can vary over a wide range,
depending on the different sources.
Similarly to ILs, several authors have also been studying the influ
ence of this factor when DES are used in extraction of sulfur compounds,
both in EDS and ODS approaches. From Table 9, where a summary of
literature studies is listed, it can be observed that the impact of the initial
S content has been essentially studied in EDS approaches. Li et al. [50]
studied the use of one acidic and one neutral DES to remove sulfur
compounds from matrices, where the initial S content ranged from 200
ppm to 1600 ppm. In the case of the neutral DES, an increase in the
initial S content leads to a very small drop on the extraction efficiency,
from 72.7% to 71.1%., while the acidic DES capacity seems to be
maximum around 70% at 1200 ppm. Thus, increasing the initial sulfur
content by almost 90% decreased the DES extraction capacity by a
maximum of 2.2%, which means that the initial sulfur content does not
play a decisive role in sulfur extraction by EDS. Similar results were also
reported by Khezeli et al. [94], Rahma et al. [101], Jiang et al. [102],
among others. However, since different DES may have different mech
anisms of sulfur removal, different behaviors might be observed. For
instance, Li et al. [93], reported a significant decrease of the EDS ca
pacity of a metal-based DES with increasing initial sulfur content. As
described in Table 9, the extraction efficiency of TBAC:PEG:FeCl3
(4:1:0.05) DES decreased from 100% to 88.4%, by increasing the sulfur
content from 400 ppm to 2000 ppm.
Regarding ODS approaches, only Xu et al. [130] studied the impact
of sulfur content, reporting a decrease from 100% to 79% in the ECODS
capacity of a ternary DES, when the initial sulfur content increased from
500 ppm to 1000 ppm. This represents a considerable drop on the system
capacity which might hinder the industrial application of this technol
ogy. Thus, more data for ODS approach is in need in order to enable the
confident draw of conclusions.
In conclusion, in a simple EDS approach, the initial sulfur content is
not, in most cases, an important parameter. However, a wider range of
initial sulfur content must be analysed since only sulfur content between
200 and 2500 ppm are studied.
5.4. Regeneration
The reuse and recycling of a DES after the desulfurization process is
very important concerning environmental and economic aspects. In the
past, the volatile nature of the organic solvents prevented their
Table 8
Comparison of the temperature effect on the sulfur extraction efficiency for ODS and EDS approaches.
Method Extraction efficiency at different temperatures (%)
20 ◦
C 25 ◦
C 30 ◦
C 40 ◦
C 50 ◦
C 60 ◦
C Time (min) S. Comp. Ref.
ODS TBAC:OA (1:2) – – – 67.0 91.0 98.0 180 DBT [105]
ChCl:PEG (1:2) – – – 53.7 99.1 74.3 180 DBT [90]
ChCl:AA (1:2) – – 86.7 100.0 92.8 – 150 DBT [107]
ZnCl2:PPA (1:2) – – 94.0 98.0 98.0 90.0 180 DBT [109]
ChCl:TfOh (1:1.5) – – 98.3 98.7 98.7 93.4 180 DBT [106]
ZnCl2:PrA (1:0.5) – – 99.4 99.4 99.4 99.4 180 DBT [110]
EDS TBAC:PEG (1:2) 71.7 70.7 – 65.0 64.1 – 30 BT [50]
TBAC:PrA (1:2) 67.7 70.2 – 64.2 61.6 – 30 BT
TBPB:SnCl2⋅2H2O (1:1) – – 40.0 40.2 40.0 39.5 100 Th [92]
– – 62.2 59.8 57.5 56.0 100 DBT
ChCl:EG (1:2) 64.9 67.0 67.7 65.6 58.2 – 20 2-MT + BT [96]
ChCl:FA (1:1) 67.0 70.5 71.7 68.8 66.4 – 30 Th [97]
78.4 79.9 79.9 78.7 77.5 – 30 DBT
79.6 81.3 82.2 80.7 79.6 – 30 BT
TBAC:PEG:FeCl3 (4:1:0.05) 89.6 89.5 89.0 86.9 84.8 82.6 30 DBT [93]
MIM:PrA (1:1) 54.7 – 53.6 52.2 51.0 – 10 DBT [102]
TBAB:PEG600 (1:2) – 82.5 81.2 79.9 78.7 77.0 30 DBT [101]
Table 9
Effect of the initial concentration of sulfur on the extraction efficiency for different DES.
Initial sulfur concentration (ppm)
DES 200 300 400 500 800 1000 1200 1500 1600 2000 2500 3000 9000 14,000 Ref.
TBAC:PEG (1:2) BT 72.7 – 72.4 – 72.4 – 71.4 – 71.1 – – – – – [50]
TBAC:PrA (1:2) BT 70.8 – 70.5 – 70.8 – 71.4 – 70.8 – – – – –
TBPB:SnCl2⋅2H2O (1:1) Th – – – 40.1 – 41.6 – 41.5 – 40.9 41.8 – – – [92]
DBT – – – 61.3 – 62.4 – 61.9 – 63.3 62.9 – – –
TBPB:FeCl3 (2:1) Th – – – 44.1 – 45.1 – 45.5 – 43.6 44.1 – – – [53]
DBT – – – 64.3 – 63.6 – 64.7 – 63.4 63.4 – – –
TBAC:PEG:FeCl3 (4:1:0.05) DBT – – 100.0 – 93.6 – 91.2 – 89.7 88.4 – – – – [93]
MIM:PrA (1:1) DBT 54.9 55.0 – 54.2 54.5 – – – – – – – – – [102]
TBAB:Im (1:1) Th – – – 45.2 – 45 – 45.4 – 46.3 47.1 – – – [99]
DBT – – – 67.8 – 68.5 – 68.9 – 69.3 69.8 – – –
TBAB:PEG600 (1:2) Th – – – 62.2 – 62.8 – 63 – 62.4 62.8 – – – [101]
DBT – – – 82.4 – 82.6 – 82.1 – 81.1 79.8 – – –
TBAC:PEG400 (1:2) Th – – – 64.3 – – – 64.5 – – – 64.6 63.6 59.0 [133]
DBT – – – 84.4 – – – 84.2 – – – 84.5 83.7 84.1
F. Lima et al.
13. Fuel 293 (2021) 120297
13
industrial application in desulfurization, as it caused major volatile
losses and made the regeneration step difficult [124]. In contrast, the
low volatility of some DES may be an advantage for their regeneration.
In order to maximize the DES lifetime in desulfurization processes,
several authors have been studying the DES reuse with and without
regeneration. Regarding an ODS approach, depending on the nature of
the process (oxidant, catalyst), the reuse of a DES without regeneration
might not be possible, since the oxidant and catalyst are generally in the
DES phase. Nevertheless, in the case of an EDS approach, the DES phase
can be directly reused without a regeneration step. In all studies, as
represented in Fig. 9, the spent DES phase could be reused, however,
with declining desulfurization rates [50,97,134]. This clearly indicates
that the sulfur compounds that accumulate in the DES phase, throughout
the reuse cycles, lead to a decrease in DES extraction capacity and em
phasizes the importance of DES regeneration. Concerning regeneration
methods in EDS, back extraction with organic solvents [50,97,124] or
precipitation using water followed by distillation [96,99,114] have been
the most reported methods. In general, as depicted in Fig. 9, the re
generated DES fully recover their sulfur extraction capacity, which is a
good indicator of DES robustness.
The regeneration of the DES after an ODS cycle is generally more
complex than in EDS, since an oxidant (aqueous solution), and possibly a
catalyst, are present. Jiang et al. [129], who studied a POM-based cat
alytic system, performed the regeneration of an acid DES phase by
adding water. In this case, the addition of water precipitated not only the
oxidized sulfur product but also the catalyst. Thereafter, an additional
step was required in order to regenerate the catalyst, which consisted of
washing the precipitate with ethyl ether. Another example is when
oxidative products, water (from the oxidant solution) and H2O2, are
present in the DES phase. In this case, it may be necessary to add more
water to allow precipitation of the oxidative products, which can then be
filtered off and removed under reduced pressure by rotary evaporation
equipment [51,111]. The use of water as a regenerating agent is trans
versal to the various approaches, as represented in Fig. 10. Nevertheless,
just like in EDS, another possibility is the use of organic solvents. Carbon
tetrachloride (CCl4) is among the most used to remove, for example,
dibenzothiophene sulfone [106,110]. However, since water is typically
present in DES phase (from aqueous solution of H2O2), it needs to be first
removed. Oxidative products are then removed by several extraction
steps with CCl4 and last the residual CCl4 present in DES is removed
under reduced pressure [109]. In general, several cycles (4 or more) of
regeneration have been tested and, contrary to DES regeneration in EDS,
a decrease in DES desulfurization capacity has been detected. For
example, Mao et al. [109], who regenerated a Brønsted-Lewis acidic
DES, detected a drop in desulfurization capacity from 99.23% to 95.25%
after five cycles, while Zaid et al. [104] found a decrease from 97.06% to
88.34% after 3 regeneration cycles. One of the most likely explanations
for this decrease is the possible loss of small amounts of DES throughout
the process, since DES is being exposed to several cycles of desulfur
ization and regeneration. In addition, the presence of some residual
oxidation products has also been pointed as a possible reason [109] and,
in our opinion, residual traces of the regenerating agent (water or
organic solvent) can also be another explanation.
In conclusion, in general, DES can be regenerated, maintaining the
same physicochemical properties, structure and, in most cases, desul
furization capacity. However, regarding the industrial feasibility of the
regeneration stage, to date, no method has been shown to be sustainable
and economically viable. If, on the one hand, water separation implies a
high cost of energy, on the other hand, the use of organic solvents in
back-extraction raises environmental concerns, since the aim of these
works is to find sustainable and greener alternatives. Thus, further
research is still needed to make the industrial application of DES
economically viable.
5.5. Real fuel matrices
High rates of extractive and oxidative desulfurization using DES have
been reported, often approaching 100% of sulfur removal. However,
these remarkable results concern model fuels, which are usually simply
composed of an aliphatic hydrocarbon (e.g. n-heptane) and a model
sulfur compound (e.g. Th or DBT). In fact, these model fuels are very
simplistic compared to real fuels, where numerous types of saturate and
aromatic hydrocarbons are present, thus affecting solubility of sulfur in
DES and ultimately the extraction efficiency of any process [136].
Moreover, diesel is very different from gasoline and their matrices can
change from source to source.
Considering that since 2013, DES have been widely explored for
sulfur removal, it would be expected that, by now, many more studies
using real matrices could be found in the open literature. However, on
average, for every five publications addressing the use of DES in EDS,
only one reports the effect of real fuel matrices in the DES capacity,
while for ODS studies, this number increases to two per five
publications.
In this limited number of works reporting the EDS technique on
commercial diesel and real gasoline a considerable drop on the desul
furization performance of DES when compared to model fuels has been
reported. For example, in 2015, Gano et al. [53], compared the EDS
capacity of TBPB:FeCl3 (2:1) DES on a multicomponent simulated fuel
(containing Th and DBT as sulfur compounds) and on commercial diesel,
reporting a drop from 50.6% to 33% on the extraction efficiency.
Similarly, other authors [99,101] performed studies with commercial
diesels and multicomponent simulated diesel reporting similar drops, as
depicted in Table 10. In addition, from Table 10, it can be observed that
when very simple model fuel matrices are used, the gap between DES
performance for simulated fuels and real fuels is large. For instance, in
the work of Li et al. [97] a gap of 55% between the extraction efficiency
for a simple model fuel and real diesel is reported. Thus, in this and
many other works [96,105] where only n-octane is used as the hydro
carbon matrix, the results for the sulfur removal efficiency are very far
from those of a real fuel.
Concerning the ODS technique, and as depicted in Table 10, the gap
of DES oxidative performances between model and real fuels is in the
same order of magnitude as that of found for EDS. However, there is an
exception, where only a small drop in extraction efficiency, from
99.99% to 97.25%, was reported [51]. The most likely explanation is the
ratio of DES:fuel used, which in this case is 1:2, one of the highest ratios
reported (Table 7).
In general, ODS scientific community has been studying the mini
mization of the extractant ratio, in order to use the minimal amount of
Fig. 9. Evolution of the EDS capacity of a DES over 4 cycles of reusing without
regeneration and after a cycle with regeneration of the DES ().
adapted from [97]
F. Lima et al.
14. Fuel 293 (2021) 120297
14
DES, which apparently works for simulated fuels, but not for real fuels.
Huaming Li’s group [137] pointed that the presence of olefins and ar
omatics in real fuels, which can also be oxidized and so competing with
sulfur compounds in the ODS system, is the most likely reason for this
performance decrease. These authors found that toluene and paraxylene
lead to a significant decrease on the oxidation of DBT (about 10%),
confirming the negative impact of aromatics. Moreover, the addition of
cyclohexene (10 wt%) proved to be much more impactful, since ODS
rates drops from 96.4% to 57.7% [137], 98.4 to 25.8% [90] and 100 to
33.4% [107] were reported. These are important conclusions confirming
that the simple and trivial model fuel matrices, that have been widely
used, do not represent the real fuel and more complex matrices should
be used.
We believe this apparent lack of interest for real fuels by the scientific
community may be due the low availability of proper methods to
quantitatively and qualitatively characterize them. In fact, considering
the studies on real matrices, they are always based on total quantifica
tions of sulfur, which does not allow to understand which types of sulfur
compounds are easier /more difficult to remove. In addition, informa
tion about cross-solubilities regarding real fuels is also needed, as it is
expected that with increasing matrix complexity, the fossil fuel solubi
lity in DES, and possibly the DES solubility in the fuel phase, will be
changed.
6. Environmental and economic analysis
For over a century, organic solvents have been used in industrial
applications. However, nowadays, their adverse effects on human health
and environment are a major concern. Several legislations have been
approved and implemented worldwide, restricting the usage of some
more toxic solvents and encouraging to find more benign replacements
for others.
The scientific community and the chemical industry have been
engaged in finding sustainable solutions to minimize the negative im
pacts of common organic volatile solvents. The commonly most reported
adverse effect from the usage of these solvents is pollution, that has
repercussions in many areas such as outdoor air pollution, soil
Fig. 10. Graphical representation of two possible regeneration methods carried out with water. On the left, according to an ODS technique (also elucidating the
oxidation/extraction mechanism) and on the right, according to an EDS technique, suggesting an integrated approach ().
adapted from [51,135]
Table 10
Comparison of DES extractive and oxidative desulfurization capacities between
a simulated fuel and a real fuel.
EE (%) Gap between
results
Simulated fuel Real
fuel
EDS Gano et al. (2015)
[53]
Multicomponent* Diesel
50.6 33 35%
Li et al. (2016)
[97]
n-octane Diesel
~78 35.27 55%
Gasoline
40.94 48%
Shu et al. (2016)
[96]
n-octane Gasoline
64 38.70 40%
Gano et al. (2017)
[99]
Multicomponent* Diesel
62 47 24%
Rahma et al.
(2017) [101]
Multicomponent* Diesel
62.36 46.75 25%
ODS Yin et al. (2015)
[51]
n-octane Diesel
99.99 97.25 3%
Zaid et al. (2015)
[138]
Dodecane Diesel
97.06 78.93 19%
Mao et al. (2016)
[110]
n-octane Gasoline
99.42 66.67 33%
Mao et al. (2017)
[106]
n-octane Diesel
98.65 68.50 31%
Xu et al. (2018)
[130]
n-octane Diesel
100.00 74.40 26%
Li et al. (2018)
[139]
n-octane +
toluene
Diesel
99.10 67.70 32%
*n-decane, cyclohexane, iso-octane and toluene in the following proportions
29.79%, 29.79%, 29.79%, 10.63
F. Lima et al.
15. Fuel 293 (2021) 120297
15
contamination, water pollution, health problems, among others. How
ever, there are other important consequences that have to be taken into
account, such as the case of the energy consumption associated with the
use of a specific solvent in a process. In fact, the whole lifecycle of a
solvent used in a process should be analysed, since energy consumption
due to solvents usage is present in several steps, such as solvent syn
thesis, transportation, application and recovery. Thus, nowadays,
techno-economics considerations are considered in parallel with sus
tainability and safety. There are several approaches that have been
explored to reduce the toxic issues related to solvents in chemical in
dustries, namely, the use of benign non-volatile organic solvents,
solvent-free processes, solvent mitigation, supercritical fluids, water-
based processes or the use of greener and sustainable solvents, such as
the case of IL and more recently DES [140,141].
Considering the focus of this review, the aim of this section is to
compile and discuss economic and sustainable considerations that have
been put forward by the scientific community regarding the use of DES
for desulfurization of fossil fuels. Unfortunately, in general, the focus has
been mainly on the technical performance of DES as substitutes of
conventional solvents/processes. DES were introduced as a “sustainable”
and “biodegradable” alternative to organic solvents and ILs [48,142], and
since then, the scientific community has generally referred to them as
“green alternative solvents”, “eco-friendly”, “environmental-friendly”,
“non-toxic”, “biodegradable”, “cheaper and easy to produce”, without
any further explanations. Those generic characteristics will be discussed
in the next paragraphs, in the context of desulfurization of fossil fuels,
according 3 big questions: a) are DES green solvents?; b) are DES non-toxic
and biodegradable solvents?; c) are DES cheap and easy to produce?
6.1. Are DES green solvents?
Nowadays, the term “green solvent” is widely used, but what is in
fact a green solvent? Are all solvents hyped as “green” actually green?
The use of specific solvents in a process is directly linked not only to their
technical performance but also to the environmental impact, cost, safety
and health issues of that process [143,144]. According to Fischer et al.
[143], a green solvent is one that allows to minimize or eliminate the
environmental impact resulting from its use. Therefore, it makes no
sense to continue labelling DES as green solvents using simple argu
ments, such as, “it is green because it is of natural origin” or “it is green
because it is a bio-product”. Thus, it is necessary to use green metrics
which allow to measure what is the impact of a green solvent, taking into
account their preparation, use, recycling and disposal [145]. A number
of green metrics have been proposed, such as the Life-Cycle Assessment
(LCA) and the Environmental, Health and Safety assessment (EHS)
[143], that allow assessing the “greenness” of a solvent. However, these
are complex and time-consuming methods, that usually require large
amounts of experimental data to provide reliable results. Nevertheless,
several authors have been proposing new methods to achieve such goal
in a simple way. For example, Fischer et al. [143], proposed an easy new
framework for the environmental assessment of solvents sustainability
which is a combination of the LCA and the EHS methods. Similarly,
Jessop [145] suggested simple and easily applicable tests which cover in
a first phase, a general comparison between the actual and the
replacement solvent in terms of energy of production, cumulative en
ergy demand and impact on health and the environment, and in addi
tional second phase, addressing the particular application. Besides its
simplicity, this last method considers the solvent́s application, which is a
step forward in terms of solvents evaluation. Focusing now on DES
literature, to date, no single work reports an LCA or EHS analysis of a
DES, which is very surprising since DES have been reported as green
solvents from day one. Thus, the need of addressing DES using green
metrics is very clear. With this review, we want to motivate the DES
community to deepen this perspective. Regarding published papers
addressing the use of DES in the desulfurization of fossil fuels, the ma
jority frequently uses the term “green” or “green and cost effective”.
However, there are no fundamentals across these works regarding the
minimization of environmental impacts, nor a simple cost analysis. Such
affirmations are based on facts such as: the easy and simple synthetic
preparation, without the need for any additional organic solvent; one or
both of the components are from a natural origin; among others. DES
should continue to be labelled as green, when: i) they can be formed
from an unlimited number of combinations of the most varied types of
compounds, ii) there is no universal environmentally sustainable recy
cling method and iii) a proper analysis of the EHS characteristics has not
yet been done? In our opinion, these bold generalizations should be
avoided. We believe that in most cases, DES are good candidates for
green solvents but, so far, they are just potential candidates.
6.2. Are DES non-toxic and biodegradable solvents?
In general, DES have been seen as “ILs analogues”, especially those
that have salts in their composition, as they share some of physico
chemical properties, such as chemical stability, low-flammability and
low-volatility, which make them good candidates when compared to
volatile solvents. In addition, depending on the DES components, they
have also been labelled as “non-toxic” and “biodegradable”, which has
been presented as an advantage over ILs. Once again, such labels have
been applied according to the nature of the individual DES components.
For example, DES-based on choline chloride have been presented as
“biodegradable and non-toxic” [90,114]. In one of those examples, DES
were prepared by the combination of several choline-like HBA (varying
the cation) with FeCl3. Can we state that they are non-toxic and biode
gradable just because one of their starting components is choline chlo
ride? If, on one hand, we must always take into account both starting
components of each DES, on the other hand the toxicity and biode
gradability of the final mixture may differ from the individual starting
compounds. In 2013, Hayyan et al. [146] performed one of the first few
studies on DES toxicity and cytotoxicity that have been published to
date. The studied DES were based on choline chloride combined with
glycerol, ethylene glycol, triethylene glycol and urea. These authors
found no toxic effect of all the studied DES on two Gram positive and two
Gram negative bacteria. However, in the case of cytotoxicity (on brine
shrimp hatches), their experiment revealed that the DES cytotoxicity
was much higher than the cytotoxicity of the individual starting com
ponents. Moreover, this increase on cytotoxicity was due to the forma
tion of DES, since when they performed the same experiment just by
adding the exact amount of each component, no significant synergic
effect was detected. Thus, the toxicological behavior of each DES may be
different than the ‘sum’ of the toxicological behaviors of the individual
components and thus no hasty conclusion about DES toxicity and
biodegradability should be taken just by considering those properties of
the starting components.
6.3. Are DES cheaper and easy to produce?
Another feature of DES, which has distinguished them from ILs, is the
low cost of production. In the case of DES, the components can simply be
mixed without any further purification, meaning no additional chem
icals are required, which reduces process risk and, in turn, the cost. In
fact, by comparison, DES production is much easier and simpler than ILs
production. In general, the only steps that involves energy consumption
are mixing and heating, which usually does not exceed 80 ◦
C and a time
period of 2 h. However, in the case of scaling up this may change, and
once again data on this subject is in need. Another advantage is related
to the price of DES starting components, that are generally much
cheaper than those of ILs. Nevertheless, there are several compounds,
which have been widely use in the DES field, which might be quite
expensive. Fig. 11 depicts the average prices of DES starting materials
most commonly applied on desulfurization. It can be seen that some of
the quaternary salts, widely used in desulfurization studies, can cost up
to € 1500 / kg. Consequently, DES are easy to produce, at least on a
F. Lima et al.
16. Fuel 293 (2021) 120297
16
laboratory scale, but depending on their starting compounds their price
may not be as low as commonly mentioned.
7. Conclusions and perspectives
An overview on the potential of using DES to remove sulfur pollut
ants from transportation fuels has been compiled, always taking into
account their similarity with ILs. DES are arguably easier to prepare and
therefore cheaper and easier to access. Furthermore, taking into account
their many desirable and shared features, it was already expected that
the use of ILs would start to decline for the benefit of the use of DES. In
academy, since 2014 the interest for ILs in the most diverse areas has
slowed down, even falling, as is the case of the desulfurization area.
From an academic point of view, and regarding lab scale, ILs provided
very good and encouraging results, mainly in the case of ODS. However,
their expensive production process and possible toxicity issues have
hindered their industrial application. In fact, these two obstacles moti
vated the use of DES in detriment of ILs. Nevertheless, since an infinite
number of different DES can be obtained, the same questions regarding
the toxicity and biodegradability of DES, as well as their production
process at large scale, have to be raised.
Concerning DES in sulfur removal from fossil fuels, similarly to ILs,
very good results have been massively reported, reaching up to almost
100% of sulfur removal, both for EDS and ODS approaches. However,
for the industrial application of DES and, in order to integrate the new
process into existing desulfurization processes, some additional major
issues must still be addressed, including not only DES stability, recycling
and disposal but also the interaction of the DES with real and complex
fuels matrices.
Despite acknowledging there is a lot of progress to be done in what
concerns real matrices, we believe that DES have the potential to
become a complementary (post-treatment) and environmental benign
alternative for the deep desulfurization of transportation fuels.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Fig. 11. Comparison of HBA and HBD average prices (prices estimated from Sigma).
F. Lima et al.
17. Fuel 293 (2021) 120297
17
Acknowledgments
The authors, F. Lima, L.C. Branco gratefully acknowledge the
financial support of FCT/MCTES (Portugal) for the PhD fellowship PD/
BDE/114355/2016, for the contract under Programa Investigador FCT
2013 (IF/0041/2013). This work was financed by CQE project (UID/
QUI/00100/2013), Lisboa/01/0145/FEDER/028367, PTDC/QUI-QFI/
29527/2017, the Associate Laboratory CICECO, Aveiro Institute of
Materials (UID/CTM/50011/2013), and Solchemar company.
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F. Lima et al.
![Fuel 293 (2021) 120297
Available online 22 February 2021
0016-2361/© 2021 Published by Elsevier Ltd.
Full Length Article
Deep desulfurization of fuels: Are deep eutectic solvents the alternative for
ionic liquids?
Filipa Lima a,b,c,d
, Luis C. Branco b,c
, Armando J.D. Silvestre d
, Isabel M. Marrucho a,*
a
Centro de Química Estrutural and Departamento de Engenharia Química, Instituto Superior Técnico, Universidade de Lisboa, Avenida Rovisco Pais, Lisboa 1049-001,
Portugal
b
Solchemar, Lda, Rua 5 de Outubro no 121C, 1◦
E, Alcácer do Sal 7580-128, Portugal
c
LAQV-REQUIMTE, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, Campus da Caparica, Caparica 2829-516, Portugal
d
CICECO-Aveiro Institute of Materials and Department of Chemistry, Universidade de Aveiro, Aveiro 3810-193, Portugal
A R T I C L E I N F O
Keywords:
Deep desulfurization
Deep eutectic solvents
Ionic liquids
Extractive desulfurization
Oxidative desulfurization
Real fuel matrices
A B S T R A C T
The impact of fossil fuels impurities on outdoor air pollution is today a matter of great concern, with the
transportation sector playing a major role. Given global efforts to reduce sulfur oxides emissions, and the
increasingly tighter legislations, removal of sulfur contaminants liquid highway fuels is a pressing issue world
wide. The purpose of this review is to provide a detailed state of the art review concerning the use of deep
eutectic solvents (DES) in the removal of sulfur pollutants from transportation fuels. Furthermore, a comparative
evaluation regarding the use of these solvents versus Ionic Liquids (ILs) is also provided, taking into account
several aspects, such as, evolution of technologies, solvents composition, extraction efficiencies and environ
mental and economic considerations. The large gap in terms of efficiency of the reported technologies, either
using ILs or DES, between simulated matrices and real fuels is also discussed, highlighting the importance of
academia and industry hand in hand work.
1. The challenge
Air pollution is a matter of great concern in today’s world. According
to the World Health Organization (WHO), every year 4.2 million of
people die due the outdoor air pollution. In 2016, 91% of the world’s
population was not breathing clean air, and more than half of the pop
ulation living in cities had been exposed to outdoor air pollution levels at
least 2.5 times higher than those recommended by WHO [1]. It is well
known that the main driving force leading to air pollution is the burning
of fossil resources (oil, coal and gas) for power generation. Up to the
present day, and beyond all efforts to introduce renewable energies, the
major source of energy is still fossil fuel [2]. The transportation sector,
specifically the road sector, is one of the main responsible for the fossil
fuels demand and concomitantly, for the increasing outdoor air
pollution.
Sulfur compounds present in transportation fuels give rise to sulfur
oxides (SOx) emission during fuel combustion, which contribute to acid
rains, global warming effect and air pollution. Their presence also
largely decreases the performance of pollution control equipments on
vehicles and affects the efficiency of numerous refinery processes,
mainly due to the poisoning of catalysts. [3]. Consequently, in 1990 and
1993, USA and UE, respectively, put forward the first constraint to sulfur
content in transportation fuels. Since then, emissions reduction has
become a global target as legislation becomes increasingly tight. Fig. 1
shows the evolution of the sulfur limit in highway fuels, in Europe.
Nowadays, the maximum sulfur content allowed in diesel and gasoline is
10 ppm in most developed countries, and this limit is spreading all over
the world. China is also tightening its standards, limiting diesel and
gasoline sulfur content to 10 ppm, by 2020 [4]. The African Refiners
Association also announced new tight limits for sulfur levels by 2020:
150 ppm for gasoline (reduced from 300 ppm) and 50 ppm for diesel
(reduced from 600 ppm) [4,5], which are still far from those mentioned
above for other countries and, thus, are an obvious matter of concern,
given the growing potential of African economy.
Although deep desulfurization is mandatory from an environmental
point of view, meeting these new stringent required specifications
mentioned above represents a real operational and economic challenge
for refineries. In addition, while the market demands for ultra-light
sulfur fuels, the quality of crude and feed streams available to the re
finers is declining [4]. This explains why, nowadays, efficient
* Corresponding author.
E-mail address: isabel.marrucho@tecnico.ulisboa.pt (I.M. Marrucho).
Contents lists available at ScienceDirect
Fuel
journal homepage: www.elsevier.com/locate/fuel
https://doi.org/10.1016/j.fuel.2021.120297
Received 27 May 2020; Received in revised form 5 January 2021; Accepted 22 January 2021](https://image.slidesharecdn.com/lima2021-220519180428-cb82db9a/85/lima2021-pdf-1-320.jpg)
![Fuel 293 (2021) 120297
2
desulfurization units are the most needed in refineries around the world.
According to the Organization of the Petroleum Exporting Countries
(OPEC) [4], until 2040, an additional global desulfurization capacity of
22.5 million barrel/day is needed, against to 19.6 million barrel/day of
total crude distillation capacity [4,6].
Hydrodesulfurization (HDS) is the current industrial technology used
for the removal of organic sulfur compounds, depicted in Fig. 2, from
refined products, such as, gasoline, diesel, jet fuel, fuel oil, and naphtha.
In this technology, the sulfur-containing compounds are catalytically
converted into hydrogen sulfide at high temperatures (>300 ◦
C), high
pressure (20–60 bar) and requiring a huge amount of hydrogen [7–9].
Hence, HDS is an expensive technology due to its high operational cost.
In addition, although the hydrogenation process is efficient for the
removal of thiols, sulfides and dissulfides, its low effectiveness in the
removal of heterocyclic sulfur compounds, such as dibenzothiophenes
(DBT) and derivatives is major drawback. Fig. 3 [10,11] depicts the
reactivity in HDS of representative chemical structures of sulfur com
pounds typically present in different types of fossil fuels. It can be seen
that HDS is not very effective in handling larger sulfur compounds
derived from DBT, typically present in diesel fuels. This is due to the very
small pores of common HDS catalysts, which hinder the diffusion of
large molecules to the active sites present in the pores. Therefore,
reducing sulfur contents to levels that comply with modern regulations
requires increased severity, energy and the use of expensive catalysts,
which makes this a highly energy and hydrogen-intensive process [12].
Furthermore, these extreme operating conditions are often not
compatible with other fuel requirements such as oxygen content, vapor
pressure, benzene content, overall aromatics content, boiling range and
olefin content for gasoline, and cetane number, density, polynuclear
aromatics content, among others [13].
The development of innovative complementary technologies for
deep desulfurization of fuels has been the focus of many research groups
and the target of many industries. Although HDS-based technologies
have been greatly investigated [14–17] and widely reviewed [7,18–22],
other alternative processes, such as extractive desulfurization (EDS)
[23,24], adsorptive desulfurization [25,26] (ADS), oxidative desulfur
ization (ODS) [27,28], biodesulfurization (BDS) [29,30] among others
[31,32], have also been researched. Among these, EDS and ODS are two
of the most promising desulfurization processes due to their simple
operation, low cost and benign effects on the final fuel quality (in the
case of EDS), no need of hydrogen and high compatibility with other
refining technologies [33,34]. However, the organic solvents typically
used as extractants raise environmental, public health, safety and cost
concerns due to their volatility and flammability. In the last two de
cades, the search for alternative solvents that can overcome the draw
backs of the organic solvents used has been intensified. Ionic Liquids
(ILs) and, more recently, Deep Eutectic Solvents (DES) have shown great
potential as possible substitutes of several hazardous organic solvents
commonly used in industrial operations, in particular in fuels
desulfurization.
ILs are organic salts with melting points below the boiling point of
water (100 ◦
C) [35,36]. They are composed of organic cations and either
organic or inorganic anions. The first reported IL was introduced around
a century ago, but only in 1982 with the publication of Wilkes et al. [37]
on dialkylimidazolium chloroaluminate melts, this class of solvents
started to have scientific impact. Nowadays, those ILs are called the first
generation, since they are water sensitive [38]. The second generation of
ILs, which contain air and moisture stable ILs, was reported by the same
research group, a decade later [38]. They are the iconic 1-ethyl-3-meth
ylimidazolium-based ILs, which today have a widespread use. Since
then, ILs have been widely investigated, and their first industrial
application occurred in 2003, by BASF, for biphasic acid scavenging
[39]. In the last two decades, research in this field focused on finding
greener, task-specific ILs, and finding alternative cheaper and simpler
synthetic routes. Typically, IL cations contain nitrogen (ammonium,
imidazolium, pyridinium or pyrrolidinium cations) and alternately
phosphorous (phosphonium cations) or sulfur (sulfonium cations).
Regarding IL anions, the most used are halides, for example, chloride
(Cl-
) and bromide (Br-
), fluorinated anions such as tetrafluoroborate
([BF4]-
), hexafluorophosphate ([PF6]-
), bis[trifluoromethylsulfonyl]
imide ([NTf2]-
) and trifluoromethanesulfonate ([TfO]-
), cyano-type an
ions such as dicyanamide ([N(CN)2]-
)- or thiocyanate ([SCN]-
) and ni
trate ([NO3]-
), among others [36].
ILs are an environmentally attractive alternative to traditional
organic solvents, once they have unique physicochemical properties,
namely, high thermal stability, non-flammability, non-volatility and
adjustable miscibility and polarity, since they can be adequately tailored
by changing the cation and/or the anion [38,40–42]. This makes ILs
extremely versatile solvents, having the capacity to dissolve a wide
range of organic and inorganic compounds, as well as polymeric mate
rials [43,44]. Therefore, these solvents have been proposed for a wide
range of different applications [45]. For example, they have been
Fig. 1. Evolution of the maximum sulfur allowed in highway fuels (in EU) overlapping with deep desulfurization research using ILs and DES, over time.
F. Lima et al.](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)