Industrial application of ionic liquids for the recoveries of spent paint solvent

Article abstract of DOI:10.1016/j.molliq.2016.03.035

The recovery of industrially valuable organic solvents from liquid waste, generated in chemical processes, is economically crucial to countries which need to import organic solvents. In view of this, the main objective of this study was to determine the ability of selected ionic liquids, namely, 1-ethyl-3-methylimidazolium ethylsulphate, [EMIM][ESO4] and 1-ethyl-3-methylpyridinium ethylsulphate, [EMpy][ESO₄] to recover aromatic components from spent paint solvents. Preliminary studies done on the liquid waste, received from a paint manufacturing company, showed that the aromatic components were present in the range of (6-21)% by volume. The separation of the aromatic components was performed with the ionic liquids listed above. The phases, resulting from the separation of the mixtures, were analysed with a gas chromatograph (GC) coupled to a FID detector. Chromatograms illustrate that the chosen ZB-Wax-Plus column gave excellent separation of all components of interest from the mixtures, including the isomers of xylene. The concentrations of aromatics recovered from the spent solvents were found to be in the % ranges of (13 - 33) and (23-49), respectively for imidazolium and pyridinium based ionic liquids. These results also show that there is a significant correlation between π-character of ionic liquids and the level of extraction. It is therefore concluded that ionic liquids have the potential for macro-scale recovery of re-useable solvents present in liquid waste emanating from paint manufacture.

Full text of DOI:10.1016/j.molliq.2016.03.035

Journal of Molecular Liquids 219 (2016) 206–210
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Industrial application of ionic liquids for the recoveries of spent
paint solvent
a,
K. Moodley a, M. Mabaso a, I. Bahadur b, , G.G. Redhi
⁎
⁎
a
Department of Chemistry, Durban University of Technology, P O Box 1334, Durban 4000, South Africa
Department of Chemistry, Material Science Innovation and Modelling (MaSIM) Research Focus Area School of Mathematical and Physical Sciences, Faculty of Agriculture, Science and Technology,
b
North-West University (Mafikeng Campus), Private Bag X2046, Mmabatho 2735, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
The recovery of industrially valuable organic solvents from liquid waste, generated in chemical processes, is eco-
nomically crucial to countries which need to import organic solvents. In view of this, the main objective of this
study was to determine the ability of selected ionic liquids, namely, 1-ethyl-3-methylimidazolium ethylsulphate,
[EMIM][ESO4] and 1-ethyl-3-methylpyridinium ethylsulphate, [EMpy][ESO4] to recover aromatic components
from spent paint solvents. Preliminary studies done on the liquid waste, received from a paint manufacturing
company, showed that the aromatic components were present in the range of (6–21)% by volume. The separation
of the aromatic components was performed with the ionic liquids listed above. The phases, resulting from the
separation of the mixtures, were analysed with a gas chromatograph (GC) coupled to a FID detector. Chromato-
grams illustrate that the chosen ZB-Wax-Plus column gave excellent separation of all components of interest
from the mixtures, including the isomers of xylene. The concentrations of aromatics recovered from the spent
solvents were found to be in the % ranges of (13−33) and (23–49), respectively for imidazolium and pyridinium
based ionic liquids. These results also show that there is a significant correlation between π-character of ionic liq-
uids and the level of extraction. It is therefore concluded that ionic liquids have the potential for macro-scale re-
covery of re-useable solvents present in liquid waste emanating from paint manufacture.
Received 28 January 2016
Received in revised form 10 March 2016
Accepted 11 March 2016
Available online xxxx
Keywords:
Synthesis
Ionic liquid
Extraction
Aromatic solvents
Spent paint organic solvents
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
in many chemical-based manufacturing processes. This study was
therefore aimed at recoveries of aromatic solvents from waste or-
Oil paint manufacturing companies are significant users of aro-
matic and aliphatic hydrocarbons as solvents in their various pro-
cessing units. This is evident from literature reports [1–3] which
focus on the health hazards of aromatic solvents used by paint and
allied industries. The pollution is not confined to the atmosphere;
there is also the danger of pollution of the land if the residues from
oil paint manufacture are dumped in landfill sites. The ‘waste’ from
oil paint making processes comprises of unused pigments and sol-
vents. In this context the solvent component is referred to as ‘spent
solvent’. It is noteworthy that the spent solvents contain significant
amounts of aromatic species such as toluene, ethyl benzene and xy-
lene isomers, known by the acronym, TEX. The spent solvents also
contain other organic species such as aliphatic hydrocarbons. Al-
though aromatic solvents are generally shunned because of their
classification as carcinogens [4], they do play an irreplaceable role
ganic mixtures emanating from oil-paint manufacture. In the inter-
est of promoting ‘green chemistry’ [5,6], ionic liquids (ILs) were
selected as extracting agents. ILs is emerged as a new class of sol-
vents for the replacement of volatile organic solvents in different
separation as well as industrial processes to reduce both economic
and environmental pollution [7]. ILs is used in a wide range of com-
mercial, industrial and incredible applications. Preliminary studies
in our laboratories showed that the concentration of aromatic com-
ponents in spent paint solvents vary between 6 and 21% by volume.
Since aromatic solvents are generally costlier to manufacture than
aliphatic ones, their recoveries using economically viable methods
is very important. In processes where aromatic solvents are utilized
as reaction media [8], it would reduce manufacturing costs if the ar-
omatic components could be recovered on a macro scale and reused.
Currently, the recovery of aromatic solvents (on an industrial scale)
from mixtures containing aliphatic solvents is generally done by dis-
tillation or by using extracting solvents such as sulfolane [9,10]. It
has been shown [11] that sulfolane is not effective in cases where
the concentration of the aromatic components is b20%. Furthermore,
distillation is not suitable for separating azeotropic mixtures.
⁎
Corresponding authors.
E-mail addresses: bahadur.indra@gmail.com (I. Bahadur), redhigg@dut.ac.za
(G.G. Redhi).
http://dx.doi.org/10.1016/j.molliq.2016.03.035
0167-7322/© 2016 Elsevier B.V. All rights reserved.

K. Moodley et al. / Journal of Molecular Liquids 219 (2016) 206–210
207
Although the recovery of organic solvents from spent paint solvents
was performed on a laboratory scale, the experimental design for this
project incorporated the following considerations:
In illustration of the principles described above, the following proce-
dure was used for the synthesis of 1-ethyl-3methylimidazolium ethyl
sulphate:
Diethylsulfate(0.44 mol, 58 mL) was slowly added drop-wise to an
ice-cooled mixture of 1-methylimidazole (0.44 mol, 35 mL) and toluene
(1.88 mol, 200 mL) under an inert atmosphere of argon. When the addi-
tion of diethylsulfate was complete, the reaction mixture was stirred for
5 h at room temperature to allow the reaction to go to completion. At
the end of this period, the lower layer (ionic liquid phase) was separated
from the mixture by using a separating funnel and then washed three
times with a total volume of 100 mL toluene. The crude product was
dried at 70 °C under reduced pressure for 3 h using a rotary evaporator;
to remove organic solvents and water before the characterization
measurements.
• The chosen laboratory method [12] for synthesis of the selected IL has
potential for adaption to an industrial-scale synthesis of the IL.
• The stability [13] of the IL was such that it was recovered unchanged
at the end of the extraction, that is, it is unaffected by exposure to
air and moisture.
• The IL meets the requirements for a ‘green solvent’ [14] so that it is not
hazardous to the environment.
• The ILs should be selected on some rational basis [15].
• The ILs shows no loss of extraction ability on repeated reuse [16].
It is noted that there are several reports on the uses of ionic liquids
for micro-extraction of BTEX [17,18] but none of them is associated
with the extraction of aromatic components from non-aqueous mix-
tures. There are also several recent reports [19–22] on the extraction
of organic liquids using ionic liquids. However they are restricted to bi-
nary and ternary systems whereas this project addresses the extraction
of organic liquids from multi-component systems. Moreover, this pro-
ject is an addition to our contributions [12,13,16] on the synthesis of
ionic liquids and their application for the extraction of organic liquids
from mixtures of organic liquids.
2.2.2. Determination of water content in the ionic liquids
The water content of the ionic liquid was determined using a Karl–
Fischer Coulometer [Metrohm 756]. Ionic liquids were tested immedi-
ately after drying under high vacuum for 48 h and it was found that
the water content was below 850 ppm for both the ionic liquids used
in this work.
2.2.3. Analysis of spent solvent waste mixtures
Spent paint solvent mixtures were analysed for the concentration by
volume of aromatic components present in each. A known volume of
each spent solvent mixture was distilled using a rotary evaporator (to
collect all volatile organic components including aromatic and aliphatic
hydrocarbons as the most important ones). The volume of the distilled
liquid was carefully measured and then analysed for the percentage
(by volume) of each aromatic component present in the distillate,
using GC fitted with capillary column and an FID detector. The concen-
trations in the raw samples were calculated using the original volume of
the raw sample. The results for the above are given in Table 1.
2. Experimental
2.1. Materials
Diethylsulphate (≥99%), 1-methylimidazole (≥99%), and 3-
methylpicoline were purchased from Fluka. Toluene (≥99.9%), benzene
(≥99.9%), ethyl benzene, m-xylene (≥99%), o-xylene (≥99%), p-xylene
(≥99%) and n-hexane (≥99%) were purchased from Merck. Karl-
Fischer reagent was purchased from Riedel-de-Haen. Samples of
spent solvents were obtained from Plascon Paints, one of the major
manufacturers of paints in South Africa. The ionic liquids 1-ethyl-3-
methylimidazolium ethylsulphate, [EMIM] [ESO4] and 1-ethyl-3-
methylpyridinium ethylsulphate, [EMpy][ESO4] were synthesized
in our laboratory using modifications of documented methods.
2.2.4. Extraction in stages using freshly-prepared ionic liquids
Aliquots of distillates from spent paint solvents were extracted
with freshly-prepared ionic liquids under controlled conditions, using
purpose-made water jacketed reaction vessels through which water
from a thermostatically controlled tank was circulated. The extraction
of aromatic components (toluene, ethyl-benzene and isomers of xy-
lenes, known by the acronym TEX) from the measured samples, using
the ionic liquids listed above, was carried out at constant temperature
of 40 °C and atmospheric pressure. Aliquots (10 mL) of the spent solvent
were placed in the reaction vessels (at 40 °C), followed by addition of
ionic liquid (20 mL). The mixture was agitated vigorously using a mag-
netic stirrer for 5 h in order to allow for intimate contact between the
two phases. The equilibrium mixture was then transferred into a sepa-
rating funnel and allowed to settle for at least 12 h at room temperature
to ensure that complete phase separation had occurred. The phases
were then separated and stored in suitable sample vials for analysis.
The ionic liquid-rich phase is referred to as the extract phase and the
alkane-rich phase as the raffinate phase. The ionic liquid was recovered
from the mixtures using a rotary evaporator to remove organic solvents
2.2. Procedures
2.2.1. Synthesis and characterization of ionic liquids
The ionic liquid, 1-ethyl-3-methylimidazolium ethylsulphate, was
synthesized using modifications of reported procedures [23,24] while
1-ethyl-3-methylpyridinium ethylsulfate was produced by adapting a
documented [25] method. Since the project was aimed at establishing
procedures suitable for industrial use, the most amenable method was
chosen. For the synthesis of imidazolium and pyridinium-based ionic liq-
uids, quarternization reactions, whereby the ionic liquid cations are
formed from the tertiary amines using alkylating agents such as alkyl ha-
lides and dialkyl sulphates, proved to be very suitable. Quarternization
of 1-ethyl-3-methylimidazolium ethylsulfate and 1-ethyl-3-methyl-
pyridinium ethylsulfate proceeds as follows:
Table 1
Concentrations of aromatic components in spent paint solvent samples.
Aromatic components
Spent solvent mixture
Peak area
% (v/v)
Benzene
Toluene
Ethyl benzene
p-Xylene
m-Xylene
o-Xylene
Nil
Nil
21.0
7.1
7.7
18.0
8.2
18493790
6168467
6652490
15830365
7260821
Total aromatics
62.0

208
K. Moodley et al. / Journal of Molecular Liquids 219 (2016) 206–210
Fig. 1. Calibration chromatogram for determining composition of the spent solvent mixture.
and water. The recovered ionic liquids were used in experiments de-
scribed under section.
were agitated for about 3 min to homogenise the contents. These
spent solvents were filtered and then distilled. The distillates were
analysed, using GC-FID, to determine the concentration (%) of aro-
matic components. A guard column was used as a pre-column to pre-
vent the non-volatile materials from entering the analytical column
and, possibly, interfering with the analysis. Standards of appropriate
concentration range of BTEX components were used to calibrate the
instrument using linear graphs of concentrations versus peak areas.
The chromatograms from the calibration experiments served to con-
firm that the aromatic components present in spent solvents were
toluene, ethyl benzene o-xylene, m-xylene and p-xylene. The con-
centrations of toluene, ethyl-benzene and isomers of xylene are
shown in Table 1. The aromatic component with the highest concen-
trations is seen to be toluene which is not only a requisite for oil paint
manufacture but is also very often used as a substitute for benzene.
This is based on the perception that toluene is less carcinogenic
than benzene. Fig. 1 shows the chromatogram for a prepared mixture
of BTEX.
2.2.5. Analysis of phase mixtures by GC-FID
The aromatic components (TEX) in each phase were analysed by
FID-GC fitted with ZB-Wax-Plus capillary column (30 m × 0.25 mm ×
0.25 μm) and a guard column.
(5 m × 0.25 mm) to protect the analytical column, with the follow-
ing parameters:
Column oven: 40 °C [2 min → 115 °C] (20 °C/min)
Carrier gas: nitrogen; column flow
Injector: 250 °C; split ratio 1:100 for all samples
Detector: FID; 250 °C
Standards of TEX, covering the concentration range of the prepared
mixtures, were used for the calibration of the instrument. The concen-
trations and the peak areas were used for the construction of the cali-
bration curves and the TEX concentrations were calculated by means
of a linear regression. Since the ionic liquids contain small amount of
solvents from the synthesis stage, a blank analysis was also carried out
to adjust the results for toluene. Furthermore, when regenerated ionic
liquids were used in the extraction, blank analyses were performed to
measure the residual aromatic components so that the extraction re-
sults are corrected. The vials were sealed with silicon septa through
which a micro syringe needle was used to draw 1 μL of each sample
for GC analysis. All analyses in this study were based on external calibra-
tion standards of the aromatic components of interest.
Table 1 also shows that the total of aromatics in the samples was
62.0% (v/v). This high result suggests that efforts to recover aromatic
solvents (even if it is a fraction of this amount), are worthwhile pursu-
ing on the grounds of high replacement costs and reduction in damage
to the environment (due to dumping of paint waste).
3.2. Extraction of TEX from mixtures using fresh ionic liquids for every stage
The two ionic liquids used were compared with regard to extraction
ability; to ascertain whether differences in structures made a significant
difference to the amounts of aromatic solvents extracted by the two
ionic liquids. This test was done over three cycles/stages under identical
conditions for both ionic liquids. For the second and third stages, the
raffinate phase from the previous extraction was treated with fresh
aliquots of ionic liquids. The results are given in Tables 2 and 3 for
[EMpy][ESO4] and [EMIM][ESO4], respectively.
3. Results and discussion
3.1. Determination of aromatic components in spent solvent mixtures
The spent solvent samples were obtained from the Plascon paint
manufacturing company which had stored them in labelled tanks. This
is referred to as ‘spent solvent’. Prior to sampling, the storage tanks
Table 2
Table 3
Extraction results using fresh [EMpy][ESO4] each for stage.
Results of the three stages of extractions from fresh [EMIM][ESO4].
Aromatic
components
1st stage % (v/v)
2nd stage % (v/v)
3rd stage % (v/v)
Total
%
(v/v)
Aromatic
components
1st stage % (v/v)
2nd stage % (v/v)
3rd stage % (v/v)
Total
%
(v/v)
Extract Recovery Extract Recovery Extract Recovery
Extract Recovery Extract Recovery Extract Recovery
Toluene
3.08
0.74
0.84
2.00
1.03
15.6
11.0
11.7
11.8
14.1
2.65
0.62
0.69
1.44
0.78
13.4
9.3
9.6
8.5
10.7
2.18
0.55
0.60
1.13
0.62
11.0
8.2
8.3
6.7
8.5
40
29
30
27
33
Toluene
2.37
0.51
0.57
0.95
0.66
12.0
7.6
7.9
5.6
9.0
1.66
0.37
0.42
0.97
0.52
8.4
5.5
5.8
5.7
7.1
1.61
0.35
0.39
0.93
0.50
8.2
5.2
5.4
5.5
6.8
29
18
19
17
23
E/benzene
p-Xylene
m-Xylene
o-Xylene
E/Benzene
p-xylene
m-xylene
o-xylene

K. Moodley et al. / Journal of Molecular Liquids 219 (2016) 206–210
209
Table 4
Table 6
Results for three-stage extractions using regenerated [EMpy][ESO4].
Total recovery for aromatic components in three stages of extractions from spent solvents.
Aromatic
components
1st stage % (v/v)
2nd stage % (v/v)
3rd stage % (v/v)
Total
%
(v/v)
Aromatic components
[EMIM][ESO4]
[EMpy][ESO4]
Extract Recovery Extract Recovery Extract Recovery
Fresh
Regenerated
Fresh
Regenerated
Toluene [% (v/v)]
E/benzene [% (v/v)]
p-Xylene [% (v/v)]
m-Xylene [% (v/v)]
o-Xylene [% (v/v)]
29
18
19
17
23
30
23
23
18
27
40
29
30
27
33
49
25
28
30
33
Toluene
4.4
20.9
11.0
12.1
13.5
15.2
3.2
15.5
7.5
8.4
9.5
10.2
2.6
12.4
5.3
6.8
7.2
7.6
49
25
28
30
33
E/benzene
p-Xylene
m-Xylene
o-Xylene
0.84
0.93
2.4
0.58
0.65
1.7
0.48
0.52
1.3
1.25
0.84
0.62
As seen in the last columns of Tables 2 and 3 the total recoveries for
three-stage extractions are in the ranges (27–40) % and (17–29) % for
[EMpy][ESO4] and [EMIM][ESO4], respectively. It is concluded from
this that the ionic liquid 1-ethyl-3-methylpyridinium ethylsulfate,
[EMpy][ESO4] shows higher extracting capacity for aromatic compo-
nents than 1-ethyl-3-methylimidazolium ethylsulfate, [EMIM][ESO4].
The pyridinium-based ionic liquid showed significant difference in
extraction ability from the imidazolium -based one, with respect to all
aromatic components recovered. The extraction ability of the former is
much higher than that of the latter. This significant difference could be
attributed to the number of π-systems in both cations; the pyridinium
ion contains six π-electrons in its 6-membered ring whereas in
imidazolium ion has four π-electrons lying in its 5-membered ring
which gives the former stronger π-systems hence the stronger the in-
teractions between the ionic liquids and aromatic components. The
overall results also indicate that, in all experiments performed, the tolu-
ene content is higher than the other aromatic components.
made by Meindersma et al., [25] that alkyl substitution on the aromatic
ring affects the molecular interaction in the preferred functional groups.
Further support for the perceived enhanced TEX interactions with the
ionic liquids may be deduced from the views of Poole, [26] and Holbrey
et al., [27]. The former suggests that hydrogen bonding between TEX
and the ionic liquid may give rise to stronger interactions, whereas the
latter proposes that ionic liquids and aromatic solutes organise them-
selves in a sandwich structure in which the ionic liquid cation and the
aromatic solutes interact in an alternating structure through π-π inter-
actions with the ionic liquid anion, arranged around this complex. In a
similar context, Deetlefs et al., [28] noted that the presence of benzene
significantly alters the structure of an imidazolium ionic liquid through
cation - cation interaction.
4. Conclusions
➢ The pyridinium-based ionic liquid has higher extraction ability than
the imidazolium based one.
3.3. Extraction of TEX using regenerated ionic liquids
➢ The ionic liquids can be recovered with their properties intact after
they have been used in an extraction procedure.
➢ It is relatively easy to recover the ionic liquids from the extraction
phases.
➢ The ionic liquids appear to retain their extraction abilities after re-
peated use.
➢ As ionic liquids are considered to be ‘green solvents’ their use in
reclaiming valuable organic solvents is good for the protection of
the environment from pollution.
➢ Since the ionic liquids can be relatively easily regenerated, there ap-
pears to be a high potential for their use in industrial scale extraction
of aromatic solvents.
To test the efficacy of regenerated solvents, the same experiments,
as described in Section 3.3 for fresh solvents was repeated with regener-
ated solvents. The results are shown in Tables 4 and 5.
It is clear from the results in Tables 3 and 4 that the regenerated sol-
vents give recoveries in the same range as that obtained by using fresh
ionic liquids for each stage of the extraction process. This is a very im-
portant result for the potential of these ionic liquids to be used in indus-
trial applications. It implies that the extraction can be done repeatedly
with a limited amount of ionic liquids. Since the cost of ionic liquids
are generally very high this result holds good promise for industrial ap-
plication of ionic liquids for recovery and reuse of ionic liquids.
It is evident from the results in Table 6 that for all the aromatic com-
ponents listed; the pyridinium-based ionic liquid gives substantially
higher extraction percentages than the imidazolium counterpart.
There is another aspect of the results in Table 6 which is worthy of con-
sideration, namely, the order of extraction of individual aromatic com-
ponents. As can be seen in Table 6, there is a general increase in the
percentages extracted in the following order for both ionic liquids: m-
xylene b ethyl benzene b p-xylene b o-xylene b toluene. This observation
can be explained in terms of steric hindrance and effects of attached
groups on the π-systems of the substituted benzenes. Furthermore, it
appears that the longer the alkyl chains which are attached to the aro-
matic rings, the more difficult it is for the molecules to organise them-
selves in a sandwich structure. This is in line with the observation
Acknowledgements
Durban University of Technology (T409) and Eskom (T196) for
financial support; Mr Jimmy Chetty for technical help.
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