Mixed Micelles of Surface Active Ionic Liquid (SAIL)–Octylphenol Ethoxylate: A Novel Reaction Medium for Selective Oxidation of Toluene to Benzaldehyde

Article abstract of DOI:10.1002/jsde.12451

Ionic liquids have been found to be suitable alternatives to volatile organic solvents in chemical transformation. Through a proper choice of cations and anions, the properties of an ionic liquid can be tuned so that it resembles an amphiphile. Such specially designed molecules are known as surface-active ionic liquids (SAIL). Like conventional surfactants, SAIL also form aggregates in an aqueous medium. Studies show that the mixing of SAIL with conventional surfactants leads to synergistic micellization. However, very few reports are available on the application of such systems as reaction media. Present study focuses on the application of mixed micelles of 1-tetradecyl-3-methylimidazol-1-ium bromide, ([C₁₄mim]Br) with nonionic surfactant, Octylphenol ethoxylate with 10 moles of ethylene oxide (OPE-10). Enhanced solubilization and selective catalytic oxidation of toluene using hydrogen peroxide as an oxidant and tungstic acid as a catalyst have been studied in detail using this system.

Full text of DOI:10.1002/jsde.12451

J Surfact Deterg (2020)
DOI 10.1002/jsde.12451
SHORT COMMUNICATION
Mixed Micelles of Surface Active Ionic Liquid (SAIL)–
Octylphenol Ethoxylate: A Novel Reaction Medium for Selective
Oxidation of Toluene to Benzaldehyde
Tushar S. Deore1 · Amid L. Sadgar1 · Radha V. Jayaram1
Received: 13 January 2020 / Revised: 15 June 2020 / Accepted: 29 June 2020
© 2020 AOCS
Abstract Ionic liquids have been found to be suitable
alternatives to volatile organic solvents in chemical trans-
formation. Through a proper choice of cations and anions,
the properties of an ionic liquid can be tuned so that it
resembles an amphiphile. Such specially designed mole-
cules are known as surface-active ionic liquids (SAIL).
Like conventional surfactants, SAIL also form aggregates
in an aqueous medium. Studies show that the mixing of
SAIL with conventional surfactants leads to synergistic
micellization. However, very few reports are available on
the application of such systems as reaction media. Present
study focuses on the application of mixed micelles of 1-
tetradecyl-3-methylimidazol-1-ium bromide, ([C14mim]Br)
with nonionic surfactant, Octylphenol ethoxylate with 10
moles of ethylene oxide (OPE-10). Enhanced solubilization
and selective catalytic oxidation of toluene using hydrogen
peroxide as an oxidant and tungstic acid as a catalyst have
been studied in detail using this system.
Introduction
Liquid phase oxidation of organic compounds is a well-
established method for the production of intermediates and
specialty chemicals (Clerici and Kholdeeva, 2013). Oxida-
tion of butane to acetic acid, cumene to phenol, cyclohex-
ane to adipic acid, p-xylene to terephthalic acid are some of
the important liquid phase oxidation processes which have
industrial relevance (Suresh et al., 2000). Liquid phase
selective oxidation of toluene to benzaldehyde is a chal-
lenge as oxidation of C H bonds requires high pressure,
high temperature, and/or oxidants with additives, which
have tendency to produce a reactive oxygen species (Gua-
jardo et al., 2015). A wide variety of oxidants are available
to the synthetic chemist ranging from molecular oxygen,
hydrogen peroxide, tert-butyl hydroperoxide (TBHP) to
organic peroxides, peroxy acids, and iodosobenzene
(Kholdeeva and Zalomaeva, 2016). Use of molecular oxy-
gen (O2) or hydrogen peroxide (H2O2) is encouraged as
they produce nonhazardous by-products. Though inexpen-
sive, molecular oxygen requires high pressure to get
homogenized in liquid phase reaction medium (Hone and
Kappe, 2018). Hydrogen peroxide is a promising alterna-
tive as it produces only water as by-product and is easy to
handle (Noyori et al., 2003). However, H2O2 is a water-sol-
uble oxidant and hence, while using it as an oxidant for the
oxidation of organic compounds that have limited or no
solubility in water, a solubilization mechanism becomes
necessary.
Keywords Surface-active ionic liquid ꢀ Mixed micelles ꢀ
Molar solubilization ratio ꢀ Partition coefficient ꢀ Catalytic
oxidation of toluene ꢀ Micellar catalysis
J Surfact Deterg (2020).
Supporting information Additional supporting information may be
found online in the Supporting Information section at the end of the
article.
Surfactants can form organized molecular system within
the water, popularly known as micelles, thus generating a
dispersed phase (Rosen and Kunjappu, 2012). These micel-
lar aggregates are of different types such as normal spheri-
cal micelles, cylindrical micelles, vesicles etc. (Ghosh
* Radha V. Jayaram
rv.jayaram@ictmumbai.edu.in
1
Department of Chemistry, Institute of Chemical Technology,
Nathalal Parekh Marg, Matugna, Mumbai, 400 019, India
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et al., 2014; Saha, et al., 2013a, Saha, et al., 2013b). In
normal micelles, the interior hydrophobic part is com-
posed of the hydrocarbon chain, the first few methylene
groups being known as palisade layer, whereas the sur-
face of the polar heads faces water. A concentric shell
of hydrophilic head groups with (1 – α)N counter ions,
where α is the degree of ionization and N is the aggrega-
tion number (number of molecules in micelles), is
called Stern layer (Acharjee et al., 2019a, b; Saha
et al., 2014). The dielectric constant of the micellar
phase is generally much lower than that of water and
hence this medium can solubilize many organic com-
pounds, which have limited solubility in water (Dwars
et al., 2005). Solubilization can occur with or without
interactions of the surfactant molecules with the solute.
Nonionic surfactants which have lower critical micelle
concentration (CMC) form larger aggregates and thus
solubilization results in a considerably high concentra-
tion of the organic solute in the micellar medium (Ullah
et al., 2019). Ionic surfactants on the other hand, form
smaller micellar aggregates, and also exhibit some sort
of interaction with polar solutes. This interaction fur-
ther enhances solubilization, which plays a vital role in
micellar catalysis (Mirgorodskaya et al., 2018; Pillai
et al., 2018).
and Hardacre, 2007). However, the high cost and viscos-
ity of ionic liquids are major constraints in their applica-
tion as reaction medium.
Ionic liquids that contain a hydrophobic chain of suitable
length exhibit surfactant-like behavior and hence are termed
as surface-active ionic liquids (SAIL). Like conventional
surfactants, they also form aggregates in water and hence
function as solubilizing agents (Gehlot et al., 2018;
Shaheen et al., 2019).
In the present study, 1-tetradecyl-3-methylimidazol-1-
ium bromide [C14mim]Br, a SAIL, was synthesized and
mixed with
a
nonionic surfactant, Octylphenol
ethoxylate with 10 moles of ethylene oxide (OPE-10).
Solubilization of toluene in this mixed surfactant system
was determined by molar solubilization ratio (MSR) and
partition coefficient values. The solubilized toluene was
then subjected to selective oxidation to benzaldehyde
using hydrogen peroxide as a green oxidant and tungstic
acid as a catalyst.
[C14mim]Br was synthesized by a reported method
(Liu et al., 2019) in which equimolar quantities of 1-
methyl imidazole and 1-bromotetradecane were mixed
in methanol and refluxed for 24 h under nitrogen atmo-
sphere. Diethyl ether was added to the viscous semi-
solid obtained after removing the solvent by rotary
evaporator. The mixture was kept aside for 72 h in a
refrigerator. The white solid obtained was filtered and
If two or more surfactants are mixed, their binary or ter-
nary mixture shows either synergism or antagonism and,
due to this property, mixed surfactant systems have been
found to be superior to single surfactant systems in many
applications (Agneta et al., 2019; Paria, 2008). The struc-
ture of micelles formed by the mixture of surfactants can be
substantially different from those of individual surfactants
(Varade and Bahadur, 2005).
1
characterized by FTIR, and H NMR techniques. CMC
of the individual surfactants and their mixtures were
determined by surface tension, specific conductivity,
and dye solubilization methods (Supporting informa-
tion). Average CMC obtained by these three techniques
are given in Table 1.
In the last few decades, ionic liquids have been exten-
sively studied due to their unique physicochemical prop-
erties (Welton, 2018). Use of ionic liquids as reaction
media is well known and in many cases the ionic liquids
not only act as solvents but also actively participate in
the formation or stabilization of intermediates (Pârvulescu
The CMC obtained are close to those reported earlier
(Sun et al., 2013). CMC of [C14mim]Br decreases with the
addition of OPE-10. This may be due to the decrease in the
mutual repulsion among the [C14mim]+ ions by the inser-
tion of OPE-10. Possible hydrogen bonding between the
ionic liquid and OPE-10 can be another reason for the
Table 1 CMC of individual surfactants ([C14mim]Br and OPE-10) and their mixtures (25 ꢁC)
Sr. No.
α([C14mim]Br)
CMC (mM)
Surface tension method
Specific conductivity method
Dye solubilization method
Average
1
2
3
4
5
6
1
2.31
0.68
0.29
0.26
0.19
0.23
2.33
0.67
0.26
0.25
0.17
—
2.0
2.21
0.64
0.25
0.23
0.16
0.21
0.8
0.6
0.4
0.2
0
0.59
0.21
0.20
0.14
0.18
Note: Where α is the mole fraction of [C14mim]Br.
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Fig 1 Solubility of toluene in pure surfactant (a) OPE-10, (b) [C14mim]Br and their mixtures by varying mole fraction of [C14mim]Br (c) α = 0.8
(d) α = 0.6 (e) α = 0.4 (f) α = 0.2
lower CMC of the mixed surfactants as compared to that of
the SAIL (Sen et al., 2019; Wang et al., 2017).
kept in a shaker for 24 h to attain equilibrium. The phases
were separated using a separating funnel and water was
added to the aqueous phase to bring the concentration
below CMC. Toluene was then extracted by petroleum
ether from the aqueous phase and the amount of toluene
present was analyzed by gas chromatography.
The solubilization capacity for toluene was determined
using various compositions of the two surfactants. Ten-
millilter of five different concentrations of each surfactant
was prepared and mixed accordingly to get a constant com-
position of mole fraction in each set. Constant volume of
toluene (1 mL) was added in each flask and the flasks were
The MSR and partition of solute in micelles—water (par-
tition coefficient, KM) are two important parameters to
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Table 2 MSR and partition coefficient of toluene in SAIL and its mixture with OPE-10
Sr. No.
α([C14MIM]Br)
MSRideal
MSRexp
R = MSRexp /MSRideal
KM
ln KM
1
2
3
4
5
6
1
—
0.22
0.28
0.41
0.43
0.49
0.38
—
1746.86
2144.97
2851.47
2948.53
3224.66
2700.09
7.46
7.67
7.95
7.98
8.07
7.90
0.8
0.6
0.4
0.2
0
0.25
0.28
0.31
0.34
—
1.13
1.33
1.38
1.41
—
measure the effectiveness of micelles toward solubilization.
MSR depicts the number of moles of solute that can be sol-
ubilized per mole of surfactant while KM is the distribution
of solute between water and micelles in terms of moles.
MSR is expressed by Eq. (1) (Maswal et al., 2015)
½STꢂ−½SMꢂ
MSR =
ð1Þ
½Cꢂ−CMC
Scheme 1 Oxidation of toluene
where ST and SM are the apparent solubilities of the solute
in water and micelles, respectively. C is the concentration
of surfactant taken and CMC its critical micelle concentra-
tion. The surfactant concentrations used for determining
MSR are higher than the CMC (C > > CMC). The slope of
the plot of concentration of solute present in the solution
versus concentration of surfactant gives the MSR (Fig. 1).
The partition coefficient is given by Eqs (2) and (3)
Xm
KM
=
:
ð2Þ
Xa
Scheme 2 Bromination of toluene
where
MSR
Xm = mole fraction of solute in micelles = ð1 + MSRÞ and
Table 3 Conversion and selectivity for the oxidation of toluene
Xa = mole fraction of solute in aqueous phase = ½ST ꢂ:Vw
Equation (2) can be rearranged as
Entry
Catalyst
Conversion
(%)
Selectivity (%)
Benzaldehyde
Benzoic
acid
MSR
KM
=
ð3Þ
1
2
Sodium
tungstate
Trace
Trace
—
—
—
ST :VW :ð1 + MSRÞ
The MSR and KM of toluene in individual and mixed
surfactants system are given in Table 2.
Ammonium
molybdate
—
The MSR and KM values reveal enhanced solubilization
of toluene in the mixed surfactant system. In the case of
SAIL, solubilization may occur with additional π-π interac-
tion between the solute and the imidazolium head group
(Łuczak et al., 2013). The higher values of MSR and KM
obtained with mixed surfactants support synergism.
Ideal value of MSR is given by Eq. (4) as (Rao and
Paria, 2009)
3
4
5
Tungstic acid
Molybdic acid
Ferric chloride
30
80
75
—
20
25
—
10
Trace
The solubilized toluene was then subjected to liquid
phase catalytic oxidation (Scheme 1). For this study, 5 mL
solution of [C14mim]Br (α = 0.6) and OPE-10 were taken
in a round bottom flask to which 0.5 mmol of toluene and
0.1 mol% of the catalyst were added and the reaction mix-
ture was kept in an ice bath. The oxidant H2O2 in appropri-
ate mole ratio was then added dropwise. The reaction was
carried out at various temperatures. The progress of
MSRideal = α1:MSR1 + α2:MSR2
ð4Þ
In the present study, MSR experimental/MSR ideal > 1.0,
indicating better solubilization of toluene in the mixed sur-
factant system.
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was changed to 1:3, benzoic acid was observed as the
major product indicating a decreased selectivity toward
benzaldehyde.
As it is well known that the oxidizing efficiency of
peroxotungstate may increase in acidic solutions (Ishimoto
et al., 2012), pH of the reaction mixture in the present study
was decreased to about 4.0 by the addition of Conc.H2SO4.
Surprisingly under such acidic conditions, bromo toluene
was obtained as reaction product confirmed by Gas Chro-
matography–MS results (Scheme 2). This can be due to the
reaction of the counterion of the SAIL with toluene under
acidic conditions.
In conclusion, MSR and partition coefficient values indi-
cate enhanced solubilization of toluene in the mixed micel-
lar system. The solubilized toluene can successfully be
oxidized by tungstic acid to benzaldehyde at 60 ꢁC.
Considering the high cost of ionic liquids, use of mixed
systems of SAIL and a conventional surfactant system can
be a cost effective alternative. This new class of reaction
medium can be extensively explored for a gamut of organic
reactions.
Fig 2 Effect of temperature on conversion and selectivity
Acknowledgement Author Tushar S. Deore would like to acknowl-
edgement to University Grant Commission (UGC), India for Senior
Research Fellowship (SRF). Amid L. Sadgar is thankful to Depart-
ment of Atomic Energy (DAE) for the Junior Research Fellow-
ship (JRF).
Conflict of Interest The authors declare that they have no conflict
of interest.
Fig 3 Effect of mole ratio of H2O2 on conversion and selectivity
Reference
reaction was monitored by gas chromatography using
biphenyl as an internal standard.
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