Deep compositional understanding of TBA: AlCl3 ionic liquid for its applications

Article abstract of DOI:10.1016/j.molstruc.2020.128936

Chloroaluminate ionic liquids (ILs) have been immensely used as homogeneous catalyst in Friedel-Crafts reaction. We have recently synthesized chloroaluminate ILs by reacting aluminium chloride with a hydrophobic neutral ligand i.e. tributylamine (TBA:AlCl₃). The current study elaborates on the investigations of the composition of the ionic liquids at various stages of their formation. The ionic liquids were synthesized using various mole ratios of tributyl amine and aluminium chloride in range of 1:1 to 1:2.3, in presence of an aromatic solvent in a one pot reaction. Various characterization techniques like Mass spectrometry, ²⁷Al Nuclear Magnetic Resonance, ³¹P Nuclear Magnetic Resonance and Fourier Transform Infrared spectroscopy were used to elucidate the formation of various moieties of the TBA:AlCl₃ Ionic Liquid. This study also elaborates on the investigations of the cationic and anionic moieties and their structure-property relationship for various applications. Various Friedel-Crafts reaction of industrial importance were performed using the ionic liquid having (Al₂Cl₇)^?moiety to assess its performance and compared with conventional processes. The synthesized products were characterised by sophisticated analytical techniques like ¹H NMR, ¹³C NMR, FTIR, GC–MS, GC-FID, to name a few. This class of ionic liquids also have importance in various electrochemical applications like aluminium deposition and aluminium batteries.

Full text of DOI:10.1016/j.molstruc.2020.128936

Journal Pre-proof
Deep Compositional Understanding of TBA:AlCl Ionic Liquid for its
3
Applications
Mangesh Sakhalkar , Rudra Prosad Choudhury ,
Vishnupriya Bhakthavatsalam , Sharad V. Lande ,
Jeevan Pradhan , Sudeshna Chandra
PII:
S0022-2860(20)31261-8
DOI:
Reference:
https://doi.org/10.1016/j.molstruc.2020.128936
MOLSTR 128936
To appear in:
Journal of Molecular Structure
Received date:
Revised date:
Accepted date:
9 May 2020
28 June 2020
18 July 2020
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Mangesh Sakhalkar ,
Rudra Prosad Choudhury ,
Vishnupriya Bhakthavatsalam , Sharad V. Lande , Jeevan Pradhan , Sudeshna Chandra , Deep
Compositional Understanding of TBA:AlCl Ionic Liquid for its Applications, Journal of Molecular
3
Structure (2020), doi: https://doi.org/10.1016/j.molstruc.2020.128936
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Highlights
1
2
3
4
5
.
.
.
.
.
Characterisation study to elucidate the formation of novel TBA:AlCl catalyst.
3
Confirmation of cation and anion composition at various mole ratio by NMR and FTIR.
Lewis acidity with compositional change for Structure-property relationship.
Green alternate catalyst for Friedel Crafts reactions with high conversion/yields.
New electrolyte system containing hydrophobic ligand for plausible Al deposition.
1

Deep Compositional Understanding of TBA:AlCl Ionic Liquid for its
3
Applications
1
,2
2
2
2
Mangesh Sakhalkar , Rudra Prosad Choudhury ,Vishnupriya Bhakthavatsalam , Sharad V. Lande ,
1
1*
Jeevan Pradhan , Sudeshna Chandra
1
Department of Chemistry, Sunandan Divatia School of Science, SVKM’s NMIMS (Deemed to-be University),
V. L. Mehta Road, Vile Parle (West), Mumbai -400056, India
2
M/s Reliance Industries Ltd, TC30C, First floor, Reliance Corporate park, Thane - Belapur road, Ghansoli,
Navi Mumbai-400701, India
Graphical abstract for characterisation paper
Abstract
Chloroaluminate ionic liquids (ILs) have been immensely used as homogeneous catalyst in
Friedel-Crafts reaction. We have recently synthesized chloroaluminate ILs by reacting
aluminium chloride with a hydrophobic neutral ligand i.e. tributylamine (TBA:AlCl ). The
3
current study elaborates on the investigations of the composition of the ionic liquids at
various stages of their formation. The ionic liquids were synthesized using various mole
ratios of tributyl amine and aluminium chloride in range of 1:1 to 1:2.3, in presence of an
aromatic solvent in a one pot reaction. Various characterization techniques like Mass
2
7
31
spectrometry, Al Nuclear Magnetic Resonance, P Nuclear Magnetic Resonance and
Fourier Transform Infrared spectroscopy were used to elucidate the formation of various
moieties of the TBA:AlCl Ionic Liquid. This study also elaborates on the investigations of
3
the cationic and anionic moieties and their structure-property relationship for various
applications. Various Friedel-Crafts reaction of industrial importance were performed using
-
the ionic liquid having (Al Cl ) moiety to assess its performance and compare with
2
7
conventional processes. The synthesized products were characterised by sophisticated
1
13
analytical techniques like H NMR, C NMR, FTIR, GC-MS, GC-FID, to name a few. This
2

class of ionic liquids also have importance in various electrochemical applications like
aluminium deposition and aluminium batteries.
2
7
Keywords: TBA:AlCl Ionic Liquid; Characterization; Al NMR; Aluminium deposition;
3
Electrodeposition
Introduction
Use of room‐temperature ionic liquids have been rapidly growing in various areas in
chemistry [1]. Chloroaluminate ionic liquids are one of the first-generation ionic liquids
having the wide range of application in catalysis [2], electrodeposition [3] and aluminium
batteries [4]. These chloroaluminates are conventionally derived from quaternary nitrogen
-
-
like alkyl amine, alkyl pyridinium, dialkyl imidazolium etc. Anions like (AlCl ) or (Al Cl )
4
2
7
-
are believed to have an active moiety of chloroaluminates where (Al Cl ) is considered
2
7
electroactive. However, anionic aluminium species have to move against the electrostatic
field during reduction in electrodeposition process. Ionic liquids having cationic aluminium is
envisaged to promote easier aluminium deposition and therefore, now-a-days,
chloroaluminates are synthesized with neutral ligands like acetamide [5], butylpyrolidine [6],
dipropyl sulphide [7] etc.
Chloroaluminate ionic liquids are reported as catalyst for several organic and Friedel–Crafts
reactions due to low catalyst requirement, higher activity, and biphasic reaction systems. The
Friedel–Crafts reactions are used to produce pharmaceuticals, fine chemicals and their
intermediates for many years. For acylation, various acylating agent like acyl chloride and
anhydrides are used while for alkylation, alkylating agents like alkyl halides, alcohols, olefins
are commonly used. These alkylated aromatics are produced conventionally by catalysing
with various Lewis acids [8]. However, use of Lewis acid catalyst results in huge waste
generation. Conventional chloroaluminate ionic liquids have demonstrated high catalytic
3

activity in various Friedel -Crafts reactions [9]. Even though, pertaining to their performance
in many Friedel–Crafts reactions, most of the studies have focused on those derived from
imidazolium based ionic liquid, which are very expensive.
Recently, we have reported a novel chloroaluminate ionic liquid synthesis in single step from
tributyl amine, a hydrophobic neutral ligand in presence of solvent [10]. Tributyl amine was
expected to give better stability towards air and water sensitivity due to hydrophobicity.
In this paper, chloroaluminates were designed and synthesized by reacting tributyl amine
with aluminium chloride in various mole ratios in presence of solvent. Various
characterization techniques were used to understand the step-wise formation of the ionic
liquid and study their structure-property relationships for specific application. Spectroscopic
2
7
31
techniques like FTIR, Al NMR, P NMR and ESI-MS were used to understand the
compositional changes of cation and anions.
The synthesized ionic liquid has an edge over conventional chloroaluminates in terms of
single step synthesis, better atom economy, effluent reduction, etc. Moreover, tributyl amine
can be recovered by neutralisation and easily reused for catalyst synthesis, resulting in
effluent reduction which is difficult in conventional catalyst. We have used this ionic liquid
as catalyst for various Friedel-Crafts reaction of aliphatic and aromatic compounds to
explore its catalytic activity as new greener alternative in industrially important processes.
Further, this ionic liquid can be plausibly used in electrodeposition of aluminium.
Experimental
Materials and Apparatus
All the reagents were procured from M/s SD Fine Chem. and used without any further
purification. All solvents (tributyl amine, benzene, cyclohexene, aniline, benzene etc ) were
4

purchased from commercial sources M/s S D Finechem and Merck. All the chemicals used
are of analytical or laboratory reagent grade.
Synthesis of TBA Chloroaluminate ionic liquid
Tributyl amine i.e. TBA and benzene were added to reactor and stirred to ensure the complete
mixing at ambient temperature. The temperature of the reactor was controlled by external
heating device assembled with the reactor. The reactor was purged with nitrogen and kept in
cabinet under dry nitrogen purging. Aluminium chloride was added as per pre-calculated
°
mole ratio to the reactor and the reactor was heated at 80 C. Reactants were thoroughly
mixed by overhead stirrer followed by heating to ensure complete formation of the ionic
liquid which was obtained as a homogeneous dark brown liquid. The reactor was cooled to
ambient temperature and ionic liquid was isolated and stored in air tight glass bottle under
2
7
nitrogen atmosphere. The ionic liquid was characterised by Mass spectroscopy, Al NMR
3
1
spectroscopy, Fourier transform spectroscopy and P NMR to understand the structural
composition and spatial arrangements of cation and anions for possible applications in
electrochemistry and synthetic (catalytic) processes. The synthesized ionic liquid was used
without any further purification for catalytic applications i. e. Friedel-Crafts reactions.
Various Friedel-Crafts reactions were carried out using the ILs and are mentioned below.
Reaction Schemes
1
2
.
.
5

3
.
4
5
.
.
6
7
.
.
Typical procedure Friedel-Crafts reactions usingTBA:AlCl Chloroaluminate Ionic liquid
3
In 100 ml reactor, reactants A and B were mixed by mechanical stirrer and assembled with
condenser with chilled water circulation. Reactant A was taken in excess mole ratio to
reactant B and which varied from 2-10 mole depending on type of reaction. Reaction was
maintained at 25-70 °C and 1-10 w/v% of freshly prepared ionic liquid was added to the
reaction mixture. The reaction was continued for 30 min and then cooled. The organic layer
was separated and de-acidified. Reaction feeds and products were analysed with gas
chromatography to estimate conversion and yield.
Alkylation of benzene with olefins were conducted by using inert solvent like paraffins. In
chlorination reaction, chlorine was produced by adding hydrogen peroxide to conc. HCl and
the chlorine gas was bubbled through reactant with catalyst. In acylation of benzene with
phthalic anhydride, ionic liquid equivalent to 3 moles of AlCl per mole of anhydride was
3
added. The upper layer containing hydrocarbon was washed with water and treated with 2N
6

sodium hydroxide solution. Sodium salt of benzoyl benzoic acid in aqueous layer was washed
with dichloromethane. The product in the aqueous layer was reconstituted by treating with
dilute HCl and recrystallized by dichloromethane – hexane solvent and confirmed by
melting point, LC- MS and NMR.
Results and discussion
The ionic liquid was examined with various characterisation techniques to identify its
composition at different mole ratios. The properties of the ionic liquids are essentially
governed by the nature of their cationic as well as anionic species. Based on the nature, cation
and anion composition, the ionic liquids can be used for specific applications.
Chloroaluminates are primarily used as a catalyst in Friedel-Crafts reaction due to its unique
properties like low catalyst requirement and easy separation. Various spectral analysis
methods, mainly UV-vis, NMR, and IR spectroscopies are traditionally used for measurement
of the acidity of the ionic liquids. The UV-vis spectroscopy method for Hammett function are
used for semi-quantitative determination of Bronsted acidity in ionic liquids, however, it is
not applicable for determining Lewis acidity of the chloroaluminate ionic liquids containing
(
Al Cl ) moiety. Similarly, cationic Al species is preferred for electrodeposition than anionic
2 7
Al species as it can move against the applied electrostatic field. Therefore, in order to get the
deep structural and compositional understanding of the ionic liquids prepared with different
2
7
31
mole ratios, they were characterised by using FTIR, Mass spectroscopy, Al NMR.
P
.
NMR was used t ascertain the Lewis acidity of the ionic liquids
Characterisation of Ionic Liquids
FTIR was used to understand the speciation of cationic and anionic moieties formed by
reaction of various mole ratios of Tributylamine with Aluminium chloride.
7

Thermo iS50 Fourier Transform Infrared Spectrometer equipped with attenuated total
reflection (ATR) with Diamond crystal with 45º single bounce mounted directly onto the
spectrometer bench compartment was used to record the IR spectra. A drop of sample, under
nitrogen blanket, was placed on the diamond crystal for measurement.
Fig 1: (A) Overlay of FTIR of various mole ratios of TBA and Aluminium chloride and (B)
-1
enlarged overlay spectrum in wave number region of 800 - 400 cm
FTIR studies of the ionic liquids as indicated in Fig 1, shows the characteristic bands which
can be related to composition as tabulated below.
-
1
Reactant
Characteristic band
Characteristic Frequency (cm )
Aliphatic C-H stretch
C-H bend scissoring
Aromatic C-H
3000-2850
1470-1450
3000-3100
TBA
Benzene
Table 1: Characteristic frequency and band in FTIR
Additionally, samples with 1:1 and 1:1.3 mole ratio of TBA and aluminium chloride shows
-1
-
peak with shoulder at 489 cm which is associated to the mononuclear aluminium (AlCl )
4
moiety [11]. The FTIR spectra for 1:1.8 and 1:2.3 mole ratio of TBA and AlCl shows peaks
3
-1
-
at 529 and 507 cm which indicates the formation of binuclear aluminium (Al Cl ) moiety
2
7
-1
[
6,11]. The peak at 489 cm disappears with higher mole ratios i.e.1:1.8 and 1:2.3. The FTIR
8

-1
spectra of these two samples also shows additional broad shoulder at 625 – 575 cm . This
broad shoulder may be due to the presence of multiple anionic components present in the
-1
samples. Distorted C-N linkage at wavenumber 1080-1360 cm with prominent and sharp
-1
peak at 675 cm are indicative of Al-N linkage which is expected in cationic moiety [12].
This has been supported by ESI-MS of ionic liquid with 2.3 mole ratio of the aluminium
chloride to tributyl amine. There are many visible peaks with various m/z values, mainly
+
+
showing TBA cation signal at m/z 186 as base peak, and the cluster [(TBA)(Al Cl )] at m/z
3
4
-
4
07 in positive ionisation mode while (AlCl ) moiety as base peak with m/z of 168 in
4
negative ionisation mode. In ESI - MS spectra for the neutral ligand based chloroaluminate
-
ionic liquids, (Al Cl ) moieties were not observed [13,14,15]. Though the phenomenon is not
2
7
clear, it may be envisaged that there might be problems with using mass spectrometry for
speciation of chlorometallate ILs. However, the phenomenon is validated for a similar system
of ILS consisting of 1-butylpyrolidine and AlCl using vibrational spectroscopy which is
3
-
considered more relevant in this context, showing the absence of acidic, (Al Cl ) anion [16].
2
7
NMR spectra were recorded using a Bruker Avance III HD spectrometer equipped with 9.1 T
2
7
and 400 MHz proton frequency. The Al NMR spectra were externally referenced to
aluminium chloride (AlCl ) solution at 0.0 ppm chemical shift. All the chemical shifts are
3
observed in 90 ppm – 110 ppm region confirming the tetrahedral symmetric arrangements in
strong ionic environment for all the moieties formed with various mole ratios of tributyl
amine and aluminium chloride.
9

2
7
Fig 2: Al NMR of various mole ratios of TBA and Aluminium chloride (A) 1:1 mole (B)
:1.3 mole (C) 1:1.8 mole (D) 1:2.3 mole
1
27
Fig. 2 represents the Al NMR spectra of 1:1 mole of TBA and AlCl which shows single
3
sharp peak at chemical shift of 103 ppm representing an adduct (TBA:AlCl ) with no other
3
peaks (Fig 2A). As the mole ratio was increased to 1.3, three clear peaks were observed (Fig
2B). Chemical shift at 103 ppm which coincides with that of the adduct was seen along with
two new chemical shifts at 109 ppm and 93 ppm are detected. Out of these , the peak at 109
+
ppm may be assigned to the cationic aluminium as [Al Cl (TBA) ] , while the peak at 93
y
z
x
-
ppm can be referred to as an anionic aluminium as (AlCl ) anion. Further increase of
4
aluminium chloride mole ratio to 1.8 and 2.3 moles, the peak at 109 ppm of cation becomes
predominant and broadens with a shoulder around 96-94 ppm, depicting the formation of two
distinct moieties. The shoulder peak has been found to shift from 93 ppm in 1.3 mole to 94-
-
9
6 ppm in higher mole ratio. The shift may be attributed to the conversion of (AlCl ) anion
4
-
to (Al Cl ) anion. The peak broadening takes place due to increase in structural in-
2
7
homogeneity with increase in the concentration of aluminium chloride. There is slight
1
0

change observed in chemical shifts of anion which may be due to the broadening of peak over
wide range of ~10 ppm which was resulting in variation while identifying the peak maxima.
It was observed that the height (Area) of the shoulder for 2.3 mole was higher than 1.8
-
27
mole, which may be attributed to more (Al Cl ) species in ionic liquid. Thus, Al NMR
2
7
shows the formation of TBA-AlCl adduct with 1:1 mole ratio. It gets transformed to [Al Cl
z
3
y
+
-
-
(
TBA) ] cation with (AlCl ) anion which further gets converted into (Al Cl ) anion on
x 4 2 7
addition of aluminium chloride [6]. The chemical shift at 103 ppm representing adduct was
clearly absent indicating complete dissociation of adduct in ionic liquid.
2
7
Al NMR spectra of chloroaluminate shows the change in chemical shift of anionic moieties
with respect to the organic cation moiety in the ionic liquid. Table 2 shows the chemical
shifts of cationic and anionic aluminium in various chloroaluminates.
Chloroaluminate
Chemical shift
92ppm
Reference
[
C mpip][NTf ]–AlCl
[17]
3
2
3
[
C mpyr][NTf ]–AlCl
94 ppm
4
2
3
–
(EMIm)Cl - AlCl3
(Al Cl ) @ 103.3 - 103.4 ppm
[18]
2
7
–
(
Al Cl ) @ 96.7 ppm
2 7
−
Urea - AlCl Melt
3
(AlCl ) @ 101.8 ppm
4
[
19]
+
[
AlCl ·(urea) ] @ 71.8 ppm
2
2
–
(
Al Cl ) @ 73.9 ppm
2 7
(
BMIm)Cl - AlCl3
[20]
[13]
−
(
AlCl ) @ 79.3 ppm
4
+
[
AlCl (Acet) ] @ 101.1 ppm
2
2
+
[
AlCl (Acet)] @ 73.6 ppm
2
Acetamide - AlCl3
-
(
AlCl ) @ 88.6 ppm
4
Table 2: Chemical shifts of Anionic and cationic Aluminium in various chloroaluminates
2
7
From FTIR, and Al NMR studies and analysis, following two types compositions of ionic
liquids are possible as
+
-
X(TBA) + nX(AlCl3)
X(TBA) + nX(AlCl3)
{(TBA) .(Al(nx-n)Cl(nx-4n))} . nAlCl
x
4
+
-
{(TBA) .(Al(nx-2n)Cl(nx-7n))} . nAl Cl
x
2
7
1
1

Further, for determination of Lewis acidic strength of the ionic liquids at various mole ratios,
3
1
the Gutmann acceptor number was determined by P NMR using triethyl phosphine oxide as
probe molecule. Chloroaluminates are believed to possess higher acidity than that of
aluminium chloride which have Gutmann acceptor number equal to 87. The chemical shift of
3
1
triethyl phosphine oxide (TEPO) dissolved in ionic liquids in
P NMR is directly
proportional to the Lewis acidity and can be identified and quantified as Gutmann acceptor
3
1
number. P NMR was used to measure the Lewis acidity of of freshly prepared ionic liquids
using various mole ratios of TBA–AlCl3
3
1
Fig 3: P NMR of various mole ratios of TBA and Aluminium chloride (A) 1:1 mole (B)
:1.3 mole (C) 1:1.8 mole (D) 1:2.3 mole
1
As shown in Figure 3, the sequential change in chemical shifts of TEPO were observed with
modified mole ratios of TBA and AlCl . All the ionic liquids possess major peak at ~80.2-
3
1
2

8
0.4 ppm which can be assigned to the cationic Al species exhibiting equivalent Gutmann
acceptor number of AlCl i.e. 87. The higher Lewis acidity of cation than AlCl have been
3
3
reported in the amide aluminium chloride ionic liquids [5]. Many other peaks were observed
at chemical shift from 75 ppm to 80 ppm for 1:1 mole ratio which possess lower Lewis
acidity. Many of these less Lewis acidic peaks were diminished with increase in mole ratio to
-
1
:1.3 with one peak emerged at 82.6 ppm which may be due to formation of (AlCl ) . Further
4
increase to 1:1.8 mole ratio shows two more peaks at 81.5 ppm and 83.4 ppm besides peaks
-
of cationic species and (AlCl ) anionic species where 83.4 ppm chemical shift represents
4
-
formation of (Al Cl ) anionic moiety. In conventional ionic liquids, a single peak at 83.48
2
7
3
1
ppm for TEPO was observed for neat Et NHCl– AlCl IL in the P NMR spectra with 1:2
3
3
molar ratio of Et NHCl to AlCl indicating that neat Et NHCl–AlCl IL contains only single
3
3
3
3
-
Lewis acid moiety i.e. [Al Cl ] . However, two moieties were observed at two peaks at 83.48
2
7
and 84.92 ppm for the neat N-methyl acetamide–AlCl ionic liquid with similar molar ratio
3
1
:2 of N-methyl acetamide to AlCl with the addition of TEPO. The presence of various
3
-
-
-
acidic moieties like (AlCl ) , (Al Cl ) , (Al Cl ) are in equilibrium with 1:2 mole ratio of
4
2
7
3
10
conventional ionic liquids [21, 22,11]. With TBA:AlCl ionic liquids, the presence of various
3
super acidic moieties was observed at 81.4 ppm, 81.6 ppm, 82.5 ppm, 83.44 ppm, 84.2 ppm
-
and 84.7 ppm. The moiety at 83.44 coincide with the (Al Cl ) . The presence of various other
2
7
-
Lewis acid moieties along with (Al Cl ) , confirms the presence in TBA -AlCl .
2
7
3
It is believed that the coordination of the triethyl phosphine oxide with a Lewis acid lead to
change in the chemical shift with change in concentration of TEPO. Therefore, various
concentration of TEPO was dissolved in chloroaluminate ionic liquid to confirm the Lewis
3
1
acidity and to minimise the concentration effect. P NMR spectra of TEPO at several small
concentrations are recorded, and the chemical shift at infinite dilution was identified by
1
3

extrapolation. It was then compared with the infinite dilution of TEPO in hexane to identify
the actual Gutmann acceptor number using formula (AN = 2.348 * ∆ chemical shift) .
3
1
Fig 4: P NMR of various concentration of TEPO in ionic liquid for 1:2.3 mole ratios of
TBA and Aluminium chloride
3
1
P NMR of 1:2.3 mole ratio of TBA and AlCl , is shown in Fig. 4 wherein many acidic
3
moieties were observed at mole ratio of 1:2.3 of TBA and AlCl between 80.3 to 85 ppm
3
having Gutmann acceptor number in the range 87-97, as calculated by formula for infinite
dilution with respect to chemical shift at infinite dilution of hexane (AN = 2.348 * ∆ ). This
inf
observation is in line with the broad shoulder observed in the FTIR spectra of this sample.
This kind of strong Gutmann acceptor number provides super acidity in the sample which is
required to drive the Friedel-Crafts reaction at lower catalyst concentration. Therefore, the
1
4

Lewis acidity of this chloroaluminates was supposed to be higher than aluminium chloride to
make it super acidic.
Catalytic activity of ionic liquid for Friedel-Crafts reactions
Friedel–Crafts reaction has been widely studied with various chloroaluminates due to distinct
advantages in Lewis acid catalysis. Therefore, to understand the catalytic properties of this
-
chloroaluminate ionic liquid having (Al Cl ) moiety, various Friedel-Crafts reactions such as
2
7
alkylation with olefins and halogenated hydrocarbons, acylation using acyl chloride and
anhydrides etc were carried out and the results are mentioned in Table 3. All the reactions
were carried with catalyst using benzene as solvent. However, different solvents can also be
used for reactions depending upon the preferential reaction mechanisms.
chloroaluminates are active catalyst for Friedel-Crafts reaction primarily due to Lewis
acidity of the chloroaluminate ionic liquid which enhance the conversion and selectivity of
-
the desired products. (Al Cl ) moiety, which provides high Lewis acidity, has been
2
7
predominantly formed when the mole fraction of AlCl exceeds to 0.5 and has been
3
considered to be highest at mole fraction of ~ 0.67 [23]. Also, the Gutmann Acceptor Number
(AN) has been considered as a representation of the Lewis acidity. Therefore, considering the
properties estimated above, the Chloroaluminate ionic liquid with 1:2.3 mole ratio of TBA to
Aluminium chloride has been used for all the reactions.
1
5

a= GC
b= NMR
c = FTIR
d= MS
e = GC MS
Reaction
Temparature
Cat Conc.
% w/v
Analysis
Method
Reactant A
Reactant B
Product
Mole Ratio
Yield
Room temp
6:1
2%
80%
a
(
~25 °C)
Room temp
~25 °C)
1
0:1
0:1
2%
2%
2%
90%
71%
95%
a
a
a
(
Room temp
~25 °C)
1
(
Room temp
~25 °C)
6
:1
(
5
0 °C)
6:1
5:1
10%
56%
93%
92%
85%
90%
92%
90%
75%
91%
90%
92%
a
b,d
a,c,e
a
As given in
Procedure
7
0 °C
Room temp
~25 °C)
2:1
2:1
2:1
5%
5%
5%
2%
2%
2%
2%
5%
5%
(
Room temp
~25 °C)
(
Room temp
~25 °C)
a
(
Room temp
~25 °C)
Cl2
Cl2
2:1
2:1
a,e
a,e
a,b,e
a
(
Room temp
~25 °C)
+
(
Room temp
~25 °C)
6
:1
:1
(
Room temp
6
(
~25 °C)
C10 H20
C4H8
C10 H20
C4H8
Oligomer
Oligomer
50 °C
-
-
a
50 °C
a
Table 3: Friedel-Crafts reactions using chloroaluminate ionic liquid
1
6

The yields of the product (Table 3) were determined by gas chromatographic analysis except
1
3
for acylation with phthalic anhydride where actual yield was mentioned. The FTIR,
C
1
NMR, H NMR and Mass spectrometry analysis of products are given in supporting material
(
Figure S1-S11). This ionic liquid has provided equivalent or better yields compared to other
available technologies for various Friedel-Crafts reactions.
Alkylation of aromatic compounds with olefins and chlorinated hydrocarbons showed higher
conversion, selectivity, and good yields in short duration of time. Alkylation of benzene
with cyclohexene showed 80% selectivity towards cyclohexyl benzene with 1:6 ratio in
comparison with conventional ionic liquids that has 70.5% selectivity with 1:16 mole ratio of
cyclohexene to benzene [24] whereas with aluminium chloride as catalyst, selectivity was
only 48.5% with 1:6 mole ratio [25]. While the reaction of benzene with cis-butene shows
slightly higher yields. Reaction of o-xylene with styrene showed 71% yield with complete
conversion of styrene without any inert solvent. Selectivity of diphenyl methane was also
found to be equivalent with conventional ionic liquids after complete conversion of benzyl
1
13
chloride [26]. Diphenyl methane was characterised by Mass spectra, H NMR and C NMR
for product confirmation. Alkylation of butyl chloride also showed higher conversion to butyl
benzene.
Acylation of aromatics with acid chlorides or anhydrides presented higher conversion to
acetophenone but with benzoyl chloride, the benzophenone yield was found to be 56% even
after longer reaction times and higher catalyst loading. Similar yields were found with
conventional chloroaluminate (BMIM:AlCl ) system in one hour reaction time [27].
3
2-benzoyl benzoic acid, a precursor for anthraquinone and its derivatives, was produced by
Friedel-Crafts reaction between benzene and phthalic anhydride [28]. Benzene acylation with
phthalic anhydride was performed using ionic liquid to get higher yield. The product was
1
7

°
purified, and the melting point was found to be 127 C which is in accordance to the material
o
o
safety data sheet from Sigma Aldrich (126 C -129 C). The mentioned products were also
1
13
confirmed by Mass spectra, H NMR and C NMR .
Alkylation of aniline was attempted by Michael addition using methyl acrylate on supported
AlCl [29]. Aluminium chloride and the aniline forms a 1:1 complex due to their Lewis acidic
3
and Lewis basic properties respectively, resulting in deactivation of AlCl catalyst and
3
terminating the reaction. Alkylation of aniline with methacrylate with this catalyst was found
to be faster (20 min) and resulted in high yield (>90 %) which has been equivalent to that of
supported AlCl . The formation of product was confirmed using GC-MS and FTIR analysis.
3
-1
FTIR of the product showed N-H band at 3366 cm corresponding to secondary amine. It
-1
also exhibited mono-substituted aromatics peak at 690 and 746 cm . Hence, the conversion
of aniline to secondary amine by N-alkylation was confirmed. GC-MS of product shows m/z
=
179 which confirms the molecular weight of desired product. Reactions with di-isopropyl
amine and cyclohexyl amine also demonstrated high conversion.
Chlorination of aromatic compounds are useful for application in fungicides, insecticides,
bulk drug and various other industries [30]. Monochlorinated compounds were obtained in
good yields( > 90%) in short reaction time and at room temperature. The products are
characterised by GC-MS. Similar high yields were achieved with conventional catalyst
(
BMIM:AlCl ) but at 80 °C [31].
3
Oligomers are widely used as the base material for manufacture of lubricants [32].
Oligomerisation of olefins was carried out with high yields using ionic liquid.
Various reactions described above demonstrates the high efficiency of the chloroaluminate in
important organic processes. Notably, the catalyst can be efficiently used for N-alkylation
where conventional catalyst i.e. metal halides form an adduct which prevents alkylation. The
1
8

catalytic activity of the ionic liquid was found to be equivalent to conventional
chloroaluminates whereas the yields were higher in few reactions. Chloroaluminate catalyst
depict similar intrinsic properties like small catalyst quantity requirement and easy separation
of catalyst in reactions as that of conventional chloroaluminates. Additionally, the major
advantage is one step synthesis process for chloroaluminates which reduces the chance of
moisture ingression as well as precursor impurities in ionic liquids when reactants are added
in stoichiometric quantities in one step. Also, the hydrophobic ligand could help in
optimising the liquid – liquid interaction which is highly desired in biphasic catalytic
reactions[33]. Lower interaction could result in high reaction time and more catalyst
requirement as observed in iso-propylation of naphthalene where reaction time was 3 hrs
[
34].
Advantages in plausible electrodeposition of Aluminium
Conventionally, aluminium deposition is performed using non-aqueous baths containing
organic solvents like chlorinated solvents, comprises volatile organic solvents which are
highly flammable. They have detrimental environment effects during applications and high
safety measures are essential. Inorganic salt melts are better options for metal depositions
requiring high temperature and eventually having high energy requirements besides the loss
of aluminium. Ionic liquids (Chloroaluminates) can be easily employed at much lower
temperatures with less safety concerns for electrodeposition and are environment friendly.
Higher current density was observed at high temperature and high mole ratio which is due to
−
more electroactive (Al Cl ) ions. Higher temperature have been required to moderate high
2
7
viscosity resulting in a faster kinetic process and high diffusivity which can lead to higher
current densities for Al plating and stripping [35]. This would also increase the conductivity
of electrolyte resulting into higher deposition rate of aluminium on the cathode as the
concentration of the electrolyte play prominent role [36]. Aromatic solvent used as co-solvent
1
9

also resulted significant improvement in quality of deposit and enhanced diffusivity of the
Lewis acidic chloroaluminate ionic liquids [37,38].
The conventional chloroaluminate ionic liquids are made from quaternary ammonium halide
salts which have high cost and hygroscopic in nature which limit its application in
aluminium deposition. Therefore, use of hydrophobic ‘‘neutral’’ ligands in development of
chloroaluminate ionic liquid was significant for the practical electroplating of Al [7].
Therefore, Tributyl amine with aluminium chloride is the ideal molecule with properties like
hydrophobicity, and neutral ligand.
The electrochemical behaviour of TBA:AlCl chloroaluminate as electrolyte was assessed
3
using cyclic voltammetry (CV) and is shown in Figure 5. The CV of ionic liquid mixture
(1:1.8mol) at 300K was carried out using a three electrode system with glassy carbon as the
working electrode (W.E), Pt as counter and Ag/AgCl as reference electrode. The electrode
-1
potential was scanned in the range of 0.0 – 0.6V at a scan rate of 10 mVs . A small broad
cathodic peak was observed at 0.235V which indicated the reduction potential of aluminium
-
(
Al Cl ) in the ionic liquid [39].
2
7
Fig 5: Cyclic voltammogram of TBA:AlCl chloroaluminate (1:1.5 molar ratio)
3
2
0

Preliminary experiments on electrodeposition of aluminium from TBA:AlCl3
chloroaluminate were conducted and the results are shown in Figure 6. Chronoamperometric
experiments have been performed to study the nucleation process of aluminium from ionic
liquid electrolyte system 1:1.8 mol at 300K. Figure 6 shows the current-time transient
-
2
obtained at 300mV (set potential) using a current density of 10mA.cm . A sharp decay of
current was observed due to the electrodeposition of aluminium from the ILs on the copper
surface. The deposition further thickens due to nucleation and growth of aluminium on the
Cu anode [40].
Fig 6: Current-time transient obtained using copper-copper-electrode system in 1:1.5mol at
-2
3
00K. Chronoamperometric deposition at a current density of 10mA.cm for 25 minutes.
A plausible reaction mechanism of electrodeposition of aluminium may be represented as
below:
-
-
0
-
Over all reaction:
Anodic Reaction:
4Al Cl + 3e
Al + 7AlCl
2
7
4
-
-
0
-
Al Cl + 3e
Al + AlCl + 3Cl
2
7
4
-
-
-
Cathodic Reaction: 3Cl + 3Al Cl
6AlCl4
2
7
2
1

A schematic representation of the electrochemical cell and the proposed mechanism is given
in Figure 7.
Another advantage of TBA:AlCl chloroaluminate is aromatic solvents which accelerates the
3
ionic liquid formation and used in synthesis process itself which also reduce its viscosity
significantly. Ionic liquid with higher concentration of aluminium chloride will provide
−
aluminium deposition by cationic Al as well the anionic species (Al Cl ) .
2
7
Fig 7: Schematic representation of the electrochemical cell and the process of
electrodeposition of aluminium
Thus, the TBA:AlCl3 chloroaluminate possesses many desired properties for
electrodeposition of aluminium such as low viscosity, aromatic solvent, minimum safety risk
and desired hydrophobic ligand in cation. Conventional ionic liquids are comparatively more
viscous, more hygroscopic and do not contain aromatic solvent. Therefore, this
chloroaluminate is a better fit for aluminium deposition than that of conventional
chloroaluminates.
Conclusions
This paper reveals the compositional changes of ionic liquids at various mole ratios. The
study confirms the adduction of amine – AlCl and step wise change over to ionic liquid. This
3
2
2

sequential changes in cationic as well as anionic moieties were authenticated by ESI-MS,
2
7
FTIR and Al NMR. The multiple super Lewis acidic sites in chloroaluminate were
3
1
confirmed by P NMR for its use in catalysis.
Chloroaluminate ionic liquid was found to be highly active and selective catalyst for many
Friedel-Crafts reaction. The catalyst meets the advantageous properties of conventional
chloroaluminate catalyst like lower catalyst requirement and biphasic reaction systems in
alkylation reactions. Pertinent to combined advantages in synthetic process of this
chloroaluminate and its high catalytic activity make it an appealing replacement in many
Friedel-Crafts reaction for industrial application. Due to its inherently beneficial properties of
this chloroaluminate, the aluminium moiety containing hydrophobic ligand can be employed
in aluminium deposition as well.
Term
Definition
Conceptualization
Methodology
Validation
Sharad Lande, Sudeshna Chandra, Vishnupriya Bhakthavatsalam
Mangesh Sakhalkar, Jeevan Pradhan, Rudra Prosad Choudhury
Mangesh Sakhalkar
Mangesh Sakhalkar, Jeevan Pradhan, Rudra Prosad Choudhury, Vishnupriya
Bhakthavatsalam
Formal analysis
Investigation
Mangesh Sakhalkar, Jeevan Pradhan, Rudra Prosad Choudhury,
Sunandan Divatia School of Science, SVKM’s NMIMS (Deemed to be) University,
Resources
Reliance Industries Limited, Mumbai
Data curation
Mangesh Sakhalkar
Mangesh Sakhalkar
Writing-original draft
Writing-Review & Editing Sudeshna Chandra, Sharad Lande
Mangesh Sakhalkar, Sudeshna Chandra, Sharad Lande, Vishnupriya
Visualization
Bhakthavatsalam
Supervision
Sudeshna Chandra, Sharad Lande
Sudeshna Chandra, Sharad Lande
Project administration
2
3

Term
Definition
Funding acquisition
Reliance Industries Ltd. Mumbai
Declaration of interests
☒
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.
☐
The authors declare the following financial interests/personal relationships which may be
considered as potential competing interests:
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