Synthesis and applications of highly efficient, reusable, sulfonic acid group functionalized Broensted acidic ionic liquid catalysts

Article abstract of DOI:10.1016/j.catcom.2011.05.030

A variety of sulfonic acid group functionalized Broensted acidic ionic liquids (BAILs) catalysts were synthesized. Catalytic activities of BAILs were assessed using condensation and esterification reactions. Catalytic activities of BAILs were high when compared with H-ZSM-5, H-BETA, sulfonic acid functionalized SBA-15 catalyst. The Hammett acidity order determined from UV-visible spectroscopy of BAILs is consistent with their activity order observed in acid-catalyzed reactions. Recycling experiments suggests that these novel BAILs can be reused without significant loss in catalytic activity. Novel BAILs offers several attractive features such as low cost, high catalytic activity and recyclability.

Full text of DOI:10.1016/j.catcom.2011.05.030

Catalysis Communications 12 (2011) 1420–1424
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Synthesis and applications of highly efficient, reusable, sulfonic acid group
functionalized Brönsted acidic ionic liquid catalysts
⁎
Rajkumar Kore, Rajendra Srivastava
Department of Chemistry, Indian Institute of Technology Ropar, Rupnagar-140001, Punjab, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A variety of sulfonic acid group functionalized Brönsted acidic ionic liquids (BAILs) catalysts were synthesized.
Catalytic activities of BAILs were assessed using condensation and esterification reactions. Catalytic activities
of BAILs were high when compared with H-ZSM-5, H-BETA, sulfonic acid functionalized SBA-15 catalyst. The
Hammett acidity order determined from UV–visible spectroscopy of BAILs is consistent with their activity
order observed in acid-catalyzed reactions. Recycling experiments suggests that these novel BAILs can be
reused without significant loss in catalytic activity. Novel BAILs offers several attractive features such as low
cost, high catalytic activity and recyclability.
Received 23 March 2011
Received in revised form 22 May 2011
Accepted 27 May 2011
Available online 6 June 2011
Keywords:
Sulfonic acid functionalized ionic liquids
Hammett acidity
© 2011 Elsevier B.V. All rights reserved.
Condensation
Esterification
1. Introduction
sulfonic acid group (one-sulfonic acid and two-sulfonic acid)
functionalized BAILs and investigate their performance in some
Ionic liquids (ILs) exhibit several unique characteristics, such as
low vapor pressure, excellent chemical and thermal stability,
recoverability, and convenience in product separation [1]. Physical
and chemical properties of ILs can be tailored by varying cations and
anions. Although ionic liquid was initially introduced as an alternative
green reaction medium [2], today it has marched far beyond showing
its significant role in controlling the reaction as a catalyst [3]. Since the
first successful use of ionic liquid, dialkylimidazolium chloroalumi-
nate, as a catalyst in Friedel-Crafts acylations [4], a number of ILs with
unique catalytic properties have been developed [3]. Some significant
research findings have been reported for acid catalyzed reaction using
ILs [5–7]. Compared to conventional homogeneous and heteroge-
neous acid catalysts, acidic ILs demonstrates several advantages such
as: reactions can be carried out under solvent-free condition, the
product can be separated by easy decantation, and ILs can be recycled.
Brönsted acidic ILs (BAILs) can be prepared by substituting alkyl or
benzyl sulfonic acid group in the imidazole ring or by replacing Cl−
with HSO4− [8–14]. Such functionalized ILs have been reported as
novel eco-friendly catalysts for some acid catalyzed reactions [8–14].
Although several such BAILs are reported in the literature, a few
key points have not been addressed. To the best of our knowledge,
there is no report in the literature that clearly state whether one-
sulfonic acid or multi-sulfonic acid group functionalized ILs exhibits
higher activity. The objective of this study is to prepare a variety of
suitable catalytic reactions. In this study, catalytic activities of BAILs
were assessed by condensation (synthesis of 3-phenacylphthalide
and synthesis of coumarin using Pechman reaction) and esterification
(benzyl alcohol with various acids) reactions. The catalytic activities
of BAILs were compared with several solid acid catalysts such as H-
ZSM-5, H-BETA, and SO3H functionalized SBA-15 (SBA-15-pr-SO3H)
catalysts.
2. Experimental
2.1. Catalyst preparation
BAIL-1 was synthesized according to the reported procedure [15].
BAIL-2 was synthesized according to the reported procedure [16],
except the sulfonation step, in which only two equivalents of H2SO4
was taken (B.P. N360 °C; overall yield=84.5%).
For the synthesis of BAIL-3, first Compound b (Scheme 1) was
synthesized using the reported procedure [17]. It was then sulfonated
using two equivalents of H2SO4 (Overall yield=82.2%).
BAIL-3: B.P.N360 °C. IR (KBr, υ, cm−1)=3400, 3152, 2954, 2850,
1710, 1562, 1498, 1456, 1154, 995, 866, 768, 719, 621. 1H NMR (D2O)
δ=9.23 (s, 1H), 7.42–7.75 (m, 6H), 5.36 (s, 2H). 13 C NMR (D2O)
δ=134.11, 133.38, 128.96, 128.21, 125.91, 121.43, 119.61, 52.27.
Elemental analysis for C10H12N2O7S2: theoretical (%): C 35.71, H 3.57, N
8.33; experimental (%): C 35.11, H 3.62, N 7.92.
For the synthesis of BAIL-4, first compound c (Scheme 1) was
synthesized using the reported procedure [17]. It was then sulfonated
using three equivalents of H2SO4 (Overall yield=77.4%).
⁎
Corresponding author. Tel.: +91 95010 18189; fax: +91 1881 223395.
E-mail address: rajendra@iitrpr.ac.in (R. Srivastava).
1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2011.05.030

R. Kore, R. Srivastava / Catalysis Communications 12 (2011) 1420–1424
1421
Scheme 1. Synthesis of Brönsted acidic ionic liquid catalysts.
Table 1
BAIL-4: B.P.N360 °C. IR (KBr, υ, cm−1)=3393, 3368, 3145, 3024,
Textural properties of various solid acid catalysts investigated in this work.
2914, 1700, 1560, 1454, 1142, 1023, 875, 709, 611. 1H NMR (D2O)
δ=8.91 (s, 1H), 7.32–7.45 (m, 10H), 5.29 (s, 4H). 13 C NMR (D2O)
δ=136.61, 135.02, 129.76, 129.49, 126.97, 123.32, 53.18. Elemental
analysis for C17H18N2O10S3: theoretical (%): C 40.32, H 3.56, N 5.53;
experimental (%): C 39.93, H 3.84, N 5.81.
Catalyst
aSBET
(m2/g) area (m2/g) volume (ml/g) species
Ext. surface Total pore
Concentration of active
H-ZSM-5
H-BETA
SBA-15-pr-SO3H 690
350
595
67
212
618
0.2
0.44
0.95
Si/Al=18b
Si/Al=14b
Organic functional
For the synthesis of BAIL-5, first compound d (Scheme 1) was
synthesized using compound b (two equivalents) and 1,6-dibromo-
hexane (one equivalent) using the reported procedure [17]. Com-
pound d was then sulfonated using four equivalents of H2SO4 (Overall
yield=75.2%).
group (0.64 mmol/g)c
a
SBET is the surface area which was calculated using the Brunauer–Emmett–Teller
equation.
b
c
Obtained from ICP.
Determined from the elemental analysis.

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R. Kore, R. Srivastava / Catalysis Communications 12 (2011) 1420–1424
2.0
1.5
1.0
0.5
0.0
reused in a further run. In the case of benzoic acid, ethyl acetate was
added in the reaction mixture to separate the product and BAILs.
Blank
BAIL-1
BAIL-2
BAIL-3
BAIL-4
BAIL-5
2.2.3. Synthesis of 7-hydroxy, 4-methylcoumarins
In a typical experiment, resorcinol (5 mmol), ethylacetoacetate
(5 mmol) and catalyst (0.5 mmol) were taken in 50 ml round bottom
flask. The reaction was conducted at 333 K for 1 h. Ethyl acetate was
added in the reaction mixture to separate the product and BAILs.
3. Results and discussion
BAILs were synthesized by multi-step synthetic protocol
(Scheme 1). N-methylimidazole/imidazole was benzylated with
benzyl chloride and the resulted quaternary salt was transformed
into BAILs by using concentrated sulfuric acid as anion exchange and
sulfonating reagents, respectively. In this study, BAILs were prepared
with equivalent amounts of the H2SO4. When samples were prepared
with excess amount of H2SO4, yields of the products were more than
the theoretical yields. This may be due to the presence of excess H2SO4
in the product. When equivalent amount of H2SO4 was taken, then the
yield was very good but slightly less than the theoretical yield. For
comparative study, solid acid catalysts such as H-ZSM-5, H-BEA and
SBA-15-pr-SO3H were prepared and characterized by using XRD, N2-
adsorption and XRF (Table 1).
Acidity of BAILs was measured using Analytikjena Specord 250
PLUS UV-visible spectrophotometer with a basic indicator by
following the concept reported in literature [21,22]. With the increase
of acidity of the BAILs, the absorbance of the unprotonated form of the
basic indicator decreased, whereas the protonated form of the
indicator could not be observed because of its small molar
absorptivity and its location, so the [I]/[IH+] (I represents indicator)
ratio can be determined from the differences of measured absorbance
after the addition of BAILs and Hammett function, H0, can be
calculated using Eq. (1). This value can be regarded as the relative
acidity of the BAILs.
300
400
500
600
700
Wavenumber (nm)
Fig. 1. Absorbance spectra of 4-nitroaniline for various BAILs in water.
BAIL-5: B.P.N360 °C. IR (KBr, υ, cm−1)=3395, 3149, 3084, 2943,
2867, 2525, 1702, 1563, 1498, 1455, 1146, 1022, 875, 708. 1H NMR
(D2O) δ=8.59 (s, 2H), 7.23–7.27 (m, 12H), 5.19 (s, 4H), 3.94 (t, 4H),
1.61 (quint, 4H), 1.05 (quint, 4H). 13 C NMR (D2O) δ=134.64, 133.37,
131.52, 128.86, 128.09, 122.15, 121.96, 52.36, 48.96, 28.34, 24.1.
Elemental analysis for C26H34N4O14S4: theoretical (%): C 41.38, H 4.51,
N 7.43; experimental (%): C 41.58, H 4.24, N 7.55.
H-ZSM-5 [18], H-BETA [19] and SBA-15-pr-SO3H [20] were
synthesized following the reported procedures. 1H, 13 C & DEPT
NMR and FT-IR spectra of unknown BAILs are given in the
supplementary information.
2.2. Catalytic reaction procedure
h
ꢀ
ꢁi
H0 = pKðIÞaq + log ðIÞ= IHþ
ð1Þ
2.2.1. Synthesis of 3-phenacylphthalide
In a typical synthesis, phthalaldehydic acid (5 mmol), acetophe-
none (5 mmol) and catalyst (0.125 mmol) were mixed in a 50 mL
round bottom flask. Reaction was conducted at 353 K for 30 min. After
the reaction, the mixture was poured over crushed ice. The solid was
filtered and recrystallized using hexane: ethyl acetate (80:20). Water
layer was washed with ethyl acetate. The aqueous layer was dried
under vacuum at 343 K to obtain the BAILs for further use.
Under the same concentration of 4-nitroanline (5 mg/L, pK
(I)aq =pKa =0.99) and BAILs (25 mmol/L) in water, H0 values of all
BAILs were determined. The maximal absorbance of the unprotonated
form of the indicator was observed at 380 nm in water. When the BAIL
was added, the absorbance of the unprotonated form of the basic
indicator decreased. As shown in Fig. 1, the absorbance of the
unprotonated form of the indicator on addition of BAILs decreased as
follows: BAIL-1NBAIL-3NBAIL-2NBAIL-4NBAIL-5. Calculations sug-
gest that the Hammett acidity (H0) of these ionic liquids follows the
order: BAIL-5NBAIL-4NBAIL-2NBAIL-3NBAIL-1 (Table 2).
The catalytic activity of BAILs was investigated in the synthesis of
3-phenacylphthalide (Scheme 2), aromatic esters (Scheme 3) and 7-
hydroxy-4-methylcoumarin (Scheme 4). All BAILs investigated in this
study were found to be active. The activity of Brönsted acidic ILs in
these catalytic investigations follows the order BAIL-1bBAIL-3bBAIL-
2bBAIL-4≈BAIL-5 (Tables 3, 4 and 5), which is consistent with the
acidity measurement using UV–visible spectroscopy. Under the
optimized condition for these reactions, solid acid catalysts H-ZSM-
5, H-BETA and SBA-15-pr-SO3H were found to be either inactive or
weakly active (Tables 3, 4 and 5). As shown in Tables 3–5, the activity
of BAIL-4/BAIL-5 is comparatively higher than that of BAIL-2.
However, it may be noted that each BAIL have different numbers of
acid sites (SO3H and HSO4−), hence all of them have different TOF per
mol. If the number of acid sites (SO3H and HSO4−) present in BAIL-4/
BAIL-5 is taken into account for calculating TOF, the activity of BAIL-2
is found to be higher than BAIL-4/BAIL-5. Acidity-activity relationship
with increase in the number of sulphonic acid group clearly illustrates
2.2.2. Esterification reaction
In a typical reaction, benzyl alcohol (10 mmol), acids (10 mmol)
and catalyst (0.5 mmol) were mixed in a 50 mL round bottom flask.
Depending upon substrates, reactions were conducted at desired
temperature (323 K or 353 K) for stipulated time period (30, 45 or
90 min). The reaction mixture became biphasic (in case of hexanoic
acid and acetic acid). Upper layer containing the ester can be isolated
just by simple decantation. The lower layer consisting of the BAIL was
Table 2
Calculation and comparison of H0 values of different BAILs in water at 298 K.
E. No.
ILs
Amax
[I] (%)
[IH+] (%)
H0
1
2
3
4
5
6
No BAIL
BAIL-1
BAIL-2
BAIL-3
BAIL-4
BAIL-5
1.696
1.588
1.260
1.543
1.041
1.001
100
0
–
93.6
74.3
90.9
61.4
59.0
06.4
25.7
09.1
38.6
41.0
2.155
1.451
1.989
1.192
1.148
Indicator: 4-nitroanline.

R. Kore, R. Srivastava / Catalysis Communications 12 (2011) 1420–1424
1423
Scheme 2. Synthesis of 3-phenacylphthalide using BAILs.
Scheme 3. Synthesis of benzyl esters using BAILs catalysts.
Scheme 4. Synthesis of 7-hydroxy-4-methylcoumarin using BAILs.
the difference in catalytic activity (Table 1, supporting information).
Theoretical modeling was able to correlate well with the acidity–
activity order obtained using catalytic investigations. Theoretical
study suggests that hydrogen bonding influence the acidity of BAILs
(supporting information). Ionic liquids were found to be more active
than H2SO4. In ionic liquids, acidity is due to the presence of –SO3H,
HSO4− and Imidazolium C–H (2-position). The experimental details
and reasons for high activity of some different kinds of Brönsted acidic
ILs have been well explained by X. Li et al. [14].
BAILs were found to be stable and reusable during these reactions
(Tables 3, 4 and 5). In addition to the reusability, stability of BAILs was
also monitored under the reaction condition. Leaching experiments
were designed at partial conversions using BAIL-2 in the synthesis of
benzyl hexonate. Initially, the reaction was performed for 15 min and
then BAIL-2 was removed from the reaction mixture and the reaction
was continued. No significant increase in the yield of benzyl hexonate
was observed (Fig. 2) after removing the catalyst from the reaction
mixture. A similar experiment was also performed after removing the
catalyst from the reaction mixture after 30 min from the start of reaction.
This clearly showed that BAIL-2 is stable under the reaction condition.
Table 3
4. Conclusions
Synthesis of 3-phenacylphthalide using different catalysts investigated in this study.
a
b
In this study, synthesis of several novel BAILs has been reported
that contain one or two aromatic sulfonic acid groups on imidazolium
cation by a simple and straight-forward procedures from cheap
starting materials in good yields. BAILs served as an efficient and
reusable catalyst realized the green synthesis of 3-phenacylphthalide,
benzyl esters and 7-hydroxy-4-methyl coumarin. The activity of
Brönsted acidic ILs follows the order BAIL-1bBAIL-3bBAIL-2bBAIL-
4bBAIL-5, which is consistent with the acidity measurement using
UV-visible spectroscopy. Based on the results obtained, one can
conclude that more numbers of –SO3H functionalization does not
really improve the catalytic activity, when the catalytic activity is
calculated per acid sites. This reaction system has several note-worthy
features: (1) reactions can be carried out at relatively lower
temperature with shorter reaction time; (2) reactions can be carried
out without solvents; (3) products can be separated easily with high
E. No.
Catalyst
Yield (%)
TOF(h−1
)
TOF(h−1
)
1
2
3
4
5
6
7
8
BAIL-1
32.1
75.3
73.2
55.4
81.6
79.1
83.2
11.5
Nil
26
60
58
44
66
63
66
9
26
30
29
22
22
21
16
9
BAIL-2
BAIL-2c
BAIL-3
BAIL-4
BAIL-4c
BAIL-5
H2SO4
9
10
11
H-ZSM-5
H-BETA
SBA-15-pr-SO3H
-
-
11
-
-
11
Nil
13.8
Reaction condition: Phthalaldehydic acid (5 mmol); acetophenone (5 mmol); catalyst
(0.125 mmol); reaction time (30 min), temperature (353 K).
a
TOF=moles of reactant converted per mole of catalyst per hour.
TOF=moles of reactant converted per mole of active species per hour.
b
c
Catalytic activity data of BAIL-2 and BAIL-4 in third cycle.

1424
R. Kore, R. Srivastava / Catalysis Communications 12 (2011) 1420–1424
100
Table 4
Synthesis of benzyl ester using different catalysts investigated in this study.
E.
No.
Catalyst
Acid
Temp.
(K)
Time
(min)
Yield
(%)
TOF
TOF
a
b
(h−1
)
(h−1
)
80
60
40
20
0
1
2
3
4
5
6
7
8
BAIL-1
BAIL-2
BAIL-2c
BAIL-3
BAIL-4
BAIL-5
BAIL-5c
H2SO4
C6H5COOH 353
C6H5COOH 353
C6H5COOH 353
C6H5COOH 353
C6H5COOH 353
C6H5COOH 353
C6H5COOH 353
C6H5COOH 353
C6H5COOH 353
C6H5COOH 353
90
90
90
90
90
90
90
90
90
90
90
23.1
78.3
75.8
46.7
88
3.1
10.4
10.1
6.3
11.7
12
11.6
1.5
-
3.1
5.2
5.1
3.2
3.9
3.0
2.8
1.5
-
(b)
(a)
90
87.2
14.1
Nil
Nil
14.1
9
10
11
H-ZSM-5
H-BETA
SBA-15-pr- C6H5COOH 353
-
1.9
-
1.9
SO3H
12
13
14
15
16
17
18
19
20
BAIL-1
BAIL-2
BAIL-3
BAIL-4
BAIL-5
H2SO4
C6H13COOH 323
C6H13COOH 323
C6H13COOH 323
C6H13COOH 323
C6H13COOH 323
C6H13COOH 323
C6H13COOH 323
C6H13COOH 323
45
45
45
45
45
45
45
45
45
28.2
81.8
50.9
91.1
93.3
5.1
Nil
3.2
17.1
7.5
7.5
10.9
6.8
8.1
6.2
1.3
-
21.8
13.6
24.2
24.8
1.3
-
0.8
4.5
H-ZSM-5
H-BETA
SBA-15-pr- C6H13COOH 323
0.8
4.5
0
15
30
45
60
75
Reaction time (minute)
SO3H
21
22
23
24
25
26
27
28
29
30
BAIL-1
BAIL-2
BAIL-2c
BAIL-3
BAIL-4
BAIL-5
H2SO4
H-ZSM-5
H-BETA
SBA-15-pr- CH3COOH
CH3COOH
CH3COOH
CH3COOH
CH3COOH
CH3COOH
CH3COOH
CH3COOH
CH3COOH
CH3COOH
323
323
323
323
323
323
323
323
323
323
30
30
30
30
30
30
30
30
30
30
20.3
68.8
66.1
38.7
73.2
75.4
7.1
Nil
3.3
14.2
8
8
27.6
26.4
15.2
29.2
30
2.8
-
1.2
5.6
13.8
13.2
7.6
9.8
7.5
2.8
-
Fig. 2. BAIL-2 was removed after (a) 15 min and (b) 30 min from the start of the
reaction of benzyl alcohol with hexanoic acid in leaching experiment.
Appendix A. Supplementary data
1.2
5.6
Supplementary data to this article can be found online at doi:10.
1016/j.catcom.2011.05.030.
SO3H
Reaction condition: Benzyl alcohol (10 mmol), acid (10 mmol), catalyst (0.5 mmol).
a
TOF=moles of reactant converted per mole of catalyst per hour.
TOF=moles of reactant converted per mole of active species per hour.
Catalytic activity data of BAIL-2 and BAIL-5 in third cycle.
References
b
c
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Table 5
Synthesis of 7-hydroxy-4-methylcoumarin using different catalysts investigated in this
study.
a
b
E. No.
Catalyst
Yield (%)
TOF(h−1
)
TOF(h−1
)
1
2
3
4
5
6
7
8
9
BAIL-1
30.1
76.2
75.1
56.1
78.5
81.7
5.1
Nil
8.3
24.6
3
3
BAIL-2
7.6
7.5
5.6
7.9
8.2
0.5
–
3.8
3.8
2.8
2.6
2.0
0.5
–
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BAIL-2c
BAIL-3
BAIL-4
BAIL-5
H2SO4
H-ZSM-5
H-BETA
SBA-15-pr-SO3H
[22] Y. Gu, J. Zhang, Z. Duan, Y. Deng, Adv. Synth. Catal. 347 (2005) 512–516.
0.8
2.5
0.8
2.5
10
Reaction condition: Resorcinol (5 mmol); ethylacetoacetate (5 mmol); catalyst
(0.5 mmol); reaction time (1 h), temperature (333 K).
a
TOF=moles of reactant converted per mole of catalyst per hour.
TOF=moles of reactant converted per mole of active species per hour.
Catalytic activity data of BAIL-2 in third cycle.
b
c