Acidic ionic liquid immobilized on nanoporous Na+-montmorillonite as an efficient and reusable catalyst for the formylation of amines and alcohols

Article abstract of DOI:10.1007/s11164-015-2116-0

In this work, nanoporous sodium montmorillonite clay (Na⁺-MMT) was used as a support for the immobilization of 1-methyl-3-(trimethoxysilylpropyl)-imidazolium hydrogen sulfate. The Na⁺-MMT chemical modification ([Na⁺-MMT-[pmim]HSO₄) was confirmed by a variety of techniques including Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), thermogravimetric analysis (TGA), scanning electron microscopy and potentiometric titration. The catalytic performance of this immobilized acidic ionic liquid was probed for the N-formylation of a variety of amines using formic acid under solvent-free conditions. This reagent is also useful for the formylation of benzylic alcohols. The procedure gave the products in excellent yields in very short reaction times. Also, this catalyst can be reused ten times without loss of its catalytic activity.

Full text of DOI:10.1007/s11164-015-2116-0

Res Chem Intermed
DOI 10.1007/s11164-015-2116-0
Acidic ionic liquid immobilized on nanoporous
Na+-montmorillonite as an efficient and reusable
catalyst for the formylation of amines and alcohols
Farhad Shirini1 Masoumeh Mazloumi1
•
•
Mohadeseh Seddighi1
Received: 17 April 2015 / Accepted: 26 May 2015
Ó Springer Science+Business Media Dordrecht 2015
Abstract In this work, nanoporous sodium montmorillonite clay (Na?-MMT) was
used as a support for the immobilization of 1-methyl-3-(trimethoxysilylpropyl)-
imidazolium hydrogen sulfate. The Na?-MMT chemical modification ([Na?-MMT-
[pmim]HSO4) was confirmed by a variety of techniques including Fourier transform
infrared spectroscopy (FT-IR), X-ray diffraction (XRD), thermogravimetric analysis
(TGA), scanning electron microscopy and potentiometric titration. The catalytic
performance of this immobilized acidic ionic liquid was probed for the N-formy-
lation of a variety of amines using formic acid under solvent-free conditions. This
reagent is also useful for the formylation of benzylic alcohols. The procedure gave
the products in excellent yields in very short reaction times. Also, this catalyst can
be reused ten times without loss of its catalytic activity.
Keywords Sodium montmorillonite Á Clay Á Immobilization of ionic liquid Á
Formamide Á O-Formylation Á Na?-MMT-[pmim]HSO4
Introduction
Formamides are one of the most important intermediates in organic synthesis. N-
Formyl compounds have been used as precursor for the synthesis of formamidines
[1], isocyanides [2], aminocarbonylation of aryl halides [3], and as reagents in the
classical Vilsmeier formylation reactions [4]. Furthermore, they are valuable
intermediates in the construction of various medicinally important compounds such
as quinolone antibiotics [5] and cancer chemotherapeutic agents [6]. Various drugs
such as acetiamine, octotiamine, fursultiamine, and formoterol, which are in clinical
&
Farhad Shirini
shirini@guilan.ac.ir
1
Department of Chemistry, College of Science, University of Guilan,
Post Box 1914, 41335 Rasht, I. R. Iran
123

F. Shirini et al.
use, have formamide groups in their skeletons. The formyl group is also an
important amino-protecting group, especially during peptide synthesis [7], because
the selective deprotection process is possible in the presence of other groups like
acetyl or benzoyl. In addition, formamides were found to be effective Lewis bases
that catalyze several useful reactions such as allylation and hydrosilylation of
carbonyl compounds [8]. Among various procedures that have been developed and
documented for the preparation of formamide derivatives [9, 10], the reaction
between amines and formic acid is the simplest and most important approach. For
this purpose, a variety of catalysts and reagents have been used to facilitate this
reaction [11–18]. Although these procedures provide an improvement in the
synthesis of these types of compounds, many of them suffer from disadvantages
such as long reaction times, harsh reaction conditions, the need for excess amounts
of the reagent, thermal instability, by-products formation, use of toxic reagents and
non-recoverability of the catalyst. Therefore, simple, efficient and mild procedures
with easily separable and reusable solid catalysts to overcome these problems are
still in demand.
During recent years, ionic liquids (ILs) have been extensively used in almost all
application fields of chemistry such as organic and inorganic syntheses, catalysis,
electrochemistry and chromatography. This is due to their remarkable properties
such as non-flammability, negligible vapor pressure, wide liquid range and high
thermal, chemical and electrochemical stability. However, their high cost, large
consumption and difficult recovery, and the separation of products, cause limitations
¨
in their large-scale application. Furthermore, the ILs (especially Bronsted acidic
ILs) have some degree of instability in the presence of air and moisture.
Immobilization of the ILs on the surface of a solid support is a useful way to
combine the advantageous characteristics of ILs and solid properties. In other
words, the immobilized ILs offer additional features compared to the pure ILs that
facilitate the handling, separation and reuse procedures, thus minimizing the amount
of IL utilized in reactions [19].
In recent years, clays as nanostructured materials have been widely used in
organic transformations as solid acid catalysts [20–23]. The main reasons for use of
clays are the large specific surface area, chemical and mechanical stability, layered
structure and high cation exchange capacity as well as accessibility, easy
modification, cheapness and non-corrosiveness. One such clay is montmorillonite
(MMT), which consists of two tetrahedral silicate sheets with a central aluminum
octahedral sheet; MMT exhibits a net negative charge on lamellar surfaces and
causes them to adsorb cations, such as Na? or Ca? for compensation. Layers of
MMT have a thickness of about 1 nm and a length of 100 nm or a little more, so
MMT is a structurally well-ordered nanoporous material. However, they require
cation exchange or surface modification to increase their active sites for better
interaction with reactants. For example, an IL was applied to modify natural clays
by their functionalization or by assisting the immobilization of noble metal
nanoparticles, which led to preparing a series of clay-supported catalysts with high
performances and good stability [24–26].
123

Acidic ionic liquid immobilized on nanoporous Na?…
Experimental
Chemicals were purchased from Fluka, Merck, and Aldrich chemical companies.
All yields refer to the isolated products. Products were characterized by comparison
of their physical constants, IR and nuclear magnetic resonance (NMR) spectroscopy
with authentic samples and those reported in the literature. The purity determination
of the substrate and reaction monitoring were accompanied by thin layer
chromatography (TLC) on silicagel polygram SILG/UV 254 plates.
Preparation of 1-methyl-3-(trimethoxysilylpropyl)-imidazolium chloride
([pmim]Cl)
A
mixture of 10-mmol 1-methylimidazole and 10-mmol (3-chloro-
propyl)trimethoxysilane was refluxed at 90 °C for 30 h. Then, the reaction mixture
was cooled down. The crude product was washed with Et2O (2 9 5 mL) and dried
under vacuum. The product (slightly yellow viscous oil) was obtained.
Preparation of 1-methyl-3-(trimethoxysilylpropyl)-imidazolium chloride
supported on sodium MMT (Na1-MMT-[pmim]Cl)
1.2 g (4 mmol) of [pmim]Cl was dissolved in 25 mL of CH2Cl2 and treated with 2 g
of sodium MMT. The reaction mixture was refluxed with stirring for 3 days. Then,
the reaction mixture was cooled to room temperature, the solid was isolated by
filtration and washed with 20 mL of boiling dichloromethane to remove the
unreacted IL. In the next step, the material was dried to obtain MMT-[pmim]Cl. The
amount of chloride in the MMT-[pmim]Cl was determined by potentiometric
titration methods [27]. For this purpose, 0.5 g MMT-[pmim]Cl in 50 mL of water
was titrated with 0.1-M AgNO3. As shown in Fig. 1, the change happened at 5.1 mL
of consumed AgNO3. On the basis of these studies, it can be concluded that Na?-
MMT-[pmim]Cl has 1.02 mmol of Cl- per gram of this reagent.
Preparation of 1-methyl-3-(trimethoxysilylpropyl)-imidazolium hydrogen
sulfate supported on sodium MMT (Na1-MMT-[pmim]HSO4)
3 g of Na?-MMT-[pmim]Cl was suspended in 20 mL of dry CH2Cl2. Under
vigorous stirring, 3 mmol of concentrated H2SO4 (97 %) was added dropwise in an
ice bath (0 °C). The mixture was then warmed to room temperature and heated
under reflux for 30 h. When the formed HCl was completely distilled off the
solution was cooled and CH2Cl2 was removed under vacuum to afford Na?-MMT-
[pmim]HSO4 as the product (Scheme 1).
General procedure for N-formylation of amines
Amine (1 mmol) was added to a mixture of Na?-MMT-[pmim]HSO4 (10 mg,
1 mol %) and formic acid (2 mmol) and the resulting mixture was stirred at 60 °C
for the appropriate time. After completion of the reaction (monitored by TLC),
123

F. Shirini et al.
Fig. 1 Potentiometric titration and its first derivative curves of Na?-MMT-[pmim]Cl with AgNO3
N
N
HSO 4
N
N
HSO4
Si
Si
O
O
O
O
O O
OH
OH
OH
2O
OH
OH
OH
OH
CH Cl
O
1) [pmim]Cl,
Reflux,
2
2
O
O
3
day
Na+
Si
H2O
Na+
10.3 oA
H
N
N
2O
OH
HSO 4
Na+
H
SO CH Cl
,
2)
2 h 2
2
4
H
OH
16.4 oA
30
Reflux,
N
OH
OH
OH
N
Na+
Si
O
HSO4
O
O
Scheme 1 Preparation of the acidic IL immobilized on Na?-MMT (Na?-MMT-[pmim]HSO4)
ethylacetate (10 mL) was added and the catalyst was separated by filtration. The
organic phase was washed with water (2 9 10 mL) and dried over Na2SO4. The
solvent was removed under reduced pressure to afford the desired product. The
spectral data of the selected and new compounds are as follows:
123

Acidic ionic liquid immobilized on nanoporous Na?…
1H NMR (DMSO-d6, 400 MHz): 7.13 (s, 1H, H1), 7.15 (d, J = 2.4 Hz, 1H, H2),
7.52–7.54 (m, 6H, H3), 8.23 (d, J = 2 Hz, 3H, H4), 8.6 (dd, J1 = 11.2 Hz,
J2 = 2.4 Hz, 1H, H5), 10.07 (d, J = 11.2 Hz, 1H, H6), 10.14 (s, 3H, H7); 13C NMR
(DMSO-d6, 100 MHz): 118.7, 119.3, 120.1, 120.6, 134.4, 134.4, 134.8, 159.7,
159.7, 162.9.
1H NMR (DMSO-d6, 400 MHz): 3.84 (s, 4H, H1), 7.11 (d, J = 8.4 Hz, 2H, H2,
H3), 7.16 (d, J = 8, 8H, H4), 7.50 (d, J = 8.4 Hz, 6H, H5), 8.24 (s, 3H, H6), 8.72
(m, 1H, H7), 10.07 (d, J = 10.8, 1H, H8), 10.13 (s, 3H, H9); 13C NMR (DMSO-d6,
100 MHz):114.59, 118.25, 118.33, 119.73, 119.81, 129.48, 130.01, 136.62, 136.73,
137.15, 137.35, 159.81, 159.91, 162.91, 162.99.
1H NMR (DMSO-d6, 400 MHz): 7.39 (d, J = 8.8 Hz, 2H, H1, H2), 7.79 (d,
J = 8.8, 6H, H3), 7.89 (d, J = 8.8 Hz, 8H, H4), 8.35 (s, 3H, H5), 8.96 (m, 1H, H6),
10.53 (d, 1H, H7), 10.65 (s, 3H, H8); 13C NMR (DMSO-d6, 100 MHz): 117,62,
119.76, 119.84, 128.79, 129.09, 129.44, 129.71, 136.12, 142.86, 142.98, 143.62,
160.77, 160.86, 163.09, 163.19.
123

F. Shirini et al.
1H NMR (DMSO-d6, 400 MHz): 7.48 (d, J = 7.2 Hz, 2H, H1), 7.54–759 (m, 4H,
H2), 7.96–8.07 (m, 7H, H3, H4), 8.50 (s, 3H, H5), 8.60 (m, 1H, H6), 10.36 (s, 3H,
H7), 10.54 (d, J = 10.6 Hz, 1H, H8); 13C NMR (DMSO-d6, 100 MHz):119.03,
119.21, 119.36, 119.91, 119.98, 120.16, 120.31, 120.65, 126.19, 126.39,
126.60,127.33, 127.44, 127.58, 128.36, 128.49, 133.31, 133.43, 134.42, 134.56,
160.79, 160.88, 164.58.
Results and discussion
Catalyst characterization
The IR spectra of Na?-MMT and Na?-MMT-[pmim]HSO4 are presented in Fig. 2.
The FT-IR spectrum of pristine clay displays a peak at 3632 cm-1 that can be
attributed to the OH units in Na?-MMT. The peaks at 3423 and 1641 cm-1
correspond to the –OH stretching vibration of free H2O onto the Na?-MMT
structure and the bands at 1043 and 919 cm-1 can be collectively attributed to Si–O
stretching vibrations [23, 28].
[pmim]HSO4 is also characterized on the basis of its FT-IR spectrum (Fig. 2).
The peaks at 1576 and 1635 cm-1 can be assigned to C=N and C=C bands of the
attached imidazolium ring. Additional bands at 3159, 2948 and 1461 cm-1 were
due to C–H stretching and deformation vibrations, confirming functionalization of
the material with the IL [29]. The absorption of the S=O asymmetric stretching
mode appeared at 1170 cm-1 and its symmetric stretching mode lies at 1060 cm-1
,
so the broad band around 1046 cm-1 can be assigned to the stretching modes of
123

Acidic ionic liquid immobilized on nanoporous Na?…
Fig. 2 FT-IR spectra of Na?-MMT, [pmim]HSO4 and Na?-MMT-[pmim]HSO4
Si–O and S=O bands which are overlapped. The S–O stretching modes of the
sulfuric acid functional group lie around 620 and 890 cm-1 proving the successful
preparation of [pmim]HSO4 [30].
123

F. Shirini et al.
Fig. 3 XRD patterns of Na?-MMT in comparison of Na?-MMT-[pmim]HSO4
Finally, in the FT-IR spectrum of Na?-MMT-[pmim]HSO4, the appearance of
the related bonds to [pmim]HSO4 showed the IL loaded on Na?-MMT.
Figure 3 represents the X-ray diffractograms of samples containing sodium
MMT and Na?-MMT-[pmim]HSO4 to demonstrate the expansion of the MMT
?
˚
layers. The calculated basal spacing for the Na -MMT is 10.3 A with 2h = 8.6°
?
˚
and for Na -MMT-[pmim]HSO4 is 16.4 A. The basal spacing of the IL-anchored
sodium MMT is obviously bigger than that of Na?-MMT, indicating that the IL has
been intercalated into MMT interlayer spaces [31]. This increase in the basal
spacing can lead to better interaction between the reactants in the acid catalysis
reactions, as compared to the MMT.
In addition to XRD, a transmission electron microscope (TEM) was also applied
to visualize the spatial morphology of Na?-MMT-[pmim]HSO4, as shown in Fig. 4.
The dark line represents the intersection of silicate layers of MMT while the grey
background represents the interlayer organic compounds. The results indicated that
after incorporation of an IL with MMT, the intercalated nanostructure was obtained.
Furthermore, clusters or agglomerated clay platelets could be seen in these images.
N2 adsorption–desorption measurements based on BET and BJH methods were
performed to obtain more information regardingthe catalyst. The adsorption–
desorption isotherm of Na?-MMT and Na?-MMT-[pmim]HSO4 are shown in
Fig. 5a. Both samples exhibited a type V isotherm with H3 hysteresis loop
categories according to the literature [32], which indicates the micro- and mesopore
structures of samples with slit-like pores [33]. These results also prove that the
textural properties of Na?-MMT were substantially maintained over immobilization
of the IL. BET surface areas, the total pore volumes (Vt), Vmicrop (estimated by the
t-plot method) and pore diameters for Na?-MMT and Na?-MMT-[pmim]HSO4 are
presented in Table 1. A decrease in BET surface area from Na?-MMT (156.0 m2/g)
to Na?-MMT-[pmim]HSO4 (133.4 m2/g) indicated that the IL may be successfully
intercalated to Na?-MMT layers. The curves of pore size distribution evaluated
from desorption data by utilizing the BJH model are also shown in Fig. 5b.
123

Acidic ionic liquid immobilized on nanoporous Na?…
Fig. 4 TEM images of Na?-MMT-[pmim]HSO4
Fig. 5 The N2 adsorption–desorption isotherm (a), and pore size distribution (b) of Na?-MMT and Na?-
MMT-[pmim]HSO4
123

F. Shirini et al.
Table 1 Pore structure parameters of Na?-MMT and Na?-MMT-[pmim]HSO4 derived from the N2
adsorption–desorption isotherms
Sample
SBET (m2/g)
Vt (cm3/g)
Vmicrop (cm3/g)
DBJH (nm)
Na?-MMT
Na?-MMT-[pmim]HSO4
156.0
133.4
56.44
48.97
0.0344
0.0275
2.51
2.50
Fig. 6 TGA curves of Na?-MMT and Na?-MMT-[pmim]HSO4
Thermogravimetric analysis (TGA) was performed for characterization of Na?-
MMT-[pmim]HSO4 in comparison with sodium MMT. Figure 6 provides the TGA
curve of pristine clay (Na?-MMT) and catalyst. The TGA curve of Na?-MMT
displays a weight loss below 100 °C corresponding to the loss of the physically
adsorbed water and interlayer H2O, also there is a slight weight loss in the range of
100–600 °C, possibly corresponding to dehydroxylation of sodium MMT [23].
TGA of Na?-MMT-[pmim]HSO4 shows completely different decomposition
from Na?-MMT. A small mass loss appears at\100 °C because of the loss of water
and a greater mass loss started at 270 °C can be attributed to the decomposition of
the immobilized IL moieties anchored to the clay surface [34]. These results
indicate that the immobilized acid catalyst has a greater thermal stability than Na?-
MMT and is apparently stable up to about 270 °C.
The samples of Na?-MMT and Na?-MMT-[pmim]HSO4 were also analyzed
using a scanning electron microscope (SEM) for determining the size distribution,
particle shape and surface morphology, as represented in Fig. 7. The SEM images
show that the original structure of Na?-MMT is changed and the particles are
clearly less stacked in the intercalated clays. This opening of the layers suggests that
the interlayer space may be occupied by the organic molecules. These images also
show that Na?-MMT-[pmim]HSO4 appears as small particles with irregular plate-
like shapes.
123

Acidic ionic liquid immobilized on nanoporous Na?…
Fig. 7 SEM images of Na?-MMT (a) and Na?-MMT-[pmim]HSO4 (b, c)
Catalytic activity
On the basis of the information obtained from the studies on Na?-MMT-
[pmim]HSO4, we anticipated that this reagent can be used as an efficient solid
acid catalyst for the promotion of the reactions which need the use of an acidic
catalyst to speed up. So we were interested in investigating the applicability of this
reagent in the promotion of the synthesis of formamides.
At first and for optimization of the reaction conditions, the reaction between
4-chloroaniline and formic acid to its corresponding N-phenyl formamide was
selected as a model reaction and the various conditions, including amount of the
catalyst and temperature, were examined (Table 2). For choosing the reaction
media, different solvents such as EtOH, CH2Cl2, CH3CN and solvent-free
conditions were used and the best results were obtained in the absence of solvent,
probably due to the increase in the concentration of the reactants and increase in the
effective contacts of reactants. Finally, the best result was obtained using 10 mg of
Na?-MMT-[pmim]HSO4 at 60 °C under solvent-free conditions (Scheme 2). Any
further increase of the catalyst or temperature did not improve the reaction time and
yield. To illustrate the efficiency of Na?-MMT-[pmim]HSO4 in these reactions, the
model reaction was also carried out in the absence of the catalyst and the reaction
was not proceeded (Table 2, entry 9). These results indicated that the catalyst is
necessary to produce the products.
123

F. Shirini et al.
Conversion (%)
Table 2 Optimization of the reaction conditions catalyzed by Na?-MMT-[pmim]HSO4
Entry
Catalyst (mg)
Temperature (°C)
Solvent
Time (min)
1
2
3
4
5
6
7
8
9
7
10
15
10
10
10
10
10
0
60
60
60
40
80
60
60
60
60
–
25
15
100
–
100
–
15
100
–
60
80
–
13
100
CH2Cl2
EtOH
CH3CN
–
70
95
60
No reaction
No reaction
No reaction
60
120
R'
R
R'
N
CHO
NH
R
MMT-[pmim]HSO4 (10 mg)
Solvent
60 °C
or
or
CHOOH
free,
R"
OH
R"
OCHO
Scheme 2 Formylation of amines and alcohols catalyzed by Na?-MMT-[pmim]HSO4
After optimization of the reaction conditions and in order to establish the
effectiveness and acceptability of the method, we explored the protocol with a
variety of simple, readily available substrates under the optimal conditions. For this
purpose, a broad range of aromatic, aliphatic and heterocyclic amines were treated
with formic acid, as shown in Table 3. It was observed that under similar
conditions, a wide range of aromatic amines containing electron-withdrawing as
well as electron-donating groups such as Cl, Br, CH3, OCH3, Et, NO2, COOH and
OH in the ortho, meta, and para positions of the benzene ring were easily converted
to the corresponding formamides in short reaction times with high isolated yields
(Table 3, entries 1–10). As can be seen, the nature of the substituents on the
aromatic ring showed obvious effects and the aromatic amines with electron-
donating groups were reacted faster than their electron-withdrawing counterpart.
Under the selected conditions, aliphatic (benzyl amine), heterocyclic (imidazole)
and polycyclic (1-naphthyl amine) amines were also converted to the corresponding
123

Acidic ionic liquid immobilized on nanoporous Na?…
Table 3 Formylation of amines and alcohols in the presence of Na?-MMT-[pmim]HSO4 under solvent-
free conditions
Entry
1
Substrate
Product
Time (min)
15
Yield (%)a
92
m.p. (°C)
[Refs.]
44–45 [11]
2
10
94
50–52 [36]
3
4
5
15
15
10
95
96
90
97–99 [35]
111–113 [35]
Oil [36]
6
12
10
3
95
91
96
88
93
93
44–46 [11]
50–52 [18]
76–78 [11]
136–138 [37]
190–192 [11]
Oil [38]
7
8
9
3
10
11
10
5
12b
30
90
136–138 [35]
13
5
3
92
93
Oil [11]
14 b
192–194 [12]
15
16
25
20
95
92
Oil [37]
Oil [39]
123

F. Shirini et al.
Table 3 continued
Entry
Substrate
Product
Time (min)
15
Yield (%)a
94
m.p. (°C)
[Refs.]
17
Oil [39]
18
19
30
55
89
93
Oil [39]
Oil [37]
20
21
4 h
4 h
90
92
Oil [40]
Oil [40]
22
23
6 h
2 h
91
–
Oil [40]
—
–
a
Isolated yields
b
4 mmol formic acid was used
products in high yields (Table 3, entries 11–14). Comparison shows that aliphatic
amines were reacted faster than aromatic ones (Table 3, entries 11 and 14 versus
entry 1).
Furthermore, this method was successfully applied for the formylation of
aliphatic alcohols with formic acid under mild reaction conditions and the
corresponding O-formylated products were obtained in high yields (Table 3, entries
15–22). It is known that these reactions give the products in longer reaction times
than formylation of amines. Phenolic –OH group formylation did not take place
(Table 3 entry 23); thus, with aromatic amines containing both an amino and a
phenolic group, an N-formylated product was selectively obtained, indicating the
chemoselectivity of the protocol (Table 3, entry 9).
This method was also found to be useful for the usage of diamines (Table 4). In
the case of 1,2-phenylenediamines, cyclization occurred instead of N-formylation
and benzimidazole derivatives were obtained as the product (Table 4, entries 1, 2).
In other cases, diamines led to diformamide products (Table 4, entries 3–6).
To check the reusability of the catalyst, the reaction of 4-chloroaniline and
formic acid under the optimized reaction conditions was studied again. When the
reaction was completed, ethyl acetate was added and the catalyst was separated by
filtration. The recovered catalyst was washed with ethyl acetate, dried and reused for
123

Acidic ionic liquid immobilized on nanoporous Na?…
Table 4 Formylation of diamines in the presence of Na?-MMT-[pmim]HSO4 under solvent-free
conditions
Entry Substrate
Product
Time Yield m.p. (°C)
(min) (%)a [References]
3
94
94
167–169 [12]
203–205 [41]
1
15
2
10
25
90
93
88
210–212 [38]
261–263
3a
4a
5a
1 h
172–174
1 h
90
274–276
6a
a
4 mmol formic acid was used
Isolated yields
b
Fig. 8 Reusability of the catalyst
123

F. Shirini et al.
Fig. 9 FT-IR spectra of fresh (a) and recovered (b) Na?-MMT-[pmim]HSO4
the same reaction. This process was carried out over ten runs and all reactions led to
the desired products without significant changes in terms of the reaction time and
yield which clearly demonstrates practical recyclability of this catalyst (Fig. 8).
Furthermore, the fresh and recovered Na?-MMT-[pmim]HSO4 was characterized
by FT-IR spectra (Fig. 9), which prove the fine reusability of the catalyst.
In order to show the efficiency of the presented method, we compared our result
obtained from the reactions of 4-nitro aniline, 4-methoy aniline and benzyl amine
with formic acid catalyzed by Na?-MMT-[pmim]HSO4 with some of the other
results reported in the literature (Table 5). It is clear that the present method is
superior in terms of the reaction time and catalyst amount. Furthermore, to compare
the applicability and efficiency of Na?-MMT-[pmim]HSO4 with the other catalysts,
we calculated the turn over frequency (TOF) of these catalysts in the mentioned
reactions. As it is clear, the TOF values showed that Na?-MMT-[pmim]HSO4 is
more effective than the compared catalysts. This comparison also clarifies an
important point about the catalyst. As can be seen, Na?-MMT, [pmim]HSO4 and
Na?-MMT-[pmim]Cl are also able to catalyze this type of reaction, but in longer
reaction times as compared to Na?-MMT-[pmim]HSO4. These results give clear
evidence to confirm the important role of the covalently bounded IL and also
-
HSO4 group in the catalyst to obtain the best performance.
123

Acidic ionic liquid immobilized on nanoporous Na?…
Table 5 Comparison of the result obtained from the N-formylation of 4-nitro aniline, 4-methoxy aniline
and benzyl amine in the presence of Na?-MMT-[pmim]HSO4 with those obtained using some of the other
catalysts
Catalyst (mol%)
Timeb
TOF
Timeb
TOF
Timeb
TOF
Refs.
(yield)c
(yield)c
(yield)c
(h−
)
(h−
)
(h−
)
1
a
1
a
1
a
γ
-Fe2O3@HAp-SO3H (0.9 mol%)
35 (94)
179
20 (97)
323
40 (92)
153
[11]
HCOONa (20 mol%)
TiO2-SO42− (7.5 mol%)
210 (92) 1.3
420 (92) 1.7
180 (99) 1.6
240 (99) 3.3
150 (85) 1.7
[12]
–
–
[13]
HClO4–SiO2 (2.5 mol%)
90 (88)
10 (91)
90 (89)
10 (90)
46 (65)
60 (92)
30 (88)
30 (92)
10 (93)
23.5
54.6
25 (93)
10 (98)
60 (97)
10 (92)
–
89
59
32
110
–
40 (92)
10 (99)
90 (91)
10 (98)
5 (85)
–
55
59
20
117
82
–
[14]
Sulfated tungstate (10 mol%)
Melamine trisulfonic acid (3 mol%)
Protic ionic liquid (5 mol%)
Silica sulfuric acid (12.5 mol%)
Na+-MMT (10 mg)
[15]
19.7
108
6.8
–
[16]
[17]
[18]
–
–
This work
This work
This work
[pmim]HSO4 (1 mol%)
–
–
–
–
–
Na+-MMT-[pmim]Cl (10 mg)
Na+-MMT-[pmim]HSO4 (1 mol%)
–
–
–
–
–
558
3 (96)
1920
5 (93)
1116 This work
a
Turn over frequency (TOF) represents the average number of substrate molecules converted into the
product per molecule of catalyst per hour
b
min
c
%
Conclusion
In conclusion, in this study, natural sodium MMT clay (Na?-MMT) was organically
¨
modified by Bronsted acidic IL and then successfully used for the simple, efficient
and chemoselective formylation of various amines and alcohols in short reaction
times with high TOF. Na?-MMT-[pmim]HSO4 shows high thermal stability and
was recovered and reused without any noticeable loss of activity. No side reactions,
a simple experimental procedure, a lower catalyst amount applied, and solvent free
conditions are other advantages of this method. Further studies on some more
practical applications of the Na?-MMT-[pmim]HSO4 catalyst in other organic
reactions are currently underway in our laboratory.
Acknowledgments We are thankful to the University of Guilan research council for the partial support
of this work.
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