The catalytic effect of anion-exchanged supported ionic liquid on aza-Michael-type addition

Article abstract of DOI:10.1007/s11164-016-2840-0

Abstract: An effective synthesis of anion-exchanged supported ionic liquids using diatomaceous earth as solid support and its catalytic effect on the aza-Michael-type addition is described. Anionic polytungstophosphate and bisulfate ion are used in the anion-exchange step in catalyst design. In addition, the aza-Michael-type addition of various amines to 2- and 4-vinyl pyridine was examined in this article. The catalytic system can be separated from the reaction mixture and recycled in subsequent reactions. The structure of anion-exchanged supported ionic liquid on diatomaceous earth was studied by XRD, FT-IR, SEM, TGA and BET techniques. The structure of organic products was determined by ¹HNMR, ¹³CNMR, FTIR, CHN and MASS spectroscopy. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s11164-016-2840-0

Res Chem Intermed
DOI 10.1007/s11164-016-2840-0
The catalytic effect of anion-exchanged supported ionic
liquid on aza-Michael-type addition
1
1
Mohammad Hadi Ghasemi · Elaheh Kowsari
Received: 29 September 2016 / Accepted: 16 December 2016
©
Springer Science+Business Media Dordrecht 2016
Abstract An effective synthesis of anion-exchanged supported ionic liquids using
diatomaceous earth as solid support and its catalytic effect on the aza-Michael-type
addition is described. Anionic polytungstophosphate and bisulfate ion are used in the
anion-exchange step in catalyst design. In addition, the aza-Michael-type addition of
various amines to 2- and 4-vinyl pyridine was examined in this article. The catalytic
system can be separated from the reaction mixture and recycled in subsequent reactions.
The structure of anion-exchanged supported ionic liquid on diatomaceous earth was
studied by XRD, FT-IR, SEM, TGA and BET techniques. The structure of organic
1
13
products was determined by HNMR, CNMR, FTIR, CHN and MASS spectroscopy.
Graphical Abstract
Keywords Anion-exchanged supported ionic liquid · Diatomaceous earth · Anionic
polytungstophosphate · Aza-Michael-type addition · Catalysis
&
Elaheh Kowsari
kowsarie@aut.ac.ir
1
Department of Chemistry, Amirkabir University of Technology, Hafez Avenue, No. 424,
Tehran, Islamic Republic of Iran
123

M. H. Ghasemi, E. Kowsari
Introduction
Diatomaceous earth (DE), known as diatomite or diatoms, is the most abundant
form of silica on earth naturally composed of amorphous hydrated silica (SiO2–
nH O) [1]. Hydrophilic properties, high porosity, strong acid resistance, low thermal
2
conductivity, low density and excellent thermal resistance are among DE unique
physical and chemical characteristics [2–5] which make it ideal for a variety of
applications such as supports, porous catalysts, and adsorbents [6, 7]. Due to these
special characteristics, DE was applied as a porous support in this research. Surface
functionalization of DE by the introduction of covalently bonded functional groups
to the outer DE surfaces is a helpful approach to increase selectivity and loading
capacity of a catalytically active species [8–10].
The polyoxometalate compounds have been used as efficient catalysts in various
chemical transformations due to their strong Brønsted acidity, thermal stabilities,
lower corrosivity and higher catalytic activity [11–16]. However, the problems in
recovery at the end of reactions limits the catalytic action of polyoxometalates [17].
In this research, (3-Mercaptopropyl)trimethoxysilane, which has a chloro group (–
Cl) at the end, was used as a linker to modify the surface of DE with acidic ionic
liquid. The efficiency of modified DE with two types of Brønsted and Lewis acidic
ionic liquids was properly investigated. So, the immobilization of the polyoxomet-
alates on the solid support will supply mild treatment in the production process and
open up a possibility for their applications in the chemical or pharmaceutical
industry.
Immobilization of ionic liquids by covalent connection on the surface of solid
supports is directed to heterogenization of homogeneous catalysts. These catalytic
systems that mix the properties of both homogeneous and heterogeneous catalysts
have the advantage of the elevated surface area, increased activity, selectivity and
sufficiently recovering the catalyst system. A number of techniques have been
published for the preparation of supported ionic liquids [18–22]. Many ionic liquid
catalytic systems containing polyoxometalate anions have been prepared and
examined in some useful organic transformations [23–28]. Although much research
has been done in the field of supported ionic liquids, the immobilization of anionic
polyphosphotungstate on the diatomaceous earth covalently functionalized with (3-
chloropropyl)trimethoxysilane (CPTMS) as reported in this manuscript, has a great
novelty and may also create new opportunities for attractive research. The activity
of heterogeneous catalysts is often comparable to their corresponding homogeneous
analogs, illustrating that the heterogenization of homogeneous catalysts could be
successful.
In this study, a supported IL design has been applied for the immobilization of an
anionic polyphosphotungstate derivative and bisulfate anion. We want to describe
an appropriate procedure for the application of anionic acids in supported IL for the
synthesis of some valuable pharmaceutical molecules (Scheme 1). Phosphotungstic
acid (H PW O ·nH O) as a polyoxometalate and sulfuric acid as a Brønsted acid
3
12 40
2
were used in this research.
1
23

The catalytic effect of anion-exchanged supported ionic…
Scheme 1 Anion exchange in supported IL covalently bounded on the DE surfaces
Much work has gone into the development of efficient techniques for the aza-
Michael-type addition of amines to activated vinyl compounds. The aza-Michael-
type addition has been catalyzed by various acids and bases in which a number of
undesired reactions may have taken place, such as polymerization of vinyl
compounds. Vinyl compounds, specially pre-activated vinyl compounds such as 2-
and 4-vinyl pyridines used in aza-Michael-type addition in this research, have a
considerable tendency to perform polymerization reactions especially in the
presence of acidic catalysts, which significantly reduces the reaction overall yield.
So, efficient approaches are needed for the development of mild catalytic systems
for the aza-Michael-type addition. A number of alternative methodologies using
catalytic systems incorporating ionic liquids have been reported [29–31]. In
continuation of our previous research on the supported ionic liquids [23, 32–35],
herein we report an efficient method for aza-Michael-type addition of amines to
2-vinyl pyridine and 4-vinyl pyridine catalyzed by anion-exchanged supported ionic
liquids (Scheme 2).
Betahistine dihydrochloride (i) as an anti-vertigo drug is synthesized via the aza-
Michael-type addition of methylamine to vinyl pyridine followed by addition of
hydrogen chloride gas to the intermediate (Scheme 3).
The search for general, effective synthesis of the abovementioned compound
under moderate conditions is of ongoing benefit for organic chemists.
123

M. H. Ghasemi, E. Kowsari
R2
N
N
R3
supported ionic liquid
o
CH CN / 50 C / 2h
3
R2
N
+
R R NH
2 3 2
R1
R3
N
N
,
R1 =
N
NH2
NH2
NH2
N ,
NH2
NH2
NH2
H
N
N
R R NH = MeNH , Me NH,
,
S
N ,
,
,
,
2
3
2
2
SO NH2
2
Scheme 2 Aza-Michael-type addition of amines to 2-vinylpyridine and 4-vinylpyridine catalyzed by
anion-exchanged supported ionic liquids
H
N
H
N
N
N
N
HCl (g)
.
2HCl
+
CH NH
3 2
Betahisꢀne.2HCl
i
Scheme 3 The synthetic method of betahistine dihydrochloride i starting with 2-vinyl pyridine
Experimental
Materials and general techniques
Materials totally were purchased from Fluka or Merck Chemical Companies.
Progression of the reactions was checked by TLC using silica gel SIL G/UV 254
plates. Melting points were recorded with an Electrothermal-9100 apparatus. NMR
spectra were recorded in CDCl on a Brucker Avance DPX instrument at 400 and
3
3
00 MHz, respectively; δ is reported in parts per million (ppm) and J in Hz. The
morphology of particles was checked employing a scanning electron microscope
AIS2300C SEI scanning electron microscope) at an accelerating voltage of 25 kV.
(
Fourier transform infrared spectra were achieved by a BRUKER EQUINOX 55
Fourier-transform spectrophotometer using the KBr pellet technique. The X-ray
diffraction (XRD) patterns were reported with a Philips X’Pert MPD diffractometer
equipped with Cu Kα radiation (λ = 0.154 nm) in the range 10–80° (2θ) at a speed of
0
.05°/min. The specific surface areas (SBET) of particles were measured by the BET
method, and the pore size distributions were determined using the Barrett–Joyner–
Halenda (BJH) method. Thermogravimetric analysis (TGA) was carried out using a
1
23

The catalytic effect of anion-exchanged supported ionic…
−1
Perkin–Elmer thermobalance at a heating rate of 10 °C min from 20 to 600 °C in a
nitrogen gas flow to evaluate the amount of organic groups bound to the DE surface.
Preparation and characterization of catalyst
Preparation and characterization of modified DE (DE-PrCl, 3)
Diatomaceous earth was functionalized with (3-chloropropyl)trimethoxysilane
(
CPTMS) according to the literature with some modifications [36]. In a three-necked
00 mL round-bottomed flask equipped with a thermometer, a reflux condenser under
dry N atmosphere, DE (1, 5.0 g) was suspended in toluene (150 mL) and vigorously
5
2
stirred for 2 h at ambient temperature. CPTMS (2, 8.5 mL) was then added to the above
mixture and was refluxed overnight. The resulted mixture was cooled and washed with
2-propanol (50 mL) and distilled water (2 9 50 mL), respectively. The DE-PrCl (3)
°
was dried in a vacuum oven at 60–70 C for 6 h.
Preparation of covalently bonded ionic liquid to the DE surfaces (DE-PrIm-Cl, 5)
A mixture of DE-PrCl (3, 2.0 g) and 1-methylimidazole (4, 0.5 mL) was suspended
in toluene (100 mL) and was refluxed for 24 h. The resulting mixture was isolated
by simple filtration and washed with diethyl ether (2 9 50 mL). The unreacted
1-methylimidazole was extracted with ethanol (50 mL) at 70 °C for 2 h. The solid
product was filtered, was washed with diethyl ether (50 mL) and was dried in the
vacuum oven at 60–70 °C for 6 h to give DE-PrIm-Cl (5).
Preparation of immobilized ion-exchanged ionic liquids DE-PrIm-HSO (6a)
4
and DE-PrIm-HPW (6b)
The compound 5 (1.0 g) was suspended in toluene (50 mL) with vigorous stirring in
an ice bath. Sulfuric acid 98% (0.5 mL) was introduced dropwise at 0–5 °C, and
then the mixture was refluxed for 48 h. The solution was cooled and completely
distilled off under vacuum to remove the solvent and formed HCl from the mixture.
The residue was extracted with benzene (3 9 10 mL), was dried in the vacuum oven
°
at 60–70 C for 6 h to produce ion-exchanged catalyst DE-PrIm-HSO (6a). The
4
preparation method of catalyst 6b is similar to the abovementioned method. The
compound 5 (1.0 g) was suspended in toluene (50 mL) with vigorous stirring in an
ice bath. Tungstophosphoric acid (0.58 g, 0.2 mmol) was introduced step by step at
0–5 °C and then was refluxed for 48 h. The resulting mixture was distilled off under
vacuum and was extracted with benzene (3 9 10 mL) and was dried in the vacuum
oven at 60–70 °C for 6 h to produce ion-exchanged catalyst DE-PrIm-HPW (6b).
Aza-Michael-type addition of amines to vinyl pyridine by catalysts 6a and 6b
In a 10 mL flask, 2-vinyl pyridine (0.53 g, 0.54 mL, 5 mmol, 1 equiv.) and aniline
(
acetonitrile (5 mL) and heated for 2 h at 50 C (Scheme 2). The progress of the
0.47 g, 0.46 mL, 5 mmol, 1 equiv.) were added to a mixture of catalyst 6b (0.1 g) in
°
123

M. H. Ghasemi, E. Kowsari
reaction was checked by TLC. After the completion of the reaction, the solid
catalyst was filtered, and the solvent was evaporated. The residue was added to
toluene: sodium hydroxide (33%) (1:1) biphasic system. Then the aqueous phase
was extracted with dichloromethane (pH = 5.0) to remove impurities. The organic
layer was evaporated, and the precipitate was washed with acetone and was dried to
afford N-(2-(pyridin-2-yl)ethyl)aniline 7a as a light brown solid (1.43 g, 95%). As
indicated in Table 3, this reaction was performed with different amines in thermal
conditions. The reaction condition of 6a approach is similar to the procedure
described above for the 6b approach. The compound 7a was synthesized as a light
brown solid (1.25 g, 83%).
Results and discussion
In the present paper, we report the fabrication of a recoverable acidic catalyst, in
which Brønsted and Lewis acids are immobilized in supported IL covalently
bounded on DE surface. We also describe an efficient simple procedure for the aza-
Michael-type addition of amines to vinyl pyridines using this catalytic system.
Characterization of catalyst
The X-ray diffraction patterns of DE, DE-PrIm-Cl, DE-PrIm-HSO and DE-PrIm-
4
HPW are shown in Fig. 1. XRD spectra of DE show that amorphous silica is the
Degrees (2θ)
4
Fig. 1 XRD pattern of a DE; b DE-PrIm-Cl; c DE-PrIm-HSO and d DE-PrIm-HPW
1
23

The catalytic effect of anion-exchanged supported ionic…
most dominant peak. The crystalline structure of compounds was characterized by
XRD analysis, with representative results shown in Fig. 1. For the crystalline
structure of DE, the main XRD diffractions of quartz were found at a 2θ of 22.04,
2
8.34 and 56.90.
Degrees (2θ)
The FT-IR spectra of the abovementioned compounds were evaluated in order to
analyze the surface functional groups of the samples (Fig. 2). The broad peaks at
−
1
about 1000–1100, 780 and 480 cm
antisymmetric stretching, symmetric stretching and bending vibrations in siloxane
in all spectra (a–e) contributed to
−1
(
Si–O–Si and Si–OH), respectively. The broad peak around 3500 cm was related
to the stretching vibrations in the structural hydroxyl groups. The presence of a peak
around 2900 cm− (C–H in propyl chain) in the spectrum b indicates that the
chloropropyl chain is covalently bounded to the DE surfaces with CPTMS via the
1
-1
Wavenumber (cm )
4
Fig. 2 FT-IR spectra of a DE; b DE-pr-Cl; c DE-PrIm-Cl; d DE-PrIm-HSO and e DE-PrIm-HPW
123

M. H. Ghasemi, E. Kowsari
method used for this work (Fig. 2b). The vibration bands of 1-methylimidazole and
propyl chain in the DE-PrIm-Cl suggest that ionic liquid 1-methylimidazolium
−1
chloride (1600 cm , imidazole ring) was successfully loaded on the DE surfaces
Fig. 2c). The FT-IR spectra of compounds DE-PrIm-HSO and DE-PrIm-HPW
(
indicate that sulfate anion (1350–1450 cm , S=O in sulfate ion) and anionic
4
−1
−1
polytungstatophosphate (800–1000 cm , Keggin type heteropolyanions) were
successfully exchanged with chloride anion in the catalytic system (Fig. 2d, e).
−
1
Wave number (cm )
The thermal stability of DE, DE-PrCl, DE-PrIm-Cl, DE-PrIm-HSO and DE-PrIm-
4
HPW was evaluated by thermogravimetric analysis (TGA) curves (Fig. 3). The
TGA curves show the mass loss of the organic materials as they decompose with an
increase in heating. The first mass loss at 25–100 °C contributes to the evaporation
of physically adsorbed water. The second mass loss at 200–400 °C is related to the
thermal decomposition of organochloropropyl fragments covalently bonded to the
DE surfaces. In the case of DE-PrIm-HSO4 (Fig. 1a), the further mass loss
contributes to the thermal decomposition in the presence of acidic HSO4.
The SEM images of natural DE and functionalized DE are displayed in Fig. 4,
showing there has been a little change in the average particle size.
The specific surface areas of particles were determined using standard BET
Brunauer–Emmett–Teller) techniques. The BET surface area and the pore diameter
(
of DE were determined as a moderate surface area for the catalytic application. The
mean pore diameter and pore volume of the DE particles were 16.33 nm and
3
.0028 cm /g, respectively. The BET specific surface area, pore diameter, and
0
volume of functionalized DE particles were fairly similar to DE. As shown in
Temperature (°C)
Fig. 3 TGA thermograms of a DE; b DE-pr-Cl; c DE-PrIm-Cl; d DE-PrIm-HPW and e DE-PrIm-HSO
4
1
23

The catalytic effect of anion-exchanged supported ionic…
a
b
d
f
c
e
Fig. 4 SEM images of a DE; b DE-pr-Cl; c DE-PrIm-Cl; d DE-PrIm-HSO
4
, e and f DE-PrIm-HPW
4
Table 1 Textural properties of DE, DE-PrIm-HSO , DE-PrIm-HPW
2
Material
S
BET (m /g)
Mean pore
diameter
nm)
Total pore
volume
3
(cm /g)
(
DE
0.70
2.83
2.50
16.33
25.66
23.23
0.0028
0.018
0.014
DE-PrIm-HSO
4
DE-PrIm-HPW
Table 1, functionalization of DE with CPTMS leads to increased surface area and
pore volume in DE-PrIm-HSO4 and DE-PrIm-HPW, respectively. The higher
surface areas of DE-PrIm-HSO and DE-PrIm-HPW can be attributed to the more
4
adsorption of N gasses by functionalized groups on the immobilized catalyst on the
2
support. The presence of organic molecules contributing the nitrogen absorption on
123

M. H. Ghasemi, E. Kowsari
the support pore surfaces resulted in more complexity and increased nitrogen
absorption and thereby increased surface area.
Effect of anion-exchanged catalytic system 6a and 6b
The anion-exchange step was carried out using sulfuric acid and tungstophosphoric
acid (Scheme 1). As illustrated in Table 2, the best results are obtained using anionic
polyoxometalates as the catalytic system. Employing anionic polyoxometalates as
inorganic compounds with high crystal lattice energies in supported ionic liquids
effectively increases their application as the catalyst in organic synthesis.
Polyoxometalates possess powerful Brønsted acidity, even stronger than conven-
tional Brønsted acids such as H SO , and conventional Lewis acids.
2
4
Polyoxometalates show excellent catalytic activity, specifically in acid-catalyzed
reactions, including aza-Michael-type addition as described throughout this paper.
Among the polyoxometalates used in this paper, phosphotungstic acid HPW was
found to be a highly effective anion-exchanged catalyst for the abovementioned
reactions. Polyoxometalates are generally more efficient proton donors than
ordinary acids; consequently, they are stronger catalysts. Additionally, anionic
polyoxometalates can stabilize intermediates in organic reactions and substan-
tially have an important function in the reaction catalysis [37]. Phosphotungstic acid
Table 2 Aza-Michael-type addition of aniline to 2-vinylpyridine catalyzed by catalytic systems 6a and
a
b
6
NH2
H
N
Catalytic systems 6a, 6b
N
N
Solvent
+
A
B
b
Entry
Catalytic system
Solvent
Mole ratio (A:B)
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
9
DE
CH
CH
CH
CH
CH
CH
CH
CH
3
3
3
3
3
3
3
3
CN
CN
CN
CN
CN
CN
CN
CN
1:1
1:1
1:1
1:1
1:1
1:1
1:2
1:1
1:1
1:1
1:1
1:1
1
1
1
1
1
2
2
2
2
2
2
2
Trace
45
H
H
2
3
SO
4
PW12
O
40
72
DE-PrIm-HSO
4
6a
83
DE-PrIm-HPW
DE-PrIm-HPW
DE-PrIm-HPW
DE-PrIm-HPW
DE-PrIm-HPW
DE-PrIm-HPW
DE-PrIm-HPW
DE-PrIm-HPW
6b
6b
6b
6b
6b
6b
6b
6b
90
95
95
c
70
THF
75
73
82
60
10
11
12
a
DMF
2
H O
2 2
CH Cl
Reactions conditions: vinyl pyridine (5 mmol, 1 equiv.), aniline (5 mmol, 1 equiv.), solvent (5 mL),
catalyst (0.1 g), 50 °C
b
Isolated yield
c
The reaction was performed at room temperature
1
23

The catalytic effect of anion-exchanged supported ionic…
HPW is the preferred acid catalyst compared to other polyoxometalates used in this
paper due to its stronger acidity, higher hydrolytic and thermal stability, and
decreased oxidation potentials.
Effect of catalytic system 6a and 6b on aza-Michael-type addition
Using DE as a heterogeneous support without any acidic catalysts was accompanied
with poor results (Table 2, Entry 1). Among various particles as heterogeneous solid
supports, 6b is an efficient and recoverable catalyst in the organic reactions
described here. Because of dual catalytic action in HPAs (Brønsted and Lewis acid),
6
b acts as an extraordinary catalyst. Considering the reaction efficiency and catalyst
recyclability, the use of 6b and 6a as the catalyst shows the better results compared
with the use of phosphotungstic acid and sulfuric acid, respectively. So, the catalytic
performances of the catalytic systems 6b and 6a were higher than for the
corresponding use of phosphotungstic acid (Table 2, entries 3 and 5) and sulfuric
acid individually (Table 2, entries 2 and 4). Due to improved results through the use
of 6b as a recoverable catalyst, some more experiments were performed by
changing the reaction conditions such as reaction time (Table 2, Entry 6), the molar
ratio of reactants (Table 2, Entry 7), reaction temperature (Table 2, Entry 8) and
solvents (Table 2, entries 9–12). The best results were obtained using 6b as catalyst
(
0.1 g) in acetonitrile as a polar solvent (5 mL) for 2 h at 50 °C (Table 2, Entry 6).
To evaluate the catalytic performance of 6b and its repeatability as Brønsted and
Lewis acids, more experiments were examined comparing the catalytic system 6a as
Brønsted acid only (Table 3, Fig. 5). In the absence of the catalytic system, the
transformations did not occur because of the activation barrier (Table 3, entry 10).
The products are easily separated in increased yields and high purity by simple
purification of the precipitated solid after evaporation of the solvent. The work-up
procedure involves simple filtration of catalytic particles and evaporation of the
organic solvent.
The mechanistic aspects
The addition reaction between amine and vinyl pyridine follows the aza-Michael-
type mechanism (Fig. 6a). The aza-Michael reaction is due to the nucleophilic
attack of a primary and/or secondary amine i to an activated vinyl compound ii. In
the case of 2-vinyl pyridine and 4-vinyl pyridine, the aza-Michael addition occurs at
the 1,4- and 1,6-position, respectively. The vinyl compound can be activated via
H-bond interaction between a nitrogen atom in the pyridine ring and the protons in
acidic heterogeneous catalysts 6a and 6b. Additionally, the vinyl compound is more
activated by the interaction between a nitrogen atom in the pyridine ring and Lewis
acidic sites, using catalyst 6b. The transition state ILTS thus obtained is well
stabilized through the polar interaction of anionic pyridinium and cationic
imidazolium in supported ionic liquid (Fig. 6b).
123

M. H. Ghasemi, E. Kowsari
Table 3 Aza-Michael-type addition of amines to 2-vinylpyridine and 4-vinylpyridine catalyzed by
a
catalysts 6a and 6b
R1
N
N
N
R2
R2
catalytic systems 6a, 6b
2
-vinylpyridine
CH CN
7a-i
3
R1
N
+
R R NH
1 2
N
N
8
a-i
4-vinylpyridine
b
Entry
1
Amine
Product
Yield (%)
6
a
6b
H
NH2
N
N
7a
83
95
H
N
8
a
90
78
80
81
75
73
63
71
68
74
98
90
93
95
89
95
90
85
94
93
N
N
H
N
N
2
3
4
5
6
CH
3
NH
2
(%40, aq)
7b
H
N
8
b
N
N
Me
2
NH
7c
N
8
c
N
N
H
N
N
H N
SO NH2
7d
2
2
SO NH
2
2
H
N
8
d
SO NH
2
2
H
N
H N
N
N
N
2
7e
S
S
H
N
N
8
e
N
S
H
N
H N
N
2
N
N
7f
N
N
1
23

The catalytic effect of anion-exchanged supported ionic…
Table 3 continued
b
Entry
Amine
Product
Yield (%)
6
a
6b
7
7g
70
90
N
N
HN
N
8
g
86
95
N
N
NH2
H
N
8
9
7h
63
82
H
N
8
h
71
90
N
NH2
H
N
N
7i
68
80
N
8
i
75
88
N
N
H
N
c
c
10
NH2
7a
Trace
Trace
a
Reactions conditions: vinyl pyridine (5 mmol, 1 equiv.), amine (5 mmol, 1 equiv.), acetonitrile (5 mL),
catalyst (0.1 g), 50 °C, 2 h
b
Isolated yield
c
The reaction was performed without any catalyst
Recyclability of the catalytic system
Catalyst recyclability is one of the most important features in heterogeneous
catalysts. The catalytic system was successfully recycled five times without any
remarkable loss of its catalytic activity (Fig. 7). At the end of each reaction, the
catalytic system was separated by a simple filtration and carefully was washed twice
with ethanol, and then was reused as a catalyst for the next reaction under the same
conditions. After reuse five times, the recycled catalysts 6b maintained their
efficiency. The structure and activity of 6b catalyst remained constant during the
reaction conditions.
Experimental data
To make reading more convenient, NMR analysis data of selected entries is
provided here.
123

M. H. Ghasemi, E. Kowsari
Fig. 5 Comparisons of catalytic activity between catalysts 6a and 6b on aza-Michael-type addition
starting with a 2-vinylpyridine and b 4-vinylpyridine
i
ILTS
(
a)
R
1
R
2
ii
H
N. .
R2
N+
R
H
1
R
1
N
R
N
-
2
N
N
H
H
+
A
-
A
-
HA
-
HA
-
HA
-
HA
-
HA
-
HA
-
HA
-
N
N
N
N
N
N
N
N
N
+
+
+
+
+
+
+
+
+
N
N
N
N
N
N
N
N
N
Si
OMe
OH
Si
Si
Si
OMe
Si
Si
Si
OMe
OH OH
Si
Si
O
OH
O
O
O
O
O
OH OH
O
O
O
O
O
O
O
O OH OH
O
O
O
O
O
OH OH
O
O
O
OH OH
O
O
Diatomaceous earth (DE)
Diatomaceous earth (DE)
Diatomaceous earth (DE)
(
b)
−
Fig. 6 The plausible mechanism for activation of 2-vinyl pyridine by DE-PrIm-HA catalytic system
N-(2-(pyridin-2-yl)ethyl)aniline (7a)
1
C H N ; mp: 40–43 °C; H NMR (300 MHz, CDCl ) δ(ppm): 3.09 (t, 2H,
1
3
14
2
3
J = 6.6 Hz), 3.54 (b, 2H), 4.30 (b, 1H), 6.67 (d, 2H, J = 7.6 Hz), 6.74 (t, 1H,
J = 7.2 Hz), 7.12 (t, 2H, J = 6.0 Hz), 7.21 (t, 2H, J = 7.5 Hz), 7.57 (t, 1H,
1
3
J = 6 Hz), 8.57 (d, 1H, J = 4.2 Hz); C NMR (75 MHz, CDCl ) δ(ppm): 37.4, 43.5,
3
1
23

The catalytic effect of anion-exchanged supported ionic…
Fig. 7 Recyclability of the catalytic system 6b for the reaction of 2-vinylpyridine with aniline
−
1
1
12.9, 117.2, 121.5, 123.4, 129.3, 136.6, 148.3, 149.3, 159.8; IR ῡ(cm ): 685, 959,
+
1
C, 78.75; H, 7.12; N, 14.13. Found: C, 78.92; H, 7.01; N, 14.05.
273, 1437, 1478, 1601, 1696, 2930, 3067, 3411; MS (M , m/z): 198; Anal. Calcd:
N-(2-(pyridin-4-yl)ethyl)aniline (8a)
1
C H N ; mp: 43 °C; H NMR (400 MHz, CDCl ) δ(ppm): 3.03 (t, 2H,
1
3
14
2
3
1
3
J = 7.2 Hz), 3.60 (t, 2H, J = 6.8 Hz), 7.05–7.27 (m, 5H), 8.49–8.55 (m, 4H);
C
NMR (100 MHz, CDCl ) δ(ppm): 33.3, 45.3, 113.3, 121.2, 123.2, 129.6, 150.3,
3
−
1
160.6, 160.7; IR ῡ(cm ): 603, 690, 998, 1148, 1215, 1437, 1492, 1601, 1696, 2936,
3057, 3242; Anal. Calcd: C, 78.75; H, 7.12; N, 14.13. Found: C, 78.61; H, 7.23; N,
14.16.
N-methyl-2-(pyridin-2-yl)ethanamine (7b)
1
C H N ; viscous yellow oil; H NMR (400 MHz, CDCl ) δ(ppm): 2.57 (s, 3H),
8
12
2
3
3
.43 (t, 2H, J = 5.2 Hz), 3.56 (t, 2H, J = 6.8 Hz), 7.96 (t, 1H, J = 6.4 Hz), 8.08 (d,
H, J = 8.0 Hz), 8.55 (t, 1H, J = 8.0 Hz), 8.85 (d, 1H, J = 5.6 Hz), 9.62 (bs, 1H);
1
1
3
C NMR (100 MHz, CDCl ) δ(ppm): 29.5, 32.6, 46.6, 126.0, 128.1, 142.3, 146.4,
3
−1
152.6; IR ῡ(cm ): 590, 999, 1109, 1418, 1604, 1656, 2850, 2938, 3030, 3462;
Anal. Calcd: C, 70.55; H, 8.88; N, 20.57. Found: C, 70.57; H, 8.76; N, 20.63.
N-methyl-2-(pyridin-4-yl)ethanamine (8b)
1
C H N ; viscous yellow oil; H NMR (400 MHz, CDCl ) δ(ppm): 2.36 (s, 3H),
8
12
2
3
1
.61–2.78 (m, 4H), 5.27 (bs, 1H), 7.00–7.06 (m, 2H), 8.38–8.41 (m, 2H); C NMR
3
2
−1
(
100 MHz, CDCl ) δ(ppm): 35.4, 36.2, 51.9, 124.1, 149.4, 149.5; IR ῡ(cm ): 591,
3
123

M. H. Ghasemi, E. Kowsari
+
8
Calcd: C, 70.55; H, 8.88; N, 20.57. Found: C, 70.65; H, 8.92; N, 20.46.
12, 996, 1110, 1413, 1607, 2854, 2948, 3035, 3442; MS (M , m/z): 135; Anal.
N,N-dimethyl-2-(pyridin-2-yl)ethanamine (7c)
1
C H N ; viscous yellow oil; H NMR (400 MHz, CDCl ) δ(ppm): 2.40 (s, 6H),
9
14
2
3
3
.54 (t, 2H, J = 7.6 Hz), 3.80 (t, 2H, J = 7.6 Hz), 7.56 (d, 1H, J = 10.4 Hz), 7.60 (d,
H, J = 7.6 Hz), 8.64 (t, 1H, J = 7.6 Hz), 8.85 (d, 1H, J = 6.4 Hz); C NMR
13
1
(
(
100 MHz, CDCl ) δ(ppm): 41.2, 45.2, 58.1, 121.5, 125.7, 136.1, 148.5, 156.4; IR ῡ
cm ): 590, 1001, 1264, 1419, 1464, 1604, 2820, 2948; Anal. Calcd: C, 71.96; H,
.39; N, 18.65. Found: C, 72.09; H, 9.31; N, 18.63.
3
−1
9
N,N-dimethyl-2-(pyridin-4-yl)ethanamine (8c)
1
C H N ; viscous yellow oil; H NMR (400 MHz, CDCl ) δ(ppm): 2.27 (s, 6H),
9
14
2
3
2
2
1
3
7
.54 (t, 2H, J = 7.6 Hz), 2.75 (t, 2H, J = 8.0 Hz), 7.11 (d, 2H, J = 4.0 Hz), 8.46 (d,
13
H, J = 2.8 Hz); C NMR (100 MHz, CDCl ) δ(ppm): 33.4, 45.3, 60.0, 124.1,
3
−1
49.5, 149.6; IR ῡ(cm ): 591, 1001, 1220, 1419, 1464, 1604, 1645, 2778, 2947,
029; MS (M , m/z): 149; Anal. Calcd: C, 71.96; H, 9.39; N, 18.65. Found: C,
2.05; H, 9.31; N, 18.59.
+
4
-(2-(pyridin-2-yl)ethylamino)benzenesulfonamide (7d)
1
C H N O S; mp: 122 °C; H NMR (300 MHz, CDCl ) δ(ppm): 3.10 (t, 2H,
1
3
15
3
2
3
J = 6.3 Hz), 3.59 (t, 2H, J = 6.6 Hz), 4.59 (bs, 2H, NH ), 5.00 (bs, 1H), 6.62 (d, 2H,
2
1
3
J = 9.0 Hz), 7.16–7.27 (m, 2H), 7.6–7.7 (m, 3H), 8.57 (d, 1H, J = 3.6 Hz);
NMR (75 MHz, CDCl ) δ(ppm): 38.0, 44.6, 111.7, 118.2, 121.5, 123.4, 129.3,
C
3
−
1
1
37.8, 157.1, 162.3, 177.0; IR ῡ(cm ): 600, 1003, 1143, 1299, 1423, 1599, 1734,
+
2861, 2930, 3335; MS (M , m/z): 277; Anal. Calcd: C, 56.30; H, 5.45; N, 15.15.
Found: C, 56.42; H, 5.51; N, 15.17.
4
-(2-(pyridin-4-yl)ethylamino)benzenesulfonamide (8d)
1
C H N O S; mp: 167–169 °C; H NMR (400 MHz, CDCl ) δ(ppm): 3.41 (t, 2H,
1
3
15
3
2
3
J = 8.8 Hz), 4.40 (t, 2H, J = 8.8 Hz), 6.63–6.70 (m, 2H), 7.10–7.34 (m, 4H), 8.45–
1
3
8
1
1
5
.56 (m, 2H); C NMR (100 MHz, CDCl ) δ(ppm): 36.7, 42.7, 109.3, 124.0, 128.1,
3
1
−
30.4, 148.2, 150.7, 156.7; IR ῡ(cm ): 600, 826, 1003, 1143, 1299, 1422, 1598,
730, 2861, 2930, 3364, 3465; Anal. Calcd: C, 56.30; H, 5.45; N, 15.15. Found: C,
6.42; H, 5.51; N, 15.22.
N-(2-(pyridin-2-yl)ethyl)thiazol-2-amine (7e)
1
C H N S; mp: 145 °C; H NMR (400 MHz, CDCl ) δ(ppm): 3.16 (t, 2H,
1
0
11
3
3
J = 6.8 Hz), 4.07 (t, 2H, J = 6.4 Hz), 5.65 (d, 1H, J = 4.0 Hz), 6.18 (d, 1H,
1
3
J = 3.6 Hz), 7.12 (m, 2H), 7.60 (m, 1H), 8.52 (d, 1H, J = 3.6 Hz); C NMR
100 MHz, CDCl ) δ(ppm): 36.2, 45.7, 97.0, 121.7, 123.9, 136.7, 139.1, 149.3,
(
3
1
23

The catalytic effect of anion-exchanged supported ionic…
−
1
1
58.2, 164.3; IR ῡ(cm ): 752, 994, 1110, 1265, 1320, 1418, 1504, 1601, 2862,
+
2932, 3030, 3332; MS (M , m/z): 205; Anal. Calcd: C, 58.51; H, 5.40; N, 20.47.
Found: C, 58.55; H, 5.53; N, 20.58.
N-(2-(pyridin-4-yl)ethyl)thiazol-2-amine (8e)
1
C H N S; mp: 177–179 °C; H NMR (400 MHz, CDCl ) δ(ppm): 2.94 (t, 2H,
1
0
11
3
3
J = 6.8 Hz), 3.46 (t, 2H, J = 7.2 Hz), 6.64–6.80 (m, 2H), 7.10–7.30 (m, 2H), 8.52–
1
3
8
1
2
5
.59 (m, 2H); C NMR (100 MHz, CDCl ) δ(ppm): 34.9, 44.0, 113.0, 117.8, 124.2,
3
−1
29.3, 129.4, 150.1; IR ῡ(cm ): 694, 752, 995, 1110, 1418, 1504, 1601, 2862,
932, 3029, 3332; Anal. Calcd: C, 58.51; H, 5.40; N, 20.47. Found: C, 58.92; H,
.43; N, 20.56.
N-(2-(pyridin-2-yl)ethyl)pyrimidin-2-amine (7f)
1
C H N ; mp: 195–199 °C; H NMR (400 MHz, CDCl ) δ(ppm): 3.08 (t, 2H,
1
1
12
4
3
J = 6.4 Hz), 3.80 (t, 2H, J = 6.4 Hz), 7.08–7.13 (m, 2H), 7.60 (t, 1H, J = 4.4 Hz),
1
.72–7.83 (m, 2H), 8.71 (s, 2H), 8.91–8.96 (m, 1H); C NMR (100 MHz, CDCl3)
3
7
δ(ppm): 37.5, 40.9, 111.4, 121.2, 122.4, 136.5, 149.4, 155.7, 158.0, 159.5; IR ῡ
−1
+
(
Anal. Calcd: C, 65.98; H, 6.04; N, 27.98. Found: C, 66.09; H, 5.90; N, 27.88.
cm ): 994, 1149, 1413, 1474, 1590, 1645, 2936, 3178, 3352; MS (M , m/z): 200;
2
-(2-(piperidin-1-yl)ethyl)pyridine (7g)
1
C H N ; viscous yellow oil; H NMR (400 MHz, CDCl ) δ(ppm): 1.39–1.63 (m,
1
2
18
2
3
6
2
3
1
H), 2.44 (m, 4H), 2.67 (t, 2H, J = 8.0 Hz), 2.94 (t, 2H, J = 7.6 Hz), 7.50–7.53 (m,
13
H), 8.46 (d, 1H, J = 4.0 Hz); C NMR (100 MHz, CDCl ) δ(ppm): 24.3, 27.0,
3
−1
1.4, 53.0, 54.5, 122.8, 124.2, 134.0, 150.1, 153.5; IR ῡ(cm ): 554, 765, 994, 1116,
+
436, 1590, 2940; MS (M , m/z): 190; Anal. Calcd: C, 75.74; H, 9.53; N, 14.72.
Found: C, 75.61; H, 10.59; N, 14.75.
4
-(2-(piperidin-1-yl)ethyl)pyridine (8g)
1
C H N ; viscous oil; H NMR (400 MHz, CDCl ) δ(ppm): 1.59 (m, 6H), 2.42 (m,
1
2
18
2
3
4
8
5
1
H), 2.53 (t, 2H, J = 8.0 Hz), 2.71 (t, 2H, J = 8.4 Hz), 7.10 (d, 2H, J = 4.4 Hz),
13
.45 (t, 2H, J = 2.4 Hz); C NMR (100 MHz, CDCl ) δ(ppm): 24.3, 25.9, 32.9,
3
−
1
4.5, 59.9, 124.2, 149.6, 149.7; IR ῡ(cm ): 759, 992, 1110, 1274, 1437, 1478,
601, 1696, 2930, 3069; Anal. Calcd: C, 75.74; H, 9.53; N, 14.72. Found: C, 75.69;
H, 9.43; N, 14.77.
N-(3,5-Dimethyl-adamantan-1-yl)-2-(pyridin-2-yl)ethanamine (7h)
1
C H N ; mp: 196–198 °C; H NMR (400 MHz, CDCl ) δ(ppm): 1.09 (s, 6H), 1.20
1
9
28
2
3
(
s, 3H), 1.26 (m, 6H), 1.42 (s, 3H), 2.10 (s, 1H), 3.64 (t, 2H, J = 9.2 Hz), 4.17 (t, 2H,
13
J = 7.6 Hz), 7.33 (d, 2H, J = 7.6 Hz), 8.56 (t, 2H, J = 2.0 Hz); C NMR (100 MHz,
CDCl ) δ(ppm): 30.1, 30.5, 32.6, 42.6, 42.9, 44.6, 49.1, 50.6, 52.3, 121.2, 122.4,
3
123

M. H. Ghasemi, E. Kowsari
−
1
1
2
8
36.5, 149.5, 155.7; IR ῡ(cm ): 803, 990, 1053, 1453, 1587, 1739, 2840, 2901,
+
942, 3436; MS (M , m/z): 284; Anal. Calcd: C, 80.23; H, 9.92; N, 9.85. Found: C,
0.09; H, 10.01; N, 9.90.
N-(3,5-Dimethyl-adamantan-1-yl)-2-(pyridin-4-yl)ethanamine (8h)
1
C H N ; mp: 210–214 °C; H NMR (400 MHz, CDCl ) δ(ppm): 0.93 (s, 6H), 1.29
1
9
28
2
3
(
s, 3H), 1.42 (m, 6H), 1.44 (s, 3H), 2.10 (s, 1H), 3.94 (t, 2H, J = 8.4 Hz), 4.20 (t, 2H,
13
J = 9.2 Hz), 7.25 (d, 2H, J = 6.0 Hz), 8.54 (d, 2H, J = 6.0 Hz); C NMR
(
100 MHz, CDCl ) δ(ppm): 30.5, 32.4, 32.6, 42.6, 42.9, 44.5, 49.1, 50.5, 52.3,
3
−1
1
24.1, 149.7, 150.1; IR ῡ(cm ): 779, 994, 1049, 1474, 1590, 1645, 2936, 3351; MS
+
(M , m/z): 284; Anal. Calcd: C, 80.23; H, 9.92; N, 9.85. Found: C, 80.26; H, 9.88;
N, 9.79.
1
-phenyl-N-(2-(pyridin-2-yl)ethyl)ethanamine (7i)
1
C H N ; mp: 233–235 °C; H NMR (400 MHz, CDCl ) δ(ppm): 1.41 (s, 3H), 2.94
1
5
18
2
3
(
m, 1H), 3.83 (t, 2H, J = 6.8 Hz), 4.12 (t, 2H, J = 6.4 Hz), 7.05–7.58 (m, 7H), 8.52
13
(
d, 2H, J = 4.8 Hz); C NMR (100 MHz, CDCl ) δ(ppm): 24.4, 37.3, 47.3, 58.3,
1
3
−
1
15.4, 119.3, 126.9, 128.4, 128.5, 145.5, 149.2, 149.5, 160.4; IR ῡ(cm ): 590, 761,
95, 1122, 1449, 1591, 2865, 2964, 3025, 3060, 3366; MS (M , m/z): 226; Anal.
+
9
Calcd: C, 79.61; H, 8.02; N, 12.38. Found: C, 79.68; H, 8.12; N, 12.05.
Conclusions
Catalysis with anionic polyoxometalate is an excellent choice with growing impor-
tance. The properly investigated organic transformations in this paper clearly show
the extensive range of hopeful applications of anionic polyoxometalate in supported
ionic liquid phase. Anionic polyoxometalate requires higher activity and selectivity
with cleaner processing. The high efficiency of polyoxometalate as an acid catalyst
is due to their extreme Brønsted acidity, which is stronger than conventional mineral
acids.
In conclusion, the method for some useful organic transformations using a
heterogeneous anion-exchanged supported ionic liquid catalyst, as reported in this
paper, is an efficient and mild procedure with simple work-up. The aza-Michael-
type addition of amines to vinyl pyridine using catalytic systems 6a and 6b as
efficient and recoverable catalysts was successfully examined in this article. The
best results were achieved using 6b as an excellent reproducible catalyst. We
prepared two supported ionic liquids 6a and 6b and mainly focused on the use of 6b
in the abovementioned reactions. This catalytic system is immiscible with the
reaction mixture; consequently, at the end of the reaction, the solid catalyst can be
easily separated from the reaction mixture. Product isolation involves simple
separation of the solid catalyst followed by purification of residue. Therefore, the
advantages of homogeneous and heterogeneous catalysis can be successfully
integrated, which greatly facilitate recovery and recycling of the catalyst. This
1
23

The catalytic effect of anion-exchanged supported ionic…
method is a useful alternative to other methods in the literature. The reactions are
usually carried out under mild conditions with elevated yields and simple work-up.
The catalytic system can be easily separated from the reaction mixture and recycled
in subsequent reactions.
Acknowledgements The authors wish to gratefully thank the Research Affairs Division at Amir Kabir
University of Technology (AUT), Tehran, for financial support.
References
1. J.F. Mukerabigwi, S. Lei, H. Wang, S. Luo, X. Ma, J. Qin, X. Huang, Y. Cao, RSC Adv. 5, 83732
(2015)
2. K. Wu, Z. Jing, L. Pan, L. Zhou, E.H. Ishida, Res. Chem. Intermed. 38, 1637 (2012)
3. S.K. Padmanabhan, S. Pal, E.U. Haq, A. Licciulli, Appl. Catal. A 485, 157 (2014)
4. N. van Garderen, F.J. Clemens, J. Kaufmann, M. Urbanek, M. Binkowski, T. Graule, C.G. Aneziris,
Micropor. Mesopor. Mater. 151, 255 (2012)
5
6
7
8
9
. Y. Yu, J. Addai-Mensah, D. Losic, Langmuir 26, 14068 (2010)
. K.L. Lin, T.C. Lee, J.C. Chang, J.Y. Lan, Environ. Prog. Sustain. Energy 32, 640 (2013)
. S.E. Ivanov, A.V. Belyakov, Glass Ceram. 65, 48 (2008)
. J. Chen, G. Qin, Q. Chen, J. Yu, S. Li, F. Cao, B. Yang, Y. Ren, J. Mater. Chem. C 3, 4933 (2015)
. G. Polizos, K. Winter, M.J. Lance, H.M. Meyer, B.L. Armstrong, D.A. Schaeffer, J.T. Simpson, S.R.
Hunter, P.G. Datskos, Appl. Surf. Sci. 292, 563 (2014)
10. D. Losic, Y. Yu, M.S. Aw, S. Simovic, B. Thierry, J. Addai-Mensah, Chem. Commun. 46, 6323
(2010)
1
1
1
1
1
1
1
1
1. S. Hegde, S.S. Joshi, T. Mukherjee, S. Kapoor, Res. Chem. Intermed. 40, 1125 (2014)
2. M. Wang, P. Wang, Q. Tian, J. Liu, J. Deng, N. Li, J. Zhou, Res. Chem. Intermed. 41, 8891 (2015)
3. J. Chen, J. Li, Y. Zhang, S. Gao, Res. Chem. Intermed. 36, 959 (2010)
4. Y. Ding, W. Zhao, W. Song, Z. Zhang, B. Ma, Green Chem. 13, 1486 (2011)
5. M.R. Farsani, E. Assady, F. Jalilian, B. Yadollahi, H.A. Rudbari, J. Iran. Chem. Soc. 12, 1207 (2015)
6. I.V. Kozhevnikov, Chem. Rev. 98, 171 (1998)
7. S. Damyanova, J.L.G. Fierro, I. Sobrados, J. Sanz, Langmuir 15, 469 (1999)
8. D.W. Kim, M.S. Park, M. Selvaraj, G.A. Park, S.D. Lee, D.W. Park, Res. Chem. Intermed. 37, 1305
(2011)
1
2
2
2
2
2
2
2
2
2
9. C. Chiappe, C.S. Pomelli, Eur. J. Org. Chem. 28, 6120 (2014)
0. M. Ghaedi, D. Elhamifar, M. Roosta, R. Moshkelgosha, J. Ind. Eng. Chem. 20, 1703 (2014)
1. H.P. Steinr u¨ ck, P. Wasserscheid, Catal. Lett. 145, 380 (2015)
2. R. Sandaroos, M.T. Goldani, S. Damavandi, J. Chem. Sci. 125, 511 (2013)
3. M.H. Ghasemi, E. Kowsari, Res. Chem. Intermed. (2016). doi:10.1007/s11164-016-2741-2
4. Y. Xiao, Y.F. Song, Appl. Catal. A 484, 74 (2014)
5. M. Li, M. Zhang, A. Wei, W. Zhu, S. Xun, Y. Li, H. Li, H. Li, J. Mol. Catal. A Chem. 406, 23 (2015)
6. Y. Mei, B. Yan, Colloid Polym. Sci. 293, 817 (2015)
7. X.M. Yan, Z. Mei, P. Mei, Q. Yang, J. Porous Mater. 21, 729 (2014)
8. J. Chen, L. Hua, W. Zhu, R. Zhang, L. Guo, C. Chen, H. Gan, B. Song, Z. Hou, Catal. Commun. 47,
1
8 (2014)
9. I.R. Siddiqui, S. Shamim, A.A. Abumhdi, M.A. Waseem, A. Srivastava, A. Srivastava, N. J. Chem.
7, 1258 (2013)
2
3
3
3
0. A. Ying, M. Zheng, H. Xu, F. Qiu, C. Ge, Res. Chem. Intermed. 37, 883 (2011)
1. M. Chelghoum, M. Bahnous, A. Bouraiou, S. Bouacida, A. Belfaitah, Tetrahedron Lett. 53, 4059
(2012)
32. M.H. Ghasemi, E. Kowsari, Res. Chem. Intermed. (2016). doi:10.1007/s11164-016-2572-1
33. M.H. Ghasemi, E. Kowsari, A. Shafiee, Tetrahedron Lett. 57, 1150 (2016)
34. M.H. Ghasemi, E. Kowsari, S.K. Hosseini, Tetrahedron Lett. 57, 387 (2016)
35. E. Kowsari, A. Zare, V. Ansari, Int. J. Hydrog. Energy 40, 13964 (2015)
36. X. Feng, G.E. Fryxell, L.Q. Wang, A.Y. Kim, J. Liu, K.M. Kemner, Science 276, 923 (1997)
37. I.V. Kozhevnikov, Catal. Rev. Sci. Eng. 37, 311 (1995)
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