Micron-particulate crystalline hexagonal aluminium nitride: A novel, efficient and versatile heterogeneous catalyst for the synthesis of some heterocyclic compounds

Article abstract of DOI:10.1007/s13738-012-0152-x

The present work reports the application of micron-particulate crystalline hexagonal aluminium nitride/aluminium as a novel, mild acidic and reusable solid heterogeneous catalyst in organic synthesis. The catalyst was synthesized by thermal plasma techniq

Full text of DOI:10.1007/s13738-012-0152-x

J IRAN CHEM SOC (2013) 10:243–249
DOI 10.1007/s13738-012-0152-x
ORIGINAL PAPER
Micron-particulate crystalline hexagonal aluminium nitride:
a novel, efficient and versatile heterogeneous catalyst
for the synthesis of some heterocyclic compounds
Nilesh S. Kanhe
Ashok B. Nawale
Rajeeta D. Ingle
•
Sunil U. Tekale
A. K. Das Sudha V. Bhoraskar
Rajendra P. Pawar
•
Naveen V. Kulkarni
•
•
•
•
•
Received: 22 March 2012 / Accepted: 25 July 2012 / Published online: 21 August 2012
Ó Iranian Chemical Society 2012
Abstract The present work reports the application of
micron-particulate crystalline hexagonal aluminium
nitride/aluminium as a novel, mild acidic and reusable
solid heterogeneous catalyst in organic synthesis. The
catalyst was synthesized by thermal plasma technique and
characterized using X-ray diffractometer and scanning
electron microscopy. It catalyzes efficiently many organic
transformations such as the synthesis of heterocyclic
compounds 2,4,5-triaryl-substituted imidazoles and 2-aryl
benzimidazoles.
acids is limited on account of their handling risks, disposal
problem, inability for recycle, toxicity and corrosiveness. It
is essential to minimize the use of environmentally harmful
acids and replace them by some novel regenerable solid
acid catalysts. Heterogeneous catalysts provide several
advantages over the homogeneous catalysts such as recy-
cling ability, easy isolation of the product, high selectivity
etc. The nano world helps to minimize such difficulties.
Recently, several nano particulate catalysts are being
extensively studied for their catalytic applications in
numerous organic transformations.
Keywords Aluminium nitride Á Heterogeneous catalyst Á
Classically aluminium nitride (AlN) is wide band gap
(6.2 eV) semiconductor material having many attractive
properties [1–7] such as high thermal conductivity, low
electrical conductivity, high dielectric constant, high
hardness, high mechanical strength, low thermal expansion
coefficient, high temperature stability, high piezoelectric
coefficient, high corrosion resistance, non-toxicity [8] and
transparency to UV light. These interesting properties
make it useful as a potential material in optoelectronic
devices [9], high power microwave devices, UV detector
2
,4,5-Triaryl imidazoles Á 2-Aryl benzimidazoles
Introduction
Heterogeneous solid acid catalysts constitute an important
role in organic synthesis. The use of conventional Lewis
[
10], biomedical research and water and air purification.
To the best of our knowledge and as per literature sur-
vey, AlN is not utilized as a catalyst in organic synthesis
besides its applications in the field of nanotechnology and
electronic devices.
N. S. Kanhe and S. U. Tekale contributed equally.
N. S. Kanhe Á N. V. Kulkarni Á A. B. Nawale Á S. V. Bhoraskar
Department of Physics, University of Pune, Ganeshkhind,
Pune, Maharashtra 411 007, India
So, apart from its utilities in semiconductor, electronic
and optical devices, in the present paper, we wish to dis-
close for the first time the application of AlN as an effi-
cient, reusable and versatile heterogeneous catalyst in
organic synthesis for carrying out several organic trans-
formations. The increased efficiency of AlN, grown in the
present method is supported on the basis of high crystal-
linity arising from high temperature processes in the ther-
mal plasma reactor [11].
S. U. Tekale Á R. D. Ingle Á R. P. Pawar (&)
Department of Chemistry, Deogiri College, Station Road,
Aurangabad, Maharashtra 431 005, India
e-mail: rppawar@yahoo.com
A. K. Das
Laser and Plasma Technology Division, Bhaba Atomic Research
Center, Trombay, Mumbai, Maharashtra 400 085, India
123

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J IRAN CHEM SOC (2013) 10:243–249
Experimental
O
O
NH4OAc
N
AlN/Al ( 20wt%)
Ar
Chemicals and apparatus
ArCHO
°
ethanol, 80 C
N
H
30-70 min
The chemicals used are commercially available (SD fine or
Sigma Aldrich) and were used without further purification.
The reactions were carried out in liquid phase by refluxing
the reactants under stirring condition for appropriate reac-
tion time. After completion of reaction as monitored by
TLC, the catalyst was separated by filtration, washed with
proper solvent and dried to recycle for several times. The
products were recovered from the filtrate after concentra-
tion on rotary evaporator followed by purification using
simple silica gel chromatography. The isolated products
were characterized by NMR (400 MHz varian spectro-
photometer), IR (Brucker spectrophotometer), ESMS
Benzil
Aldehyde
2, 4, 5- triaryl imidazole
Scheme 1 Synthesis of 2,4,5-triaryl-substituted imidazoles using
AlN/Al catalyst
concentrated to get crude product which was further puri-
fied by crystallization from ethanol. All the aldehydes
reacted smoothly to afford high purity products. The
advantages associated with this protocol are high yield,
short reaction time, mild conditions, ease of product iso-
lation and potential for recycling of the catalytic system.
The spectral data some principal compounds:
(
Shimadzu) and comparison of their melting points repor-
ted in the literature. The catalyst was found to promote the
following synthetic reactions:
4
-(4,5-Diphenyl-1H-imidazol-2-yl) phenol (1a)
-
M.P.
1
2
67–268 °C; IR (cm ) 1,213, 1,662, 2,998, 3,592;
1
H NMR (DMSO-d , 400 MHz) 6.80–6.84 (d, J = 8 Hz,
Synthesis of 2,4,5-triaryl-substituted imidazoles
6
2H), 7.20–7.70 (m, 10 H), 7.95–7.99 (d, J = 8, 2H), 9.68
(s, 1H, OH), 12.40 (br, s, 1H). ESMS: 313.39 (M ? 1).
2
,4,5-Triaryl and 1,2,4,5-tetraaryl substituted imidazoles
constitute a class of biologically potent molecules due to
their pharmacological properties [12]. Besides a wide range
of their activities (analgesic, fungicidal, herbicidal, anti-
inflammatory etc.), 2,4,5-triaryl-substituted imidazole ring
skeletons are also used in photography and photosensitive
compounds. 2,4,5-Triaryl-substituted imidazoles are syn-
thesized from the benzoin or benzil, ammonium acetate,
aromatic aldehydes in the presence of various catalysts
such as InCl Á3H O [13], ZrCl [14], NiCl Á6H O [15],
2
-(4-Chlorophenyl)-4,5-diphenyl-1H-imidazole (2a) M.P.
-1
1
2
57–259 °C; IR (cm ) 3,470, 3,059, 1,602; H (DMSO-
d6, 400 MHz) 7.20–7.40 (m, 10H, Ar–H), 7.55 (d, 2H,
J = 10 Hz), 12.78 (br, s, NH), ESMS: 331.25 (M ? 1).
2
,4,5-Triphenyl-1H-imidazole (3a) M.P. 273–275 °C,
-1 1
IR (cm ) 3,450, 3,057, 1,601; H (DMSO-d , 400 MHz)
6
7
2
.20–7.80 (m, 15H, Ar–H), 12.79 (br, s, NH), ESMS:
97.30 (M ? 1).
3
2
4
2
2
p-TSA [16], Ce Mg 9 Zr
O
[17], L-proline [18],
2
1
1-x
Synthesis of 2-aryl benzimidazoles
potassium dihydrogen phosphate [19], microwave irradia-
tion [20–22], molecular iodine [23] etc. Many of the previ-
ously employed catalysts suffer from several disadvantages
such as non-recyclable nature, strong Lewis acids which may
create environmental hazards, longer reactions time etc. The
mild Lewis acidity associated with the aluminium nitride
encouraged us to use it as a catalyst to carry out certain
organic transformations. Herein, we wish to report the syn-
thesis of 2,4,5-triaryl-substituted imidazoles using AlN as a
novel catalyst (although it is known in nanotechnology for
electronic and optical devices, it is till never used in organic
synthesis for catalytic applications) (Scheme 1).
Substituted benzimidazoles heterocyclic moiety containing
compounds are of pharmacological and biological interest
as some of them are HIV-1 RT inhibitors [24], dipeptidyl
peptidase IV inhibitors [25], antimalarial, cytotoxic and
antitubercular agents [26] and also inhibitors of MDA-MB-
231 human breast cancer cell proliferation [27]. Benzimi-
dazoles are normally synthesized by the condensation
reaction of orthophenylene diamine with aldehydes or by
the reductive cyclization of ortho nitro anilines. Various
reagents/catalysts reported for this transformation include
CAN [28], CuI/L-proline [29], copper (II) oxide nanopar-
In a typical model condensation reaction, a mixture of
benzil (1 mmol), aromatic aldehydes (1 mmol) and
ammonium acetate (2.5 mmol) with a catalytic amount of
the microcrystalline AlN catalyst was refluxed under stir-
ring in ethanol for appropriate reaction time as specified in
Table 1. After completion of reaction as monitored by
TLC, the reaction mass was diluted with hot ethanol, fil-
tered off to separate the catalyst and the filtrate was
ticles [30], hypervalent reagents like PhI(OAc) [31], NaH
2
[
32], iron(II) bromide [33], p-TSA [34], polymer support/
MW [35], NaHSO /MW [36], and TBAF/ultrasound irra-
3
diation [37].
In our present study, we have demonstrated microcrys-
talline AlN/Al synthesized by the high temperature plasma
route as a new catalyst for the synthesis of benzimidazoles
1
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J IRAN CHEM SOC (2013) 10:243–249
245
Table 1 Microcrystalline AlN catalyzed synthesis of 2,4,5-triaryl substituted imidazoles
o
M.P. ( C) [References]
Sr. no.
1
Aldehyde
Product
Time (min)
60
Yield (%)
92
267–268 [18]
CHO
CHO
N
OH
HO
N
H
1
a
a
2
45
94
257–259 [18]
N
Cl
Cl
N
H
2
3
4
45
50
86
89
273–275 [16]
189–190 [18]
CHO
N
N
H
3
a
CHO
N
F
F
N
H
4
a
5
6
75
30
90
92
181–183 [16]
CHO
OH
HO
N
N
H
5
a
CHO
228–230 [16]
N
3
H CO
OCH
3
N
H
6
a
(
Scheme 2). The reaction was performed by simply re-
the previous section. Almost all the aldehydes reacted
cleanly to afford high yield with excellent purity of the
benzimidazoles.
fluxing an equimolar mixture of orthophenylene diamine
(
(
OPD), aromatic aldehydes and a catalytic amount
20 wt%) of micron-particulate AlN particles for appro-
The spectral data some principal compounds:
priate reaction time as specified in Table 2. After com-
pletion of reaction as monitored by TLC (30–50 % EA:
petroleum ether), the catalyst was separated as described in
2
-(4-Methoxyphenyl) benzimidazole (7a)
-1
M.P.
1
82–184 °C; IR (cm ) 3,320, 2,963, 1,610, 1,479, 1,295;
1
H
(DMSO-d , 400 MHz) 3.85 (s, 3H), 6.9 (d, 2H),
6
7
.08–7.25 (m, 4H), 7.72 (d, 2H), 12.76 (brs, NH); ESMS:
O
NH
2
2
N
AlN/Al (20 wt%)
225 (M ? 1).
H
ethanol, reflux
30-90 min
N
H
NH
X
X
2
-(4-Hydroxyphenyl) benzimidazole (8a)
-1
M.P.
1
aldehyde
2-aryl benzimidazole
1
69–171 °C; IR (cm ) 3,270, 3,052, 1,597, 1,470; H
NMR (DMSO-d , 400 MHz) 4.4 (s, 1H, OH), 6.6–6.75
Scheme 2 AlN/Al catalyzed synthesis of 2-arylbenzimidazoles
6
123

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J IRAN CHEM SOC (2013) 10:243–249
Table 2 Micro crystalline AlN catalyzed synthesis of 2-arylbenzimidazole
Sr. no.
1
Aldehyde
Product
Time (min)
50
Yield (%)
89
M.P. (°C) [References]
223–224 [37]
CHO
N
OCH
3
N
H
3
H CO
7
a
a
2
3
45
35
90
92
169–171 [28]
234–236 [28]
CHO
N
OH
N
H
HO
O
8
CHO
O
N
O
N
O
H
9
a
4
5
6
50
90
30
89
87
93
289–290 [36]
253–255 [28]
291–293 [36]
CHO
N
N
H
1
0a
CHO
CHO
N
NO
Cl
2
N
O N
2
H
1
1a
N
N
Cl
H
1
2a
(
d, 2H), 6.8–6.95 (m, 4H), 7.3 (d, 2H), 9.9 (1H, s, NH);
The aluminium anode was melted by the plasma
impingement and at sufficient high temperature, nitrogen
from the plasma started reacting with aluminium. The
growth of the anode block was seen to occur. The crys-
tallites of aluminium nitride as well as those of aluminium
were identified by their appearance (luster and color).
Morphology of the crystallites was studied with the help of
scanning electron microscopy (SEM), whereas the crys-
talline structure was ascertained from the X-ray diffraction
(XRD) analysis.
ESMS: 211 (M ? 1).
2
-(3,4-Methylenedioxyphenyl) benzimidazole (9a) M.P.
-
1
1
2
34–236 °C; IR (cm ) 2,905, 1,610, 1,466, 1,282; H
(
DMSO-d , 400 MHz) 5.96 (s, 2H), 6.45 (d, 1H), 6.6
6
(
s, 1H), 6.8 (d, 1H), 7.1–7.8 (m, 4H), 12.75 (br, 1H, NH);
ESMS: 211 (M ? 1).
Catalyst preparation
The ingot was then purified by treating it with HCl and
crushed into the powder. Aluminium was dissolved off and
the crystals of hexagonal AlN (h-AlN) were obtained for
the further catalytic application.
The catalyst was synthesized by known method in a DC-
transferred arc plasma reactor (DCTATPR). The reactor
consists of a double-walled, water-cooled stainless steel
cylindrical chamber with a hemispherical top flange. The
vertical movable plasma torch was mounted on the top flange
and aluminium block was placed on the graphite anode. The
Catalyst characterization
plasma forming gas was N , the flow of which was main-
2
X-ray diffraction studies
tained at 5 lpm. The reactor chamber was pre-evacuated to a
pressure of 0.01 mbar and the pressure in the reactor was
maintained at a level close to atmospheric pressure, by
purging nitrogen into the chamber with a controlled flow rate.
The operating parameters of the reactor are given in Table 3.
The as-grown product and post-process samples were
characterized at room temperature by powder XRD anal-
ysis. Plots in Fig. 1a and b correspond to the standard line
pattern for h-AlN and cubic Al (c-Al) as referred to JCPDS
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J IRAN CHEM SOC (2013) 10:243–249
247
Table 3 Typical operating
condition for synthesis of AlN/
Al crystals
Sr. no.
Operating parameter
Range of values
1
2
3
4
5
6
7
Plasma current
70, 100 A
50–60 V
5 lpm
Torch voltage
2
Flow rate of plasma forming gas (N )
Flow rate of ambient gas (N
Operating pressure
2
)
15 lpm
710 mbar
70–80 mm
Arc length
Size of aluminium anode
Disc with diameter 50 mm and 12 mm thick
been confirmed from the XRD analysis; however, there
seems to be added traces of aluminium as found in XRD
pattern in Fig. 1c. In order to remove the traces of Al, the
ingot was post-processed (etched) with HCl. Figure 2b
shows the SEM micrograph of the post-processed sample.
Black holes are formed as a result of etching, and provide
improved reaction sites for catalytic applications.
(
d)
(
c)
(111)
(b)
(200)
(220)
(311)
Results and discussion
(a)
The transformations described herein are normally reported
in strong acidic (both conventional and Lewis) conditions,
using various ionic liquids, microwave or ultrasound pro-
moted synthesis. Strong acids are not beneficial as far as
environmental safety is concerned. Also the microwave or
ultrasound promoted synthesis requires the additional use
of microwave oven or sonicator. As a part of our attempts
to develop versatile and efficient methods for the synthesis
of heterocyclic compounds, we have successfully synthe-
sized 2,4,5-triaryl-substituted imidazoles and benzimidaz-
oles using microcrystalline AlN as a novel, heterogeneous,
reusable and efficient catalyst. The yield of products using
this practical approach for the synthesis of various phar-
macologically important heterocyclic molecules is com-
parable with the previous reported methods.
2
0
30
40
50
θ (degree)
60
70
80
2
Fig. 1 XRD pattern of a standard hexagonal AlN (JCPDS #79-2479),
b standard cubic Al (JCPDS #85-1327), c as grown crystals and
d HCl-etched powder sample
data’s from #79-2497 and # 85-1327, respectively. The
indices of h-AlN and c-Al are marked in line pattern.
Figure 1c refers to the XRD pattern recorded for the as-
synthesized crystalline ingot. From XRD pattern, it is seen
that it consists of both h-AlN and c-Al phases. Calculated
lattice parameters for h-AlN: a = 3.11 A, c = 4.97 A, c/a
ratio = 1.60 and for c-Al: a = 2.4 A are consistent with
wurtzite AlN and c-Al structure [2, 7, 12–15]. Volume
fraction of Al content in the samples was calculated by
comparing the observed intensity of AlN (100), AlN (101),
Al (111) and Al (200) lines. Average volume fraction of
c-Al and h-AlN phase is found to be 30 and 70 %,
respectively. Figure 1c shows the XRD pattern of HCl-
treated powder sample. The sample is seen to consist of
pure h-AlN phase and is free of c-Al traces.
Although the laboratory synthesis of catalyst seems to
be difficult, there are certain advantages associated with the
catalyst. This catalyst is mild Lewis acidic in nature. An
important advantage of the catalyst is that it is heteroge-
neous owing to which it facilitates the separation process
from the reaction mass after completion of the reaction
simply by diluting with organic solvent (ethanol in this
case) and filtering. The catalyst after drying offers good
reusability results without any loss of its catalytic activity.
It is stable in hydrogen and carbon dioxide atmospheres up
to 980 °C. Furthermore, melting point of the catalyst being
high (2,200 °C), its effective catalytic activity can be
studied up to high temperature; maintaining its heteroge-
neous nature. It provides a new route for the synthesis of
2,4,5-triaryl imidazoles and 2-aryl benzimidazoles. On the
basis of above stated facts, the catalyst is found to be
Scanning electron microscopy
Figure 2a shows the SEM micrograph of the as-synthesized
crystalline sample obtained from the ingot. Hexagonally
facetted crystalline structure having dimensions of
approximately 100–150 lm in size are observed. These
pallets most probably belong to crystalline AlN as already
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J IRAN CHEM SOC (2013) 10:243–249
Fig. 2 a SEM micrograph of grown crystallites, b SEM micrograph of etched powder sample
Table 4 A comparative study of the effect of catalyst (AlN) on the synthesis of 2,4,5-triaryl-substituted imidazoles
Sr. no.
Catalyst
Conditions
Yield (%)
References
1.
2.
3.
4.
5.
6.
7.
L-Proline
EtOH/reflux, 2–5 h
MeOH/r.t., 8.3–9.4 h
TBAI, 140 °C, 1–4 h
180 °C, MW, 5 min
75–94
54–82
75–95
87–97
80–94
58–92
86–94
[18]
InCl
3
Á3H
2
O
[13]
p-TSA
[16]
AcOH
[20]
Ce
1
Mg 9 Zr1-x
O
2
EtOH:H
2
O, 70 °C, 35–50 min
[17]
Silica gel
AlN
MW, 8 min
[21]
EtOH/reflux, 30–70 min
Present work
superior and hence can be further used for the synthesis of
various heterocyclic compounds. A comparative study of
the catalytic effect of our catalyst with the literature
reported protocols for the synthesis of 2,4,5-triaryl imida-
zoles is summarized in Table 4. The reduced time and
enhanced yield of the corresponding products can be
accounted on the basis of small particle size.
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