Polyaniline-supported zinc oxide (ZnO) nanoparticles: An active and stable heterogeneous catalyst for the Friedel-Crafts acylation reaction

Article abstract of DOI:10.1080/00397911.2010.540695

We report herein the synthesis of zinc oxide (ZnO) nanoparticles incorporating a polyaniline (PANI) matrix in aqueous medium and its catalytic performance toward Friedel-Crafts acylation reactions. The acylation reactions demonstrate that polyaniline/ nano-ZnO composites are very active catalysts and are able to activate the reactant at ambient temperature. The obtained nanocatalyst was characterized by Fourier transform-infrared, scanning electron microscopy, transmission electron microscopy, Brunauer-Emmett-Teller, X-ray diffraction, and particle-size analysis techniques. The catalyst was reusable for five cycles without appreciable loss in activity. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397911.2010.540695

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Polyaniline-Supported Zinc Oxide
(ZnO) Nanoparticles: An Active and
Stable Heterogeneous Catalyst for the
Friedel–Crafts Acylation Reaction
Seyed Jamal Tabatabaei Rezaei a , Mohammad Reza Nabid a ,
Seyedeh Zahra Hosseini a & Mahvash Abedi a
a Department of Chemistry, Shahid Beheshti University, Tehran, Iran
Accepted author version posted online: 21 Oct 2011.Published
online: 25 Jan 2012.
To cite this article: Seyed Jamal Tabatabaei Rezaei , Mohammad Reza Nabid , Seyedeh Zahra Hosseini
& Mahvash Abedi (2012) Polyaniline-Supported Zinc Oxide (ZnO) Nanoparticles: An Active and Stable
Heterogeneous Catalyst for the Friedel–Crafts Acylation Reaction, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 42:10, 1432-1444,
DOI: 10.1080/00397911.2010.540695
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Synthetic Communications1, 42: 1432–1444, 2012
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2010.540695
POLYANILINE-SUPPORTED ZINC OXIDE (ZnO)
NANOPARTICLES: AN ACTIVE AND STABLE
HETEROGENEOUS CATALYST FOR THE
FRIEDEL–CRAFTS ACYLATION REACTION
Seyed Jamal Tabatabaei Rezaei, Mohammad Reza Nabid,
Seyedeh Zahra Hosseini, and Mahvash Abedi
Department of Chemistry, Shahid Beheshti University, Tehran, Iran
GRAPHICAL ABSTRACT
Abstract We report herein the synthesis of zinc oxide (ZnO) nanoparticles incorporating a
polyaniline (PANI) matrix in aqueous medium and its catalytic performance toward
Friedel–Crafts acylation reactions. The acylation reactions demonstrate that polyaniline/
nano-ZnO composites are very active catalysts and are able to activate the reactant at
ambient temperature. The obtained nanocatalyst was characterized by Fourier transform–
infrared, scanning electron microscopy, transmission electron microscopy, Brunauer–
Emmett–Teller, X-ray diffraction, and particle-size analysis techniques. The catalyst was
reusable for five cycles without appreciable loss in activity.
Keywords Friedel–Crafts acylation; green synthesis; heterogeneous catalyst; polymer-
supported; zinc oxide nanoparticles
INTRODUCTION
Nano-zinc oxide (nano-ZnO), one of the multifunctional inorganic nanoparti-
cles, has drawn increasing attention in recent years because of its interesting physical
and chemical properties, such as chemical stability, unique catalytic activity, low
dielectric constant, effective antibacterial and bactericide properties, intensive
Received August 26, 2010.
Address correspondence to Mohammad Reza Nabid, Department of Chemistry, Shahid Beheshti
University, P. O. Box 1983963113, Tehran, Iran. E-mail: m-nabid@sbu.ac.ir
1432

POLYMER-SUPPORTED NANOCATALYST
1433
ultraviolet (UV) and infrared (IR) absorption, and its low cost.[1] Therefore,
nano-ZnO can be potentially used as catalyst, gas sensor, semiconductor, varistor,
piezoelectric device, antibacterial and bactericide agent, and UV-shielding
material.[2]
ZnO nanoparticle, like other nanoparticles, has high surface energy, which may
cause the agglomeration of particles when ZnO nanoparticles are dispersed in
organic solvents and matrices. Because catalytic activity depends highly on disper-
sion and surface area,[3] it is necessary to immobilize the respective catalytic system
on suitable supports to prevent the formation of agglomerated nanoparticles. The
most commonly used stabilizers and supports are self-assembled monolayers,[4] poly-
mers,[5] dendrimers,[6,7] and functional organic polymers.[8]
Supported metal and metal oxide catalysts play a crucial role in many indust-
rially important chemical reactions.[9–11] The support enables the metal particles to
be highly dispersed and thermostable to retard sintering so that they can be used
with high efficiency. The most promising advantage of supported metal and metal
oxide catalysts is easy separation from the reaction mixture, allowing the recovery
of the solid and eventually its reuse, provided that the recovered catalyst was not
deactivated during the course of the reaction. This is particularly important with
expensive metal and metal oxide nanocatalysts.
Conducting polymers have been used as the supporting matrix in different
composites for the intercalation of catalytically important nanoparticles so that
the catalytic activity can be retained in the composite.[12] Among p-conjugate poly-
mers, polyaniline (PANI) is one of the most widely studied conducting polymers
because of its excellent electronic and optical applications as well as environmental
stability and interesting redox properties.[13] It also catches a lot of attention in
organic synthesis as a support because of its easy preparative protocol from inexpen-
sive starting materials.[14–16]
Friedel–Crafts acylation is a fundamental and useful method for the synthesis
of aromatic ketones, which are important intermediates for preparing fine chemicals
in the field of pharmaceuticals, agrochemicals, and fragrances. Typically, these reac-
tions are performed using acyl chloride in the presence of a little more than 1 equiva-
lent of Lewis acids, such as anhydrous AlCl3, TiCl4, and FeCl3. These methods are
limited by high amounts, toxicity, and corrosion of the catalysts that cannot be easily
recovered and recycled. To solve this problem, some catalysts [including LiClO4-
acylhydride complex;[17] heteropolyacid salts;[18] sulfated ZrO2 or Fe2O3;[19] sulfated
Al2O3–ZrO2 (or TiO2);[20] Fe=MCM-41;[21] cation-exchanged clays;[22] ZnCl2, CuCl2,
and MgCl2 supported on montmorillonite K10;[23] HY, H-beta, and H-ZSM-5 zeo-
lites;[24] polyvalent-metal salts of heteropolyacid;[25] and GaAlClx-grafted montmor-
illonite K-10 clay[26]] have been employed for Friedel–Crafts acylation. Although
there was some progress for efficient Friedel–Crafts acylation reaction, developing
economical and environmentally friendly procedures remains a challenge. For these
reasons and as a part of our ongoing research program on the application of cata-
lysts for development of useful new synthetic methodologies,[27–29] herein we are
interested in the development of highly efficient, green, and inexpensive hetero-
geneous catalysts for Friedel–Crafts acylation that (a) would avoid the use of added
acids and bases, (b) would avoid chromatographic purification, (c) are easy to per-
form, and (d) are economical for large-scale preparations.

1434
S. J. T. REZAEI ET AL.
RESULTS AND DISCUSSION
Catalyst Preparation
In the present study, we first attempted synthesis of zinc oxide nanoparticles by
a simple precipitation method. Then, the PANI=nano-ZnO composites were pre-
pared by adding ZnO nanoparticles into aniline monomer, followed by chemical oxi-
dation polymerization. The main issue of the present method is how much
improvement of catalytic properties can be expected by surface modification of
ZnO nanoparticles. After preparation, the products were characterized by Fourier
transform (FT)–IR, Scanning electron microscopy (SEM), Transmission electron
microscopy (TEM), X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET),
and particle-size analyzer techniques.
Catalyst Characterization
Figure 1 shows the FT-IR spectra of pure ZnO nanoparticles, PANI, and
PANI=nano-ZnO composites. Figure 1a is the FT-IR absorption spectra of ZnO
nanoparticles. The broad peak between 3350 and 3500 cmꢀ1 is due to the stretching
vibrations of the –OH group on the surface of ZnO nanoparticles. Figure 1b is the
FT-IR absorption spectra of pure PANI. The band at 3450 cmꢀ1 is attributable to
N-H stretching. The bands at 1577 and 1485 cmꢀ1 correspond to quinoid and ben-
zenoid structures of PANI, respectively. Also, the band at 1310 cmꢀ1 is assigned
to C-N stretching of a secondary aromatic amine. The characteristic peaks at
1549, 1452, and 1288 cmꢀ1 appear in the FT-IR spectrum of the PANI=nano-ZnO
composites (Fig. 1c), shifting to the lower wave numbers in comparison to the cor-
responding peaks of pure PANI at 1577, 1485, and 1310 cmꢀ1. This is probably
caused by the hydrogen actions existing between the ZnO nanoparticles and
PANI.[30]
Figure 1. FTIR spectra of (a) pure ZnO nanoparticles, (b) pure PANI, and (c) PANI=nano-ZnO composites.

POLYMER-SUPPORTED NANOCATALYST
1435
Figure 2. XRD patterns of (a) ZnO nanoparticles and (b) PANI=nano-ZnO composites.
Figure 2 shows the XRD pattern of pure ZnO nanoparticles and PANI=
nano-ZnO composites. Figure 2b offers the XRD pattern of the PANI=nano-ZnO
composites. Except for a weak amorphous peak appearing at 2h ¼ 20ꢁ to 28ꢁ, the
XRD pattern of the composite sample presents the same profile observed in the pure
ZnO (Fig. 2a), indicating that the crystal structure of ZnO was not modified by the
PANI. Also, these results indicate that the PANI is amorphous in the nanocompo-
sites, suggesting that the addition of the ZnO nanoparticles impedes the crystalliza-
tion of the PANI molecular chain.[31]
The size distribution of ZnO nanoparticles and PANI=nano-ZnO composites
are shown in Fig. 3. It is clearly seen that ZnO particles in Fig. 3a are nano size
(10–35 nm) with a narrow size distribution. In contrast, PANI=nano-ZnO particles
Figure 3. Size distribution of (a) ZnO nanoparticles and (b) PANI=nano-ZnO composites.

1436
S. J. T. REZAEI ET AL.
Table 1. BET surface area measurements of the catalysts
Sample
Particle size (nm)
BET surface area (m2 gꢀ1
)
Bulk ZnO
Nano-ZnO
PANI=nano-ZnO
—
20
35
6
45
197
in Figure 3b are larger with a broad size distribution (30–65 nm). The size of PANI=
ZnO nanoparticles obtained from TEM was about 30 nm, which was smaller than
that of PANI=ZnO nanoparticles in the composites. An explanation for this is that
the ZnO nanoparticles in the composites were coated by PANI.
Surface areas of the catalysts were measured by BET surface area technique.
Surface areas of various prepared catalysts are mentioned in Table 1. The results
in Table 1 demonstrate that the supported form of nano-ZnO is preferable because
of its high surface area compared with the bulk ZnO (or pure nano-ZnO) and better
accessibility of reactants to the active sites.
Morphological studies were done with SEM and TEM, which confirm the for-
mation of nanocomposite. Scanning electron micrographs of ZnO nanoparticles are
shown in Fig. 4a and those of PANI=nano-ZnO composite in Fig. 4b. Comparing
these images showed the growing of PANI on ZnO nanoparticles’ surfaces. Also,
PANI= ZnO nanoparticles in Fig. 4b show much better dispersion. It demonstrates
that after growing PANI on ZnO nanoparticle surfaces, the aggregation of ZnO
nanoparticles was greatly reduced. Figure 4 shows the TEM image of ZnO nanopar-
ticles (4c) and PANI=nano-ZnO composites (4d). In accordance with our studies,
after polymerization of aniline in the presence of ZnO nanoparticles, dimension
increased almost 10–15 nm. As shown in SEM and TEM images, polyaniline layers
on the ZnO nanoparticle surface attached together and generated the porous PANI=
nano-ZnO composites.
Catalytic Activity: Optimization of the Reaction Parameters
To find the standard experimental procedure, as a model reaction, we chose
toluene as the substrate and benzoyl chloride as the acylating agent and treated these
reactants with catalytic amounts of different catalysts including bulk ZnO, ZnO
nanoparticles, and PANI=nano-ZnO composites in solvent-free conditions
(Table 2, entries 1, 2, and 3). As a result, these catalysts can effectively catalyze
the reactions. Also, the results in Table 2 demonstrate that among these catalysts,
PANI=nano-ZnO is the best in terms of yield and selectivity. This might be due to
the high content of ZnO exposed on the surface of PANI (polyaniline nanocompo-
sites have greater surface area and higher conductance, Table 1) and several other
factors such as polymer support, particle size, and morphology of polymer-ZnO par-
ticles, which have a huge impact on the catalytic properties of supported metal oxide
in organic transformations.[32,33]
Next, we studied the reaction of toluene with benzoyl chloride catalyzed by
PANI=nano-ZnO composites in different solvents as summarized in Table 2. This
reaction was also performed without solvent. The best yields were obtained when

POLYMER-SUPPORTED NANOCATALYST
1437
Figure 4. SEM images of (a) nano-ZnO particles and (b) PANI=nano-ZnO composites particles. TEM
images of (c) nano-ZnO particles and (d) PANI=nano-ZnO composites particles.
the acylation was performed without solvent. The reactions carried out in acetoni-
trile and dichloromethane gave moderate yields of ketone (Table 2, entries 4 and
5) whereas the reaction in tetrahydrofuran (THF) gave trace yields of ketone
(Table 2, entry 6).
It was demonstrated that PANI=nano-ZnO composites are superior catalysts
among all the catalysts screened in this transformation. It should be noted that
20 wt% (with respect to benzoyl chloride) of PANI=nano-ZnO was efficient enough
to catalyze the reaction, and increasing the amount of catalyst did not improve the
yield significantly (Table 2, entry 7). Finally, we achieved optimized conditions using
20 wt% of PANI=nano-ZnO composites as a catalyst in solvent-free conditions.
Then, the effect of temperature was studied by carrying out the model reaction in
the presence of PANI=nano-ZnO at 40 and 60 ꢁC (Table 2, entries 8 and 9). It
was observed that the yield was slightly increased as the reaction temperature raised,
but above 40 ꢁC no change was observed on the yield and rate of the reaction.
The Friedel–Crafts acylation catalyzed by PANI=nano-ZnO was explored with
a variety of aromatic compounds to evaluate the scope and limitations of this

1438
S. J. T. REZAEI ET AL.
Table 2. Friedel–Crafts acylation reaction of toluene as the substrate and benzoyl chloride as the
acylating agent under different reaction conditionsa
Entry
Catalyst (wt%)
Solvent (10 ml) Temp. (ꢁC) Time (min) Yieldsb (%) (para=ortho)
1
2
PANI=nano-ZnO (20)
Nano-ZnO (0.096 mmol)
Bulk ZnO (0.096 mmol)
PANI=nano-ZnO (20)
PANI=nano-ZnO (20)
PANI=nano-ZnO (20)
PANI=nano-ZnO (30)
PANI=nano-ZnO (20)
PANI=nano-ZnO (20)
None
—
—
Rt
Rt
Rt
Rt
Rt
Rt
Rt
40
60
Rt
10
10
10
10
10
10
10
10
10
10
92 (20:1)
76 (17:1)
60 (17:1)
40 (20:1)
55 (20:1)
Trace
3
—
c
4
CH2Cl2
5
CH3CNc
THFc
—
6
7
93 (20:1)
93 (20:1)
93 (20:1)
Trace
8
—
—
9
10
—
aThe reactions were carried out with 1 mmol of benzoyl chloride and 5 mmol of aromatic compound.
bIsolated yields. The ratio of para=ortho-isomers was determined by 1H NMR.
cThe reactions were carried out with 1 mmol of benzoyl chloride and 2 mmol of aromatic compound.
method (Table 3). The results summarized in Table 3 show that all of the aromatic
compounds react very rapidly within 10–90 min. It is very exciting to find that unac-
tivated benzenes such as chlorobenzene react smoothly in the presence of PANI=
nano-ZnO composites to afford the corresponding aromatic ketone in good yields
(Table 3, entries 7 and 11). Acylation occurs with good selectivity at the position
para to -OMe, -Me, and -Cl for all of the studied compounds in almost good yields.
However, in cases where the para positions are blocked (Table 3, entries 4, 8, and
12), the acyl group is introduced in the ortho position. This procedure is also good
enough for the acylation of heterocyclic compounds such as furan (Table 3, entry
6), producing the corresponding 2-acylated product in excellent yield.
Mechanistically, we suggest initial activation of the acyl chloride by the coor-
dinative strongly acidic Lewis acid ZnO. This activation can occur in two steps. One
step involves the carbonyl group coordination to the active sites from ZnO nanopar-
ticles on the polyaniline matrix; in the next step, the complex formed reacts with the
aromatic compound, yielding the corresponding acylated product.
The lifetime and leaching of active metal oxide species into solution are impor-
tant issues to consider when heterogeneous catalysts are used, particularly for prac-
tical applications of the Friedel–Crafts acylation reaction. Our preliminary
investigations demonstrated that catalyst PANI=nano-ZnO is very stable to air
and moisture. Moreover, in a separate experiment, the catalyst was filtered off after
ꢂ50% conversion at the reaction temperature. Further treatment of the filtrate under
similar reaction condition did not proceed significantly. Atomic absorption spec-
troscopy of the filtrate also confirmed that the ZnO content in the solution was
below the detection limit. Therefore, we may conclude that any zinc oxide species

POLYMER-SUPPORTED NANOCATALYST
1439
Table 3. Synthesis of aromatic ketones in the presence of a catalytic amount of PANI=nano-ZnO
composites at rt
Entry
1
R
ArH
Product
Time (min)
10
Yieldsa (%) (para=ortho)
PhCO
95 (25:1)
2
PhCO
10
92 (20:1)
3
4
PhCO
PhCO
80
20
78
82
5
6
7
PhCO
90
10
10
75
94
PhCO
PhCH2CO
90 (17:1)
8
9
PhCH2CO
PhCH2CO
2-Cl-PhCO
CH3CO
10
10
10
10
96
96 (25:1)
93 (25:1)
88 (18:1)
10
11
12
13
CH3CO
CH3CO
10
10
95
96 (25:1)
aIsolated yields. The ratio of para=ortho-isomers was determined by 1H NMR.

1440
S. J. T. REZAEI ET AL.
Figure 5. Reusability study of PANI=nano-ZnO for acylation of toluene.
that leached into the reaction mixture is not an active homogeneous catalyst and that
the observed catalysis is truly heterogeneous in nature.
The recyclability and reusability of the PANI=nano-ZnO catalyst was tested
for the acylation of toluene with benzoyl chloride up to five cycles. The results are
shown in Fig. 5, demonstrating that after every run the yield of acylated product
does not change, indicating fair stability of PANI=nano-ZnO catalyst under experi-
mental conditions.
CONCLUSION
In conclusion, we have described a novel and highly efficient solvent-free pro-
tocol for Friedel–Crafts acylation of aromatic compounds using nontoxic and inex-
pensive PANI=nano-ZnO composites, which can be prepared by a simple operation
from commercially available and relatively cheap starting materials. The catalyst
shows high thermal stability (up to 500 ꢁC) and also is stable in both organic and
water milieus. It can also be recovered and reused for at least five reaction cycles
without considerable loss of reactivity.
EXPERIMENTAL
Instruments and Characterization
Melting points were measured on an Elecrtothermal 9200 apparatus and are
1
uncorrected. H NMR spectra was recorded on a Bruker DRX-300 Avance spec-
trometer at 300.13 MHz. IR spectra were recorded on a Bomem MB-Series FT-IR
spectrophotometer. The mean size and size distribution of nanoparticles were mea-
sured by a particle-size analyzer (BI-90Plus, Brookhaven Instruments Co., USA).
Surface area measurements were carried out by nitrogen adsorption on a Micromeri-
tics ASAP 2010 instrument at an adsorption temperature of 77 K. The SEM images
were recorded using a Cambridge scanning electron microscope, model stereo scan
360, and the TEM studies were performed using a Phillips transmission electron

POLYMER-SUPPORTED NANOCATALYST
1441
microscope. XRD data were collected on an XD-3A diffractometer using Cu Ka
radiation.
Preparation of Zinc Oxide (ZnO) Nanoparticles
In the present investigation, ZnO nanoparticles were prepared by the homo-
geneous precipitation method.[34] Typically, aqueous solutions of Zn(NO3)2 and urea
were added into a flask under vigorous stirring condition (300 rpm=min). The molar
ratio of Zn2þ to urea was about 1:4. To inhibit the growth of the ZnO crystallite dur-
ing the course of precipitation, a certain amount (10 wt% with respect to zinc salt) of
sodium dodecyl sulfonate (SDS) was added into reaction mixture. Then, the reaction
mixture was heated and maintained at 95 ꢁC. After stirring for 2 h, a semitransparent
zinc hydroxide colloid was obtained. The precipitates were then filtered, washed with
distilled water and alcohol in four rounds, dried in air at 80 ꢁC, and finally calcined
at 350 ꢁC for 2 h to achieve samples with 10- to 35-nm particle size.
Preparation of Polyaniline/Nano-ZnO Composites
Aniline 0.5 ml was injected into 20 ml phosphate buffer (pH ¼ 4.3) containing
100 mg nano-ZnO under constant stirring conditions for 15 min at ambient tempera-
ture. To reduce aggregation of ZnO nanoparticles, ultrasonic action was needed for
30 min. Then, 10 ml phosphate buffer solution containing ammonium persulfate
(1.14 g) was added very slowly into this solution at a temperature of 0–5 ꢁC. After
all the oxidant was added, the reaction mixture was stirred at a constant temperature
for 4 h. The oxidation of aniline is highly exothermic and thus the rate of addition of
the oxidant was adjusted to prevent any increase in the temperature of the reaction
mixture. The precipitated nanocomposite was recovered from the polymerization
vessel by filtration and then washed with distilled water until the washing liquid
was colorless. To remove oligomers and other organic by-products, the precipitate
was washed with methanol until the methanol solution was colorless. Finally, the
resulting green powder was washed twice with acetone and subsequently dried
at 60 ꢁC until it achieved a constant mass (yield 90%, 0.55 g). Typical loading of
ZnO was determined using atomic spectroscopy (AA) and shows a loading
3.44 ꢃ 0.01 mmol gꢀ1
.
General Procedure for Acylation Using Polyaniline/Nano-ZnO
Composites
Aromatic compound (5 mmol) was treated with benzoyl chloride (1 mmol) in
the presence of nano-ZnO supported on polyaniline (20 wt% with respect to acid
chloride, 0.028 g equal to 0.096 mmol ZnO) at room temperature for a certain period
of time as shown in Scheme 1. The progress of the reaction was monitored by
thin-layer chromatography (TLC). After completion, the reaction mixture was
diluted with dichloromethane (CH2Cl2), the solution was filtered to separate the
catalyst, and the dichloromethane extract was then washed with an aqueous solution
of sodium bicarbonate and dried over anhydrous sodium sulfate. Evaporation of sol-
vent furnished practically pure corresponding product. All the acylated products

1442
S. J. T. REZAEI ET AL.
Scheme 1. Friedel–Crafts acylation reactions.
were characterized by spectral methods as well as by comparison of their spectral
data with those reported earlier.[35,36]
Selected Spectral Data
1-(2,5-Dimethoxy)-2-phenyl-1-ethanone (Table 3, Entry 8). IR (KBr)
(nmax=cmꢀ1): 1696 (C O) cmꢀ1; H NMR (300.13 MHz, CDCl3): dH 3.80 (s, 6H),
4.35 (s, 2H), 6.88–7.60 (8H, Ar-H).
1
=
2-Chlorophenyl (4-methoxy phenyl) methanone (Table 3, Entry 10). IR
(KBr) (nmax=cmꢀ1): 1686 (C O) cmꢀ1; H NMR (300.13 MHz, CDCl3): dH 3.82 (s,
3H), 6.94 (d, 2H, Ar-H), 7.27–7.45 (4H, Ar- H), 7.80 (d, 2H, Ar-H).
1
=
ACKNOWLEDGMENT
We gratefully acknowledge financial support from the Research Council of
Shahid Beheshti University.
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