Spinel-type zinc aluminate (ZnAl2O4) nanoparticles prepared by the co-precipitation method: A novel, green and recyclable heterogeneous catalyst for the acetylation of amines, alcohols and phenols under solvent-free conditions

Article abstract of DOI:10.1016/j.apcata.2010.05.005

Zinc aluminate (ZnAl₂O₄) nanoparticles with an average particle size of about 8 nm were easily prepared by the co-precipitation method using aqueous ammonia solution as the precipitating agent. This nanosized spinel-type oxide was characterized by TGA, XRD, FT-IR, TEM, and surface area measurement and used as the heterogeneous catalyst for the acetylation reaction. Efficient acetylation of various amines, alcohols and phenols was carried out over ZnAl₂O₄ nanoparticles using acetic anhydride and/or acetyl chloride as the acetylating agents at room temperature without the use of a solvent. The method is highly selective, allowing the alcoholic hydroxyl group to be protected while the phenolic hydroxyl group remains intact, and the amine group can be acetylated in the presence of the hydroxyl group. This method is fast and has a high yield. It is also clean, safe, cost effective, compatible with substrates that have other functional groups and very suitable for practical organic synthesis. In addition, the catalyst can be reused without significant loss of activity. Indeed, the catalytic activity of the ZnAl₂O₄ nanoparticles is higher than that of bulk ZnAl₂O₄.

Full text of DOI:10.1016/j.apcata.2010.05.005

Applied Catalysis A: General 382 (2010) 293–302
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Spinel-type zinc aluminate (ZnAl2O4) nanoparticles prepared by the
co-precipitation method: A novel, green and recyclable heterogeneous catalyst
for the acetylation of amines, alcohols and phenols under solvent-free conditions
Saeid Farhadi∗, Somayeh Panahandehjoo
Department of Chemistry, Lorestan University, Khoramabad 68135-465, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Zinc aluminate (ZnAl2O4) nanoparticles with an average particle size of about 8 nm were easily prepared
by the co-precipitation method using aqueous ammonia solution as the precipitating agent. This nano-
sized spinel-type oxide was characterized by TGA, XRD, FT-IR, TEM, and surface area measurement and
used as the heterogeneous catalyst for the acetylation reaction. Efficient acetylation of various amines,
alcohols and phenols was carried out over ZnAl2O4 nanoparticles using acetic anhydride and/or acetyl
chloride as the acetylating agents at room temperature without the use of a solvent. The method is
highly selective, allowing the alcoholic hydroxyl group to be protected while the phenolic hydroxyl
group remains intact, and the amine group can be acetylated in the presence of the hydroxyl group. This
method is fast and has a high yield. It is also clean, safe, cost effective, compatible with substrates that
have other functional groups and very suitable for practical organic synthesis. In addition, the catalyst can
be reused without significant loss of activity. Indeed, the catalytic activity of the ZnAl2O4 nanoparticles
is higher than that of bulk ZnAl2O4.
Received 24 November 2009
Received in revised form 5 May 2010
Accepted 8 May 2010
Available online 13 May 2010
Keywords:
Spinel-type oxide
Zinc aluminate
Nanoparticles
Acetylation
Solvent-free conditions
© 2010 Elsevier B.V. All rights reserved.
The protection of –OH and –NH2 functional groups is one of
the most important transformations in organic chemistry, both
for fundamental research and industrial manufacturing [1,2]. This
transformation is usually performed with acetic anhydride and/or
acetyl chloride in the presence of either basic or acidic catalysts.
Numerous catalytic systems are available for this transformation
[3–26], but most of them are homogeneous and non-recoverable,
and they often have disadvantages, such as prolonged reaction
times, low yields, harsh conditions, use of harmful organic sol-
vents, tedious work-up procedures, the requirement for excess
reagents/catalysts, and the use of explosive, moisture-sensitive, or
expensive catalysts.
Heterogeneous catalytic materials play a very important role
in the selective protection of functional groups in ways that are
economically advantageous and environmentally friendly [27,28].
They can be recovered easily from the reaction mixture by sim-
ple filtration, and reused several times, making the process more
economically and environmentally viable. Heterogeneous catalysts
such as yttria–zirconia based catalysts [29], simple metal oxides
[30–32], montmorillonite K10 and KSF [33–35], HClO4-SiO2 [36],
H2SO4-SiO2 [37], AlPW12O40 [38], zeolites [39,40], Nafion-H [41],
HBF4-SiO2 [42], KF-Al2O3 [43], silica embedded-triflate catalysts
[44], MoO3-Al2O3 [45], NaHSO4-SiO2 [46], sulfated zirconia [47],
(NH4)2.5H0.5PW12O40 [48], [TMBSA][HSO4] ionic liquid [49], silica-
bonded cobalt(II) salen [50], and silica-bonded N-propyl sulfamic
acid and S-propyl sulfuric acid [51,52] have been reported for the
acetylation of alcohols, phenols, thiols, and amines. From a practical
viewpoint, the development of simple, inexpensive, widely appli-
cable and environmentally benign catalysts/procedures is still an
active area of research.
Among various inorganic solids, spinel-type mixed oxides
(AB2O4) are well known for their rich catalytic action. These oxides
are non-toxic, inexpensive, very stable materials with strong resis-
tance to acids and alkalis, and they have high melting points
and relatively high surface areas. These properties make them
suitable for use as solid heterogeneous catalysts for organic trans-
formations. Among them, zinc aluminate (ZnAl2O4) has been used
extensively as a heterogeneous catalyst in many reactions, such as
cracking, dehydration, hydrogenation, dehydrogenation, dehydro-
genative condensation of normal alcohols, methylation of phenolic
compounds and N-alkylation of 2-hydroxypyridine with methanol
[53,54]. These catalytic applications prompted us to test this spinel-
type mixed oxide as a heterogeneous catalyst for the acetylation
reaction.
In this work, which is a continuation of our studies of the cat-
alytic acetylation reaction using inorganic solids as heterogeneous
catalysts [55,56], we wish to describe the preparation of spinel-type
zinc aluminate (ZnAl2O4) nanoparticles by the co-precipitation
∗
Corresponding author. Tel.: +98 661 2202782; fax: +98 661 6200098.
E-mail address: sfarhad2001@yahoo.com (S. Farhadi).
0926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.05.005

294
S. Farhadi, S. Panahandehjoo / Applied Catalysis A: General 382 (2010) 293–302
method and its application as a heterogeneous catalyst for the
efficient acetylation of alcohols, phenols and amines under mild
conditions (solvent-free, atmospheric pressure, room tempera-
ture). This article demonstrates that spinel-based catalysts have
great potential for use “green” organic transformations. To the best
of our knowledge, this is the first report of catalytic acetylation
using a spinel-type AB2O4 mixed oxide.
powder was dispersed in ethanol in an ultrasonic bath for 30 min,
and few drops of the resulting suspension were placed onto a
carbon-coated copper grid. Specific surface area was calculated
by the BET method using N2 adsorption–desorption experiments
carried out at −196 ◦C on a Micromeritics ASAP 2010. Before each
measurement, the sample was outgassed at 200 ◦C for 3 h. The
specific surface area of the ZnAl2O4 nanoparticles was 86 m2/g.
1. Experimental
1.3. Catalytic tests
All chemicals and solvents were purchased from either Merck
or Fluka and used as received. 1H NMR spectra were recorded on
a Bruker 500-MHz instrument. GC–MS analysis was carried out
on a Shimadzu QP 5050 GC–MS instrument. Melting points were
obtained using an Electrothermal-9200 instrument.
Amine, alcohol and/or phenol (10 mmol) were added to a
mixture of ZnAl2O4 nanoparticles (0.1 g) and acetic anhydride
(10 mmol) or acetyl chloride (15 mmol). The mixture was stirred
for an appropriate time at room temperature. The progress of
the reaction was monitored by TLC or GC–MS. When the reac-
tion was completed, 10 mL of ethyl acetate were added, and the
mixture was filtered to separate the ZnAl2O4 catalyst. The solid
catalyst was washed twice with 5 mL of ethyl acetate. The com-
bined organic phases were washed with a 10% solution of sodium
hydrogen carbonate, and then the organic phases were dried over
Na2SO4. The solvent was removed to obtain the product. If further
purification of the product was needed, it was dissolved in ethyl
acetate and passed through a short column of silica gel using carbon
tetrachloride-ethyl acetate as an eluent. All products were charac-
terized on the basis of GC–MS, FT-IR and 1H NMR spectral data and
by comparing the spectra to those of authentic samples or reported
data. The results are summarized in Tables 1–5.
1.1. Catalyst preparation
Zinc aluminate nanoparticles were prepared in aqueous solution
from metal nitrates by co-precipitation method using ammonia as a
precipitating agent as follows. Zn(NO3)2·6H2O (20 mmol) dissolved
in 10 mL of distilled water was added to a solution of Al(NO3)3·9H2O
(40 mmol) in 10 mL of distilled water. Then, the appropriate amount
of aqueous ammonia solution (25 wt%) was added to the above
solution, and the mixture was stirred until complete precipitation
occurred at a pH between 8 and 9. The precipitate was filtered,
washed with distilled water, and dried. The dry precipitate was
calcined at 600 ◦C for 4 h to obtain the ZnAl2O4 nanoparticles.
For a comparison, commercial ZnAl2O4 (S.D. Fine Chemicals,
India; 99.5%) with surface area of 4.2 m2/g, and average particle size
of 65 ␮m) and four nanosized ZnAl2O4 samples prepared according
to the reported methods in the literature [57–60] have also been
studied as the heterogeneous catalysts in the acetylation reaction.
In a similar manner, the acetylation reaction of some substrates
was also carried out in the presence of commercial bulk-ZnAl2O4
and nano-ZnAl2O4 samples prepared by other methods. The results
are compared in Tables 6 and 7.
2. Results and discussion
1.2. Catalyst characterization
2.1. Characterization of ZnAl2O4 nanoparticles
The X-ray diffraction studies were performed on a Bruker
D8 Advance X-ray diffractometer using Cu K␣ radiation
(ꢀ = 0.15418 nm). Infrared spectra were recorded on a Shimadzu
system FT-IR 8400 spectrophotometer using the KBr pellet method.
The particle size and morphology of the ZnAl2O4 nanoparticles
were investigated by using a LEO-906E transmission electron
microscope (TEM) with an accelerating voltage of 80 kV. In the
process of preparing the TEM specimen, a small amount of the
Inorganic nanoparticles have been receiving increased attention
because they exhibit unusual physical and chemical properties that
are significantly different from those of relatively larger particles of
the same materials. These unusual properties have been attributed
to the extremely small sizes and the high specific surface areas
of the nanoparticles. It is well known that the catalytic activity of
inorganic materials depends strongly on particle size, morphology,
and microstructure. Therefore, the preparation and characteriza-
Table 1
Acetylation of 4-nitroaniline with acetic anhydride over ZnAl2O4 nanoparticles under different conditions.a,b
.
Entry
Amount of catalyst (g)
Solvent
Time (min)
Conversionc (%)
Yieldd (%)
1
2
3
4
5
6
7
8
9
–
–
–
–
–
–
–
30
30
30
8
8
8
120
90
80
90
20
58
65
100
99
99
65
50
17
52
61
92
93
92
60
44
55
52
0.01
0.05
0.1
0.2
0.3
0.1
0.1
0.1
0.1
Acetonitrile
Toluene
Acetone
61
58
10
Dichloromethane
a
Reaction conditions: 4-nitroaniline (10 mmol), acetic anhydride (10 mmol), with or without solvent at rt.
In all cases, selectivity was over 99% determined by GC–MS analysis of the crude product mixture.
Conversions were determined by GC–MS analysis of the crude product mixture.
Yields are for isolated pure product.
b
c
d

S. Farhadi, S. Panahandehjoo / Applied Catalysis A: General 382 (2010) 293–302
295
Table 2
Acetylation of amines with acetic anhydride over nano-ZnAl2O4.a,b
.
Entry
Amine
Product
Time (min)
Conversionc (%)
Yieldd (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
G = H
4-Me
2-Me
4-Et
5, (5, 5, 5)e
98, (98, 97, 98))e
94, (93, 91, 90)e
3
3
5
3
3
5
4
8
6
4
6
8
8
10
100
100
100
100
100
99
99
98
99
97
96
95
96
96
96
93
93
92
92
90
91
88
86
88
4-Pri
4-MeO
4-Cl
2-Cl
2,4-Cl2
4-Br
4-I
3-NO2
4-CN
4-CHO
3-MeCO
97
95
96
96
16
8
5
96
90
17
98
92
18
19
8
98
97
90
87
10
20
7
98
92
a
Reaction conditions: amine (10 mmol), acetic anhydride (1 equiv. per NH2 group), catalyst (0.1 g), without solvent at rt.
In all cases, selectivity was ≥99% determined by GC–MS analysis of the crude product mixture.
Conversions were determined by GC–MS analysis of the crude product mixture.
Yields are for isolated pure product.
b
c
d
e
The reactions was carried out with the recovered catalyst in three consecutive runs.
tion of nanoparticles with well-controlled size, shape and chemical
This finding confirms that the decomposition reaction of the dou-
ble hydroxide precursor is completed at about 550 ◦C. Therefore,
we selected 600 ◦C as the calcination temperature for preparing
the ZnAl2O4 nanoparticles.
The purity and crystallinity of the ZnAl2O4 nanoparticles were
examined by using powder X-ray diffraction (XRD), as shown in
Fig. 2. It can be seen in Fig. 2 that the diffraction peaks are broad-
ened markedly due to the small size effect of the particles. The peaks
appearing at 2Â = 31.32◦, 36.90◦, 44.88◦, 49.10◦, 55.65◦, 59.45◦,
65.25◦, 74.07◦, and 77.35◦ can be indexed as (2 2 0), (3 1 1), (4 0 0),
(3 3 1), (4 2 2), (5 1 1), (4 4 0), (6 2 0), and (5 3 3) crystal planes of
homogeneity are interesting and important for catalytic purposes.
Co-precipitation is one of the simplest techniques for preparing
nanoparticles of numerous inorganic materials. In this simple and
inexpensive method, salts of the required metals are dissolved in
water and co-precipitated by adding a precipitating agent. In fact,
the precipitates obtained are solid solutions that contain the cations
mixed together, essentially on an atomic scale. Because of the high
degree of homogenization, much lower temperatures are sufficient
for the reaction to occur. For these reasons, the co-precipitation
method was selected for the preparation of ZnAl2O4 nanoparticles
in this work. The overall process for the preparation of ZnAl2O4
from Al3+ and Zn2+ nitrates and ammonium hydroxide as the pre-
cipitating agent can be written as follows:
First, thermal gravimetric analysis (TGA) of the co-precipitated
Zn(OH)2·2Al(OH)3 precursor was conducted, as shown in Fig. 1. In
the TGA curve, there are two distinct weight loss steps. The first
weight loss that occurred between 50 and 110 ◦C is attributed to
the evaporation of physically adsorbed water molecules. The sec-
ond weight loss step that occurred between 200 and 550 ◦C is due
to the decomposition of anhydrous Zn(OH)2·2Al(OH)3 to ZnAl2O4.
Fig. 1. TGA curve of ZnAl2O4 precursor.

296
S. Farhadi, S. Panahandehjoo / Applied Catalysis A: General 382 (2010) 293–302
Table 3
Acetylation of alcohols with acetyl chloride over nano-ZnAl2O4.a,b
.
Entry
Alcohol
Product
Time (min)
Conversionc (%)
Yieldd (%)
1
R = H; G = H
18, (18, 20, 20)e, 20f
100, (99, 98, 99)e, 0f
90, (90, 88, 88)e, 0f
2
3
4
5
R = H; G = 4-Me
R = H; G = 4-tert-Bu
R = H; G = 4-MeO
R = H; G = 2,4-(MeO)2
R = H; G = CN
15
16
15
16
100
100
100
98
95
94
94
94
6
23
92
87
7
8
9
R = H; G = 3-NO2
R = H; G = 2-NO2
R = H; G = 4-CF3
R = H; G = 4-F
R = H; G = 4-Cl
R = H; G = 4-Br
15
20
35
20
16
15
95
93
91
94
98
96
90
88
85
88
92
90
10
11
12
13
14
15
16
17
R = CH3; G = H
20, (22, 22, 24)e
99, (99, 98, 98)e
94, (92, 90, 89)e
R = C2H5; G = H
R = C6H5; G = H
R = 4-Cl-C6H4; G = H
R = 4-MeOC6H4; G = 4-MeO
20
15
14
12
98
97
98
98
92
90
91
92
18
19
30
30
94
93
86
88
20
21
48
20
82
98
72
92
22
23
32
26
94
96
86
88
24
25
25
30
96
98
88
89
26
27
28
98
90
26
30
98
90
91
85
28
a
Reaction conditions: alcohol (10 mmol), acetyl chloride (1.5 equiv. per OH group), catalyst (0.1 g), without solvent at rt.
In all cases, selectivity is ≥99% determined by GC–MS analysis of the crude product mixture.
Conversions were determined by GC–MS analysis of the crude product mixture.
Yields are for isolated pure product.
The reaction was carried out with the recovered catalyst in three consecutive runs.
Control experiment in the absence of catalyst.
b
c
d
e
f
the cubic crystalline structure of ZnAl2O4, respectively, in accor-
very high purity. The average particle size was calculated by X-
ray diffraction line broadening using the Debye–Scherrer equation
[61], d = (0.9ꢀ)/(h1/2cos Â), where d is the grain size; ꢀ is the wave-
length of the X-ray (Cu K␣, 0.15418 nm); Â is the diffraction angle
of the peak; and h1/2 is the full-width at half-height of the peaks.
The average particle size calculated using the most intense peak
dance with the standard JCPDS card of cubic spinel-type ZnAl2O4
(JCPDS File No. 05-0669). The XRD pattern shows that the prod-
uct is single-phase and that the only peaks detected were the
characteristic peaks of the cubic phase ZnAl2O4. This result con-
firms that the ZnAl2O4 nanoparticles obtained in this work are of

S. Farhadi, S. Panahandehjoo / Applied Catalysis A: General 382 (2010) 293–302
297
Table 4
Acetylation of phenols with acetyl chloride over nano-ZnAl2O4.a,b
.
Entry
Phenol
Product
Time (min)
Conversionc (%)
Yieldd (%)
1
2
3
4
5
6
7
8
G = H
4-Me
4-Cl
4-MeO
4-Br
2,4-(Cl)2
4-NO2
2,4-(NO2)2
36, (36, 37, 37)e, 36f
34
35
30
99, (98, 98, 99)e, 0f
98
99
100
95, (95, 94, 94)e, 0f
93
96
93
38
42
100
98
95
94
46, (46, 48, 47)e
53
93, (93, 90, 92)e
90
85, (85, 83, 84)e
82
9
40
40
95
94
86
86
10
a
Reaction conditions: phenol (10 mmol), acetyl chloride (15 mmol), catalyst (0.1 g), without solvent at rt.
In all cases, selectivity is ≥99% determined by GC–MS analysis of the crude product mixture.
Conversions were determined by GC–MS analysis of the crude product mixture.
Yields are for isolated pure product.
The reaction was carried out with the recovered catalyst in three consecutive runs.
Control experiment in the absence of catalyst.
b
c
d
e
f
(3 1 1) at 2Â = 36.90◦ was 7.6 nm. This value is in accordance with
TEM observations (see Fig. 4).
particles. From this image, it is evident that the particles have a
semi-spherical morphology, a narrow size distribution, and homo-
geneous shape.
Fig. 3 shows the FT-IR spectrum of the ZnAl2O4 nanoparti-
cles obtained by calcining the precursor at 600 ◦C for 4 h. Below
1000 cm−1, two strong bands around 670 and 560 cm−1 were
observed, which are related to Al–O stretching and O–Al–O bend-
ing vibrations of AlO6 groups in the spinel-type ZnAl2O4 structure,
respectively [62]. This result is in agreement with the result
obtained from the XRD analysis. It is noted that the number and
shape of FT-IR bands of ZnAl2O4 is dependent to its preparation
method. It shows two or three bands in range of 450–700 cm−1. In
some reported methods, ZnAl2O4 was formed as a mixture of the
normal spinel (Al3+ ions in octahedral sites of AlO6 groups) and the
inverse spinel (Al3+ ions in octahedral AlO6 and tetrahedral AlO4
sites) [63,64]. In these cases, the number of bands below 700 cm−1
increases. However, in most published papers, ZnAl2O4 showed
From the data obtained by TEM micrograph, a particle-size
histogram can be drawn, and the mean size of the particles can
be determined. Fig. 5 shows the particle-size distribution of the
ZnAl2O4 nanoparticles. It can be seen that the particle sizes pos-
sess a narrow size distribution in a range from 4 to 12 nm, and the
mean particle diameter is about 8 nm. Actually, the mean particle
size determined by TEM is very close to the average particle size
calculated by the Debye–Scherrer formula from the XRD pattern.
The specific surface area of the ZnAl2O4 nanoparticles measured
by the BET method, was 86 m2/g. This relatively high specific sur-
face area of ZnAl2O4 is beneficial to its catalytic activity.
2.2. The acetylation reaction over ZnAl2O4 nanoparticles
two main bands with or without shoulder in range 450–700 cm−1
.
In present work, two bands were appeared in the FT-IR spectrum,
confirming that the pure spinel-type ZnAl2O4 was formed.
In order to investigate the activity of spinel-type ZnAl2O4
nanoparticles as a heterogeneous catalyst, the acetylation of
4-nitroaniline (10 mmol) with acetic anhydride (10 mmol) was
chosen as the model reaction at room temperature. In the absence
of the ZnAl2O4 catalyst about 17% of 4-nitroacetanilide was
obtained after 30 min (Table 1, entry 1). Then, we examined
It is well known that the catalytic activity of AB2O4 spinels
is strongly dependent on the shape, size, and size distribution of
the particles. The size and shape of the ZnAl2O4 particles were
investigated by TEM, as shown in Fig. 4. The TEM image reveals
that the powder is composed of loosely aggregated extremely fine
Fig. 2. XRD pattern of ZnAl2O4 nanoparticles.
Fig. 3. FT-IR spectrum of ZnAl2O4 nanoparticles.

298
S. Farhadi, S. Panahandehjoo / Applied Catalysis A: General 382 (2010) 293–302
Table 5
Inter- and intramolecular competitive acetylation over nano-ZnAl2O4.a.
Entry
Substrate
Product
Time (min)
Yieldb (%)
1
2
3
5
5
6
93c
90
94
4
5
6
4
5
91
95c
92c
20
7
16
20
90
8
(45 + 43)c
a
Reaction conditions: substrate (10 mmol), catalyst (0.1 g), acetic anhydride (10 mmol for entries 1–5) and acetyl chloride (15 mmol for entries 6–8), without solvent at rt.
Yields are for isolated pure products.
GC–MS yields.
b
c
this reaction in the presence of different loadings of ZnAl2O4
some organic solvents such as acetonitrile, toluene, acetone, and
dichloromethane (Table 1, entries 7–10). However, the best result
in terms of reaction time, substrate conversion, and product yield
was achieved when the reaction was carried out without solvent.
Therefore, the acetylation reactions were carried out hereafter
under solvent-free conditions.
To evaluate the scope of this method, the acetylation of pri-
mary aromatic amines containing both electron-donating as well
as electron-withdrawing groups and primary aliphatic amines with
acetic anhydride over ZnAl2O4 nanoparticles was investigated at
room temperature under solvent-free conditions (Table 2). All of
catalyst (Table 1, entries 2–6). As shown in Table 1, the con-
version of 4-nitroaniline and the yield of 4-nitroacetanilide
were increased with the increase of the amount of nanocata-
lyst from 0.01 to 0.1 g/10 mmol of 4-nitroaniline (Table 1, entries
2–4). However, amounts greater than 0.1 g of catalyst produced
no significant increase in the conversion and yield of prod-
uct (Table 1, entries 5 and 6). The optimal amount of ZnAl2O4
was 0.1 g/10 mmol of 4-nitroaniline which resulted in a 92% iso-
lated yield of 4-nitroacetanilide after 8 min at room temperature
under solvent-free conditions. This reaction was also studied using
Table 6
Comparison of the acetylation over nano-ZnAl2O4 and commercial bulk-ZnAl2O4.a.
Entry
Substrate
Nano-ZnAl2O4
Time (min)
Bulk-ZnAl2O4
Yieldb (%)
Time (min)
Yieldb (%)
1
2
3
4
5
3-Nitroaniline
Benzyl alcohol
1-Phenylethanol
Phenol
6
18
20
36
50
91
90
94
95
85
45
65
60
90
120
80
78
76
70
61
4-Nitrophenol
a
Reaction conditions: substrate (10 mmol), acetic anhydride (10 mmol for entry 1) and acetyl chloride (15 mmol for entries 2–5), catalyst (0.1 g), without solvent at rt.
Yields are for isolated pure products.
b

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299
Table 7
Comparison of the catalytic activity of ZnAl2O4 nanoparticles prepared by five different methods in acetylation of 4-nitroaniline and benzyl alcohol as the model substrates.a,b
.
Entry
Preparation method [Ref.]
SBET (m2 g−1)/particle size (nm)
4-Nitroaniline
Time (min)
Benzyl alcohol
Time (min)
Conversionc (%)/yieldd (%)
Conversionc (%)/yieldd (%)
1
2
3
4
5
Co-precipitation methode
Sol–gel technique [57]
Alkoxide route [58]
Citrate technique [59]
Hydrothermal method [60]
86/8
58/20
126/28
230/5–20
99/10
8
13
6
4
7
100/92
97/88
100/94
100/97
99/93
18
28
14
11
16
100/90
97/86
100/95
100/96
99/92
a
Reaction conditions: substrate (10 mmol), acetic anhydride (10 mmol for 4-nitroaniline) or acetyl chloride (15 mmol for benzyl alcohol), ZnAl2O4 nanoparticles as the
catalyst (0.1 g), without solvent at rt.
b
In all cases, selectivity is ≥99% determined by GC–MS analysis of the crude product mixture.
c
Conversions were determined by GC–MS analysis of the crude product mixture.
d
Yields are for isolated pure product.
This work.
e
the amines that we studied were acetylated at selectivity greater
than 99% and conversions exceeding 95% with reaction times of less
than 10 min. In each case, the excellent yields of the corresponding
acetylated derivative were obtained (88–96%). The high activity of
ZnAl2O4 nanoparticles was demonstrated by high conversions and
isolated yields obtained for anilines having electron-withdrawing
groups (Table 2, entries 12–15). Other primary aromatic amines
and aliphatic amines gave the corresponding acetamides in high
yield under the present reaction conditions (Table 2, entries 16–20).
As can be seen in Table 2, functional groups such as –Ome, –CHO,
–COMe, –CN, and –NO2 remained unchanged under the reaction
conditions. The conversion of aniline to acetanilide on a 100-mmol
scale proceeded just as well as the 10 mmol reaction (14 min, 95%
isolated yield).
To explore the potential of this catalytic system, we studied the
acetylation of alcohols under solvent-free conditions. The reaction
with acetic anhydride was too sluggish for practical application.
In order to overcome this drawback, acetyl chloride was used as
an acetylating agent. In a short reaction time, the desired esters
were obtained in excellent yields from the reaction of a variety
of benzylic, primary, secondary, and hindered tertiary alcohols
(10 mmol) with acetyl chloride (15 mmol) in the presence of an
optimal amount of ZnAl2O4 (0.1 g). The conversions of the alco-
hols and isolated yields of products are given in Table 3. As can be
seen in Table 3, all primary benzylic alcohols were selectively con-
verted to the corresponding acetates in quantitative yields without
any evidence of the formation of side products (Table 3, entries
1–12). In all cases, electron-rich benzylic substrates as well as
electron-deficient substrates were converted to the acetate deriva-
tives with high yields. Various secondary alcohols were converted
to the corresponding acetates as well (Table 3, entries 13–18). In a
controlled-blank experiment, the acetylation of benzyl alcohol with
acetyl chloride under similar reaction conditions did not proceed
in the absence of catalyst (Table 3, entry 1).
This acetylation protocol is also efficient for allylic systems. For
example, cinnamyl alcohol was selectively converted to the cor-
responding acetate and the double bond remained intact under
the reaction conditions (Table 3, entry 19). It is very interesting
to note that a sterically hindered tertiary alcohol such as triphenyl-
methanol can also be acetylated with moderate yield, albeit with
longer reaction time (Table 3, entry 20). Aliphatic alcohols were
also converted into the corresponding acetate compounds with
high yields under the same reaction conditions (Table 3, entries
21–24). It is noteworthy that in the case of an optically active alco-
hol, the reaction proceeded well with retention of configuration
(Table 3, entry 25). The efficiency of the catalyst can be seen clearly
in the acetylation of di-hydroxy compounds under similar condi-
tions (Table 3, entries 26 and 27). Furthermore, the heteroaromatic
alcohol 4-pyridine methanol was converted to the corresponding
acetate with high yield (Table 3, entry 28). Among the various
alcohols studied, primary benzylic alcohols were found to be most
reactive, giving the corresponding acetylated products with shorter
reaction times. In each case, no by-products were found by GC–MS
analysis.
Fig. 4. TEM image of ZnAl2O4 nanoparticles.
The scope of this reaction was further extended for the acety-
lation of phenols. As shown in Table 4, phenol and differently
substituted phenols were acetylated with excellent conversions
(≥93%) and high yields (82–96%), albeit after longer reaction times
in comparison with alcohols. In all cases, selectivity for the acety-
lated products was greater than 99%. The excellent activity of
Fig. 5. Particle-size distribution of ZnAl2O4 nanoparticles.

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S. Farhadi, S. Panahandehjoo / Applied Catalysis A: General 382 (2010) 293–302
Fig. 6. The variation of conversion, selectivity and yield of product versus reaction time for (a) 4-nitroaniline, (b) 4-methylbenzyl alcohol and (c) phenol. Reaction conditions:
substrate (10 mmol), acetic anhydride (10 mmol for 4-nitroaniline) or acetyl chloride (15 mmol for 4-methyl benzyl alcohol and phenol), ZnAl2O4 nanoparticles as the catalyst
(0.1 g), without solvent at rt.
ZnAl2O4 nanoparticles was demonstrated by the high conversions
and yields obtained for phenols that have electron-withdrawing
groups (Table 4, entries 7 and 8).
the recycled ZnAl2O4 catalyst did not show significant change after
the third run in comparison with the fresh catalyst (Figs. 2 and 3).
This observation confirmed that the structure of the nanoparticles
was stable under the reaction conditions and was not affected by
the reactants.
In order to examine the chemoselectivity of the present method,
inter- and intramolecular competitive acetylations of substrates
containing –NH2 and –OH groups were studied. Selective acetyla-
tion of –NH2 groups in the presence of –OH groups was observed at
room temperature with one equivalent of acetic anhydride (Table 5,
entries 1–5). This might be due to the fact that the –NH2 group has
greater nucleophilicity than the –OH group. This selective acety-
lation of a primary –NH2 group over a primary –OH group by this
process is of considerable synthetic importance and is difficult to
achieve with many other reagents. Also, an alcoholic –OH group
was selectively acetylated in the presence of a phenolic –OH group
(Table 5, entries 6 and 7). No selectivity was observed between
primary and secondary alcohols, such as benzyl alcohol and 1-
phenylethanol (Table 5, entry 8).
The effect of reaction time on the conversion of substrate, iso-
lated yield and selectivity of acetylated product was also studied
in the acetylation of 4-nitroaniline, 4-methylbenzyl alcohol and
phenol as the model substrates (Fig. 6). It is evident from the
graphs that conversions and yields increased with time and that the
highest conversions, as well as the highest yields of products, for
4-nitroaniline, 4-methybenzyl alcohol, and phenol were obtained
after 8, 15, and 36 min, respectively. Selectivity to acetylated prod-
uct is more than 99% and remained almost constant with time.
The reusability of the catalyst was investigated in the acetylation
of aniline, benzyl alcohol, 1-phenylethanol, and phenol under opti-
mized conditions. After reaction with each of these substrates, ethyl
acetate was added, and the mixture was filtered to separate the cat-
alyst. The recovered catalyst was washed with acetone, dried, and
activated at 200 ◦C for 1 h. It was reused as the catalyst in the next
run under the same conditions. The catalytic results indicate that
there is no appreciable difference in either activity or selectivity
even after three runs (Table 2, entry 1; Table 3, entries 1 and 13;
Table 4, entries 1 and 7). As shown in Fig. 7, XRD and FT-IR spectra of
For a comparison and to investigate the small size effect of
ZnAl2O4 nanoparticles on the reaction, the acetylation of several
substrates was also investigated on a commercial bulk ZnAl2O4
with surface area of 4.2 m2/g and average particle size of 65 ␮m
under the optimized conditions. The results obtained are compared
in Table 6. Although acetylated products were obtained through
both conditions, the isolated yields were improved and the reac-
tion times were shortened when the ZnAl2O4 nanoparticles were
used. Clearly, the reaction times using the bulk ZnAl2O4 were eight
times longer than the times required when the ZnAl2O4 nanoparti-
cles were used. For complete comparison, the XRD, FT-IR, and SEM
of this conventional catalyst are shown in Fig. 8. As can be seen in
Fig. 8, the XRD and FT-IR spectra of the conventional catalyst are
different mainly due to its large particle sizes.
To evaluate the effect of preparation method on the catalytic
activity, we prepared nanosized ZnAl2O4 powders using four dif-
ferent methods that have been reported in the literature [57–60].
The activity of these nano-ZnAl2O4 catalysts was compared with
ZnAl2O4 prepared in the present study via the acetylation of
4-nitroaniline and benzyl alcohol under the optimized reaction
conditions. The results are summarized in Table 7. Although the
differences in the activities of various nanosized ZnAl2O4 catalysts
in terms of conversion and product yield are not remarkable, the
reaction time was shortened in the case of catalysts with higher
surface area (Table 7, entries 3 and 4). It is obvious from Table 7
that the surface area and activity of nano-ZnAl2O4 catalysts depend
on the preparation method. Further work in this area with various
substrates is under the way.
Although the exact mechanism for this reaction over ZnAl2O4
nanoparticles is not very clear, we think that it is similar to that
of other catalysts as proposed in the literature [32]. As shown in

S. Farhadi, S. Panahandehjoo / Applied Catalysis A: General 382 (2010) 293–302
301
Fig. 7. (a) XRD pattern and (b) FT-IR spectrum of the recovered ZnAl2O4 catalyst
after the third cycle.
Scheme 1, catalytic acetylation is initiated by coordinating carbonyl
group of acetic anhydride/acetic chloride to unsaturated aluminum
and zinc ions on the surface of the nanoparticles as Lewis acidic
sites. This coordination of carbonyl groups to the acidic sites on
the surface of the catalyst changes its character from being a low-
reactive electrophile into a moderately or reactive electrophile. This
activated carbonyl group is attacked by heteroatom (N or O) of
substrate, which is a reactive nucleophile, to produce the corre-
sponding acetate. The reaction steps are evident without comment.
From the results of Tables 6 and 7, we conducted that the
chemisorption of the carbonyl group of acetylating agents upon the
surface of the ZnAl2O4 catalyst is very important to the outcome of
the reaction. In a manner that is similar to other heterogeneous
catalytic reactions, this reaction was takes place mainly on the
surface of the particles of the catalyst, and surface atoms make a dis-
tinct contribution to its catalytic activity. In fact, the surface atoms
behave as Lewis acid centers where the chemical reaction can be
activated catalytically. On the other hand, the number of surface
atoms in the nanoparticles is a larger fraction of the total, and they
provide more contact area for reactants and catalyst, mainly due
to their high surface-to-volume ratio and high surface area com-
pared to bulk particles. Therefore, the greater catalytic activity of
ZnAl2O4 nanoparticles compared to the activity of bulk ZnAl2O4
can be attributed to the better coordination of the carbonyl group
to nano-ZnAl2O4 due to the participation of more surface Lewis
acidic sites in the reaction. This statement is also confirmed by the
results that are presented in Table 7.
Fig. 8. (a) XRD pattern, (b) FT-IR spectrum and (c) SEM of the commercial bulk-
ZnAl2O4 used as a catalyst.
3. Conclusions
In this research, ZnAl2O4 nanoparticles prepared by the co-
precipitation method were successfully used as a novel, efficient,
and recyclable heterogeneous catalyst for the acetylation of alco-
hols, phenols, and amines with acetic anhydride and/or acetyl
chloride under solvent-free conditions. Among the various sub-
strates, the acetylation of anilines and primary aliphatic amines
proceeded the most rapidly. Selective acetylation of –NH2 groups
in the presence of –OH groups and alcoholic –OH in the presence of
phenolic –OH was observed. The ZnAl2O4 catalyst was easily pre-
Scheme 1. The proposed catalytic cycle for the acetylation reaction over ZnAl2O4
nanoparticles.

302
S. Farhadi, S. Panahandehjoo / Applied Catalysis A: General 382 (2010) 293–302
pared and reusable without loss of activity. XRD and FT-IR analyses
confirmed that the structure of the catalyst did not change after
reaction. In this paper, we have provided the first description of the
acetylation of amines, alcohols, and phenols with acetic anhydride
or acetyl chloride over a nano-ZnAl2O4 catalyst.
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