ZnAl2O4@SiO2 nanocomposite catalyst for the acetylation of alcohols, phenols and amines with acetic anhydride under solvent-free conditions

Article abstract of DOI:10.1016/S1872-2067(12)60758-X

A ZnAl₂O₄@SiO₂ nanocomposite was prepared from metal nitrates and tetraethyl orthosilicate by the sol-gel process, and characterized by X-ray diffraction, Fourier transform infrared, transmission electron microscopy, and N₂ adsorption-desorption measurements. The nanocomposite was tested as a heterogeneous catalyst for the acetylation of alcohols, phenols, and amines under solvent-free conditions. Under optimized conditions, efficient acetylation of these substrates with acetic anhydride over the ZnAl₂O₄@SiO₂ nanocomposite was obtained. Acetylation of anilines and primary aliphatic amines proceeded rapidly at room temperature, while the reaction time was longer for the acetylation of alcohols and phenols, showing that an amine NH₂ group can be selectively acetylated in the presence of alcoholic or phenolic OH groups. The catalyst can be reused without obvious loss of catalytic activity. The catalytic activity of the ZnAl₂O₄@SiO₂ nanocomposite was higher than that of pure ZnAl₂O₄. The method gives high yields, and is clean, cost effective, compatible with substrates having other functional groups and it is suitable for practical organic synthesis.

Full text of DOI:10.1016/S1872-2067(12)60758-X

Chinese Journal of Catalysis 35 (2014) 368–375
ava i l a b l e a t w w w. s c i e n c ed i r e c t . c o m
j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e /c h n j c
Article
ZnAl2O4@SiO2 nanocomposite catalyst for the acetylation of
alcohols, phenols and amines with acetic anhydride under
solvent‐free conditions
Saeed Farhadi*, Kosar Jahanara
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:
A ZnAl2O4@SiO2 nanocomposite was prepared from metal nitrates and tetraethyl orthosilicate by
the sol‐gel process, and characterized by X‐ray diffraction, Fourier transform infrared, transmission
electron microscopy, and N2 adsorption‐desorption measurements. The nanocomposite was tested
as a heterogeneous catalyst for the acetylation of alcohols, phenols, and amines under solvent‐free
conditions. Under optimized conditions, efficient acetylation of these substrates with acetic anhy‐
dride over the ZnAl2O4@SiO2 nanocomposite was obtained. Acetylation of anilines and primary
aliphatic amines proceeded rapidly at room temperature, while the reaction time was longer for the
acetylation of alcohols and phenols, showing that an amine NH2 group can be selectively acetylated
in the presence of alcoholic or phenolic OH groups. The catalyst can be reused without obvious loss
of catalytic activity. The catalytic activity of the ZnAl2O4@SiO2 nanocomposite was higher than that
of pure ZnAl2O4. The method gives high yields, and is clean, cost effective, compatible with sub‐
strates having other functional groups and it is suitable for practical organic synthesis.
© 2014, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Received 21 September 2013
Accepted 28 November 2013
Published 20 March 2014
Keywords:
Spinel‐type oxide
Zinc aluminate
Silica
Sol‐gel process
Nanocomposite
Heterogeneous catalyst
Acetylation
Published by Elsevier B.V. All rights reserved.
Solvent‐free condition
1. Introduction
One of the most promising solutions to overcome these
problems is the use of heterogeneous solid catalysts [14,15],
The selective protection of alcohols, phenols, and amines
has received attention for its fundamental importance and also
for its role in multistep synthesis [1,2]. This important trans‐
formation is typically performed using acetic anhydride and/or
acetyl chloride in the presence of either basic or acidic cata‐
lysts. Numerous catalytic systems are available for this trans‐
formation [3–13], but most of these are homogeneous catalysts
and non‐recoverable, and they have the disadvantages of pro‐
longed reaction time, low yields, harsh conditions, use of
harmful organic solvents, tedious work‐up procedures, exces‐
sive reagents or catalysts, and the use of explosive, mois‐
ture‐sensitive, or expensive catalysts.
which can be recovered easily from the reaction mixture by
simple filtration and reused to make the process more eco‐
nomical and environmentally viable. Solid catalysts such as
HClO4‐SiO2 [16], montmorillonites [17–19], metal oxides
[20–22], H2SO4‐SiO2 [23], zeolites [24,25], HBF4‐SiO2 [26],
MoO3‐Al2O3 [27], NaHSO4‐SiO2 [28], sulphated zirconia [29],
(NH4)2.5H0.5PW12O40 [30], silica‐bonded Co(II) salen [31], sili‐
ca‐bonded N‐ and S‐propyl sulfamic acids [32,33], poly(4‐vi‐
nylpyridinium) perchlorate [34], polystyrene‐supported GaCl3
[35], borated zirconia modified with ammonium metatungstate
[36], rice husk [37], V(IV) tetraphenylporphyrin [38],
TiIV(salophen)(OTf)2 [39], polystyrene‐bound electron‐defi‐
* Corresponding author. Tel: +98‐661‐6200111; Fax: +98‐661‐6200112; E‐mail: sfarhadi48@yahoo.com
This work was supported by the Lorestan University Research Council and Iran nanotechnology Initiative Council.
DOI: 10.1016/S1872‐2067(12)60758‐X | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 35, No. 3, March 2014

Saeed Farhadi et al. / Chinese Journal of Catalysis 35 (2014) 368–375
cient Sn(IV) porphyrin [40], poly(vinylpolypyrrolidoniume)
2.2. Characterization of the catalyst
tribromide [41], modified attapulgite [42], succin‐
imide‐N‐sulfonic acid [43], acylimidazolium acetate [44],
Ph3P(OAc)2 [45], poly(4‐vinylpyridinium) tribromide [46],
sulfonated ordered nanostructured carbon [47], iron oxide
nanoparticles [48] and Ni/Silica [49], Cu based metal‐organic
framework [50] have been used in the acetylation of alcohols,
phenols, and amines. However, each of these catalysts or rea‐
gents has disadvantages and limitations.
In recent years, transition metal mixed oxides with the per‐
ovskite (ABO3) or spinel (AB2O4) structure have attracted con‐
siderable attention as promising catalytic materials for organic
transformations due to their high thermal and hydrothermal
stability and relatively low cost compared with their noble
metal counterparts [51–56]. However, their potential catalytic
applications are limited by their small surface areas. One possi‐
ble way to circumvent this problem is to disperse these oxides
on a medium with high specific surface area. In recent years,
silica‐based materials have been reported to be promising
supports for a number of catalytically active species because of
their high surface area and stable and well‐ordered pore struc‐
ture [57–59].
X‐ray diffraction (XRD) patterns were recorded on a Rigaku
D‐max C III X‐ray diffractometer using Ni‐filtered Cu K radia‐
tion (λ = 1.5406 Å) to determine the phases in the samples.
Infrared (IR) spectra were recorded on a Shimadzu system
FT–IR 160 spectrophotometer using KBr pellets. The particle
size and morphology of the ZnAl2O4@SiO2 nanocomposite were
determined by a transmission electron microscope (TEM,
Philips CM10) with an accelerating voltage of 80 kV. To prepare
the TEM specimen, a small amount of the powder was dis‐
persed in ethanol in an ultrasonic bath for 30 min, and a few
drops of the resulting suspension were placed on a carbon
coated copper grid. The specific surface area of the catalyst was
obtained by the BET method using N2 adsorption‐desorption at
–196 °C with a surface area analyzer (Micromeritics ASAP
2010). Before each measurement, the sample was degassed at
200 °C for 1 h.
2.3. Catalytic tests
Alcohol, phenol, and/or amine (1 mmol) were added to a
mixture of the ZnAl2O4@SiO2 nanocomposite (100 mg) and
acetic anhydride (1 mmol). The mixture was stirred at 75 °C
(for alcohols and phenols) or at room temperature (for amines)
for a time. The progress of the reaction was monitored by TLC
and/or GC‐MS. When the reaction was completed, ethyl acetate
(10 mL) was added and the mixture was filtered to separate off
the catalyst. The catalyst was washed twice with 7.5 mL ethyl
acetate. The combined organic phases were washed with a
10% solution of NaHCO3 and then dried over MgSO4. The sol‐
vent was removed to yield the product. If further purification
was needed, the product was passed through a short column of
silica gel. All products were characterized on the basis of
In the present work, we report that spinel‐type ZnAl2O4
embedded in a silica matrix (ZnAl2O4@SiO2) is a highly efficient
catalyst for the acetylation reaction. The ZnAl2O4@SiO2 nano‐
composite was prepared by the sol‐gel method and used as a
heterogeneous catalyst for the acetylation of alcohols, phenols,
and amines with acetic anhydride under solvent‐free condi‐
tions.
2. Experimental
2.1. Preparation of the catalyst
1
The reagents used in the synthesis of ZnAl2O4@SiO2 were
Al(NO3)3·9H2O, Zn(NO3)2·6H2O, and tetraethylorthosilicate
(Si(OC2H5)4, TEOS, >99.5%). All these reagents were of purity
>98% from Merck. Alcohols, phenols, amines, and solvents
were purchased with the highest purity available (>98%) from
Merck Chemical Company and used without further purifica‐
tion.
GC‐MS, FT‐IR, and H‐NMR spectral data by comparing these
spectra with those of standard samples or literature data.
3. Results and discussion
3.1. Characterization of the ZnAl2O4@SiO2 nanocomposite
The ZnAl2O4@SiO2 nanocomposite was synthesized by the
sol‐gel method as follows. A mixture of TEOS (0.35 mol, SiO2
content 21 g), ethanol (60 mL), and water (10 mL) was pre‐
pared in a 1‐L beaker, and its acidity was adjusted to pH 2 using
nitric acid (0.1 mol/L). In order to hydrolyze the TEOS, the
mixture was stirred at room temperature for 1 h. Then, a solu‐
tion of Al(NO3)3·9H2O (0.05 mol, 18.75 g) and Zn(NO3)2·6H2O
(0.025 mol, 7.27 g) dissolved in 90 mL of water was added. The
resulting mixture was stirred for 1 h and allowed to gel at room
temperature over 5 d. After gelation, it was dried and calcined
at 600, 800, or 900 °C for 5 h to fix the silica network. It was
washed with hot water (80 °C) three times to give ca. 30 g of
the ZnAl2O4@SiO2 nanocomposite. From the solid mass ob‐
tained and the initial amount of the two metal nitrates em‐
ployed, we estimated that the ZnAl2O4@SiO2 material prepared
contained 30 wt% ZnAl2O4.
Figure 1 shows the XRD patterns of the ZnAl2O4@SiO2 sam‐
ples calcined at 600, 800, and 900 °C for 5 h. It is clear that the
sample calcined at 600 °C was amorphous, with one broad re‐
flection at 2θ = 22.5°, which is the characteristic diffraction of
the amorphous SiO2 matrix. Calcining the dried gel sample at
800 °C resulted in gradual crystallization of the sample. The
increase of calcination temperature to 900 °C resulted in an
increase in the crystallinity. All the peaks could be indexed as
the (220), (311), (400), (422), (511), and (440) planes assigned
to the ZnAl2O4 spinel (ICCD Card File No. 5‐0669). No other
crystalline phase was detected in the calcined samples. The
result meant that the materials consisted of ZnAl2O4 nanocrys‐
tals and an amorphous phase of SiO2. Furthermore, the diffrac‐
tion peaks of the ZnAl2O4 phase were markedly broadened due
to the small size of the particles. The average particle size cal‐
culated based on the Debye‐Scherrer equation [60] using the

Saeed Farhadi et al. / Chinese Journal of Catalysis 35 (2014) 368–375
ZnAl2O4
(3)
(2)
(1)
10
20
30
40
50
60
70
80
2/(o )
Fig. 1. XRD patterns of ZnAl2O4‐SiO2 samples calcined at different tem‐
peratures for 5 h. (1) 600 °C; (2) 800 °C; (3) 900 °C.
Fig. 3. TEM image of the ZnAl2O4@SiO2 nanocomposite.
most intense peak (311) at 2θ = 36.90° was 20 nm.
N2 adsorption‐desorption isotherm was measured to get the
surface area of the ZnAl2O4@SiO2 nanocomposite. As shown in
Fig. 4, the specific surface area of the ZnAl2O4@SiO2 nanocom‐
posite calculated by the BET method was 147.5 m2/g, which
was much higher than that of the commercial bulk ZnAl2O4
sample (ca. 5 m2/g). The high specific surface area of the nano‐
composite product showed that it could be an efficient catalyst.
The isotherm was Type IV with an evident hysteresis loop in
the 0.5–1.0 range of relative pressure, indicating that the exist‐
ence of SiO2 prevented the ZnAl2O4 nanoparticles from aggre‐
gating, and this can be expected to give good catalytic activity
to the ZnAl2O4@SiO2 nanocomposite.
The XRD analysis confirmed that a high crystallinity
ZnAl2O4@SiO2 nanocomposite was formed by calcining the
dried gel at 900 °C for 5 h. For further confirmation, the FT‐IR
spectrum of this sample was recorded. As shown in Fig. 2,
bands appeared at 1078 and 800 cm–1 were assigned to the
stretching and bending vibrations of the Si–O–Si bonds of the
silica network [61]. These bonds were formed by the hydroly‐
sis and condensation reactions of the TEOS precursor. In addi‐
tion, three bands appeared at 680, 560, and 485 cm–1 that were
related to the Al–O stretching and O–Al–O bending vibrations of
the AlO6 octahedron group and the Zn–O stretching vibration of
the ZnO4 tetrahedron group in the spinel‐type ZnAl2O4 struc‐
ture, respectively [61,62]. This result confirmed the formation
of a ZnAl2O4@SiO2 nanocomposite in the silica network in
agreement with the XRD result.
Since the catalytic properties of inorganic nanomaterials are
highly dependent on their size, shape, and crystalline structure,
TEM was conducted to determine the particle sizes and mor‐
phology of the ZnAl2O4@SiO2 nanocomposite. Figure 3 shows
the TEM image of the ZnAl2O4@SiO2 nanocomposite obtained
by calcining the gel at 900 °C. The result revealed that the
ZnAl2O4@SiO2 particles had a semi‐spherical morphology and
homogeneous shape. The nanocomposite particles have a nar‐
row size distribution in a range from 10 to 30 nm with an av‐
erage particle size of 20 nm, which was in agreement with XRD
result.
3.2. Acetylation reaction over ZnAl2O4@SiO2 nanocomposite
The aim of this work was to evaluate the catalytic activity of
ZnAl2O4@SiO2 in the acetylation of alcohols, phenols, and
amines. At the beginning, an optimization reaction was carried
out to get the best conditions with benzyl alcohol as a model
substrate. A mixture of benzyl alcohol (1 mmol) and acetic an‐
hydride (1 mmol) in the presence of various amounts of the
catalyst was stirred under solvent‐free conditions. The acetyla‐
tion at room temperature was too slow for practical applica‐
tion. In order to overcome this drawback, the reaction was
carried out at 75°C, which resulted in the best yield. The pro‐
gress of the reaction was monitored by TLC. The yield of benzyl
acetate was increased from 43% to 64% and then 92% with
200
S
BET = 147.5 m2/g
160
120
80
40
0
0.0
0.2
0.4
0.6
0.8
1.0
3600 3200 2800 2400 2000 1600 1200 800 400
Wavenumber (cm1)
Relative pressure (p/p0)
Fig. 4. N2 adsorption‐desorption isotherm for the ZnAl2O4@SiO2 nano‐
Fig. 2. FT‐IR spectrum of the ZnAl2O4@SiO2 nanocomposite.
composite.

Saeed Farhadi et al. / Chinese Journal of Catalysis 35 (2014) 368–375
the increase of the amount of catalyst from 25 to 50 and then to
reaction time and product yield were achieved without the use
of any solvent (20 min, 92%). So, we performed the reactions
under solvent‐free conditions. The low activity of the catalyst in
the presence of a solvent can be attributed to the existence of
various interactions of the solvent e.g. solvation or formation of
a complex with the reactants and/or product. When the reac‐
tion was performed without a solvent, these interactions or
interferences were eliminated, so a higher efficiency of the cat‐
alytic system was achieved. There was probably also an in‐
100 mg, respectively, but further increase did not improve the
yield. The appropriate amount of catalyst in the reaction mix‐
ture was 100 mg per 1 mmol of alcohol. The essential role
played by the catalyst was evident from the extremely low yield
of benzyl acetate found in the absence of the catalyst. In order
to choose the best medium, the reaction was also studied in
various organic solvents such as acetonitrile, toluene, acetone,
and dichloromethane. However, the best results in terms of
Table 1
Acetylation of alcohols and phenols with acetic anhydride over the ZnAl2O4@SiO2 nanocomposite.
Time
(min)
Yieldb
(%)
Time Yieldb
Entry
1
Substrate
Product a
Entry
16
Substrate
Product a
(min)
(%)
OAc
OH
OH
OAc
20
15
92
12
92
OH
OAc
OH
OAc
t-Bu
t-Bu
2
3
94
17
18
15
15
94
92
Cl
Cl
OH
OAc
OH
OAc
MeO
MeO
15
88
OMe
OMe
OMe
OMe
OMe
OMe
OH
OAc
OH
OH
OAc
OAc
4
5
6
20
20
20
92
90
92
19
20
21
30
25
30
86
92
86
OH
OAc
F
F
Cl
Cl
OH
OAc
OH
OAc
Cl
Cl
OAc
OAc
OH
Cl
Cl
OH
OAc
7
20
89
22
36
85
OH
OH
OAc
OAc
OH
F3C
F3C
8
9
35
20
85
90
23
32
32
89
90
NO2
NO2
OH
OAc
OH
OAc
24 c
NO2
NO2
OH
OAc
OH
OAc
10
11
12
15
25
20
90
90
90
25
26
27
38
35
45
86
88
82
OH
OAc
OAc
Me
OH
Me
CN
CHO
CN
CHO
OAc
OH
OH
O2N
OAc
O2N
OAc
OH
COMe
COMe
OH
OH
OAc
OAc
13
18
92
28
40
92
OAc
SAc
OH
14
15
15
15
94
92
29
30
50
45
88
80
OH
OAc
SH
Reaction conditions: substrate 1 mmol, acetic anhydride one equiv. per OH, catalyst 100 mg, solvent‐free, 75 °C.
a All products were characterized by GC‐MS, IR and 1H‐NMR spectral data and comparison with those of standard samples or literature data.
b Isolated yield from weight of pure product obtained.
c The reaction was carried out on a scale of 20 mmol benzyl alcohol.

Saeed Farhadi et al. / Chinese Journal of Catalysis 35 (2014) 368–375
crease in the number of effective collisions between reactants
their corresponding acetates (Table 2, entries 13–18). Cinnam‐
yl alcohol, an α,β‐unsaturated alcohol, was selectively convert‐
ed to the corresponding acetate, and C=C bond remained intact
under the reaction conditions (Table 1, entry 19). Also, non‐
benzylic primary alcohols were selectively converted to their
corresponding acetates with high efficiency at the same reac‐
tion conditions (Table 1, entries 20 and 21). A sterically hin‐
dered tertiary alcohol such as triphenylmethanol was also
acetylated with high yield but it took a longer reaction time
(Table 1, entry 22). In this case, no elimination product was
found in the reaction mixture by GC–MS analysis. The efficiency
of the catalyst can be seen clearly in the acetylation of a
di‐hydroxy compound under similar conditions (Table 1, entry
23). Among the various alcohols studied, secondary benzylic
alcohols were the most reactive, giving the corresponding acet‐
ylated products with shorter reaction times. As we can see in
Table 1, functional groups such as ‐OMe, ‐CHO, ‐COMe, ‐CN, and
‐NO2 remained unchanged under the reaction conditions. Also,
and catalyst. It is noteworthy that the acetylation of benzyl
alcohol with acetic anhydride using a pure ZnAl2O4 sample was
too slow for practical application under the optimized condi‐
tions.
The generality of this protocol was shown by utilizing vari‐
ous primary, secondary and tertiary alcohols, as well as diols
and an allylic alcohol (Table 1). The results in Table 1 showed
that all the substrates were selectively converted to the corre‐
sponding acetates in high yields without any side products. The
acetylation of a wide range of ring‐substituted primary benzyl
alcohols having various electron‐donating and electron‐with‐
drawing groups was investigated with acetic anhydride over
the ZnAl2O4@SiO2 nanocomposite. These alcohols were effi‐
ciently converted to their corresponding acetates in high yields
and the nature of the substituent had no significant effect on
the reaction time and yields (Table 1, entries 1–13). In addition,
various secondary alcohols were converted in high yields to
Table 2
Acetylation of amines with acetic anhydride over the ZnAl2O4@SiO2 nanocomposite.
Time
(min)
Yield a
(%)
Time
(min)
Yield a
(%)
Entry
1
Amine
Product
Entry
12
Amine
Product
NHAc
NC
NH2
NC
NHAc
NH2
3
3
96
5
5
90
NO2
NO2
Me
NH2
Me
NHAc
2
3
94
13
14
95
NH2
NHAc
Me
Me
OH
OH
3
93
6
91
H2N
AcHN
HS
NHAc
NH2
Et
NHAc
HS
NH2
NHAc
Et
NH2
NH2
4
5
3
3
96
96
15
16
3
4
95
95
i-Pr
NHAc
NHAc
i-Pr
HO
NH2 HO
NHAc
OH
OH
MeO
Br
NH2
MeO
Br
6
7
3
4
96
94
17
18
3
7
96
92
NH2
NHAc
NH2
NHAc
NHAc
NHAc
NH2
NH2
Cl
Cl
N
NHAc
Cl
Cl
N
NH2
8
3
6
7
95
94
89
19
20
21
5
90
88
85
Cl
Cl
CH2NH2
NH2
CH2NHAc
NHAc
9
10
12
NH2
NH2
NHAc
NHAc
O
H
O
H
10
O
O
Me
Me
NH2
NHAc
11
8
90
22
H2N
AcHN
8
87
NH2
NHAc
Reaction conditions: amine 1 mmol, acetic anhydride 1 eqiuv. per NH2 group, catalyst 100 mg, without solvent, room temperature.
a Yields are for isolated pure products.

Saeed Farhadi et al. / Chinese Journal of Catalysis 35 (2014) 368–375
the conversion of benzyl alcohol to benzyl acetate on a 20
Table 3
mmol scale proceeded just as efficiently as the 1 mmol reaction
(Table 1, entry 24).
Protection of benzyl alcohol with various anhydrides by the ZnAl2O4@
SiO2 nanocomposite.
CH2OH
CH2OC(=O)R
The acetylation of phenols was also investigated using the
present experimental conditions. Phenol, substituted phenols
with electron‐donating and electron‐withdrawing groups, α‐
and β‐naphthol and also thiophenol were acetylated with high
yields (80%–92%), although with longer reaction time in com‐
parison to alcohols (Table 1, entries 25–30). The excellent ac‐
tivity of the ZnAl2O4@SiO2 nanocomposite was demonstrated
by the high yield obtained for para‐nitrophenol that has an
electron‐withdrawing group (Table 1, entry 27).
ZnAl2O4@SiO2 nanocomposite
(RC=O)2O, Solvent-free, RT
Entry
Anhydride
Ac2O
Time (min)
Yield b (%)
92
1
2
3
4
5
6
20
25
25
32
40
45
(CF3CO)2O
(EtCO)2O
(iso‐PrCO)2O
88
86.5
81
(tert‐BuCO)2O
80
78.5
The scope of this reaction was further extended to the acet‐
ylation of amines. A wide range of anilines containing various
electron‐donating and withdrawing groups and also primary
aliphatic amines were treated with acetic anhydride at room
temperature under solvent‐free conditions (Table 2). In all
cases, excellent yields (85%–96%) of the corresponding acety‐
lated derivatives were obtained within 3–12 min. The excellent
activity of the ZnAl2O4@SiO2 nanocomposite was demonstrated
by the high yields obtained for anilines that have elec‐
tron‐withdrawing groups (Table 2, entries 10–13). Interesting‐
ly, the acetylation of substrates containing –NH2, –OH, and –SH
groups with one equivalent of acetic anhydride produced the
corresponding acetamides as sole products; the hydroxyl or
thiol moiety remained untouched (Table 2, entries 14–17). This
may be due to that the –NH2 group has greater nucleophilicity
than the –OH and –SH groups. The selective acetylation of a
–NH2 group in the presence of –OH and –SH groups is of con‐
siderable synthesis importance. Furthermore, 1‐naphthylamine
and para‐aminopyridine were converted to the corresponding
acetamide in high yields (Table 2, entries 18 and 19). Benzyla‐
mine, n‐butylamine and also 1,4‐phenylenediamine gave the
corresponding acetamides in high yields under the present
reaction conditions (Table 2, entries 20–22). 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 20 mmol scale
proceeded just as efficiently as the 1 mmol reaction (10 min,
95% isolated yield). In contrast to primary aromatic amines
which were readily acetylated, secondary amines such as di‐
phenylamine did not undergo any change.
(PhCO)2O
Reaction conditions: benzyl alcohol 1 mmol, anhydride 1 mmol, catalyst
100 mg, without solvent, 75 °C.
a Isolated yield of corresponding acylated product.
b 2 equiv. of (PhCO)2O used at 85 °C.
posed to one equivalent of Ac2O were due to the combined ef‐
fect of the steric and electronic factors of the phenyl group in
(PhCO)2O. The phenyl group makes the carbonyl group in
(PhCO)2O less electrophilic by a resonance effect.
The recycling of the catalyst was investigated with the acet‐
ylation of benzyl alcohol. After completion of the reaction, ethyl
acetate was added and the mixture was filtered to separate off
the catalyst. The recovered catalyst was dried and reused in
more runs. As can be seen in Table 4, no appreciable decrease
in activity of the catalyst was observed after four runs.
We were also interested in testing the nature of the recov‐
ered catalyst. Hence, we performed XRD analysis on the spent
catalyst after the fourth run. As shown in Fig. 5, the recovered
catalyst did not show any significant change after four cycles
Table 4
Recyclability of the ZnAl2O4@SiO2 nanocomposite.
Cycle
0
1st
2nd
3rd
4th
Yielda (%)
92
90
90
88
88
Reaction conditions: benzyl alcohol 1 mmol, acetic anhydride 1 mmol,
catalyst 100 mg, 75 °C, 20 min.
a Yields are for isolated pure benzyl acetate.
The activity of the ZnAl2O4@SiO2 nanocomposite as a gen‐
eral catalyst was tested by the acetylation of benzyl alcohol
with other anhydrides under the same reaction conditions (Ta‐
ble 3). In comparison to acetic anhydride, the reactions with
higher anhydrides took longer time at room temperature.
However, the reactions were completed in 20 to 45 min under
solvent‐free conditions and afforded the corresponding acylat‐
ed derivatives with high yields (78.5%–92%). The rate of acyl‐
ation was influenced by steric and electronic factors of the an‐
hydrides and followed the order Ac2O > (CF3CO)2O > (EtCO)2O
> (iPrCO)2O > (tert‐BuCO)2O > (PhCO)2O. The longer times re‐
quired for the reaction with (CF3CO)2O, (EtCO)2O, (iso‐PrCO)2O,
and (tert‐BuCO)2O were mainly due to the steric effect of their
alkyl groups (Table 3, entries 1–4). The longer reaction time
and the requirement of two equivalents of (PhCO)2O as op‐
ZnAl2O4
10
20
30
40
2/(o )
50
60
70
80
Fig. 5. XRD pattern of the recovered ZnAl2O4@SiO2 catalyst after the
fourth run.

Saeed Farhadi et al. / Chinese Journal of Catalysis 35 (2014) 368–375
Table 5
Comparison of the result obtained for the acetylation of benzyl alcohol in the present work with those obtained by some reported heterogeneous
catalysts.
Alcohol:Ac2O
molar ratio
Catalyst amount
(g)
Time
(min)
Yield
(%)
Entry
Heterogeneous catalyst
Conditions
Ref.
1
2
3
4
5
6
7
Mont. KSF
SiO2‐H2SO4
FER zeolite
Solvent‐free, r.t.
CH2Cl2, r.t.
1:2
1:2
1:1.5
1:1
1:1
1:3
1:1
0.1
0.2
0.15
0.05
0.5
60
240
120
10
90
90
91
93
92
94
92
[18]
[23]
[24]
Solvent‐free, 75 °C
Solvent‐free, r.t.
Solvent‐free, r.t.
Solvent‐free, 80 °C
Solvent‐free, 75 °C
Sulphated ZrO2
(NH4)2.5H0.5PW12O40
Rice husk
[29]
60
60
20
[30]
[37]
this work
0.3
0.1
ZnAl2O4@SiO2
compared to the fresh one (Fig. 1). This indicated that the
structure of the nanocomposite was stable under the reaction
conditions and was not affected by the reactants. In addition,
the XRD result confirmed there was no organic impurities in
the recovered catalyst.
In order to show the advantage of the present method, we
compared our results in the acetylation of benzyl alcohol with
those of some reported heterogeneous catalysts in the litera‐
ture (Table 5). From the reaction conditions, substrate/Ac2O
molar ratio, reaction time, and product yield, it can be seen that
the present method is superior. The reaction over the most of
other reported catalysts required longer times for completion.
The other advantages of the catalyst in this work in comparison
with other previously reported catalysts are its easy prepara‐
tion, no moisture sensitivity, it is not explosive or expensive,
and it has very low toxicity.
Similar to other heterogeneous catalytic reactions, this reac‐
tion takes place on the surface of the catalyst and small parti‐
cles make a distinct contribution to its catalytic activity
[20–22]. The surface atoms behave as Lewis acid centers where
the chemical reaction is activated. As shown in Scheme 1, cata‐
lytic acetylation is initiated by coordination of the carbonyl
group of acetic anhydride to unsaturated Al and Zn ions on the
surface of the nanocomposite as Lewis acidic sites, which acti‐
vates it. The activated carbonyl group reacts with the substrate,
which is a reactive nucleophile, to produce the corresponding
acetate.
of the reaction. Due to the high specific surface area of the
ZnAl2O4@SiO2 nanocomposite, the number of surface atoms is a
larger fraction of the total, and they provide more contact area
for the reactants on the catalyst. Therefore, the higher catalytic
activity of the ZnAl2O4@SiO2 nanocomposite compared to the
activity of bulk‐ZnAl2O4 can be attributed to more coordination
of the carbonyl group to the ZnAl2O4 due to the participation of
more surface Lewis acidic sites in the reaction.
4. Conclusions
A ZnAl2O4@SiO2 nanocomposite was prepared by the sol‐gel
method, and was successfully applied as an efficient and recy‐
clable heterogeneous catalyst for the acetylation of alcohols,
phenols, and amines with acetic anhydride under solvent‐free
conditions. The acetylation of amines proceeded rapidly,
namely, selective acetylation of a –NH2 group in the presence of
an –OH group was observed. The ZnAl2O4@SiO2 nanocomposite
was reused without loss in activity. XRD showed that the
structure of the catalyst did not change after the reaction. This
paper provided the first description of the acetylation of
amines, alcohols and phenols with acetic anhydride over a
nanocomposite catalyst.
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Saeed Farhadi et al. / Chinese Journal of Catalysis 35 (2014) 368–375
Graphical Abstract
Chin. J. Catal., 2014, 35: 368–375 doi: 10.1016/S1872‐2067(12)60758‐X
ZnAl2O4@SiO2 nanocomposite catalyst for the acetylation of
alcohols, phenols and amines with acetic anhydride under
solvent‐free conditions
ZnAl2O4/SiO2
nanocomposite
Saeed Farhadi*, Kosar Jahanara
Lorestan University, Iran
ROAc
ArOAc
ROH
ArOH
RNH2
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