Effect of dispersed ZnO in Cu-Zn composite oxides on catalytic activity of anisole acetylation

Article abstract of DOI:10.1039/d0nj00084a

Cu-Zn composite oxide catalysts were prepared by a co-precipitation method and used in the anisole acetylation reaction. Through experimental study and characterization analysis, we have found that the activity of the catalysts derived from the Lewis acid

Full text of DOI:10.1039/d0nj00084a

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Effect of dispersed ZnO in Cu–Zn composite
oxides on catalytic activity of anisole acetylation†
Yanqiu Chen,a Yue Yang,a Zhou Zhou,a Wei Luo,a Junhua Liu *a and Fang Wang*b
Cite this: New J. Chem., 2020,
44, 10492
Cu–Zn composite oxide catalysts were prepared by a co-precipitation method and used in the anisole
acetylation reaction. Through experimental study and characterization analysis, we have found that the
activity of the catalysts derived from the Lewis acidity of the octahedrally coordinated Cu2+ (CuO2+) and
2+
ZnO plays an important role in promoting the dispersion of CuO active sites, adjusting the surface
structure and coordination of the catalysts. Due to the integration of ZnO and CuO with different
degrees of dispersion, some of the tetrahedrally coordinated Cu2+ (CuT2+) species are converted to a
2+
Received 7th January 2020,
Accepted 22nd May 2020
low binding energy CuO on different levels. When ZnO is completely dispersed in the CuO crystalline
2+
lattice, the amount of CuT is at its lowest and catalyst activity is highest, with the conversion of anisole
DOI: 10.1039/d0nj00084a
reaching 92.8% with a selectivity for p-methoxyacetophenone (p-MAP) of 97.0% under solvent-free
rsc.li/njc
conditions. More importantly, the catalyst can be reused three times without any treatment.
anhydride catalyzed by heteropolyacids, with a conversion and
selectivity of more than 90%. However, heteropolyacid catalysts
Introduction
Friedel–Crafts acylation is a type of electrophilic substitution have a small specific surface area, poor thermal stability and
reaction occurring on an aromatic ring and is an important many limitations. In addition, for the anisole acetylation reaction,
reaction in the chemical industry. Its products, aromatic zeolite molecular sieves have received much attention, but the
ketones, are widely used in pesticides, pharmaceuticals, spices acylation process was inevitably inhibited because of their fast
and dyes.1–3 In recent decades, researchers have been paying deactivation and difficulties for reuse.9
attention to this reaction and numerous related studies have
Recently, composite metal oxides have been widely used
appeared. Generally speaking, the traditional metal halide in both academic and industrial fields due to their simple
catalysts (AlCl3, ZnCl2, FeCl3) and protic acid catalysts (HCl, preparation and low cost. This is because the interaction
HF, H2SO4) used in the existing processes encounter many between oxides increases the acidity and basicity of the catalyst
disadvantages, such as equipment corrosion, environmental surface, providing superior catalytic activity compared with
pollution, and non-recyclability.1,4,5 However, heterogeneous other catalysts.10 However, little research has focused on
solid acid catalysts not only can avoid these problems effec- Friedel–Crafts acylation using composite metal oxides as catalysts.
tively, but also have the advantages of high catalytic activity and Pavuluri Srinivasu’s group reported a SnO2–SiO2 catalyst for
simple separation. Therefore, they are a good substitute for Friedel–Crafts acylation of anisole with acid chloride at 80 1C.
traditional liquid acids. Yadav6 used various ion exchange The yield of para-product reached 92.0% with nitromethane as
resins for the anisole acetylation reaction with the yield of the solvent, but the preparation of the catalyst is cumbersome.11
para-product being close to 100%. Unfortunately, these resin Meanwhile, Arnold E. Ruoho claimed that P2O5/Al2O3 successfully
catalysts are difficult to get for widespread use due to the high realized a catalytic Friedel–Crafts reaction, but the para-product
price of the resin and their easy swelling in organic solvents. was only produced in 62.0% yield.12
Admas7 used ionic liquids to carry out acetylation of a series
In this study, a series of Cu–Zn composite oxides with
of aromatic compounds such as anisole, good conversion different molar ratios were prepared and this is the first time
was obtained but serious catalyst deactivation happened. that Cu–Zn composite oxides have been used in the anisole
Parida8 also performed the reaction of anisole and acetic acylation reaction under solvent-free conditions. These cata-
lysts show not only perfect results, but also high stability after
a College of Chemistry and Materials Science, Nanjing Normal University,
three cycles without any treatment, which effectively avoids the
serious deactivation and other problems, such as high reaction
Nanjing 210023, China. E-mail: liujh2007@aliyun.com
b College of Chemistry and Molecular Engineering, Nanjing Tech University,
temperature, high cost and solvent toxicity, of previously used
Nanjing 211816, China. E-mail: wangfang2012@njtech.edu.cn
2+
catalysts. The synergistic catalysis of ZnO and CuO is an
important reason for the high activity of this reaction.
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0nj00084a
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Experimental
Catalyst preparation
added to a flask in which a magneton was placed in advance.
Then, the flask was heated and stirred in an oil bath at 100 1C
for 6 h, with the whole process was carried out under the
protection of a nitrogen atmosphere. The catalyst was removed
from the reaction solution by centrifugation in order to recycle
test. Finally, we analyzed the product using a gas chromato-
graph (GC 9560), equipped with a hydrogen flame ionization
detector and an SE-54 capillary column.
All catalysts were prepared by a co-precipitation method.
Taking the composite oxide of Cu/Zn with a molar ratio of
1 as an example: 5 mmol of zinc nitrate hexahydrate and
5 mmol of copper nitrate trihydrate were dissolved separately
in a beaker containing 200 ml of distilled water, and the mixed
solution was sonicated for 0.5 h and then added dropwise to
aqueous sodium hydroxide solution. Subsequently, the mixture
was heated and stirred for 2 hours at 75 1C. After cooling, the
products were obtained by filtration, washed several times until
the pH of the solution became neutral, and dried at 1001 for
12 hours. Finally, the ZnCuOx catalysts were obtained by
calcination at 6501 for 6 hours (5 1C minÀ1). Other catalysts
with different Cu/Zn molar ratios were prepared by the same
method and are denoted as ZnCu2Ox, ZnCu3Ox and ZnCu4Ox,
respectively.
Results and discussion
Catalyst characterization
X-ray diffraction was used to obtain material composition, and
to determine the internal atomic or molecular structure. XRD
patterns of Cu–Zn composite oxides with different Cu/Zn molar
ratios are displayed in Fig. 1. The diffraction peaks at 2y = 31.71,
34.41, 36.21, 47.51, 56.61, 62.91, 57.91, 69.01 and 77.01 are
assigned to zincite ZnO (JCPDS: 36-1451).13,14 The diffraction
peaks at 2y = 32.51, 35.51, 38.71, 48.71, 53.41, 58.31, 61.51, 65.51,
66.21, 68.01 and 75.01 are assigned to tenorite CuO (JCPDS:
48-1548).15 From the XRD patterns (Fig. 1a–f), it can be clearly
seen that all the diffraction peaks are attributable to ZnO or
CuO, and that no new diffraction peaks are found. However,
when the Cu/Zn molar ratio is 3 (Fig. 1d), only the crystal plane
of CuO is observed, which indicates that the ZnO is completely
dispersed in the CuO crystal lattice.16 Table 1 illustrates the
grain size of different complex oxides calculated using the
Scherrer equation, and only the ZnCu3Ox has a larger grain
size compared to pure CuO, which is further confirmation that
ZnO is completely dispersed in the CuO lattice.17
Catalyst characterization
The crystal structures of the catalysts were studied by X-ray
diffraction (XRD) on a Rigaku/Max-3 A with Cu Ka radiation
(l = 0.15418 nm) and operated in the 2y range 5–801and with a
scanning rate of 0.51 minÀ1
. Fourier transform-infrared
spectroscopy (FT-IR) information was collected using a Bruker
Tensor 27 spectrometer with KBr disks and the Raman spectro-
scopy data were obtained using a Raman spectrometer (HR800)
with 20 mW laser (532.14 nm). The surface electronic structure
and chemical composition of the samples were determined
using a ThermoFisher ESCALAB250XI X-ray photoelectron
spectrometer which had a K-Alpha surface analysis system with
X-ray monochromatisation. The spectrogram was recorded after
the samples were purged under vacuum and the calibration
reference for binding energies was C1s at 284.8 eV. The morphol-
ogy of the catalysts was measured by scanning electron micro-
scopy (SEM) (JSM-7600F) and transmission electron microscopy
(TEM), and elemental mapping was carried on a FEI Talos F200X
apparatus. The acidity of all catalysts was detected using
NH3-temperature programmed desorption (NH3-TPD) on an Auto
Chem II 2920 with a thermal conductivity detector. Typically,
100 mg of the sample was purged with helium for 60 min at
120 1C and then cooled to 50 1C, then 10% NH3 (CO2)/He gas was
introduced into the reactor for chemical adsorption for 30 min.
Then, the samples were subjected to desorption of ammonia
or carbon dioxide from 50 1C to 700 1C with a heating rate
of 10 1C minÀ1. Inductively coupled plasma optical emission
spectroscopy (ICP-OES) analyses were carried out to measure the
composition of Cu and Zn for the introduced catalysts and
leaching of Cu and Zn species during the process of cyclic
experiments, using a Varian ICP-OES 720ES apparatus.
Raman spectroscopy was used to conduct a more compre-
hensive study of the coordination environment in the Cu–Zn
Fig. 1 XRD patterns of (a) ZnOx, (b) ZnCuOx, (c) ZnCu2Ox, (d) ZnCu3Ox,
(e) ZnCu4Ox and (f) CuOx.
Table 1 The crystal size (D) of catalysts
Reaction procedure
Catalyst
CuO
4.49
ZnCuOx ZnCu2Ox ZnCu3Ox ZnCu4Ox
Ba/rad (Â 10À3
)
4.89
5.11
28.43
4.22
34.42
5.27
27.56
The acylation experiment of anisole was carried out in a 25 ml
single-neck round-bottomed flask equipped with a condenser.
A typical reaction procedure was as follows. Catalyst (0.15 g),
Db/nm
32.35 29.71
a
b
Data is from MDI Jade 6. Result is calculated by the Scherrer
anisole (5.9 ml), and acetyl chloride (1.3 ml) were successively equation.
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dispersed in the CuO lattice, the peak position moves to varying
degrees, and the peak intensity also changes, which may be due
to the interaction between ZnO and CuO being enhanced.29 The
broad band at 3446 cmÀ1 is ascribed to the O–H stretching
vibration of water molecules and the band at 1620 cmÀ1 is
attributed to the bending vibration of water molecules.17
X-ray photoelectron spectroscopy (XPS) was used to analyze
the metal valence, content and coordination environment
of the catalyst surface. Fig. 4A depicts the Zn 2p spectrum of
the single metal zinc oxide and Cu–Zn composite oxides.
Obviously, the main peaks of Zn 2p3/2 and Zn 2p1/2 are 1010 and
1040 eV, respectively, corresponding to the Zn2+ 22,30
Notably,
.
Fig. 2 Raman spectra of (a) ZnOx, (b) ZnCuOx, (c) ZnCu2Ox, (d) ZnCu3Ox,
(e) ZnCu4Ox and (f) CuOx.
with dispersion of ZnO in the CuO phase, the binding energy of
Zn2+ shifts to a higher level, and this may be due to the strong
interaction between ZnO and CuO.21,31 This is also consistent
with the FT-IR results.
oxides. It can be seen from Fig. 2 that the Raman peak bands
for all the catalysts are well defined. The frequencies 378, 416,
439, 578 and 751 cmÀ1 are attributed to ZnO crystals,18 and the
characteristic peaks at 290 (Ag mode), 342, 618 (Bg mode) cmÀ1
are assigned to the CuO crystal.19 The strong peak at 700 cmÀ1
is a typical vibration mode of metal and oxygen tetrahedral
coordination (A1g).20,21 The peaks at 289, 451, 581 and 617 cmÀ1
are all ascribed to the octahedral lattice vibration mode.20,22,23
Obviously, the characteristic peak intensity of the tetrahedral
environment gradually weakens but the octahedral peak is
enhanced with the increase in Cu/Zn molar ratio up to 3, at
2+
which point the content of CuT is at its lowest and that of
CuO2+ is at its highest. This result indicates that dispersed ZnO
2+
promotes the conversion of CuT to CuO2+, because the ZnO
species, which are completely dispersed in the CuO crystal
phase, cause the interaction of copper ions in the tetrahedral
and octahedral lattices.24
The FT-IR spectra are related to the internal molecular
structure and motion of the materials. From the position,
intensity and change of band, the variation in the molecular
structure can be inferred.25 From Fig. 3f, we can see that copper
oxide has two strong and one weak absorption band at 523 and
598, and 446 cmÀ1, respectively. This can be attributed to the
stretching vibration of Cu–O bonds in copper oxide.26 More-
over, the tensile mode of zinc oxide is at 430 cmÀ1 27
All the
.
peaks from 400 to 600 cmÀ1 are considered to be metal–oxygen
(M–O) vibration modes.28 However, as zinc ions become
Fig. 4 X-ray photoelectron spectra of (A) Zn 2p for catalysts (a) ZnOx,
(b) ZnCuOx, (c) ZnCu2Ox, (d) ZnCu3Ox and (e) ZnCu4Ox. (B) Cu 2p for
catalysts (a) CuOx, (b) ZnCuOx, (c) ZnCu2Ox, (d) ZnCu3Ox and (e) ZnCu4Ox.
Fig. 3 FT-IR spectra of (a) ZnOx, (b) ZnCuOx, (c) ZnCu2Ox, (d) ZnCu3Ox,
(e) ZnCu4Ox and (f) CuOx.
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2+
2+
Table 2 The content of CuT and CuO in all catalysts
Atomica/%
2+
2+
2+
2+
Catalyst
CuO
CuT
CuT2+/CuO + CuT
CuOx
82.2
82.8
89.9
96.1
92.8
17.8
17.2
10.1
3.9
17.8
17.2
10.1
3.9
ZnCuOx
ZnCu2Ox
ZnCu3Ox
ZnCu4Ox
7.2
7.2
a
Data were obtained by Thermo Avantage.
As can be seen from Fig. 4B, the Cu 2p spin orbits of all the
Cu–Zn oxides and the single metal copper oxide catalysts are
composed of two main peaks, at 932.9–935.4 and 952.8–953.2 eV.
Where the peaks at 940 and 943 eV are the characteristic
satellite peaks of Cu 2p3/2, which may be due to the vibrational
transition of electrons in the copper ion.32 The copper ions in
2+
all catalysts are divalent, and can be divided into CuT and
CuO2+, with the corresponding peak positions of 935.0–935.4
and 932.9–933.3 eV,33 respectively. It is worth noting that in
2+
single metal copper oxide, CuT2+/CuO is 17.8% (Table 2), and
when the Cu/Zn molar ratio increases, the peak intensity of
2+
CuT decreases at first and then increases. This can be
Fig. 5 X-ray photoelectron spectra of (A) O 1s for catalysts (a) ZnOx,
(b) CuOx, (c) ZnCuOx and (B) O 1s for catalysts (d) ZnCu2Ox, (e) ZnCu3Ox
and (f) ZnCu4Ox.
attributed to the dispersion of ZnO within the crystal phase
of CuO, which promotes the dispersion of the CuO phase itself,
adjusts the structure and coordination of the catalyst surface
and enhances the dispersion of active sites,34 since the highly
dispersed Cu2+ species has lower binding energy than the Cu2+
ones in CuO phase,35 the former can exhibit stronger coordina-
Table 3 Distribution of acidic sitesa in the catalyst
Weak sites
Moderate sites
Total acidity
tion ability of oxyanion36 leads to more CuT translating into
2+
Catalyst
(NH3/mmol gÀ1
)
(NH3/mmol gÀ1
)
(NH3/mmol gÀ1
)
stable CuO2+ with more acidity.36,37 ZnO disperses completely in
the CuO lattice when the Cu/Zn molar ratio is 3, with the
ZnOx
CuOx
ZnCuOx
ZnCu2Ox
ZnCu3Ox
ZnCu4Ox
—
—
7.1
7.2
9.3
7.0
—
—
18.1
40.8
56.7
59.0
54.4
18.1
47.9
63.9
68.3
61.4
content of CuT being the least at this time, at only 3.9%,37
2+
which is in keeping with the Raman results.
Fig. 5 shows the XPS diagram for the O element. After
de-stacking, three peaks are obtained, which are the lattice
oxygen peak (Olatt), surface oxygen peak (Osur) and adsorbed
oxygen (Oads), and their peak positions are 529, 530 and 531 eV,38
a
Determined by NH3-TPD.
respectively. As the molar ratio of Cu/Zn increases, the peak CuO phase promotes the dispersion of acidic sites,34 which
intensity of Olatt and Osur undergoes some subtle changes, and makes the distribution of surface acid intensity more extensive,
Olatt decreases and Osur increases gradually, especially when the and is very favorable for the reaction between anisole and acetyl
Cu/Zn molar ratio is 3. The oxygen atom in the catalysts mainly chloride. Furthermore, the dispersion of zinc oxide promotes
forms a hydroxyl group on the surface of the metal, which has a better dispersion of copper oxide itself and fully excites the Lewis
certain acidity and plays an important role in the reaction acidity of Cu2+,39 thus playing a role in activating acetyl chloride.
process. Meanwhile, the decrease in lattice oxygen means that Meanwhile, it is evident that the desorption peaks are usually
the oxygen vacancy increases, which is beneficial for improving classified into three types: weak acid sites (50–200 1C), medium
the reaction between electrons and surface-adsorbed substances, acid sites (200–550 1C) and strong acid sites (550–700 1C),40–42 and
increases the migration of interface electrons and inhibits the in the catalysts shown in Fig. 6 they range from 200 to 300 1C,
electron–hole recombination process.
The acidities of Cu–Zn composite oxides and single metal these results are consistent with the XRD and XPS data.
oxides are given by the NH3-TPD pattern, with all catalysts The morphology of the Cu–Zn composite oxides and single
which can be attributed to moderately acidic sites. Therefore,
exhibiting broad desorption peaks. Significantly, when zinc metal zinc oxide are a rod-shaped coral structure. Remarkably,
oxide disperses completely within the copper oxide at the with the dispersion of zinc oxide, the structure of the catalyst
Cu/Zn molar ratio of 3, both the width of the desorption peak changes gradually, becoming more and more sheet-like in
and the total number of acidic sites reach their highest values appearance, and when the Cu/Zn molar ratio is 3 the sheet
(Table 3). This indicates that the dispersion of ZnO within the structure is at a maximum.
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Table 4 Activity of different catalysts for anisole acylationa
Selectivity (%)
Catalyst
Conversion (%)
p-MAP
Others
Blank
ZnOx
CuOx
ZnCuOx
ZnCu2Ox
ZnCu3Ox
ZnCu4Ox
9.1
13.7
35.2
70.5
72.6
82.4
77.8
70.0
97.0
96.7
95.0
95.5
96.0
95.3
30.0
3.0
3.3
5.0
4.5
4.0
4.7
a
Reaction conditions: anisole 5.9 ml, acetyl chloride 1.3 ml, catalyst
0.15 g, 80 1C, 6 h.
Fig. 6 NH3-TPD of (a) ZnOx, (b) ZnCuOx, (c) ZnCu2Ox, (d) ZnCu3Ox,
(e) ZnCu4Ox and (f) CuOx.
is 70.0%. The conversion is 45.2% and 13.7% in the presence of
single metal oxides CuO and ZnO, respectively. Indeed, the
catalytic activity of CuO is significantly higher than that of ZnO,
and this is because the Cu2+ ion in copper oxide has a certain
Lewis acidity,39 which can provide acidic sites for the reaction.
For Cu–Zn composite oxides, the catalytic activity improves
at first and then declines with increasing Cu/Zn molar ratios.
When the Cu/Zn molar ratios are 1 : 1, 2 : 1 and 4 : 1, the
conversion of anisole is 70.5%, 72.6% and 77.8% respectively.
When the molar ratio reaches 3, the conversion of anisole can
reach 82.4% and the selectivity for p-MAP can reach 96.0%.
This may be due to the fact that copper species play a major role
in the reaction and the dispersed zinc species play a supporting
role. According to the XRD data, the characteristic peak of ZnO
is no longer visible as the molar ratio reaches 3, as it is
completely dispersed within the CuO phase, and so the acidity
2+
increases correspondingly. XPS shows that the ratio of CuT
/
(CuO + CuT2+) in ZnCu3Ox is higher than for other catalysts
with different Cu/Zn molar ratios. NH3-TPD also accounts for
the effect of acidity on the reactivity.
2+
Fig. 7 SEM images of catalysts (a) CuO, (b) ZnCu4Ox, (c) ZnCu3Ox,
(d) ZnCu2Ox, (e) ZnCuOx and (f) ZnO. TEM images of catalysts (g) ZnO,
(h) CuO and (i) ZnCu3Ox.
In general, all the changes in acidity are caused by the
dispersed ZnO in the CuO lattice, and these changes promote
the progress of the reaction.
The lattice structures of the catalysts were characterized by
TEM. It can be seen from Fig. 7(g)–(i) that the lattice fringe of
0.26 nm can be attributed to the (002) crystal plane of the
zincite ZnO (JCPDS: 36-1451).43 The lattice fringe of the (111)
crystal plane (JCPDS: 48-1548) of the tenorite CuO is 0.25 nm.
Significantly, the lattice fringes of the ZnCu3Ox catalyst are
mainly covered by the (111) crystal plane of the CuO phase.
Nevertheless, the crystal plane of ZnO is not seen, which further
proves that Zn2+ species are completely dispersed in the CuO
lattices. Additionally, elemental mapping images (Fig. S1, ESI†)
also indicate that the Cu and Zn elements in catalyst are
uniformly distributed, which further confirms the above
conclusion.
Optimization experiment
In order to obtain the maximized activity of the catalyst to this
reaction, we carried out a series of condition optimization
experiments over the most active ZnCu3Ox catalyst, such as
calcination, temperature, weight of catalyst, and time. As can be
seen from Fig. 8a, different calcination temperatures result in
different catalyst activities. With increase in the calcination
temperature from 450 to 650 1C, conversion of anisole follows a
straight line increase but the selectivity changes little. However,
when the calcination temperature exceeds 650 1C, the catalyst
decomposes due to thermal instability. This may be due to
ZnO being well-dispersed in the CuO phase at a calcination
temperature of 650 1C (Fig. S2, ESI†), so we selected 650 1C as
the calcination temperature. As shown in Fig. 8b, the conver-
Catalytic activity
As we can see, the main product is p-MAP and the by-product is sion of anisole increases from 42.1–92.8% with a rise in
mainly o-methoxyacetophenone (o-MAP). Table 4 shows the temperature from 60–100 1C. However, as the temperature
different catalytic performance of Cu–Zn composite oxides on continues to increase to 140 1C, the conversion of anisole
anisole and acetyl chloride. The conversion of anisole is 9.1% hardly changes. This low conversion of anisole at low tempera-
without the presence of catalyst, and the selectivity for p-MAP tures may be attributed to the inefficiency of Lewis acid Cu2+
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Table 5 Reuse of the ZnCu3Ox catalysta
Cycles
Conversion (%)
Selectivity (%)
1
2
92.0
91.7
91.1
92.1
96.5
96.2
96.1
96.3
3
4b
a
Reaction conditions: anisole 5.9 ml, acetyl chloride 1.3 ml, catalyst
b
0.15 g, 100 1C, 6 h. Catalyst reused: data are from the third repeated
use after calcining.
Comparison of the activity of Cu–Zn composite oxides with the
reported catalyst
The activity of Cu–Zn composite oxides was compared with
other reported catalysts (Table 6). Catalysts of microporous
zeolites (BEA and H-ZSM-5), mesoporous ordered materials
(Al-MCM-41) and hybrid zeolitic-mesostructured materials
(HZM) show high selectivity with low conversion for anisole.
While metal organic framework catalysts (Cu-MOF-74, HKUST-1)
show both high conversion and selectivity, the preparation
process is complicated and the cost is high. In addition, nitro-
benzene was used as solvent in the reaction process, and the
reaction temperature was higher than that in our research. The
catalytic activity of our ZnCu3Ox catalyst is good and exhibits
92.8% anisole conversion and 97.0% P-MAP selectivity without
solvent at 100 1C.
Fig. 8 (a) Effect of calcination temperature: anisole 5.9 ml, acetyl chloride
1.3 ml, catalyst 0.15 g, 100 1C, 6 h. (b) Effect of temperature: anisole 5.9 ml,
acetyl chloride 1.3 ml, catalyst 0.15 g (calcined at 650 1C), 6 h. (c) Effect of
catalyst amount (calcined at 650 1C): anisole 5.9 ml, acetyl chloride 1.3 ml,
100 1C, 6 h. (d) Effect of time: anisole 5.9 ml, acetyl chloride 1.3 ml, catalyst
0.15 g (calcined at 650 1C), 100 1C.
catalyzing the acylation, which means 100 1C is determined to
be the optimal reaction temperature. A series of experiments on
the amount of the catalyst (mass of catalyst/mass of substrate)
were also carried out, and the results are shown in Fig. 8c. As
the mass ratio increases, the conversion of anisole increases at
first and then decreases. This is because when catalytic activity
is maximized, too much catalyst accumulates and blocks the
original active sites. As expected, with extending the reaction
time, the conversion of anisole increases until a stable state is
reached, which indicates that the reaction has reached equili-
brium at 6 h (Fig. 8d). Therefore, 2.0 wt% of catalyst amount
(0.15 g) and 6 h of reaction time were chosen. Under these
optimal conditions, the conversion of anisole reaches 92.8%
and the selectivity of p-MAP can reach 97.0%.
Reaction mechanism
Based on the above experimental data and reports in the related
literature,46 we propose a feasible reaction mechanism for
anisole acetylation (Scheme 1). As mentioned above, completely
dispersed Zn2+ in the catalyst enhances the interaction of
copper oxide in the crystal lattices and promotes the transfor-
2+
mation of CuT to CuO2+, so that more acetyl chloride mole-
cules are adsorbed on acidic sites of the catalyst, which are
2+ 39
provided by the Lewis acid CuO
.
The in situ-generated
chloride is adsorbed onto the outer surface of the catalyst, so
that the surface of the catalyst can provide a negative charge.
A similar process was also proposed by Reza Khalifeh’s group.
Then, the p electron in the carbon–carbon double bond (CQC)
on the aromatic ring acts as a nucleophile to attack the acyl
Catalyst reusability
To further investigate the performance of the catalysts, we tested cation and form a cyclohexadienyl carbon cation intermediate.
repeated use of the ZnCu3Ox catalyst. After each reaction, the Finally, the negatively charged catalyst attracts the intermediate,
reaction solution was poured into a centrifuge tube and centri-
fuged to obtain the catalyst. Without drying and washing, the
catalyst was washed with anisole into a round-bottomed flask to
continue the reaction, and the activity was almost unchanged
Table 6 Comparison of the activity of the Cu–Zn composite oxides with
the reported catalysts44,45
after recycling three times (Table 5). After this, the catalyst was
washed and calcined, and then it can be reused again (Table 5,
entry 3). Fig. S3 (ESI†) shows that after three cycles, the structure
of the catalyst (after calcination) hardly changes. Moreover, the
concentrations of zinc and copper ions in each filtered reaction
solution were measured by ICP-OES, and no data were obtained
because the values are lower than the detection limit. This result
shows that the amount of Cu and Zn species leaching from the
initial catalyst was negligible after each cycle.
Anisole
conversion (%)
p-MAP
Catalyst
T (1C)
t (h)
selectivitya (%)
Cu-MOF-74
HKUST-1
BEA
H-ZSM-5
Al-MCM-41
HZM
120
120
120
120
120
120
100
5
5
5
5
5
3
6
99.0
94.5
85.0
57.0
48.0
22.5
92.8
98.0
97.0
99.0
99.0
98.0
98.0
97.0
Cu–Zn
a
p-MAP = p-methoxyacetophenone.
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