The cascade synthesis of α,β-unsaturated ketones via oxidative C-C coupling of ketones and primary alcohols over a ceria catalyst

Article abstract of DOI:10.1039/c5cy01607j

We herein report the oxidative C-C coupling of ketones and primary alcohols to produce α,β-unsaturated ketones in the absence of base additives. This cascade synthetic reaction was conducted at 150 °C in 12 h using a heterogeneous CeO₂ catalyst. The conversion of acetophenone reached 74% with 89% selectivity to chalcone. A correlation between the CeO₂ crystal plane and catalytic performance is established as the catalytic activities decrease in the sequence of (110) > (111) > (100). Characterization using Raman spectroscopy, CO₂ temperature-programmed desorption (CO₂-TPD), and in situ active site-capping tests has shown that the unusual catalysis of the CeO₂ catalyst is attributed to the coexistence of basic and redox active sites. These sites synergistically catalyze the oxidation of alcohols to aldehydes and the aldol condensation to ketones. Moreover, the CeO₂ catalyst can be reused several times after calcination to remove the surface-adsorbed substances.

Full text of DOI:10.1039/c5cy01607j

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The cascade synthesis of α,βꢀunsaturated ketones via
oxidative C–C coupling of ketones and primary
alcohols over ceria catalyst
Cite this: DOI: 10.1039/x0xx00000x
Zhixin Zhang,a,b Yehong Wang,b Min Wang,b Jianmin Lu,b Chaofeng Zhang,b
Lihua Li,b Jingyang Jiang,a and Feng Wang*b
Received 00th January 201x,
Accepted 00th January 201x
DOI: 10.1039/x0xx00000x
We herein report the oxidative C‒C coupling of ketones and primary alcohols to produce α,βꢀ
unsaturated ketones in the absence of base additives. This cascade synthetic reactions were
conducted at 150 oC in 12 h using heterogeneous CeO2 catalyst. The conversion of
acetophenone reached 74% with 89% selectivity of chalcone. A correlation between CeO2
crystal plane and catalytic performance is established as the catalytic activities decrease in the
sequence of (110) > (111) > (100). The characterizations with Raman spectroscopy, CO2ꢀ
temperature programmed desorption (CO2ꢀTPD), and inꢀsitu activeꢀsiteꢀcapping tests have
shown that the unusual catalysis of CeO2 catalyst is attributed to the coexistence of basic and
redox active sites. These sites synergistically catalyze the oxidation of alcohol to aldehyde and
the Aldol condensation to ketones. Moreover, the CeO2 catalyst can be reused for several times
after calcination to remove the surfaceꢀadsorbed substances.
www.rsc.org/
homogeneously in the reaction system and the use of chromium
is environmental unfriendly. Therefore, it is desirable to
Introduction
α, βꢀUnsaturated ketone is a class of important chemicals,
which have numerous applications such as pesticides and food
additives.[1] They are traditionally prepared by the classical
Aldol condensation reaction between ketones and aldehydes.[2]
The direct CꢀC bond coupling of ketones with primary alcohols
is a more promising and economical process in synthesizing α,
βꢀunsaturated ketones. The tandem reactions combine the
oxidation (or dehydrogenation) of alcohols to aldehydes, and
the Aldol condensation of aldehydes with ketones with minimal
workꢀup and less waste production (e.g., efficiency in catalyst
utilization, without isolation of aldehyde intermediates).[3] The
tandem reactions have been recently studied using bifunctional
catalysts, which consist of precious metals with active sites for
oxidative conversion, and alkaline supports with active sites for
condensation reaction, such as the catalysts of Pd/AlO(OH),[4]
Pd/MgOꢀAl2O3,[5] Pt/CeO2,[6] or Au/AlO(OH).[7] But the
majority of the reported processes still require alkali
additives.[8]
develop an active, robust, and green catalyst for the cascade
synthesis of ketones.
In general, the oxidative dehydrogenation of alcohols is a
redox process and the Aldol condensation of aldehydes with
ketones is catalyzed by acids or bases. Therefore, the expected
catalyst for this tandem reaction is required to possess both
redox sites and acid or base sites. CeO2 has both redox and
acidꢀbase sites and has been widely used in various catalytic
reactions, which have been well reviewed by Laurence Vivier
and other researchers.[11] We and other investigators have
recently discovered that CeO2 is a highly active and selective
catalyst in the Prins condensation,[12] transformylation,[13] CO2
fixation[14] and other organic reactions[15] because of its redox
and acidꢀbase properties.
Herein, we report that CeO2 is an efficient catalyst for the
cascade synthesis of α, βꢀunsaturated ketones from ketones and
primary alcohols without base additive. Various ketones and
primary alcohols were transformed to the corresponding α, βꢀ
unsaturated ketones over CeO2. Besides, the catalyst is robust
and can be reused several times with comparable results.
Detailed studies have revealed that the performance of CeO2 in
tandem reactions is attributed to its unique catalytic properties.
The development of inexpensive and highly efficient nonꢀ
precious metal catalysts have drawn great attention in the α, β
ꢀ
unsaturated ketones synthesis, such as earlyꢀtransitionꢀmetal
nitrides (TiN, NbN, and NbCN).[9] But the preparation of these
materials involves the nitridation of the parent oxides under
fierce conditions. Recently, Chen et al.[10] used the metal oxide
CrO3 as a catalyst. However, the chromium species is dissolved
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DOI: 10.1039/C5CY01607J
was flushed with Ar (30 mL minꢀ1) for 10 min. The CO2
desorption was carried out by increasing sample temperature
from 30 to 600 oC (ramping rate: 10 oC minꢀ1) in an Ar gas flow
(30 mL minꢀ1). The gas effluents during the CO2 adsorption and
desorption were analyzed by an onꢀline mass spectrometer
(GSD320 Thermostar).
Experimental
Materials
Chemicals and oxides were of analytical grade, purchased from
Aladdin Chemicals, and used without further purification.
CeO2 preparation
Catalyst characterizations
The CeO2 sample was prepared by a conventional precipitation
method.[12] Briefly, 5.0 g of Ce(NO3)3·6H2O was dissolved in
100 mL of Milliporeꢀpurified water (18 mꢁ·cm), and the
solution was adjusted to pH = 11.0 by the addition of NH4OH
(3.4 M) under magnetic stirring at room temperature. The
resulting gel mixture was washed with pure water, dried in an
oven at 120 °C for 12 h, and calcined at 500 °C in air (50 mL
minꢀ1) for 4 h. To obtain CeO2 of different morphologies,
Ce(NO3)3·6H2O was used as the cerium source. The
preparation of different morphologies of CeO2 samples was
Powder Xꢀray diffraction (XRD) patterns were acquired using a
PANalytical XꢀPert diffractometer, applying CuꢀKα radiation at
40 kV and 40 mA. Continuous scans were collected over the 2
θ
range of 5° to 80°. Transmission electron microscopy (TEM)
and highꢀresolution TEM (HRTEM) images were recorded on a
JEOLꢀ2100F electron microscope operating at 200 kV. Raman
spectra were recorded on
a microꢀRaman spectrometer
(Renishaw) equipped with a CCD detector using a He/Ne laser
with a wavelength of 532 nm.
[12ꢀ13, 16]
previously reported by our group and other groups.
Results and discussion
Briefly, 0.868 g of Ce(NO3)3·6H2O and an appropriate amount
of NaOH (0.016–15 g) were dissolved in 5 mL and 35 mL of
deionized water, respectively. Then, these two solutions were
Catalyst screen and reaction condition optimizations
mixed in a Teflon bottle, and this mixture was stirred for 30 In a preliminary experiment, the C‒C coupling of acetophenone
min at room temperature, and finally formed a milky slurry. and benzyl alcohol was conducted over various solid oxides at
o
Subsequently, the Teflon bottle with this mixture was held in a 130 C in 24 h (Table 1, entries 1–11). The reaction did not
stainless steel vessel autoclave, which was then placed in an occur in the absence of catalyst (Table 1, entry 1). CeO2
oven at temperatures in the range of 100ꢀ180 °C for 12 h or 24 showed the highest activity with 64% acetophenone (
1)
h. After the hydrothermal treatment, the resulting precipitate conversion and 86% chalcone ( ) selectivity (Table 1, entry 2).
3
was separated by centrifugation, thoroughly washed with MgO, MnO2, and Y2O3 gave 36%, 21%, and 18%
deionized water and ethanol, followed by drying at 60 °C in air acetophenone conversions, respectively (Table 1, entries 3–5).
for 12 h.
Other oxides, such as Nb2O5, Al2O3, V2O5, ZrO2, SiO2, and
TiO2 exhibited far lower activities (Table 1, entries 6–11).
Different solvents were examined in the synthesis of
chalcone from acetophenone and benzyl alcohol (Table 1,
Catalytic reactions and product analyses
In a typical reaction, the catalyst was added into a screw capped
glass pressure vessel with a stirring bar. A solution of 58 ꢂL
acetophenone (0.5 mmol) and 78 ꢂL benzyl alcohol (0.75
mmol) in 2 mL solvent was added, and the reactor was screwed
tightly and heated to the set value in an oil bath. The products
of the reaction were analyzed by gas chromatography (GC,
Agilent 7890A) and gas chromatographyꢀmass spectrometry
(GCꢀMS) using an Agilent 7890A/5975C instrument equipped
with an HPꢀ5MS column (30 m in length, 0.25 mm in
diameter). Authenticated samples were used to quantify the
reactants and products. The calculation of conversion and
selectivity is based on acetophenone.
entries 12–18). pꢀXylene offered the highest conversion (74%)
and the best chalcone selectivity (89%) at 150 oC in 12 h (Table
1, entry 12). Although 2ꢀpropanol gave the moderate
conversion (49%), the chalcone selectivity was poor (8%)
(Table 1, entry 13). Methanol and acetonitrile gave the low
acetophenone conversions (~10%) and moderate chalcone
selectivities (~65%) (Table 1, entries 14–15). In addition, the
coupling reaction were much worse in dioxane, dimethyl
formamide (DMF), and dimethyl sulfoxide (DMSO) (Table 1,
entries 16–18). This is perhaps due to that these polar solvent
molecules strongly adsorb on catalyst via coordination to
coordinatively unsaturated metal ions (eg. (CH3)2–S=O→Ce3+,
(CH3)2–N–C=O→Ce3+)[17] and compete with polar substrates
and reaction intermediates. In addition, these molecules would
CO2 temperatureꢀprogrammed desorption (CO2ꢀTPD)
The chemisorption of CO2 on CeO2 with different
morphologies, fresh CeO2, and used CeO2 was conducted in a
Uꢀtype quartz tube. In a typical CO2ꢀTPD experiment, 35 mg of
dried sample was placed in the quartz tube and pretreated in
flowing Ar at 210 oC for 1 h. Then, the sample was cooled to 30
oC and successive pulses of CO2 were injected, using Ar (30
mL minꢀ1) as the carrier gas, until no change in the CO2
concentration was detected. After CO2 adsorption, the sample
block the redox process of CeO2.
generated large amount of 1ꢀphenylethanol (21% GC yield) and
dihydrochalcone (15% GC yield) (Table 1, entry 19), which
Reaction in Ar gas
indicates this is
a hydrogenꢀtransfer reaction between
acetophenone and benzyl alcohol. Oxygen is used to oxidize the
active surface hydrogen species. Reaction in pure oxygen was
detrimental to the conversion of acetophenone (Table 1, entry
20). This is perhaps due to the use of pure O2 gas enhances the
2 | J. Name., 2015, 00, 1-3
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o
oxidation rate of benzyl alcohol, benzaldehyde as the main discussion above, 12 h at 150 C in pꢀxylene under air were
oxidation product would accumulate and would be oxidized to chosen as the suitable reaction conditions for further
benzoic acid, and benzoic acid would poison the basic sites of experiments.
CeO2.
The following optimization reactions focused on CeO2
catalyst in order to achieve high yield of chalcone. Fig. 1 shows
the catalytic results at temperatures of 40–150 oC. The
100
conversion of acetophenone increased from 5% to 74% with the
80
increase of temperature. Simultaneously, the selectivity of
chalcone decreased from 100% to 89%. Dihydrochalcone (
4)
60
40
20
0
and 1ꢀphenylethanol ( ) as byproducts were generated at high
5
conversion of
1
temperature, indicating transfer hydrogenation reactions take
place. Similar results have been obtained by several studies.[8c,
selectivity of
selectivity of
selectivity of
3
4
5
8f, 8g]
Table 1 C‒C coupling of acetophenone and benzyl alcohol over
40
60
80
100
120
140
160
various oxides a
Reaction temperature (oC)
Fig. 1 Effect of reaction temperature on the catalytic performance of
CeO2 in the C‒C coupling of acetophenone and benzyl alcohol.
Reaction conditions:
mL), 24 h, air.
1 (58 ꢂL), 2 (78 ꢂL), CeO2 (100 mg), pꢀxylene (2
Sel.(%)
Conv.
(%)b
0
Entry
Catalyst
Solvent
3
ꢀ
4
ꢀ
5
ꢀ
1
2
ꢀ
p
ꢀxylene
ꢀxylene
ꢀxylene
ꢀxylene
ꢀxylene
ꢀxylene
ꢀxylene
ꢀxylene
ꢀxylene
ꢀxylene
ꢀxylene
ꢀxylene
CeO2
MgO
MnO2
Y2O3
Al2O3
Nb2O5
V2O5
ZrO2
SiO2
p
p
p
p
p
p
p
p
p
p
p
64
36
21
18
2
86
90
100
45
100
ꢀ
5
ꢀ
8
100
80
3
10
ꢀ
4
ꢀ
5
ꢀ
55
ꢀ
6
ꢀ
60
7
4
ꢀ
100
ꢀ
8
0
ꢀ
ꢀ
conversion of
1
40
9
0
ꢀ
ꢀ
ꢀ
selectivity of 3
selectivity of 4
selectivity of 5
filtration of the catalyst
10
11
12 c
13 d
14 d
15 d
16 d
17 d
18 d
19 e
20 f
0
ꢀ
ꢀ
ꢀ
TiO2
CeO2
CeO2
CeO2
CeO2
CeO2
CeO2
CeO2
CeO2
CeO2
0
ꢀ
ꢀ
ꢀ
20
74
49
11
10
8
89
8
5
ꢀ
6
2ꢀPropanol
Methanol
Acetonitrile
Dioxane
DMF
92
33
36
100
ꢀ
0
0
2
4
6
8
10
12
67
64
ꢀ
ꢀ
Reaction time (h)
ꢀ
ꢀ
Fig. 2 Reaction profiles for the C‒C coupling of acetophenone and
benzyl alcohol over CeO2. Reaction conditions: (58 ꢂL), (78 ꢂL),
CeO2 (100 mg),
ꢀxylene (2 mL), 150 oC, air.
0
ꢀ
ꢀ
1
2
DMSO
0
ꢀ
ꢀ
ꢀ
p
p
p
ꢀxylene
ꢀxylene
64
51
45
89
23
4
32
7
a Reaction conditions:
1
(58 ꢂL),
2
(78 ꢂL), catalyst (100 mg), solvent (2 mL),
Coupling of various ketones and primary alcohols
130 oC, 24 h, 1 atm air. The conversion was based on acetophenone,
determined by GC with the internal standard method. c150 oC for 12 h,
isolated: 44 %.d 150 oC for 12 h. e 150 oC for 6 h in Ar. f 150 oC for 6 h in
oxygen.
b
In order to explore the substrate scope and functional group
tolerance, we extended the CeO2 catalyzed C‒C coupling
protocol to a wide range of ketones and primary alcohols (Table
2). We employed two different strategies to explore the
efficiency of the CeO2 for the synthesis of diverse α, βꢀ
unsaturated ketones. In the first strategy, various substituted
benzyl alcohols were allowed to react with acetophenone.
Substituted benzyl alcohols bearing an electronꢀdonating or ꢀ
withdrawing group reacted with acetophenone to afford the
corresponding α, βꢀunsaturated ketones in moderate
conversions (> 57%) and selectivities (> 64%) (3a, 3b, 3c). For
o
Optimization of the reaction time was conducted at 150 C. As
can be seen from the Fig. 2, the chalcone selectivity was above
84% throughout the reaction. The acetophenone conversion
rapidly increased to 73% after 6 h and leveled off at 74% after
12 h, probably due to the strong adsorption of organic species
on CeO2.[18] CeO2 behaves as a truly heterogeneous catalyst, as
proved by the hot filtration test (Fig. 2). According to the
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the
reaction is inhibited (3d) (11% acetophenone conversion), removed by calcination. Therefore, before any successive use,
because the condensation of acetophenone
with the catalyst is calcined at 300 oC to remove the adsorbed
pꢀnitrobenzyl alcohol, however, the desired coupling reaction intermediate adsorbed on CeO2 surface can be
nitrobenzaldehyde is much more difficult than benzaldehyde organic species. The calcined CeO2 can be reused for several
(Supporting information, Table S1), which causes the times (Fig. 3). Although the catalytic activity of CeO2 gradually
accumulation of intermediates and seems to be detrimental to decreased slightly during such repeated uses. The possible
the coupling reaction. In the second strategy, the benzyl alcohol reason is that the specific area of the CeO2 gradually decreased
remained unchanged and was allowed to react with various slightly during repeated calcination processes. These results
ketones, such as
cyclic ketones. The conversions of
are in a medium level (> 55%) (3e 3f
ꢀnitroacetophenone (3h) (27% conversion). In the presence of
ꢀnitroacetophenone, benzaldehyde yield was remarkably
p
ꢀsubstituted acetophenone, alkyl ketones, and indicate CeO2 is a robust heterogeneous catalyst for this
ꢀsubstituted acetophenone reaction.
3g) with the exception of
p
,
,
Effects of the CeO2 crystal planes
p
p
The relationship between the catalytic activity and the
morphology (rod, octahedron, and cube)/crystal planes of
nanostructured CeO2 was investigated by performing the C‒C
coupling reaction. CeO2 samples with different morphologies
were synthesized according to procedures previously reported
in the literature.[12ꢀ13, 16] The Xꢀray diffraction (XRD) patterns
(Fig. S2) confirm the formation of ceria based on the diffraction
improved due to the oxidation of benzyl alcohol. When acetone
was used as a substrate, the conversion of acetone reached 76%
(
3i) and dibenzylideneacetone was produced as the byproduct.
For other alkyl ketones and cyclic ketones, such as 2ꢀ
heptanone, cyclohexanone, and 1ꢀtetralone, the desired
products were obtained but in low conversions (23% ~31%)
(3j, 3k, 3l).
peaks observed at 2θ
= 28.7o, 33.2o, 47.7o and 56.6o (JCPDS
65ꢀ2975), corresponding to the (111), (200), (220) and (311)
planes of a CeO2 fluoriteꢀtype structure, respectively, with no
significant impurities observed. According to highꢀresolution
transmission electron microscope (HRTEM) images (Fig. S3),
these samples have different exposed crystal plane: (110) and
(100) for rods, (111) for octahedra, and (100) for cubes. This
result is in good agreement with Yan et al.[19] The chalcone
yield over the rod sample is 48% (Table 3, entry 1), whereas the
octahedron sample and the cube sample only had 6% and 2%
chalcone yield, respectively (Table 3, entries 2–3). In
consideration of the BET surface area of CeO2, the generation
rate of chalcone per added surface area per hour (yield) was
also calculated and compared. It can be found that the
generation rate of chalcone over the rod sample is 3.953 ꢂmol
m–2 h–1, which is about 3ꢀfold and 4ꢀfold of octahedron (1.190
ꢂmol m–2 h–1) and cube (0.926 ꢂmol m–2 h–1), respectively. The
sequence of catalytic activity (rod > octahedron > cube) and the
HRTEM result indicated that the (110) crystal plane of CeO2
nanorods is the most active one for this reaction in these crystal
planes.
Table 2 Scope of the coupling reaction of ketones with alcohols
catalyzed by CeO2
a
3a conv. / sel.:
68% / 81%
(IY 33%)b
3c 66% / 64%
3f 55% / 88%
3b 57% / 79%
3e 55% / 87%
3d 11% / 100%
3g 69% / 77%
3h 27% / 58%
3k 27% / 100%
3i 76% / 84%
3l 23% / 80%
3j 31% / 88%
a Reaction conditions:
1 (0.5 mmol), 2 (0.75 mmol), CeO2 (100 mg), pꢀxylene (2
mL), 150 oC, in 12 h, air. The conversions based on ketones consumption were
determined by GCꢀMS. b Yield of isolated product.
Catalyst Stability
The recyclability of the CeO2 catalyst was investigated. After
reaction, spent CeO2 was recovered by centrifugation, washed
with ethanol, dried, and then used for a subsequent cycle. As
depicted in Fig. 3, the fresh catalyst reached 66% yield of
chalcone. However, deactivation of the catalyst (21% yield),
with a color change from pale yellow to grey yellow, was
observed in the first reuse. The possible reason of catalyst
deactivation is that the reaction intermediate species blocks
active sites on the catalyst surface. Accordingly, TPOꢀMS
analysis indicates the obvious peaks of COx (x = 1, 2) appearing
at around 255 oC (Figure S1). This test suggests that the
4 | J. Name., 2015, 00, 1-3
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90
80
70
60
50
40
30
20
10
0
1-phenyl ethanol (5)
dihydrochalcone (4)
chalcone (3)
315
88
CeO2-rod
CeO2-octahedron
CeO2-cube
Fresh
1st*
1st
2nd
3rd
50 100 150 200 250 300 350 400 450 500 550
Temperature (oC)
Catalyst reuse cycle
Fig. 3 Catalyst recycling for the C‒C coupling of acetophenone and
benzyl alcohol over CeO2. Reaction conditions: (58 ꢂL), (78 ꢂL),
CeO2 (100 mg), catalyst (100 mg), ꢀxylene (2 mL), 150 C, 12 h, air.
1
2
o
Fig. 4 CO2ꢀTPD profiles of the CeO2 with different morphology.
p
1st* was the first run of CeO2 without calcination. 1st was the first run
after calcination.
Furthermore, the effect of crystal plane on the basic properties
of the CeO2 catalysts was investigated by the CO2ꢀTPD
technique. The CO2ꢀTPD peaks representing the base strength
distribution of CeO2 catalysts with different morphologies are
presented in Fig. 6. In general, the numbers of weak, medium,
and strong catalyst basic sites are estimated from the CO2
desorption peak area under their TPD curves for the
The formation energy of surface oxygen vacancy is a key
parameter for the oxidation reactions.[20] DFT theoretical
studies indicate that the formation energies of ceria surface
oxygen vacancies in the (110) surface are the lowest, followed
by (100) and (111),[21] which is in agreement with the results
that the (110) plane exhibited the highest activity. In addition,
the concentration of surface oxygen vacancy was also
determined by Raman spectroscopy in the present study (Fig.
S4). Two bands were observed in these three samples, which
were ascribed to the Ramanꢀactive F2g vibrational mode of the
CeO2 fluoriteꢀtype structure (the intense band at 462 cm‒1)[22]
and surface oxygen vacancy sites (the weak band at 590 cm‒
1),[23] respectively. The ratio of the integrated peak areas under
the bands at 590 cm‒1 and 462 cm‒1 (A590/A462) can be used to
quantify the relative concentration of surface oxygen vacancy
sites.[24] The calculated values were listed in the Table 4. The
tendency of the A590/A462 ratio matches well with the catalytic
performance of these CeO2 samples.
o
temperature range of 100 ~ 250, 250 ~ 400, and >400 C.[25]
According to Fig. 6, the CO2ꢀTPD profile for CeO2ꢀrod showed
two peaks centred around 88 oC and 315 oC, which were
attributed to the weak and medium basic sites, respectively.
However, there was no obvious CO2 desorption peak for the
CeO2ꢀoctahedron and CeO2ꢀcube samples in the range of 50‒
o
550 C. In detail, the distribution of the number of basic sites
for the CeO2 catalysts are listed in Table 4. As revealed in Fig.
6 and Table 4, the different distribution of basic sites for each
catalyst indicates that the basicity and base strength distribution
are influenced by the morphology of the CeO2 catalysts and the
(110) places shows a large number of weak and medium basic
sites as compared to the (111) and (100) places.
Table 3 C‒C coupling of acetophenone and benzyl alcohol over
Table 4 Concentration of oxygen vacancy and basic sites of CeO2
with different morphology
different crystal planes of the CeO2 catalyst a
Basic sites (ꢁmol/g)b
Chalcone
a
b
S (m2 g–1
)
Entry
Catalyst
A595/A462
Entry
Morphology
Crystal plane
yield. (%)c
weak
659
0
medium
total
1339
0
1
2
3
CeO2ꢀrod
CeO2ꢀoctahedron
CeO2ꢀcube
0.077
0.003
0.001
680
0
1
2
3
rod
octahedron
cube
(110)/(100)=2/1
(111)
86
21
9
48 (3.953)d
6 (1.190)
0
0
0
(100)
2 (0.926)
a
A595/A462 means intrinsic oxygen vacancy concentration of CeO2 samples based
a
Reaction conditions:
1
(58 ꢂL),
2
(78 ꢂL), CeO2 (100 mg),
p
ꢀxylene (2 mL), 150
b
on Raman spectroscopy. Determined by CO2ꢀTPD expressed in micromoles CO2
oC, for 12 h, under air. b S = BET surface area. c Yield = conversion
×
selectivity, the
desorbed per gram catalyst.
d
conversions based on acetophenone consumption were determined by GC. Date in
parenthesis indicated the molar product per reaction time (h) per added surface area
of CeO2 (unit: ꢂmol m–2 h–1).
To sum up, the (110) crystal plane is considered as the most
active plane of CeO2 due to its substantial enhanced redox and
basic properties.
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DOI: 10.1039/C5CY01607J
Table 5 Effect of capping agent on the catalytic performance of
species occurs on the redox sites of CeO2 catalyst. Benzyl
alcohol removes the lattice oxygen, leading to the formation of
oxygen vacancy, then the oxygen vacancy is replenished with
molecular oxygen through oxygen mobility.
CeO2 in the CꢀC coupling of acetophenone with benzyl alcohol. a
Capping agent
(0.58 mmol)
ꢀ
Sel. (%)
Entry
Conv. (%)b
3
96
99
ꢀ
4
2
ꢀ
5
1
2
3
39(43)c
37(31)c
0(11)c
2
Pyridine
1
a
Table 6 Benzyl alcohol oxidation over CeO2
Benzoic acid
ꢀ
ꢀ
a
Reaction conditions:
1
(58 ꢂL),
2
(78 ꢂL), CeO2 (100 mg),
p
ꢀxylene(2 mL), 100
b
oC, for 12 h, under Air. The conversions based on acetophenone consumption
c
were determined by GCꢀGS. Date in parenthesis indicated the conversion of
Sel. (%)
benzyl alcohol.
Entry
Catalyst
Conv. (%)b
6
7
1
2
Fresh CeO2
22
92
8
c
Used CeO2
29
92
8
a Reaction conditions:
2
(0.5 mmol), Catalyst (100 mg), p
ꢀxylene (2 mL), 150 oC, for 5
In situ capping tests
h, 1 atm air. b The conversions and selectivity based on benzyl alcohol consumption
were determined by GC. c The used CeO2 was recovered by centrifugation, washed with
ethanol for three times, and dried at 100 oC.
In order to make clear the influence of the surface acidꢀbase
properties of CeO2 for the coupling reaction, the inꢀsitu acidꢀ
base sites capping experiments have been performed. Usually,
pyridine strongly reacts with the acidic sites of the catalyst,[26]
while benzoic acid poisons the basic sites.[27] In the present
work, the CꢀC coupling of acetophenone and benzyl alcohol
was performed in the presence of pyridine or benzoic acid. The
conversion was almost the same with or without the addition of
pyridine. (37% in Table 5, entry 2 vs. 39% in Table 5, entry 1).
In contrast, the addition of benzoic acid completely suppressed
the reaction (Table 5, entry 3). These results indicate that the
basic sites of CeO2 are the catalytically active center for the
reaction. Otherwise, the conversions based on the benzyl
alcohol consumption were also showed in the column of
conversion (Table 5). In the presence of benzoic acid (0.58
mmol), the aerobic oxidation of benzyl alcohol was
constrained.
For the Aldol condensation of acetophenone with
benzaldehyde over basic sites of CeO2, the organic species
produced by benzaldehyde oxidation would blocks basic sites
on the basic sites. CO2ꢀTPD of used CeO2 (CeO2ꢀR) and fresh
CeO2 (CeO2ꢀF) (See Fig. 5) proved this point. In addition, the
deposited organic species desorption were confirmed by
comparison between TPD of CeO2ꢀR and CO2ꢀTPD of CeO2ꢀR.
As depicted in Fig. 5, the peaks around 356 and 522 C were
attributed to the organic species desorption. The concentration
of basic sites for the CeO2ꢀF and CeO2ꢀR were calculated based
o
o
on the desorption peak of CO2 around 120 C, and the values
were 85.6 ꢂmol/g and 24.8 ꢂmol/g, respectively. We think the
concentration difference (60.8 ꢂmol/g) between this two values
could reflect the amount of deposited organic species.
Calcination is a common manner of catalyst regeneration. The
organic species which blocks basic sites on the catalyst surface
would be removed by calcination and the catalyst would be
recovered. Thus, we suppose that the CꢀC coupling reaction
was a catalytic reaction over CeO2.
Reaction mechanism
In this study, CeO2 acted as bifunctional catalyst and played
two roles: oxidative dehydrogenation of alcohol and Aldol
condensation of benzaldehyde with acetophenone. In the first
step, although the alcohol is oxidized by lattice oxygen, the
CeO2 acts as catalyst. Because if the benzyl alcohol oxidation
was conducted in the absence of oxygen, the conversion of
benzyl alcohol was below 1 %. In addition, the recycling of
CeO2 without calcination in the benzyl alcohol oxidation also
proved this point (See Table 6). Fresh CeO2 gave 22 % benzyl
alcohol conversion (Table 6, Entry 1). After reaction, used
CeO2 was recovered by centrifugation, washed with ethanol,
dried, and then used for a subsequent cycle. As depicted in
Table 6, Entry 2, the used CeO2 obtained 29% benzyl alcohol
conversion. It can be concluded that CeO2 worked as catalyst.
Moreover, it is generally accepted that benzyl alcohol oxidation
over ceria proceeds via Mars−van Krevelen mechanism.[28]
Tamura et al have also reported that CeO2 acts as a reusable and
efficient heterogeneous catalyst for the imine formation from
benzyl alcohol and aniline.[15a] In the first step of this reaction,
oxidative dehydrogenation of benzyl alcohol with oxygen
CO2-TPD of CeO2-F 85.6 µmol/g
24.8 µmol/g
CO2-TPD of CeO2-R
120 oC
522 o
C
TPD of CeO2-R
356 o
C
100
200
300
400
500
600
Temperature (oC)
Fig. 5 CO2ꢀTPD of CeO2ꢀF and CeO2ꢀR and TPD of CeO2ꢀR
6 | J. Name., 2015, 00, 1-3
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DOI: 10.1039/C5CY01607J
Scheme 1. Tentative reaction mechanisms for the C‒C coupling of
acetophenone and benzyl alcohol over CeO2.
Kinetic analysis of the alcohol oxidation and Aldol reaction
over CeO2 catalyst were carried out at 150 oC. The initial
concentrations of benzyl alcohol and acetophenone were 0.36
mol/L and 0.23 mol/L, respectively. The reaction rate values
are based on data from Fig. 6. and are calculated to be 2.48 ×
10ꢀ4 mol Lꢀ1 minꢀ1 and 9.79 × 10ꢀ5 mol Lꢀ1 minꢀ1, respectively.
Thus, it can be concluded that the Aldol condensation is a
slower step and could be the rate determining step.
Conclusions
In summary, we have reported an efficient C‒C coupling of
ketones and primary alcohols to produce α,βꢀunsaturated
ketones over CeO2 catalyst in the absence of alkaline additives.
Strong crystal plane effects of CeO2 on the C‒C coupling
reaction have been identified. The (110) plane of CeO2
nanorods is the most active plane for the reaction among the
three low index planes (110), (111), and (100). Raman spectra
and CO2ꢀTPD characterizations have suggested that the
catalytically active sites of CeO2 consist of surface redox and
base sites. The reaction involves the oxidation dehydrogenation
of benzyl alcohol on the redox sites and the Aldol condensation
of benzaldehyde and acetophenone on the base sites. This study
will be helpful for the understanding of catalytic reactions over
ceria catalyst.
0.36
Benzyl alcohol oxidation
= 2.48
10-4 mol L-1 min-1
0.34
0.32
0.30
0.28
0.26
0.24
0.22
0.20
0.18
0.16
γ
×
Aldol condensation
= 9.79
10-5 mol L-1 min-1
γ
×
Acknowledgements
0
50
100
150
200
250
300
This work was supported by the National Natural Science
Foundation of China (21303189, 21422308) and Dalian
Excellent Youth Foundation (2014J11JH126).
Reaction time (min)
Fig. 6 The kinetic analysis of the oxidation of benzyl alcohol
and the Aldol condensation of acetophenone with
benzaldehyde.
Reaction conditions: 1) Benzyl alcohol oxidation: benzyl
Notes and references
o
alcohol 0.75 mmol,
p
ꢀxylene 2 mL, CeO2 100 mg, 150 C, 1
a State Key Laboratory of Fine Chemicals, College of Chemistry, Faculty
of Chemical Environmental and Biological Science and Technology,
Dalian University of Technology, Dalian 116024, Liaoning, China.
b State key Laboratory of Catalysis, Dalian National Laboratory for Clean
Energy, Dalian Institute of Chemical Physics, Chinese Academy of
Sciences, Dalian 116023, Liaoning, China.
atm Air; 2) Aldol condensation: acetophenone 0.5 mmol,
benzaldehyde 0.75 mmol,
oC, Ar.
pꢀxylene 2 mL, CeO2 100 mg, 150
On the basis of the results above, a reaction mechanism for
the C‒C coupling of acetophenone and benzyl alcohol over
CeO2 catalyst can be proposed as in Scheme 1. First, the
formation of benzaldehyde by the oxidative dehydrogenation of
benzyl alcohol with oxygen species occurs on the redox sites of
CeO2 and Ce will be reduced from tetravalent (+4) to trivalent
(+3). Then, the obtained benzaldehyde reacts with
acetophenone to afford the chalcone over the basic sites of
CeO2. At last, the target product chalcone desorbs and the
active site is regenerated by calcination to remove the
chemisorbed organic species for next cycle. This possible
reaction mechanism is similar to the mechanism of imine
formation from benzyl alcohol and imine over CeO2 reported
by Tamura.[15a]
*Corresponding author. Tel: +86ꢀ411ꢀ84379762; Fax: +86ꢀ411ꢀ
84379762; Eꢀmail: wangfeng@dicp.ac.cn
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
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Graphical Abstract