Efficient aldol condensation by using modified CaO as solid-base catalysts

Article abstract of DOI:10.1007/s11164-011-0430-8

A new type of solid-base catalyst for aldol condensation reaction was prepared by modifying commercial CaO with benzyl bromide in a simple way. It was found that modified CaO can effectively catalyze the aldol condensation of acetophenone and benzaldehyde to produce chalcone with a high conversion and good selectivity. The catalyst gave a higher yield (90.5%) of chalcone than commercial CaO. The high catalytic activity and stability of this catalyst was related to the organic modifier with a hydrophilic functional group that improved the diffusion of grease to the catalyst surface and prevented its hydration. The influence of several reaction parameters, such as temperature, catalyst loading and the moisture absorption rate of modified CaO, was investigated. From the results, the basic centers of modified CaO are stable and hardly poisoned by CO2 unlike commercial CaO. The catalyst was completely recyclable without significant loss in activity up to five reaction cycles. Moreover, this catalyst showed a promising future in providing an environmentally clean process for the industrial sector. Springer Science+Business Media B.V. 2011.

Full text of DOI:10.1007/s11164-011-0430-8

Res Chem Intermed (2012) 38:937–946
DOI 10.1007/s11164-011-0430-8
Efficient aldol condensation by using modified CaO
as solid-base catalysts
•
Ying Tang Gang Chen Yong Lu
•
Received: 20 August 2011 / Accepted: 27 October 2011 / Published online: 12 November 2011
Ó Springer Science+Business Media B.V. 2011
Abstract A new type of solid-base catalyst for aldol condensation reaction was
prepared by modifying commercial CaO with benzyl bromide in a simple way.
It was found that modified CaO can effectively catalyze the aldol condensation of
acetophenone and benzaldehyde to produce chalcone with a high conversion and
good selectivity. The catalyst gave a higher yield (90.5%) of chalcone than com-
mercial CaO. The high catalytic activity and stability of this catalyst was related to
the organic modifier with a hydrophilic functional group that improved the diffusion
of grease to the catalyst surface and prevented its hydration. The influence of several
reaction parameters, such as temperature, catalyst loading and the moisture
absorption rate of modified CaO, was investigated. From the results, the basic
centers of modified CaO are stable and hardly poisoned by CO2 unlike commercial
CaO. The catalyst was completely recyclable without significant loss in activity
up to five reaction cycles. Moreover, this catalyst showed a promising future in
providing an environmentally clean process for the industrial sector.
Keywords Aldol condensation ꢀ CaO ꢀ Surface modification
Introduction
In the fine chemical industry, the aldol condensation of acetone is an important
reaction involving carbon–carbon bonds formation for synthesis of a variety of
organic compounds [1–3]. This kind of reaction can take place catalytically in the
Y. Tang (&) ꢀ G. Chen
College of Chemistry and Chemical Engineering, Xi’an Shiyou University, Xi’an, Shaanxi, China
e-mail: tangying78@xsyu.edu.cn
Y. Lu
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry,
East China Normal University, Shanghai 200062, China
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Y. Tang et al.
presence of a strong base or acid in liquid phase [4–6]. However, high operating
costs and serious environmental issues associated with product separation,
purification, corrosion, and waste generation have attracted great efforts toward
the development of processes mediated by heterogeneous catalysts. Solid-base
catalysts constitute a class of heterogeneous catalysts that are capable of catalyzing
the C–C bond-forming reactions. Different families of solid bases have been found
active in this reaction, including alkali-exchanged zeolites [1], ion-exchange resins
[2, 3], alumina-supported hydroxide [7, 8], Mg–Al hydrotalcites [9, 10], and
alkaline-earth metal oxides [11, 12]. In particular, alkaline-earth metal oxides have
been regarded as the most potential catalysts for industrial application for their
strong basicity and low cost, especially for CaO. However, this solid base has not
widely reached the industrial scene because of technical aspects concerning the
stability of these catalysts upon exposure to moist air. The basic sites of alkaline-
earth metal oxides or hydrotalcite-like samples surface can be easily poisoned by air
due to the adsorption of CO2 and H2O on the surface [13–15]. Furthermore, the
single contact between reagents and catalysts causes the catalyst to be less active
than a homogeneous strong base like sodium hydroxide. Many efforts have been
taken toward the activity enhancement of such solid base catalysts by increasing
their basicity. Diez et al. [16] reported that magnesium oxide doped with 0.5 wt%
lithium formed an active catalyst for aldol condensation. The presence of Li?
correlates with an increase in the density of strongly basic sites. It should be stressed
that all operations during preparation, processing, storage, and use of the catalyst
were undertaken extremely carefully in an inert gas atmosphere to avoid contact
with air.
In this work, a high-efficiency CaO was developed by a simple method using an
organic reagent as modifier over commercial CaO. After modifying, the three-phase
reaction system will be weakened and the stability of the modified catalyst in moist
air is greatly improved. The modification conditions and various reaction conditions
as well as characterization of the samples have also been investigated.
Experimental
Modification of CaO
The surface modification of CaO was carried out by stirring 1 g commercial CaO
particles (160–200 mesh) in a certain amount of benzyl bromide (0.01–0.5%) in
2.5 mL hexane at room temperature. After 24 h, the mixture was filtrated and
washed with hexane to remove unreacted modifier, and then the modified CaO was
obtained after a vacuum drying process.
Humidity test
Commercial CaO and modified CaO particles were kept in a container containing
saturated humidity at room temperature for several days to accelerate the moisture
123

Efficient aldol condensation
939
absorption. The tested samples were weighed at regular intervals, and the moisture
absorption degree (%) was evaluated by the following equation.
56Dm
18m0
wt% ¼
ꢁ 100%
In the equation, the Dm was signed as increased weight, and the m0 as the initial
weight of the sample.
Characterizations
X-ray diffraction (XRD) patterns were collected on a D/Max-3C X-ray powder
diffractometer (Rigalcu, Japan) using a Cu-K a source fitted with an Inel CPS 120
hemispherical detector. Data were collected in the 2h range of 10–80° with an
angular step of 0.01° and a counting time of 10.4 s per step. The Fourier transform-
infrared (FT-IR) spectra were recorded by using Nicolet Nexus 670 spectropho-
tometers. Scanning electron microscope (SEM) photographs were taken by Quanta
200 SEM equipped with an energy dispersive spectrometer (Philips-FEI, the
Netherlands).
Catalytic activity measurement
Amounts of 5.0 mmol acetophenone and 7.5 mmol purified benzaldehyde were
added to 20 mL MeOH, and then 20% catalyst was added under stirring. After
reacting for 6 h at room temperature, the liquid was flitted from the mixture and
analyzed by gas chromatography (Agilent 6890) using a FID detector and HP-5
column. The yield and selectivity were calculated by area normalization method on
a carbon basis and the carbon balances within 100 5%.
Results and discussion
The loading of modifier, benzyl bromide, on CaO was investigated for moisture
absorption and catalytic activity for aldol condensation. Too much modifier will
lead to the catalytic activity decreasing for the occupation of active sites of CaO,
while less modifier is insufficient for the formation of the hydrophobic layer over
the catalyst surface. On the other hand, too much benzyl bromide will lead to a large
amount of CaBr2 forming over the CaO surface, which is highly water-absorptive
and decreases the resistance to moisture of the modified CaO. Humidity tests were
carried out for various coated products with modifier amounts varying from 0.01 to
0.5%; the unmodified sample was also included as a comparison. The tested samples
were kept in a container at saturated humidity condition at room temperature to
accelerate the moisture absorption. The weight of each CaO sample modified with
various amounts of benzyl bromides at different times are shown in Fig. 1. It was
noted that the moisture absorption continues rising for all samples at different time
periods. For the commercial CaO particles, the moisture absorption degree reached
nearly 100% within 50 h, while for the modified CaO, only a slight weight increase
123

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Y. Tang et al.
100
80
60
40
20
0
0.5% modifier
0.2% modifier
0.1% modifier
0.01% modifier
commercial CaO
0
20
40
60
80
100 120 140 160 180 200
time(h)
Fig. 1 Moisture absorbing rate over commercial CaO and modified CaO with various benzyl bromides
CaO
Modified CaO
4000
3500
3000
2500
2000
1500
1000
500
Wave number (1/cm)
Fig. 2 IR pattern of commercial CaO and modified CaO
can be found over the same period. It is obvious that the surface modification can
improve the moisture resistance of commercial CaO effectively.
Figure 2 shows the IR spectra of commercial CaO and modified CaO. The bands
at 1,621 and 3,460 cm-1 are associated with the adsorbed water. The spectra display
bands at 867 and 1,477 cm-1 correspond to vibration modes of mono and bidentate
carbonates. This is also evidenced from the characteristic absorption of C=O
between 2,000 and 1,500 cm-1, which indicates the presence of calcium carbonate
in the two samples. The vibration of aromatic carbon double-bond bands causes an
increase of intensity at 1,590 cm-1. The sp3 hybridized C–H stretching vibration at
2,800–3,000 cm-1 and bending at 1,440 cm-1 [17] of modified CaO indicates the
successfully chemical bonding of the modifier and CaO [18].
123

Efficient aldol condensation
941
The TG/DSC thermogram profiles for the modified CaO sample is illustrated in
Fig. 3. As shown in Fig. 3, no obvious peak corresponding to the elimination of the
surface loosely held water, at a temperature below 473 K, was observed in the
thermal analysis profiles, suggesting that the surface physisorbed water was
removed during the course of drying. In addition, two steps in the TG curve between
the temperature range from 400 to 700 °C were due to the loss of H2O and CO2
remaining in the form of CaO. Furthermore, the DSC curve of the modified CaO
presents two broad peaks at 450 and 700 °C corresponding to the decomposition of
Ca(OH)2 and CaCO3 due to the hydration and carbonation of CaO [19], which
moves to higher temperature ranges compared to commercial CaO reported by
Ngamcharussrivichai et al. [20]. The results suggest that the thermal stability of
CaO is enhanced after modifying. By comparing with the commercial samples, the
percentage of water loss of 2.414% in modified CaO is the same as in commercial
CaO samples. Since the percentage of water weight loss is directly correlated to the
amounts of Ca(OH)2, these results suggest that Ca(OH)2 formed during the
modifying procedure but there is relatively little storage. Furthermore, the step
and peak in the TG/DSC thermogram profiles corresponding to decomposition of
modifier do not appear, suggesting the amount of modifier over the CaO surface is
too small.
The optimized reaction condition of the amount of catalyst was investigated over
commercial CaO after reacting for 6 h at room temperature with acetophenone/
benzaldehyde molar ratio of 1:1.5 (Table 1). With a low CaO amount (\10%),
relatively low conversion of acetophenone (60.2%) was observed. On increasing the
catalyst amount to 20%, conversion of acetophenone increased up to 75.7% with
99.1% selectivity of chalcone (1,3-diphenyl-2-propen-1-one). Too much catalyst
produces a large number of basic sites, which lead to the selectivity of chalcone
decrease with the formation of 1,3,5-triphenylpentan-1,5-dione by Michael addition
[20]. The Michael reaction is usually considered to require a strong basicity. The
effect of reaction temperature on the conversion of acetophenone was investigated
20
10
9
8
7
6
5
DSC(mV/mg)
0
-10
-20
-30
-40
-50
TG(%)
100
200
300
400
500
600
700
800
Temperaure [oC]
Fig. 3 TG/DSC thermogram of modified CaO
123

942
Y. Tang et al.
Table 1 Effects of the amount of catalyst on yield and selectivity for condensation of acetophenone
using commercial CaO as a catalyst
Catalyst (wt%)
Yield of acetophenone (%)
Selectivity to chalcone (%)
5
10
20
40
60
60.2
68.5
75.7
72.3
70.6
99.4
98.9
99.1
98.9
99.5
The reactions were performed with 5.0 mmol acetophenone, 7.5 mmol purified benzaldehydein methanol
(20 mL) at room temperature for 6 h
Table 2 Effects of reaction temperatures on yield and selectivity for condensation of acetophenone
using commercial CaO as a catalyst
Temperature (°C)
Yield of acetophenone (%)
Selectivity to chalcone (%)
30
40
50
60
65
73.0
74.4
76.2
80.8
77.4
99.1
99.5
98.9
99.4
98.8
The reactions were performed with 5.0 mmol acetophenone, 7.5 mmol benzaldehydein methanol
(20 mL) and catalyst (20%) for 6 h
with a 20% amount of catalyst, and the results are shown in Table 2. Conversion
of acetophenone increases as the reaction temperature rises, and reaches the highest
value of 80.8% at the reaction temperature of 60 °C, while the selectivity to
chalcone increased slightly along with the conversion. Further increases of the
temperature cause a slight decrease of the yield and selectivity. To assess the
influence of the molar ratio on the conversion of acetophenone, the acetophenone/
benzaldehyde ratio was varied from 1:1.1 to 1:2.0 at 60 °C (Table 3).
The conversion of acetophenone was 72.1% with an acetophenone/benzaldehyde
ratio of 1:1.1. The conversion of acetophenone enhances as the acetophenone/
benzaldehyde ratio increases, and the ratio of 1:1.5 results in a conversion as high
as 80.8%. In comparison, Lopez et al. [21] obtained the conversion of 80%
over KF/Al2O3 at an extremely high reaction temperature of 473 K with an
acetophenone/benzaldehyde ratio of 1:1.
In our work, the influence of the substitutes of aldehydes on the conversion
and the selectivity was also tested using modified CaO as catalyst (Table 4).
The p-nitrobenzaldehyde, o-nitrobenzaldehyde and 2-furaldehyde (with strong
electron-withdrawing groups, entries 2, 3, and 10) involved reactions giving close to
100% conversion of acetophenone but poor selectivity to chalcone, while aldehydes
with relatively weak electron-withdrawing groups, such as p-chlorobenzaldehyde,
o-chlorobenzaldehyde, and naphthaldehyde, lead to high yields of chalcone
123

Efficient aldol condensation
943
Table 3 Effects of the acetophenone/benzaldehydein ratio on yield and selectivity for condensation of
acetophenone using commercial CaO as a catalyst
Acetophenone/benzaldehydein ratio
Yield of acetophenone (%)
Selectivity to chalcone (%)
1:1.1
1:1.2
1:1.3
1:1.5
1:1.8
1:2.0
72.1
73.8
77.2
80.8
78.6
77.9
98.4
98.1
98.8
99.4
98.8
98.3
The reactions were performed with catalyst (20%) at 60 °C for 6 h
(conversion of acetophenone 9 selectivity to chalcone, entries 4, 5, and 9). On the
other hand, the reactions of p-methoxybenzaldehyde, p-methybenzaldehyde, and
piperonyl aldehyde (with electron-donor groups, entries 6, 7, and 8) give moderate
conversion of acetophenone. In general, electron-withdrawing groups of benzalde-
hyde can activate the aldehyde group leading to high conversion. However, with the
basicity of the catalyst as high as CaO, the selectivity to chalcone decreases due to
more side reactions. So, the relative weak electron-withdrawing groups lead to the
highest yields.
The catalytic data over commercial CaO and modified CaO with different
amounts of benzyl bromide at the reaction temperature of 60 °C with an
acetophenone/benzaldehyde molar ratio of 1:1.5 are displayed in Table 5. From
the results, it can be seen that the conversion of acetophenone over modified CaO is
much higher than that of commercial CaO, and the catalytic activity increases with
the amount of modifier. The highest yield of chalcone (90.5%) was obtained over
modified CaO with 0.2% benzyl bromide. The main reason for the high activity for
modified CaO can be attributed to the role of the benzyl groups, which form a
hydrophobic layer over the CaO surface to improve the absorption of grease to the
catalyst surface and so increase the efficiency of basic sites over the CaO surface.
As the reaction completed, the reaction mixture was filtered by vacuum and the
catalysts were washed with 5 mL MeOH and used directly in subsequent aldol
reactions without adding any new catalyst, in order to reuse the catalyst and study
the catalyst’s stability. In each reuse, the same amounts of substrate were used. The
results showed that the catalyst essentially retained its catalytic activity without any
decrease of selectivity and yield for five cycles (Table 6).
The possible reaction mechanism of aldol condensation between acetophenone
(A) and benzaldehyde (B) over CaO is illustrated in Scheme 1. In this case, the solid
base catalysis captures a proton and promotes the rearrangement reaction of
acetophenone to form C, which is the active media of the aldol reaction. After a
classical nucleophile attacking of C to benzaldehyde (B), intermediate D is formed.
Then, the Ca2? picks off an O- from D to form a carbocation E, and the
deprotonation of E finally results in the formation of product F.
123

944
Y. Tang et al.
Table 4 Aldol condensation reaction of acetophenone with various aldehydes
Entry
Product
Conversion of
acetophenone (%)
Selectivity to
chalcone (%)
O
1
2
3
90.8
*100
*100
99.1
59.7
79.0
O
O
NO2
NO2
O
O
Cl
4
5
91.5
93.4
99.0
99.0
Cl
O
O
O
6
7
8
90.7
88.6
88.5
98.5
99.3
99.5
CH3
OCH3
O
O
O
O
9
92.5
95.0
99.5
78.5
10
O
The reactions were performed with 5.0 mmol acetophenone, 7.5 mmol purified benzaldehydein methanol
(20 mL) and catalyst (20%) at 60 °C for 6 h
123

Efficient aldol condensation
945
Table 5 Reaction results of the commercial CaO and modified CaO with different amounts of benzyl
bromides for condensation of acetophenone
Catalyst
Yield of acetophenone (%)
Selectivity to chalcone (%)
Commercial CaO
80.8
88.0
90.3
90.5
89.6
88.3
85.1
97.1
98.1
99.1
99.0
98.8
97.6
95.4
CaO ? 1% benzyl bromides
CaO ? 0.5% benzyl bromides
CaO ? 0.2% benzyl bromides
CaO ? 0.1% benzyl bromides
CaO ? 0.001% benzyl bromides
CaO ? 0.00001% benzyl bromides
The reactions were performed with 5.0 mmol acetophenone, 7.5 mmol purified benzaldehydein methanol
(20 mL) and catalyst (20%) at 60 °C for 6 h
Table 6 Recycling and reuse of modified CaO
Run
Yield of acetophenone (%)
Selectivity to chalcone (%)
1
2
3
4
5
6
90.5
90.0
89.7
89.2
88.9
88.1
99.0
98.8
98.4
97.9
97.4
96.4
The reactions were performed with 5.0 mmol acetophenone, 7.5 mmol purified benzaldehydein methanol
(20 mL) and catalyst (20%) at 60 °C for 6 h
A
B
C
B
D
-
O
O
O
O
H
-
O
O
Ca2+
O2-
Ca2+
Ca2+
Ca2+
Ca2+
O2-
O2- Ca2+
CaO
CaO
CaO
+
E
O
H
O
Ca2+
O2-
Ca2+
F
CaO
Scheme 1 Possible reaction mechanism of aldol condensation
123

946
Y. Tang et al.
Conclusion
This work developed an efficient catalyst for the aldol condensation of acetophe-
none and benzaldehyde by modifying commercial CaO with benzyl bromide.
The results showed that the conversion of acetophenone over modified CaO was
greatly enhanced, to 90.5% compared with 80.8% over commercial CaO, under the
same reaction condition. Part of the explanation for such a finding is the role of the
benzyl group which improves the adsorption of reactant to the catalyst surface and
resists the adsorption of water to ther catalyst surface. Our strategy to design a
highly efficient heterogeneous catalyst with good stability offers the possibility of
industrial application and environmentally benign carbon–carbon bond-forming
reactions.
Acknowledgments This work was financially supported by grants from Natural Science Research Plan
Projects of Shaanxi Science and Technology Department (2011JQ2014), Scientific Research Program
Funded by Shaanxi Provincial Education Department (No. 11JK0591) and the Open Founds of the
Shanghai Key Laboratory of Green Chemistry and Chemical Process.
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