Modified calcium oxide as stable solid base catalyst for Aldol condensation reaction

Article abstract of DOI:10.1007/s12039-013-0362-5

A highly efficient and stable solid-base catalyst for Aldol condensation was prepared by modifying commercial CaO with benzyl bromide in a simple way. It was found that modified CaO can effectively catalyse the Aldol condensation of cyclohexanone and benzaldehyde, as well as various benzaldehydes, to produce 2-benzylidenecyclohexanone with a good selectivity and high yield. Higher yield of 95.8% was obtained over modified CaO after 3 h, which is short compared with the yield of 92.1% after 12 h over commercial CaO. The influence of several reaction parameters, such as temperature, catalyst loading, was investigated. The humidity test over modified CaO reveals that the basic centres of modified CaO are stable for CO₂ and moisture. From the results of Fourier transform-infrared (FT-IR) and Thermogravity analysis (TG) characterization, the modifier was bonded on surface of CaO chemically and almost no Ca(OH) ₂ formed during the modification process. The type of aldehyde has great influence on the yield of aldol condensation.

Full text of DOI:10.1007/s12039-013-0362-5

c
J. Chem. Sci. Vol. 125, No. 2, March 2013, pp. 313–320. ꢀ Indian Academy of Sciences.
Modified calcium oxide as stable solid base catalyst for Aldol
condensation reaction
YING TANG∗, JINGFANG XU and XUEFAN GU
College of Chemistry and Chemical Engineering, Xi’an Shiyou University, Xi’an Shaanxi, China
e-mail: tangying78@xsyu.edu.cn
MS received 19 February 2012; revised 15 May 2012; accepted 15 June 2012
Abstract. A highly efficient and stable solid-base catalyst for Aldol condensation was prepared by modifying
commercial CaO with benzyl bromide in a simple way. It was found that modified CaO can effectively catalyse
the Aldol condensation of cyclohexanone and benzaldehyde, as well as various benzaldehydes, to produce 2-
benzylidenecyclohexanone with a good selectivity and high yield. Higher yield of 95.8% was obtained over
modified CaO after 3 h, which is short compared with the yield of 92.1% after 12 h over commercial CaO. The
influence of several reaction parameters, such as temperature, catalyst loading, was investigated. The humidity
test over modified CaO reveals that the basic centres of modified CaO are stable for CO2 and moisture. From the
results of Fourier transform-infrared (FT-IR) and Thermogravity analysis (TG) characterization, the modifier
was bonded on surface of CaO chemically and almost no Ca(OH)2 formed during the modification process.
The type of aldehyde has great influence on the yield of aldol condensation.
Keywords. Aldol condensation; CaO; surface modification.
1. Introduction
poisoned by air due to the adsorption of CO2 and
H2O on the surface.13 Furthermore, the single contact
between reagents and catalysts causes the catalyst less
active than homogeneous strong base, such as sodium
hydroxide. Many efforts have been taken towards the
activity enhancement of such solid base catalysts by
increasing their basicity. The published method for Mg–
Al hydrotalcite activation is liquid-phase reconstruc-
tion to created surface base sites with uniformly dis-
tributed strength as suggested by Ebitani.14 It should be
stressed that all operations during preparation, process-
ing, storage and use of the catalyst was taken extremely
carefully in inert gas atmosphere to avoid contacting
with air.
Important role of Aldol condensation in the reaction
involving C–C bond formation makes it necessary for
obtaining many fine chemicals of commercial interest.
This kind of reaction can catalytically undergo in the
presence of a strong base or acid in liquid phase.1–3
However, high operating costs and serious environmen-
tal issues associated with product separation, purifi-
cation, corrosion and waste generation attract great
efforts toward the development of processes medi-
ated by heterogeneous catalysts. A recent topic of
interest in the Aldol condensation is the use of solid
base catalysts. Different families of solid bases have
been found active in this reaction, including alkali-
exchanged zeolites,4 ion-exchange resins,5,6 alumina-
supported hydroxide,7,8 Mg–Al hydrotalcites9,10 and
alkaline-earth metal oxides.11,12 In particular, alkaline-
earth metal oxides and hydrotalcite have been regarded
as the most potential catalyst for industrial applica-
tion for its strong basicity and low cost, especially for
CaO. However, this solid base has not broadly reached
the industrial scene for the technical aspect about the
stability of these catalysts upon exposure to mois-
Here, the aim of this study is to develop an activated
CaO with high efficiency for the Aldol condensation
of cyclohexanone and various benzaldehydes with good
stability in moisture air. The modifying conditions and
various reaction conditions were also investigated.
2. Experimental
ture air. The basic sites of alkaline-earth metal oxides 2.1 Modification of CaO
or hydrotalcite-like samples surface are easy to be
The surface modification procedure was as follows.
Appropriate amount of commercial CaO particles
(2.8 g, 100–160 mesh, China National Medicines Co.,
∗For correspondence
313

314
Ying Tang et al.
CH2 Ph
O
O
O
+
CH2
Ph
Br
Ca
Ca
O
+
Ca
Ca
O
Catal
Br
R
+
R
Scheme 1. The reaction between commercial CaO and
benzyl bromide.
Scheme 2. Aldol condensations of cyclohexanone with
benzaldehydes.
Ltd.) were added into the benzyl bromide (Sigma-
Aldrich, AR)/methanol solution (40 mL) under stir-
ring at room temperature for activation suggested as in
scheme 1. The content of benzyl bromide was varied
from 0.01% to 0.5%. After 24 h, the mixture was sepa-
rated and washed with methanol to remove excess modi-
fier, and the modified CaO was obtained after a vacuum
drying process. In order to increase the reproducibili-
ty, the particles were sieved before use. Particles of
100–160 mesh size were selected for all experiments.
2.4 Catalysts characterization
The Fourier transform-infrared (FT-IR) spectrophoto-
meter (Nicolet Nexus 670, USA) was used to iden-
tify the surface group over the catalyst. TGA experi-
ments were carried out using Q600 SDT thermal ana-
lysis machine (TA Instruments, USA) under a flow of
air in the temperature range from 25◦C to 800◦C with a
ramping rate of 10◦C min−1.
3. Results and discussion
2.2 Humidity test
3.1 Humidity test
Amount of commercial CaO and modified CaO parti-
cles were kept in a container under saturated humidity at
room temperature for several days to allow the absorp-
tion of water on the surface. The samples were weighed
at regular intervals of time. The absorbing moisture
rate (%) of the samples was evaluated by the following
equation, using the ꢀm as increase weight, as well as
the m0 as the initial weight.
For CaO, a water-absorptive material, the poisoning
effect of H2O adsorption during catalyst preparation
process are the most important problems to be resolved.
The loading of modifier, benzyl bromide, are an impor-
tant factors for the catalyst moisture absorption rate
so to its catalytic activity for Aldol condensation.
Too much modifier will lead the catalytic activity to
decrease for the occupation of active sites on CaO sur-
face, while fewer modifiers are insufficient for the for-
mation of hydrophobic layer over catalyst surface. Fur-
thermore, the resistance to moisture of the modified
CaO will decrease due to a large amount of highly
water-absorptive CaBr2 forms when too much benzyl
bromide is used over CaO surface. Humidity tests were
carried out over various modified CaO with modifier
amount in the range from 0.01 to 0.5%. For compari-
son, the unloaded sample was also included. The tested
samples were kept in a vapour-saturated container at
room temperature to allow the hydration and carbon-
ation. The weight of each samples were measured at
regular intervals of time, and the results were shown
in figure 1. It was noted that the moisture absorption
rate continues rising for all samples along with time.
For the unmodified CaO particles, the moisture absorp-
tion rate increases rapidly and reaches to nearly 100%
within 50 h, while nearly no weight incensement can be
found over modified CaO at the same time, and 0.2%
benzyl bromide modified CaO gets the best moisture
resistance. It is obvious that the surface modification
can improve the moisture resistance of commercial CaO
effectively.
56ꢀm
w% =
× 100%
18m0
2.3 Aldol condensation activity test
As shown in scheme 2, 0.6 mmol cyclohexanone (99%)
and 0.5 mmol purified benzaldehyde (99.9%) (molar
ratio of benzaldehyde: cyclohexanone = 1.2: 1) were
added into 20 ml MeOH in a 50 ml round bottom flask
equipped with magnetic stirrer and condenser, and then
about 0.25 mmol catalysts was introduced in a con-
stant temperature. The reaction mixture was heated at
required reaction temperature ranging from 20 to 65◦C
at atmospheric pressure. After the completion of reac-
tion, the liquid was cooled and filtrated from the mix-
ture and analysed by gas chromatography using a flame
ionization detector and HP-5 capillary column of 30 m
length and 0.25 mm diameter, programmed oven tem-
perature of 50–280◦C and N2 (1.5 ml/min) as a carrier
gas. The conversion and selectivity were calculated by
area normalization method on a carbon basis and the
carbon balances are within 100 5% (scheme 2).

Modified CaO for Aldol condensation
315
100
80
60
40
20
0
9
20
10
DSC(mV/mg)
8
7
6
5
0
0.5% modifier
0.2% modifier
0.1% modifier
0.01% modifier
commercial CaO
-10
-20
-30
-40
-50
TG(%)
100
200
300
400
500
600
700
800
0
20
40
60
80
100
120
140
160
180
200
Temperaure [oC]
Time(h)
Figure 3. TG/DSC thermogram of modified CaO.
Figure 1. Moisture absorbing rate over commercial CaO
and modified CaO with various benzyl bromides.
evidence of the phenyl groups, which suggesting suc-
cessful modification over commercial CaO surface.16
Figure 3 shows the TG/DSC thermogravity analysis
of commercial CaO and modified CaO. There are two
steps in the TG curve in the temperature range from
400◦C to 700◦C due to the loss of H2O and CO2. Fur-
thermore, the DSC curve of the modified CaO presents
two broad peaks at 450◦C and 700◦C corresponding to
the decomposition of Ca(OH)2 and CaCO3 formed by
the hydration and carbonation of CaO in the storage
and preparation. The peaks move to higher tempera-
ture range compared with commercial CaO (339◦C and
614◦C), which indicates the enhancement of thermal
stability of modified CaO. Furthermore, from the per-
centage of weight loss of the two samples, 2.305% for
commercial CaO and 2.414% for modified CaO, it can
be concluded that the relatively little Ca(OH)2 forms
during the modification procedure.
The figure 2 shows the FT-IR spectra of commer-
cial CaO and modified CaO. The spectra display bands
at 867 cm−1 and 1477 cm−1 corresponding to vibration
modes of mono and bidentate carbonates. It is also evi-
denced from the characteristic absorb of C=O between
2000 and 1600 cm−1 that indicates the presence of
calcium carbonate formed in both catalysts. However,
there is no great difference between their intensity, sug-
gesting carbonation by CO2 in air is very slight over
modified CaO. The bands at 1621 and 3460 cm−1 are
associated with adsorbed water. The important features
of the modified CaO appear in the C–H (alkane) stretch-
ing (2800–3000 cm−1) and bending (1440 cm−1).15 The
vibration of aromatic carbon double bond bands causes
the incensement of intensity at 1600 and 1580 cm−1 and
the aromatic carbon–hydrogen stretch (3060 cm−1) are
3.2 Activity comparison of different catalysts
Particularly, the Aldol condensation can be carried
out on homogeneous basic catalysts with good yield
and selectivity. A condensation of cyclohexanone and
benzaldehyde was operated with kinds of bases at
room temperature for 12 h with catalyst amount of
20 wt% as shown in figure 4. From the results, it
can be seen that among the homogeneous catalysts
the reaction proceed increased with the basicity of
catalyst with yield above 30%, and the most effi-
cient catalyst is NaOH. Low catalytic activity for
Na2CO3 was due to its weak basicity. Surprisingly,
CaO showed highest activity of 83.6% yield of 2-
benzylidenecyclohexanone among the tested basic cata-
lyst, indicating the basicity of CaO is strong and the
most important factor to the active sites to improve the
CaO
Modified CaO
4000
3500
3000
2500
2000
1500
1000
500
Frequence(1/cm)
Figure 2. IR pattern of commercial CaO and modified
CaO.

316
Ying Tang et al.
tively low yield (36.6%) was observed. On increas-
100
ing the catalyst amount to 20%, the yield increased up
to 94.5%. However, too higher catalyst amount also
leads to lower yield due to the decreased selectivity
of 2-benzylidenecyclohexanone with the formation of
1,3,5-triphenylpentan-1,5-dione by Michael addition as
suggested by Lopez.17
90
80
70
60
50
40
30
20
10
0
3.4 Effect of reaction temperature
The influence of the reaction temperature was
investigated using
a cyclohexanone/benzaldehyde
molar ratio of 1.3: 1 and 20 wt% CaO. Yield of 2-
benzylidenecyclohexanone was observed due to
increase with reaction temperatures ranging from 20◦C
to 65◦C as inferred from figure 6. The results show that
a yield of 90.1% at 20◦C was obtained after 12 h. With
increase of the reaction temperature above 50◦C, the
incensement of yield is not as high as the change in the
low temperature range, which is similar to the results
of Aldol condensation of citral and acetone reported by
Raju et al.7 The optimum reaction temperature is 65◦C
for following experiments.
Na2CO3
KOH
NaOH
CaO
catalyst
Figure 4. The catalytic performances of various catalysts in
Aldol reaction of cyclohexanone and benzaldehyde (cata-
lyst amount = 20%, reaction time = 12 h, cyclohexanone/
benzaldehyde = 1.2: 1, reaction temperature = 65◦C).
abstraction of an α–proton form the ketone molecule at
the initial reaction and help the dehydration at the end
of the reaction.
3.3 Effect of catalyst amount
3.5 Effect of cyclohexanone/benzaldehyde ratio
The optimum amount of catalyst was investigated
over commercial CaO with reaction time of 12 h
at room temperature and cyclohexanone/benzaldehyde
molar ratio of 1.2: 1 (figure 5). The yield of 2-
benzylidenecyclohexanone increases along with the
amount of catalyst. Lower CaO amount (<2%), rela-
To evaluate the effect of reagent concentrations on
the yield of 2-benzylidenecyclohexanone, the reaction
was carried out with the cyclohexanone/benzaldehyde
ratio varied from 1.1 to 2.0. Figure 7 shows that
the yield of 2-benzylidenecyclohexanone is the low-
est with a cyclohexanone/benzaldehyde ratio of 1:
100
90
80
70
60
50
40
90
89
88
87
86
85
84
83
82
81
80
20
30
40
50
60
70
0
5
10
15
20
25
catalyst amount/%
reaction temperature /oC
Figure 5. Effects of catalyst amount on Aldol conden- Figure 6. Effect of reaction temperature on Aldol conden-
sation of cyclohexanone and benzaldehyde using commer- sation of cyclohexanone and benzaldehyde using commercial
cial CaO as catalyst (reaction time = 12 h, cyclohex- CaO as catalyst (catalyst amount = 20 wt%, reaction time =
anone/benzaldehyde = 1.2: 1, reaction temperature = 65◦C). 12 h, cyclohexanone/benzaldehyde = 1.2: 1).

Modified CaO for Aldol condensation
317
100
95
90
85
80
75
70
95
90
85
80
75
70
65
60
10-5
10-4
10-3
10-2
10-1
100
1.0
1.2
1.4
1.6
1.8
2.0
modifier loading/%
molar ratio of cyclohexanone/benzaldehyde
Figure 8. Effect of modifier loading on Aldol condensa-
tion of cyclohexanone and benzaldehyde (catalyst amount =
20 wt%, reaction time = 12 h, reaction temperature = 65◦C).
Figure 7. Effect of molar ratio on Aldol condensation of
cyclohexanone and benzaldehyde using commercial CaO as
catalyst (catalyst amount = 20 wt%, reaction time = 12 h,
reaction temperature = 65◦C).
3.7 Catalytic performance over different
benzaldehydes
1.1. The yield enhances obviously as the cyclo-
hexanone/benzaldehyde ratio increases. The highest
yield was obtained with relatively low cyclohexa-
none/benzaldehyde ratio of 1: 1.3 in comparison with
reported high acetone/aldehyde ratio.1 The results show
that commercial CaO is an effective catalyst for the
Aldol condensation of cyclohexanone and benzalde-
hyde. Much higher cyclohexanone/benzaldehyde ratio
has increased the yield slightly, so 1: 1.3 was chosen as
the optimum cyclohexanone/benzaldehyde ratio.
The Aldol condensations of cyclohexanone with differ-
ent benzaldehydes over commercial CaO and 0.1% ben-
zyl bromide modified CaO were investigated at the opti-
mum reaction condition (catalyst amount = 20 wt%,
reaction time = 3 h, reaction temperature = 65◦C) and
shown in table 1. From this table, it was found that
the modified CaO can catalyse the reaction in a very
short time to give high yields, while the yields over
CaO are much lower in 3 h, in other word, the modified
CaO accelerates this condensation effectively. Further
more, we can find the substitutes of benzaldehyde also
affect the yield. The benzaldehyde with strong electron-
withdrawing groups -NO2, (entries 1 and 2) give higher
yield, but for entry 3 the low yield may be due to the
poor selectivity to 2-benzylidenecyclohexanone. Rela-
tively weak electron-withdrawing groups lead to mode-
rated yields and electron-donor groups give lower con-
version. Besides, the ortho substituted groups lower the
yield for the stereochemistry (entries 3, 4, 6, 7, 11).
3.6 Effect of modifier loading
The catalytic data over commercial CaO and modi-
fied CaO with different amount of benzyl bromide
at the reaction temperature of 65◦C with cyclohexa-
none/benzaldehyde molar ratio of 1: 1.3 after 3 h were
shown in figure 8. From the results, it can be seen that
the time to a high yield of 2-benzylidenecyclohexanone
(>94%) was greatly shorted from 12 h for commercial
CaO to 3 h for modified CaO. Furthermore, the catalytic
activity of modified CaO increases along with the modi-
fier’s amount, and the highest activity (94.7% yield of
2-benzylidenecyclohexanone) was obtained over 0.1%
benzyl bromide modified CaO. The high activity of
modified CaO should be attributed to the phenyl groups
forming a hydrophobic layer over CaO surface with
similar structure to reactance, which greatly improve
the diffusion of reactance to the catalyst surface and
make the utilization of active sites over catalyst surface
efficient.
3.8 Catalytic mechanism
The possible reaction mechanism of Aldol condensa-
tion between cyclohexanone (A) and benzaldehyde (B)
over CaO was illustrated in scheme 3. In this case, the
CaO captures a proton and promotes the rearrangement
reaction of cyclohexanone to form C, which is an active
intermediate of the Aldol condensation. Through a clas-
sical nucleophile attacking of C to benzaldehyde, inter-
mediate D is formed. Then, the Ca2+ accept an O− from

318
Ying Tang et al.
Table 1. Aldol condensations of cyclohexanone with different benzaldehydes.
Yield /%
Entry
1
Benzaldehydes
CaO
86.4
Modified CaO
95.8
2
3
85.5
80.2
98.5
95.0
4
5
6
81.9
81.4
86.4
92.0
98.6
90.6
7
58.2
81.1
83.5
83.8
83.4
98.8
93.3
91.9
8
9
10

Modified CaO for Aldol condensation
319
Table 1. (continued).
Yield /%
Entry
Benzaldehydes
CaO
48.7
Modified CaO
87.0
11
12
81.2
98.6
13
14
88.8
89.1
99.4
99.1
A
B
C
D
-
O
H
O
O
O
-
O
O
Ca2+
Ca2+ O2- Ca2+
Ca2+
Ca2+
O2-
O2- Ca2+
CaO
CaO
CaO
E
+
O
H
O
Ca2+
Ca2+ O2-
F
CaO
Scheme 3. The possible reaction mechanism of Aldol condensation between cyclohexanone and benzaldehyde over CaO.
D to form a carbocation E, and the deprotonation of E the same condition. The modified CaO shown good
results in the formation of product F at last.
stability in air. It can be attributed to the benzyl groups
on CaO surface, which defend the catalyst from water
and improve the adsorption of reactance to catalyst sur-
face. The aldhydes with electron-donor group give good
yield. The characterization by FT-IR and TG shows that
the modifier bonds to surface of CaO chemically and
the slight hydration of CaO during modifying process.
4. Conclusions
In this work, an efficient catalyst for the Aldol conden-
sation of cyclohexanone and benzaldehyde using com-
mercial CaO and modified CaO as catalysts are shown.
It showed that the commercial CaO is an efficient cata-
lyst for this reaction, and the catalytic activity over
Acknowledgements
modified CaO enhances greatly and shortens the reac- This work was financially supported through grants
tion time from 12 h to 3 h with an yield of 90% under from Scientific Research Program Funded by Natural

320
Ying Tang et al.
Science Research Plan Projects of Shaanxi Science
and Technology Department (No. 2011JQ2014) and
PetroChina Innovation Foundation.
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