High efficient Aldol condensation reaction utilizing modified calcium oxide as stable solid base catalyst

Article abstract of DOI:10.1134/S0023158416040145

Aldol condensation of acetophenone and benzaldehyde, as well as various benzaldehydes was carried out efficiently to produce chalcone with a good selectivity and high yields by using modified CaO as a solid base catalyst. Stability and catalytic activity of commercial CaO were significantly improved after modifying calcium oxide with bromobenzene in a simple way. An Aldol yield higher than 98.9% was obtained after the reaction was conducted for 3 h. This time interval is considerably shorter when compared to a period of 12 h needed for the commercial CaO to reach 92.1% yield under optimum activation. The high catalytic activity of modified CaO suggests that heterogeneous aldolisation was greatly improved by changing its hydrophilic properties. The influence of several reaction parameters, such as temperature and catalyst loading, was investigated. The humidity test over modified CaO reveals that the basic sites of modified CaO are resistant to CO₂ and moisture. The type of aldehyde has great influence on the yield of Aldol condensation. Based on the results of characterization by Fourier transform-infrared spectrometry (FT-IR) and thermogravimetric measurements (TG), it was concluded that the modifier was chemically bonded to the surface of CaO and nearly no Ca(OH)₂ was formed during the modification process.

Full text of DOI:10.1134/S0023158416040145

ISSN 0023-1584, Kinetics and Catalysis, 2016, Vol. 57, No. 4, pp. 439–445. © Pleiades Publishing, Ltd., 2016.
High Efficient Aldol Condensation Reaction Utilizing Modified
Calcium Oxide as Stable Solid Base Catalyst1
Jin Wanga, Tianlan Yana, Ying Tanga, *, and Yanqing Miaob
aCollege of Chemistry and Chemical Engineering, Xi’an Shiyou University, Xi’an Shaanxi, 710065, China
bDepartment of Pharmacology, Xi’an Medical University, Xi’an, 710068, China
*e-mail: tangying78@xsyu.edu.cn
Received October 17, 2015
Abstract—Aldol condensation of acetophenone and benzaldehyde, as well as various benzaldehydes was car-
ried out efficiently to produce chalcone with a good selectivity and high yields by using modified CaO as a
solid base catalyst. Stability and catalytic activity of commercial CaO were significantly improved after mod-
ifying calcium oxide with bromobenzene in a simple way. An Aldol yield higher than 98.9% was obtained after
the reaction was conducted for 3 h. This time interval is considerably shorter when compared to a period of
12 h needed for the commercial CaO to reach 92.1% yield under optimum activation. The high catalytic activ-
ity of modified CaO suggests that heterogeneous aldolisation was greatly improved by changing its hydro-
philic properties. The influence of several reaction parameters, such as temperature and catalyst loading, was
investigated. The humidity test over modified CaO reveals that the basic sites of modified CaO are resistant
to CO2 and moisture. The type of aldehyde has great influence on the yield of Aldol condensation. Based on
the results of characterization by Fourier transform-infrared spectrometry (FT-IR) and thermogravimetric
measurements (TG), it was concluded that the modifier was chemically bonded to the surface of CaO and
nearly no Ca(OH)2 was formed during the modification process.
Keywords: Aldol condensation, CaO, surface modification
DOI: 10.1134/S0023158416040145
by air due to adsorption of CO2 and H2O [11]. Calcium
oxide, an alkaline-earth metal oxide with strong basic-
ity and low cost, is widely used in various base-cata-
lyzed reactions. However, the industrial application of
this solid base is limited since it is not stable upon
exposure to moist air. It was repeatedly reported that
all operations during preparation, processing, storage
and use of the catalyst were conducted extremely care-
fully in an inert gas atmosphere to avoid contact with
air. Nevertheless, independently of the catalyst nature
a single contact between reagents and catalysts cannot
be ruled out and that contact can cause loss of the cat-
alytic activity to a level below that of homogeneous
strong base, such as sodium hydroxide. Herein, the
aim of this study was to develop an active CaO with a
high efficiency for the Aldol condensation of ace-
tophenone and various benzaldehydes and good sta-
bility in moist air. The modifying conditions and vari-
ous reaction conditions were also investigated.
The Aldol condensation involving C-C bond for-
mation has numerous applications in the synthesis of
fine chemicals. This kind of reaction can proceed cat-
alytically in the presence of a strong base or acid in liq-
uid phase [1–3]. For the aim of environmental protec-
tion it is desirable to find solid bases which could sub-
stitute liquid catalysts and would also preserve all the
advantages of heterogeneous catalysis, i.e. ease of sep-
aration of the products, insignificant corrosion of the
reactor, and possible regeneration of the catalyst. Dif-
ferent families of solid bases were found to be active
catalysts in this reaction, including alkali-exchanged
zeolites [4], ion-exchange resins [5, 6], alumina-sup-
ported hydroxide [7, 8], Mg–Al hydrotalcite sand
alkaline-earth metal oxides [9]. Because of its strong
basicity hydrotalcite was regarded as the most suitable
candidate for industrial application. The published
method for Mg–Al hydrotalcite activation included a
liquid-phase reconstruction to create surface sites with
a narrow base-strength distribution [10]. It is worth
noting that at high conversions reached with these sol-
ids the yields are generally modest. Furthermore, the
basic sites on the surfaces of alkaline-earth metal
oxides or hydrotalcite-like samples are easily poisoned
EXPERIMENTAL
Modification of CaO [12, 13]
A simple method was used for the surface modifica-
tion of CaO. An aliquot (2.8 g) of the 0.15–0.095 mm
1
The article is published in the original.
439

440
Absorption rate, %
JIN WANG et al.
rated from the solid and analyzed by gas chromatogra-
phy using a flame ionization detector, HP-5 capillary
column (30 m long, i.d. 0.25 m) programmed from
initial temperature of 50 to final temperature of 280°C
with N2 (1.5 mL/min) as a carrier gas. The yield was
calculated by area normalization method on a carbon
basis and the carbon balances data were good at
around 100 5%.
100
80
60
40
20
Catalysts Characterization
The Fourier transform-infrared (FT-IR) spectro-
photometer Nicolet Nexus 670 (Nicolet, United
States) was used to identify the surface group of the
catalyst. Thermogravimetric (TG) experiments were
carried out using Q600 SDT thermal analysis machine
(TA Instruments, United States) under a flow of air
attemperatures ranging from 25 to 800°C with a ramp-
ing rate of 10 deg/min.
0.001%
0.10%
CaO
0.01%
0.2%
0
50
100
150
200
Time, h
Fig. 1. Moisture absorbing rate over commercial CaO and
modified CaO with various benzyl bromides.
RESULTS AND DISCUSSION
Humidity Test
fraction of commercial CaO (China National Medi-
cines Co., Ltd.) was added into the bromobenzene
(Sigma-Aldrich, AR)/methanol solution (40 mL)
under stirring at room temperature. The content of
bromobenzene was varied from 0.01 to 0.5%. The mix-
ture was stirred for 24 h, the solid was then filtered and
washed with methanol to remove excess modifier.
Vacuum drying yielded the modified CaO catalyst.
Since CaO can sorb considerable amounts of water,
the poisoning effect of H2O adsorption during catalyst
preparation is the most important issue. In order to
improve stability of CaO in air, the surface of CaO was
modified by different amounts of brombenzene used
as a hydrophobic reagent. The effect of modifier load-
ing on the stability of modified CaO in air was also
investigated. Humidity tests were carried out with the
samples of CaO modified by different amounts of ben-
zyl bromide (from 0.001 to 0.2 wt %). The extent of
moisture adsorption for these samples as a function of
time is illustrated in Fig. 1. It can be readily seen that
the degree of moisture absorption calculated from the
incremental weight increase of CaO continues to rise
with time for all samples. However, obvious differ-
ences in the patterns of behaviour of commercial CaO
and calcium oxide modified by benzyl bromide can be
traced. For the unmodified CaO material, the extent
of moisture absorption increases rapidly and reaches a
nearly 100% level within 50 h, while very low weight
increment can be found over modified CaO at the
same time. The sample of CaO modified with
0.1% bromobenzene shows the slightest moisture
resistance. It can be concluded that the surface modi-
fication greatly improves the moisture resistance of
commercial CaO and this improvement can be
explained by the formation of hydrophobic layer over
catalyst surface caused first of all by the introduction
of the aromatic group of bromobenzene. A further
increase in the modifier loading deteriorates the resis-
tance to moisture of the modified CaO because the
reaction between bromobenzene and hydroxyl over
surface of CaO results in the increased formation of
CaBr2 species, which have high affinity to water.
Humidity Test
Fractions of commercial CaO and modified CaO
catalyst were transferred into the container and stored
at room temperature over saturated water vapour for
several days to maintain the adsorption equilibrium
with water. The samples were weighed at regular time
intervals. The extent of moisture adsorption (%) was
evaluated using the following equation:
56Δm
18m0
,
×100%
w% =
where ∆m denotes an increase in the weight of the
solid, and m0 is the initial weight of the solid.
Activity Test
In a typical procedure, 0.6 mmol of acetophenone
(99%) and 0.5 mmol of purified benzaldehyde
(99.9%) (mole ratio of benzaldehyde: acetophenone =
1.2 : 1) were added into 20 mL MeOH in a 50-mL
round bottom flask equipped with magnetic stirrer and
condenser. The mixture was stirred for 15 min and
then about 0.25 mmol of catalyst was introduced at a
constant temperature. The reaction mixture was
heated at a desired reaction temperature (20–65°C) at
atmospheric pressure. When the reaction was com-
Figure 2 shows the FT-IR spectra of commercial
pleted the mixture was cooled, the solution was sepa- CaO and modified CaO. The spectra display bands at
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HIGH EFFICIENT ALDOL CONDENSATION REACTION
441
Transmission
160
DSC, mV
20
Weight loss, mg
9
DSC (mV/mg)
140
120
100
80
10
0
8
7
6
5
–10
2
TG (%)
–20
60
1
40
–30
20
–40
0
–50
3900 3400 2900 2400 1900 1400 900 400
100 200 300 400 500 600 700 800
Wave number, cm–1
Temperature, °C
Fig. 2. IR pattern of commercial CaO (1) and modified
CaO (2).
Fig. 3. TG/DSC thermogram of modified CaO.
867 and 1477 cm–1 corresponding to vibration modes
of mono- and bidentate carbonates. The presence of
carbonates is also evident from the characteristic
absorption of C=O between 2000 and 1600 cm–1 that
indicates the presence of calcium carbonate formed in
both catalysts. However, a minor difference in the
intensity of bands suggests that carbonate formation
over modified CaO involving CO2 from air is negligi-
by modifying CaO it is possible to improve thermal
stability of calcium oxide.
Catalytic Activity
Homogeneous basic catalysts are efficient for cata-
lysing Aldol condensation with good yield and selec-
tivity. Different basic catalysts were investigated in our
work with results of condensation of acetophenone
and benzaldehyde shown in Fig. 4. The reaction was
conducted at room temperature for 6 h, the amount of
catalyst in all runs was 20 wt %. It can be easily rec-
ognised that in the presence of the homogeneous cat-
alysts the yields increased with increasing catalyst
ble. 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) stretching
(2800–3000 cm–1) and bending (1440 cm–1) vibra-
tions [14]. Vibrations of aromatic carbon double bond
cause the increase in the intensity of bands at 1600 and
1580 cm–1 where as stretching vibrations of the aro-
matic carbon-hydrogen (3060 cm–1) provide evidence
of the formation of phenyl groups [15]. Another differ-
ence between the spectral pictures of modified CaO and
commercial CaO is the vibration around 1080 cm–1,
which is assigned to the stretching vibration of –C–O as
a result of the reaction between modifier and hydrox-
yls on the surface of CaO. All these observations sug-
gest that modification of commercial CaO was suc-
cessful.
Yield of chalcone, %
80
70
60
50
40
30
20
10
0
Figure 3 shows the results of TG/DSC thermo-
gravmetric analysis of commercial CaO and modified
CaO. The samples are characterized by TG curves that
exhibit two steps between 400 and 800°C due to the
loss of H2O and CO2. Furthermore, the DTG curve of
the modified CaO shows two broad peaks at 450 and
700°C corresponding to the decomposition of
Ca(OH)2 and CaCO3 formed by the hydration and
carbonation of CaO in the course of storage and
preparation. Both peaks are shifted to higher tempera-
tures compared with those exhibited by commercial
CaO (339 and 614°C). It can be therefore inferred that
Na2CO3
NaOH
KOH
CaO
Catalyst
Fig. 4. The catalytic performances of various catalysts in
Aldol reaction of bromobenzene and benzaldehyde. Cata-
lyst amount—20%, reaction time—12 h, bromoben-
zene/benzaldehyde = 1.2 : 1, reaction temperature—60°C.
KINETICS AND CATALYSIS Vol. 57 No. 4 2016

442
JIN WANG et al.
Table 1. Effect of the catalyst amount on the yield of chal-
cone in condensation of acetophenone using commercial
CaO as a catalyst
Effect of Catalyst Amount
To optimize the catalyst amount the reaction was
conducted over commercial CaO for 6 h at room tem-
perature with acetophenone/benzaldehyde mole ratio
of 1.2 : 1 (Table 1). It can be seen that increased
amounts of the catalyst favour enhanced yields of
chalcone whereas low catalyst loadings have unfavour-
able effect. When the amount of CaO was lower than
20%, chalcone in 62.4% yield was obtained, while the
yield of chalcone as high as 75.8% was recorded after
the catalyst amount was increased to 20%. However,
excessive amounts of the catalyst leads to lower yields
due to side reactions, such as self-condensation of ace-
tophenone that occurs under moderate concentra-
tions of modified CaO.
Catalyst, wt %
Yield of chalcone, %
5
62.4
68.5
75.8
71.4
67.1
10
20
40
60
Reaction conditions: 5.0 mmol acetophenone, 7.5 mmol purified
benzaldehyde/methanol (20 mL), ambient temperature, 6 h.
Table 2. Effect of reaction temperature on the yield of chal-
cone in the condensation of acetophenone in the presence
of commercial CaO catalyst
Effect of the Reaction Temperature
Temperature, °C
Yield of chalcone, %
The effect of the reaction temperature was investi-
gated using acetophenone/benzaldehyde mole ratio of
1.2 : 1, reaction time of 6 h and 20 wt % CaO. The yield
of chalcone increased when the reaction temperature
was increased from 30 to 65°C (Table 2). At 30°C a
yield of 76.1% was observed and at 60°C 82.3% yield
was recorded. With reaction temperatures above 60°C
the incremental yield increase was slower than that
found for the lower temperature range. Similar effects
of increasing temperature on the product yields were
reported for Aldol condensation of citral and acetone
[17]. Based on the reaction efficiency the optimum
reaction temperature of 60°C was chosen for the fur-
ther experiments.
30
40
50
60
65
76.1
78.4
80.1
82.3
82.6
Reaction conditions: 5.0 mmol acetophenone, 7.5 mmol benzal-
dehyde/methanol (20 mL), 20% of the catalyst, 6 h.
Table 3. Effect of acetophenone/benzaldehyde mole ratio
on yield of chalcone in the condensation of acetophenone
using commercial CaO as a catalyst
Acetophenone/benzaldehyde
Yield of chalcone, %
ratio
1 : 1.1
1 : 1.2
1 : 1.3
1 : 1.5
1 : 1.8
1 : 2.0
78.8
82.3
83.5
85.8
81.9
78.9
Effect of Acetophenone/Benzaldehyde Ratio
To evaluate the effect of reagent concentrations on
the yield of chalcone, the reaction was carried out with
the acetophenone/benzaldehyde mole ratio varied
from 1.1 to 2.0. The results are summarized in Table 3.
It can be seen that the yield of chalcone was the lowest
with the acetophenone/benzaldehyde ratio of 1 : 1.1
and it increased as the acetophenone/benzaldehyde
mole ratio was increased. The highest yield was
obtained with a relatively low acetophenone/benzal-
dehyde ratio of 1 : 1.5 whereas at higher acetone/alde-
hyde ratios the yields were lower. The results show that
commercial CaO is an effective catalyst for the Aldol
condensation of acetophenone and benzaldehyde.
Since higher acetophenone/benzaldehyde ratio makes
a negligible contribution to the yield increase a ratio of
1 : 1.5 was chosen as the optimum acetophe-
none/benzaldehyde ratio.
o
Reaction conditions: 20% of the catalyst, 60 C, 6 h.
basicity and the most efficient catalyst was KOH. Low
catalytic activity observed in the presence of Na2CO3
is due to its weak basicity. Contrary to the expectation,
CaO, a heterogeneous catalyst, showed the highest
yield (75.8%) among the basic catalysts tested. The
reason can be sought in the strong basicity of CaO. It
can be speculated that the active sites favour the
abstraction of α-proton form the ketone molecule at
an initial step of the reaction and promote the dehy-
dration at the end of the reaction. However, neither
benzyl alcohol nor benzoic acid was observed in the
reaction products when CaO was used as a catalyst. It
appears that CaO shows a moderate basicity and for
this reason suppresses Cannizzaro reaction, which
Effect of Modifier Loading
In order to improve the catalytic efficiency of CaO,
the surface of calcium oxide was modified and the
effect of the modifier loading on the yield of chalcone
otherwise proceeds under strong basic environment in was investigated at the reaction temperature of 60oC
the condensation reaction [16].
with acetophenone/benzaldehyde mole ratio of 1 : 1.5.
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HIGH EFFICIENT ALDOL CONDENSATION REACTION
443
Table 4. The effect of the amount of bromobenzene in the
modified CaO on yield of chalcone in the condensation of
acetophenone
The relevant results are given in Table 4. It can be seen
that the catalytic activity of modified CaO increases as
the modifier amount increases, and the highest yield
of chalcone of 98.9% was obtained with CaO modified
by 0.001% bromobenzene. The reason for the high
activity of modified CaO should be attributed to the
phenyl groups of modifier which formed a hydropho-
bic layer over CaO surface. Because of structural sim-
ilarity of modifier and reactants one might expect an
increased rate of diffusing reactants to the catalyst sur-
face. Accordingly, more effective performance of the
surface active sites results in a higher catalytic activity
of modified CaO. However, excessive amounts of the
modifier reduce the catalytic activity of modified CaO
since modifier molecules can occupy a fraction of
active sites on the surface of CaO.
Catalyst
Yield of chalcone, %
Commercial CaO
85.8
86.4
88.7
92.1
98.9
92.7
90.6
CaO + 0.5% bromobenzene
CaO + 0.2% bromobenzene
CaO + 0.1% bromobenzene
CaO + 0.001% bromobenzene
CaO + 0.00001% bromobenzene
CaO + 0.000001% bromobenzene
Reaction conditions: 5.0 mmol of acetophenone, 7.5 mmol of
purified benzaldehyde/methanol (20 mL), 20% of the catalyst,
o
60 C, 6 h.
Effect of the Nature of Benzaldehyde Substituents
yields, whereas lowest yields were observed with elec-
tron-donor groups.
The Aldol condensations of acetophenone with
different benzaldehydes over commercial CaO and
0.1% bromobenzene modified CaO were investigated
at the optimum reaction condition (20% of the cata-
lyst, 60oC, 6 h). The results and shown in Table 5. It
can be concluded that the modified CaO can catalyze
the reaction with high rates and excellent yields inde-
pendent on the nature of aldehyde substituents. Minor
differences in the yield of chalcone in the condensa-
tion acetophenone with various aldehydes can be
accounted for by different properties of benzaldehyde
Catalytic Mechanism
A possible reaction mechanism of Aldol condensa-
tion between acetophenone (A) and benzaldehyde (B)
over CaO is illustrated in Scheme. According to the
scheme, CaO captures a proton and promotes a rear-
rangement reaction of acetophenone to form C, which
is an active intermediate in the Aldol condensation.
Through a classical nucleophile attack of C on benzal-
substituents. The benzaldehydes with strong electron- dehyde, an intermediate D is formed. Then, Ca2+ ion
abstracts an O- from D to form a carbocation E, and
withdrawing groups (–NO2, entry 2 and 3) are charac-
terized by higher yields. The presence of relatively finally the deprotonation of E results in the formation
weak electron-withdrawing groups leads to moderated of product F.
The possible reaction mechanism of Aldol condensation between acetophenone and benzaldehyde over CaO
A
B
C
D
O
O
H
–O
O
O–
O
2+
2+
2–
2–
Ca2+
Ca
2+
2–
O2– Ca2+
Ca
Ca
O
2+
2–
O2– Ca22++
2+
2+
CCaa2+
Ca2+
O
O
Ca
Ca
O
Ca
CaO
CaO
CaO
E
H
O
O
O2–
2+
2+
2+
2+
2–
Ca
Ca
Ca
Ca
O
F
CaO
Scheme.
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444
JIN WANG et al.
Table 5. Aldol condensation reaction of acetophenone with various aldehydes
Entry
1
Product
O
Yield of chalcone, %
92.1
O
NO
2
2
3
4
5
~100
~100
94.2
95.6
O
NO
2
O
Cl
O
Cl
O
6
7
8
92.5
90.1
90.2
CH
3
O
OCH
3
O
O
O
O
9
94.6
97.8
O
10
O
o
Reaction conditions: 5.0 mmol of acetophenone, 7.5 mmol of purified benzaldehyde/methanol (20 mL), 20% of the catalyst, 60 C, 6 h.
KINETICS AND CATALYSIS Vol. 57 No. 4 2016

HIGH EFFICIENT ALDOL CONDENSATION REACTION
445
5. Podrebarac, G.G., Ng, F.T.T., and Rempel, G.L.,
CONCLUSIONS
Chem. Eng. Sci., 1997, vol. 52, p. 2991.
In this work, a heterogeneous Aldol condensation
of acetophenone and benzaldehyde was effected in the
presence of commercial CaO and modified CaO.
Results suggest that the commercial CaO is an effi-
cient catalyst for this reaction and by modifying CaO
it is possible to significantly enhance the catalytic
activity achieving 98.9% yield under the same condi-
tions. The modified CaO have good stability in air
attributable to the presence of benzyl groups on the
surface of CaO. These groups increase resistance of
the catalyst to water and improve adsorption of reac-
tants on the catalyst surface. The best yields of chal-
cone were recorded for aldhydes with electron-with-
drawing groups. The characterization by FT-IR and
TG shows that the modifier is attached to surface of
CaO by chemical bonds and the hydration of CaO
during modifying process is negligible.
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