Direct Henry reactions with modified calcium oxide as solid catalyst

Article abstract of DOI:10.1007/s11164-012-0875-4

Commercial CaO was modified simply with benzyl bromide. The modified CaO had good water resistance, and characterization by FTIR and TG revealed the modifier was chemically bonded to the CaO surface. Commercial CaO and CaO modified with benzyl bromide were investigated as catalysts for the Henry reaction between benzaldehyde and nitromethane. It was found that the catalytic activity of the modified CaO was greatly improved, with high conversion of benzaldehyde to the (E)-phenyl nitroolefin and 1-phenyl-2-nitroethanol, and with different selectivity from commercial CaO. The effect of modification and reaction conditions on yield, selectivity, and mechanism were studied thoroughly.

Full text of DOI:10.1007/s11164-012-0875-4

Res Chem Intermed
DOI 10.1007/s11164-012-0875-4
Direct Henry reactions with modified calcium oxide
as solid catalyst
•
Ying Tang Xuefan Gu Mei Meng Jingfang Xu
•
•
Received: 30 April 2012 / Accepted: 15 October 2012
Ó Springer Science+Business Media Dordrecht 2012
Abstract Commercial CaO was modified simply with benzyl bromide. The
modified CaO had good water resistance, and characterization by FTIR and TG
revealed the modifier was chemically bonded to the CaO surface. Commercial CaO
and CaO modified with benzyl bromide were investigated as catalysts for the Henry
reaction between benzaldehyde and nitromethane. It was found that the catalytic
activity of the modified CaO was greatly improved, with high conversion of
benzaldehyde to the (E)-phenyl nitroolefin and 1-phenyl-2-nitroethanol, and with
different selectivity from commercial CaO. The effect of modification and reaction
conditions on yield, selectivity, and mechanism were studied thoroughly.
Keywords Henry reaction ꢀ Heterogeneous catalyst ꢀ CaO ꢀ Modification
Introduction
The Henry reaction is an important C–C bond-formation reaction producing b-
nitroalcohol chiral carbon atoms by adding a nitroalkane to a carbonyl compound,
for example benzaldehyde. Further reduction of the nitro group of the b-nitroal-
cohols can lead to the formation of aminoalcohols; this occurs widely with natural
and synthetic products and with chemical intermediates [1–3]. Henry reactions with
impressively high conversion are realized by use of a variety of homogeneous
bases, for example alkali metal oxides, alkaline earth oxides, and hydroxides, which
are corrosive, toxic, nonreusable, and also produce neutralization waste [4].
Y. Tang (&) ꢀ X. Gu ꢀ M. Meng ꢀ J. Xu
College of Chemistry and Chemical Engineering, Xi’an Shiyou University,
Xi’an 710065, Shaanxi, China
e-mail: tangying78@xsyu.edu.cn
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Y. Tang et al.
Ignoring the high cost and rigorous preparation process, the Henry reaction
catalyzed by chloroaluminate ionic liquids, especially mixtures of substituted
organic species with AlCl3, results in good yields with efficient recycling [5, 6].
To solve the problem associated with homogeneous bases, several solid bases, for
example ion-exchange resin, amorphous silica [7], and zeolite [8] has been reported.
Choudary et al. [9] developed efficient Mg–Al hydrotalcite by incorporation of t-
butoxide by use of an exchange process to increase the basicity of the hydrotalcite.
Nanocrystalline MgO and its modified particles are effective and reusable catalysts for
the Henry reaction. However, the nature of the active sites on the modified MgO
surfaces is unknown and the catalytic properties of MgO materials depend
substantially on the method of preparation [10, 11]. CaO, with strong basicity and
low cost, has much potential for catalyzing the Henry reaction. However, this kind of
solid base has not been widely used industrially because of the technical problem of the
stability of the material on exposure to moist air. The basic sites of alkaline-earth metal
oxide or hydrotalcite-like surfaces are easily poisoned by air because of adsorption of
CO2 and H2O on the surface [12]. Furthermore, for the two-phase reaction with solid
catalyst and liquid reactants, the single contact between reagents and catalysts is a
major limitation of reaction efficiency, which results in the catalyst being less active
than a homogeneous strong base, for example sodium hydroxide.
In this work, commercial CaO was investigated as a catalyst for the Henry
reaction of benzaldehyde with nitromethane, and the effects of reaction conditions
were investigated. The effects of modification and reaction conditions on yields,
selectivity, and mechanism were also studied.
Results and discussion
Catalyst characterization
Modification of CaO by benzyl bromide was as suggested in Scheme 1. IR spectra of
commercial CaO and modified CaO are shown in Fig. 1. The spectra contain bands at
867 and 1,477 cm-1 corresponding to vibration modes of mono and bidentate
carbonates. It is also apparent from the characteristic absorption of C=O between 2,000
and 1,600 cm-1 that calcium carbonate is present in both catalysts. However, there is no
great difference between their intensity, suggesting little carbonation of modified CaO
by CO2 in air. The bands at 1,621 and 3,460 cm-1 are associated with adsorbed water.
An important feature of the modified CaO is the C–H (alkane) stretching
(2,800–3,000 cm-1) and bending (1,440 cm-1) [13]. Bands from vibration of aromatic
carbon double bonds result in increased intensity at 1,600 and 1,580 cm-1 and this and
the aromatic carbon–hydrogen stretch (3,060 cm-1) are evidence of benzyl groups,
suggesting successful modification of commercial CaO [14].
Figure 2 shows results from TG/DSC thermogravimetric analysis of commercial
CaO and modified CaO. There are two steps in the TG curve in the temperature
range from 400 to 700 °C because of loss of H2O and CO2. Furthermore, the DSC
curve of the modified CaO contained two broad peaks at 450 and 700 °C
corresponding to decomposition of Ca(OH)2 and CaCO3 formed by hydration and
123

Direct Henry reactions by using modified calcium oxide
CH2 Ph
+
CH2
Ph
Br
Ca
Ca
Br
O
+
Ca
Ca
O
Scheme 1 The reaction between commercial CaO and benzyl bromide
CaO
Modified CaO
4000
3500
3000
2500
2000
1500
1000
500
Frequence [cm-1]
Fig. 1 IR spectra of commercial CaO and modified CaO
9
8
7
6
20
10
DSC(mV/mg)
0
-10
-20
-30
-40
-50
TG(%)
5
800
100
200
300
400
500
600
700
Temperaure [°C]
Fig. 2 TG/DSC thermogram of modified CaO
carbonation of CaO during storage and preparation, which moves to higher
temperature range compared with commercial CaO (339 and 614 °C). This
indicates the thermal stability of modified CaO is enhanced.
123

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. 3 Moisture absorption by commercial CaO and modified CaO
Humidity test
A major disadvantage of CaO limiting its widespread application in a variety of
reactions as a solid basic catalyst is adsorption and the poisoning by H2O during
storage and use. In this work, surface modification was investigated by anchoring an
organic layer on the CaO surface to improve its stability in air. Loading with a
modifier, benzyl bromide, has an important effect on moisture absorption by the
catalyst during its use as a catalyst for the Henry reaction. So humidity testing of
modified CaO loaded with modifier in the range 0.01–0.5 % was conducted; the
results are summarized in Fig. 3. For comparison, the unloaded sample was also
tested. All of the tested samples were kept in a vapor-saturated container at room
temperature to enable hydration and carbonation, and the weight of each sample was
measured at regular intervals. From the results it can be seen that absorption of
moisture continues for all samples with time, indicating unavoidable poisoning by
the atmosphere H2O and/or CO2. However, for the unmodified CaO particles,
moisture absorption reaches to nearly 100 % within 50 h whereas only a very slight
weight increase was observed for modified CaO in that time, especially for 0.2 %
benzyl bromide-modified CaO, and only 5.29 % moisture absorption was observed
even after 200 h. It is obvious that the surface modification can effectively improve
the moisture resistance of commercial CaO.
Comparison of the activity of different catalysts
The Henry reaction can be carried out on homogeneous and heterogeneous basic
catalysts, for example NaOH and Na2CO3, with good yield and selectivity, as shown
in Scheme 2. The catalytic activity of conventional bases and commercial CaO in
the Henry reaction between benzaldehyde and nitromethane was investigated under
reflux conditions with 10 wt% catalyst. From the results (Fig. 4), it can be seen that
yield by the catalysts is not very high under these reaction condition, and the most
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Direct Henry reactions by using modified calcium oxide
O
O
N
OH
O
N
O
O
Catal
O
N
R
R
R
+
+
O
Scheme 2 Henry reaction of benzaldehyde and nitromethane
60
50
40
30
20
10
0
Na2CO3
K2CO3
NaOH
KOH
CaO
catalyst
Fig. 4 Performance of different catalysts in the Henry reaction of benzaldehyde and nitromethane.
(amount of catalyst = 10 wt%, reaction time = 12 h, benzaldehyde-to-nitromethane = 0.05:1, reaction
temperature = 102 °C)
efficient liquid base catalyst, NaOH, only yields 38.9 %. Surprisingly, CaO, a
heterogeneous catalyst, had the highest activity—60.1 % yield. This can be
attributes to the strongly basic sites of CaO, which are the active sites for abstraction
of an a-proton from the nitromethane molecule in the initial reaction and for aiding
the dehydration at the end of the reaction.
Effect of amount of catalyst
To check the effect of the amount of catalyst on reactivity, the optimum amount of
catalyst was investigated for commercial CaO with a reaction time of 12 h, a
benzaldehyde-to-nitromethane molar ratio of 0.05:1, and under reflux. As indicated
by Table 1, for the smallest amounts of catalyst, i.e. 1 %, only 59.1 % conversion of
benzaldehyde was achieved. On increasing the amount of CaO to 5 %, conversion
of benzaldehyde increases to 89.2 %. The increased conversion with increasing
catalyst weight can be attributed to increased availability of the catalytically active
sites required for this reaction. However, when the amount of catalyst was [10 %
conversion of benzaldehyde increased, which may be attributed to Michael addition
between the (E)-b-phenyl nitroolefin and nitromethane, as suggested by Lopez
et al. [15].
123

Y. Tang et al.
Table 1 Effect of amount of
commercial CaO on Henry
reaction of benzaldehyde and
nitromethane
Catalyst
weight
ratio
Conversion of
benzaldehyde
(%)
Yield of (E)-b- Yield of
phenyl nitroolefin 1-phenyl-2-
(%)
nitroethanol (%)
1
59.1
89.2
94.8
84.0
59.2
26.1
29.8
82.2
53.8
34.9
33.0
59.4
12.6
30.2
24.3
5
10
15
20
Reaction time = 12 h,
benzaldehyde-to-
nitromethane = 0.05:1, reaction
temperature = 102 °C
Table 2 Effect of reaction
temperature on Henry reaction
of benzaldehyde and
Reaction
temperature benzaldehyde
Conversion of Yield of (E)-b-
Yield of
1-phenyl-2-
nitroethanol (%)
phenyl
nitroolefin (%)
(%)
nitromethane catalyzed by
commercial CaO
30
40
50
60
70
80
90
102
71.1
72.1
80.0
88.1
90.8
93.9
96.8
93.8
50.6
53.3
58.7
57.9
63.8
77.6
80.5
83.2
20.5
18.8
22.3
30.2
27.0
16.3
16.3
10.6
Reaction time = 12 h,
benzaldehyde-to-
nitromethane = 0.05:1, dosage
of CaO = 10 wt%
Effect of reaction temperature
The effect of reaction temperature on conversion of benzaldehyde was investigated
at temperatures ranging from 30 to 102 °C, using benzaldehyde-to-nitromethane
molar ratio of 0.05:1 and 10 wt% CaO. Conversion of benzaldehyde to both (E)-b-
phenyl nitroolefin and 1-phenyl-2-nitroethanol increases when reaction temperature
is below 60 °C, as shown in Table 2. However, conversion of benzaldehyde to its
addition product, 1-phenyl-2-nitroethanol, was observed to decrease with increasing
reaction temperatures above 60 °C until the temperature reached the boiling point of
nitromethane, 102 °C. The reason is that 1-phenyl-2-nitroethanol tends to dehydrate,
being transformed to (E)-b-phenyl nitroolefin at high temperatures. So the optimum
reaction temperature was selected as 102 °C in subsequent experiments.
Effect of modifier loading
The loading of modifierhas a substantial effect on the reactivity ofmodified CaO. Results
for CaO catalyst modified with different amounts of benzyl bromide at a benzaldehyde-
to-nitromethane molarratio of 0.05:1 under reflux are listed in Table 3. Fromtheresults it
can be seen that compared with commercial CaO conversion of benzaldehyde over
modified CaO is enhanced and the reaction time is greatly shortened from 12 to 5 h with
highest conversion of 99.8 % when 0.001 % benzyl bromide was used for modification
of the CaO. The high activity of modified CaO can be attributed to the hydrophobic layer
123

Direct Henry reactions by using modified calcium oxide
Table 3 Effect of modifier
loading on Henry reaction of
benzaldehyde and nitromethane
Modifier
loading
(%)
Conversion of
benzaldehyde
(%)
Yield of (E)-b- Yield of
phenyl nitroolefin 1-phenyl-2-
(%)
nitroethanol (%)
1
93.7
98.1
98.5
99.8
97.4
96.9
56.8
28.0
15.6
36.7
33.7
52.0
60.0
36.9
70.1
82.9
62.1
63.7
44.9
36.3
0.1
0.01
0.001
0.0001
0.00001
Reaction time = 5 h,
benzaldehyde-to-
nitromethane = 0.05:1, reaction
temperature = 102 °C
0.000001 96.3
formed by phenyl groups of benzyl bromide, which has the potential to improve
absorption of the hydrophobic benzaldehyde and nitromethane by the CaO surface and
diffusion of the compounds in the surface. There is also a very interesting change in
product selectivity. Selectivity for 1-phenyl-2-nitroethanol is enhanced by use of the
modified CaO catalyst, and the highest yield reaches 82.9 % for CaO catalyst modified
with 0.01 % benzyl bromide; more or less modifier is of no benefit to selectivity for
1-phenyl-2-nitroethanol. Compared with the results in Table 2, the Henry reaction
catalyzed by commercial CaO selectively produces (E)-b-phenyl nitroolefin, which
indicates that modification reduced the basicity of CaO and resulted in reduction of
deprotonation of 1-phenyl-2-nitroethanol to produce (E)-b-phenyl nitroolefin.
Catalytic performance with different benzaldehydes
The Henry reaction of nitromethane with different benzaldehydes over commercial
CaO and 0.01 % benzyl bromide-modified CaO were investigated under the
optimum reaction conditions (amount of catalyst = 10 wt%, reaction time = 5 h,
reaction temperature = 102 °C); the results are shown in Table 4. From the results,
it was found that modified CaO can catalyze the reaction giving higher yields of the
dehydrated and hydrated products in a much shorter time than commercial CaO. It
was, furthermore, found that substituents on the benzaldehyde substantially affect
yield and the selectivity. Higher conversion is observed for benzaldehydes with
electron-withdrawing groups (entries 6, 7, 8, 9) whereas electron-donor groups
result in lower conversion. In reactions catalyzed by commercial CaO, irrespective
of the type of benzaldehyde, selectivity for (E)-b-phenyl nitroolefins was higher
than for 1-phenyl-2-nitroethanol. For the modified CaO, however, selectivity for
(E)-b-phenyl nitroolefins is lower than for 1-phenyl-2-nitroethanol.
Catalytic mechanism
A possible mechanism of the Henry reaction between nitromethane (A) and
benzaldehyde (B) over CaO is illustrated in Scheme 3. CaO captures a proton and
promotes rearrangement of the nitromethane to form C, which is an active
intermediate of the Henry reaction. Via classical nucleophilic attack of C on
benzaldehyde, intermediate D is formed. In route (i), the O- of D picks up an H?
123

Y. Tang et al.
Table 4 Henry reaction of nitromethane and different benzaldehydes
CaO
Modified CaO
Entry
Benzaldehydes
Conversion
of
Yield of
Yield of
Conversion
of
Yield of
Yield of
(E)-β-phenyl
1-phenyl-2
(E)-β-phenyl
1-phenyl-2
benzaldehyde (%) -nitroolefin (%) -nitroethanol (%) benzaldehyde (%) -nitroolefin (%) -nitroethanol (%)
O
HO
1
2
88.0
89.0
91.2
74.9
86.6
92.4
96.3
94.1
97.9
86.9
51.6
53.8
62.1
45.2
57.7
80.7
53.8
73.6
56.3
74.7
36.4
35.2
29.1
29.7
38.9
12.7
42.5
20.5
41.6
97.9
93.4
98.9
80.7
96.3
87.8
99.8
97.7
99.6
87.2
40.3
22.6
29.4
31.8
31.3
10.1
33.1
16.9
20.7
8.9
57.6
70.8
69.5
48.9
65.0
77.7
66.7
80.8
78.9
78.3
O
HO
OCH3
O
HO
HO
3
O
H3CO
H3CO
4
O
O
O
5
O
6
Cl
O
7
Cl
O
8
NO2
O
9
O2N
NO2
O
10
12.2
from CaO to form E; in route (ii), the Ca2? picks off an O- from D to form a
carbocation F, and deprotonation of F results in the formation of product G.
Conclusions
In this work, an efficient catalyst for the Henry reaction of benzaldehyde and
nitromethane was developed by modifying commercial CaO with an organic
123

Direct Henry reactions by using modified calcium oxide
A
B
C
D
O
O
N
O
O
N
N
O
(i)
H
(ii)
O
H
O
-O
O-
H
Ca2+
O2-
Ca2+
Ca2+
Ca2+
Ca2+
O2-
O2- Ca2+
CaO
CaO
CaO
(i)
(ii)
O
N
O
+
O
N
OH
O
N
O
F
G
O
H
E
Ca2+
O2-
Ca2+
CaO
(E)-β-phenyl-nitro-olefin
1-phenyl-2-nitroethanol
Scheme 3 Possible mechanism of the Henry reaction between benzaldehyde and nitromethane over CaO
modifier. Compared with commercial CaO, the catalytic activity of the modified
CaO was substantially enhanced, so the reaction time was reduced from 12 to 5 h
with 99.8 % conversion of benzaldehyde under the same conditions. Selectivity for
(E)-b-phenyl nitroolefins and 1-phenyl-2-nitroethanol were quite difference for CaO
and modified CaO. Furthermore, the modified CaO has good stability in air, which is
very important to organic reactions. Characterization by FTIR and TG showed that
the modifier bonds to the surface of CaO chemically and that hydration of CaO
during the modification process is very slight.
Experimental
Modification of CaO
Commercial CaO (purity 99 %) was purchased from Sigma-Aldrich. The surface
modification procedure was as follows. An appropriate amount of commercial CaO
particles without thermal pretreatment (2.8 g, 100–160 mesh) was added directly
to a solution of benzyl bromide in methanol (40 mL) with stirring at room
temperature. The amount of benzyl bromide was varied in the range from 0.01 to
0.5 %. After 24 h, the oxide was isolated, washed with methanol to remove excess
modifier, and the modified CaO was obtained after vacuum drying. To increase
reproducibility, the particles were sieved before use. Particles of 100–160 mesh size
were selected for all experiments.
Humidity test
Commercial CaO and modified CaO particles were kept in a container under
saturated humidity at room temperature for several days to enable absorption of
123

Y. Tang et al.
water by the surface. The samples were weighed at regular intervals. The amount of
moisture absorbed as a percentage of sample weight was evaluated by use of the
following equation, where Dm is the increase in weight and m0 is the initial weight.
56Dm
18m0
w% ¼
ꢁ 100
Henry reaction activity test
The Henry reaction of benzaldehyde and nitromethane shown in Scheme 1 was
conducted as follows. First, 0.5 mmol benzaldehyde (99.5 %) and 100 mmol
nitromethane (99.0 %) were placed in a 50-mL round-bottomed flask equipped with
a magnetic stirrer and condenser. The flask was immersed in a constant temperature
bath, then approximately 0.25 mmol catalyst was introduced at room temperature.
The reaction mixture was monitored by TLC. The catalyst was isolated by filtration
and washed with dichloromethane (3 9 10 mL). The filtrate was then concentrated
under reduced pressure. The crude product was purified by column chromatography
(silica gel, 60–120 mesh; petroleum ether–ethyl acetate 5:1 v/v). The products were
characterized by comparing 1H NMR and mass spectrometry data with those
reported in the literature.
Characterization of the catalysts
FTIR spectrophotometry (Nicolet Nexus 670, USA) was used to identify the surface
group on the catalyst surface. TGA experiments were performed with Q600 SDT
thermal analysis equipment (TA Instruments, USA) under a flow of air in the
temperature range 25–800 °C and with a ramp rate of 10 °C min-1
.
Acknowledgments This work was financially supported by grants from a scientific research program
funded by Shaanxi Provincial Education Department (No. 11JK0591), natural science research plan
projects of Shaanxi Science and Technology Department (No. 2011JQ2014), and the open funds of the
Shanghai Key Laboratory of Green Chemistry and Chemical Process.
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