Design of zeolite catalysts for nitroaldol reaction under mild condition

Article abstract of DOI:10.1016/j.apcata.2012.05.010

NaY and KL zeolites were modified by impregnating with potassium fluoride for investigating their basicity and catalytic activity towards Henry nitroaldol reaction under different reaction conditions. Effects of solvents, temperature, catalysts and ratio

Full text of DOI:10.1016/j.apcata.2012.05.010

Applied Catalysis A: General 433–434 (2012) 122–127
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Design of zeolite catalysts for nitroaldol reaction under mild condition
Rasna Devi, Ruli Borah, Ramesh C. Deka∗
Department of Chemical Sciences, Tezpur University, Napaam, Tezpur 784 028, India
a r t i c l e i n f o
a b s t r a c t
Article history:
NaY and KL zeolites were modified by impregnating with potassium fluoride for investigating their basic-
ity and catalytic activity towards Henry nitroaldol reaction under different reaction conditions. Effects
of solvents, temperature, catalysts and ratio of substrate versus reagents were studied to optimize the
reaction conditions. The catalysts were characterized by FTIR, XRD and SEM techniques and the products
were characterized by 1H NMR, 13C NMR and FTIR techniques. It was observed that NaY/KF zeolite with
H2O–MeOH pair as solvent showed good conversion and selectivity at room temperature. Both XRD and
FTIR study showed no change of spectral patterns for both modified and parent zeolites. We have again
showed a plot of Hammett free energy linear relationship in terms of percentage yield and Hammett
substituent constant.
Received 19 March 2012
Received in revised form 9 May 2012
Accepted 10 May 2012
Available online 18 May 2012
Keywords:
Zeolite
Base catalyst
Henry nitroaldol reaction
␤-Nitro alcohols
Hammett’s constant
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
Homogeneous catalysts like NaOH with surfactant catalyst hex-
adecyltrimethylammonium chloride, benzyltrimethylammonium
Nitroaldol reaction is a classic C C coupling reaction between
a nitroalkane and an aldehyde or a ketone (Scheme 1) [1–3].
The importance of the reaction is due to the synthetic utility of
nitroaldol product which can be easily converted into ketones, ␤-
amino alcohols, carboxylic acids, nitro alkenes, etc. and also can
act as suitable intermediate for synthesis of various biological com-
pounds [4–9]. Again, possible competitive reactions like Cannizzaro
reaction, base catalyzed elimination of water from nitroaldol and
polymerization of nitroalkenes may occur simultaneously during
progress of the reaction [10–15]. Thus to minimize side products
an investigation of environmentally acceptable reaction conditions
and effective catalysts is under the scope of study during these days.
Asymmetric Henry reaction has been carried out through two
strategies from very early days, i.e. by pre-activation of nitroalkane
species with a fluoride ion source and direct reaction of different
aldehydes with unmodified nitroalkanes. Shibasaki’s rare earth cat-
alyzed metal complexes involving La with BINOL system and Li
[16,17], metal catalyzed reactions like Zn and Cu based complexes,
Co-SALEN complexes, organocatalysts like chiral guanidinium salts
and bifunctional amine–thiourea complexes have been found
successful for Henry reaction [18–22]. Again various heteroge-
neous catalysts like Amberlyst A-21, functionalized SBA-15, amine
supported on silica gel, hydrotalcite catalysts in ionic liquid, sup-
ported BINOL complexes, etc. also have been reported [23–34].
hydroxide, tris-(2,4,6-trimethoxyphenyl) phosphine (TTMP), etc.
are reported previously as efficient catalysts for Henry reaction
[35,36]. However they have their limitations in recovery, as well as
long reaction time, formation of multiple products, low yield, etc.
In this report we have tried to optimize Henry reaction conditions
using zeolite as base catalyst.
In recent years, zeolites and modified zeolites have been used as
acid and base catalysts for a number of organic reactions [37–39].
However, their uses as base catalysts are limited as compared to
acid catalyzed reactions. Due to their peculiar shape selectivity,
thermal stability and ordered pore structures, reactions can occur
inside the pores and give selective products [40,41]. Although many
reactions have been carried out through exchanging extra frame-
work cations of zeolites by alkali and alkaline earth metal cations,
their uses in Henry nitroaldol reaction is not very common. In our
work, we have modified NaY and KL zeolites by a wet impregna-
tion method from aqueous solution of KF and used them for Henry
reaction [42].
2. Experimental
2.1. Chemicals and reagents
NH4Y zeolite was purchased from Sigma Aldrich, Germany;
aldehydes from Merck and Sigma Aldrich, Germany; Nitromethane
from G.S. Chemicals Testing Lab & Allied Industries, New Delhi;
potassium fluoride (KF), sodium sulfate (Na2SO4), silica gel, iodine
∗
Corresponding author. Tel.: +91 3712 267008; fax: +91 3712 267005/6.
E-mail address: ramesh@tezu.ernet.in (R.C. Deka).
0926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2012.05.010

R. Devi et al. / Applied Catalysis A: General 433–434 (2012) 122–127
123
Scheme 1. Henry reaction catalyzed by zeolites.
and solvents were purchased from RANKEM and KL zeolite was
obtained from National Chemical Laboratory, Pune.
Fig. 1. XRD pattern of (a) NH4Y, (b) NaY, (c) NaY/KF, (d) KL zeolite and (e) KL/KF.
2.2. Preparation of catalysts
2.4. Catalyst testing
+
At first NH4 ion of parent NH4Y zeolite were exchanged with
Na+ ion. In this procedure 5 g of NH4Y zeolite was stirred with
100 mL of 1 M NaNO3 solution for 6 h at 90 ◦C. The exchanged zeo-
lites were then filtered and again stirred with fresh NaNO3 solution.
This was repeated for 4 times, filtered, washed and dried in oven for
overnight at 110 ◦C. The modified NaY and KL basic zeolites were
prepared by an impregnation method from aqueous solution of KF.
Zeolite amount of 500 mg was taken to impregnate 0.40 mmol of
KF. It was done for 24 h and slurries were dried in an oven at 100 ◦C
for 14 h.
The reactions were performed under two conditions, i.e. with
solvents at room temperature and with solvents on increasing tem-
perature. Solvent study was carried out by taking 1 mmol of the
aldehyde, 1 mmol of the nitroalkane and 20 mg of the catalysts with
various solvents at room temperature. All reactions were carried
out for 48 h for comparison and reaction conditions were optimized
by varying temperature, amount of Nitromethane and catalysts.
The reactions were monitored via thin layer chromatography and
yields were obtained from 1H NMR analyses of the crude reaction
mixture.
2.3. Catalysts characterization
3. Results and discussion
All catalysts samples were characterized by X-ray diffraction
method using a Rigaku (miniflex UK) X-ray diffractometer with
Cu K␣ radiation (1.5418 A) at a scan speed of 2 min
The powder X-ray diffraction analyses were performed for both
parent and KF modified zeolite samples. Results obtained from XRD
analysis are shown in Fig. 1. XRD pattern of parent NaY zeolite
corroborate well with the literature reports [44]. The peak posi-
tions of parent and KF modified zeolites are not altered which
proves that modification does not affect the size and shape of
the unit cell. Again no other additional peaks for KF are observed
which indicates its well dispersion in zeolites. However, it is obvi-
ous from the decreased peak intensities of the modified samples
that crystallinity of KF modified samples is decreased somewhat.
Fig. 2 shows crystallinity differences of modified samples with that
of references obtained by comparing intensities of some highest
intensity peaks. Peak intensities of the modified samples decrease
meagerly. It may be due to decrease in surface area and pore volume
after modification.
Fig. 3 shows FTIR spectra of NH4Y, NaY, NaY/KF, KL and KL/KF
zeolites. All spectra were recorded after drying overnight in an
oven at 100 ◦C. Infrared bands shown here in the region from
500 to 1200 cm−1 provide information of the structural features
of zeolite framework. Strong vibrations at ∼950 to 1050 cm−1
and at ∼650 to 700 cm−1 are due to asymmetric and symmet-
ric T O (Si or Al) stretching mode, respectively, in tetrahedral
co-ordination and these both arise due to internal tetrahedral vibra-
tions. These stretching modes are responsible for Si/Al composition
of the framework and may shift to higher or lower frequency
depending on increase or decrease of Si to Al ratio. Again band
at ∼500 to 600 cm−1 is due to double ring and bands at ∼1100
to 1200 cm−1 and 750–800 cm−1 are, respectively, due to asym-
metrical and symmetrical stretching vibrations of T O in external
linkages. As reported earlier there is a possibility of dealumination
in case of NaY/KF and KL/KF due to reaction of KF with aluminum
in zeolites [44]. However no distinct shifting of these bands has
been observed in our case, i.e. no dealumination occurs (Fig. 3).
This may be due to small amount of KF loading in this study. Thus,
FTIR spectra also agree well with our observation that KF is well
◦
−1
and 2ꢀ
˚
range of 10–40◦. FTIR spectra of various catalysts and products were
recorded on a Nicolet Impact Model-410 spectrometer with 1 cm−1
resolution and 32 scans in the mid IR (400–4000 cm−1) region using
the KBr pellet technique. The SEM measurements were carried
out using a JEOL JSM-6390LV scanning electron microscope. The
images were taken at an accelerator voltage of 15 kV and magnifi-
cation of micrographs ranges from 10,000× to 5500×. The catalysts
were secured onto brass stubs, sputter coated with Platinum and
viewed in JEOL JSM-6390LV microscope. Flame photometry was
carried out with FP-114 Flame Photometer.1H and 13C NMR spectra
were recorded on a JEOL JNM-ECS 400 taking Me4Si as the internal
standard in CDCl3 and CD3OD solvents. Preparative column chro-
matography was carried out with silica gel 60–120 mesh size and
thin-layer chromatography (TLC) was carried out on glass plates
with silica gel.
The strength of basic sites was measured qualitatively using
Hammett indicator method [43]. In this technique, 25 mg of solid
catalyst calcined at 450 ◦C was shaken with 1 mL solution (0.1%
in methanol) of the indicator and left to equilibriate for 2 h. The
color of the catalyst was then noted. The following Hammett indica-
tors were used: neutral red (pKBH+ = 6.8–8.0) and phenolphthalein
(pKBH+ = 8.0–9.6). The base strength is reported as stronger than the
weakest indicator which exhibits a color change and weaker than
the strongest indicator which exhibits no color change. The aqueous
soluble amount of basic sites was determined by acid–base titra-
tion technique [42]. In this procedure 100 mg of calcined sample
was shaken in 10 mL of water at room temperature for 24 h and
the catalyst was separated by centrifuge. The filtrate was neutral-
ized with 10 mL of 0.05 M aqueous HCL. Then the remaining acid
was titrated with 0.01 M of standard aqueous NaOH. Oxalic acid
dihydrate was taken for standardization of NaOH and a methanol
solution of phenolphthalein (1 mL, 0.1 mg/mL) was taken as indi-
cator.

124
R. Devi et al. / Applied Catalysis A: General 433–434 (2012) 122–127
Table 1
Effect of solvents in Henry reaction of 4-nitrobenzaldehyde and Nitromethane cat-
alyzed by NaY/KF.
Entry
Solvents
Time (h)
Conversion
(%)a
Selectivity
(%)b
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
No
No
MeOH
EtOH
(Me)2CHCH2OH
CH2Cl2
CHCl3
THF
DMF
10 minc
10 mind
48
48
48
48
48
48
48
48
12
48
48
48
48
0
0
0
0
92
90
90
71
78
64
78
71
98
98
88
82
15
73
85
86
66
78
80
67
57
88
76
60
80
–
CH3CN
H2O–MeOH
H2O–CHCl3
H2O–EtOH
H2O–(Me)2CHCH2OH
H2O
a
Reaction were carried out with 1 mmol scale with molar ratio 1:1 of
4-nitrobenzaldehyde/Nitromethane, 2 mL solvent and 0.02 g NaY/KF at room tem-
perature.
b
Yields and selectivities were obtained from 1H NMR data of crude reaction
mixture (byproducts formed are dehydrated product and polymerized product 1,3-
dinitro-2-(4-nitrophenyl)-propane).
c
Reaction with p-nitrobenzaldehyde.
Reaction with 1-naphthaldehyde.
d
dispersed in zeolite and their framework structures are not affected
after modification.
The scanning electron micrographs of NaY, NaY/KF, KL and
KL/KF zeolites are shown in Fig. 4. Magnification of micrographs
at 10,000× and 5500× clearly shows homogeneity in shapes and
highly crystalline nature of all samples. From the micrographs no
aggregation of salt particles on support is observed, i.e. KF is highly
dispersed on support.
Fig. 2. The intensity ratio of the modified zeolites with parent zeolites at some points
for (a) NaY zeolites and (b) KL zeolites.
The strength of basic sites and their amounts were measured
using acid–base titration and the Hammett indicator methods.
As expected from our catalytic experiment, the base strength
of KF modified NaY and KL zeolites are found to be higher
(8.0 < pKBH+ < 10.0) than the parent zeolites (6.8 < pKBH+ < 8.0). In
neutral red solution NaY remains as red, KL slight yellow, KL/KF
comparatively brighter than the previous two and NaY/KF shows
the brightest yellow color. This indicates that NaY/KF possesses
the highest base strength among the four catalysts with pKBH+
value greater than 8.0 but less than 10.0. Moreover, no change in
color of the four catalysts in phenolphthalein solution indicates that
the base strengths (pKBH+) of the catalysts are less than 10.0. The
amount of soluble basicity in all the catalysts is almost same.
In our attempt to investigate environmentally benign reaction
conditions, we have first used NaY/KF catalyst to study whether the
reaction proceeds without solvent or not. We have ground 1 equiv.
of p-nitrobenzaldehyde and 1 equiv. of Nitromethane with 20 mg
catalyst using mortar and pestle for 10 min. Thin layer chromatog-
raphy observation showed no conversion for both solid and liquid
substrates (Table 1, entries 1 and 2). It may be due to inadequate
contact between catalysts and reactants to precede the reaction
further. Therefore we have initially studied the effect of solvents to
optimize conditions for Henry reaction. For this purpose we have
chosen p-nitrobenzaldehyde and Nitromethane as model reactants
and performed the reaction at room temperature with different
organic solvents and 50% aqueous solution of organic solvents.
Both polar aprotic and protic solvents, less polar organic solvents
and organic solvents with water were screened to understand the
effects. As observed from Table 1, dichloromethane (CH2Cl2) and
chloroform (CHCl3) having similar polarity gives comparable con-
version of 71% for CH2Cl2 and 78% for CHCl3. Aprotic polar solvents
DMF, acetonitrile, THF have similar effects on reaction rate and we
Fig. 3. FTIR spectra of (a) NH4Y, (b) NaY, (c) NaY/KF, (d) KL and (e) KL/KF.

R. Devi et al. / Applied Catalysis A: General 433–434 (2012) 122–127
125
Fig. 4. SEM images of (a) NaY, (b) NaY/KF, (c) KL and (d) KL/KF.
found enhanced rate and selectivity with 50% aqueous alcoholic
solution. From Table 1, it is observed that conversion increases in
the order MeOH–H2O > EtOH–H2O > isobutanol–H2O. It may be due
to steric effect of alkyl groups from MeOH to isobutanol which
affects the activation of catalysts through solvation process. The
reaction in H2O was very slow. This is due to the fact that the differ-
ences in hydrophobic nature of the reactant molecules place them
in heterogeneous phases. Among different solvents, MeOH–H2O
(1:1 ratio) gives the highest percentage of conversion and selec-
tivity. Therefore MeOH–H2O pair is the best solvent in our study
and hence we performed other subsequent reactions presented in
Tables 2–6 in this solvent.
As shown in Table 2, temperature effect was studied by taking
Nitromethane, p-nitrobenzaldehyde and MeOH–H2O (1:1) as sol-
vent. It can be seen from Table 2 that temperature has a great effect
on reaction time. Reaction time decreases from 12 h at RT to 1 h at
60 ◦C (entries 1 and 4) and selectivity decreases slightly on increas-
ing temperature. Formation of side products of similar nature was
observed both at room temperature and at higher temperatures.
However, formation of side products decreases significantly with
higher amount of Nitromethane at room temperature. At higher
temperatures both speed of reaction and formation of side prod-
ucts become faster. So, the further studies were performed at room
temperature.
We next investigated the effect of amount of catalyst on reac-
tion time and percentage conversion. The results are summarized
in Table 3. As the catalysts amount increases, the conversion and
selectivity also increases. We have selected 20 mg catalyst as our
optimum amount for further studies.
We have also performed stoichiometry study with 20 mg of the
catalyst at room temperature to observe the effect of molar equiv-
alent of Nitromethane on yield of Henry reaction. From Table 4 it
is seen that with increasing the amount of Nitromethane the reac-
tion time gradually decreases. It can be seen that the best result was
obtained from 1:5 and 1:10 ratio (entries 4 and 5) of aldehyde and
nitroalkane. However, in order to have maximum atom economy
we chose 2 mmol Nitromethane as our optimum amount for Henry
reaction.
In order to study the effect of pore architecture we performed
Henry reactions in two large pore zeolites namely faujasite and
zeolite-L having supercage and straightly channel, respectively. The
basicity of both the zeolites was enhanced by impregnating KF.
Table 2
Table 3
Effect of temperature on Henry reaction of 4-nitrobenzaldehyde and Nitromethane
catalyzed by NaY/KF.
Effect of the amount of catalyst used in the conversion (%) and selectivity (%) of
Henry reaction.
Entry
Temperature (◦C)
Time (h)
Conversion
(%)a
Selectivity
(%)b
Entry
Catalyst (mg)
Time (h)
Conversion (%)a
Selectivity (%)b
1
2
3
4
10
15
20
25
15
13
12
12
91
94
98
99
78
82
88
92
1
2
3
4
RT
40
50
60
12
5
2.5
1
98
98
99
97
88
80
77
63
a
Reactions were carried out in 1 mmol scale with molar ratio 1:1 of 4-
nitrobenzaldehyde/Nitromethane and 2 mL solvent (MeOH–H2O) and 0.02 g NaY/KF
at room temperature.
a
Reactions were carried out in 1 mmol scale with molar ratio 1:1 of 4-
nitrobenzaldehyde/Nitromethane, 2 mL solvent (MeOH–H2O) and 0.02 g NaY/KF.
b
b
Yields and selectivities were obtained from 1H NMR data of crude reaction
Yields and selectivities were obtained from 1H NMR data of crude reaction
mixture.
mixture.

126
R. Devi et al. / Applied Catalysis A: General 433–434 (2012) 122–127
Table 4
Effect of amount of Nitromethane on conversion (%) and selectivity (%) for various Henry reactions.
Entry
4-Nitrobenzaldehyde
(mmol)
Nitromethane
(mmol)
Time
(h)
Conversion
(%)a
Selectivity
(%)b
1
2
3
4
5
1
1
1
1
1
1
2
3
5
10
12
4
3
3
98
99
99
99
99
88
91
93
91
93
45 min
a
Reactions were carried out in 1 mmol 4-nitrobenzaldehyde with different molar amount of Nitromethane, 2 mL solvent (MeOH–H2O) and 0.02 g NaY/KF at room
temperature.
b
Yields and selectivities were obtained from 1H NMR data of crude reaction mixture.
Table 5
Effect of catalysts on Henry reaction of 4-nitrobenzaldehyde and Nitromethane.
Entry
Zeolite
Time (h)
Conversiona (%)
Selectivityb (%)
1
2
3
4
KL
NaY
KL/KF
NaY/KF
12
12
7
87
90
95
99
63
75
67
91
4
a
Reactions were carried out in 1 mmol scale with molar ratio 1:2 of 4-
nitrobenzaldehyde/Nitromethane, 2 mL solvent (MeOH–H2O) and with different
catalysts at room temperature.
b
Yields and selectivities were obtained from 1H NMR data of crude reaction
mixture.
NaY/KF is found to be the best catalyst among all. This may be
due to the presence of the supercages in NaY which allow bulkier
transition states to form during the reaction. Table 5 shows effect
of different catalysts on Henry reaction. NaY/KF again was found
to show superior conversion and selectivities among the four. In
all cases, we have observed from Tables 1–5 that there are very
less amount of formation of dehydrated and polymerized prod-
ucts. Further studies with different substrates were carried out with
NaY/KF.
Fig. 5. Hammett plot for Henry reaction.
Table 6 shows Henry reaction with variety of aldehydes, e.g.
simple aromatic aldehydes, heterocyclic aldehyde and polycyclic
aromatic aldehydes. As expected reactions with substrates having
electron withdrawing groups (entries 1, 2, 3) are faster giving up
to 92% yield (entries 1) and reactions with substrates having elec-
tron donating groups (entries 6, 7, 8) are slower (entry 6, 53% yield).
Reactions for 1-naphthaldehyde and 2-furaldehyde rates are some-
what faster giving 78% and 72% yield, respectively. From results of
Table 6 for various meta- and para-substituted benzaldehydes it
would be interesting to see whether there is a linear relationship
between the percentage yield and the Hammett substituent con-
stants. As observed from Fig. 5 Henry reaction with these modified
zeolites shows a linear relationship between the percentage yield
and the Hammett substituent constants (ꢁ).
The recyclability of NaY/KF was investigated for three times
and performed the reaction with 4-nitrobenzaldehyde and
Nitromethane (1:5 ratio) using MeOH–H2O solvent. Used Catalyst
was dried for 24 h in an oven at 150 ◦C before performing the reac-
tion. Reaction time gradually increases after each run. It may be due
to slight leaching or deactivation of active sites of zeolite through
adsorption of polar solvent molecules. To investigate the leaching
phenomena we performed flame photometry for K+ ion. Negligi-
ble amount of K+ leaching (0.001 g/L) was detected and it has not
hamper the reaction rate considerably.
4. Conclusions
In conclusion we have found NaY/KF as an efficient catalyst
for nitroaldol reaction in MeOH–H2O solvent at room temperature
with good to moderate yields. The presence of H2O with alcohol as
reaction medium greatly effects the reaction rate and selectivities.
Both temperature and molar ratio of Nitromethane with aldehydes
play important role in the reaction.
Table 6
Henry reaction of Nitromethane with different aldehydes.
Entry
Aldehyde
Time (h)a
Yield (%)b
1
2
3
4
5
6
7
8
4-Nitobenzaldehyde
2-Nitrobenzaldehyde
3-Nitrobenzaldehyde
1-Naphthaldehyde
2-Furfuraldehyde
4-Hydroxybenzaldehyde
p-Tolualdehyde
4-Chlorobenzaldehyde
4
7
7
24
24
48
48
48
92
90
84
78
72
53
63
70
Spectral data
2-Nitro-1-(4-nitrophenyl)ethan-1-ol (entry 1, Table 6)
a
1H NMR (400 MHz, CDCl3): ı 3.52 (s, 1H), ı 4.58–4.60 (m, 2H),
ı 5.59–5.61 (m, 1H), ı 7.62 (d, JHH = 8 Hz, 2H), 8.22 (d, JHH = 8 Hz,
2H); 13C NMR (400 MHz CDCl3): 69.7, 80.3, 123.8, 126.7, 144.9,
Reactions were carried out in 1 mmol scale with molar ratio 1:2 of 4-
nitrobenzaldehyde/Nitromethane, 2 mL solvent (MeOH–H2O) and 0.02 g NaY/KF at
room temperature.
b
Isolated yields after TLC purification. All products were characterized by FT-IR,
147.7 ppm; IR (KBr): 1338, 1536, 1604, 3444 cm−1
.
1H NMR and 13C NMR. Byproducts formed were not isolated.

R. Devi et al. / Applied Catalysis A: General 433–434 (2012) 122–127
127
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