Nitro compounds reduction via hydride transfer using mesoporous mixed oxide catalyst

Article abstract of DOI:10.1016/j.molcata.2006.06.033

The catalysts NiCo₂O₄ were prepared by co-precipitation and surfactant approach. These catalysts were characterized by crystal size, BET surface area, pore volume, average pore size, XRD and SEM. The catalytic activity of co-precipit

Full text of DOI:10.1016/j.molcata.2006.06.033

Journal of Molecular Catalysis A: Chemical 261 (2007) 232–241
Short communication
Nitro compounds reduction via hydride transfer
using mesoporous mixed oxide catalyst
∗
Nivedita S. Chaubal, Manohar R. Sawant
Department of Chemistry, Institute of Chemical Technology, Matunga, Mumbai 400019, Maharashtra, India
Received 29 May 2006; accepted 13 June 2006
Available online 11 September 2006
Abstract
The catalysts NiCo
2
O
4
were prepared by co-precipitation and surfactant approach. These catalysts were characterized by crystal size, BET
and surfactant-assisted
surface area, pore volume, average pore size, XRD and SEM. The catalytic activity of co-precipitated NiO, CoO, NiCo
2 4
O
NiCo was tested for the reduction of 4-Chloro-nitrobenzene using variety of hydrogen donor and basic promoters. The reaction was carried out
2
O
4
at different temperatures and catalysts concentrations. Reusability, recyclability of the catalyst and reductions of variety of nitro arenes was also
studied.
©
2006 Published by Elsevier B.V.
Keywords: NiCo2O4; Co-precipitation; Surfactant; Reduction
1
. Introduction
also require sophisticated techniques and sometime hazardous
chemicals. Activity and selectivity of CHTR of nitrobenzene
and substituted nitrobenzene using FeCl3·6H2O/In [9], Ni [10],
Ru3(CO)12/chelating diimines [11], Te [12] have been studied
but the regio- and stereoselectivity suffers. Selvam and co-
workers [13] have made good attempt to enhance the selectivity
of amines by incorporating Co in hexagonal molecular sieves
(HMS) but the synthesis of the catalyst by hydrothermal treat-
ment and incorporation of Co inside the cavity of the HMS
increases the cost of the catalyst. Further as Co is supported
inside the cavity of HMS, subsequent recycling can leach the
Co.
Reduction of nitro compounds is carried industrially to obtain
hydroxylamine, azo-, azoxycompounds and amines, which find
applications in dyes, agrochemicals, pharmaceuticals and pho-
tographic chemicals. Extensive reduction methodologies are
being practiced using metal/HCl system, H2 gas, sulfides and
polysulfides. Due to the stringent environmental concerns con-
stant efforts are being made to find safer alternative to the
above-mentionedprocesses. Catalytichydridetransferreduction
(
CHTR) has received continuous recognition as an alternative
for the clean production of variety of chemicals [1]. CHTR is
facilitated by hydride transfer from the readily available hydro-
gen sources such as hydrocarbons [2], primary or secondary
alcohols [3], formic acid and its salts [4].
Nitro compounds CHTR is an important reaction for the syn-
thesis of amines which are used as bulk chemical as well as an
intermediate in pharmaceuticals, dye stuff industries, etc. [5].
CHTR have been demonstrated by various greener processes,
namely reduction in super critical water [6], ultrasound pro-
moted reduction [7], use of ionic liquid [8]. These processes are
efficient to laboratory scale but lack commercial viability and
In order to improve regio-, stereoselectivity and prevent cat-
alyst leaching we have synthesized mesoporous NiCo2O4 using
novel sugar based surfactant (alkyl polyglucoside, C10) act as a
template.
2. Experimental
2.1. Chemicals
Nickel chloride, cobalt chloride, sodium hydroxide, ethanol,
&2-propanol, 1-butanol, formic acid, acetic acid, glucose,
1
∗
Vitamin C, pyridine, liquid ammonia, triethyl amine, trimethyl
amine, calcium and magnesium nitrates, for the preparation of
Corresponding author. Tel.: +91 2224145616; fax: +91 2224145614.
E-mail address: mrsawant2@rediffmail.com (M.R. Sawant).
1
381-1169/$ – see front matter © 2006 Published by Elsevier B.V.
doi:10.1016/j.molcata.2006.06.033

N.S. Chaubal, M.R. Sawant / Journal of Molecular Catalysis A: Chemical 261 (2007) 232–241
233
methanol; to this 50% of decylpolyglycoside aqueous solution
was added with vigorous stirring for 1 h. The resulting solution
◦
was gelled at 50 C for 20 h (catalyst preparation is optimized).
◦
This as made bulk samples were calcined at 300 C for 6 h in air
to remove the surfactant.
2.4. Catalyst characterization
Scheme 1. Probable reaction mechanism on the surface of the catalyst contain-
ing electron rich and electron deficient sites ꢀ− is electron rich site and ⊕ is
electron deficient site on the surface of the catalyst.
All the catalysts were characterized by XRD and BET sur-
face area measurements. %Composition of each element in the
catalysts was done by ICP-AES. SEM and FT-IR of NiCo2O4
(
a) and (b) are reported.
From the XRD, NiO, CoO and NiCo2O4 (a) were crys-
theirhydroxidesbyco-precipitationmethod, wereobtainedfrom
m/s S.D. Fine Chemicals Ltd., Mumbai. Cognis, Germany, is
acknowledged for alkyl polyglycoside (C10).
talline with the sharp peaks in wide angle whereas NiCo2O4
(b) gave no peaks. Small angle XRD of NiCo2O4 (b) gave a
◦
peak at 2θ = 2.0 , thus the catalyst NiCo2O4 (b) prepared by
surfactant templated approach is mesoporous in nature. Crys-
tallite size is calculated by Scherrer formula (Fig. 1; Table 1).
The nitrogen adsorption–desorption isotherm shown in Fig. 2
is a type-IV isotherm, as defined by the IUPAC, with a sharp
step at intermediate relative pressures with an H2 hysteresis
loop [19] due to capillary condensation. The isotherm, shows
three well-defined stages can be identified: (a) a slow increase
in nitrogen uptake at 0.0–0.2 relative pressures, corresponding
2
.2. Choice of catalyst
Cobalt and nickel based catalysts are attracting much atten-
tion in the recent years. They are used in various industrially
important reactions such as hydrodesulphurization [14], reduc-
tion of nitriles [15] and azocompounds [16]. Literature for
hydride transfer reduction over cobalt and nickel based cata-
lysts shows lack of stereo- as well as regioselectivity due to their
non-porous nature; if porosity of these catalysts increases, these
catalysts can be used to selectively reduce a particular functional
group as well as bulky moieties can be reduced effectively with
good yields. Hence we have extended the amphiphilic template
approach to cobalt nickel mixed oxide catalyst to improve the
mesoporosity and to enhance the surface area, so that the bulky
moieties will be reduced selectively with high yields. Individual
cobalt and nickel were used for the hydrogenation; no attempt
has been made to study the synergism in the activity of the mixed
cobalt and nickel catalyst in the reaction. So we have thought it
desirable to study the effect of mixed cobalt and nickel oxide
catalyst in the transfer hydrogenation with an aim to obtain
maximum selectivity and high yield for the reduced products
(Scheme 1).
2
2
.3. Catalyst preparation
.3.1. Co-precipitation [17]
Ni and Co nitrates were taken in the 1:2 molar ratio, respec-
tively. Ten percent NaOH solution was added as a precipitating
agent under vigorous stirring until pH of 9.0–9.5 was achieved.
◦
The solution was kept on a water bath maintained at 80 C for
3
h and then oxidized by adding requisite amount of 30% H2O2
drop wise with constant stirring and aged for 24 h. The resul-
tant precipitate was filtered and washed with distilled water to
remove excess of precipitating agent and nitrate ions. The pre-
◦
◦
cipitate was dried at 110 C for 3 h and calcined at 300 C for
0 h. Similar procedure was applied for preparation of NiO and
CoO. H2O2 addition gives single-phase catalyst.
1
2
.3.2. Templated route [18]
In a typical preparation, Co and Ni nitrates were taken in
stoichiometric amounts (2:1) and were dissolved in 10 ml of
Fig. 1. XRD of all the catalysts.

2
34
N.S. Chaubal, M.R. Sawant / Journal of Molecular Catalysis A: Chemical 261 (2007) 232–241
Table 1
Elemental analysis of the catalysts NiCo2O4 (b)
Compound
%Co
%Ni
ICP-AES
Theoretical
ICP-AES
Theoretical
NiCo2O4 (b) (fresh)
NiCo2O4 (b) (used)
28
28
28
–
14.7
14.7
14
–
to monolayer–multilayer adsorption on the pore walls, (b) a
sharp step at 0.2–0.6 relative pressures indicative of capillary
condensation in mesopores and (c) a plateau at 0.6–0.9 relative
pressures associated with multilayer adsorption on the external
2
surface. Characteristic nitrogen BET surface area is 202.9 m /g.
As observed from BJH adsorption pore size distribution, a major
part of the surface area is contributed by pores with a diameter
˚
characteristic of pores ranging from 35 to 65 A. There is also a
˚
significant part due to mesopores ranging from 65.0 to 100.0 A,
as shown the pore size distribution (Fig. 3). The catalyst is essen-
tially mesoporous.
Fig. 3. Pore size distribution.
This narrow (PSD) shows the particles are uniform. Theoreti-
cal and observed percentage of Co and Ni by ICP-AES are given
in Table 1. From the results obtained from elemental analysis we
can say that catalyst is stable under the given reaction condition.
Three SEM photographs of (a) and (b) are shown in
Figs. 4 and 5, respectively. SEM of (a) shows the aggregation
of the particles due to which not many active sites are being
exposed to the substrate and reaction yield is low. SEM of sam-
ple b shows the rod-like morphology of the particles with two
lobes at one end suggesting the increasing surface area with large
exposure of active sites for the substrate to react on the surface.
(
a) and (b) correspond to the co-precipitated catalyst and
−
1
templated catalyst. Sharp peaks in the region 1600–1700 cm
indicate the presence of Bronsted acidity and in the region
−
1
1
400–1500 cm indicate the presence of Lewis acidity the cat-
alyst (see Fig. 6).
Fig. 4. SEM of NiCo2O4 (a).
Fig. 2. N2 adsorption–desorption isotherms.
2 4
Fig. 5. SEMs of NiCo O (b).

N.S. Chaubal, M.R. Sawant / Journal of Molecular Catalysis A: Chemical 261 (2007) 232–241
235
Fig. 6. IR spectra of NiCo2O4 (a) co-precipitated catalyst and (b) templated
catalyst.
2
.5. Experimental procedure
Fig. 7. Effect of various catalysts.
The reactor consisted of a standard flat bottom cylindrical
3.1. Effect of various catalysts
vessel of 6 cm internal diameter of 100 ml capacity equipped
with four equi-spaced baffles, a pitched bladed turbine impeller
and a condenser. The assembly was kept in an isothermal oil
bath at a known temperature and mechanically agitated with an
electric motor. In a typical experiment, the reaction mixture con-
sisted of 15 mmol 4-Chloro1-nitrobenzene and 15 mmol sodium
hydroxide. Thetotalvolumeofthereactionmixturewasmadeup
to 15 ml with 2-propanol which acts as a hydrogen donor as well
The effect of various catalysts on transfer hydrogenations was
studied over CoO, NiO, NiCo2O4 (a) and NiCo2O4 (b) keeping
other reaction parameters constant (Fig. 7; Table 2). NiCo2O4
a) was prepared by co-precipitation method whereas NiCo2O4
b) was prepared in the presence of the alkyl polyglucoside (C10)
template. A trend dependent on surface area was seen from the
result, as the surface area increases yield of amines goes on
increasing. Thus NiCo2O4 (a) was selected to optimize other
parameters of the hydrogenations. In absence of either catalyst
or base no reaction occurred as reported.
A standard experiment was done wherein the reaction was
started with 4-Chloro1-nitrobenzene and 2-propanol under alka-
line conditions at refluxing temperature without NiCo2O4. After
(
(
◦
as a solvent. The reaction was carried out at 82 C at a speed of
3
7
00 rpm with 0.05 g/cm as the standard catalyst loading. Sam-
ples of the reaction mixture were withdrawn periodically for
analysis.
3
. Results and discussions
1
.5 h no conversion of 4-Chloro1-nitrobenzene was observed
The main objective of the work was to investigate the activ-
due to absence of catalyst. Same reaction was repeated without
base in the presence of the catalyst. It was found that the catalyst
alone was unable to reduce 4-Chloro1-nitrobenzene via hydride
transfer from the H-donor. Hence both catalyst as well as base
ity of NiCo2O4 and probable mechanism of hydrogen transfer
reduction of 4-Chloro1-nitrobenzene. The reaction is given in
Scheme 2.
Table 2
Transfer hydrogenation of 4-Chloro1-nitrobenzene to amine with 2-propanol over different catalysts
Crystallitea
size (A)
Unit cell
parameter (a) (A)
Porosityb
Pore
size (A)
Unreacted 4-Chloro1-
nitrobenzene (mass%)
Yield of amine
d
Catalyst
BET surface
area (m /g)
2
˚
˚
c
˚
(mass%)
No catalysts/no NaOH
CoO (a)
NiO (a)
NiCo2O4 (a)
NiCo2O4 (b)
–
13.3
15
20.1
202.9
–
–
0.15
0.2
0.25
0.46
100
34.2
33.1
10.1
4.0
00.0
65.8
66.9
89.9
94.0
0.4
0.59
10
4.012
4.23
8.111
8.056
12
20
22
65
35
(
a) Prepared by co-precipitation method and (b) prepared by templated approach. Reaction conditions: 4-Chloro1-nitrobenzene (15 mmol), NaOH pellets (15 mmol)
3
refluxed in 15 ml of 2-propanol and catalyst (0.05 g/cm ).
a
Determined by Schererr equation, a is determined by XRD.
The porosity is estimated from the pore volume determined using the adsorption branch of the N2 isotherm curve at the P/P0 = 0.986 single point.
Average pore size by BET.
b
c
d
All yields are isolated, purified by column chromatography and analyzed by IR spectroscopy.

2
36
N.S. Chaubal, M.R. Sawant / Journal of Molecular Catalysis A: Chemical 261 (2007) 232–241
Scheme 2. Reduction of nitrobenzene by catalyst transfer hydrogenation.
was necessary for promoting hydride ion from alcohol to nitro
moiety to obtain the reduced product.
3.3. Effect of pH, [base], [4-Chloro1-nitrobenzene] and
[2-propanol]
3
.2. Effect of various hydrogen donors and bases
It was observed that in the absence of base reaction did not
proceeded. The pH of the system was varied to study its effect on
the reduction. As the pH of the reaction system was increased,
the selectivity towards amines increased. At lower pH formation
of hydroxybenzene was favored. At acidic pH no reduction was
observed. The best results were obtained above pH 9.0. Con-
centration of the base was varied to study the dependence of the
reaction. From Fig. 8, it can be said that as the concentration
of the base increases conversion of 4-Chloro1-nitrobenzene to
4-Chloro1-aminobenzene increases. At lower concentration the
partially reduced compounds are obtained. We can conclude that
the rate of reaction is dependent on the concentration of the base
till the optimum concentration (15 mmol) beyond that the rate of
reaction was independent of the concentration of base. Change
in the concentration of either nitrobenzene or 2-propanol do not
affect the reduction reaction, i.e. rate of reaction is independent
of the concentration of 4-Chloro1-nitrobenzene and 2-propanol.
The entries in Table 3 report the effect of different hydro-
gen donors and bases used; under otherwise similar condi-
tions it was concluded that 2-propanol/NaOH was the efficient
pair for transfer hydrogenation of 4-Chloro1-nitrobenzene to
4
-Chloro1-aminobenzene. Amongst alcohols 2-propanol was
the most suited hydrogen donor as primary alcohols dehydro-
genate to aldehyde which acts as a catalysts poison. HCOOH
and CH3COOH fail to produce aniline with Et3N/Me3N/NH3 or
with NaOH. d-Glucose and Vitamin C were unsuccessful with
pyridine as a promoter, but with NaOH yield of azoxy benzene
was more as compared to aniline. 2-Propanol with Ca(OH)2 and
Mg(OH)2 yielded less amount of 4-Chloro1-aminobenzene due
to their lower basicity than NaOH. With Et3N, Me3N or NH3
2
-propanol did not give any product. Hence all further reactions
were carried with 2-propanol/NaOH system.

N.S. Chaubal, M.R. Sawant / Journal of Molecular Catalysis A: Chemical 261 (2007) 232–241
237
Table 3
Effect of different hydrogen donors and different bases on transfer hydrogenation of 4-Chloro1-nitrobenzene to amine
c
H-donor
Promoter
Yield (mass%)
Amine
a
b
Azoxycompound /azocompound
4-Chloro1-nitrobenzene
CH3CH2OH
CH3CHOHCH3
CH3CHOHCH3
CH3CHOHCH3
CH3CHOHCH3
CH3CHOHCH3
CH3CHOHCH3
CH3CHOHCH2CH3
HCOOH
HCOOH
HCOOH
HCOOH
CH3COOH
CH3COOH
CH3COOH
CH3COOH
d-Glucose
NaOH
NaOH
Ca(OH)2
Mg(OH)2
Et3N
Me3N
NH3
NaOH
NaOH
Et3N
Me3N
NH3
NaOH
Et3N
Me3N
NH3
NaOH
Pyridine
NaOH
Pyridine
72.8
96.0
17.0
15.0
–
–
–
45.0
–
–
–
–
–
–
–
–
4.0
–
10.0
–
–
–
27.0
3.8
40.0a
42.0
48.1
100.0
100.0
99.8
54.0
100.0
100.0
99.0
99.0
100.0
100.0
99.6
98.7
61.2
99.0
61.0
99.9
36.0a
–
–
–
–
–
–
–
–
–
–
–
–
34.0a
–
28.0a
–
d-Glucose
Vitamin C
Vitamin C
◦
3
Reaction conditions: 4-Chloro1-nitrobenzene (15 mmol), promoter (15 mmol) refluxed at 80 C in 15 ml of 2-propanol and catalyst (0.05 g/cm ).
a
Azoxy conpound.
azo compound.
b
c
All yields are isolated, purified by column chromatography and analyzed by IR spectroscopy.
The ratio of base and 4-Chloro1-nitrobenzene should be main-
tained at 1, below that partially reduced compound are obtained
predominantly and above which the reduction is independent.
Fig. 9 shows the effect of [catalyst] on the conversion of 4-
Chloro1-nitrobenzene. The conversion increased with increase
in[catalyst], whichisduetotheproportionalincreaseinthenum-
3
ber of active sites. However, beyond of 0.05 g/cm [catalyst],
3
.4. Effect of [catalyst]
there was no significant increase in the conversion and hence all
further experiments were carried out at this catalyst loading. At
Rate of reaction is directly proportional to [catalyst] base
on the entire liquid phase volume. The catalyst concentration
3
was varied from 0.01 to 0.06 g/cm on the basis of total vol-
ume of the reaction mixture keeping all other conditions similar.
Fig. 8. Effect of [base].
Fig. 9. Effect of [catalyst].

2
38
N.S. Chaubal, M.R. Sawant / Journal of Molecular Catalysis A: Chemical 261 (2007) 232–241
Fig. 11. Reusability of the catalyst.
3
.7. Catalytic hydrogen transfer reduction of substituted
Fig. 10. Effect of temperature.
nitro compounds
The selective and rapid reduction of nitro compounds is an
interesting area of research, particularly when other potentially
reducible moieties are present in the molecule (see Table 4).
Nitro group attached to the aromatic ring can withdraw more
strongly from benzene as compared to other reducible groups.
Hence, can be easily adsorbed on the catalyst as soon as the
product is formed, it is desorbed, the reaction is chemo selec-
tive. The catalyst also promises regioselectivity as can be seen
from the dinitro compounds. It was found that the activity was
significantly influenced by the position of the substituents on the
aromatic ring. Halo nitro aryls has the tendency of hydrogenoly-
sis of C–X bond (Cl, Br, I), but no hydrogenolysis was observed
in the case of 4-Chloro1-nitrobenzene. It was also observed that
presence of electron releasing group reduces the reduction of
nitro compounds thus electron releasing group at para position
enhanced the reduction. Hence it is possible that rate of reduc-
tion is decided by initial adsorption of the substrate which is rate
determining.
lower catalyst concentration partially hydrogenated product was
also obtained along with amine. From the results obtained we
can say that as the concentration of catalyst increases, the con-
version of 4-Chloro1-nitrobenzene to 4-Chloro1-aminobenzene
with 100% selectivity.
3
.5. Effect of temperature
Catalytictransferhydrogenationishighlytemperaturedepen-
dant. The reaction did not proceed at room temperature and at
lower temperature partially hydrogenated product was obtained,
thus the effect of temperature was studied from 323 to 363 K
under otherwise similar conditions (Fig. 10). The conversion of
4
-Chloro1-nitrobenzene increased with increase in the temper-
ature of the reaction. Beyond 353 K there was no increase in the
conversion of nitrobenzene.
Aliphatic nitro were totally reluctant for reduction may be
due to structural similarity between substrate, 2-propanol and
acetone formed in the reaction, thus this method cannot be used
for obtaining aliphatic amines.
3
.6. Reusability and activity of the catalyst
The reusability of the catalyst was studied by recycling
the filtered and acetone washed catalyst under otherwise sim-
ilar conditions. Fig. 11 shows the conversion of nitrobenzene
against time, and that the conversion has been decreased from
4. Plausible reaction mechanism on the surface of the
catalyst
9
6 to 76% from the fresh to the third reuse of the catalyst.
There was attrition of the catalyst; hence some catalyst was
lost during filtration. There was a loss of ∼5% catalyst dur-
ing filtration. Thus in the next experiment, the volume of the
On the surface NiCo2O4 we have two types of sites electron
rich (electrons provided by lattice oxygen) and electron deficient
(holes as nature of NiCo2O4 is P-type semiconductor).
reaction mixture was adjusted to make the catalyst loading
3
0
.05 g/cm , the standard value. It was found that the conver-
sion were practically the same as in the previous run, thereby
suggesting that there was no loss in activity. Thus it can be con-
cluded that there was little decrease in the catalyst activity with
reuse.
Step 1. An electron deficient site forms weak adsorption bond
with nitro compound through the oxygen of nitro com-
pound due to presence of electron pair on it. An elec-
tron rich site on the surface of the catalyst forms

N.S. Chaubal, M.R. Sawant / Journal of Molecular Catalysis A: Chemical 261 (2007) 232–241
239
Table 4
Catalytic hydride transfer reduction of substituted nitro benzenes
Substrate (15 mmol) Product (15 mmol)
Time (min)
Yield (mass%)
97.0
3
5
5
0
96.0
70.0
90.0
120
9
9
0
0
89.0
9
0
85.0
78.0
92.0
1
1
50
20
9
0
87.0
92.0
91.0
2
2
70
10
300
69.8

2
40
N.S. Chaubal, M.R. Sawant / Journal of Molecular Catalysis A: Chemical 261 (2007) 232–241
Table 4 (Continued )
Substrate (15 mmol)
Product (15 mmol)
Time (min)
Yield (mass%)
81.0
100
1
1
80
50
72.0
85.5
1
50
50
83.0
86.0
1
◦
3
Reaction conditions: 4-Chloro1-nitrobenzene (15 mmol), base (15 mmol) refluxed at 80 C in 15 ml of 2-propanol and catalyst (0.05 g/cm ). All yields are isolated,
purified by column chromatography and analyzed by IR spectroscopy.
weak adsorption bond with OH group of 2-propanol.
Na OH is also adsorbed on electron deficient site.
Acknowledgments
+
−
+
−
−
Step 2. Na OH transfers OH to 2-propanol, which in turn
transfers H to nitro group to form hydroxyl intermediate
and acetone is given out as a side product.
The authors would like to acknowledge Tata Institute of Fun-
damental Research, Mumbai, for XRD and SEM. RSIC IIT,
Mumbai, forICP-AES. AuthorsarethankfultoProf. B.M. Bhan-
age for detailed discussions.
−
+
+
−
Step 3. OH is retransferred to Na to form Na OH . H from
hydroxyl compound is also retransferred to an electron
rich site to form H2O and azoxy intermediate.
Step 4. In this step azoxy intermediate is reduced completely
to amine and desorbed from the surface and oxygen is
gained back by the catalyst lattice. Again the surface is
ready to adsorbed new reactant molecules and the pro-
cess continues till optimum conversion of nitro arenes to
amines. Throughout the catalytic cycle NaOH promotes
the transfer of hydrogen from 2-propanol.
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donor and NaOH. NiCo2O4 prepared by the alkyl polyglucoside
[
(
C10) template was the best catalyst that gave 100% selectivity
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