Solvent-free oxidation of aldehydes to acids by TBHP using environmental-friendly MnO-14-exchanged Mg-Al hydrotalcite catalyst

Article abstract of DOI:10.1007/s12039-012-0268-7

A number of hydrotalcite (Mg-Al, Mn-Al, Co-Al, Ni-Al, Mg-Fe, Mg-Cr and Cu-Al) catalysts, with or without MnO^-1₄-exchange, were evaluated for their performance in the solvent-free oxidation of benzaldehyde to benzoic acid by tert-butyl hydroperoxide under reflux in the absence of any solvent. The MnO^-1₄ - exchanged Mg-Al-hydrotalcite (Mg/Al = 10) showed high activity in the oxidation of different aromatic and aliphatic aldehydes to their corresponding acids and also showed excellent reusability in the oxidation process which is environmental-friendly. Indian Academy of Sciences.

Full text of DOI:10.1007/s12039-012-0268-7

c
J. Chem. Sci. Vol. 124, No. 4, July 2012, pp. 835–839. ꢀ Indian Academy of Sciences.
Solvent-free oxidation of aldehydes to acids by TBHP using
environmental-friendly MnO−4 1-exchanged Mg-Al hydrotalcite catalyst
VASANT R CHOUDHARY∗, DEEPA K DUMBRE and VIJAY S NARKHEDE
Chemical Engineering and Process Development Division, National Chemical Laboratory, Pune 411 008, India
e-mail: vr.choudhary@ncl.res.in; vrc0001@yahoo.co.in
MS received 22 November 2010; revised 11 November 2011; accepted 22 November 2011
Abstract. A number of hydrotalcite (Mg–Al, Mn–Al, Co–Al, Ni–Al, Mg–Fe, Mg–Cr and Cu–Al) catalysts,
with or without MnO−4 1-exchange, were evaluated for their performance in the solvent-free oxidation of ben-
zaldehyde to benzoic acid by tert-butyl hydroperoxide under reflux in the absence of any solvent. The MnO4−1
-
exchanged Mg–Al-hydrotalcite (Mg/Al = 10) showed high activity in the oxidation of different aromatic and
aliphatic aldehydes to their corresponding acids and also showed excellent reusability in the oxidation process
which is environmental-friendly.
Keywords. Aldehyde; acid; oxidation; TBHP; MnO−4 1-exchanged Mg–Al–hydrotalcite catalyst.
1. Introduction
have also been reported on the oxidation of benzalde-
hyde by quaternary ammonium permangate,8 KMnO4
in aqueous solutions of surfactant9 and basic perman-
ganate10 in stoichiometric amounts. The stoichiometric
reactions have severe limitations, such as the formation
of large amounts of liquid and solid wastes and the cor-
rosive nature of the reaction medium. It is practically
very interesting and important to use permanganate in
catalytic amounts for the oxidation of different alde-
hydes to their corresponding acids without using any
solvent to make the aldehyde oxidation environmen-
tally benign. We report here that different aromatic
and aliphatic aldehydes can be selectively oxidized
to their corresponding acid with high yields by tert-
butyl hydroperoxide (TBHP) using MnO−4 1-exchanged-
hydrotalcite as a catalyst in the absence of any solvent.
The catalyst can be easily separated by simple filtration
and reused in the reaction several times.
Oxidation of aldehyde to acid is one of the frequently
used transformations in organic synthesis. It is one of
the most important steps in the synthesis of pharmaceu-
tically important compounds like α-arylpropionic acids
e.g., ibuprofen, flurbiprofen, etc. These are known to be
good antiinflammatory agents.1
Aldehydes can be oxidized to carboxylic acids by
atmospheric oxygen but with very low product yield.2
Several oxidizing reagents have been reported for
obtaining good product yields in the oxidation of alde-
hydes. However, the commonly used oxidants like
chromic acid, potassium permanganate in acidic, basic
and neutral solution, bromine and nitric acid are not
suitable for the large scale preparation of carboxylic
acid because of the formation of hazardous waste.
Balicki Roman3 achieved mild oxidation of aromatic
and heteroaromatic aldehydes to the corresponding
acids using urea hydrogen peroxide in formic acid.
Howarth4 reported the oxidation of aromatic aldehy-
des, containing both electron donating and electron
withdrawing substituents at para position by oxygen
at atmospheric pressure using the [Ni(ace)2] catalyst
in an ionic liquid. A few studies have been reported
on the oxidation of benzaldehyde using homogeneous
or heterogeneous catalysts, such as Na2WO42H2O,5
Amberlite IR-120(acidic form)6 and cerium(IV) sul-
fate in aqueous sulfuric acid.7 A number of studies
2. Experimental
The hydrotalcite catalysts were characterized for their
crystalline structure by XRD (using a Philips 1730
series) diffractometer and CuKα radiations), for their
surface area by the single-point N2 adsorption method
(using a surface area analyzer; Quanta Chrome, USA)
and for their basicity by measuring the pH of their
suspension in water (0.15 g of catalyst in 10 ml of
deionized water at room temperature) and also for their
concentration of carbonate anions (by treating the cat-
∗For correspondence
835

836
Vasant R Choudhary et al.
alyst with 4N HNO3 and measuring quantitatively the 2.2 General procedure for oxidation reaction
evolved CO2).
The catalytic oxidation of benzaldehyde was carried
out in a magnetically stirred round bottom flask (capa-
city: 25 cm3), provided with a mercury thermometer
for measuring the reaction temperature, in the absence
2.1 General procedure for catalyst preparation
of any solvent, at the following reaction conditions:
The Mg–Al–hydrotalcite was synthesized by adding
two aqueous solutions, one containing magnesium
nitrate and aluminum nitrate with the required Mg/Al
ratio and the other containing potassium hydroxide and
reaction mixture = 10 mmol aldehyde + 15 mmol
TBHP (70% TBHP in water) + 0.1 g catalyst, tem-
perature = under reflux (bath temperature of 97◦C),
reaction time = 1–10 h. The reaction was monitored
potassium carbonate, drop-wise into a flask contain-
by TLC. After completion of the reaction, the product
ing deionized water under vigorous stirring at 40◦C,
was isolated and purified using column chromatogra-
while maintaining a constant pH of 11–12, by a pro-
phy. The oxidation of substituted aromatic and aliphatic
cedure similar to that described earlier.11 The result-
aldehyde was also carried out as above. All the prod-
ing white gel was aged for 0.5 h and then filtered,
ucts were known compounds and were characterized by
thoroughly washed and dried at 80◦C in a vacuum oven
their melting point and/or NMR spectroscopy.
for 12 h. The preparation and characterization of other
hydrotalcites containing different divalent [M (II)] and
trivalent [M (III)] metallic elements (viz. Mn–Al, Co–
3. Results and discussion
Al, Ni–Al, Mg–Fe, Mg–Cr and Cu–Al) were described
earlier and their crystal structure as hydrotalcite w−as1
confirmed by XRD.11–13 In these hydrotalcites, MnO4
anions were incorporated by the procedure described
earlier.11,14
Results showed the performance of the different hydro-
talcite (with or without MnO−4 1-exchange) catalysts
in the oxidation of benzaldehyde to benzoic acid by
Table 1. Performance of different catalysts in the oxidation of benzaldehyde to benzoic acid by
TBHP (reaction conditions: benzaldehyde = 10 mmol, TBHP = 15 mmol, catalyst = 0.1 g, bath
temperature = 97◦C).
Reaction
time (h)
Conversion
of benzaldehyde (%)
No.
Catalyst
1
2
3
4
5
6
7
8
MnO−4 1(1.55 mmol/g)/ Ni-Al HT (Ni/Al = 3)
MnO−4 1(0.94 mmol/g)/ Ni-Al HT (Ni/Al = 10)
MnO−4 1(0.42 mmol/g)/ Cu-Al HT (Cu/Al = 3)
MnO−4 1(0.77 mmol/g)/ Mn-Al HT (Mn/Al = 3)
MnO−4 1(0.23 mmol/g)/ Co-Al HT (Co/Al = 3)
MnO−4 1(0.14 mmol/g)/ Co-Al HT (Co/Al = 10)
MnO−4 1(2.11 mmol/g)/ Mg-Cr HT (Mg/Cr = 3)
MnO−4 1(0.77 mmol/g)/ Mg-Fe HT (Mg/Fe = 3)
MnO−4 1(0.71 mmol/g)/ Mg-Fe HT (Mg/Fe = 10)
MnO−4 1(0.80 mmol/g)/ Mg-Al HT (Mg/Al = 3)
MnO−4 1(0.40 mmol/g)/ Mg-Al HT (Mg/Al = 10)
Mg-Al-HT (Mg/Al = 10)
5
3
4
4
4
4
4
4
4
3
3
4
4
4
4
4
4
3
10
3
90.5
92.7
81.0
90.4
90.6
95.5
90.9
87.8
99.0
94.2
99.9
11
20
23
12
19
11
99.5
60b
9
10
11
12
13
14
15
16
17
18
19
20
Ni-Al-HT (Ni/Al = 10)
Co-Al-HT (Co/Al = 10)
Mg-Fe-HT (Mg/Fe = 10)
Co-Al-HT (Co/Al = 3)
nil
MnO−4 1(0.40 mmol/g)/ Mg-Al HT (Mg/Al = 10)a
MnO−4 1(0.40 mmol/g)/ Mg-Al HT (Mg/Al = 10)
c
KMnO4
50.5
a5th reuse of the catalyst. bWhen O2 was used as the oxidizing agent, cAmount used was equivalent
to that of MnO−4 1(0.40 mmol/g)/Mg-Al HT (Mg/Al = 10) catalyst

Oxidation of aldehydes to acids
837
TBHP in the absence of any solvent (table 1). No
product other than benzoic acid was observed. The
MnO−4 1-exchanged hydrotalcites were characterized for
their XRD phases, d(001) spacing and pH of water
slurry (table 2). The characterization of the hydrotalcite
catalysts before their MnO−4 1-exchange was reported in
talcites. Because of the immobilization of the
active catalyst component, the separation/removal
of the catalyst from the reaction mixture is easy,
simply by filtration. Moreover, the catalyst can
be reused in the reaction several times without a
significant loss in its activity.
our earlier studies.11–14 From these results, the follow- (iv) The MnO−4 1-exchanged Mg–Al–HT (Mg/Al = 10)
ing important observations are made.
also showed good oxidation activity when molec-
ular oxygen is used as the oxidizing agent (entry
no.19). However, in this case, the reaction was
found to be relatively quite slow.
(i) The hydrotalcites without MnO−4 1-exchange
showed very poor catalytic activity in the oxida-
tion. However, after the MnO−4 1-exchange, their
activity was drastically increased (from 11–23% to
81–99.9% conversion).
The XRD analysis of all the catalysts (entry nos. 1–
16 in table 1) showed the presence of pure hydrotal-
cite phase in all the catalysts except for the MnO−4 1-
exchanged Cu–Al–HT (which was found to contain
hydrotalcite phase along with copper hydroxide and
aluminum hydroxide phases).
The surface area of the catalyst before use was
26 m2/g. After the use in the oxidation (entry no.18),
the surface area of the catalyst remained almost the
same. Also the concentration of Mn in the catalyst
(0.40 mmol/g) was not changed. Moreover, the XRD
spectra for the MnO−4 1-exchanged Mg–Al–HT (before
and after its 5th reuse in the benzaldehyde oxidation
reaction) were found to be almost similar. These facts
revealed that, after its 5th use in the oxidation, the cata-
lyst retained its hydrotalcite structure and also had high
stability during the oxidation.
(ii) The Mg–Al–HT (Mg/Al = 10) showed a little
or no activity in the oxidation (the benzaldehyde
conversion is almost the same as that observed
in the absence of any catalyst). However, after
the MnO−4 1-exchange, it showed highest activity
(99.9% conversion) in the oxidation. The catalyst
also showed excellent reusability in the reaction;
after its fifth reuse, it showed no significant loss in
its activity (entry no. 18). Other MnO−4 1-exchanged
hydrotalcites with Ni/Al, Co/Al and Mg/Fe = 10
(entry nos. 2, 6 and 9) also showed good activity
(92.7, 95.5 and 99.0%, respectively). It is interest-
ing to note that all these MnO−4 1-exchanged hydro-
talcites showed better activity when their diva-
lent/trivalent metal ratio was higher. This may be
attributed to their higher basicity.11–14
Since the MnO−4 1-exchanged Mg–Al–HT (Mg/Al =
10) showed the best performance in the benzaldehyde
to benzoic acid oxidation, further work on the oxida-
tion of other aromatic and aliphatic aldehydes by TBHP
to their corresponding acids (scheme 1) in the absence
of any solvent was carried out using this catalyst. The
results are presented in table 3. In this case, the prod-
uct was isolated from the reaction mixture and purified
using column chromatography. The results in table 3
reveal that the catalyst shows high activity even in the
(iii) When KMnO4 [equivalent to the MnO−4 1 ions
present in the MnO−4 1-exchanged Mg–Al–HT
(Mg/Al = 10)] was used as a homogeneous cat-
alyst in the reaction, the conversion under the
similar reaction conditions was 50.5%; which is
much lower than that obtained using the differ-
ent MnO−4 1-exchanged hydrotalcites. This reveals
that the catalytic activity of MnO−4 1 ions increases
appreciably after their immobilization in the hydro-
Table 2. Characterization of different MnO−4 1-exchanged hydrotalcite catalysts.
XRD
phases
d(001)
spacing (nm)
pH of water
slurry
Catalyst
MnO−4 1(1.55 mmol/g)/ Ni-Al HT (Ni/Al = 3)
MnO−4 1(0.94 mmol/g)/ Ni-Al HT (Ni/Al = 10)
MnO−4 1(0.23 mmol/g)/ Co-Al HT (Co/Al = 3)
MnO−4 1(0.14 mmol/g)/ Co-Al HT (Co/Al = 10)
MnO−4 1(0.77 mmol/g)/ Mg-Fe HT (Mg/Fe = 3)
MnO−4 1(0.71 mmol/g)/ Mg-Fe HT (Mg/Fe = 10)
MnO−4 1(0.80 mmol/g)/ Mg-Al HT (Mg/Al = 3)
MnO−4 1(0.40 mmol/g)/ Mg-Al HT (Mg/Al = 10)
Pure HT
Pure HT
Pure HT
Pure HT
Pure HT
Pure HT
Pure HT
Pure HT
0.780
0.784
0.752
0.756
0.764
0.769
0.765
0.767
8.0
8.0
7.8
7.8
9.6
9.6
9.7
10.5

838
Vasant R Choudhary et al.
Table 3. Performance of the MnO−4 1-exchanged Mg-Al-hydrotalcite catalyst in the oxidation of various aldehydes to corre-
sponding acids by TBHP (reaction conditions: aldehyde = 10 mmol, TBHP = 15 mmol, catalyst = 0.1 g, bath temperature =
97◦C).
Reaction
time (h)
Melting point
Isolated product
yield (%)
Entry
Aldehyde
Product
of product (◦C)
CHO
COOH
1
2
3
3
121
183
95
87
CHO
COOH
MeO
MeO
MeO
MeO
COOH
COOH
CHO
CHO
MeO
MeO
OMe
OMe
3
3
170
87
MeO
HO
MeO
HO
4
5
3
4
212
133
86
90
COOH
CH
CH CH
CHO
CH
CHO
COOH
Br
Br
6
7
5
1
150
Liq.
80
87
CH3.CH2.CHO
CH3.CH2.COOH
CHO
CHO
COOH
COOH
8
9
2
1
Liq
73
87
86
CH
CH
3
3
N
CHO
N
COOH
10
3
192
80
COOH
CHO
Me
Me
11
2
172
88
CHO
CHO
COOH
COOH
12
13
14
2
2
2
Liq
Liq
Liq
85
86
85
CHO
COOH
CHO
COOH
NO
NO
2
2
15
5
138
79
CHO
CHO
COOH
COOH
MeO
MeO
16
17
2
4
182
Liq
85
85

Oxidation of aldehydes to acids
839
CHO
THBP, catalyst, reflux
COOH
R
R
R = H, OMe, Me, NO2
Scheme 1. Oxidation of various aldehydes to acids.
oxidation of different aromatic and aliphatic aldehydes dation. It also showed high activity for the solvent-
to their corresponding acids in the absence of any free oxidation of different aromatic and aliphatic
solvent. A small decrease in the product yield is, how- aldehydes to their corresponding acids. This cata-
ever, observed because of the substitution in aromatic lyst is environmental-friendly because of its excellent
ring of the aromatic aldehydes.
reusability in the oxidation and operation in the absence
When benzaldehyde was refluxed with the MnO−4 1- of any solvent. The oxidation over this catalyst most
exchanged hydrotalcites catalyst under N2 atmosphere probably involves redox mechanism.
(in the absence of TBHP and air), an appreciable
amount of benzoic acid was formed. This indicates
that the catalytic aldehyde to acid oxidation by TBHP
Acknowledgements
mostly involves a redox mechanism with following
redox reactions:
VRC and DKD are grateful to the National Academy
of Sciences (NASI), India, for Senior Scientist
Platinum Jubilee Fellowship and Project Assistantship,
respectively.
Mn (VII) O−4 1 + PhCHO
→ Mn (V) O−3 1 + PhCOOH
(1)
Mn (V) O−3 1 + (CH3)3 COOH
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Both the hydrotalcites (viz. Mg–Al–HT, Ni–Al–HT,
Cu–Al–HT, Co–Al–HT, Mn–Al–HT, Mg–Cr–HT and
Mg–Fe–HT) and KMnO4 alone show poor catalytic
activity in the solvent-free oxidation of benzalde-
hyde to benzoic acid by TBHP. However, when
the MnO−4 1 anions are immobilized in the hydrotal-
cites through anion exchange, the resulting MnO−4 1-
exchanged hydrotalcites show high catalytic activity
in the oxidation. The MnO−4 1-exchanged Mg-Al-HT
(Mg/Al = 10) showed highest activity in the oxi-