Highly active nanostructured Co3O4 catalyst with tunable selectivity for liquid phase air oxidation of p-cresol

Article abstract of DOI:10.1246/cl.2008.310

This is a first report of highly efficient heterogeneous nanostructured Co₃O₄ catalyst (6-8 nm) having high surface area (95 m²/g) developed for selective liquid phase air oxidation of p7-cresol under atmospheric pressure conditions. Copyright

Full text of DOI:10.1246/cl.2008.310

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Chemistry Letters Vol.37, No.3 (2008)
Highly Active Nanostructured Co O Catalyst with Tunable Selectivity
3
4
for Liquid Phase Air Oxidation of p-Cresol
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ꢀ2
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ꢀ1
Vikas S. Kshirsagar, Subramanian Vijayanand, Hari S. Potdar, Pattayil A. Joy, Kashinath R. Patil, and Chandrashekhar V. Rode
1
Chemical Engineering & Process Development Division, National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411008, India
2
Physical and Materials Chemistry, National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411008, India
Centre for Materials Characterization, National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411008, India
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(Received December 14, 2007; CL-071386; E-mail: cv.rode@ncl.res.in, hs.potdar@ncl.res.in)
This is a first report of highly efficient heterogeneous nano-
structured Co3O4 catalyst (6–8 nm) having high surface area
(95 m /g) developed for selective liquid phase air oxidation of
p-cresol under atmospheric pressure conditions.
A-16
A-8
2
A-4
A-2
A-0
Developing new catalysts for oxidative technologies for the
production of hydroxybenzaldehydes is of continuing interest
due to the potential advantages of achieving better selectivities
Bulk
2
0
30
40
50
/degree
60
70
80
1
,2
and reduced effluent problems. Studies on the oxidation of
p-cresol have been reported in the literature, since its oxidation
products such as p-hydroxybenzyl alcohol (PHBAlc) and p-hy-
droxybenzaldehyde (PHB) are used as building blocks for sever-
2
θ
Figure 1. XRD patterns of Co3O4 nanoparticles prepared with
different digestion times of 0 h (A0), 2 h (A2), 4 h (A4), 8 h
(
A8), and 16 h (A16).
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,4
al key organic products. Catalytic oxidation of p-cresol is also
a challenging reaction due to presence of a deactivating phenolic
prepared in this work, and the XRD peaks were indexed on
the basis of spinel structure with the space group of Fd3m. The
1
,2,5–8
–OH and formation of undesired side products.
In those lit-
˚
eratures, mainly cobalt-based catalysts, both in homogeneous as
well as heterogeneous forms, have been studied including CO
lattice parameter obtained, a ¼ 8:072 A, is in good agreement
˚
with the reported value for Co3O4 powder (a ¼ 8:072 A; JCPDS
9
oxidation. However, the preparation of such catalysts is either
No. 76-1802).
a multistep procedure or cobalt metal leached out under reaction
conditions, and the selectivity to PHB is affected by the forma-
tion of nonoxidation side products such as p-hydroxybenzyl
The relatively broader peaks of A0, A2, A4, A8, and A16
samples indicate nanocrystalline nature of the prepared catalyst
samples.
1
0
methyl ether (PHBME), and tarry materials. Although, a
The HRTEM image of Co3O4 particles (Figure 2a) indicates
rod-type aggregates (diameter: 4–5 nm, length: 20–30 nm) that
are formed from 2–3 nm primary particles. The parallel lattice
fringes across almost all the primary particles are clearly visible
(Figure 2b) which confirm the perfectly oriented aggregation of
the nanoparticles of Co3O4. It is clearly seen that the aggregated
Co3O4 rod-like particles are composed of many small Co3O4
nanoparticles. These interconnected nanoparticles of Co3O4
form pores having an average diameter of 15 nm.
.
homogeneous CoCl2 6H2O catalyst has been widely used for
p-cresol oxidation, it has been found that after the oxidation re-
action, cobalt(II) chloride gets converted to a solid black residue
which inhibits the oxidation reaction. To overcome these
drawbacks, we have recently reported the use of an insoluble co-
balt oxide, Co3O4, in its bulk form, which showed activity under
1
1
higher (830 kPa) partial pressure of oxygen. Due to inherent
advantages of nanomaterials (e.g., high surface to volume
ratio) we have designed nanostructured Co3O4 prepared by a
coprecipitation/digestion method, which was found to be highly
active heterogeneous catalyst for liquid phase air oxidation of
p-cresol.
Figure 3 shows the Co 2p XPS spectrum exhibiting the Co
2p3=2 and Co 2p1=2 doublet core level peaks at binding energies
(BE) of 780.2 and 795.3 eV with a difference of 15.1 eV between
the 2p3=2 and 2p1=2 binding energies. These binding energies are
comparable to that reported for Co3O4 indicating the presence of
The nanostructured Co3O4 was prepared from Co(NO3)2
and K2CO3 by a simple protocol without using any template,
involving a simultaneous coprecipitation/digestion technique
2þ
3þ
13
both Co and Co species in the catalyst. This redox couple
1
2
followed by calcination at 573 K in air. Bulk Co3O4 was avail-
able commercially from M/s SD Fine Chemicals, India. All the
catalytic oxidation experiments were conducted at atmospheric
pressure in a 100-mL-capacity three-necked-glass reactor fitted
with a cooling condenser. In a typical experiment, p-cresol
(
27.7 mmol), NaOH (111 mmol), and n-propanol (50 mL) were
charged to the reactor and the contents were refluxed for 1.5 h.
To this 0.02 g of catalyst was added followed by bubbling the
air for oxidation. Reaction progress was monitored by HPLC
2
method using RP-18 column.
Figure 1 shows the XRD patterns of the Co3O4 catalysts
Figure 2. HRTEM images of (a) nanorod aggregates of Co O ;
3
(b) primary nanoparticles of Co3O4.
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Copyright Ó 2008 The Chemical Society of Japan

Chemistry Letters Vol.37, No.3 (2008)
311
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1
1
1
00000
80000
60000
40000
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00000
one C–H bond is activated and dissociates, PHBAlc is formed
while activation and dissociation of another C–H bond gives
PHB. In case of nanostrutured Co3O4 catalyst, the dynamic equi-
XPS
2
p3/2
2
p1/2
2þ
3þ
ꢁ
librium of Co , Co , and O2 gets modified even at mild con-
ditions in such a way that the tuning of selectivity to alcohol and
aldehyde could be achieved as shown in Table 1. Further inves-
tigations to elucidate the reaction mechanism are in progress.
In summary, Co3O4 nanoparticles which aggregated to form
stable nanorods were prepared by a simple coprecipitation–
digestion method without using a template or a capping
agent. These nanoparticles were found to be highly active for
air oxidation of p-cresol under mild conditions while the selec-
tivity to the oxidation products could be altered by changing
the digestion time during the preparation of the catalyst. A leach-
ing test was also carried out by hot filtration to remove the
catalyst after an intermittent p-cresol conversion of 17%. No
further conversion of p-cresol was observed for the hot filtrate
without catalyst, confirming no leaching of Co occurred under
the reaction conditions. Catalyst reuse experiment showed the
same activity as that for the fresh catalyst. Reproducibility of
catalyst preparation with respect to its physicochemical charac-
terization as well as the activity measurement for oxidation was
found to be excellent. This approach allows the preparation of
nanosize metal oxides with controlled morphology having
potential applications in catalysis and other electronic devices.
7
75
780
785
790
795
800
Binding Energy /eV
Figure 3. Co 2p XPS spectrum of Co3O4.
Table 1. Comparison of catalyst activities and selectivities for
a
p-cresol oxidation
Sr. Co3O4 Digestion
% Selectivity
ꢁ
1
TOF/h
No catalyst time/h
PHBAlc PHB PHBacid
1
2
3
4
5
6
7
bulk
A0
A2
A4
A8
A16
A8
—
0
2
4
8
8
18
21
20
31
9
67
59
50
42
33
56
16
33
41
49
58
66
44
83
—
—
01
—
01
—
01
16
8
174
aReaction conditions: temperature 368 K; mole ratio, NaOH:
p-cresol, 4; catalyst weight, 0.02 g; total reaction volume,
5 cm ; air flow rate, 30–40 mL/min; Po2, 20.2 kPa (Except
Entry 7); initial p-cresol concentration, 27.7 mmol, solvent
n-propanol.
3
Financial support from DST (Department of Science and
Technology, New Delhi) is gratefully acknowledged. Authors,
VSK and SV thank Council of Scientific and Industrial Research
5
(
New Delhi) for the award of fellowships to them.
of cobalt is responsible for catalyzing the oxidation reaction.
Table 1 shows the comparison of the activity (expressed as
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twice activity than that of the bulk catalyst (Entry 1). This can be
attributed to the lower particle size (6–8 nm) and very high
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TOF, 31 h ) was obtained for the digestion time of 8 h (Entry
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ꢁ1
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reversed as the TOF was increased for Co3O4 nanocatalysts.
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7
6
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,11
2þ
3þ
and Co coexist in the spinel-type Co3O4 and a dynamic equi-
2
þ
3þ
librium is set up involving Co , Co , and the lattice oxygen
ꢁ
14
(O2 ) species under liquid phase oxidation conditions. Oxida-
tion proceeds through activation of C–H bond, its dissociation
and formation of C–O bond on an active site of a catalyst. If
1
5
Published on the web (Advance View) February 16, 2008; doi:10.1246/cl.2008.310