Selective oxidation of n-hexane by Cu (II) nanoclusters supported on nanocrystalline zirconia catalyst

Article abstract of DOI:10.1166/jnn.2015.10205

Cu (II) nanoclusters supported on nanocrystalline zirconia catalyst (with size ～15 nm), was prepared by using cationic surfactant cetyltrimethylammonium in a hydrothermal synthesis method. The catalyst was characterized by XRD, XPS, TGA, SEM, TEM, FTIR and ICP-AES. The catalyst was found to be efficient in selective oxidation of n-hexane to 2-hexanol. An n-hexane conversion of 55%, with a 2-hexanol selectivity of 70% was achieved over this catalyst in liquid phase, without the use of any solvent. The catalyst can be reused several times without any significant activity loss.

Full text of DOI:10.1166/jnn.2015.10205

Article
Journal of
Nanoscience and Nanotechnology
Vol. 15, 5816–5822, 2015
Copyright © 2015 American Scientific Publishers
All rights reserved
Printed in the United States of America
www.aspbs.com/jnn
Selective Oxidation of n-Hexane by Cu (II) Nanoclusters
Supported on Nanocrystalline Zirconia Catalyst
Shankha Shubhra Acharyya, Shilpi Ghosh, Shubhadeep Adak, Raghuvir Singh,
∗
Sandeep Saran, and Rajaram Bal
Catalyst Conversion and Process Division, CSIR-Indian Institute of Petroleum, Dehradun 248005, India
Cu (II) nanoclusters supported on nanocrystalline zirconia catalyst (with size ∼15 nm), was pre-
pared by using cationic surfactant cetyltrimethylammonium in a hydrothermal synthesis method.
The catalyst was characterized by XRD, XPS, TGA, SEM, TEM, FTIR and ICP-AES. The catalyst
was found to be efficient in selective oxidation of n-hexane to 2-hexanol. An n-hexane conversion
of 55%, with a 2-hexanol selectivity of 70% was achieved over this catalyst in liquid phase, without
the use of any solvent. The catalyst can be reused several times without any significant activity
loss.
Keywords: Selective Oxidation, Liquid Phase, n-Hexane, Cu (II) Nanoclusters, Nanocrystalline
Zirconia, Heterogeneous Catalysis.
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exhibits mechanical stability, with moderately high specific
1
. INTRODUCTION
Copyright: American Scientific Publishers
28
At the nano scale, materials exhibit fascinating optical,
electronic, and magnetic properties that are often radically
different from their bulk counterparts. So, controllable
synthesis of metal nanoparticls and their catalytic perfor-
mances has drawn a tremendous amount of attention over
surface and good semiconducting properties. Cu/ZrO
bimetallic catalyst can be synthesized in various methods
2
2
9ꢀ30
31
like impregnation method,
co-precipitation method,
3
2ꢀ33
sol–gel process,
etc. Herein, we report cationic sur-
factant CTAB-mediated synthesis of Cu(II) nanoclusters
supported on nanocrystalline zirconia catalyst (with size
∼15 nm) in hydrothermal route. Although there are sev-
1
–5
the past two decades.
Zirconium oxides have always
been a very important ceramic material due to its broad
6
–8
9ꢀ10
range of applications for catalysis, oxygen sensors,
eral reports on the preparation of Cu/ZrO catalysts, with
2
1
1ꢀ12
13ꢀ14
2+
fuel cells,
biological materials,
automobile parts
Cu as an active component, there is no report on cationic
etc. Copper catalysts (nanoparticles) are widely employed
commercially for the dehydrogenation of cyclohexanol to
surfactant CTAB mediated hydrothermal preparation of
Cu/ZrO catalyts with ∼15 nm particles size and its cat-
2
1
5
16
cyclohexanone, oxidation of cyclohexane, the synthe-
alytic use in n-hexane oxidation reaction.
1
7
sis of methanol, the direct decomposition of NO to
The selective transformation of inert C H bonds
of alkanes into useful functional groups has attracted
much attention because alkanes are less expensive and
more readily available than the current petrochemical
N2ꢀ1
8–20
CO oxidation etc.21 In addition, they can also
be used in hydrogen fuel cells to generate energy for
vehicles.2
2–24
The catalytic properties of the active cop-
3
4ꢀ35
per phase can be greatly influenced by the nature of
the supported oxide and the dispersion of the active
feedstocks.
However, selective catalytic oxidation of
3
sp hybridized carbons, within hydrocarbons, to alco-
hols, aldehydes, or ketones remains a challenging topic
in contemporary chemical research,
2
5
component. However, the nature of the active species
of these catalysts is still the subject of extensive inves-
3
6ꢀ37
because desired
tigation by many researchers. Cu/ZrO nanoparticles cat-
products sequentially convert to CO2 and the position
of oxygen attachment is dictated solely by C H bond
energies. n-Hexane is one of the hydrocarbons from
2
alyst showed interesting catalytic behavior for CO and
2
6
38
CO hydrogenation. The catalyst can also be used for
2
the upgradation of bio-oil e.g., hydrogenation of levulinic
which a number of useful oxygenates can be prepared.
2
7
acid to ꢁ-valerolactone. Furthermore, Cu/ZrO catalyst
Several researchers reported oxidations of n-hexane with
2
3
9–41
different oxidants;
and the production of specific oxy-
∗
Author to whom correspondence should be addressed.
genate selectively with high yield is highly demanding.
5816
J. Nanosci. Nanotechnol. 2015, Vol. 15, No. 8
1533-4880/2015/15/5816/007
doi:10.1166/jnn.2015.10205

Acharyya et al.
Selective Oxidation of n-Hexane by Cu (II) Nanoclusters Supported on Nanocrystalline Zirconia Catalyst
Herein, we report a n-hexane conversion of 55% with 2-
hexanol selectivity of 70% over our so prepared Cu/ZrO2
nanoparticles catalyst at room temperature with H O as
lanthanumhexaboride (LaB ꢀ as an X-ray source, fitted
6
with an ETD detector with high vacuum mode using sec-
ondary electrons and an acceleration tension of 10 or
30 kV. Samples were analyzed by spreading them on a
carbon tape. Energy dispersive X-ray spectroscopy (EDX)
was used in connection with SEM for the elemental anal-
ysis. The elemental mapping was also collected with the
same spectrophotometer.
2
2
oxidant at liquid phase without using any solvent.
2
2
. EXPERIMENTAL DETAILS
.1. Materials
Zirconium (IV) oxychloride octahydrate [ZrO Cl ·8H O],
2
2
2
copper (II) nitrate trihydrate [Cu (NO ꢀ ·3H O], cetrimo-
3
2
2
2
.3.3. Transmission Electron Microscopy ꢂTEMꢀ
niumbromide, ammonia solution (25%), hydrazine (80%
aqueous solution), n-hexane and hydrogen peroxide were
purchased from Sigma Aldrich. All chemicals were used
as received without further purification.
TEM images were collected using a JEOL JEM 2100
microscope, and samples were prepared by mounting
an ethanol-dispersed sample on a lacey carbon Formvar
coated Cu grid.
2
.2. Catalyst Preparation
2
.3.4. X-Ray Photoelectron Spectroscopy ꢂXPSꢀ
The catalyst was prepared followed by our own prepara-
4
2
XPS spectra were recorded on a Thermo Scientific
K-Alpha X-Ray photoelectron spectrometer and binding
energies (± 0.1 eV) were determined with respect to the
position C 1s peak at 284.8 eV.
tion method. In a typical experiment, 1.5 g of Cu(NO ꢀ ·
3
2
3
H O and 26 g ZrOCl ·8H O were dissolved (taking 4%
2
2
2
Cu loading on ZrO by weight) in 34 g deionized water at
2
RT to give a clear blue solution. The pH of solutions was
measured by pH Meter Eutec (Code: 08001-07), which
was standardized for pH measurement prior to use. The pH
2
.3.5. Inductively Coupled Atomic Absorption
Spectroscopy ꢂICP-AESꢀ
of the medium was made 9, by gradual addition of NH OH
4
Chemical analyses of the metallic constituents were car-
ried out by Inductively Coupled Plasma Atomic Emission
Spectrometer; model: PS 3000 uv, (DRE), Leeman Labs,
solution dropwise; the colour of the solution became for-
est green gradually. Then 1.7 g of CTAB was dissolved in
water by intensive stirring for 2 h. A solution of hydrazine
monohydrate (80% aqueous solution) was added drop wise
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(
0.6 g) to the well stirred mixture at RT by simultaneous,
Copyright: American Scientific Publishers
2
.3.6. Thermogravimetric Analyses ꢂTGAꢀ
vigorous agitation. All the reagents were used maintain-
ing the ratio: Cu: CTAB: hydrazine: H O = 1:0.75:1:300.
The uncalcined catalyst were carried out in a Pyris Dia-
mond, Perkin Elmer Instruments, and Technology by SII
[(SEIKO Instruments INC), USA] instrument-balance by
2
The mixture was stirred vigorously for 30 min and sub-
sequently sealed in a Teflon lined stainless-steel autoclave
ꢀ
−1
(
250 mL capacity). The autoclave was heated to and main-
heating 2.15 mg samples at 5 C min in flowing air.
ꢀ
tained at 180 C for 24 h and then allowed to cool to
RT. The sky-blue fluffy solid products (precipitates) were
collected by centrifugation at 5000 rpm and washed with
2.3.7. Fourier Transform Infra-Red
Spectroscopy ꢂFT-IRꢀ
water and ethanol several times prior to drying in air at
The FTIR spectra were recorded on a Thermo Nicolet
ꢀ
6
0 C for 6 h. The resulting dry powder was transferred to
8
700 (USA) instrument with the operating conditions: res-
−1
ꢀ
a quartz reactor inside a tubular resistance furnace for cal-
olution: 4 cm , scan: 36, operating temperature: 23–25 C
ꢀ
−1
cination. The calcination was operated at 800 C under O
2
and the frequency range: 4000–400 cm . Spectra in the
lattice vibrations range were recorded for wafers of sample
mixed with KBr.
ꢀ
−1
atmosphere at ramp of 1 C min . The obtained oxides
were stored for further characterization.
2
.3. Catalyst Characterization Techniques
2
.4. Liquid Phase Catalytic Reaction
2
.3.1. X-Ray Power Diffraction (XRD)
The catalytic reactions were carried out batchwise in
a 25 ml round bottomed flask with continuous stirring
(1100 rpm). In a typical run, 50 mg of the catalyst was
dispersed in a solution (total volume = 2 ml) containing
1 g of n-hexane, 0.8 g of H O (50% aqueous solution).
Aliquots were periodically collected every 1 h, centrifuged
to remove the solid particles and analyzed by GC (GC,
Agilent 7890) using a capillary column, HP5 (5% phenyl-
methylsiloxane (30 m length, 0.28 mm id, 0.25 ꢃm film
thickness), equipped with FID detector. The identities of
Powder X-ray diffraction patterns were collected on a
Bruker D8 advance X-ray diffractometer fitted with a Lynx
eye high-speed strip detector and a Cu K radiation source.
Diffraction patterns in the 2 –80 region were recorded at
a rate of 0.5 degrees (2q) per minute.
ꢁ
ꢀ
ꢀ
2
2
2
.3.2. Scanning Electron Microscopy ꢂSEMꢀ
Scanning electron microscopy images were taken on a
FEI Quanta 200 F, using tungsten filament doped with
J. Nanosci. Nanotechnol. 15, 5816–5822, 2015
5817

Selective Oxidation of n-Hexane by Cu (II) Nanoclusters Supported on Nanocrystalline Zirconia Catalyst
Acharyya et al.
the products were established by GC, compared with the
authentic products and GC-MS (Hewlett Packard). The
total conversion and product distribution (estimated in
moles) were evaluated with a calibration curve (obtained
by injecting known amounts of authenticated standard
compounds), the individual yields being calculated and
normalized with respect to the GC response factors.
show Cu2p and Zr3d XPS spectra of the Cu/ZrO cata-
2
lyst respectively. The binding energy of the Cu2p3/2 peak
at 934.2 and the characteristic shakeup feature at a bind-
2
+
30
ing energy of 944 eV are indicative for Cu species. Zr
3d5/2 and Zr 3d3/2 binding energy values are in the range
of 181.8 and 184.5 eV, respectively. This provides an evi-
dence for the presence of Zirconium oxide in 4+ oxidation
3
0
state.
A representative SEM (Fig. 3) image has been plot-
ted to show the morphology of the catalyst. SEM images
revealed that the sample is composed of almost homo-
geneously distributed uniform nanoparticles with size
3
. RESULTS AND DISCUSSION
3
.1. Catalyst Characterization
XRD pattern of the prepared and used Cu/ZrO catalyst
samples are shown in Figure 1. The appearance of the
2
1
0–20 nm. SEM-EDX diagram has been plotted (Fig. 3(c))
ꢀ
ꢀ
ꢀ
indexed diffraction lines at 2ꢀ = 30ꢁ16 , 34.45 , 35.3 and
as an evidence of the presence of Cu, Zr and O in the
sample. We also noticed that there was no morpholog-
ical change of the catalyst even after 5 recycles from
its corresponding SEM image (Fig. 3(d)). SEM images
ꢀ
5
0.37 indicate the presence of the crystalline phases of
3
0
tetragonal ZrO in the sample. The particle size eval-
uated from the Scherrer equation using the peak at 2ꢀ
value of 50.37 having the maximum intensity, and it was
2
ꢀ
of the Cu/ZrO catalyst prepared using acidic pH (pH =
2
found to be 16.8 nm, which beared consistency with the
HRTEM results (Fig. 5). In addition to the characteris-
tic peaks of tetragonal zirconia, XRD peaks due to the
crystalline CuO phase were also noticed with 2ꢀ value of
1
ꢁ0) (Fig. 4) has been plotted to compare the morpho-
logical difference with the former catalyst; formation of
larger particle with non-uniform particles was noticed.
More interestingly, no peak due to the presence of Cu
was detected in the SEM-EDAX diagram (Fig. 4(d)). This
observation actually states the necessity of basic medium
ꢀ ꢀ
5.5 and 38.7 which probably the active phase, supported
3
3
0
on nanocrystalline zirconia. We also noticed that, with
the increment in Cu loading three distinct peaks of Cu(II)
gradually increased (Fig. 1). Furthermore, no significant
change in the XRD pattern of the spent catalyst (only neg-
for the synthesis of the uniform Cu/ZrO nanoparticles
2
catalyst.
Transmission Electron Microscopy (TEM) images of the
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ligible decrement in intensity) was observed (Fig. 1(d));
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which ascertained the fact that, there wC a os p ny or i pg hh at :s eA cmh ea nr i gc ea n Scientific Publishers
ticle size with an average size of ∼10 nm. From the
HRTEM diagram (Fig. 4(c), inset), two different patches
with corresponding to two different interplanar spacings,
d = 0ꢁ29 nm and d = 0ꢁ23 nm are noticed; these may be
attributed due to the presence of tetragonal zirconia with
during catalysis.
The catalyst surface compositions as well as the oxida-
tion state were investigated by XPS. Figures 2(a) and (b)
(
[
111) plane [JCPDS. 17-923] and CuO with (111) plane
JCPDS. 89-2530]. Furthermore, the TEM diagram of the
Cu(II)Oxide
spent catalyst (Fig. 4(d)) was almost same with that of
the fresh one (Figs. 4(a)–(c)), indicating the fact that, the
catalyst was devoid of any severe structural change during
catalysis (Table I).
It is worth mentioning that, the embedment of the
CTAB molecules on the uncalcined catalyst can be ana-
lyzed by the FTIR analysis (Fig. 6). A comparison of
the FTIR-spectra of dried uncalcined sample, with that of
pure CTAB was analyzed, which not only confirmed the
presence, but also revealed the nature of interaction of
CTA- molecules with the Cu–Zr surface. The peaks of the
Zr(IV) Oxide (tetragonal)
(
d)
c)
(
(
b)
−
1
+
sample at 809, 1062 cm can be assigned to the C
N
4
3
stretching modes of CTAB molecules. The peak at 1378
and at 1462 cm were assigned to symmetric mode
of vibration of the head groups of the methylene moi-
−1
(
a)
+
43
ety (N
CH ꢂ and CH scissoring mode respectively.
3 2
−1
−1
2
0
40
60
80
The frequencies above 1600 cm to 3000 cm can be
2
Theta (degree)
attributed due to CH symmetric antisymmetric vibrations
2
region. It is to be noted that, the shift of vibrations to
lower frequency suggested that, alkyl chains experienced
Figure 1. XRD diagram of Cu/ZrO catalyst with Cu loading (a) 2.6%,
2
(
b) 3.6%, (c) 9.8% and (d) spent Cu/ZrO catalyst (after 5 reuse).
2
5818
J. Nanosci. Nanotechnol. 15, 5816–5822, 2015

Acharyya et al.
Selective Oxidation of n-Hexane by Cu (II) Nanoclusters Supported on Nanocrystalline Zirconia Catalyst
Zr3d5/2
(
a) 2000
(b) 20000
Cu2p3/2
1
1
500
000
15000
Zr3d3/2
10000
Shakeup Peaks
5
00
5000
0
0
950
945
940
935
930
180
182
184
186
Binding Energy (eV)
Binding Energy (eV)
Figure 2. XPS diagram: (a) Cu2p and (b) Zr3d core level spectra.
a more hydrophobic environment in Cu-Zr blocks upon
the surface of which the CTAB moieties were supposed
(
a)
(b)
4
3
to be bounded. These typical frequencies were absent
ꢀ
when the material was calcined at 750 C in presence of
oxygen (fresh catalyst) in the case of the prepared cata-
lyst, which indicated that, the embedded CTAB moieties
have been completely removed from the catalyst surface
during calcination. The IR spectra of the calcined sam-
1
µm
100 nm
−1
ple showed a strong absorption at 470 cm due to the
Zr–O vibration, which was expected to be generated after
(
c)
(d)
4
4
calcination. Furthermore, the absorption bands at 350,
−1
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zirconia, which further supported the XRD data.
Copyright: American Scientific Publishers
The embedment of CTAB molecules on the pre-calcined
catalyst surface was further confirmed from TGA analy-
sis. TGA analysis (Fig. 7) was operated to understand the
various decomposition regimes. The TGA diagram showed
1
00 nm
Figure 3. (a)–(b): SEM images at different magnifications (c) SEM-
EDAX of the CuO/ZrO2 catalyst and (d) SEM of the spent CuO/ZrO2
catalyst (after 5 recycles).
Figure 4. (a)–(c): SEM images and (d) SEM-EDAX diagram of the
catalyst prepared at pH = 1.
Figure 5. TEM images of (a)–(c) fresh and (d) spent (after 5 recycle)
Cu/ZrO catalyst.
2
J. Nanosci. Nanotechnol. 15, 5816–5822, 2015
5819

Selective Oxidation of n-Hexane by Cu (II) Nanoclusters Supported on Nanocrystalline Zirconia Catalyst
Acharyya et al.
a
Table I. Reaction conditions of catalytic oxidation of n-hexane .
c
SP(%)
b
d
Entry
Catalyst
Cu loading(%)
C (%)
1-ol
2-ol
2-one
3-ol
Others
Eo
H
1
2
3
CuOCOM
>99
>99
–
3
3.5
6
4.5
55
48
18
57
–
–
–
–
10
8
15
12
17
22
20
22
15
12
–
22
28
28ꢀ5
24
5
53
52
42ꢀ5
41ꢀ5
4
4ꢀ95
7
23
–
0ꢀ3
0ꢀ28
0ꢀ7
0ꢀ6
COM
Cu O
2
COM
ZrO2
12
12.5
70
65
72
55
–
45
32
–
e
PM
4
Cu/ZrO2
15
–
f
NP
5
Cu/ZrO2
3ꢀ6
3ꢀ6
2ꢀ6
9ꢀ8
–
3ꢀ6
3ꢀ6
3ꢀ6
1
38ꢀ5
g
NP
6
Cu/ZrO2
0ꢀ05
–
0ꢀ08
–
–
–
8
6
31ꢀ2
12ꢀ9
31ꢀ3
–
9ꢀ3
21ꢀ4
–
NP1ꢀ6
7
8
Cu/ZrO2
NP9ꢀ8
Cu/ZrO2
10
–
25
22
–
10
11
12
13
No Catalyst
h
i
NP
Cu/ZrO2
62
67
–
18
28
–
12
18
–
NP
Cu/ZrO2
NP
Cu/ZrO2
–
Notes: aTypical reaction conditions: substrate (n-hexane) = 1 g, catalyst = 0.05 g, n-hexane: H O (molar ratio) = 1:1, reaction temperature = RT, time = 6 h. bCH
=
2
2
c
Conversion of n-hexane based upon the FID-GC using chloroform as external standard = [Moles of n-hexane reacted/initial moles of n-hexane used]×100.
S
= Selectivity
P
d
to 2-hexanol = [Moles of products produced/moles of n-hexane reacted]× 100;
E
=H O efficiency = [Moles of 2-hexanol formed/total moles of H O added] × 100;
o
2
2
2
2
e
Cu/ZrO2 catalyst prepared by physical mixing (PM) of CuO and ZrO ; Cu(II) nanoclusters supported on nanocrystalline ZrO fresh catalyst; gCu(II) nanoclusters supported
on nanocrystalline ZrO2 spent (after 5 recycle) catalyst; hn-hexane: H O (molar ratio) = 1:3 and reaction temperature = 50 C.
f
2
2
i
ꢀ
2
2
that the weight loss has three regimes, first being the loss
of water followed by the decomposition of reactants to
by using H O as oxidant. Blank experiment was per-
2 2
formed without the catalyst and it was found that there was
ꢀ
form NO and organic phases at 150 to 250 C and finally
no conversion of the reactant. Commercial CuO, Cu O,
x
2
ꢀ
ꢀ
the combustion of CTAB between 250 –350 C. A fur-
ZrO did not show any activity for n-hexane oxidation
2
ꢀ
ꢀ
ther small mass loss is noticed between 400 –550 C due
to the elimination of remaining carbon and organic com-
pounds. Further weight loss was not observed when the
temperature was further increased from 780 –900 C, indi-
cating the generation of the Cu/ZrO Cc ao t pa ly yr si gt h( zt :i rAc om n ei ar i ca at n Scientific Publishers
(Table I, Entry 1–3). Commercial CuO, mixed with com-
mercial zirconia by physical mixing (Table I, Entry 4)
and was introduced as a catalyst in the oxidation reac-
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ꢀ
ꢀ
tion; but still, no improvement in the catalytic activity was
IP: 46.243.173.185 On: Tue, 07 Jun 2016 10:58:17
noticed. Introducing the prepared Cu/ZrO nanoparticles
2
2
NP
2
catalyst (Cu/ZrO ꢁ, significant increment in the catalytic
its tetragonal phase) in that temperature range. The total
ꢀ
activity was noticed (Table I, entry 5). Room tempera-
mass loss is 21.2% upto 800 C.
ꢀ
ture (35 C) as well as low concentration of oxidant ren-
dered the production of 2-hexanol. Increment of either of
these parameters lead to the decrement in the selectivity
of 2-hexanol, although conversion of n-hexane increased.
Six hour reaction time was proved to be the optimum
one. It was also noticed that, with time, the conversion
of n-hexane increased, but the selectivity to the desired
3
.2. Activity of the Catalyst
Table I shows the activities of the Cu/ZrO nanoparticles
catalyst in the direct oxidation of n-hexane in liquid phase
2
(d)
1
00
9
9
8
8
7
7
6
5
0
5
0
5
0
5
(
(
c)
b)
2
1.2%
Weight Loss
(
a)
1
000
2000
Wavenumber (cm )
3000
4000
-
1
150
300
450
600
750
900
Temperature (ºC)
Figure 6. FTIR images of (a) pure CTAB, (b) uncalcined Cu/ZrO cata-
2
lyst, (c) calcined Cu/ZrO catalyst and (d) Cu/ZrO catalyst after 5 reuse.
Figure 7. TGA of the uncalcined Cu/ZrO catalyst.
2
2
2
5820
J. Nanosci. Nanotechnol. 15, 5816–5822, 2015

Acharyya et al.
Selective Oxidation of n-Hexane by Cu (II) Nanoclusters Supported on Nanocrystalline Zirconia Catalyst
1
00
Table III. The comparative studies on the selective oxidation of n-
hexane.
Reaction Yield of
Oxidant conditions 2-Hexanol
8
6
4
2
0
0
0
0
0
Entry
1
Catalyst
References
ꢀ
MnAlPO-18
O2
100 C,
0ꢂ7
Nature 398, 227
(
1999).
1
.5 MPa
Pressure
ꢀ
2
Vanadium
phosphorus
oxide
H O
65
C
–
New J. Chem.
27, 525 (2003).
2
2
ꢀ
3
4
Polyoxometalate
H
O
69
C
18ꢂ5
38ꢂ5
Nature Chem. 2, 478
2
2
2
(
2010)
Cu-ZrO2
H O
RT
Our work (2014)
2
0
5
10
15
20
25
possessing highly stable tetragonal phase.22 Recycling and
reusability of the catalyst was examined by introducing
the used catalyst subsequently 5 times to carry out the
catalytic oxidation (Table II). The reusability of the cata-
lyst was studied without regeneration of the catalyst in the
same experimental condition. Before each recycle, the cat-
alyst was recovered easily from the reaction mixture (after
Time (h)
Figure 8. Effect of time on n-hexane oxidation. ꢀꢀꢁ Conversion of n-
hexane; ꢀ•ꢁ Selectivity to 2-hexanol; ꢀꢁꢁ yield of to 2-hexanol. Reac-
tion Condition: n-hexane = 1 g; Catalyst = 0ꢂ05 g; n-hexane: H O mole
2
2
ꢀ
ratio = 1:1; temperature = RT (35 C).
product was considerably less due to the formation of other
oxygenates with time (Fig. 8). Moreover, higher loading of
copper also affects the reaction. Higher loading of copper
indicates the increment of active copper sites on the cat-
alyst, which renders the conversion of the reactant; but
selectivity to 2-hexanol decreases considerably (Table I,
Entry 8).
6
h) by centrifugation, repeatedly washed with ethanol and
ꢀ
acetone and kept for drying at 120 C for overnight. The
catalyst showed ∼65% selectivity even after 5 successive
runs and the catalyst was proved to be truly of heteroge-
neous nature.
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The oxidation path probably follows participation of
IP: 46.243.173.185 On: Tue, 07 Jun 2016 10:58:17
4
. CONCLUSION
free radicals in the presence of the oxidant H O , over
Copyright: A
2
2
merican Scientific Publishers
4
5
In conclusion, we have successfully prepared Cu (II)
nanoclusters supported on nanocrystalline ZrO2 by
hydrothermal synthesis method in presence of surfactant
cetyltrimethylammonium bromide and found that the cat-
alyst can convert n-hexane to 2-hexanol without using
any solvent. The catalyst was thoroughly characterized by
XRD, XPS, SEM, TEM, FTIR, TGA and ICP-AES. A
n-hexane conversion of 55% with 2-hexanol selectivity
of was achieved over this catalyst. The catalyst did not
change any significant activity even after 5 reuses, show-
ing the true heterogeneity of the catalyst.
transition metals. Addition of small quantities of hydro-
quinone, a free-radical scavenger, greatly hinders the oxi-
dation (Table I, Entry 13), and thereby confirms the
involvement of free radicals in the oxidation reaction.
3
.3. Reusability Test
At the end of the reaction, the catalyst was filtered in
the hot condition and the resulting filtrate was indepen-
dently analyzed by ICP-AES for free or dissolved metal
ions. Only trace amounts of Cu2+ (<3 ppb) were detected;
which indicated the fact that, the catalyst is almost devoid
of leaching properties; this may be attributed by the fact
that, Cu2+ remains strongly anchored upon ZrO support,
Acknowledgments: Shankha Shubhra Acharyya thanks
CSIR and Shilpi Ghosh thanks UGC, New Delhi, India,
for their respective fellowships. Rajaram Bal thanks CSIR,
New Delhi for the financial support in the form of 12 FYP
Project (CSC-0125).The Director, CSIR-IIP is acknowl-
edged for his help and encouragement. The authors thank
Analytical Section Division, IIP for the analytical services.
2
a
Table II. The reusability of Cu/ZrO catalyst.
2
c
S (%)
b
H
P
Recycling
no.
C
%Cu Loading
(ICP-AES)
(%) 1-ol 2-ol 2-one 3-ol Others
1
2
3
4
5
6
55
54
52.5
50
1
1
1
70
70
68.5
68
20
20
20
5
5.5
6
4
3ꢂ6
3ꢂ6
3ꢂ6
3ꢂ6
3ꢂ6
3ꢂ6
3ꢂ5
4ꢂ5
4ꢂ4
5ꢂ4
4ꢂ95
References and Notes
0ꢂ08
21.5
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