Highly selective and direct oxidation of cyclohexane to cyclohexanone over vanadium exchanged NaY at room temperature under solvent-free conditions

Article abstract of DOI:10.1016/j.molcata.2014.05.027

Vanadium exchanged zeolite Y has been synthesized by simple ion-exchange on NaY and resulting V-Y material has been employed as a heterogeneous catalyst for highly selective and direct one-pot liquid phase oxidation of cyclohexane to cyclohexanone under s

Full text of DOI:10.1016/j.molcata.2014.05.027

Accepted Manuscript
Title: Highly selective and direct oxidation of cyclohexane to
cyclohexanone over vanadium exchanged NaY at room
temperature under solvent-free conditions
Author: Nabanita Pal Malay Pramanik Asim Bhaumik
Mahammad Ali
PII:
S1381-1169(14)00234-9
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2014.05.027
MOLCAA 9127
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
17-3-2014
26-5-2014
27-5-2014
Please cite this article as: N. Pal, M. Pramanik, A. Bhaumik, M. Ali, Highly selective
and direct oxidation of cyclohexane to cyclohexanone over vanadium exchanged NaY
at room temperature under solvent-free conditions, Journal of Molecular Catalysis A:
Chemical (2014), http://dx.doi.org/10.1016/j.molcata.2014.05.027
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Graphical Abstract
Highly selective and direct oxidation of cyclohexane to cyclohexanone over
vanadium exchanged NaY at room temperature under solvent-free conditions
a
b
b
*a
Nabanita Pal, Malay Pramanik, Asim Bhaumik* and Mahammad Ali
a
Department of Chemistry, Jadavpur University, Kolkata 700 032, India.
b
Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur,
Kolkata, 700 032, India.
+
VO exchanged Y zeolite has been synthesized by ion exchange over NaY zeolite with VOSO
2
4
under aqueous medium in open air. The material shows extraordinary efficiency for the solvent-
free highly selective direct oxidation of cyclohexane to cyclohexanone at room temperature.
1
Page 1 of 44

Research Highlights




 Vanadium exchanged zeolite Y synthesized through simple ion-exchange.
 One step highly selective oxidation of cyclohexane to cyclohexanone
 High selectivity of cyclohexanone catalyzed by V-Y in the presence of TBHP oxidant.
 Reaction proceeds over V-Y under solvent-free condition at room temperature.
2
Page 2 of 44

Highly selective and direct oxidation of cyclohexane to cyclohexanone over vanadium
exchanged NaY at room temperature under solvent-free conditions
a
b
b
a
Nabanita Pal, Malay Pramanik, Asim Bhaumik* and Mahammad Ali*
a
Department of Chemistry, Jadavpur University, Kolkata 700 032, India.
b
Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur,
Kolkata, 700 032, India.
*
Corresponding authors.
E-mail: msab@iacs.res.in (A. Bhaumik)
mali@chemistry.jdvu.ac.in (M. Ali).
3
Page 3 of 44

Abstract
Vanadium exchanged zeolite Y has been synthesized by simple ion-exchange on NaY
and resulting V-Y material has been employed as a heterogeneous catalyst for highly selective
and direct one-pot liquid phase oxidation of cyclohexane to cyclohexanone under solvent-free
conditions at room temperature. The catalyst (V-Y) has been thoroughly characterized by
powder X-ray diffraction (PXRD), N sorption, transmission electron microscopic (TEM),
2
Fourier transform infrared (FT IR), UV-visible, X-ray photoelectron spectroscopic (XPS) and
atomic absorption spectroscopic (AAS) analyses. The selective oxidation reaction proceeds
smoothly over this V-containing heterogeneous catalyst in the presence of tert-
butylhydroperoxide (TBHP) as oxidant and the progress of the reaction has been monitored
thoroughly by gas chromatographic (GC) and gas chromatography coupled with mass
spectrometric (GC-MS) analyses. The recycling efficiency of the V-Y catalyst has been tested by
conducting the catalytic reaction repeatedly with the recovered catalyst, where almost retention
of the original catalytic activity and selectivity after five reaction cycles has been observed. The
effect of oxidant amounts on the catalytic reaction has been studied. Mechanistic pathway for the
catalytic reaction over this inexpensive, non-air sensitive, eco-friendly and reusable vanadium
exchanged zeolite has been proposed, suggesting future potential of V-Y in solvent-free liquid
phase selective oxidation reactions under very mild reaction conditions.
Keywords: Cyclohexane oxidation, cyclohexanone, NaY zeolite, ion-exchanged Y, liquid phase
oxidation, solvent-free conditions.
Introduction
4
Page 4 of 44

Saturated hydrocarbons like n-hexane, cyclohexane etc. can be catalytically converted to
a mixture of aliphatic and aromatic fine chemicals through a wide variety of chemical reactions
[1], but it’s very challenging to oxidize them selectively into a value added chemicals in a
controlled fashion under mild reaction conditions [2-4]. Selective liquid phase oxidation of
cyclohexane to cyclohexanone via activation of relatively inert C-H bond is a very important
reaction in chemical industry. Because, a great majority of cyclohexanone combined with
cyclohexanol (a mixture called ‘KA oil’, K= ketone, A= alcohol) is consumed for synthesis of ε-
caprolactam and the remaining portion is supplied for the preparation of adipic acid, both of
which are employed as important precursors for the manufacturing nylon 6 and nylon 6,6
polymers [5]. Conventionally, cyclohexanone is produced in homogeneous medium from the
aerobic oxidation of cyclohexane at elevated temperature (423-433 K) and high pressure (1-2
MPa) using cobalt or metal-boric acid catalysts [6]. But this process suffers from the
disadvantages like low conversion, poor selectivity towards main products, catalyst separation
and environmental hazards [7]. Reactions carried out in heterogeneous media in the presence of
Co, Fe-based catalysts give lower yield of the main products or require involvement of toxic
solvents [8], which are environmentally not acceptable.
In recent times a wide range of catalytic processes have been developed globally for the
environment benign green synthesis of cyclohexanone over different heterogeneous catalysts
[
9,10]. A solvent-free high temperature oxidation of cyclohexane using 1.0 MPa O pressure
2
over transition metal containing ZSM-5 catalyst has been reported by Yuan et al., where the total
conversion does not exceed 10 mol% even after using tert-butylhydroperoxide (TBHP) as
initiator [11]. Similar attempts have also been made using Au/MgO [12], Cu-supported SBA-15
[13], gold C-scorpionate complexes [14], Au nanoparticle supported silica [15], Ce/AlPO-5 [16],
5
Page 5 of 44

Bi-SBA-15 [17], etc. in the presence of peroxides or under O pressure for selective oxidation of
2
cyclohexane. Hattori et al. have reported a green pathway for cyclohexane oxidation via photo-
induced reaction over reactive Fe/TiO catalyst [10]. Similarly, Shiraishi et al have reported the
2
photocatalytic oxidation of cyclohexane with molecular oxygen over WO loaded with Pt
3
nanoparticles under irradiation of visible light, which leads to mixture of cyclohexananol and
cyclohexanone [18]. However, most of these methods neither require high temperature nor bear a
significantly low conversion level of cyclohexane. In this context, remarkably high catalytic
conversion (>80%) of cyclohexane together with high selectivity for ketone product has been
observed by Sarkar et al. who have performed the reaction at room temperature using Cu
nanocluster supported over Cr O [19]. But they could not avoid considerable amount of
2
3
cyclohexanol along with the main ketone product and involvement of organic solvent in this
partial oxidation reaction. On the other hand, we have observed high selectivity for cyclohexanol
in the catalytic oxidation of cyclohexane over Cr-MCM-41 under liquid phase reaction
conditions using TBHP as oxidant [20]. Hence, a suitable environment friendly
microporous/mesoporous catalyst is very demanding, which can work efficiently under solvent-
free conditions in the selective catalytic transformation of cyclohexane to cyclohexanone at room
temperature.
Microporous zeolites, which bear cation exchange sites in the crystalline alumnosilicate
frameworks can be the ideal host to carry out liquid phase catalytic reactions under environment
friendly conditions [21-24]. Thus, keeping all these developments in mind we have synthesized
vanadium exchanged NaY zeolite material, which could act as an efficient catalyst for the liquid
phase oxidation reactions under very mild conditions. There are number of successful reports on
liquid phase oxidation of cyclohexane over V-based catalysts, which require relatively severe
6
Page 6 of 44

reaction conditions [25]. Due to high oxidizing property of 3d transition elements, when they are
loaded in the aluminosilicate framework of NaY zeolites they can often employed in the liquid
phase oxidation reaction [26]. V-containing materials especially showed excellent catalytic
properties for various critical oxidation reactions, like hydroxylation of benzene to phenol [27],
functionalization of n-butane to maleic anhydride [28], etc. Zeolitic framework being highly
stable, inexpensive, non-corrosive, environment friendly and have inherent ability for high
product selectivity could be employed as good support for different heteroelements [29,30].
Thus, when the vanadyl cation has been exchanged for Na in NaY the resulting material can
serve as efficient heterogeneous catalyst for hydrocarbon oxidation with very high K/A ratio.
Herein, we report a very simple, environment friendly, highly selective and one-pot route for the
oxidation of cyclohexane to cyclohexanone over V-Y zeolite. In addition, investigations have
been carried out to understand the role of oxidant, catalyst amounts and porous solid supports for
this highly selective conversion of cyclohexane to cyclohexanone. Further attempts are also
+
made to test the recyclability of the catalyst and to check the probability of the leaching of VO -
2
species from the catalyst surface.
Experimental section
Materials used. NaY zeolite and VOSO .5H O used for preparation of the catalyst were
4
2
purchased from Sigma-Aldrich and Loba Chemie (India), respectively. Cyclohexane (Sigma-
Aldrich) and TBHP (5.0-6.0 M in decane, Sigma-Aldrich) were used as substrate and oxidizing
agent, respectively. All the chemicals were used without further purification.
+
Catalyst synthesis. VO -exchanged NaY zeolite (designated as V-Y) was prepared exchanging
2
+
the Na ions of NaY by V(IV) ions via aqueous ion-exchange method followed by calcination in
air. First, NaY was activated by heating at 773 K in air for 6 h. In a typical synthesis, 10.0 mmol
7
Page 7 of 44

of VOSO .5H O was dissolved in 30 ml of distilled water followed by addition of 1.0 g activated
4
2
NaY. Then this mixture was stirred at 363 K for 24 h during which the color of the mixture
changed from blue to deep green. Then the reaction mixture was cooled to room temperature,
filtered and washed thoroughly with hot distilled water till the filtrate became colorless. This ion-
exchange process was repeated twice more to get higher loading of vanadium into zeolite Y. The
material thus obtained was dried at 373 K in air for overnight and finally heated at 773 K for
about 4 h to get yellow vanadium-exchanged material V-Y. A simple schematic representation of
formation of V-Y catalyst is shown in Scheme 1.
Scheme 1: Schematic representation depicting the formation of V-exchanged Y zeolite (V-Y).
Physicochemical characterizations. Powder X-ray diffraction (PXRD) patterns of the
vanadium exchanged NaY zeolite as well as NaY were recorded by a Bruker AXS D-8 Advance
diffractometer operated at 40 kV voltage and 40 mA current, calibrated with a standard silicon
sample using Ni-filtered Cu-K ( = 0.15406 nm) radiation. The diffraction data was collected in
α
8
Page 8 of 44

0
0
-1
0
-1
a 2θ range of 5-60 with scan rate and step size of 0.1 s and 0.02 step , respectively. BET
surface areas of the parent zeolite and V-exchanged samples were measured from nitrogen
adsorption/desorption isotherms obtained by using a Quantachrome Autosorb 1C instrument at
7
7 K temperature. Prior to gas adsorption, the sample was degassed for 5 h at 413 K. Total pore
volume was estimated from the N uptake at the relative pressure of P/P = 0.99. UV–visible
2
0
diffuse reflectance spectra (DRS) were obtained by using a Shimadzu UV 2401PC
spectrophotometer with an integrating sphere attachment; a BaSO pellet was used as
4
background standard and analysis was done with solid sample in the wavelength range of 200-
8
00 nm at room temperature (303 K). A Perkin-Elmer 3100 atomic absorption spectrometer was
used to measure the percentage of vanadium in our catalyst. For this chemical analysis the
material was digested with H SO , HF and few drops of H O and then dissolved in water to
2
4
2
2
prepare an aqueous solution of V-Y. Fourier transform infrared (FT IR) spectra of the samples
were recorded at room temperature on KBr pellets by using a Perkin Elmer Spectrum BX FT IR
spectrophotometer. X-ray photoelectron spectroscopic (XPS) analysis of V-Y was performed on
an Omicron nanotech system operated at 15 kV and 20 mA with a monochromatic Al-K (1486.6
α
eV) X-ray source. Prior to the analysis, the powder sample was dispersed in ethanol for 30 min
under sonication and a thin film was prepared by spin coating method. The thin film was heated
at 393 K under vacuum for 4 h before transferring it into the XPS instrument.
Oxidation of cyclohexane. A mixture of cyclohexane (1.0 mmol) and 0.75 g of TBHP (in
decane) was taken in a 25 mL round bottomed flask. Then 0.05 g of pre-activated catalyst heated
at 423 K for 2 h was poured into the mixture. The mouth of the flask was closed with glass
stopper and Teflon tape. Then the mixture was stirred vigorously through a magnetic stirrer at
3
03 K for definite period of time. Progress of the reaction has been monitored by withdrawing
9
Page 9 of 44

aliquots from the reaction mixture at regular intervals and analyzing with help of an Agilent
890A gas chromatograph (FID detector) fitted with a capillary column. The products were
7
identified using known standards and also by using a Perkin Elmer Clarus 680 gas
chromatograph attached with Clarus SQ 8 T mass spectrometer (GC/MS) which records mass
+
spectra of the sample in EI mode.
Results and Discussion
PXRD analysis. In Figure 1 the powder XRD patterns of pure NaY and vanadium exchanged V-
0
0
0
0
Y are shown. Diffraction peaks for NaY showed at 2θ values of 6.18 , 10.13 , 11.96 , 15.64 ,
0
0
0
0
0
2
0.30 , 23.70 , 27.02 , 31.30 , 34.14 , which represent the characteristic reflection planes for
(111), (220), (311), (331), (511), (440), (533), (642), (555) and (666), respectively of typical
Na-Y
V-Y
10
20
30
40
50
60
2 in degree
Figure 1. Powder XRD patterns of (a) Na-Y and (b) V-Y samples. Vertical lines indicate the
peak positions in both the samples.
1
0
Page 10 of 44

NaY zeolite [31]. The PXRD of V-Y exhibits an essentially similar pattern suggesting that the
regularity of crystal structure of NaY zeolite is maintained in V-Y after the introduction of
vanadium at the exchange sites. However, a little change in relative intensities of the peaks upon
+
exchange with VO species in the zeolite cages is observed. The high loading of vanadium via
2
+
systematic replacement of randomly distributed extra framework Na ions in the zeolite causes
the decrease in peak intensity of V-Y with respect to YZ sample [32]. The cell parameters (a )
533
2
2
2
2
1/2
for NaY and V-Y calculated (a = {(d )  (h + k + l )} ) are 2.4608 nm and 2.4567 nm,
hkl
hkl
respectively. Very little deviation in cell parameter of NaY zeolite suggested that metal exchange
occurs at the extra framework sites.
Porosity and BET surface area. The porosity and related surface properties of NaY zeolite and
vanadium exchanged V-Y materials have been obtained from the N adsorption/desorption
2
analysis at 77 K and respective isotherms are shown in Figure 2A. The BET surface area,
250
200
150
100
a
b
B
A
4
2
40
38
50
0
36
0.2
0.4
0.6
0.8
1.0
0.4
0.8
1.2
1.6
2.0
Relative pressure [P/P0]
Statistical Thickness (nm)
1
1
Page 11 of 44

Figure 2. A: N adsorption (●)-desorption (○) isotherms of the parent zeolite Na-Y (a) and V-Y
2
(
b) material. For clarity, Y-axis points of V-Y sorption have been 130. B: Adsorbed N gas
2
volume Vs statistical thickness of pores in the material.
Table 1. Physico-chemical data of the catalyst V-Y.
Sample
name
2
-1
S
S
Dp
[nm]
V
V
Presence1
micro
ext
t
micro
-1
SBET [m g ]
2
-1
2
-1
3
-1
3
[m g ]
[m g ]
[cm g ] [cm g ] of V (%)
Na-Y
V-Y
686
672
14.4
2.25
2.59
0.34
0.32
-
139.4
135.0
3.80
0.063
0.060
15.10
SBET = BET surface area, Smicro = micropore surface area, Smicro = external surface area, D =
p
average pore diameter corresponding to the maximum of PSD curve, V = total pore volume,
t
Vmicro = micropore volume.
micropore surface area, external surface area, average pore diameter, total pore volume and
micropore volume estimated from this surface analysis are summarized in Table 1. The sorption
isotherm followed typical type I isotherm, characteristic of microporous materials [33]. Here, in
case of V-Y material nitrogen uptake takes place in a large amount at very low relative pressure
P/P of N corresponding to the filling of micropores [34]. But after monolayer adsorption at low
0
2
P/P range N uptake is almost constant and no hysteresis is found in the region of higher relative
0
2
pressure, indicating the absence of mesopores or interparticle porosity in V-Y. BET surface area
2
-1
of V-Y is 139 m g , which is decreased sharply with respect to that of pure NaY zeolite (686
2
-1
m g ). This could be attributed to the decrease of crystallinity in the V-exchanged sample as
+
noticed from the PXRD pattern (Figure 1) as well as high loading of VO species having high
2
+
molar mass vis-à-vis Na . The t-method micropore analysis revealed that the material is
2
-1
consisting of internal micropore area of 135.2 m g together with external mesopore area of 3.8
1
2
Page 12 of 44

2
-1
-1
m g . The corresponding micropore volume is 0.06 ccg . The graphical plot of adsorbed gas
volume with respect to the statistical thickness obtained from sorption analysis is shown in
Figure 2B. From this figure it is clear that the maximum volume of the adsorbed gas (N ) has
2
been occupied at the micropores of V-Y.
Electron microscopic analysis. In order to investigate the nanoscale structural features of the
sample the HR TEM analysis of V-Y has been carried out and the results are shown in Figure 3.
High resolution image of NaY specimen is shown in Figure 3A, where micropores of sub-
A
B
Figure 3. A: TEM image of the sample V-Y and B: TEM image of the sample showing reflection planes.
1
3
Page 13 of 44

a
b
c
d
e
K e V
Figure 4. EDS elemental mapping of V-Y sample. a: O, b: Al, c: Si and d: V elements,
respectively. e: the whole EDX spectrum of the sample V-Y.
nanometer scale is observed all over the sample. On the other hand crystalline well-resolved
lattice planes of vanadium exchanged Y is observed in the Figure 3B. A little perturbation
1
4
Page 14 of 44

observed in the image is due to the presence of magnetic V species in the sample. From the EDS
elemental mapping (Figure 4) the homogeneous distribution of all the elements, mainly
distribution of V throughout the framework of the material is clearly observed. The EDS
spectrum shown in Figure 4e indicates that all the Na ions of NaY zeolite have been successfully
exchanged during ion-exchange method by V ion.
FT IR spectroscopic data. Information about framework vibrations as well as bonding/
connectivity in NaY and V-Y samples could be obtained from the FT IR analysis. FT IR spectra
-
of both Y zeolites are shown in Figure 5. A broad absorption band in the range of 2900-3600 cm
1
-1
and a sharp peak near 1630 cm which could be attributed to the O-H stretching frequency
(
a) NaY
(b) V-Y
c) V-YR
1
00
(c)
(
8
6
4
2
0

0 (a)
0


0
0 (b)
-20
4
000 3500 3000 2500 2000 1500 1000
-1
Wavenumber (cm )
Figure 5. FT IR data of (a) Na-Y, (b) the catalyst V-Y and (c) the recovered catalyst V-YR. (*)
indicates the peak for CO2.
-1
of the physisorbed water molecules [28]. The broad and strong band near 1000 cm could be
attributed to the asymmetric stretching vibration of (Si/Al)O units of zeolite framework [35].
4
-1
The other peaks at ca. 1150 and 790 cm are also characteristic of typical zeolite structure [36].
1
5
Page 15 of 44

Almost no shift in the absorption peaks in the case of V-Y with respect to NaY is observed,
clearly suggesting that zeolite framework is remained intact upon the loading of the vanadium
-1
ions. But the absorption band for V=O species cannot be traced in the region of ~1000 cm due
to superimpose of this peak with a strong peak due to zeolite framework at this position [36].
XPS analysis. Additional confirmation of the chemical environment and the valence state of the
vanadium ion in V-Y zeolite can be obtained from the electron spectroscopic analysis at the
surface. Figure 6A-D exhibits high resolution core level X-ray photoelectron spectra of V 2p, Si
16000
A
V 2p1/2
B
Si 2p
1
8000
1
4000
2000
0000
000
000
000
1
17000
1
16000
V 2p3/2
8
15000
6
4
14000
1
14 111 108 105 102 99
528
524
520
516
Binding energy (eV)
Binding energy (eV)
D
O 1s
4
000 C
Al 2p
30000
3900
25000
3800
20000
3700
15000
3600
10000
84 82 80 78 76 74 72 70
5
40 536 532 528
Binding energy (eV)
Binding energy (eV)
Figure 6. X-ray photoelectron spectra of A: V 2p, B: Si 2p, C: Al 2p and D: O 1s core level
electrons of V-Y.
1
6
Page 16 of 44

2
p, Al 2p along with O 1s electrons of V-Y, respectively and the binding energy values in eV
corresponding to the individual peak (after charge correction with reference to O 1s peak
referenced at 532.0 eV) are presented in Table 2. High resolution V 2p spectrum consisting two
peaks near 516.51 for V 2p and 523.42 eV for V 2p suggesting the presence of intra-zeolite
3/2
1/2
V(V) oxide in the material [37,38]. This binding energy values suggest that V(IV) of vanadyl
sulphate has been oxidized to V(V) during heating in air at temperature at 773 K. Besides Si 2p
electron showing broad peak near 102.85 eV, Al 2p peak at 74.30 eV and O 1s peak at 532.0 eV
strongly support the evidence for zeolite framework of the catalyst and these values matched
quite well with the literature data of NaY [39]. The surface atomic composition of V-zeolite
analyzed with the help of CasaXPS software showed Si:O:Al:V molar ratio of 4.0:11.73:1.70:1.0,
suggesting the presence of exchanged V-sites almost at the quantitative level.
Table 2. Binding energies of core level electrons obtained from XPS study of V-Y zeolite (with
reference to the O 1s binding energy at 532.0 eV)
Core level
electron
Binding energy
(eV)
5
5
16.51
23.42
V 2p
Si 2p
Al 2p
O 1s
102.85
74.30
532.00
Cyclohexane oxidation. In addition to the XPS analysis, we have carried out the AAS analysis
for V-Y and the material showed a significantly large amount of V loading (~15%, Table 1).
High V-loading has prompted us to explore the liquid phase catalytic oxidation of vanadium
1
7
Page 17 of 44

exchanged NaY zeolite in the solvent-free selective oxidation of cyclohexane (CH) to
cyclohexanone (CHone) using TBHP as oxidant. The conversions under different reaction
conditions are calculated from gas chromatographic (GC) analysis and these are presented in
Table 3. All catalytic experiments are carried out at room temperature (303 K). It is observed that
V-Y showed a very large cyclohexane conversion (about 98%) together with 100% selectivity
for cyclohexanone when the reaction was carried out at 303 K for 72 h (Entry 1). For
convenience, we have monitored the progress of the reactions after 24 h intervals. Formation of
Table 3. Catalytic oxidation of cyclohexane (CH) to cyclohexanone (CHone) over different
catalysts.
Entry Catalyst
Reaction time (h)
CH conversion (%)
CHone selectivity (%)
1
2
3
V-Y
V-Y
NaY
-
72
24
24
24
24
24
24
97.76
38.10
3.23
1.81
0.00
2.36
9.65
100
100
-
a
4
-
b
5
V-Y
V-Y
V-Y
-
c
6
100
100
d
7
Conversion (%) = [Peak area of product obtained from GC/Summation of peak area of substrate
and product obtained from GC]*100.
Reaction conditions: cyclohexane = 1 mmol, TBHP = 0.75 g, catalyst = 0.05 g, room
temperature,
a
blank reaction carried out without any catalyst.
b
reaction carried out without TBHP.
c
reaction carried out using H O instead of TBHP.
2
2
d
reaction carried out with 1:1 mixture of H O and TBHP.
2
2
1
8
Page 18 of 44

cyclohexanone as the only product with no other intermediate oxidation product like
cyclohexanol and this is confirmed from the GC-MS (Figure S1) and the mass spectroscopic
analysis (Figure S2). Blank reactions have been carried out without catalyst or TBHP (Entry 4
and 5) and they show almost no conversion, which implies that the oxidation reaction cannot
proceed without the aid of either TBHP oxidant or V-catalyst alone. Pure NaY zeolite also failed
to show any catalytic activity for the cyclohexane oxidation (Entry 3). We have carried out the
reaction under other mild oxidant H O , (Entry 6) or with a mixture of H O and TBHP (Entry 7).
2
2
2
2
However, in both cases we could not get the desired yield of product cyclohexanone. These
results suggested obvious catalytic role of vanadium species and TBHP as an efficient oxidant in
this selective catalytic reaction.
The effect of changing reaction parameters like reaction time and the amount of TBHP on
the selective oxidation of cyclohexane has also been examined. It is observed that, CHone
formation is gradually increasing with reaction time, which is presented graphically in Figure 7A
by % yield as against the reaction time in this plot. One of the reasons of linear increase of
ketone percentage may be due to the direct conversion of CH to CHone during this reaction and
thus no alcohol is detected as intermediate/side product. Effect of reaction time in the selectivity
of cyclohexanone yield has already been mentioned in previous studies. On the other hand, the
influence of oxidant amount on the cyclohexane conversion is also tested, which is displayed in
Figure 7B. The plot shows a steep increase of CHone amount on changing the oxidant amount
from 0.45 g to 0.75 g. But on further increase of the TBHP amount to 1.125 g the reaction rate
did not follow the linear rise, which indicates that in the presence of large amount of oxidant a
number of side products are formed and this caused a decrease in the actual yield of the desired
product.
1
9
Page 19 of 44

100
A
80
60
40
20
0
10 20 30 40 50 60 70
Time (h)
4
2
9
6
3
0
7
B
3
3
3
3
2
0
.50
0.75
1.00
1.25
Amount of oxidant (g)
Figure 7. A: Effect of reaction time and B: the amount of TBHP on the % yield of
cyclohexanone over V-Y catalyst.
Presence of electron-withdrawing counter ion (here oxo anion) [40], high loading of V or
V
strong Lewis acidity of the oxo-metal catalyst (V =O) [41] as well as strong oxidizing potential
of TBHP influence the product ratio of ketone to alcohol greatly in this cyclohexane oxidation
reaction. 5.0-6.0 M solution of TBHP in organic solvent like nonane or decane is a
comparatively strong and thermally stable oxidizing agent than conventional oxidant like H O ,
2
2
2
0
Page 20 of 44

containing large amount of active oxygen atoms [42,43]. TBHP being soluble in organic media
can be readily activated by high-valent oxo-metal species and therefore it can be successfully
used as oxidant in many selective oxidation reactions mediated by V-exchanged catalyst [43]. In
the present case, a solvent-free complete organic mixture of TBHP/decane oxidant and
V
cyclohexane along with V -exchanged Y zeolite could be responsible for the high selectivity of
CHone formation. The catalytic activity of V-Y material has been compared to the other reported
vanadium or oxovanadium species present in the solid heterogeneous supports (Table 4) [44-47].
Although, vanadium containing mesoporous materials showed good catalytic activity in this
liquid phase cyclohexane oxidation, in all the cases mixture of cyclohexanol and cyclohexanone
Table 4. Comparative catalytic performance of reported oxidation catalysts with our materials.
CH
CHone
Time Temp
Entry
Catalyst
Oxidant
Solvent
conversion selectivity Ref.
(h)
(K)
(%)
(%)
1
2
3
4
V-MCM-41 12
373
413
333
333
H O
Acetic acid
No solvent
Acetonitrile
Acetonitrile
99.0
9.34
1.9
25
46
47
48
2
2
V-HMS
VS-2
4
4
O2
60.3
-
1
H O
28
2
2
VOPO on
24
H O
59.0
9.7
4
2
2
alumina
5
V-MCM-41 12
373
TBHP
H O
Acetone or
cyclohexane
44.6
61.29
49
49
6
7
V-MCM-41 12
373
303
Acetone
11.7
97.7
57.14
100.0
2
2
V-Y
72
TBHP
No solvent
This
work
1
TON = Turn over number, moles of cyclohexane converted per mole of V.
were main products together with low selectivity for cyclohexanone. As seen from the table that
V-Y showed very high selectivity together with excellent conversion level under very mild
2
1
Page 21 of 44

reaction conditions compared with other V-containing microporus and mesoporous supports. The
high catalytic activity of V-Y zeolite together with excellent selectivity compared to mesoporous
V-MCM-41 could be attributed to the fact that hydrophobic Y-zeolite surface facilitates stronger
interaction with cyclohexane vis-à-vis hydrophilic MCM-41 support. Additionally high Brønsted
acidity of Y zeolite over MCM-41 [50] could facilitate the protonation of the reaction
intermediates. Under solvent-free conditions over V-Y the concentration of peroxide is much
higher, which could help to proceed the reaction at relatively low temperature. Higher catalytic
activity and enhanced selectivity of the zeolite surfaces over other molecular sieve supports and
for the desired products is observed over other microporous materials also [51]
A mechanistic pathway for the one-pot oxidation of cyclohexane has been presented in
Scheme 2. The reaction pathway for this direct-oxidation reaction in the presence of TBHP
oxidant is supposed to proceed through free-radical chain reaction pathway as proposed by
V
Nowotny et al [48]. The first step is the initiation step where V =O species react with TBHP to
t
●
t
●
generate BuOO and BuO radical species. In propagation step these radicals abstract H atom
●
from cyclohexane to generate highly reactive cyclohexyl (C H ) radical which readily
6
11
●
converted to cyclohexyl peroxy radical (C H -OO ) on reaction with aerial oxygen dissolved in
6
11
the medium. Cyclohexyl peroxy radical is the main propagator of this chain reaction. Since only
ketone is our only final product instead of a mixture ketone and alcohol mixed Russell
termination step is most likely to happen after propagation rather than normal termination [49].
●
t
●
So the reaction is terminated when two free radical C H -OO combines with BuOO to form
6
11
cyclohexanone with release of one molecule oxygen and tert-butanol as byproduct (Scheme 2).
2
2
Page 22 of 44

Scheme 2: Proposed mechanism for cyclohexane oxidation via free-radical reaction pathway
over V-Y catalyst.
We have successfully employed the used V-Y catalyst for recycling experiments by
simply recovering it from the reaction mixture through filtration followed by washing thoroughly
with alcohol and acetone. The recovered catalyst has been dried and activated by heating at 773
K for 2 h for further use it the next cycle. FT IR spectrum of V-YR (Figure 5) shows that the
recovered catalyst retains its framework and bonding with respect to the original one (V-Y). The
2
3
Page 23 of 44

reusability test was performed with V-YR up to another four reaction cycle showing almost same
extent (decrease of conversion after each cycle is < 2%) in cyclohexane conversion level (Figure
8
) while maintaining exactly similar reaction conditions for each cycle. Little decrease in the
conversion level is within the experimental error of GC analysis and could cause due to a very
1
st run
2
nd run
40
35
30
25
20
15
10
5
0
3
rd run
4
th run
Figure 8. Recycling efficiency of V-Y in one pot oxidation of cyclohexane to cyclohexanone.
For convenience all cycles are continued for 24 h only.
small amount of vanadium ion leaching. However, the V-Y catalyst does not lose its structural
stability, which is indicated from the resemblance of its powder XRD and FT IR data with
25
reference to that of the fresh one. From this result it is evident that V-Y can act as an efficient
+
heterogeneous catalyst for selective oxidation of cyclohexane with minimum leaching of VO2
active species from the zeolite Y extraframework sites.
Conclusions
+
A highly reactive V-Y zeolite bearing VO at the exchange site of NaY zeolite has been
2
synthesized through a simple ion-exchange method. V-Y showed excellent catalytic activity in
highly selective one-pot oxidation of cyclohexane to cyclohexanone by using tert-
2
4
Page 24 of 44

butylhydroperoxide as oxidant at room temperature. The reaction proceeds under solvent-free
conditions, without any aid of toxic organic solvent and thus provide a very significant pollution-
free route for the large scale production of cyclohexanone. The impact of catalytic reaction
parameters and explanations of the highly selective one-pot synthesis of cyclohexanone are also
discussed to get a clear overview in this eco-friendly oxidation reaction. High reactivity of V-Y
under mild liquid phase oxidation reaction may motivate the researchers to explore its potential
in other value added selective oxidation reactions.
Acknowledgements
NP is grateful to University Grants Commission (UGC), New Delhi for Dr. D. S. Kothari
postdoctoral fellowship. MP thanks CSIR, New Delhi for a senior research fellowship. MA
wishes to thank DST, New Delhi for providing research funding. MA and NP are thankful to Dr.
T. K. Paine of Indian Association for the Cultivation of Science for providing GCMS facility in
his laboratory. AB thanks DST, New Delhi for providing instrumental facilities through DST
Unit on Nanoscience and DST-SERB project grants.
2
5
Page 25 of 44

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