Efficient room temperature oxidation of cyclohexane over highly active hetero-mixed WO3/V2O5 oxide catalyst

Article abstract of DOI:10.1016/j.catcom.2014.05.031

An efficient room temperature catalyzed oxidation of cyclohexane to cyclohexanone (K) and cyclohexanol (A) was achieved over hetero-mixed tungsten-vanadia (WO₃/V₂O₅) using H ₂O₂ oxidant. WO₃/V₂O₅ exhibited high catalytic activity to initiate the free-radical oxyfunctionalization of cyclohexane to afford up to 90% conversions within 6 h. The KA selectivity was found to depend on reaction time and the amount of catalyst. The WO₃/V₂O₅ catalyst was highly recyclable with consistent catalytic activity.

Full text of DOI:10.1016/j.catcom.2014.05.031

Efficient room temperature oxidation of cyclohexane over highly active
hetero-mixed WO3/V2O5 oxide catalyst
P.R. Makgwane, S. Sinha Ray
PII:
DOI:
Reference:
S1566-7367(14)00212-X
doi: 10.1016/j.catcom.2014.05.031
CATCOM 3925
To appear in:
Catalysis Communications
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Revised date:
Accepted date:
20 March 2014
28 May 2014
30 May 2014
Please cite this article as: P.R. Makgwane, S. Sinha Ray, Efficient room temperature
oxidation of cyclohexane over highly active hetero-mixed WO3/V2O5 oxide catalyst,
Catalysis Communications (2014), doi: 10.1016/j.catcom.2014.05.031
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ACCEPTED MANUSCRIPT
Efficient room temperature oxidation of cyclohexane over
highly active hetero-mixed WO3/V2O5 oxide catalyst
P.R. Makgwane1,2 and S. Sinha Ray1,2
1 DST-CSIR National Centre for Nano-Structured Materials, Council for Scientific and
Industrial Research, P. O. Box 395, Pretoria, 0001, South Africa.
2 Department of Applied Chemistry, University of Johannesburg, Doornfontein, 2018,
South Africa
* Corresponding author:
Fax: +27 12 841 2229; Tel: +27 12 841 2693
E-mail: pmakgwane@csir.co.za
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Abstract
An efficient room temperature catalyzed oxidation of cyclohexane to cyclohexanone
(K) and cyclohexanol (A) was achieved over hetero-mixed tungsten-vanadia
(WO3/V2O5) using H2O2 oxidant. WO3/V2O5 exhibited high catalytic activity to initiate
the free-radical oxyfunctionalization of cyclohexane to afford up to 90% conversions
within 6 h. The KA selectivity was found to depend on reaction time and the amount of
catalyst. The WO3/V2O5 catalyst was highly recyclable with consistent catalytic activity.
Keywords: vanadium, tungsten, cyclohexane, oxidation, hetero-mixed, nanostructured
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Introduction
Selective liquid-phase oxidation (LPO) of cyclic hydrocarbons inert C-H is of major
industrial importance in the production of oxygenates [1]. The challenge in these
reactions is to attain improved process efficiency and products selectivity. In general,
selectivity is in the trade-off to high conversions in the hydrocarbons LPO thus
conversions are restricted to low [2]. An example is the LPO of cyclohexane into
cyclohexanone (K) and cyclohexanol (A) that facilitate the production of adipic acid, an
intermediate to nylon-6 and nylon-6.6 [3,4]. The industrial-scale LPO of cyclohexane is
carried out as non-catalysed process with conversions restricted to 5-10% in order to
ensure the KA selectivity in the range of 80-90% [4]. As a result, the development in
catalytic LPO of cyclohexane has been one of the most studied hydrocarbons oxidation
reactions [5]. In order to address the various shortcomings in the cyclohexane oxidation
several heterogeneous catalysts using hydrogen peroxide (H2O2) as oxidant have been
explored [5]. Shi et al.[6] developed a titanium silicalate with hallow structure (HTS)
for cyclohexane oxidation using H2O2 to afford up to 60% conversions. In the other
study, vanadium phosphorous oxide demonstrated effective catalytic activity with near
complete cyclohexane conversions [7]. However, the catalyst displayed poor KA
selectivity due to high formation amount of cyclohexyl peroxide. Furthermore,
Carabineiro et al.[8] reported excellent activity of gold nanoparticles supported on
carbon materials for cyclohexane oxidation although their conversions were relatively
poor with KA yields of less than 10% achieved after 6 h. Parida and co-workers [9]
developed an intercalated Zn-Al layer double hydroxide Fe(III)-Schiff base complex as
effective catalyst to afford up to 45.4% conversions with 100% KA selectivity in 8 h. In
line with these reported cyclohexane oxidation process aspects it is apparent that there is
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a need for continuing catalyst development in order to improve on reaction times,
conversion, KA selectivity and catalyst re-usability for cyclohexane LPO. Apart from
these factors critical to obtaining efficient cyclohexane LPO process the design of
catalyst that can be active at room temperature would definitely make the process
economically competitive.
The uses of vanadium based oxides and metal complexes for LPO of hydrocarbons are
well known [10-13]. Furthermore; the activity of supported V2O5 has been reported for
cyclohexane oxidation although with low conversions of up to 30% [14]. Whilst V2O5
proved to possess interesting catalytic properties, explorative studies on catalytic
performance of hetero-mixed oxide of V2O5 with WO3 as catalyst for cyclohexane LPO
had not been reported. Likewise, WO3 is a versatile oxidation catalyst in the direct
conversion of cyclohexene into adipic acid using H2O2 [15]. Thus, the combination of
WO3/V2O5 oxide composite and atom-efficient H2O2 oxidant would undoubtedly
provide a green heterogeneous catalysed LPO process of cyclohexane into KA oil. In
this paper, the catalytic performance of WO3/V2O5 for LPO of cyclohexane with the
view of improving on poor conversion was investigated. The influences of the different
reaction variables on the oxidation rates were evaluated. In order to comprehend the
WO3/V2O5 catalytic effect, its resulting structure properties obtained using X-ray
diffraction (XRD), field emission scanning electron microscopy (FESEM), hydrogen-
temperature reduction (H2-TPR), transition electron microscopy (TEM) and N2
physisorption were correlated to the catalytic results in the cyclohexane oxidation.
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2 Experimental
2.1. Preparation of the catalysts
The preparation of WO3, V2O5 and WO3/V2O5 oxides was achieved by using ethylene
glycol solvent and polyvinylpolypyrrole (PVP) as structure templating agent. Typically,
ammonium metavandate (NH4VO3) in oxalic acid solution was mixed with sodium
tungstate dehydrate (Na2WO4∙2H2O, Aldrich) in 1:1 metal ratio into ethylene glycol
followed by addition of 5 wt% PVP based on the total metals content. The solution
mixture’s pH was adjusted to 11 using ammonium hydroxide and stirred vigorously for
48 h under reflux. The solids were recovered by vacuum filtration, washed with ethanol
and followed by drying at 100 ºC for 12 h. Similarly, the individual oxides were
prepared following the same procedure. The dried samples were calcined under air
condition heated from room temperature to 500 ºC at a heating rate of 10 ºC/min and
held for 4 h at 500 ºC.
2.2. Catalyst characterisation
The Brunauer–Emmett–Teller (BET) surface area and micropores volume were
measured by nitrogen (N2) physisorption at 77 K, using a Micromeritics TriStar II
surface area and porosity analyzer (USA). Before analysis, samples were degassed at
o
120 C for 6 h under N2 flow. XRD experiments were recorded on a PAnalytical
XPERT-PRO diffractometer using Ni filtered CuKα radiation (λ = 1.5406 Å) at
40 kV/50 mA. The diffraction measurements were collected at room temperature in a
Bragg-Brentano geometry with scan range, 2 = 10-90° using continuous scanning at a
rate of 0.02o/s. H2-TPR studies were performed on a Micromeritics Autochem AC2920
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o
apparatus (USA) gaseous mixture of 10%H2/Ar at a heating rate of 10 C/min from 50
o
to 1000 C and total gases flow rate of 50 ml/min. The H2 consumption was monitored
by a calibrated thermal conductivity detector (TCD). The morphology and structure of
the catalysts were characterised JEOL-JSM 7500 high resolution FESEM operated at an
accelerating voltage of 2 keV coupled to EDX for elemental mapping analysis. Before
the analysis samples were sputter coated with gold in order to minimize their possible
charging. TEM measurements were performed by JEOL JEM 2100 operated at an
accelerating voltage of 200 kV. Powder samples were sonicated for 5 min dissolved in
ethanol and dispersed on copper grid for TEM analysis.
2.3. Catalytic activity testing
Catalytic testing was performed in a 50 ml two-necked flask equipped with magnetic
stirrer, reflux condenser and thermometer. To a reactor were added cyclohexane
(10 mmol, 0.84 g), acetonitrile solvent (5 ml), catalyst (0.05 g) and 50% H2O2 oxidant
(30 mmol). The oxidation products samples were treated with triphenylphosphine
before gas chromatograph (GC) coupled to flame ionization detector (FID) analysis
using a Supelco SPB-20 column (30 m × 0.25 mm i.d. × 0.25 μm). No differences in
products quantitative amounts were observed for the samples analysed directly without
the triphenylphosphine to those treated indicating that the catalyst was effective to
decompose readily the formed cyclohexyl peroxide into carbonyls.
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3 Results and discussion
3.1 Catalyst characterisation
Figure 1a illustrates the XRD patterns of calcined WO3, V2O5 and WO3/V2O5 catalysts
at 500 ºC. The patterns of WO3 exhibited those of previously reported monoclinic phase
[16], while typical reflections belonging to V2O5 phase were confirmed by the XRD.
Both XRD of WO3 and V2O5 showed the formation of well-defined crystalline peaks.
The redox H2-TPR profiles of the catalysts are illustrated in Figure 1b. The V2O5
exhibited three-stage reduction peaks at 496 C, 576 C and 710 C, respectively. This
reduction pattern has been associated to the sequential reducibility of vanadia surface
oxygen removal which corresponds to the V+5/V+4/V+3 oxidation states, respectively. In
comparison to the previous reported reduction temperature regions, our catalyst showed
signification low shift reduction temperatures [17]. This could perhaps be attributed to
the observed nanostructure particles morphology that can results into easily reducible
exposed surface vanadia species as opposed to usual bulk V2O5. For WO3, only one
high temperature reduction peak was observed which corresponded to tungsten carbide
WC formation from WO3. For the WO3/V2O5 composite, the reduction peaks indicating
both oxides were observed but with slightly temperature shifts to high that could be the
indication of possible internal structure properties alterations and strong solids
interactions. Such interactions in the mixed oxides of V and W metals have been
elaborated previously for solid solution formation with V2O5 dissolving in the WO3
structural framework or WO3 dissolving in V2O5 depending on the composition ratios
[18].
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The microstructure morphology and particles shape of the catalysts was further
characterised by FE-SEM (Figure 2a-g). Some particles with nano-needle shape
morphology of 150–200 nm size in length were observed by FE-SEM for V2O5 (Figure
2a-b). These particles were grouped together to form irregular round shape with the
nano-needle growing outwards as illustrated by the high magnification image (Figure
2b). For WO3, the formations of platelets morphology-like particles were observed
which showed the shape of nanorods for high magnification image (Figure 2c-d). The
composite WO3/V2O5 exhibited distinctive particles morphology to those of the pure
WO3 and V2O5 oxides that were clustered nanoparticles with size diameter of less 100
nm (Figure 2f-g). The elemental mapping of the composite WO3/V2O5 (Figure 2h)
confirmed uniform metals distribution interface. This was further illustrated by EDX
(Figure 2g) of the mapped area, which indicated the presence of both elements at
composition closer to the theoretical 1:1 ratio. The formation of well-dispersed WO3
nanoparticles of less than 20 nm in size on the surface of irregular spherical V2O5 nano-
platelets with particles diameter size in a range of 40-100 nm were observed (Figure 3).
The BET surface areas and micropores volume of the catalysts are summarised in Table
1. The V2O5 exhibited the highest surface area of 37 m2/g while WO3 had the low of 14
m2/g and the WO3/V2O5 catalyst exhibited the in-between surface area of 23 m2/g. The
micropores volume and size corresponded relatively with the catalysts surface area and
their respective porosity morphology as observed in the FE-SEM results especially for
the WO3/V2O5 composite catalyst (Figure 2).
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3.2 Catalytic activity
Figure 4a-d list the catalytic results of the tungsten-vanadia catalysts tested in the
cyclohexane oxidation using H2O2 at room temperature. All the catalysts improved the
cyclohexane oxidation rates with up to 65% conversions achieved after 3 h when
compared to less than 1% for non-catalysed reaction (Figure 4a). In terms of conversion
efficiency, the catalysts followed the increasing order of WO3/V2O5 > WO3> V2O5. The
V2O5 exhibited both poor KA selectivity and conversion when compared to all other
catalysts. On the other hand, both WO3 and WO3/V2O5 displayed closely related KA
selectivity, however with WO3/V2O5 exhibiting the highest cyclohexane conversion.
The observed KA selectivity and cyclohexane conversion rates somehow did not follow
the obvious trend of the catalysts surface areas, thus indicated that there were other
factors of the materials structural properties which were critical to obtaining the catalyst
with highly active sites. By considering the redox recycle results (Figure 4a), it was
clear that the combination of the two metals had an effect on their modified redox that
could also suggest possible alteration of electron charge transfer of the individual metals
in the composite catalyst, thus enhancing on their mobility during the
oxidation/reduction cycle to improve the oxidation rates. Another intriguing aspect
worth mentioning was the possible contribution of the oxides acidity which was
however not investigated that could have influenced the composite catalyst performance
significantly. Continuous studies are underway to elaborate further on this catalyst
aspect. Such structure-property effect would require the studies on the influence of
oxides composition on the overall performance of the catalyst activity. This was
however, demonstrated by the poor KA selectivity and activity of V2O5 when used
alone in relation to WO3, of which when combined both enhanced relatively the
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conversion and KA selectivity. The main role of the catalyst in H2O2 catalysed LPO is
∙
to improve on the controlled decomposition rates of H2O2 into reactive OH/HOO∙
species to initiate the H-abstraction of hydrocarbons C-H into free-radicals that in turn
traps them to form stable oxygenates. In order to determine the effect of the catalyst
amount on the cyclohexane oxidation rates and products selectivity the mass of the
catalyst was varied over a range of 25-100 mg (Figure 4b). It can be seen that
conversion increased from 30% at 25 mg of catalyst to 71% at 100 mg (Figure 3b).
However, the high catalyst amount proved to be detrimental to KA selectivity, thus 50
mg appeared to give satisfying process performance.
Next, we evaluated the time-course evolution of cyclohexane conversion and KA
selectivity up to 5 h reaction time including the formation of other over oxidation
products (Figure 4c). It can be seen that after 5 h the reaction showed near completion
substrate conversion forming plateau at around 90%. At 24% conversion (1 h), the KA
selectivity was still about 98% of which thereafter began to drop gradually with both
increasing reaction time and conversion. After 6 h reaction time at about 90%
cyclohexane conversion the KA selectivity had dropped significantly to 63% and the
other by-products accumulated to the total amount of 37%. Based on the observed
results it could be suggested that the gradual drop of KA selectivity was the factor of
both high conversion and prolonged reaction time that facilitated further overoxidation
of the formed KA possibly accelerated by the catalyst itself. Some of the identified
over-oxidation products due to high substrate conversion and prolonged reaction time
included ϵ-caprolactone, 6-hydroxyhexanoic acid and adipic acid (Scheme 1 and Figure
4c). As a result, to obtain satisfying KA selectivity of above 90% in the present study
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the conversion of cyclohexane would need to be limited to approximately 40% as is
usually practiced in hydrocarbons LPO. This conversion of 40% was found achieved
within reasonable reaction time of 2 h in the present study at room temperature, thus
making the catalyst productivity space-time-yield efficient (Figure 4c). Whilst the
catalysed LPO of cyclohexane using H2O2 as oxidant present green protocol, on the
other hand the effectiveness of the catalyst for this process would depend on its ability
to control the efficiency of H2O2 conversion into the desired cyclohexane products
rather than to H2O by-products. As a result, we controlled this issue by adding H2O2
drop-wise in order to minimize its waste decomposition into water. Such method of
adding H2O2 slowly has been found to be beneficial in maximizing its usage efficiency
for cyclohexane oxidation [19]. Finally, we investigated the recyclability performance
of WO3/V2O5 in number of reaction cycles. The catalyst was highly recyclable
maintaining its initial activity even after re-used for four consecutive reaction cycles
(Figure 4d). The ICP analysis of the hot filtrate solution for leaching after the reaction
was stopped at the desired time showed trace amount of only vanadium metal ( 54
ppb) and further oxidation of this solution without the catalyst proved to be slow
reaction rates with negligible increase in conversion. Thus, this showed that the
presence of any possible leached WO3/V2O5 catalyst into the solution was not adequate
to catalyze effectively the cyclohexane LPO at room temperature.
Conclusion
In summary, an efficient room temperature oxidation process of cyclohexane to
cyclohexanone (K) cyclohexanol (A) by highly active nanostructured WO3/V2O5
composite has been demonstrated that afforded up to 90% conversions. The catalytic
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results appeared to suggest a possible existence of strong interaction effect of the two
combined metal oxides that enhanced their performance when compared to individual
oxides. The enhanced performance of WO3/V2O5 was partly attributed to the redox and
possible structural modifications due to the metals strong interaction. We believe that
these achieved initial catalytic results provide the basis for further studies on WO3/V2O5
material development as a promising catalyst for liquid-phase oxidation of typical
hydrocarbons inert C-H such as cyclohexane.
Acknowledgements
Authors are grateful to the financial support from the NRF and the DST both of the
South African government. NCNSM unit is thanked for assisting with characterisation
of the powder samples.
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Caption to Figures, Tables and Schemes
Table 1. N2 physisorption characterisation data of the tungsten-vanadia based catalysts
Figure 1. (a) XRD and (b) H2-TPR analysis of tungsten-vanadia based catalysts.
Figure 2. FESEM images of (a, b) low and high magnification V2O5, (c,d) low and high
magnification of WO3, (e,f) low- and high-magnification of WO3/V2O5, (g) elemental
mapping of WO3/V2O5, (h) EDX of WO3/V2O5.
Figure 3. TEM image of WO3-V2O5 nanostructured catalyst.
Figure 4. (a) Comparison performance of the tungsten-vanadia based catalysts activity
in cyclohexane oxidation. (b) Effect of WO3/V2O5 catalyst amount, (c) Effect of
reaction time on WO3/V2O5 performance, and (d) Recyclability performance of
WO3/V2O5. Other conditions used otherwise described: Cyclohexane (10 mmol),
catalyst (50 mg), 50% H2O2 (30 mmol), acetonitrile solvent (5 ml), time (3 h) and all
reactions were performed at room temperature.
Scheme 1. Plausible mechanism route to cyclohexane liquid-phase oxidation products
formation.
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Table 1.
Entry Catalyst
BET surface areas
Pores volume
(cm3/g)
0.12
Pores diameter size
(m2/g)
(nm)
24.53
16.28
21.14
1
2
3
WO3
14
V2O5
37
0.26
WO3/V2O5
23
0.19
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Scheme 1
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Figure 1
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Figure 2
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Figure 3
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Figure 4
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Graphical abstract
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Research Highlights
 Nanosized structured WO3/V2O5 mixed oxide was synthesized.
 WO3/V2O5 was active for oxidation of cyclohexane with high substrate conversion.
 Mixing of WO3 and V2O5 enhanced both conversion and selectivity.
 WO3/V2O5 was highly recyclable with consistent catalytic activity.
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