Mixed Cu-Ni-Co nano-metal oxides: A new class of catalysts for styrene oxidation

Article abstract of DOI:10.1016/j.catcom.2013.06.008

A series of mixed nano-metal oxides of Cu-Ni-Co on γ-alumina with different metal loadings have been synthesized through the ultrasonic cavitation-impregnation method. All the prepared catalysts were analyzed by BET, ICP, XRD, SEM-EDS and TEM. The heterogeneous catalytic oxidation of styrene was investigated using tert-butyl hydroperoxide (TBHP) as an oxidizing agent and acetonitrile as a solvent under mild conditions. The influence of the metal loadings, reaction temperatures, oxidizing agents and styrene/oxidant molar ratio was investigated on the styrene conversion and on the selectivity to benzaldehyde and styrene oxide.

Full text of DOI:10.1016/j.catcom.2013.06.008

Mixed Cu-Ni-Co nano-metal oxides: A new class of catalysts for styrene
oxidation
Jignesh Valand, Hitesh Parekh, Holger B. Friedrich
PII:
DOI:
Reference:
S1566-7367(13)00213-6
doi: 10.1016/j.catcom.2013.06.008
CATCOM 3529
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Catalysis Communications
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Revised date:
Accepted date:
12 April 2013
7 June 2013
11 June 2013
Please cite this article as: Jignesh Valand, Hitesh Parekh, Holger B. Friedrich, Mixed
Cu-Ni-Co nano-metal oxides: A new class of catalysts for styrene oxidation, Catalysis
Communications (2013), doi: 10.1016/j.catcom.2013.06.008
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Mixed Cu-Ni-Co nano-metal oxides: A new class of catalysts for styrene oxidation
Jignesh Valand, Hitesh Parekh and Holger B. Friedrich*
School of Chemistry & Physics, University of KwaZulu-Natal, Westville Campus,
Durban 4000, South Africa
Corresponding Author:
 Prof. Holger B. Friedrich
Tel.: +27 31 2603107, Fax: +27 31 2603091
E-mail address: friedric@ukzn.ac.za (H.B. Friedrich)
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Abstract
A series of mixed nano-metal oxides of Cu-Ni-Co on -alumina with different metal
loadings have been synthesized through the ultrasonic cavitation-impregnation method. All the
prepared catalysts were analyzed by BET, ICP, XRD, SEM-EDS and TEM. The heterogeneous
catalytic oxidation of styrene was investigated using tert-butyl hydroperoxide (TBHP) as an
oxidizing agent and acetonitrile as a solvent under mild conditions. The influence of the metal
loadings, reaction temperatures, oxidizing agents and styrene/oxidant molar ratio was
investigated on the styrene conversion and on the selectivity to benzaldehyde and styrene oxide.
Key words: Mixed metal oxides, Oxidation, Styrene conversion, C=C cleavage, Benzaldehyde
1.
Introduction
Due to increasing demand for clean technologies in chemical process engineering, much
attention has recently been paid to catalytic processes, which include removal of toxic and
expensive reagents, minimization of by-products, and simplification of workup procedures [1].
Olefin oxidation is an important transformation in the production of fine and pharmaceutical
grade chemicals. Metal-catalyzed oxidation of olefins can give rise to a whole variety of organic
products [2,3], e.g. aldehydes, ketones, alcohols, epoxides, and acids.
Styrene oxidation at the side chain is of considerable interest. Traditionally, many waste
byproducts are produced during the reaction over homogeneous catalysts [4]. This reaction has
been studied using tert-butyl hydroperoxide (TBHP) over supported gold nanocatalysts in
benzene at reflux [5]. Solvent free oxidation of styrene over undecamolybdophospate supported
on neutral alumina has been reported recently [6], as has the oxidation of styrene to
benzaldehyde by anhydrous hydrogen peroxide on -alumina-supported V2O5 nanoparticle
catalysts [7]. Simple transition metal oxides, like NiO, CoO and MoO3 are known for the
selective oxidation of styrene by TBHP and among these NiO showed the best performance [8].
Most systems tend to give styrene oxide [9-11], although, dendrimer derived titania-supported
Au catalysts show 41% styrene conversion with TBHP in cyclohexane to give benzaldehyde as
major product [12].
Benzaldehyde is a very valuable chemical that has widespread application in perfumery,
pharmaceuticals, dyestuffs, and agro chemicals. It is the second most important aromatic
molecule (after vanillin) used in the cosmetics and flavor industries [6]. Styrene oxide can be
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used for producing epoxy resin diluting agents, ultraviolet absorbents, flavoring agents, etc., and
is also an important intermediate for organic synthesis, pharmacochemistry and perfumery.
Being a terminal olefin, styrene is difficult to oxidize. Despite many efforts to increase
the conversion and selectivity to desired products, only few catalytic systems with multi
transition metal oxides have been developed. Many reports exist on mesoporous materials as
catalyst supports with high surface areas and defined pore-sizes [13-19] and these, with transition
metal oxides, might enhance catalytic activity in styrene oxidation. It seemed likely that the use
of nano-phased and well dispersed catalysts would benefit the catalytic activity due to the
increase in catalyst surface area and physical-chemical interactions. Thus, by mixing the nano-
metal oxides into alumina using ultrasonic cavitation-impregnation, we aimed at achieving good
metal dispersion. We now report in this study a new class of inexpensive mixed nano-metal
oxides of 5-30 wt% loadings of Cu-Ni-Co on -Al2O3, as highly active and selective
heterogeneous catalysts for styrene oxidation using TBHP as co-oxidant with high selectivity to
benzaldehyde.
2.
Experimental details
2.1. Catalyst preparation
The ultrasonic cavitation-impregnation method was used. Appropriate amounts of the
metal precursors, Cu(NO3)23H2O (Sigma-Aldrich), Ni(NO3)26H2O (Sigma-Aldrich) and
Co(NO3)26H2O (ACE ) were dissolved in distilled water and added to γ-Al2O3 (Alfa-Aesar).
To get uniform dispersion, magnetic stirring of 300 rpm and ultrasonic cavitation at 43
KHz frequency (MRC Ultrasonic System, D150H) was applied at room temperature. The excess
water of the slurry was evaporated at 70°C under continuous stirring. The catalysts were dried at
110°C for 16 h, then calcined at 550°C for 5 h, to give the 5, 10, 20 and 30 wt% ternary metallic
nanocatalysts. The catalysts compositions are given in Table 1. The codes 5CuNiCoA,
10CuNiCoA, 20CuNiCoA, 30CuNiCoA, mean 5, 10, 20, 30 wt% total transition metal oxides on
the γ-Alumina (A) support. The intermetallic ratio of copper oxide, nickel oxide and cobalt
oxide was kept constant in the range.
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2.2. Characterizations techniques
The Brunauer-Emmett-Teller (BET) surface area and porosity was determined using a
Micromeritics Tristar II instrument. The metal loading was determined by a Perkin Elmer
Inductive coupled plasma Optical Emission Spectrometer Optima 5300 DV. The metal
dispersion and crystallite size were determined by CO chemisorption using a Micromeritics
ASAP 2020. All catalysts were viewed under a Zeiss ‘Ultra plus’ SEM and a Jeol JEM-1010
Electron Microscope (TEM). Powder XRD was performed on a Bruker D8 Advance instrument.
2.3. Catalyst testing
The oxidation of styrene was usually carried out at 40°C (reaction mixture temperature)
using catalyst (5 mg) and styrene (96 mmol) in CH3CN (2.5 ml) for 24 h in a two-neck flask
equipped with a condenser in an oil bath with stirring. The molar ratio of styrene to TBHP (70%
in water) was kept at 1:1.5. The reactant and products of the reaction were analyzed by gas
chromatography (Perkin Elmer Autosystem). Only products with a benzene ring were considered
in the calculation of selectivity.
3.
Results and discussion
3.1. Characterization
The surface areas and pore volumes of the ternary catalysts decreased from 5 wt% to 30
wt% total metal loadings of copper, nickel and cobalt on -Al2O3 (Table 1). The support surface
was completely covered due to the higher metallic loading in 30CuNiCoA, giving the lowest
surface area. Fig. 1a shows the nitrogen adsorption-desorption isotherms of the ternary catalysts
calcined at 550°C for 5 h. The isotherms show IUPAC type IV patterns with sharp inflections
of nitrogen adsorbed volume at P/P0 of about 0.65 (type H2 hysteresis loop), indicating the
existence of mesopores and these play a significant role in catalyst activity in the presence of
metal oxides. All the prepared materials have mesopores with narrow pore size distribution (Fig.
1b). BET results revealed that the texture of the catalysts was strongly dependent on the total
metal loadings, resulting in significant changes of the surface areas as well as the total pore
volumes. Major changes in textural properties were observed in 10, 20 and 30 wt% catalyst
samples. ICP results obtained were in the expected range (Table 1).
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The powder XRD patterns (Fig. S1) of the prepared ternary oxides on alumina support
show the different metal oxides phases of CuO (JCPDS card no. 5-066), NiO (JCPDS 4-00835),
Co3O4 (JCPDS 42-1467) and -Al2O3 (JCPDS 10-425). The increase in the metal loadings
resulted in a slight increase in the 2 values in the XRD diffractograms. Some overlapping of
diffraction peaks was found among the various metal oxides in all the samples and similar
patterns were observed for all ternary catalysts. Because of the low metal loadings in the case of
5CuNiCoA and 10CuNiCoA, no peaks of metal oxides were seen, due to the detection limit of
XRD. The CO chemisorption study of the ternary catalysts revealed a decrease in the average
crystallite size of all metals from higher metal loadings to lower metal loadings (Table 2) and
further information is given in the supplementary data.
The morphologies of the ternary Cu-Ni-Co oxide catalyst particles on the -Al2O3 surface
are visible in the higher magnification SEM images (Fig. 2a-c) and their dispersion on alumina
was also confirmed by EDS mapping, shown for 10CuNiCoA (Fig. 3). This revealed that all
metal particles are well distributed throughout the surface of the support. As the metal loadings
increases from 10 to 30 wt%, the size of the catalyst particles also increases to an extent due to
agglomeration of the small metal oxide nanoparticles. In the case of low metal loaded samples,
5CuNiCoA and 10CuNiCoA show good individual particle dispersion and very little
agglomeration is observed. The TEM images (Fig. 2d-f) of all the catalyst powders show the
effect of metal loadings on the size of catalyst particles and their dispersion on the support. On
higher magnification, the metal oxide particles appear very clear. All images show round shaped
metal oxide particles, as well as elongated rod/needle-like alumina particles. The size of these
mixed metal oxide particles is in the range of 10-30 nm and more agglomeration is seen in the
case of 30CuNiCoA.
3.2.
Catalyst evaluation
3.2.1. Optimization
The catalysts were tested in the oxidation of styrene. Effects of solvent, oxidant, catalyst
amount, reaction temperature, reaction time and the styrene/oxidant molar ratio were optimized
using 10CuNiCoA as catalyst.
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The effect of the amount of catalyst on the oxidation reaction was explored by using 5%
(5 mg), 10% (10 mg), 15% (15 mg) of 10CuNiCoA, by weight of styrene, keeping all other
parameters fixed. The conversion and selectivity were not affected by the amount of catalyst, due
to the good metal dispersion on the high surface area support. Therefore, 5 mg catalyst was taken
for all further studies.
Initially, the reaction was carried out using dichloromethane and acetonitrile, which are
used commonly in these types of reactions. The best performance of the catalyst was found in
acetonitrile, as it fully dissolves both styrene and TBHP and thus facilitates the oxidation
reactions.
Two different oxidizing agents were initially employed, namely H2O2 and TBHP, at
40°C. The oxidation reaction with hydrogen peroxide gives 42 % conversion after 24 hour but
very poor selectivity towards benzaldehyde and styrene oxide. It forms a whole variety of
oxidized products like benzaldehyde, styrene oxide, acetophenone, phenyl acetaldehyde and 1-
phenyl-ethane-1,2-diol. However, with TBHP, best conversion (73%) and good selectivity
towards benzaldehyde (72%) and styrene oxide (19%) was obtained after 24 hours for
10CuNiCoA.
The effect of styrene/oxidant molar ratio was also examined to get maximum conversion
and selectivity towards benzaldehyde and styrene oxide. After 24 h at 40°C, the 1.0:0.5, 1.0:1.0,
and 1.0:1.5 molar ratios gave 46%, 60%, and 73% styrene conversion, respectively, but no
influence on the selectivity to desired products was observed. A molar ratio of 1.0:1.5
styrene/oxidant was thus chosen for further studies.
The effect of temperature was investigated at room temperature, 40 and 55°C, on the
oxidation of styrene using acetonitrile as a solvent with 5 mg catalyst and a styrene to TBHP
ratio of 1.0:1.5. At RT, 40 and 55°C, the styrene conversion was found to be 22%, 53% and
81%, respectively, after 9 h of reaction (Fig. 4). At 40°C, selectivity to benzaldehyde was 50%
and 7% was to styrene oxide and at 55°C, conversion was good but selectivity to benzaldehyde
was 39% with 16% to styrene oxide. At 55°C, because of the higher temperature, the reaction
followed both reaction pathways shown in Scheme 1 simultaneously and hence a decrease in
benzaldehyde selectivity was observed. So, a 40°C reaction temperature is the preferred
temperature for the oxidation of styrene over these catalysts.
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The ideal conditions for maximum conversion of styrene to benzaldehyde and styrene
oxide are thus a mole ratio of styrene to TBHP of 1.0:1.5, 40°C, with 5 mg catalyst, in
acetonitrile.
3.2.2. Role of ternary catalysts for styrene oxidation
The utility of this new class of ternary metal oxide catalysts for styrene oxidation was
further explored under the above mentioned, optimized reaction conditions. Since the Cu-Ni-Co
oxides are on an alumina support, the behavior of the support (without transition metal oxide)
was investigated under similar reaction conditions and it showed only 4% styrene conversion.
Hence, the conversion exhibited with the catalysts is clearly dependent on the nature of the
mixed metal oxides and not the alumina itself.
As the metal loading increased from 5 to 30 wt%, the conversion of styrene also
increased (Fig. 5). This can be due to the increased availability of the ternary metal oxide active
sites, providing more active centres for reaction. The benzaldehyde selectivity decreases from 72
to 58% with increasing total metal loadings at the end of the 24 h reaction period. The selectivity
of styrene oxide and 1-phenylethane-1,2-diol was found in the range of 18-19% and 8-9%,
respectively, in all reactions after 24 h. Other products, i.e. acetophenone and phenyl
acetaldehyde, total selectivity was found to be 11% and 14% in the case of 20CuNiCoA and
30CuNiCoA catalysts respectively, but the 10CuNiCoA catalyst showed only 1% total selectivity
to these two products. For all the catalysts, turnover frequencies (TOF), defined as mmols of
styrene converted per gram of catalyst per hour, range between 5 to 7, comparable to other
systems [8], and the carbon balance of all reactions is between 91 to 93%, the balance being CO2
(Table S1). To identify the role of single metal catalysts, a series of monometallic catalyst with 6
wt% copper, 3 wt% nickel and 1 wt% cobalt supported on alumina were prepared by the same
method and tested under similar reaction conditions. Results show 42%, 24% and 25% styrene
conversion, respectively, with selectivities of benzaldehyde in the range of 30-50%, styrene
oxide in the range of 11-20% and other products in the range of 6-35%.
Overall best catalytic activity and benzaldehyde selectivity was found for 10CuNiCoA.
An appropriate metal loading, as well as uniform metal dispersion, tends to facilitate specific
product selectivity. Optimum metal loadings may affect the surface properties and number of
reactive sites. Activity increases due to the increased number of active sites. The addition of the
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copper decreased the crystal sizes of both Ni and Co and increased the metal dispersion, as
measured by CO chemisorption (Table 2). In addition to the smaller particle sizes, the increased
metal dispersion could contribute towards the better activity of the catalysts. The homogenous
dispersion of small sized metal particles not only increases the activity by exposing a large
number of active sites, but also the possible oxygen transfer to the support.
In general, oxides of Ni and Cu show greater Brønsted acidity than Co oxide [20]. Due
to the high Lewis basic nature of the cobalt, the catalyst contains an even distribution of Lewis
basic sites and Brønsted acidic sites. The oxidant adsorbs on the surface of the catalyst due to its
Brønsted acidic nature and the oxygen from the oxidant desorbs to the support due to the Lewis
acidic nature. Thus, this catalytic reaction follows a probable pathway similar to the Eley-Rideal
mechanism [21]. The reactive sites would favor the formation of benzaldehyde as a major
product in agreement with the experimental results.
It is known that the styrene oxidation at the side chain can lead to various reaction
products, depending on the catalyst and reaction conditions. Two major reactions take place.
They are the oxidative C=C cleavage into benzaldehyde and epoxidation [6]. However, under the
present catalysis conditions, the major oxidation product obtained was benzaldehyde. This may
be due to (i) direct oxidative cleavage of C=C of styrene and (ii) fast conversion of styrene oxide
to benzaldehyde.
Under the optimized reaction conditions, the control experiment with styrene oxide as a
substrate was carried out to confirm the reaction pathway (Scheme 1). The results indicate that
styrene oxide converts into acetophenone and phenyl acetaldehyde. This proves that
benzaldehyde formed through only the direct oxidative cleavage of the C=C bond of styrene, but
not through the fast conversion of styrene oxide to benzaldehyde. These catalysts thus have the
special property of stimulating direct oxidative cleavage of C=C so that benzaldehyde is the
major product.
4.
Conclusion
In summary, novel heterogeneous mixed Cu-Ni-Co nano-metal oxide catalysts have been
successfully synthesized, characterized and proven highly successful for the oxidation of styrene
with TBHP as an oxidant under mild reaction conditions. All ternary catalysts showed good
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oxidative activity even at low temperature, due to good dispersion and a cooperative effect
produced by the mixed metals on the high surface area mesoporous support.
The main products of the styrene oxidation reaction were benzaldehyde and styrene
oxide. The conversion and selectivity towards benzaldehyde was highest i.e. 73% and 72%,
respectively, with minimal byproducts formed in the case of 10CuNiCoA. The superiority of this
inexpensive catalyst is likely due to synergistic interaction between the metals coupled with high
dispersion.
Acknowledgements
We thank Sasol Technology and THRIP for financial support and the University of
KwaZulu-Natal for a postdoctoral fellowship (HP).
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Captions
Table captions:
Table 1 - Surface area and metal composition of the catalysts
Table 2 - Metal dispersion of -Alumina supported ternary catalysts
Figure captions:
Fig. 1 (a) N2 Adsorption-desorption isotherms and (b) pore size distribution of ternary catalysts
Fig. 2 (a,b,c) SEM and (d,e,f) TEM images of the 10CuNiCoA, 20CuNiCoA, 30CuNiCoA
catalysts respectively
Fig. 3 SEM-EDS mapping of the 10CuNiCoA catalyst
Fig. 4 Effect of reaction temperature on styrene conversion over the 10CuNiCoA catalyst
Fig. 5 Effect of ternary catalysts in styrene oxidation using 5 mg catalysts and 1.5 mole
equivalent of TBHP at 40°C after 24 h
Scheme 1. Proposed reaction scheme for styrene oxidation catalyzed by the supported mixed
nano-metal oxides
Supplementary Data:
1.CO chemisorption analysis
Fig. S1 XRD patterns of the (a) -Al2O3 (b) 30CuNiCoA and (c) 20CuNiCoA catalysts
Table S1 TOF and carbon balance of styrene oxidation
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Table 1 Surface area and metal composition of the catalysts
Catalyst
BET Surface
Area (m2/g)
Total Pore
Volume
(cm3/g)
Metal
Composition
by ICP-OES
(Cu:Ni:Co) wt%
---
3.23:1.58:0.55
6.56:3.41:0.96
13.05:6.43:2.01
16.98:8.25:2.57
263
213
200
172
153
0.84
0.64
0.59
0.51
0.44
-Alumina
5CuNiCoA
10CuNiCoA
20CuNiCoA
30CuNiCoA
Table 2 Metal dispersion of -Alumina supported ternary catalysts
Catalyst
Metal Dispersion*
(%)
Average Crystallite Size*
(nm)
Cu
9
Ni
19
21
10
Co
Cu
9.6
Ni
5.0
Co
1.3
1.5
2.0
10CuNiCoA
20CuNiCoA
30CuNiCoA
70
67
49
7
13.9
19.6
4.7
5
10.0
*calculated using CO chemisorption (more information in supplementary data)
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(a)
(b)
Fig. 1 (a) N2 Adsorption-desorption isotherms and (b) pore size distribution of
ternary catalysts
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(a)
(d)
(b)
(e)
(f)
(c)
Fig. 2 (a,b,c) SEM and (d,e,f) TEM images of the 10CuNiCoA, 20CuNiCoA and
30CuNiCoA catalysts respectively
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Fig. 3 SEM-EDS mapping of the 10CuNiCoA catalyst
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Fig. 4. Effect of reaction temperature on styrene conversion and selectivity to major
products over the 10CuNiCoA catalyst
*Others: Acetophenone, Phenyl acetaldehyde
Fig. 5. Effect of ternary catalysts in styrene oxidation using 5 mg catalysts and 1.5 mole
equivalent of TBHP at 40°C after 24 h
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H
Oxidative
C=C Cleavage
O
O
Benzaldehyde
CH3
OH
OH
Rearrangement
Styrene
Oxida
[O]
tion
CHO
O
+
Styrene oxide
1-Phenylethane-1,2-diol
Acetophenone
Phenyl
acetaldehyde
Scheme 1. Proposed reaction scheme for styrene oxidation catalyzed by the supported
mixed nano-metal oxides
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Graphical Abstract
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Highlights:




Supported Cu-Ni-Co oxides were synthesized with good dispersion of the metals
Ternary nanocatalysed approach was appropriate for styrene oxidation
Catalysts stimulate direct oxidative cleavage of C=C bond to benzaldehyde
Enhancement in benzaldehyde selectivity is due to synergistic effect of metals
20