Excellent product selectivity towards 2-phenyl-acetaldehyde and styrene oxide using manganese oxide and cobalt oxide NPs for the selective oxidation of styrene

Article abstract of DOI:10.1016/j.apcata.2018.04.027

MnO_x and Co₃O₄ nanoparticles (NPs) were synthesized using the temperature controlled co-precipitation method and were immobilized on the mesoporous metal oxide supports that were synthesized using the inverse surfactant micelle method. The synthesized catalysts were characterized using TEM, SEM, pXRD, N₂-sorption, XPS, and H₂-TPR. From TEM the well-dispersed metal oxide NPs were observed and the interaction of the NPs with the support was confirmed with H₂-TPR. These metal oxide NPs and immobilized metal oxide NPs were employed in the selective oxidation of styrene using tert-butyl hydrogen peroxide (TBHP) as an oxidant. Higher selectivity towards styrene oxide was obtained using Co₃O₄ NPs. MnO_x NPs gave higher selectivity towards 2-phenyl-acetaldehyde while the immobilized metal oxide NPs gave a mixture of products. The catalytic activities of the compared catalysts followed the following decreasing order of MnO_x NPs (646.9 h^?1) > Co₃O₄ NPs (338.1 h^?1) > Co-MnO₂_350 (54.1 h^?1) > Mn-Co₃O₄_350 (44.1 h^?1) ? MnO₂_350 (35.8 h^?1) > Co₃O₄_350 (21.3 h^?1) after 8 h of reaction. The catalytic activities and selectivity profiles under various reaction conditions such as solvent type, catalyst amount, styrene to TBHP ratio and temperature are discussed. The catalytic recyclability of immobilized metal oxide NPs on mesoporous metal oxides showed that they are stable, and the structure of the catalyst was retained.

Full text of DOI:10.1016/j.apcata.2018.04.027

AppliedCatalysisA,General559(2018)175–186
Contents lists available at ScienceDirect
Applied Catalysis A, General
journal homepage: www.elsevier.com/locate/apcata
Excellent product selectivity towards 2-phenyl-acetaldehyde and styrene
oxide using manganese oxide and cobalt oxide NPs for the selective
oxidation of styrene
Ngonidzashe Masungaa, Bryan P. Doyleb, Emanuela Carleschib, Reinout Meijbooma,⁎
a
Department of Chemistry, University of Johannesburg, P.O. Box 524, Johannesburg, South Africa
Department of Physics, University of Johannesburg, P.O. Box 524, Auckland Park 2006, Johannesburg, South Africa
b
A R T I C L E I N F O
A B S T R A C T
Keywords:
MnOx and Co3O4 nanoparticles (NPs) were synthesized using the temperature controlled co-precipitation
method and were immobilized on the mesoporous metal oxide supports that were synthesized using the inverse
surfactant micelle method. The synthesized catalysts were characterized using TEM, SEM, pXRD, N2-sorption,
XPS, and H2-TPR. From TEM the well-dispersed metal oxide NPs were observed and the interaction of the NPs
with the support was confirmed with H2-TPR. These metal oxide NPs and immobilized metal oxide NPs were
employed in the selective oxidation of styrene using tert-butyl hydrogen peroxide (TBHP) as an oxidant. Higher
selectivity towards styrene oxide was obtained using Co3O4 NPs. MnOx NPs gave higher selectivity towards 2-
phenyl-acetaldehyde while the immobilized metal oxide NPs gave a mixture of products. The catalytic activities
of the compared catalysts followed the following decreasing order of MnOx NPs (646.9 h−1) > Co3O4 NPs
(338.1 h−1) > Co-MnO2_350 (54.1 h−1) > Mn-Co3O4_350 (44.1 h−1) ˃ MnO2_350 (35.8 h−1) > Co3O4_350
(21.3 h−1) after 8 h of reaction. The catalytic activities and selectivity profiles under various reaction conditions
such as solvent type, catalyst amount, styrene to TBHP ratio and temperature are discussed. The catalytic re-
cyclability of immobilized metal oxide NPs on mesoporous metal oxides showed that they are stable, and the
structure of the catalyst was retained.
Manganese oxide nanoparticles
Cobalt oxide nanoparticles
Mesoporous metal oxides
Styrene oxide
2-Phenyl-acetaldehyde
Styrene oxidation
1. Introduction
and benzoic acid are a result of further oxidation of styrene oxide while
2-phenyl-acetaldehyde and acetophenone are a result of the iso-
The development of heterogeneous catalysts has been an area of
great interest over the past decades, due to the ease of recycling of these
heterogeneous catalysts. However, there have been great strides in the
field of catalysis such as the successful drive to move from the tradi-
tional complex compounds to nanoscale materials [1]. The ability to
prepare NPs with well-defined shapes, with sizes that range between
1–100 nm in diameters, have attracted great attention from researchers
[2]. These NPs have unique properties such as electronic, magnetic and
catalytic properties that distinguish them from their corresponding bulk
materials [2–4].
The selective oxidation of styrene to its products has been much
studied in the past decades as its products are of industrial importance
[5]. It has been also used as a model reaction for ethylene epoxidation
due to it being a liquid at room temperature [3]. Styrene oxidation
produces multiple products such as styrene oxide, benzaldehyde, 2-
phenyl-acetaldehyde, acetophenone, benzoic acid [3,6–8], depending
on the reaction conditions and the type of catalyst used. Benzaldehyde
merization of styrene oxide. The schematic illustration of styrene oxi-
dation is given in Scheme 1.
Styrene oxide is mainly used in the epoxy resins industry as a
plasticizer and also, as a raw material for phenethyl alcohol, styrene
glycol, and its derivatives. It is also used as a chemical intermediate for
cosmetics, surface coatings, treatment of fibers and textiles, and agri-
cultural and biological chemicals and is, therefore, an important pro-
duct for the pharmaceutical and fine chemical industry [9–11]. While,
2-phenyl-acetaldehyde, the isomer of styrene oxide, is mainly used in
the agricultural industry for monitoring and controlling insects because
of its aroma that attracts them. It is also used as an intermediate in the
biochemical pathways, including those involved in the metabolism of
carbohydrates, vitamins, steroids, amino acids, benzylisoquinoline al-
kaloids, hormones, and lipids [12]. Therefore, it is important to selec-
tively oxidize styrene to styrene oxide and 2-phenylacetaldehyde while
achieving high styrene conversion.
We intend to exploit the properties of manganese oxide (MnOx) NPs
⁎
Corresponding author.
E-mail addresses: rmeijboom@uj.ac.za, reinout.meijboom@gmail.com (R. Meijboom).
https://doi.org/10.1016/j.apcata.2018.04.027
Received 18 January 2018; Received in revised form 20 April 2018; Accepted 21 April 2018
Availableonline23April2018
0926-860X/©2018ElsevierB.V.Allrightsreserved.

N. Masunga et al.
AppliedCatalysisA,General559(2018)175–186
Scheme 1. Schematic illustration of the catalytic oxidation of styrene.
Fig. 1. HR-TEM images of (A): MnOx NPs, (C): Co3O4 NPs, and their particle size distribution histograms in (B) and (D) respectively.
and cobalt oxide (Co3O4) NPs which include high surface to volume
ratio and excellent processability to selectively produce styrene oxide
and 2-phenylacetaldehyde. From literature, it can be noted that dif-
ferent forms of manganese oxide catalysts have been employed in
styrene oxidation and different products were observed. Benzaldehyde
and styrene oxide were the major products observed using manganese
oxide catalytic systems [13–18]. For example, the role of manganese
oxide octahedral molecular sieves in styrene epoxidation [19], man-
ganese catalyzed epoxidation of alkenes in bicarbonate solutions [20],
etc. The Mn3O4 and Co3O4 nanocrystals which were synthesized using
the sonochemical method were used for the epoxidation of styrene
[21]. The MnOx NPs have also been used in hydrogenation reactions
such as reduction of carbon dioxide to methanol [22] and also as
electrocatalysts for the oxygen reduction reaction in alkaline medium
[23]. However, in this work, the manganese oxide nanoparticles which
were synthesized using the heat mediated co-precipitation method was
investigated for the oxidation of styrene.
Different forms of cobalt catalysts which include cobalt complex,
cobalt nanocrystals, cobalt oxides, etc, have also been employed in the
oxidation of styrene to styrene oxide and benzaldehyde as major pro-
ducts [7,16,17,24,25]. However, cobalt oxide NPs synthesized using the
heat mediated co-precipitation method has not been used for studying
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AppliedCatalysisA,General559(2018)175–186
Fig. 2. TEM images A: Mn-Co3O4_350 and B: Co-MnO2_350.
2.2. Synthesis of manganese oxide NPs (MnOx)
Table 1
The table of BET surface areas, BJH pore sizes, pore volumes and crystallite
sizes of the synthesized mesoporous metal oxides catalysts.
Manganese oxide NPs were synthesized using Mn-oleate as a source
of manganese. Mn-oleate was synthesized by dissolving 7.92 g
(40 mmol) MnCl2.4H2O and 24.36 g (80 mmol) sodium oleate in a
mixture of 30 mL of ethanol, 40 mL of water, and 70 mL n-hexane that
was preheated to 70 °C. The mixture was left to react for 4 h at 70 °C
under magnetic stirring. The solution of Mn-oleate was collected using
a separation funnel. Then, the Mn-oleate salt was collected using a
rotary evaporator.
The MnOx NPs were synthesized by dissolving 1.24 g of Mn-oleate in
12.67 mL of 1-octadecene in a 50 mL round bottom flask. The solution
was stirred at 70 °C and degassed under nitrogen for 1 h. The tem-
perature of the reaction mixture was ramped to 260 °C (1.9 °C/min).
The color of the solution changed from pink to dark brownish. The
solution was left to cool for 10 min. The MnOx NPs were collected by
centrifugation and they were dispersed in 10 mL of chloroform [22,30].
Catalyst
BET Surface
areas (m2/g)
BJH pore
size (nm)
Pore Volume
(cm3)
Crystallite sizea
(nm)
MnO2 _ 350
Co-MnO2_350
Co3O4 _ 350
37
36
42
22.4
22.1
26.4
25.9
0.21
0.20
0.27
0.32
11.5
12.0
20.8
19.5
Mn-Co3O4 _ 350 38
a
Crystallite sizes were calculated from p-XRD using the Scherer equation.
the oxidation of styrene. Cobalt NPs have been used as an efficient
electrocatalyst for oxygen evolution reactions [26,27]. Other transition
metal NPs that have been investigated in styrene oxidation include Pt
and Pd by Noh [3] and supported gold NPs [2,28,29]. Therefore, it is
important to investigate the behavior of our unsupported MnOx and
Co3O4 NPs in styrene oxidation. Also, supporting these NPs on an active
metal oxide support will enhance our knowledge on the influence of the
created interface between the support and the metal NPs. The MnOx
NPs were supported on mesoporous cobalt oxide, while Co3O4 NPs was
supported on mesoporous manganese oxide to investigate the influence
of the interfacial domain that is created by the change in electronic
structure through the exchange of electrons between the two metals
[22]. Hence, exploration of the catalytic behavior of MnOx and Co3O4
NPs in catalytic oxidation of styrene in this study will enrich the sci-
entific knowledge in the field of oxidation by colloidal metal oxide NPs.
2.3. Synthesis of cobalt oxide NPs
Cobalt oxide NPs were synthesized by degassing 150 μL of oleic acid
under nitrogen for 1 h that was placed in a Schlenk tube connected to a
condenser. Then, 12 ml of dichlorobenzene (DCB) was transferred into
the flask and the temperature was raised to 169 °C (5 °C/min). Finally,
0.444 g of Co2(CO)8 dissolved in 3 mL DCB was injected into a reaction
vessel using an airtight syringe and the reaction was left to react for
20 min before the heat was turned off. DCB (13 mL) was poured into the
flask, and the colloidal suspension was left to cool. The Co3O4 NPs were
collected by centrifugation and redispersed in 10 mL chloroform for
further use [22].
2. Experimental details
2.1. Materials and instrumentation
All chemicals were used as received without further purification and
were of the analytical grade. Styrene (≥99%), TBHP (70% in water),
styrene oxide (≥97%), benzaldehyde (≥99%), acetophenone (≥99%),
2-phenyl-acetaldehyde (≥98%), dichloromethane (DCM) (≥99.8%),
dimethyl carbonate (DMC) (≥99%), methanol (≥99.5%), cyclohexane
(≥99.8%), 1-butanol (≥99.5%), dichlorobenzene (≥99%), ethanol
(≥99.8%), n-hexane (≥97%), 1-octadecene (≥98%), toluene
(≥99.8%), chloroform (≥99%), and acetonitrile (≥99.9%), manga-
nese(II) nitrate tetrahydrate (≥97%), cobalt(II) nitrate hexahydrate
(≥98%), MnCl2·4H2O (≥98%), dichlorobenzene (DCB) (≥99%) so-
dium oleate (≥82%), Co2(CO)8 (stabilized with 1–5% hexanes), oleic
acid (≥ 99%), and poly (ethylene glycol)-block-poly (propylene
glycol)-block-poly (ethylene glycol) (P123) were all purchased from
Sigma-Aldrich.
2.4. Immobilization of NPs on mesoporous metal oxides
Immobilization of the NPs was achieved using the capillary inclu-
sion method. Briefly, 5 mL (0.570 mmol) of colloidal MnOx NPs dis-
persed in chloroform was diluted with 40 mL n-hexane and mixed with
1 g of mesoporous cobalt oxide. The mixture was sonicated for 3 h at
room temperature. The precipitate was collected by centrifugation and
washed with a 50/50 v/v mixture of ethanol and acetone five times.
The resultant pellet was dried overnight at 80 °C. The same procedure
was followed for immobilizing Co3O4 NPs on mesoporous manganese
oxide [22]. The synthesis of mesoporous metal oxides is detailed in the
supplementary information. The description of the instruments used for
characterization is found in the supplementary information.
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Fig. 3. Characterization of immobilized metal oxide NPs on MMOs. A: N2 sorption isotherms; B: BJH pore volume distributions, and C: BJH pore sizes distributions.
Fig. 4. Characterization of immobilized metal oxide NPs mesoporous metal oxides. A: graphs of low angle p-XRD and B: graphs of wide angle p-XRD patterns.
2.5. Experimental procedure for styrene oxidation
silicone oil bath was heated to 90 °C prior to the reaction and the re-
actions were heated under reflux with stirring. The reaction was sam-
pled after every 2 h for 8 h. The catalytic conversion of styrene and
selectivity was determined using a Shimadzu GC-2010 Plus equipped
with a flame ionization detector (FID) using a Restek - 800-356-1688
capillary column (30 m * 0.25 mm, the film thickness of 0.25 μm).
Temperatures of 250 °C and 300 °C were used for injection port and FID
respectively. The column oven temperature was set at 300 °C and ni-
trogen was used as a carrier gas. Reaction products were confirmed
The oxidation of styrene was conducted under aerobic conditions
using a 25 mL 2-neck round bottom flask equipped with a magnetic
stirrer, thermometer, and a heated silicon oil bath. For colloidal MnOx
and Co3O4 NPs, 0.114 mmol of each was used. For immobilized MnOx
and Co3O4 NPs on mesoporous metal oxides, typically 0.1 g of catalyst,
1 mmol styrene, and 2 mmol TBHP as an oxygen source was added to a
25 mL 2-neck round bottom flask filled with 5 mL DMC as a solvent. The
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Fig. 5. H2-TPR profiles of mesoporous metal oxides and when immobilized with metal oxide NPs. A: cobalt oxide and B: manganese oxide.
using a Shimadzu gas chromatography (GC–MS) using a Rxi-5Sil MS
capillary column (30 m * 0.25 mm ID * 0.25 μm film thickness) and
injection port temperature of 200 °C was used. The ion source and in-
terface temperature were 200 °C and 250 °C, respectively.
NPs. Fig. 3A shows that both immobilized mesoporous metal oxides
exhibited the characteristic type IV adsorption isotherms, with a hys-
teresis loop of the ordered mesoporous structure.
From Fig. 4A, it was observed that the synthesized materials diffract
in a low angle region. The peak observed at around 2θ showed that both
immobilized metal oxide NPs on mesoporous metal oxides are crystal-
line in nature. The crystallite sizes increase did not change significantly
after immobilization of the metal oxide NPs as shown in Table 1. These
crystallite sizes were calculated from wide angle p-XRD shown in
Fig. 4B, using the Scherrer equation. Fig. 5 shows the H2-TPR profiles of
mesoporous metal oxides and when immobilized with metal oxide NPs
and it can be noted that two bands are observed. It is observed that
immobilization of metal oxide NPs on the mesoporous metal oxide
support increases the reduction temperature of the mesoporous metal
oxides.
X-ray photoelectron spectroscopy (XPS) was carried to provide in-
formation on the electronic structure of the catalysts. The O 1s spectra
for all four catalysts are as expected [32]. The largest peak in Fig. 6, at
the lowest binding energy, is due to stoichiometric oxygen. The peak at
around 531 eV is due to oxygen vacancies and/or other defects [33,34].
Finally, the peak at highest binding energy is known to be from che-
misorbed oxygen [35]. In a similar way the Mn 2p spectra, shown in
Fig. 6, show spin-orbit split peaks. These are due to Mn in a 2 + oxi-
dation state, a 3 + oxidation state and a satellite at high binding en-
ergy. Note that for Mn the 2 + oxidation state is at lower binding en-
ergy, which is the opposite to what occurs with Co [36]. The oxidation
state % ratio of Mn 2p are presented in Table 2. It can be noted that the
two samples with MnO2 have similar oxidation states, whereas the
MnOx NPs samples have a slightly different oxidation state. Fig. 6 shows
the Co 2p spectra of those catalysts with Co present. The spectra were
fitted with three components. Each component is split into the Co 2p3/2
and 2p1/2 peaks. These components are due to Co in the 3 + oxidation
state (at lower binding energy), Co in the 2 + oxidation state (at higher
binding energy) and a satellite at high binding energy. The spectra are
consistent with what is expected from literature [32]. The oxidation
state % ratio of Co 2p are presented in Table 2, and they were found to
be similar across all 3 samples.
3. Results
3.1. Synthesis and characterization
The metal oxide NPs were successfully synthesized using the tem-
perature-controlled co-precipitation method. The synthesized MnOx
NPs were well dispersed as noted from the TEM image in Fig. 1A and
were found to have an average size of 6.2 nm as shown in Fig. 1B. The
synthesized Co3O4 NPs were also well dispersed, and they were found
to have an average size of 8.4 nm. The purity of the metal oxide NPs
was confirmed using EDX as shown in Fig. S1. From Fig. S1A, the
manganese and oxygen peaks were observed in the EDX spectrum
which confirms the presence of pure MnOx NPs. The copper peaks
which were observed came from the Cu grid. In Fig. S1B, the cobalt and
oxygen peaks were observed which confirms the presence of Co3O4 NPs
The elemental analysis of immobilized metal oxide NPs on meso-
porous metal oxide supports was confirmed using EDX as shown in Fig.
S1. From the ICP-OES analysis, 1.2 wt% of Mn was found to be sup-
ported on mesoporous cobalt oxide and 1.2 wt% of Co was found to be
supported on mesoporous manganese oxide support. Fig. 2 shows the
TEM images of the immobilized NPs on the metal oxides support. It is
important to note the difficulty in observing the supported NPs on the
support and measuring them since the NPs suffer from low contrast due
to the high electron density from the support. The difference in focal
planes of the NPs and the support when acquiring images adds to the
difficulty of measuring the NPs [31]. In Fig. 2A and B) the small black
dots are observed on the support which signifies the presence of the
MnOx NPs and Co3O4 NPs on the support respectively. The SEM images
in Fig. S2A and B show the morphology of the immobilized MnOx NPs
and Co3O4 NPs on the support respectively.
The nitrogen sorption experiments showed that the BJH pore sizes
decreased from 22.4 nm for mesoporous MnO2_350 to 22.1 nm after the
Co3O4 NPs were immobilized on it. For mesoporous Co3O4_350 it de-
creased from 26.4 to 25.9 nm after MnOx NPs were immobilized, as
shown in Table 1. The surface area for mesoporous MnO2_350 de-
creased from 37 to 36 m2/g after immobilization of Co3O4 NPs. While
the surface area of Co3O4_350 decreased from 42 to 38 m2/g after im-
mobilization of MnOx NPs. These changes in the physical properties of
the mesoporous metal signify successful immobilization of metal oxide
3.2. Optimization of the reaction conditions
The oxidation of styrene in the presence of air using metal oxide NPs
and immobilized metal oxide NPs on mesoporous metal oxides support
as catalysts were conducted with various experimental parameters such
as solvent type, catalyst amount, etc, to obtain the optimum conditions.
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Fig. 6. The O 1 s, Mn 2p, and Co 2p core level electrons XPS spectra for pure mesoporous metal oxides and for immobilized metal oxide nanoparticles on a
mesoporous metal oxide support.
phenyl-acetaldehyde was lower (81.6%) than that of DMC (90.6%) as
shown in Fig. 7A. From Fig. 7A, it is noted that DMC as a solvent gave
high selectivity (90.6%) towards 2-phenyl-acetaldehyde while its
styrene conversion is moderate as compared to others. Solvents with a
high polarity index such as acetonitrile (polarity index 5.8) and me-
thanol (polarity index 5.1) showed low styrene conversion of 50.9%
and 38.7% respectively while their product selectivity was moderate
compared to others. Toluene and cyclohexane which are nonpolar
solvents with a polarity index of 2.4 and 0.2 respectively gave a rela-
tively high styrene conversion and a moderate selectivity towards 2-
phenyl-acetaldehyde as shown in Fig. 7A. After 4 h of reaction, small
brown particles were observed in methanol while in acetonitrile, large
black particles were observed. The overall results showed that DMC as a
solvent gave the highest product selectivity and a moderate styrene
conversion, hence it was selected as an optimum solvent for further
Table 2
The percentage composition of the oxidation states of the synthesized catalysts.
Orbital type
Co 2p
Catalyst
Oxidation state (2+)
%
Oxidation state (3+)
%
Co3O4 _350
28
21
24
66
62
53
72
79
76
34
38
47
Mn-Co3O4 _350
Co-MnO2_350
MnO2_350
Co-MnO2_350
Mn-Co3O4 _350
Mn 2p
3.2.1. Solvent type
The influence of solvent on the oxidation of styrene was investigated
using various types of solvents. Chloroform was found to give the
highest styrene conversion of 67.8%, however, its selectivity towards 2-
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Fig. 7. Optimization of the reaction conditions. A: solvent variation; B: catalytic amount variation; C: mole ratio of styrene to TBHP variation; D: temperature
variation. Reaction conditions: catalyst (0.114 mmol of MnOx NPs), solvent (5 mL DMC), 1 mmol styrene, 2 mmol TBHP, T = 90 °C, time = 4 h, except when the
condition is investigated.
investigations.
3.2.4. The optimum temperature
The influence of temperature on the styrene conversion and product
selectivity was investigated. Styrene conversion was found to increase
with increase in temperature. The highest selectivity towards 2-phenyl-
acetaldehyde (91.7%) was achieved at 80 °C. From the observed results,
no correlation can be deduced between temperature and product se-
lectivity as the product selectivity remains relatively constant as shown
in Fig. 7D. Therefore, 90 °C was used as the reaction temperature for
further investigations on styrene oxidation considering the boiling point
of the solvent and styrene conversion.
3.2.2. Optimum amounts of reagents
The influence of the catalyst amount on the styrene conversion and
product selectivity was investigated and it was observed that the
styrene conversion and product selectivity towards 2-phenyl-acet-
aldehyde increase as the catalyst amount is increased as shown in
Fig. 7B. The reaction was performed without the catalyst under the
same reaction conditions and a conversion of 8.5% was observed. When
the catalyst amount was increased beyond 0.114 mmol, a large decrease
in selectivity towards 2-phenyl-acetaldehyde was observed. Hence
0.114 mmol was chosen as the amount of catalyst for further research
on styrene oxidation.
3.3. Styrene oxidation catalyzed by various catalysts
From Fig. 8 it can be noted that styrene conversion increases with an
increase in reaction time for all catalysts. The highest styrene conver-
sion (59.3%) was obtained using MnOx NPs as a catalyst after 8 h. The
catalytic activities of the compared catalysts were decreasing in the
order of MnOx NPs (646.9 h−1) > Co3O4 NPs (338.1 h−1) > Co-
3.2.3. The optimum ratio of styrene to TBHP
The influence of styrene to TBHP ratio in the styrene conversion and
product selectivity was examined by keeping the concentration of
styrene constant while varying the amount of TBHP. From Fig. 7C, it is
noted that the styrene conversion increases with an increase in TBHP
amount. Also, the selectivity towards 2-phenyl-acetaldehyde was af-
fected to a lower extent. However, a large decrease in selectivity to-
wards 2-phenyl-acetaldehyde was observed when the styrene to TBHP
ratio is at 1:4 as shown in Fig. 7C. Hence, the ratio of 1:1 was found to
be the optimum condition for further research on the styrene oxidation.
MnO2_350 (54.1 h−1) > Mn-Co3O4
_
350 (44.1 h−1) ˃ MnO2_350
(35.8 h−1) ˃ Co3O4 _ 350 (21.3 h−1) after 8 h of reaction. The turnover
frequency (TOF), which is defined as the number of moles of styrene
converted to all products per mole of the catalyst per hour were cal-
culated after 8 h of reaction. When the mesoporous metal oxide sup-
ports were used in styrene oxidation as catalysts, poor styrene
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Fig. 8. The comparison of styrene conversion and product selectivity for different metal NPs and after being immobilized on mesoporous metal oxide supports. A:
MnOx NPs, B: Co3O4 NPs, C: Mn-Co3O4_350, D: Co-MnO2_350, and E: turnover frequency. Reaction conditions: catalyst (1 mL for metal oxide NPs and 0.1 g for
immobilized metal oxide NPS), solvent (5 mL DMC), 1 mmol styrene, 1 mmol TBHP, T = 90 °C.
conversion and product selectivity were observed. From Fig. 8E it can
also be noted that immobilizing metal oxide NPs on the mesoporous
metal oxides supports reduces the activity of the metal oxide NPs in
styrene oxidation. From the obtained results, it can be noted that the
selectivity of the products formed during oxidation of styrene depends
on the catalyst used.
observed as a major product while when colloidal Co3O4 NPs were
used, styrene oxide was obtained. When these metal oxide NPs were
immobilized on the mesoporous metal oxides support they lost their
selectivity and a mixture of products were obtained as shown in Fig. 8
(C and D). For metal oxide NPs, the product selectivity remains constant
towards one product over the course of the reaction from 2 to 8 h.
However, with immobilized metal oxide NPs, the product selectivity
When colloidal MnOx NPs were used, 2-phenyl-acetaldehyde was
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Fig. 9. The influence of catalyst recycles test on styrene conversion and product selectivity. A: Mn-Co3O4_350 and B: Co-MnO2_350. Reaction conditions: 100 mg
catalyst, Solvent (5 mL DMC), 1 mmol styrene, 1 mmol TBHP, T = 90 °C, Time = 4 h.
changed with the change in time. When Mn-Co3O4_350 was used as a
catalyst, the formation of 2-phenyl-acetaldehyde increased while the
formation of styrene oxide decreased with an increase in reaction time.
For the Co-MnO2_350 catalyst, the formation of other products in-
creases with increase in reaction time while product selectivity towards
styrene oxide and 2-phenyl-acetaldehyde decreased from 4 to 8 h.
Hence, immobilizing metal oxide NPs on mesoporous metal oxide
supports decreased both styrene conversion and product selectivity.
Representative GC–MS spectra of 2-phenyl-acetaldehyde and styrene
oxide are given in Figs. S3 and S4, respectively of the supplementary
materials.
techniques. From the nitrogen, sorption experiments the observed Type
IV adsorption isotherms signify the regular ordered mesoporous nature.
The presence of the regular ordered mesoporous structure was sup-
ported by the presence of a low angle peak observed in the p-XRD [38].
After immobilization of metal oxide, NPs on the mesoporous metal
oxides, the surface area, and pore volumes decreased. This is because
the metal oxide NPs occupy space inside pores. [40]. The purity of the
metal oxide NPs and immobilized metal oxide NPs on the support was
confirmed by EDX analysis which showed only the peaks of oxygen and
the corresponding metals under investigation as shown in Fig. S1.
The H2-TPR was performed to investigate the differences in redox
properties of the immobilized metal oxide NPs on mesoporous metal
oxide supports because the catalytic activity of the catalyst towards
redox reactions depends on the reducibility. For spinel metal oxides,
usually, the first reduction signal is assigned to the most electropositive
cation. The first band appearing at 268 °C for mesoporous cobalt oxides
can be ascribed to the reduction of Co3+ species to Co2+ while the
second band at 348 °C can be ascribed to the reduction of Co2+ to
metallic cobalt [37,39]. For mesoporous manganese metal oxide, the
first band appearing at 272 °C can be ascribed to the reduction of Mn3+
to Mn2+ while the second band 405 °C can be ascribed to the reduction
of Mn2+ to metallic manganese [38,41]. The obtained oxidation states
are supported by the data obtained from the XPS measurements shown
in Table 2. From Fig. 5, the increase in reduction of the mesoporous
metal oxide supports after immobilization of the metal oxide NPs can be
attributed to the change in the electronic structure of the material due
to the interaction of the NPs with the support [22].
3.3.1. Catalyst recycle test
The influence of catalyst recycling on styrene conversion and pro-
duct selectivity was investigated by recycling the catalysts four times
for immobilized metal oxide NPs. From Fig. 9, it can be noted that
styrene conversion diminished, and product selectivity showed insig-
nificant changes even after the 4th cycle for Co-MnO2_350. Mn-
Co3O4_350 showed higher selectivity towards 2-phenylacetaldehyde in
the first two cycles and then, the selectivity changed towards styrene
oxide for the third and fourth cycles. The immobilized metal oxide NPs
on mesoporous metal oxide catalysts remained unchanged as shown in
the TEM images (Fig. 10E and F). The TEM images taken after 8 h of the
reaction showed slight agglomeration for Co3O4 NPs while most of the
NPs remain well dispersed in solution as seen in Fig. 10A. The Co3O4
NPs increased in size from 8.4 to 9.7 nm during the reaction. The shape
of MnOx NPs remained unchanged after the reaction as shown in
Fig. 10C and their particle size also increases from 6.2 to 7.8 nm after
the reaction. This showed the stability of the metal oxide NPs in the
reaction.
4.2. Selective oxidation of styrene
4.2.1. Optimization of the reaction conditions: solvent type, catalyst
amount, the ratio of styrene to TBHP and temperature variation
4. Discussion
In literature, the influence of solvent type used for the oxidation of
styrene was found to influence the styrene conversion and the product
selectivity of the reaction. Higher styrene conversion was reported to be
obtained for solvents with a high polarity such as acetonitrile, DMF etc
[17,18,24,42,43]. Therefore, considering the results of these papers,
polar solvents and non-polar solvents were investigated. The lower
styrene conversion obtained using acetonitrile and methanol as solvents
can be attributed to the agglomeration of NPs in solution which results
in them being inactive catalysts. DMC which is also a polar solvent with
a polar index of 3.9 gave higher 2-phenylacetaldehyde selectivity and
moderate styrene conversion. The good dispersion of NPs in DMC and
4.1. Synthesis and characterization of colloidal NPs and MMOs
immobilized with metal oxide NPs
The mesoporous metal oxide supports used to immobilize metal
oxide NPs in this study were synthesized using the inverse micelle sol-
gel method. The principle behind this method is that it uses HNO3 to
prevent condensation while the metal oxo-clusters are stabilized and
the clusters are formed inside hydrated inverse micelles and they in-
teract with the surfactant via hydrogen bonding [37–39].
The synthesized materials were characterized using a range of
183

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AppliedCatalysisA,General559(2018)175–186
Fig. 10. TEM images were taken after the recycle test of styrene oxidation: A: Co3O4 NPs, C: MnOx NPs E: Mn-Co3O4_350, F: Co-MnO2_350. B and D are particle size
distribution histograms of Co3O4 NPs and MnOx NPs respectively.
chloroform can be attributed to the absorption of these solvents to the
surface of the NPs which inhibit aggregation of the NPs [44]. The
brown and black particles observed when methanol and acetonitrile
were used as solvents are a result of NP aggregation [3]. While non-
polar solvents gave moderate selectivity towards 2-phenylacetaldehyde
and styrene conversion which can be attributed to the interaction of the
solvent with the reactants and catalyst.
The increase in styrene conversion with an increase in catalyst
amount can be attributed to the increase in the active site of the catalyst
for the reaction to take place before they are saturated. When the
temperature of a catalyzed reaction is increased, it increases the colli-
sion frequency of the reactants by supplying them with kinetic energy
which results in an increase in the collision of reactants with catalyst
hence an increase in styrene conversion was observed with increase in
temperature. A similar trend of results had been reported by Noh et al
[3]. The decrease in 2-phenylacetaldehyde selectivity can be attributed
to further oxidation of the product and hydrolysis to form other pro-
ducts [45].
184

N. Masunga et al.
AppliedCatalysisA,General559(2018)175–186
Scheme 2. The proposed reaction mechanism for styrene oxidation using metal oxide nanoparticles and immobilized nanoparticles [46].
the metal oxide NPs, thus more active sites are available for the reaction
to take place. The immobilized metal oxide NPs have lower activities,
which can be attributed to the NPs passivation [3]. Also, the change in
binding energy due to metal to metal interaction after immobilization
of the metal oxide NPs influenced the activities of the catalysts.
The higher styrene conversion obtained with MnOx NPs as com-
pared to Co3O4 NPs can be ascribed to the fact that Mn has a lower
reduction potential (+1.51) for Mn3+ to Mn2+ as compared to Co
(+1.81) for Co3+ to Co2+. Hence based on the reduction potentials, Mn
is expected to be a better catalyst for the oxidation reaction compared
to Co. The obtained results give an insight that the product selectivity of
styrene oxidation depends mainly on the type of the metal used. The
difference in the structure of MnOx and Co3O4 NPs might be responsible
for different product selectivity. The structure of MnOx NPs allows the
opening of the ring through the rearrangement of hydrogens which
gave rise to 2-phenyl-acetaldehyde. This can be explained by the XPS
data which showed that for Mn the 2 + oxidation state is at lower
binding energy, which is the opposite to what occurs with Co [36].
Hence after, immobilizing the metal oxide NPs, a mixture of products
was observed because both the metal oxide NPs and the support played
a role in influencing the product formed. Moreover, the interaction
between the metal oxide NPs and the support played a role in the
product formed. The decrease in selectivity and conversion that was
observed after the third and fourth cycles can be attributed to the
leaching of the nanoparticles from the support to the solvent [3]. The
oxidation of styrene using metal oxide NPs and immobilized NPs on
mesoporous metal oxide supports was expected to follow the me-
chanism shown in Scheme 2 which was proposed by Nepak et al [46].
The two bonds between the TBHP oxygen and the nanoparticles are
formed due to enhanced electron density on the oxygen π* orbitals
caused by the positive inductive effect of the alkyl groups. The oxygen
will donate electrons to the empty d orbitals of the metal oxide nano-
particles. This eventually weakens the OeO bond which results in the
formation of nanoparticle oxo species. On the metalloepoxy inter-
mediate, oxygen is transferred to the olefin double bond to form styrene
oxide. ß- hydride elimination at the metalloepoxy results in the for-
mation of 2-phenyl-acetaldehyde [46].
Table 3
Comparison of metal NPs for selective oxidation of styrene.
Catalyst
oxidant Con. (%) Product selectivity
STO PHA BZA others
TOF
Ref.
(h−1
)
MnOx NPs
Co3O4 NPs
TBHP
TBHP
59.3
42.3
–
90.6
–
–
9.4
646.9
338.1
This
work
This
work
[3]
[3]
[3]
[3]
[47]
100
–
–
G5-OH Pd80
G6-OH Pt160
Pd wo =10
Pt wo = 5
Au NPs@XAD- O2
4-G3.0
Au/MgO
(HDP)
Au/SrO (HDP) TBHP
Co/N-HCS300 O2
1% Ag-
METMS
Mn-
Co3-
O4_350
Co-MnO2_350
TBHP
TBHP
TBHP
TBHP
64.3
33.4
47.2
27.2
9.5
67.4
59.5
73.3
70.5
48
–
–
–
–
–
30.2 2.4
33.6 6.8
23.8 2.9
25.0 4.0
285.1
271.4
209.3
219.5
63.4
33
19
TBHP
62.6
54.3 16.8 10.8 18.1
297.1
[48]
53.0
99
60
44.8 26.0 11.7 17.4
373.8
232.6
129.2
[48]
[7]
[10]
86
–
–
–
–
–
–
H2O2
˃ 99
TBHP
31
30.4 46.3
32.3 22.5
–
–
23.3
45.1
44.1
54.1
This
work
TBHP
38.9
This
work
Con. = conversion of styrene, STO = styrene oxide, PHA = 2-phenyl-acet-
aldehyde, BZA = benzaldehyde. G5-OH and G6-OH mean fifth and sixth gen-
eration hydroxyl terminated polyamidoamine dendrimers. W0 = water to sur-
factant
ratio,
HDP = homogeneous
deposition
precipitation,
METMS = methyltrimethoxysilane, XAD-4-G3.0 = third generation PAMAM
dendrimers on the surface of amberlite XAD-4 resin, N-HCS300 = nitrogen
doped carbon hollow spheres.
4.2.2. Comparison of the catalytic activities of the synthesized catalysts
The increase in styrene conversion with the increase in reaction
time for most catalysts shows that styrene oxidation requires time for
the reaction to take place. The higher styrene conversion observed with
colloidal metal oxide NPs as compared to the immobilized metal oxide
particles may be attributed to the high surface area to volume ratio of
185

N. Masunga et al.
AppliedCatalysisA,General559(2018)175–186
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Acknowledgments
Authors are grateful for the financial support we received from the
University of Johannesburg and the National Research Foundation of
South Africa (Grant specific numbers 5386 and 93205). We would like
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