Selective cyclohexene oxidation with O2, H2O2and: Tert -butyl hydroperoxide over spray-flame synthesized LaCo1- x Fex O3nanoparticles

Article abstract of DOI:10.1039/d0cy00906g

The elimination of waste and by-product generation and reduced dependence on hazardous chemicals are the key steps towards environmentally sustainable chemical transformations. Heterogeneously catalysed oxidation of cyclohexene with environmentally friendly oxidizing agents such as O2, H2O2 and tert-butyl hydroperoxide (TBHP) has great potential to replace existing processes using stoichiometric oxidants. A series of spray-flame synthesised nanoparticulate LaCo1-xFexO3 catalysts was employed for cyclohexene oxidation, and the comparative results showed that TBHP led to the highest initial activity and allylic selectivity, but O2 resulted in higher conversion for longer reaction times. Furthermore, the influence of Fe substitution was studied, which did not show any beneficial synergistic effects. LaCoO3 was found to be the optimum catalyst for cyclohexene oxidation with O2, following first-order reaction kinetics with an apparent activation energy of 57 kJ mol-1. The catalyst showed good reusability due to its highly stable particle size, morphology and perovskite structure. 7-Oxabicyclo[4.1.0]heptan-2-one was identified to be formed by the oxidation of 2-cyclohexene-1-one with 2-cyclohexene-1-hydroperoxide.

Full text of DOI:10.1039/d0cy00906g

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Technol., 2020, DOI: 10.1039/D0CY00906G.
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ISSN 2044-4761
PAPER
Chaoqiu Chen, Yong Qin et al.
Highly dispersed Pt nanoparticles supported on carbon
nanotubes produced by atomic layer deposition for hydrogen
generation from hydrolysis of ammonia borane
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DOI: 10.1039/D0CY00906G
ARTICLE
Selective Cyclohexene Oxidation with O2, H2O2 and tert-Butyl
Hydroperoxide over Spray-Flame Synthesized LaCo1-xFexO3
Nanoparticles
Received 00th January 20xx,
Accepted 00th January 20xx
Julia Büker,[a] Baris Alkan,[b] Qi Fu,[a] Wei Xia,[a] Jonas Schulwitz,[a] Daniel Waffel,[a] Tobias Falk,[a]
Christof Schulz,[b] Hartmut Wiggers,[b] Martin Muhler,[a],[c] and Baoxiang Peng*[a],[c]
DOI: 10.1039/x0xx00000x
The elimination of waste and by-product generation and reduced dependence on hazardous chemicals are the key steps
towards environmentally sustainable chemical transformations. Heterogeneously catalysed oxidation of cyclohexene with
environmentally friendly oxidizing agents such as O2, H2O2 and tert-butyl hydroperoxide (TBHP) has great potential to replace
existing processes using stoichiometric oxidants. A series of spray-flame synthesised nanoparticulate LaCo1-xFexO3 catalysts
was employed for cyclohexene oxidation, and the comparative results showed that TBHP led to the highest initial activity
and allylic selectivity, but O2 resulted in higher conversion for longer reaction times. Furthermore, the influence of Fe
substitution was studied, which did not show any beneficial synergistic effects. LaCoO3 was found to be the optimum catalyst
for cyclohexene oxidation with O2, following first-order reaction kinetics with an apparent activation energy of 57 kJ mol-1.
The catalyst showed good reusability due to its highly stable particle size, morphology and perovskite structure.
7-Oxabicyclo[4.1.0]heptan-2-one was identified to be formed by the oxidation of 2-cyclohexene-1-one with
2-cyclohexene-1-hydroperoxide.
are formed. However, the sustainability of an oxidizing agent
cannot only be classified by its degradation products but also by
the environmental impact of its production. H2O2 is produced in
Introduction
Cyclohexene oxidation provides access to many fine chemicals
including adipic acid, which is an important monomer for the
manufacture of nylon-6,6.[1] Adipic acid is usually produced by
oxidising a mixture of cyclohexanol and cyclohexanone,
commonly known as K/A oil, with nitric acid.[1]–[7] This leads to
the formation of environmentally harmful nitrous oxide (N2O)
that is known today as one of the greenhouse gases.[8]
Heterogeneously catalysed oxidation of cyclohexene with more
environmentally friendly oxidants such as molecular O2, H2O2 or
tert-butyl hydroperoxide (TBHP) offers an attractive and
sustainable route by replacing the application of hazardous
stoichiometric oxidants.[2],[3],[9],[10] In terms of their degradation
products, all of the oxidising agents are classified as
environmentally friendly, because only H2O, O2 and tert-butanol
industry via the anthraquinone oxidation (AO) process[11], which
can hardly be considered as a green route, because it requires a
high energy input and generates large amounts of waste.[12]
Since TBHP is the reaction product of tert-butyl alcohol with
H2O2 in acidic solution, it suffers from similar environmental
restrictions as H2O2.[13] Molecular oxygen is the ultimate green
oxidant as it is inexpensive, abundant and completely harmless
to the environment.[14]–[18]
Scheme 1. Reaction pathways for the oxidation of cyclohexene.[19]
cyclohexene
epoxidation path
O
allylic path
a. J.Büker, Q. Fu, W. Xia, J. Schulwitz, D. Waffel, T. Falk, Prof. M. Muhler, Dr. B. Peng
Laboratory of Industrial Chemistry Ruhr University Bochum
Universitätsstraße 150,
O
OH
44780 Bochum (Germany)
E-mail: baoxiang.peng@techem.rub.de
b. B. Alkan, Prof. C. Schulz, Prof. H. Wiggers
IVG, Institute for Combustion and Gas Dynamics – Reactive Fluids and CENIDE,
Center for Nanointegration Duisburg-Essen
University of Duisburg-Essen
2-cyclohexene-1-hydroperoxide
cyclohexene oxide
ion
- H2O
Carl-Benz-Straße 199,
decomposit
47057 Duisburg (Germany)
c. Prof. M. Muhler, Dr. B. Peng
O
+ O
HO
Max Planck Institute for Chemical Energy Conversion
Stiftstraße 34-36,
45470 Mülheim an der Ruhr (Germany)
OH
- H2O
OH
O
O
adipic acid
2-cyclohexene-1-ol
2-cyclohexene-1-one
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Among these oxidants, TBHP was found to have higher performed with respect to the environmental factors of the
View Article Online
DOI: 10.1039/D0CY00906G
reactivity for liquid-phase oxidation reactions compared with O2 three oxidizing agents. Then, the influence of Fe substitution on
(singlet vs. triplet ground state) and to be more stable cyclohexene conversion and the resulting selectivities was
compared with H2O2 towards decomposition.[20],[21] To the best studied. Furthermore, kinetic studies and reusability tests were
of our knowledge, the effect of these oxidants on cyclohexene carried out for aerobic cyclohexene oxidation over LaCoO3. The
oxidation has not been systematically investigated.
formation pathway of 7-oxabicyclo[4.1.0]heptan-2-one was
Due to the presence of multiple reactive centres, cyclohexene identified through the decomposition of the hydroperoxide.
oxidation always proceeds in several pathways and the
selectivity is hard to control.[15],[22],[23] The oxidation at the allylic
C-H centre leads to the formation of the intermediate
Results and Discussion
2-cyclohexene-1-hydroperoxide, which can subsequently
decompose to 2-cyclohexene-1-ol and 2-cyclohexene-1-one
(Scheme 1).[19] Additionally, 2-cyclohexene-1-ol can be further
oxidized to 2-cyclohexene-1-one. On the other hand, oxidation
can also take place at the olefinic C=C bond leading to the
formation of cyclohexene oxide, which can also be formed by
the reaction of 2-cyclohexene-1-hydroperoxide with another
cyclohexene molecule. Via further oxidation steps and a ring-
opening reaction, cyclohexene oxide can be transferred into
adipic acid. This reaction network is rather complex, and it is
therefore challenging to reach high selectivity to a specific
product.
Catalyst Characterization
The XRD patterns of the LaCo1-xFexO3 perovskites indicate the
presence of a dominant perovskite phase (Figure 1). Similar
catalysts have already been reported for cinnamyl alcohol and
ethanol oxidation in our previous studies, but the investigated
perovskite samples in this study benefit from higher phase
purity originating from optimised synthesis conditions.[30],[43] In
the Fe-substituted samples, the slight shift of the reflections to
lower 2θ values with increasing Fe amount can be explained by
the incorporation of Fe3+ into the perovskite lattice because of
its larger ionic radius compared with Co3+.[43] For LaCoO3, the
reflections have minor shifts to the lower 2θ compared to the
reference sample, which is most likely due to the influence of
oxygen vacancies in LaCoO3. The physical properties of the
perovskite catalysts with varying amounts of Fe are summarized
in Table 1. The perovskite samples containing 0, 30 and 40 at%
Fe show similar specific surface areas and estimated average
particle sizes, while the perovskite containing 20 at.% Fe has a
significantly larger particle size and a lower surface area.
Industrially, cyclohexene is oxidised by using a stoichiometric
amount of KMnO4 resulting in considerable waste
production.[6],[7] Heterogeneously catalysed reactions are
currently based on Ti and expensive metals such as Ru.[16],[24],[25]
This work focuses on noble metal-free catalysts such as the
oxides of first-row transition metals as attractive alternatives
because of their lower prices.[26]–[28] Recently, perovskite
materials were used in the thermal catalytic oxidation of
alcohols because of their high structural stability at elevated
temperatures.[29]–[31] Perovskite oxides are known to be active
in the oxidation of volatile organic compounds and in oxidative
exhaust gas treatment removing CO, NO or NH3 at relatively
high temperatures,[32]–[35] but their applications in liquid-phase
oxidations have been rarely investigated because of their
relatively low activities under mild conditions.[36]–[38] To improve
their catalytic activity, partial substitution of metal ions in the
perovskite structure is a promising strategy, which could enable
possible synergistic effects between the incorporated metal
ions.[32],[39],[40] In addition to partial substitution, the synthesis of
the presented catalysts is improved by an alternative
manufacturing process.[41] Generally, perovskite nanoparticles
are synthesised by wet-chemical techniques such as
precipitation or by solid-state synthesis routes.[33],[39] Both
techniques require high calcination temperatures causing
sintering and lower specific surface areas. By comparison,
spray-flame synthesis is an environmentally friendly method,
because it is continuous one-step synthensis, avoiding post-
treatment and intensive solvent washing. Moreover, the
synthesized nanoparticles have well-controlled compositions,
larger surface areas and smaller particle sizes down to 10 nm.[42]
In the present work, we report the oxidation of cyclohexene
with TBHP, H2O2 and O2 over a series of spray-flame synthesized
LaCo1-xFexO3 perovskites in acetonitrile under mild conditions in
the liquid phase. A comparative study concerning the activity
and selectivity over LaCoO3 with TBHP, H2O2 and O2 was first
Figure 1. XRD patterns of the LaCo1-xFexO3 perovskites with the reference pattern
of LaCoO3 (COD: 1533633).
Table 1. Specific surface areas (BET method) and calculated mean particle sizes of the
LaCo1-xFexO3 perovskites.
Catalyst
Specific surface
area
Particle size[a]
[m²/g]
56.3
27.7
62.0
68.4
[nm]
14.7
30.5
13.7
12.5
LaCoO3
LaCo0.8Fe0.2O3
LaCo0.7Fe0.3O3
LaCo0.6Fe0.4O3
[a] Estimated from the specific surface areas assuming non-porous spherical particles.
2 | J. Name., 2012, 00, 1-3
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In addition to the qualitative phase analysis by XRD, particle sizes and
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50 nm
(a)
dp : 9.2 nm
size distributions of the LaCo1 xFexO3 sampleDsOwIe: 1r0e.1d0e3t9e/rDm0iCnYe0d0u9s0i6nGg
TEM (Figure 2). All samples show agglomerated small particles. The
obtained particle size distribution histograms were fitted with
log-normal functions and show average particle sizes dp ranging from
9 to 18 nm. The particle sizes derived from N2 physisorption and the
TEM images were found to match very well for 30 and 40 at% Fe
samples, while they exhibited a noticeable deviation for LaCoO3 and
LaCo0.8Fe0.2O3. For LaCoO3, this deviation can be attributed to the
presence of a high content of smaller nanoparticles as detected by
TEM. For LaCo0.8Fe0.2O3, the determined σg of 1.6 suggests a quite
broad polydisperse size distribution due to presence of both big and
small nanoparticles. All other samples show the expected self-
preserving particle size distributions with a σg value of 1.4.[45]
Importantly, chemical compositions of these nanoparticles
determined by EDX measurements in the STEM mode indicate that
LaCo0.8Fe0.2O3 shows a 20 at% La-rich composition, while other
samples show about 10-14 at% La-rich compositions (Table S1). The
observed La-rich composition can be related to the presence of RP-
type compounds (La2CoO4 and La4Co3O10 phases), while no pure La
oxides were detected in the XRD patterns (Figure 1). Higher
La/(Co+Fe) at% ratios were specifically found for larger particles of
about 30-40 nm, which can explain the more La-rich composition of
LaCo0.8Fe0.2O3. Apart from La-rich large particles, all inset images in
Figure 2 confirm the single-crystalline perovskite structure of LaCo1
-xFexO3. X-ray photoelectron spectra (XPS) were recorded to assign
the chemical species and the oxidation states of the involved
transition metals in the Co 2p and Fe 2p regions (Figure 3). All the
observed Co 2p XP signals indicate the presence of cobalt in the
oxidation states +2 and +3.[46] While we attribute the XP signals of
Co3+ to cobalt ions in LaCo1 xFexO3 perovskite, those of Co2+ can be
associated with oxgen-deficient and/or La-rich perovskite
compositions as mentioned above. Apart from LaCo0.7Fe0.3O3, the
other samples not only show broadened Co 2p XPS peaks, but also
broadened La 3d peaks (Figure S1, La 3d). These findings indicate a
higher content of Co2+ and a La-rich surface composition, which is
most likely due to RP-type surface compositions (La2CoO4 and
La4Co3O10) as deduced from the EDX analyses. However, a Co3+
satellite peak at 789.5 eV is visible in the spectrum of LaCo0.6Fe0.4O3,
(e)
g : 1.4
0
10 20 30 40 50 60 70 80
Diameter / nm
3 nm
40
30
20
10
0
dp : 18.1 nm
50 nm
(f)
(b)
(c)

g : 1.6
0
10
20
30
40
50
60
70
3 nm
Diameter / nm
100
80
60
40
20
0
50 nm
dp : 13.2 nm
(g)
g : 1.4
0
10
20
30
40
50
60
70
3 nm
Diameter / nm
(d)
50 nm
dp : 13.3 nm
100
80
60
40
20
0
(h)
g  1.4
0
10
20
30
40
50
60
70
3 nm
Diameter / nm
Figure 2. TEM images and particle size distribution of LaCoO3 (a+e), LaCo0.8Fe0.2O3
(b+f), LaCo0.7Fe0.3CoO3 (c+g) and LaCo0.6Fe0.4O3 (d+h) nanoparticles. Mean particle
size and geometric standard deviation are abbreviated as dp and σg. HAADF STEM
images are shown as inset, representing the crystal structure of nanoparticles
oriented in 100 direction.
suggesting
a a
higher content of surface Co3+ ions and
correspondingly stoichiometric perovskite for the sample with 40 at%
Fe.[47] Additionally, the increasing Fe content in the Fe-substituted
samples can be confirmed by the Fe 2p XPS peaks. For all Fe contents,
the binding energies of the Fe 2p3/2 peaks higher than 710.0 eV
suggest that Fe3+ ions are predominantly present in the samples.[48]
The spin-orbit splitting observed in the La 3d spectra is typical for
perovskites, while broadened La 3d5/2 and La 3d3/2 core-level peaks
are indicative of mixed signals of perovskite and RP-type compounds
mentioned above (Figure S1, La 3d). The O 1s peak at 528.6 eV
indicates surface oxygen anions of LaCo1 xFexO3 perovskite, whereas
the O 1s peak at 531.3 eV is associated with surface carboxylate
oxygen species, hydroxide ions and low-coordinated oxygen, being
observed in higher intensities in LaCoO3 and LaCo0.8Fe0.2O3 (Figure S1,
O 1s).
LaCo0.6Fe0.4O3
LaCo0.7Fe0.3O3
LaCo0.8Fe0.2O3
LaCoO3
LaCo0.6Fe0.4O3
LaCo0.7Fe0.3O3
LaCo0.8Fe0.2O3
Co 2p
Fe 2p
Co2p3/2
Fe 2p1/2
Co3+
Co2+
Co2p1/2
Fe 2p3/2
730
725
720
715
710
705
800
795
790
785
780
775
Binding energy [eV]
Binding energy [eV]
The synthesized nanoparticles were further investigated by ATR-FTIR
spectroscopy (Figure S2). All samples show similar spectra at high
wavenumbers with bands that can be assigned to asymmetric and
Figure 3. XP spectra of the Co 2p region and the Fe 2p region for LaCo1 xFexO3. The
reference XP lines of Co2+ and Co3+ were drawn referring to a previous report.[44]
This journal is © The Royal Society of Chemistry 20xx
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symmetric C-H stretching vibrations (2961, 2922, 2891 and butanol and H2O. However, due to their different chemical
View Article Online
2853 cm-1). The bands between 1700 and 1000 cm-1 can be nature, these three oxidants show diffDerOeI:n1t0.e10ff3i9c/iDen0CciYe0s0.9T06hGe
attributed to symmetric and asymmetric O-C=O stretches (1543 and comparative study was performed over LaCoO3 as the reference
1418 cm-1), most probably resulting from combustion residuals such catalyst. Figure 4 shows the results obtained with cyclohexene
as acetate and carboxylate groups of precursors and solvent.[49]
oxidation at 60 °C under otherwise standard conditions. After
1 h, the highest cyclohexene conversion of 15 % was achieved
using TBHP as oxidizing agent. The oxidation using O2 resulted
in 9.2 % conversion, while H2O2 led to the lowest conversion of
6.5 % after 1 h. These results confirm the hypothesis that
cyclohexene oxidation with TBHP results in the highest
conversion due to its higher reactivity compared with O2 and its
higher stability compared with H2O2. Although TBHP led to the
highest initial conversion, the increase in conversion after 6 h
was more pronounced for the oxidation with O2 (32 % vs. 22 %
for TBHP). This observation indicates a strong decrease in
reaction rate after longer reaction periods for the oxidation
with TBHP. In terms of selectivity, the use of TBHP as oxidizing
agent led to the highest allylic selectivity over the whole
reaction period. After 1 h, the oxidation with H2O2 resulted in
the lowest selectivity to the main product 2-cyclohexene-1-one
(33 %), while O2 (43 %) and TBHP (47 %) formed higher amounts
of ketone. Especially with TBHP, the selectivity to
2-cyclohexene-1-one increased strongly over the investigated
period. With an additional selectivity of 32 % to 2-cyclohexene-
1-ol, TBHP showed an allylic selectivity of 79 % after 1 h, which
increased to 92 % after 6 h. Since the use of H2O2 resulted in the
formation of cyclohexene oxide and 7-oxabicyclo[4.1.0]heptan-
2-one, the total allylic selectivity was slightly lower in the range
of 80 % to 85 %. Furthermore, different ratios of the allylic
products were observed. A ketone/alcohol ratio of 1.5 was
obtained for TBHP after 1 h, while H2O2 led to a much lower
ratio of 0.7. For both oxidizing agents, the ketone/alcohol ratio
increased over time. After 6 h, a ratio of 2 was obtained with
TBHP, while H2O2 formed both reaction products in equal
amounts. As the selectivity to 2-cyclohexene-1-ol decreases
over time with H2O2, the oxidation of the alcohol to the
corresponding ketone 2-cyclohexene-1-one seems to be
favoured.
The oxidation with O2 leads to a different product distribution.
The highest allylic selectivity of 64 % was determined after 1 h,
which decreased to 57 % after 6 h. Moreover, the oxidation with
O2 leads to the highest 2-cyclohexene-1-hydroperoxide
selectivity, suggesting that the hydroperoxide formation using
O2 has the highest rate compared with the other oxidizing
agents. The formed ketone/alcohol ratio of 1.6 after 1 h was
very similar to the one using TBHP, but it increased strongly to
4.2 after 6 h. This result indicates a favoured oxidation of
2-cyclohexene-1-ol to 2-cyclohexene-1-one. Another special
feature of the aerobic oxidation was the formation of
7-oxabicyclo[4.1.0]heptan-2-one with a comparatively high
selectivity of 12 %, which was not detected with TBHP and the
selectivity stayed below 5 % with H2O2. The formation of this
oxidation product is discussed in detail later.
Comparison of TBHP, H2O2 and O2 as Oxidizing Agents for
Cyclohexene Oxidation
TBHP, H2O2 and molecular O2 all enable cyclohexene oxidation
with the generation of harmless by-products such as tert-
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cyclohexene conversion
2-cyclohexene-1-ol
2-cyclohexene-1-hydroperoxide
7-oxabicyclo[4.1.0]heptan-2-one
2-cyclohexene-1-one
(b)
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The obtained data show a high initial catalytic activity and allylic
selectivity over LaCoO3 with TBHP. The allylic selectivity can
further increase with the progress of the reaction, as the
intermediate 2-cyclohexene-1-hydroperoxide decomposes to
Time [h]
Figure 4. Comparison of cyclohexene oxidation at 60 °C with TBHP (a), H2O2 (b)
and O2 (c) as oxidizing agents.
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70
the corresponding allylic products. Conversion, however, stays
at a low level over the investigated period. The oxidation
reaction using H2O2 as oxidizing agent results in even lower
conversion and lower allylic selectivity compared with TBHP.
Although the initial conversion using O2 is lower compared with
TBHP, it increases significantly for longer reaction periods and
leads to the highest conversion of 32 % after 6 h. Furthermore,
the use of O2 in an autoclave allows performing the reaction at
higher temperatures to reach higher conversion.
In summary, cyclohexene oxidation with O2 results in the
highest conversion for longer reaction periods as well as a broad
product distribution, which allows to study the selectivity over
LaCoO3. Moreover, O2 is the ultimate green oxidizing agent with
respect to its sustainability. Therefore, O2 was chosen for the
further investigation of cyclohexene oxidation over a series of
LaCo1-xFexO3 catalysts.
View Article Online
(a)
DOI: 10.1039/D0CY00906G
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Influence of Fe Content on Cyclohexene Oxidation with O2
LaCoO3 nanoparticles show a promising activity towards
cyclohexene oxidation using O2 as oxidizing agent. To study
possible synergistic effects between Co and Fe, three more
perovskite samples LaCo1-xFexO3 containing 20, 30 and 40 at%
Fe were investigated (Figure 5). To achieve higher conversion,
the reaction was carried out at 80 °C. Compared with the
obtained data at 60 °C (Figure 4), the higher reaction
temperature leads to a significantly higher conversion over
LaCoO3. The degree of conversion doubled in the investigated
period of time. The degrees of conversion are independent of
the incorporation of Fe and are rather similar over all four
catalysts after 1 h. After 6 h, there is a minor trend indicating a
slightly higher conversion for the samples containing higher
amounts of Fe. However, this trend is small and can also be
explained by the slightly different specific surface areas and
particle sizes of the catalysts (Figure S3). The allylic selectivity
decreases as a function of time over all LaCo1-xFexO3 samples,
while the selectivity to epoxidation products increases.
Moreover, the allylic selectivity decreases with increasing Fe
amount, indicating that the presence of Fe promotes the
formation of epoxidation products. Additionally, as the
Fe-containing catalysts show significantly higher hydroperoxide
selectivities after 1h, Fe seems to suppress the hydroperoxide
decomposition at the beginning of the reaction. The oxidation
reaction performed in the absence of a catalyst clarifies the
decisive role of the catalyst in the hydroperoxide decomposition
and confirms that the autocatalytic formation of the
hydroperoxide (Figure S5a). After 6 h, the hydroperoxide
selectivities of the Fe-containing samples equal to the one over
LaCoO3. The formation of hydrogenation products such as
cyclohexanone can only be observed over Fe-containing
catalysts.
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cyclohexene conversion
2-cyclohexene-1-ol
2-cyclohexene-1-hydroperoxide
7-oxabicyclo[4.1.0]heptane-2-one
2-cyclohexene-1-one
(d)
cyclohexene oxide
In summary, the incorporation of Fe does not lead to a strong
increase in cyclohexene conversion but significantly decreases
the total allylic selectivity due to its weaker ability to
decompose the hydroperoxide intermediate. LaCoO3 leads to a
conversion of 66 % and the highest allylic selectivity of 54 %
after 6 h. Additionally, the Fe-containing catalysts promote the
1
2
3
4
5
6
Time [h]
Figure 5. Comparison of different LaCo1-xFexO3 catalysts containing 0 at% (a),
20 at% (b), 30 at% (c) and 40 at% (d) Fe in cyclohexene oxidation with O2.
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Journal Name
100
products such as cyclohexene oxide by an additional reaction
View Article Online
(a)
100 °C
80 °C
with cyclohexene (Scheme 1). The incDreOaI:s1e0.1i0n39t/eDm0CpYe0r0a9t0u6rGe
additionally led to an increase in the ketone/alcohol ratio. It
resulted in a ratio of 4.2 at 60 °C, while ketone/alcohol ratios of
8.6 and 11.6 were obtained at 80 °C and 100 °C, respectively
(Figures S4a-c). It can be concluded that further oxidation of the
formed 2-cyclohexene-1-ol to 2-cyclohexene-1-one is favoured
by increasing reaction temperatures. Moreover, higher
temperatures lead to a strong increase in the selectivity of
7-oxabicyclo[4.1.0]heptan-2-one, which has not been reported
in literature yet regarding its formation route. The formation of
this reaction compound is discussed later.
80
60
40
20
0
60 °C
The linearized plot for first-order kinetics is well suited for linear
regression (R2 > 0.98), indicating that cyclohexene oxidation
over LaCoO3 follows first-order reaction kinetics (Figure S4d).
The Arrhenius analysis results in a plot with high accuracy
(R² = 0.999) and reveals an apparent activation energy of
57.0 kJ mol-1 (Figure 6b).
0
1
2
3
4
5
6
Time [h]
R2 = 0.999
EA = 57.0 ± 2.4 kJ mol-1
(b)
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
Liquid-phase oxidation of cyclohexene using molecular O2 as
oxidizing agent has received attention in academic research and
was often reported using Au nanoparticles as catalysts. For
example, the allylic oxidation of cyclohexene over
Au/SBA-15-Py was carried out in acetonitrile under atmospheric
oxygen pressure at 100 °C. It was shown that Au/SBA-15-Py
exhibited moderate catalytic activity with 40.2 % cyclohexene
conversion and 38.0 % selectivity to the main product
2-cyclohexene-1-one.[50]
Similarly,
immobilized
Au
nanoparticles enable solvent-free cyclohexene oxidation with a
high conversion of 64.0 % and a high ketone selectivity of 65.0 %
after 8 h at 80 °C and 10 bar O2[51], showing similar activity and
selectivity towards cyclohexene oxidation as the LaCoO3
catalyst. However, the use of LaCoO3 is economically much
favourable compared to Au-based catalysts. Cyclohexene
oxidation with CoO nanoparticles has been reported to result in
a low conversion of 17.2 %, but high selectivity of 93.5 % to
2-cyclohexene-1-one after 8 h at 75 °C and 3 bar O2[52], whereas
CuO nanoleaves exhibited moderate cyclohexene conversion of
52.8 % and ketone selectivity of 51.4 % after 6 h at 1 bar O2 and
80 °C.[53] Our results show that cyclohexene oxidation using
transition metal perovskite oxides such as LaCoO3 also enables
higher degrees of conversion of 65.8 % and 47.8 % selectivity to
the main product 2-cyclohexene-1-one under comparable
reaction conditions of 80 °C, 10 bar O2 and 6 h. LaCoO3 leads to
significantly higher catalytic activity and moderate selectivity
compared with other transition metal oxides. To the best of our
knowledge, this is reported for the first time for liquid-phase
cyclohexene oxidation.
2.65
2.70
2.75
2.80
2.85
2.90
2.95
3.00
3.05
103 T-1 [K-1]
Figure 6. Kinetic analysis of liquid-phase cyclohexene oxidation over LaCoO3, (a)
the degree of cyclohexene conversion as function of time at different
a
temperatures, (b) Arrhenius plot based on first-order reaction kinetics.
formation of undesired products such as cyclohexane-1,2-diol
and cyclohexene-1,2-dione. As the presence of Fe is not
beneficial towards cyclohexene oxidation, LaCoO3 was selected
as an optimum catalyst for further detailed kinetic studies.
Kinetic Investigations over LaCoO3
The continuous increase in cyclohexene conversion over LaCoO3
as a function of temperature is shown in Figure 6 and
Figures S4a-c. A high cyclohexene conversion of 89 % was
obtained at 100 °C (Figures 6a and S4c). The increase in
temperature led to different product distributions over LaCoO3.
It has to be pointed out that the XRD patterns of the used
catalysts at different temperatures did not show any changes
(Figure S4f).
Higher temperatures led to stronger hydroperoxide
decomposition, and accordingly, the hydroperoxide selectivity
decreased from 25 % to < 2 % with increasing temperature from
60 to 100 °C after 6 h (Figure S4c). Meanwhile, the selectivity to
the epoxidation products increased, while the allylic selectivity
decreased. This observation clearly supports the decomposition
of the hydroperoxide to allylic products, but also to epoxidation
Formation of 7-Oxabicyclo[4.1.0]heptan-2-one
7-Oxabicyclo[4.1.0]heptan-2-one was detected during
cyclohexene oxidation using H2O2 and especially using O2 as
oxidizing agents. The selectivity of H2O2 remained below 5 % at
60 °C, while the use of O2 already led to 12 % selectivity under
similar conditions (Figure 4). When increasing the temperature
to 100 °C in O2, the selectivity to 7-oxabicyclo[4.1.0]heptan-
2-one even increased to 24 % representing one of the main
6 | J. Name., 2012, 00, 1-3
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concentration did not change at all during the reaction.
O
View Article Online
DOI: 10.1039/D0CY00906G
Therefore,
it
can
be
concluded
that
2-cyclohexene-1-hydroperoxide is also able to act as an
oxidizing agent and to oxidize the corresponding ketone to
7-oxabicyclo[4.1.0]heptan-2-one, which is not possible for O2.
a.
b.
O2
O2

O
O
Catalyst Reusability

O
Since LaCoO3 was found to be the most promising catalyst in
terms of cyclohexene conversion and hydroperoxide
decomposition, recycling tests were carried out to determine its
reusability under standard conditions (Figure 8a). The
conversion of cyclohexene slightly decreased from 62.4 % in the
first run to 59.1 % in the second run and was almost constant at
58.1 % in the third run despite the significant loss of catalyst,
suggesting that the catalyst is highly reusable. A significant
increase in hydroperoxide selectivity was observed after the
first run, which in turn resulted in a lower 2-cyclohexene-1-one
selectivity indicating a lower hydroperoxide decomposition rate
for the reused catalyst, but it remained constant for the third
run. Due to the slower hydroperoxide decomposition, a lower
selectivity to the epoxidation products was also obtained after
the first run. XRD patterns of the recycled catalyst did not show
any changes in crystallinity or phase purity (Figure 8b). STEM
images were recorded for the fresh catalyst and the recycled
OH
O

O
c.
Scheme 2.
Possible
reaction
pathways
for
the
formation
of
7-oxabicyclo[4.1.0]heptan-2-one.
by-products, as only 2-cyclohexene-1-one had
a higher
selectivity of 50 % after 6 h (Figure S4c). Since the formation
route of 7-oxabicyclo[4.1.0]heptan-2-one has not yet been
reported in literature, its formation pathway is studied in detail.
Three possible reaction pathways are presented in Scheme 2.
The oxidation of 2-cyclohexene-1-one with O2 to
7-oxabicyclo[4.1.0]heptan-2-one was first excluded, because
only a conversion of < 1 % was observed mainly forming
hydrogenation products under similar conditions as for
cyclohexene oxidation (Figure S6). Alternatively, the compound
could be formed by the oxidation of cyclohexene oxide with O2,
but control experiment showed nearly no conversion even after
6 h (< 0.5 %) suggesting a different pathway (Figure S7). These
results lead to the assumption that the formation of
7-oxabicyclo[4.1.0]heptan-2-one could be triggered by the
oxidation of one reaction product with the reaction
catalyst after three runs (Figures 8c and 8e). Neither
a
significant increase in particle size nor a significant change in
particle morphology was observed for the used catalysts
compared with the fresh one. EDX mapping of the used catalyst
does not show any phase separation compared with the fresh
catalyst (Figures 8d and 8f). Overall, LaCoO3 is not only highly
active, but it also has high stability with respect to particle size,
intermediate (Scheme 2c). 2-cyclohexene-1-hydroperoxide can morphology and bulk structure ensuring a good reusability.
also act as oxidizing agent, because it has a reactive peroxide
group. To synthesize 2-cyclohexene-1-hydroperoxide with high
purity, cyclohexene was oxidised with O2 using 2,2-azobis-(2-
methylpropionitrile) as initiator.[54]
During the synthesis, the reaction progress was monitored by
GC analysis (Figure S8). After 50 h, a mixture containing 52.0 %
2-cyclohexene-1-hydroperoxide and 28.2% 2-cyclohexene-1-
one as main components was obtained. The storage of the
mixture in a refrigerator over 2 months resulted in a
hydroperoxide content of 45.9 %. This mixture was then used to
study the reaction over LaCoO3 in the absence of O2.
Figure 7 shows the changes in composition as a function of
time. It becomes clear that the hydroperoxide was consumed
over time (- 19.9 %) resulting mainly in the formation of
2-cyclohexene-1-one (+ 13.4 %) and 2-cyclohexene-1-ol
(+ 2.8 %) after 6 h. This result underlines the assumption
2-cyclohexene-1-ol
2-cyclohexene-1-one
2-cyclohexene-1-hydroperoxide
cyclohexene oxide
7-oxabicyclo[4.1.0]heptan-2-one
20
15
10
5
0
-5
-10
-15
-20
already
presented
in
the
decomposes
literature
into
that
the
2-cyclohexene-1-hydroperoxide
0
1
2
3
4
5
6
corresponding alcohol and ketone. Importantly, the
concentration of 7-oxabicyclo[4.1.0]heptan-2-one increased by
1.9 %, indicating its formation as an oxidation product of
Time [%]
Figure 7. Decomposition of 2-cyclohexene-1-hydroperoxide over LaCoO3.
2-cyclohexene-1-hydroperoxide
2-cyclohexene-1-one, since
with
cyclohexene
presumably
oxide
the
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90
80
70
60
50
40
30
20
10
0
70
Conclusions
View Article Online
DOI: 10.1039/D0CY00906G
cyclohexene conversion
2-cyclohexene-1-one
2-cyclohexene-1-ol
2-cyclohexene-1-hydroperoxide
cyclohexene oxide
7-oxabicyclo[4.1.0]heptan-2-one
(a)
A series of LaCo1-xFexO3 nanoparticulate materials was prepared
by spray-flame synthesis, characterised and applied in the
oxidation of cyclohexene with three different oxidants in the
liquid phase under mild conditions. The substitution of different
amounts of Fe3+ ions into LaCoO3 showed no beneficial
synergistic effect between Co and Fe for cyclohexene
conversion, which was more dependent on the specific surface
area and the particle size of the catalysts. On the contrary, the
presence of Fe3+ suppressed the decomposition of 2-
60
50
40
30
20
10
0
cyclohexene-1-hydroperoxide, resulting in
a lower allylic
product selectivity. Detailed kinetic studies revealed that the
aerobic oxidation follows first-order reaction kinetics with an
apparent activation energy of 57.0 kJ mol-1 over LaCoO3. The
conversion of cyclohexene decreased only by 4.3 % after three
runs, suggesting high reusability. The formation of
7-oxabicyclo[4.1.0]heptan-2-one was mainly found for aerobic
cyclohexene oxidation originating from the oxidation of
1
2
3
Run [-]
(b)
LaCoO3 fresh
2-cyclohexene-1-one
with
the
intermediate
2-cyclohexene-1-hydroperoxide. The comparison of TBHP, H2O2
and O2 as oxidizing agents for cyclohexene oxidation confirmed
the highest initial activity of TBHP as expected, but O2 reached
higher conversion for longer reaction periods. Moreover, O2
provides the possibility to perform the reaction at higher
temperatures in a closed reaction system, in which higher
conversion up to 90 % was achieved at 100 °C after 6 h. Thus,
valuable mechanistic insights were gained for cyclohexene
oxidation, and spray-flame synthesized LaCoO3 was found to be
a suitable catalyst to reach high conversion and selectivity to 2-
cyclohexene-1-one using environmentally friendly O2 as
oxidizing agent.
LaCoO3 recycled
20
30
40
50
60
70
2 Theta [°]
(c)
(d)
Experimental Section
Materials
Cyclohexene (99 %), 2-cyclohexene-1-one (98 %), cyclohexene
oxide (98 %), triphenylphosphine (99 %), 1,2-dichlorobenzene
(99 %),
hydrogen
peroxide
(30 %
in
water),
2,2-azobis-(2-methylpropionitrile) (98 %) as well as the
precursors La(CH3COO)3 ・ x H2O (99.9 %), Co(CH3COO)2 ・
4 H2O (> 98 %) and Fe(CO2CH3)2 (95 %) were supplied by
Sigma-Aldrich. The crystallization water content in lanthanum
(III) acetate hydrate was determined by TGA/DSC/QMS
analyses, and its chemical formula was determined as
La(CH3COO)3 ・ 1.09 H2O. As solvents, 1-propanol (99.7 %),
octanoic acid (> 99 %) and ultrapure DI water by Sigma-Aldrich
were employed. Tert-butyl hydroperoxide (70 % in water) was
supplied by Arcos Organics. Acetonitrile in analytical reagent
grade was obtained from Fisher Chemicals.
(e)
(f)
La Co O C
Figure 8. Reusability test for cyclohexene oxidation over LaCoO3, (a) cyclohexene
conversion and product selectivity in three runs, (b) XRD patterns of fresh and the
recycled LaCoO3 after three runs, (c) STEM image and (d) EDX mapping of fresh LaCoO3,
(e) STEM image and (f) EDX mapping of LaCoO3 after three runs.
Catalyst Synthesis
The synthesis of the LaCo1-xFexO3 perovskites was carried out in
a spray-flame synthesis set-up equipped with a SpraySyn
burner-type flame, in which all gas flows, the flow of the liquid
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precursor solution and the reaction pressures can be fully After the addition of the oxidising agent, the reaction vessel was
View Article Online
controlled.[43],[55],[56] The La-, Co- and Fe-containing precursors closed and transferred into the preheated heating block. The
DOI: 10.1039/D0CY00906G
were added in a freshly prepared solvent mixture (120 mL reaction mixture was stirred with 600 rpm at 60 °C for 3 h.
1-propanol, 80 mL propionic acid, 40 mL DI water) leading to a Samples were taken after 0.5, 1, 2 and 3 h. The reaction was
total metal loading of 0.45 M. The La/(Co+Fe) ratio was kept at stopped by quenching using an ice bath. For the GC analysis,
1 for all samples containing different amounts of 0, 20, 30 and two samples of 1.5 mL each were taken after the catalyst was
40 at% Fe in the solution. The solutions were stirred for at least filtered off by a syringe filter (200 nm). The first sample was
2 h. They were then injected into the burner nozzle through a purely analysed, while 2 mmol of triphenylphosphine was
hollow needle at the end of the pump syringes with a flow rate added to the second sample to decompose the produced
of 3 sccm. A flow rate of 2 slm CH4 and 4 slm O2 was set for the 2-cyclohexene-1-hydroperoxide into 2-cyclohexene-1-ol.
pilot flame and 5 slm oxygen dispersion gas was utilized. For the reaction with O2, 60 mg of catalyst was filled in a Teflon
Compressed air was used with flow rates of 125 slm and liner and suspended in 30 mL of acetonitrile. 20 mmol
170 slm, respectively, as sheath and quench gas. The synthesis cyclohexene and 4.5 mmol of dichlorobenzene as internal
was carried out at a constant pressure of 950 mbar abs.. The standard for the GC analysis were added. The liner was inserted
formed particles were collected on a circular filter membrane into a 100 mL autoclave reactor (Parr Instruments). The
and dried at 900 mbar abs. for 30 min.
autoclave was purged three times with O2 and then pressurized
to 10 bar. Subsequently, the autoclave was heated to 80 °C. For
the comparison of the oxidising agents, the reaction was carried
out at 60 °C. At a temperature of 5 °C below the chosen reaction
temperature, the stirrer was switched on to 600 rpm and the
reaction time was started. Samples were taken through an
online sampling system after 1, 2, 4 and 6 h. For the GC analysis,
two samples of 1.5 mL were taken after the catalyst was filtered
off by a syringe filter (200 nm) and analysed by GC as previously
described without and with triphenylphosphine treatment. The
reaction was stopped after 6 h and the reactor was cooled in a
water-bath to 30 °C. The catalyst was separated by
centrifugation.
X-Ray Diffraction (XRD) Measurements
X-Ray Diffraction measurements were carried out with a
PANanalytical X’Pert PRO and a Bruker D8 DISCOVER X-ray
diffractometer. As X-ray source K radiation with a wavelength
of 1.5401 Å was used. A scan step size of 0.03 ° was set using a
continuous scan mode with a scan time of 2 s per step and a
rotation of 10 rpm.
Transmission Electron Microscopy (TEM)
A JEM-2200FS (JEOL) microscope was employed to perform
TEM and energy-dispersive X-ray spectroscopy (EDX) in the
STEM mode. The nanoparticles were dispersed in ethanol by 10
min sonication and the stable dispersion was dropped on
carbon-coated copper grid and let it dry under ambient
conditions.
Synthesis of 2-Cyclohexene-1-Hydroperoxide
To
synthesize
2-cyclohexene-1-hydroperoxide,
50 g
cyclohexene were filled in a 100 mL two-neck-flask. 1 g of
2,2-azobis-(2-methylpropionitrile) was added as initiator. The
flask was equipped with a reflux condenser, a thermostat which
was cooled to 4 °C and a plastic tube to bubble molecular
oxygen through the solution. The reaction mixture was heated
to 60 °C and the oxygen flow rate was set to 30 mL/min using a
rotameter. The reaction was run for 50 h and samples were
taken after 2, 4, 6, 22, 24, 26, 28, 30, 46, 48 and 50 h. Each time,
two GC samples were prepared, both containing 300 μL of the
reaction solution and 10 μL 1,2-dichlorobenzene diluted in 1 mL
acetonitrile. GC analysis was carried out as previously described
without and with triphenylphospine treatment.
X-Ray Photoelectron Spectroscopy (XPS)
XPS was carried out using
a VersaProbe II (Ulvac-Phi,
Chanhassen, USA) XPS equipment with Al K radiation (photon
energy of 1486.6 eV) to obtain La 3d, Co 2p, O 1s and C 1s
spectra. For Fe 2p, the same equipment with a Mg K radiation
(1253.6 eV) was used to eliminate the overlapping of the Co
LMM Auger peak in the Fe 2p spectra. All the spectra were
recorded using a 11.75 eV pass energy. All spectra were
calibrated using the C 1s binding energy of 284.8 eV.
Attenuated Total Reflectance Fourier Transform Infrared
(ATR-FTIR) Spectroscopy
Oxidation of 2-Cyclohexene-1-One and Cyclohexene Oxide
The oxidation was carried out similar to cyclohexene oxidation
with O2 using 20 mmol of 2-cyclohexene-1-one and cyclohexene
oxide as reactants, respectively.
ATR-FTIR spectra were recorded on a Bruker Vertex 80
spectrometer equipped with
a diamond crystal in the
4000-400 cm-1 wavenumber range with 4 cm-1 resolution.
Decomposition of 2-Cyclohexene-1-Hydroperoxide
Catalytic Oxidation Reactions
2 g of the 2-cyclohexene-1-hydroperoxide-containing mixture
(45.9 % hydroperoxide) and 200 μL 1,2-dichlorobenzene as
internal standard were dissolved in 15 mL acetonitrile. The
reaction mixture was filled in a 25 mL flask equipped with a
reflux condenser and a thermostat which was cooled to 4 °C.
30 mg LaCoO3 were added and the solution was heated to 80 °C.
For the reaction with TBHP and H2O2, 1 mmol cyclohexene and
0.05 mmol 1,2-dichlorobenzene as internal standard were
dissolved in 3 mL acetonitrile in an open glassy vial. 10 mg of the
catalyst was dispersed to form a suspension. 1 mmol of TBHP
and H2O2, respectively, were added to initiate the reaction.
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Journal Name
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The stirrer was set to 600 rpm. The reaction was run over 6 h
and samples were taken after 0, 1, 2, 4 and 6 h. Each time, two
GC samples were prepared, both containing 1.5 mL of the
reaction solution. GC analysis was carried out as previously
described without and with triphenylphospine treatment.
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For the reusability test, three reaction runs were carried out
under standard conditions for the oxidation reaction with O2.
After each run, the catalyst was centrifuged, washed with
acetonitrile and dried overnight. The initial catalyst amount
decreased from initially 60 mg to 51 mg in the second run and
41 mg in the third run.
Gas Chromatography
Gas chromatographic analysis was carried out in a 7820 A GC
from Agilent Technologies. It was equipped with an Agilent
DB-XLB column (30 m x 180 μm x 0.18 μm) and an FID detector.
The injection volume was set to 0.5 μL with a split ratio of 75:1,
a split flow of 30 mL/min and an inlet temperature of 260 °C.
The column was first kept at 80 °C for 5 min. Subsequently, the
oven was heated to 170 °C with a rate of 15 °C/min. Afterwards,
it was heated with a ramp of 30 °C/min up to 300 °C to avoid
deposits of the triphenylphosphine in the column. The end
temperature was kept for 1 min.
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Conflicts of interest
There are no conflicts to declare.
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57, 1908.
Acknowledgements
This
research
was
funded
by
the
Deutsche
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