Towards Mechanistic Understanding of Liquid-Phase Cinnamyl Alcohol Oxidation with tert-Butyl Hydroperoxide over Noble-Metal-Free LaCo1–xFexO3 Perovskites

Article abstract of DOI:10.1002/cplu.201900429

Noble-metal-free perovskite oxides are promising and well-known catalysts for high-temperature gas-phase oxidation reactions, but their application in selective oxidation reactions in the liquid phase has rarely been studied. We report the liquid-phase oxidation of cinnamyl alcohol over spray-flame synthesized LaCo_1–xFe_xO₃ perovskite nanoparticles with tert-butyl hydroperoxide (TBHP) as the oxidizing agent under mild reaction conditions. The catalysts were characterized by XRD, BET, EDS and elemental analysis. LaCo_0.8Fe_0.2O₃ showed the best catalytic properties indicating a synergistic effect between cobalt and iron. The catalysts were found to be stable against metal leaching as proven by hot filtration, and the observed slight deactivation is presumably due to segregation as determined by EDS. Kinetic studies revealed an apparent activation energy of 63.6 kJ mol^?1. Combining kinetic findings with TBHP decomposition as well as control experiments revealed a complex reaction network.

Full text of DOI:10.1002/cplu.201900429

Accepted Article
Title: Towards Mechanistic Understanding of Liquid-Phase Cinnamyl
Alcohol Oxidation with tert-Butyl Hydroperoxide over Noble Metal-
Free LaCo1–xFexO3 Perovskites
Authors: Daniel Waffel, Baris Alkan, Qi Fu, Yen-Ting Chen, Christof
Schulz, Hartmut Wiggers, Martin Muhler, and Baoxiang Peng
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Towards Mechanistic Understanding of Liquid-Phase Cinnamyl
Alcohol Oxidation with tert-Butyl Hydroperoxide over Noble-
Metal-Free LaCo1–xFexO3 Perovskites
Daniel Waffel,[a] Baris Alkan,[b] Qi Fu,[a] Yen-Ting Chen,[a] Stefan Schmidt,[a] Christof Schulz,[b] Hartmut
Wiggers,[b] Martin Muhler,[a],[c] and Baoxiang Peng*[a],[c]
Abstract: Noble metal-free perovskite oxides are promising and
well-known catalysts for high-temperature gas-phase oxidation
reactions, but their application in selective oxidation reactions in the
liquid phase has been rarely studied. We report the liquid-phase
oxidation of cinnamyl alcohol over spray-flame synthesized
LaCo1–xFexO3 perovskite nanoparticles with tert-butyl hydroperoxide
(TBHP) as the oxidizing agent under mild reaction conditions. The
catalysts were characterized by XRD, BET, EDS and elemental
analysis. LaCo0.8Fe0.2O3 showed the best catalytic properties
indicating a synergistic effect between Co and Fe. The catalysts
were found to be stable against metal leaching as proven by hot
filtration and the observed slight deactivation is presumably due to
segregation as determined by EDS. Kinetic studies revealed an
apparent activation energy of 63.6 kJ mol-1. Combining kinetic
findings with TBHP decomposition as well as control experiments
revealed a complex reaction network.
the absence of metal-containing byproducts.[2,6,7] These
approaches currently rely on expensive noble metal-based
catalysts such as Au, Pd, Pt, or Ru, which have high catalytic
activities.[8–16] However, the oxides of first-row transition metals
are attractive alternatives because of their significantly lower
price despite their inferior intrinsic catalytic properties.[2,4,17,18]
Transition metal-based perovskite oxides are widely used in all
fields of catalysis because of their high structural stability and
chemical versatility.[19] Stoichiometric perovskites have the
general chemical formula of ABO3 with a larger cation A, usually
an earth alkali metal or a lanthanide, and a smaller cation B,
usually a first-row transition metal.[19] The cation at the A site is
mostly related to structural stability, while the cation at the B site
is mostly regarded as the catalytically active site.[20] Partial
substitution of the A and B site cations is possible, leading to the
more general formula AxA´1–xByB´1–yO3. The strategy of
isomorphic substitution allows the formation of mixed metal
perovskites, which enable synergistic effects between the
incorporated metal ions. Therefore, substitution is considered a
promising way of improving the catalytic properties of
perovskites.[21]
Introduction
Selective oxidation is a key step in chemical industry, which is
considered to be among the technologies with the largest
potential of improvement.[1] Among the various oxidation
reactions, the oxidation of alcohols in the liquid phase is an
important reaction for synthetic organic chemistry to produce
pharmaceuticals and other fine chemicals.[2] While metal oxides
as oxidizing agents in stochiometric amounts are used in
established processes for these reactions,[3–6] alternative
catalytic methods with oxidizing agents such as molecular
oxygen, hydrogen peroxide or tert-butyl hydroperoxide (TBHP)
have been extensively investigated in the past decades due to
Syntheses of single and mixed oxide perovskite compounds are
typically performed by wet-chemical methods and solid-state
synthesis routes, both of which require long-time thermal
treatments at high temperatures resulting in relatively low
specific surface areas around 20 m2 g–1.[21–25] Spray-flame
synthesis has been developed as an alternative single-step
synthesis of ligand-free transition metal oxide nanoparticles with
smaller particle sizes and larger specific surface areas of about
100 m2 g–1.[26–29] Recently, spray-flame synthesized LaCoO3 and
La1–xCexMnO3 perovskites with well-tuned properties by
substitutional doping[30] have shown good catalytic performance
in the oxidation of methane[29] and benzene[30] in the gas phase.
Further reactants for gas-phase oxidation reactions catalyzed by
perovskites are CO,[20,22] NO,[20] and NH3.[22] While transition
metal-based perovskites are frequently applied in high-
temperature gas-phase oxidation reactions, liquid-phase
oxidation reactions catalyzed by perovskites under mild
conditions have been rarely reported.[31,32]
[a]
D. Waffel, Q. Fu, Dr. Y.-T. Chen, S. Schmidt, Prof. M. Muhler,
Dr. B. Peng
Laboratory of Industrial Chemistry
Ruhr-University Bochum
Universitätsstr. 150
44801 Bochum (Germany)
E-mail: baoxiang.peng@techem.rub.de
B. Alkan, Prof. C. Schulz, Dr. H. Wiggers
IVG, Institute for Combustion and Gas Dynamics – Reactive Fluids
and CENIDE, Center for Nanointegration Duisburg-Essen
University of Duisburg-Essen
Carl-Benz-Straße 199
47057 Duisburg (Germany)
Prof. M. Muhler, Dr. B. Peng
Max Planck Institute for Chemical Energy Conversion
Stiftstr. 34-36
TBHP has been found to be the superior oxidant for liquid-phase
oxidation because of the higher reactivity compared with O2
(singlet vs. triplet ground state) and the higher stability
compared to H2O2 towards decomposition to O2 and the
respective byproducts.[7,33–36] The oxidizing ability of TBHP
depends strongly on the used solvent. Compared with many
other solvents, acetonitrile (MeCN) has been found to be the
best solvent for liquid-phase oxidation with TBHP,[33–36]
presumably because of its highly polar, but aprotic nature.[33,38]
Cinnamyl alcohol oxidation is often used as probe reaction to
assess the selective activation of the OH group or of the C=C
double bond.[7,16,18,32,33,39–42] Furthermore, cinnamyl alcohol is
[b]
[c]
45470 Mülheim an der Ruhr (Germany)
Supporting information for this article is given via a link at the end of
the document.
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naturally occurring in cinnamon oil[43] and can be used as a
sustainable feedstock to replace chemicals derived from fossil
sources. The desired product cinnamaldehyde is used as
cinnamon flavor, as insecticide and as an important intermediate
in organic syntheses.[44,45] For the aerobic oxidation of cinnamyl
alcohol over Pd-based catalysts, a dehydrogenation mechanism
LCO
LCFO-02
LCFO-04
LCFO-06
has
been
demonstrated.[46–49]
The alcohol
is
first
dehydrogenated over Pd to form the aldehyde, and the catalyst
is subsequently reoxidized by O2 yielding the coupled product
water. Contrary to the extensive knowledge about the aerobic
oxidation over Pd, the reaction network and especially the
mechanism of the oxidation with TBHP catalyzed by transition
metal oxides have been hardly explored.
In the present work, we report the liquid-phase oxidation of
cinnamyl alcohol with TBHP over spray-flame synthesized
LaCo1–xFexO3 perovskites (LCFO) in MeCN under mild reaction
conditions. The perovskites with different Fe content were first
screened to investigate the synergistic effect between the Co
and Fe ions in the catalysts. The stability of the catalysts was
also tested, and possible deactivation mechanisms were
investigated. Kinetic studies of cinnamyl alcohol oxidation and
TBHP decomposition as well as control experiments were
performed providing detailed mechanistic insight into the liquid-
phase oxidation of cinnamyl alcohol with TBHP.
10
20
30
40
50
60
2 q / °
Figure 1. XRD patterns of the synthesized perovskite oxides, black dotted
bars: reference pattern of LaCoO3 (ICSD: 01-075-0279).[52]
Table 1. Physical properties of the perovskite oxides.
Sample
Nominal
composition
La[a]
wt%
/
Co[b]
wt%
/
Fe[b]
wt%
/
SBET
/
Particle
m2 g–1
size[c]
nm
/
LCO
LaCoO3
56.1
55.7
54.8
55.0
23.9
18.8
14.0
9.3
<0.01
4.4
64.3
115.3
116.2
82.2
12.9
7.4
Results and Discussion
LCFO-02
LCFO-04
LCFO-06
LaCo0.8Fe0.2O3
LaCo0.6Fe0.4O3
LaCo0.4Fe0.6O3
8.8
7.3
Catalyst Characterization
13.3
10.6
The physical properties of the spray-flame synthesized
LaCo1–xFexO3 perovskites with varying the nominal ratio of
Fe/(Fe+Co) from 0 to 60 % are summarized in Table 1. The
metal compositions determined by elemental analysis agree well
with the theoretical calculated values. The specific surface areas
determined by N2 physisorption are found to be in the range
from 64 to 116 m2 g–1 and first increase and then decrease with
increasing Fe content. Compared with wet-chemical methods,
the large specific surface areas as a result of the combustion
synthesis[21,22,24] are advantageous for catalytic applications. For
oxide nanoparticles prepared by spray-flame synthesis, the
calculation of an average particle size based on the assumption
of spherical particles is well justified.[50] Accordingly, average
particle sizes of the perovskites between 7 and 13 nm can be
estimated, which agree well with our previously reported particle
size distributions determined by TEM.[51] The XRD patterns of
the synthesized perovskites (Figure 1) indicate the presence of a
dominant perovskite phase with a minor content of La2O3.[52] The
shift of the reflections towards lower diffraction angles for
samples with higher Fe content verifies the incorporation of Fe3+
into the perovskite lattice because of the larger ionic radius of
Fe3+ compared with Co3+ (0.645 Å vs. 0.61 Å).[21] The XP spectra
of the perovskite catalysts reported in our previous study show
that there is an enrichment of Co especially Co2+ at the surface
for all iron-containing samples,[51] which is most pronounced for
the LCFO-02 sample.
[a] Determined by UV/Vis spectroscopy
[b] Determined by AAS
[c] Estimated from the specific surface areas assuming spherical particles
Reaction Condition Optimization and Catalyst Screening
TBHP was chosen as the oxidizing agent, since the aerobic
oxidation of cinnamyl alcohol over LCO in toluene only resulted
in (3 ± 0.1) % conversion at 110 °C and ambient pressure after
6 h, indicating that the perovskites cannot catalyze the aerobic
oxidation in toluene even at elevated temperatures. This is
consistent with literature reporting that La-based perovskites are
not suitable catalysts for aerobic oxidation reactions in
toluene.[32] The reaction conditions for the liquid-phase oxidation
of cinnamyl alcohol with TBHP were first optimized using LCO
as a benchmark catalyst. The results of the reaction condition
optimization are summarized in Table 2. In comparison to an
equimolar and two-fold excess of TBHP (entries 1 and 2), a
five-fold excess of TBHP (entry 3) was found to achieve the
highest degree of conversion, which was therefore chosen as
the optimum TBHP-to-cinnamyl alcohol ratio. While a lower
amount of cinnamyl alcohol (entry 4) slightly decreased the
production rate of cinnamaldehyde, higher amounts of cinnamyl
alcohol were found to increase the cinnamaldehyde productivity
(entries 5 and 6). However, higher amounts of cinnamyl alcohol
were not chosen as the standard conditions because of safety
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concerns related to large amounts of TBHP.[37,53] In all the
experiments, the cinnamaldehyde selectivity was similar ranging
between 69 – 78 %. Other products were benzaldehyde and
3-phenylglycidol. Based on these results, the conditions in
entry 3 were set as the standard reaction conditions for further
studies. For blank reactions under standard conditions at 50 °C,
(10.6 ± 0.4) % conversion after 30 min and (11 ± 0.4) %
conversion after 6 h were obtained, and the selectivity towards
cinnamaldehyde was (73 ± 4.7) %. As alternatives to MeCN,
toluene and water were also used as solvents for the reaction
under the standard conditions with LCO. In both solvents, the
degree of conversion was 27 % after 30 min (with the highest
deviation being 0.7 %) and slightly higher than that of
(23 ± 0.6) % in MeCN (entry 3). However, the selectivity towards
cinnamaldehyde was significantly lower (43 – 47 %, deviation
≤ 3 %) compared with the reaction in MeCN ((71 ± 3.3) %), while
the over-oxidation to benzaldehyde was much more severe
correspondingly. Based on these results, these solvents were
not considered for further studies.
Figure 2. Comparison of different perovskite catalysts for cinnamyl
alcohol oxidation with TBHP. Reaction conditions: 10 mg catalysts, 20 mg
CA, TBHP:CA = 5 (n/n), 3 mL MeCN, 50 °C, 1 bar, 30 min.
Table 2. Cinnamyl alcohol (CA) conversion and selectivities of
cinnamaldehyde (CAld) as a function of the reaction conditions.
oxidation reactions in the gas phase.[54] Note that LaFeO3 was
reported as being not suitable as a catalyst for liquid-phase
alcohol oxidations with TBHP.[38] Similar synergistic effects have
been observed for gas-phase oxidation reactions over LCFO
catalysts.[21] When using H2O2 instead of TBHP as the oxidizing
agent over LCFO-02, only negligible degrees of conversion were
observed because of the poor stability of H2O2 towards
decomposition to O2 and water.[33–35,55] In the screening
experiments using the different catalysts, the selectivity to
cinnamaldehyde of approximately 70 % was hardly influenced,
and the selectivities to benzaldehyde and 3-phenylglycidol were
around 25 % and 5 %, respectively.
For liquid-phase alcohol oxidation reactions, it is known that
basic properties, either by adding a base or using an intrinsically
basic catalyst, are beneficial for the oxidation reaction.[13] The
basicity of the perovskite catalysts was investigated by the
Knoevenagel condensation between malononitrile and
benzaldehyde in MeCN yielding benzylidenemalononitrile. Since
the Knoevenagel condensation is a base-catalyzed reaction, it
can be used to determine the relative basicity of different
catalysts,[56] which has been reported earlier as a suitable
method to determine the basic properties of zeolite materials in
the liquid phase.[57] The averages of the yields of
benzylidenemalononitrile (Figure S2) follow the trend of the
conversion of cinnamyl alcohol over the perovskite catalysts
(Figure 2). However, due to comparably large errors of the
measurements, this trend cannot be considered conclusively.
Thus, the basicity of the catalysts might play a role in the
oxidation of cinnamyl alcohol, but a certain assignment cannot
be made.
Reaction
Condition
Rate[a]
/
Entry
X (CA) / %
10 ± 0.3
15 ± 0.5
23 ± 0.6
34 ± 0.8
25 ± 0.8
22 ± 0.6
S (CAld) / %
78 ± 4.2
75 ± 3.9
71 ± 3.3
77 ± 3.1
73 ± 4.1
69 ± 3.2
–1
mmol h–1 gcat
20 mg CA,
TBHP 1:1
1
2
3
4
5
6
2.88 ± 0.29
20 mg CA,
TBHP 2:1
4.53 ± 0.21
6.87 ± 0.18
5.09 ± 0.15
14.9 ± 0.5
20.0 ± 0.5
20 mg CA,
TBHP 5:1
10 mg CA,
TBHP 5:1
40 mg CA,
TBHP 5:1
60 mg CA,
TBHP 5:1
Reaction conditions: 10 mg LCO, 3 mL MeCN, 50 °C, 1 bar, 30 min.
[a] Cinnamaldehyde formation rate
Figure 2 shows the comparison of different perovskite samples
applied in liquid-phase cinnamyl alcohol oxidation with TBHP
under standard conditions. LCFO-02 achieved the highest
degree of conversion and comparable selectivity to
cinnamaldehyde and was therefore chosen for further reusability,
kinetic and mechanistic studies. The initial reaction rate of
cinnamyl alcohol oxidation exhibits a maximum for the LCFO-02
sample, and it shows a clear dependence on the relative surface
Fe or Co amount (Figure S1). Combing the fact that Fe-free
LCO was found to have inferior catalytic properties compared
with LCFO 02, it can be concluded that a synergistic effect
between Co and Fe is present in the perovskite catalysts, which
presumably leads to a surface enrichment of Co especially Co2+
as shown by XPS. The resulting oxygen deficiency at the
surface is possibly beneficial for the catalytic properties of the
perovskite catalysts, which has been demonstrated for other
Catalyst Stability
To test the leaching stability of the perovskite catalysts, hot-
filtration tests were performed: The reaction was first performed
for 2 h in the presence of the catalyst, and then the catalyst was
filtered off after taking a sample. The remaining filtrate was
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stirred for additional 2 h and another sample was taken. It was
found that the degrees of cinnamyl alcohol conversion only
increased slightly, after the catalysts had been filtered off
(Figure 3). For comparison, the high degrees of conversion after
6 h amounting to, e.g., (89 ± 0.2) % for LCFO-02 suggest that
the conversion of cinnamyl alcohol was not finished after 2 h.
Therefore, it can be concluded that the perovskite nanoparticles
are highly stable against leaching under the present conditions
and that the reaction proceeds heterogeneously catalyzed.
In addition, reusability tests were performed with LCFO-02 under
standard conditions for 2 h. The degree of conversion decreased
from (45 ± 1.0) % in the first run to (39 ± 0.8) % in the second
run and to (31 ± 1.1) % in the third run, showing that the
perovskite catalyst is moderately reusable. TEM images
(Figure S3) of the used catalyst showed no significant change in
the average particle size, excluding agglomeration of the
particles as a possible deactivation mechanism. The XRD
pattern of the used catalyst (Figure S4) reveals that the
perovskite structure is mostly retained after the reaction,
suggesting that the perovskite oxide is the active phase for the
reaction. By comparing the EDS elemental mapping of the fresh
LCFO-02 (Figure 4a) and of the spent catalyst (Figure 4b), it
was observed that presumably the corresponding mono- or
bimetallic oxides were formed partly under reaction conditions.
However, this was not observed consistently (Figure 4c). It can
therefore be concluded that phase segregation took place, but
not to a large extent, and only on the catalyst surface and not in
the bulk. This surface phase segregation can be considered as a
possible deactivation mechanism of the perovskite catalysts. It
has also been reported by other groups that perovskite catalysts
have superior catalytic properties compared with their
corresponding single metal oxides.[31]
Figure 4. EDS elemental mapping of the LCFO-02 perovskite, a) fresh
sample, b) and c) sample after 3 uses in the liquid-phase oxidation of
cinnamyl alcohol. Blue: La, green: Co, red: Fe.
Kinetic Investigations
The continuous increase of cinnamyl alcohol conversion over
LCFO-02 as a function of temperature is shown in Figure 5a. A
plot of the kinetic data in the linearized form for first-order
kinetics (Figure S5) is well suited for linear regression
(R2 > 0.99), demonstrating that the oxidation of cinnamyl alcohol
over LCFO-02 follows first-order kinetics. An Arrhenius analysis
performed with the first-order kinetic data (Table S1) results in a
plot well suited for linear regression (Figure 5b), from which an
apparent activation energy of (63.6 ± 2.3) kJ mol–1 was derived.
The addition of 6 % (n/n) hydroquinone, a well-known radical
scavenger,[58] to the oxidation of cinnamyl alcohol with TBHP
catalyzed by LCO resulted in a significant decrease of the
cinnamyl alcohol conversion from (84 ± 0.5) % to (29 ± 0.7) %,
suggesting that the reaction proceeds via a radical mechanism.
This is consistent with similar phenomena observed by other
groups and indicates that the primary role of the catalyst is to
decompose TBHP into reactive oxygen species (ROS).[7,31,34,59]
The essentially constant selectivity to cinnamaldehyde observed
for all catalysts during the catalyst screening indicates that the
same radical mechanism applies to all utilized perovskite
catalysts.
100
80
60
40
20
0
LCO
LCFO-02
LCFO-04
LCFO-06
As the primary role of the catalyst is presumably the
decomposition of TBHP, TBHP decomposition kinetics were also
investigated. Because the main product of the TBHP
decomposition besides tert-butanol is O2,[60,61] the kinetics of the
TBHP decomposition can be monitored by measuring the
evolved gas volume over time.[62] The data obtained from these
measurements (Figure S6) show that the decomposition of
TBHP reaches a plateau towards the end of the reaction. The
Figure 3. Hot-filtration tests of the perovskite catalysts investigated.
Black bars: cinnamyl alcohol conversion after 2 h reaction time,
diagonally hatched bars: cinnamyl alcohol conversion after additional 2 h
reaction time without a catalyst, horizontally hatched bars: cinnamyl
alcohol conversion after 6 h reaction time with a catalyst. Reaction
conditions: 10 mg catalysts, 20 mg CA, TBHP:CA = 5 (n/n), 3 mL MeCN,
50 °C, 1 bar.
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60
50
40
30
20
10
0
Conversion Cinnamyl Alcohol
Conversion TBHP
0
20
40
60
80
100
120
Time / min
Figure 6. Comparison of the conversion of cinnamyl alcohol and TBHP
over LCFO-02 at 50 °C.
first-order kinetics. To differentiate which reaction step is rate-
determining during the progress of the reaction, the degrees of
conversion during TBHP decomposition and cinnamyl alcohol
oxidation both over LCFO-02 at 50 °C are compared in Figure 6
as a function of time. TBHP conversion and cinnamyl alcohol
conversion increase equally fast within the first 10 min,
indicating that the decomposition of TBHP is rate-determining in
alcohol oxidation during the first 10 min. Similar phenomena
have been reported for the oxidation of benzyl alcohol with
TBHP over Cu-loaded LaFeO3, in which the conversion of TBHP
and of the alcohol match at the beginning of the reaction.[38]
Afterwards, the degree of cinnamyl alcohol conversion is lower
than the degree of TBHP conversion, suggesting that the
reaction of cinnamyl alcohol with the reactive radicals is rate-
determining after 10 min.
The reaction between the organic substrates and the reactive
radicals might also occur noncatalytically in the bulk of the
reaction solution. However, it was observed that surface-bound
radicals play an important role in toluene oxidation with TBHP
over AuPd/TiO2.[63] Therefore, a test for surface-bound radicals
was performed for the present catalytic system. TBHP was first
decomposed at 50 °C for 30 min followed by the filtration of the
solution. Then, cinnamyl alcohol was added to the filtrate. The
filtrated catalyst was added to a solution of cinnamyl alcohol in
MeCN. Both mixtures were stirred for another 30 min at 50 °C.
For the test with the filtrated catalyst, no detectable conversion
of cinnamyl alcohol was observed, indicating that surface-bound
radicals are not present. For the filtrate of TBHP decomposition,
only small degrees of cinnamyl alcohol conversion below 5 %
were observed. It can be concluded that the reactive radicals
can react in the liquid bulk as in the blank reaction, but with a
much lower rate compared with that in the presence of the
catalyst. Thus, the secondary role of the catalyst in addition to
peroxide decomposition is also to heterogeneously catalyze the
reaction between the ROS and the organic substrates.
Figure 5. Kinetics analysis of the liquid-phase cinnamyl alcohol oxidation
over LCFO-02, a) degree of cinnamyl alcohol conversion as a function of
time at different temperatures, b) Arrhenius plot based on first-order
reaction kinetics. Reaction conditions: 50 mg catalysts, 100 mg CA,
TBHP:CA = 5 (n/n), 15 mL MeCN, 1 bar.
autoretardation of the TBHP decomposition at 60 – 80 %
conversion has also been observed by other groups.[61] The
lowest degree of conversion of TBHP is obtained with LCFO-06,
which fits to the lowest degree of cinnamyl alcohol conversion in
the screening experiment and supports the conjecture
concerning the primary role of the catalyst. For the other
catalysts, the produced amounts of O2 are very similar, but
LCFO-02 leads to the highest degree of TBHP conversion as
determined by GC.
To analyze which reactants are part of the rate-determining step
(rds) of the oxidation reaction, it is useful to compare the
cinnamaldehyde formation rates during the optimization of the
reaction conditions (Table 2). The specific reaction rate
decreases for lower amounts of TBHP (entries 1 and 2),
suggesting that TBHP is involved in the rds of the reaction.
Furthermore, the reaction rate decreases for lower amounts of
cinnamyl alcohol (entry 4), while it increases for increased
amounts of cinnamyl alcohol (entries 5 and 6) as expected for
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Scheme 1. Proposed reaction network for the oxidation of cinnamyl alcohol with TBHP over LaCo1–xFexO3 perovskites, rds 1: rds during the first 10 min,
rds 2: rds after about 10 min.
When the temporal evolution of the selectivities to cinnam-
aldehyde and benzaldehyde in the kinetic experiments with
LCFO-02 is taken into consideration (Figure S7), it can be
observed that the selectivity to cinnamaldehyde decreases over
time, while the selectivity to benzaldehyde increases. This
observation indicates that the formation of benzaldehyde is
probably due to the consecutive conversion of cinnamaldehyde.
When cinnamaldehyde was used as reactant and oxidized
applying the same procedure as for cinnamyl alcohol,
benzaldehyde was observed as the main product with
approximately 70% selectivity, supporting the postulated
consecutive reaction pathway. The oxidative cleavage of
cinnamaldehyde to benzaldehyde has been also reported by
other groups.[64–66] Upon oxidation of the epoxide
3-phenylglycidol under the same conditions, benzaldehyde was
also obtained as product, but in lower amounts compared with
cinnamaldehyde conversion. Furthermore, when styrene was
oxidized analogously, benzaldehyde was also observed as the
main product alongside with styrene oxide, presumably being
the intermediate of the oxidative cleavage. These observations
suggest that the oxidative cleavage of the C=C bond may
proceed via an epoxide intermediate, which has also been found
for other similar reactants.[64,67–70] Based on these results, the
epoxide of cinnamaldehyde can be proposed as the common
intermediate in the oxidative cleavage of cinnamaldehyde and
3-phenylglycidol, which has never been observed experimentally
probably because of its instability.[71] The observation that less
benzaldehyde is formed from 3-phenylglycidol than from
cinnamaldehyde can be rationalized by the allylic position of the
double bond in cinnamaldehyde. Cinnamaldehyde is presumably
easier to be epoxidized than the oxidation of 3-phenylglycidol to
the aldehyde, because a radical intermediate is formed in both
cases, which is stabilized in the case of cinnamaldehyde, but not
in the case of 3-phenylglycidol. Therefore, the oxidative
cleavage of cinnamaldehyde proceeds faster. The direct
oxidative cleavage of 3-phenylglycidol without the aldehyde
intermediate can be neglected because of the lack of allylic
stabilization of a formed radical during the cleavage of the
epoxide in 3-phenylglycidol.
Combining all information obtained from the kinetic studies and
control experiments, the following reaction pathway can be
proposed (Scheme 1): initially, TBHP is decomposed by the
perovskite catalyst to ROS, which is the rds in the first 10 min of
the reaction. The ROS react with cinnamyl alcohol on the
surface of the catalyst forming cinnamaldehyde and
3-phenylglycidol in parallel, which is the rds after 10 min. Both
formed primary products react further with the ROS on the
catalyst surface to the common intermediate cinnamaldehyde
epoxide, which is mainly formed from cinnamaldehyde. This
unstable intermediate rapidly reacts with further ROS to
benzaldehyde. Further studies are in progress to reveal the
mechanisms of the underlying surface reactions.
Conclusions
Mechanistic insights in the liquid-phase oxidation of cinnamyl
alcohol with TBHP under mild reaction conditions over
noble-metal-free LaCo1–xFexO3 perovskites were gained by
performing kinetic experiments. The presence of Fe3+ ions was
found to be beneficial for the catalytic properties of the
perovskite catalysts with a maximum synergistic effect for an Fe
content of 20 %. The catalysts were stable against metal
leaching and moderately reusable. Deactivation presumably
occurred because of the segregation of binary metal oxides on
the surface. Kinetic investigations showed that the oxidation
over LCFO-02 follows first-order reaction kinetics in cinnamyl
alcohol with an apparent activation energy of 63.6 kJ mol–1.
Combining detailed kinetic investigations and control
experiments with reaction intermediates, a reaction network with
the short-lived intermediate cinnamaldehyde epoxide was
proposed. By comparing the kinetics of cinnamyl alcohol
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10.1002/cplu.201900429
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and LaFeO3.[52] TEM images and EDS elemental mapping were
measured with a JEM-2800 (JEOL, Akishima, Japan). A dispersion of the
solid samples in ethanol was drop coated on the copper grid and dried.
The beam energy was 200 keV. The effective area of the detector was
200 nm2 and yielded a resolution of 0.09 nm in TEM. The TEM images of
the used catalysts were taken with a Gatan One View camera with
4000×4000 pixel. EDS was performed with two large angle silicon drift
detectors, yielding a resolution of 123 eV.
oxidation and of TBHP decomposition, it was found that TBHP
decomposition is the rate-determining step initially, while the
reaction of cinnamyl alcohol with the formed reactive oxygen
species is rate determining after approximately 10 min. Surface-
bound radicals were not detected, but the reaction between the
ROS in the liquid bulk and the organic substrate was identified
as being catalyzed heterogeneously by the perovskite catalysts
applying hot filtration.
Catalytic Oxidation Reactions
Experimental Section
For the optimization of the reaction conditions, 10 mg LCO, the desired
amounts of cinnamyl alcohol and TBHP in 3 mL MeCN were used in
these experiments performed at 50 °C and 1 bar for 30 min. Afterwards,
a sample of approximately 1.5 mL was taken and filtrated through a
syringe filter (200 nm). The obtained clear solution was analyzed by GC.
In catalyst screening experiments (standard conditions), cinnamyl alcohol
(20 mg, 0.15 mmol) was dissolved in 3 mL MeCN and 10 mg of catalyst
was used to form a suspension. 103 µL of TBHP (70 wt% in water,
0.75 mmol) were added to initiate the reaction that was run at 50 °C and
1 bar for 30 min or 6 h. To investigate the effect of the solvent for the
reaction, HPLC-water and toluene were used under standard conditions
as with MeCN. For kinetic investigations, cinnamyl alcohol (100 mg,
0.75 mmol) was dissolved in 15 mL MeCN. 50 mg of LCFO-02 were
dispersed in the solution to match the standard conditions. Upon initiation
of the reaction by the addition of TBHP (70 wt% in water, 514 µL,
3.75 mmol), the reaction mixture was heated to 30, 40, 50 or 60 °C and
stirred at 1 bar. GC samples were taken after 10, 20, 30, 60 and 120 min.
For reusability experiments, the LCFO-02 catalyst was removed from the
reaction mixture by centrifugation and dried overnight in air. The used
LCFO-02 was utilized in cinnamyl alcohol oxidation with an amount of
reactants and solvent adjusted to the recovered catalyst amount to match
the standard conditions. Hot-filtration tests were performed by letting the
reaction run with a catalyst for 2 h under standard conditions at 50 °C.
After 2 h, a GC sample was drawn as mentioned. From the rest of the
solution, the catalyst was filtered off by a syringe filter (200 nm) and the
filtrate was stirred for another 2 h at 50 °C and 1 bar before taking
another GC sample.
Materials
Lanthanum(III) acetate hydrate (99.9 %), cobalt(II) acetate tetrahydrate
(>98 %), iron(II) acetate (95 %), propionic acid (ACS reagent, >99.5 %),
1-propanol (99.7 %), cinnamyl alcohol (98 %), trans-cinnamaldehyde
(99 %), TBHP 70 wt% in water (LuperoxR TBH70X), hydrogen peroxide
30 wt% in water (p.a.), hydroquinone (ReagentPlusR, ≥ 99 %),
benzylidenemalononitrile (98 %) and styrene (ReagentPlusR, ≥ 99 %)
were obtained from Sigma-Aldrich. (2R,3R)-3-phenylglycidol (96 %) was
supplied by Acros Organics. Benzaldehyde (99 %) was obtained from
Riedel-deHaen. Acetonitrile (99.99 %) and Malononitrile (99 %) was
purchased from ThermoFisher. HPLC-water and toluene (100.0 %) was
obtained from Fisher Chemicals. During catalyst synthesis, deionized
water with 18.2 M resistance was used.
Catalyst Synthesis
The LaCo1–xFexO3 perovskites were prepared in a spray-flame synthesis
setup based on the SpraySyn-burner concept[72] with full control over all
gas flows and the reaction pressure.[50,73] The syringe pumps were filled
with the precursor solutions of La(OAc)3, Co(OAc)2 and Fe(OAc)2 in the
desired ratios with a total metal concentration of 0.1 M in propionic
acid/1-propanol (1/1 V/V). A hollow needle was mounted to the end of the
syringe, through which the solution was injected into the burner nozzle
with 3 mL min–1 flow rate. For the pilot flame, 1 L min–1 CH4 and 2 L min–1
O2 was used. As dispersion gas O2 was utilized with a flow of 4 L min–1.
As sheath gas and quench gas, synthetic air was utilized with a flow rate
of 125 L min–1 and 170 L min–1, respectively. The synthesis was
performed at a constant pressure of 950 mbar abs. After the solutions
had been fully consumed, the filter was removed and dried for 30 min in
air. Afterwards, the particles were mechanically removed by scratching.
For the reaction with hydrogen peroxide instead of TBHP, 76.6 µL of
H2O2 (30 wt% in water, 0.75 mmol) was used under similar conditions as
for catalyst screening with LCFO-02. A GC sample was collected after
6 h.
To verify the radical reaction mechanism, a reaction was performed
under standard conditions with LCO but adding 1 mg hydroquinone (6 %
n/n related to cinnamyl alcohol). The reaction mixture was stirred at
50 °C for 6 h before collecting a GC sample.
Catalyst Characterization
To test for surface-bound radicals, 103 µL TBHP (70 wt% in water, 0.75
mmol) were first stirred with 10 mg LCO in 3 mL MeCN at 50 °C and
1 bar for 30 min. Afterwards, the catalyst was filtrated from the reaction
mixture. To the filtrate, 20 mg cinnamyl alcohol (0.15 mmol) were added.
The filtrated LCO was added to a solution of 20 mg cinnamyl alcohol in
3 mL MeCN. Both reaction mixtures were stirred at 50 °C and 1 bar
before taking a GC sample after 30 min.
Cinnamaldehyde, 3-phenylglycidol and styrene were oxidized with LCO
as the catalyst under the same conditions as cinnamyl alcohol for 6 h.
The aerobic oxidation of cinnamyl alcohol (100 mg, 0.75 mmol) was
performed with 50 mg LCO in 15 mL toluene. At 110 °C, 20 mL min-1 O2
were bubbled through the solution under stirring at 600 rpm. After 6 h, a
sample was taken by filtering 1.5 mL of the solution through a PTFE
syringe filter (200 nm).
Elemental analysis for Co and Fe was performed by AAS with a
PerkinElmer AAS Model Analyst200 after an acid digestion. The La
amount was determined via
a Wurzschmitt digestion with an
AnalytikJena Model Specord 50 plus UV/Vis spectrometer. XRD patterns
were recorded with a PANalytical X´Pert PRO diffractometer. As X-ray
source, Ni-filtered Cu Kα radiation (40 kV, 40 mA) without
a
monochromator was used. The diffraction patterns were obtained by
scanning from 10° – 90° in 0.05° steps. The XRD pattern of the used
catalyst was obtained using a Bruker D8 Discover diffractometer with Cu
Kα radiation by scanning from 5° - 85° in 0.05° steps. The determination
of the specific surface areas of the catalysts by N2 physisorption
measurements was performed with a Quantachrome NanoWin. Before
the measurement, the samples were degassed at 180 °C for 1 h under
N2 atmosphere. For the estimation of the particle size based on the
specific surface areas, the densities of Co- and Fe-containing perovskites
were calculated by a weighted average of the two densities of LaCoO3
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10.1002/cplu.201900429
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Knoevenagel Condensation
Acknowledgements
The Knoevenagel condensation was performed with 10 mg catalyst,
189 µL malononitrile (3 mmol) and 303 µL benzaldehyde (3 mmol) in
1 mL MeCN at 80 °C and 1 bar by stirring for 2 h at 600 rpm. Afterwards,
a sample was taken as described.
This
research
was
funded
by
the
Deutsche
Forschungsgemeinschaft (DFG, German Research Foundation)
– Projektnummer 388390466 – TRR 247 and supported by
International Max-Planck Research School for Interface-
Controlled Materials for Energy Conversion (IMPRS-SurMat).
D.W. acknowledges financial support as a Kekulé fellow of the
Fonds der Chemischen Industrie im Verband der Chemischen
Industrie e.V.
TBHP Decomposition Reactions
The decomposition of TBHP was performed in a 25 mL two-neck flask
that was placed inside of a water bath heated to 50 °C (Figure S8). One
connection of the flask was mounted to a reflux condenser, the other one
was sealed with a septum. The top outlet of the reflux condenser was
connected via a plastic tube to a system of two burettes filled with water,
serving as a u-pipe pressure gauge. 44 mg of catalyst and 12 mL of
MeCN were heated to 50 °C inside of the flask. A solution of 452 µL
TBHP (70 wt% in water, 0.66 mmol) in 1.2 mL MeCN was added through
the septum into the sealed system to initiate the reaction. The evolved
gas volumes were measured by keeping the height of the liquid inside of
the two burettes equal at any time to ensure atmospheric pressure inside
of the reaction vessel. This was accomplished by constantly removing
water from the open burette with a syringe connected to a plastic tube.
The evolved gas volumes were recorded in appropriate time periods.
After 6 h, the measurement of the gas volume was stopped, and a GC
sample was drawn as mentioned.
Keywords: Heterogeneous catalysis • oxidation • perovskite
phases • reactive intermediates • Spray-flame synthesis
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Entry for the Table of Contents
FULL PAPER
Versatility of Perovskites:
LaCo1–xFexO3 perovskites were
found to be active and selective
for the liquid-phase oxidation of
cinnamyl alcohol to
Daniel Waffel, Baris Alkan, Qi Fu,
Yen-Ting Chen, Stefan Schmidt,
Christof Schulz, Hartmut Wiggers,
Martin Muhler, and Baoxiang Peng*
Page No. – Page No.
cinnamaldehyde with tert-butyl
hydroperoxide, yielding high
degrees of conversion of up to
89 %. A complex reaction network
leading to benzaldehyde via
cinnamaldehyde epoxide was
identified. The peroxide
Towards Mechanistic
Understanding of Liquid-Phase
Cinnamyl Alcohol Oxidation with
tert-Butyl Hydroperoxide over
Noble-Metal-Free LaCo1–xFexO3
Perovskites
decomposition is the rate-
determining step initially, while the
reaction of the organic substrates
with the formed organic radicals is
rate determining after 10 min.
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