In situ formed Co clusters in selective oxidation of Α-C–H bond: Stabilizing effect from reactants

Article abstract of DOI:10.1016/j.mcat.2019.03.016

Aerobic oxidation of α-C–H bond of organic compounds to valuable chemicals is widely investigated in both fundamental research and industry. Due to the good stability of molecular oxygen, severe reaction conditions are generally required. Herein, by in situ synthesis we used molecular oxygen to induce cobalt nanoclusters with the sensitive catalysis in mild selective oxidation. The cobalt containing clusters with an average diameter around 0.9 nm are in situ prepared in the presence of cis-cyclooctene epoxidation and cyclooctene dimer oxide is formed at the interface to stabilize Co clusters with electron donation as an oil-soluble surfactant. The soluble clusters exhibit high activity in selective oxidation of α-C–H bond of ethylbenzene into acetophenone and turnover number (TON) reaches about 7 × 10⁴ during 50 h’ reaction at 373 K, which is around 960 times more active than the one using CoCl₂ salt as the catalyst, resulting from efficient mass transportation, π bond interaction and oxygen gas activation. Extended work based on this understanding demonstrates that cobalt nanoclusters also effectively catalyze aerobic oxidation of cyclohexene.

Full text of DOI:10.1016/j.mcat.2019.03.016

MolecularCatalysis470(2019)1–7
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
In situ formed Co clusters in selective oxidation of α-CeH bond: Stabilizing
effect from reactants
Zhijie Wanga, Anxiang Guana, Mayfair C. Kungb, Anyang Pengb, Harold H. Kungb, Ximeng Lva,
Gengfeng Zhenga, Linping Qiana,⁎
a Department of Chemistry, Laboratory of Advanced Materials, Fudan University, 2005 Songhu Road, Shanghai, 200438, China
b Department of Chemical and Biological Engineering, Northwestern University, 2145 Sheridan Road, Evanston, IL, 60208-3120, United States
A R T I C L E I N F O
A B S T R A C T
Keywords:
Aerobic oxidation of α-CeH bond of organic compounds to valuable chemicals is widely investigated in both
fundamental research and industry. Due to the good stability of molecular oxygen, severe reaction conditions are
generally required. Herein, by in situ synthesis we used molecular oxygen to induce cobalt nanoclusters with the
sensitive catalysis in mild selective oxidation. The cobalt containing clusters with an average diameter around
0.9 nm are in situ prepared in the presence of cis-cyclooctene epoxidation and cyclooctene dimer oxide is formed
at the interface to stabilize Co clusters with electron donation as an oil-soluble surfactant. The soluble clusters
exhibit high activity in selective oxidation of α-CeH bond of ethylbenzene into acetophenone and turnover
number (TON) reaches about 7 × 104 during 50 h’ reaction at 373 K, which is around 960 times more active than
the one using CoCl2 salt as the catalyst, resulting from efficient mass transportation, π bond interaction and
oxygen gas activation. Extended work based on this understanding demonstrates that cobalt nanoclusters also
effectively catalyze aerobic oxidation of cyclohexene.
Aerobic oxidation
Co nanoclusters
Ethylbenzene
Cyclohexene
cis-Cyclooctene
1. Introduction
(Co@GCNs) which showed efficient oxidation of ethylbenzene to
acetophenone under 0.8 MPa O2 pressure [23]. Thus, designing a green
Green oxidation of aromatic alkyl CeH bond is an important step in
synthetic organic chemistry [1]. Acetophenone, a product of ethyl-
benzene oxidation, is widely used in the manufacturing of pharma-
ceuticals, resins, fragrance and as a solvent for cellulose ethers [2]. It is
traditionally produced by acylation of benzene via the Friedel-Crafts
reaction, which generates a large amount of waste [3]. Replacing the
Friedel-Crafts process by selective oxidation of styrene or ethylbenzene
has been explored extensively using stoichiometric oxidants [4–11], but
the green usage of molecular oxygen is still a challenge [1,9,12–14].
Various transition metals including Co, Mn, Cr and Fe have been
examined as catalysts for ethylbenzene oxidation [5,6,15–25]. Bhoware
et al. synthesized 1.0–2.0 wt% cobalt-containing hexagonal mesoporous
molecular sieves (Co/HMS) and observed high activity in ethylbenzene
oxidation using tert-butyl hydroperoxide as the oxidant [20]. For the
usage of molecular oxygen as an oxidant, the reaction was carried out
under severe reaction conditions [19,23,26]. High pressure of 1 MPa O2
was required, when Ma et al. used 1.6 wt% Co/SBA-15 to catalyze the
ethylbenzene oxidation [19]. Lin et al. prepared cobalt nanoparticles
encapsulated into graphic nitrogen-doped carbon nanotubes
catalyst as a highly dispersed Co species through the encapsulating
treatment might be efficient for selective oxidation with atmospheric
pressure O2. However, it is difficult to find a suitable support or ligand
not only for metal species anchoring but also for the synergistic pro-
motion.
There has been no report to date of using solubilized transition
metal clusters as catalysts for ethylbenzene oxidation. But there are
many interesting reports of catalysis by quantum-sized metal clusters
[27–30]. Corma et al. found that gold clusters of low atomicity had
extraordinarily high activity in thiophenol reaction with molecular
oxygen [27]. Oliver-Meseguer et al. reported that small gold clusters (3
to 10 atoms) catalyzed the ester-assisted hydration of alkynes with a
high turnover frequency (TOF) of about 105 per hour [28]. Liu et al.
synthesized atomically dispersed palladium catalysts that were about
55 times more active than the commercial Pd catalysts in the hydro-
genation of aldehydes [29]. Unusual catalytic activities were found for
metal nanoclusters catalysis in the oxidation reactions [27,31–34].
Previously, we found that leached Au from Au/SiO2 in situ forms sui-
table Au7-8 clusters on which the superoxo species is formed to oxidize
⁎ Corresponding author.
E-mail address: lpqian@fudan.edu.cn (L. Qian).
https://doi.org/10.1016/j.mcat.2019.03.016
Received 1 February 2019; Received in revised form 13 March 2019; Accepted 13 March 2019
2468-8231/©2019ElsevierB.V.Allrightsreserved.

Z. Wang, et al.
MolecularCatalysis470(2019)1–7
further analysis.
2.3. Characterization of Co clusters
The fluorescence spectra of the filtrate solutions containing Co
clusters were recorded on Cary Eclipse Fluorescence
a
Spectrophotometer (Varian Inc.). The emission spectrum was obtained
using the excitation light wavelength that generated the highest in-
tensity emission. Excitation spectrum was collected at the emission
peak in the above test through changing the excitation wavelength.
Fourier transform infrared (FT-IR) spectroscopy of the precipitated
clusters was recorded on a Schimadz Prestige-21 spectrometer using
KBr pellets at room temperature.
Co concentrations in the solution were determined by the
PerkinElmer Optima 8000 Inductively Coupled Plasma Atomic
Emission Spectrometer (ICP-AES). The sample around 1 ml, was pre-
treated by calcining at 1123 K for 4 h in a muffle furnace to remove all
organic compounds. After that, 0.2 ml concentrated HCl and 0.2 ml
concentrated HNO3 were used to dissolve the Co species. The obtained
solution was diluted to a total volume of 50 ml with doubly distilled
(DDI) water.
Transmission electron microscopy (TEM) analysis was taken on a
JEOL JEM-2011 transmission electron microscope. Before test, 0.1 ml
Co clusters solution was dried onto 0.1 g nanofumed silica quickly
under Ar gas by increasing the temperature to 573 K. After that, the
prepared sample was mounted on carbon-coated copper grid by drying
a droplet of an ethanol suspension of the Co cluster-containing silica.
Matrix-Assisted Laser Desorption and Ionization Time-Of-Flight
Spectrometry (MALDI-TOF) measurements were carried out on an
Applied Biosystems MALDI-TOF mass spectrometer (Voyager DE-STR)
equipped with a nitrogen laser delivering 3 ns laser pulses at 337 nm.
Before tests, Co containing nanoclusters were loaded on the plate di-
rectly without a matrix by drying an ethanol suspension of the pre-
cipitated nanoclusters on the disperser. The spectra were collected in
positive mode on about 100 shots. The mass fragments were assigned
through comparing the experimental spectra with the calculated isotop
patterns by Data Explorer 4.3 software based on the Yergey algorithm
[36].
XPS measurements were recorded on a RBD 147 upgraded PHI-
5000C ESCA system equipped with a dual X-ray source. The photo-
electrons produced by Mg Kα (1253.6 eV) anode were analyzed with a
hemispherical energy analyser. Each sample of Co containing silica was
treated below 10−6 Pa. The spectrum was obtained at a pass energy of
93.00 eV using C1s peak at 284.6 eV as the calibration of the binding
energy.
Fig. 1. In situ formed Co nanoclusters in aerobic oxidation of α-C–H. (A the
formation pathway and selective oxidation application, B the mechanism of
selective oxidation of ethylbenzene).
α-CeH of cis-cyclooctene under atmospheric pressure O2 [32,35].
However, there is less work about the stabilizer structure of the in situ
formed clusters due to the difficulty in the sample accumulation. The
induction effect of the reactants on the solubility and activity of metal
clusters has not been disclosed. In this work, in situ synthesis method
was used to prepare the small soluble Co-containing clusters in aerobic
oxidation of cis-cyclooctene. After that, the clusters were precipitated
on the disperser with the tiny pore, analyzed by TEM, Fourier Trans-
form Infrared Spectroscopy (FT-IR), Matrix-Assisted Laser Desorption
and Ionization Time-Of-Flight (MALDI-TOF), and XPS measurements,
and further tested in aerobic oxidation of organic compounds (Fig. 1A).
2. Experimental section
2.1. Materials
The source and purity of the chemicals used are: CoCl2﹒6H2O (≥
99.0%, Sinopharm Chemical Reagent Co. Ltd), cis-cyclooctene (≥ 95%,
Aladdin Industrial Co. Ltd), cyclooctene oxide (≥ 99.0%, Aldrich),
triphenylphosphine (> 99.0%, Aladdin Industrial Co. Ltd), ethylben-
zene (≥ 98.5%, Sinopharm Chemical Reagent Co. Ltd), n-decane (≥
99.0%, Aladdin Industrial Co. Ltd), cyclohexene (≥ 99.5%, Sinopharm
Chemical Reagent Co. Ltd), cis-cyclooctane diol (99.0%, Aldrich), CDCl3
(99.8%, 0.03% TMS, Aladdin Industrial Co. Ltd), oxygen gas (≥
99.999%, Qinglei Special Gas Company Affiliated with Shanghai
Research Institute of Chemical Industry), nano fumed silica (≥ 99.8%,
7–40 nm, Aladdin Industrial Co. Ltd), n-Octadecyl-β-(3,5-ditert-butyl-4-
hydroxyphenyl)-propionate (Irganox-1076,
≥
98.0%, Aladdin
Industrial Co. Ltd), tert-butyl hydroperoxide (TBHP in nonane, 5.5 M,
Aldrich).
2.4. Catalytic test
The catalytic oxidation of ethylbenzene with molecular oxygen was
carried out in a three-neck glass cylindrical shaped vessel as that used in
the Co clusters preparation. Typically, 12 ml ethylbenzene, 1.2 ml n-
decane, 3 ml of the prepared Co clusters solution and a Teflon-coated
magnetic stirrer were loaded into the reactor. The mixture was soni-
cated for about 10 min, and then placed in the oil bath for 30 min.
When the temperature reached 373 K, molecular oxygen was bubbled in
at a flow rate of 30 ml/min to begin the reaction. The reaction mixture
was analyzed periodically by 1H NMR (Mercury plus 400 M). 0.1 ml
samples of the mixture were retrieved and diluted to 0.5 ml by CDCl3
for the analysis. Cyclohexene oxidation using Co clusters was carried
out as the above procedure.
2.2. Co clusters preparation
Co clusters were in situ formed using the procedure for cis-cy-
clooctene epoxidation starting with CoCl2﹒6H2O and a cis-cyclooctane
diol promoter [32]. 10 ml cis-cyclooctene, 1 ml n-decane, and 181 mg
diol were added into a three-neck glass cylindrical shaped vessel shown
in Fig. S1. After heating for 10 min. to dissolve the diol, 9 mg CoCl2﹒
6H2O was added. With stirring, the mixture was heated to 373 K and
then O2 was introduced through a fine glass disperser tube (Fig. S1) to
begin the reaction. The exit gas was passed through a reflux condenser
maintained at around 268 K (Greatwall DLSB-10/20 refrigerated cir-
culator, Zhengzhou Greatwall Scientific Industrial and Trade Co. Ltd) to
control the evaporation of the reagents. The reaction was allowed to
proceed for 23 h to reach a conversion of around 60% during which
time dissolved cobalt clusters are formed. The soluble fraction after
filtration were collected and used as the Co cluster catalyst. Some Co
clusters precipitated on the disperser was washed out by ethanol for the
The ethylbenzene conversion, acetophenone yield, turnover number
(TON) and turnover frequency (TOF) are defined as follows:
the amount of ethylbenzene consumed(mol)
the amount of ethylbenzene input(mol)
Conversion(%) =
× 100%
2

Z. Wang, et al.
MolecularCatalysis470(2019)1–7
Fig. 2. Characterization of the prepared Co clusters. (A TEM image of the Co clusters sample loaded on fumed silica, B TEM image of the Co clusters sample
precipitated on the disperser, C FT-IR spectra of cyclooctene oxide (red), cis-cyclooctene (blue) and the precipitated Co clusters sample (black), D FT-IR spectra
amplified from 1600 cm−1 to 1800 cm−1 for the Co clusters sample (black, below) and cis-cyclooctene (blue, above), E MALDI-TOF (positive) of the precipitated Co
clusters sample, the range of test covers up to 2000 Da but no peaks > 400 Da were detected, F XPS spectra of the Co clusters sample loaded on fumed silica).
the amount of acetophenone produced(mol)
the amount of ethylbenzene input(mol)
Fig. 2A and Fig. S2 A) with an average diameter of 0.9 nm smaller than
the one precipitated on the disperser (total count of 163 nanoclusters in
Fig. 2B and Fig. S2B). No large Co aggregates were observed indicating
that the Co-containing clusters are highly dispersed in the organic so-
lution [27,28,32]. The size distribution corresponds to 46 atom-cluster
on average using the average nearest neighbour distance of 0.25 nm as
in Co43 clusters [37] and cubic model. This is in line with the fact that
Co nanoparticle of 1.5 nm contains 55 Co atoms [38]. 60% conversion
of cis-cyclooctene was achieved after 23 h indicating Co clusters with
suitable size are active in selective oxidation process especially for the
key step of oxygen gas activation.
Acetophenone Yield(%) =
× 100%
the amount of ethylbenzene consumed or
acetophenone/2-cyclohexene-1-one produced(mol)
TON =
the amount of Co used(mol)
the amount of 2-cyclohexene-1-one produced(mol)
-1
TOF(h ) =
the amount of Co used(mol) × reaction time(h)
The structure and composition of Co clusters were further studied
by FT-IR, MALDI-TOF and XPS measurements. FT-IR measurement of
the precipitated clusters (Fig. 2C, black curve) shows a broad peak
3. Results and discussion
around 3350 cm−1 and three characteristic peaks around 2930 cm−1
,
The filtrate with cobalt containing clusters, in situ formed during cis-
cyclooctene oxidation in the presence of CoCl2, has a Co concentration
of 93 μg/ml as determined by ICP. When the filtrate was loaded onto
silica, TEM analysis shows a very narrow size distribution of the Co
clusters ranging from 0.7 to 1.3 nm (total count of 112 nanoclusters in
1715 cm−1 and 1665 cm−1, attributing to the vibrational mode of
ν(OH) and ν(CH), stretching C]O bond and ν(C]C) bond of the formed
organic compounds, respectively. Compared with cis-cyclooctene
(Fig. 2D, blue curve), there is a blue shift for vibrational mode of C]C
3

Z. Wang, et al.
MolecularCatalysis470(2019)1–7
bond of the formed compounds, which is attributed to the electron
donation effect from the clusters plasmon [39]. MALDI-TOF spectrum
of the precipitated Co nanoclusters was shown in Fig. 2E. Three main
peaks of 268.1566, 331.1778 and 346.2053 were found, which are
much greater than the atomic weight of Co and molecular weight of
cyclooctene and cyclooctene oxide, indicating the occurrence of the
oligomerization of the reactants or Co species. Based on FT-IR mea-
surement, C16H32O3Co and C16H31O4Co in Fig. S3B’ and Fig. S3C’ are
the metal-organic compounds containing alkene and hydroxyl/carbonyl
group such as alcohol/ketone. Mass fragments and fragmentation pat-
terns indicate that cyclooctene dimer oxide forms to anchor Co species.
The formation pathway was described as follows. Initially, C]C bond
of cyclooctene coordinates with Co salt [40] and α-CeH dissociates to
cyclooctene radical due to the weak bond [32,35,41]. After that, the
free radical interacts with cis-cyclooctene to generate the dimer, and
meanwhile Co cluster forms. Finally, oxygen gas is activated on Co
clusters surface into the reactive species which further oxidizes the
dimer. Water was observed in the mass fragment assigning for the
formed dimer oxide occurs in the interface between hydrous Co salt and
hydrophobic cis-cyclooctene. Therefore, the stabilizer bridges the mass
transportation between hydrophilic Co species and hydrophobic cis-
cyclooctene as an oil-soluble surfactant (Fig. 1A). Fig. 2F shows Co2p
XPS binding energy from 765 to 815 eV for Co clusters loaded on fumed
silica. The peaks around 777.1 and 789.8 are ascribed to Co(2p3/2) and
Co(2p1/2) of metallic Co species [42,43]. The binding energies and the
absence of satellite peaks suggest that metallic Co clusters occur in
these samples. However, it is possible that an oxidic cobalt present in
the sample is reduced by the X ray beam in the XPS. Noticeably, a large
peak around 770.9 eV was observed towards lower energy than metallic
cobalt, which could be ascribed to the electron donation of Co na-
noclusters promoted by the surrounding stabilizer. This feature pro-
motes oxygen gas activation through donating the electron from Co
nanoclusters to π* orbit of O2 [44], which is the key step in the aerobic
oxidation [35]. The fluorescence spectrum of the prepared cobalt
clusters is shown in Fig. S4. The emission scan was carried out at the
excitation wavelength of 410 nm which shows maximum intensity,
while the excitation scan was taken at the emission of 480 nm. For the
filtrate solution with no Co species, the emission peak intensity around
450 nm is low [32]. Thus, the observed spectra could be attributed as
arising from Co containing clusters. The excitation spectrum is sig-
nificantly blue shifted from the band ascribed to surface plasmon re-
sonance of larger Co nanoparticles, attesting to the small size of the Co
containing clusters. The formed surface plasmon is stabilized by the
formed dimer oxide, which assists the electron transfer during the ex-
citation of the light wave [39,45].
Based on the above analysis, mass transportation and electron do-
nation of Co clusters was promoted with the formed cyclooctene dimer
oxide, which might be used for aerobic oxidation of organic compounds
efficiently. The result of solvent-free aerobic oxidation of ethylbenzene
using in situ formed Co clusters catalyst are shown in red label of
Fig. 3A. Co containing nanoclusters show high activity with no induc-
tion period. With the time lasting, the ethylbenzene conversion and the
acetophenone (Fig. 3A, red curve) and 1-phenylethylhydroperoxide
yield (Fig. 3B, red curve) increase. The selectivity of acetophenone is
enhanced from 31% to 63%. The product distribution of ethylbenzene
oxidation is shown in Table S1. The carbon balance is good despite
evaporative losses, less than 5.0% around 50% conversion and
1.5% around 20% conversion. The control experiment, without the
catalyst, showed no products after 50 h of reactions. Similarly, low
conversion of ethylbenzene about 5% and low yield of acetophenone
about 1% were observed when 9 mg of CoCl2 salt was used as the
catalyst (Fig. 3A, blue curve) and interestingly no Co cluster was ob-
servable in the TEM (Fig. S5) after reaction despite the fact that the
solution contains 85 μg/ml of soluble cobalt. This indicates that cobalt
salt is poorly active for ethylbenzene oxidation under our mild reaction
conditions. In contrast, when 3 ml of a solution containing in situ
formed Co clusters derived from cis-cyclooctene oxidation reaction was
added to the ethylbenzene reaction solution (Fig. 3A, red curve), oxi-
dation reaction was observed with the introduction of O2. After 50 h,
ethylbenzene conversion reaches around 50% and the yield of acet-
ophenone is about 30%.
The hydroperoxide concentration in the in situ formed Co clusters
solution is around 0.3 mol/L as determined by triphenylphosphine ti-
tration [46]. When the same amount of tert-butyl hydroperoxide
(TBHP) was added as an initiator to the ethylbenzene oxidation solution
in place of the Co cluster catalyst [32], only 1% ethylbenzene conver-
sion results after 50 h, indicating that hydroperoxide is not an effective
promoter in ethylbenzene oxidation and Co clusters solubilized in the
solution plays the major role in dissociating α-CeH bond to form the
acetophenone of interest. The turnover number (TON) of acetophenone
is about 6000 after 50 h, calculated using the total number of cobalt
atoms in the reaction solution, which is around 110 times higher than
the one using cobalt salt as the catalyst.
The activity and stability of in situ formed Co clusters was further
investigated through cis-cyclooctene addition. When the prepared Co
clusters solution was diluted 10 times by cis-cyclooctene and 3 ml of the
diluted catalyst solution was added to the ethylbenzene solution, a 22%
conversion of ethylbenzene (Fig. 3C, orange curve) and a TON of
20,000 for acetophenone production were obtained after 50 h. This is
significantly higher than expected based on the amount of Co species.
With 100 times dilution by cis-cyclooctene, the conversion of ethyl-
benzene and the yield of acetophenone after 50 h decrease slightly to
19% and 12% respectively (Fig. 3C, purple curve), but the TON of
acetophenone reaches 1.4 × 105, suggesting that there might be other
species in the filtrate solution that are active for ethylbenzene oxidation
[30]. It is noticeable that 1-phenylethylhydroperoxide yield does not
increase after 20 h’ reaction in 100 times dilution sample (Fig. 3D,
purple curve). Previously we have shown that during cis-cyclooctene
oxidation, cyclooctene hydroperoxide is also formed that could initiate
hydrocarbon oxidation [32]. In order to delineate the roles between
cobalt clusters and other organics such as cyclooctene hydroperoxide,
we prepared a separate solution from cis-cyclooctene auto-oxidation
without any Co salt. When 3 ml of the auto-oxidation-generated solu-
tion was added to ethylbenzene without Co, the ethylbenzene conver-
sion is around 13% and the yield of acetophenone is about 6% after
50 h of reaction (Fig. S6). These are lower than when in situ formed
cobalt filtrate was used. Consequently, in situ formed Co clusters played
a significant role in ethylbenzene oxidation and TON of acetophenone
of around 7 × 104 was achieved after substracting the activity from the
organics in the filtrate, which is about 960 times higher than the direct
usage of cobalt salt (Fig. 3A). The excitation and emission spectra after
ethylbenzene oxidation reaction (Fig. S7) show no significant change as
compared with that before reaction (Fig. S4), indicating that the Co
clusters stabilized with the dimer oxide are active and stable under
reaction conditions.
According to the literatures [47–52], the ethylbenzene oxidation
proceeds by first formation of free radicals of ethylbenzene, which traps
a molecule of oxygen to generate a peroxide. Then, the product of
acetophenone is generated through the propagator of 1-phenyl-ethyl-
hydroperoxide. Thus, 1-phenyl-ethylhydroperoxide may be observed in
the oxidation reaction. However, small amount of 1-phenyl-ethylhy-
droperoxide was observed in 100 times dilution sample and the content
does not increase after 20 h’ reaction (Fig. 3D, purple curve). Al-
shammari found that 3 × 10−5 mol of 6-bis(t-butyl)-4-methylphenol
(BHT) radical scavenger was able to completely quench the Au/C cat-
alyzed oxidation of cyclopentene [53]. To test the role of free radicals in
the reaction, Irganox-1076, a radical scavenger was added to remove
the free radicals. When Irganox-1076 (˜ 3 × 10−5 mol) was added to
the ethylbenzene reaction after 25 h, the rate of ethylbenzene conver-
sion and the content of 1-phenyl-ethylhydroperoxide decrease notice-
ably (Fig. 3E, black curve). This phenomenon can be explained by the
scavenger which stops the conversion through phenylethyl radical. But
4

Z. Wang, et al.
MolecularCatalysis470(2019)1–7
Fig. 3. Ethylbenzene oxidation with molecular
oxygen using Co species ( , and in Fig. A,
B, E and F,
and in Fig. C and D,
D, and ●, ○ and
,
and
in Fig. A and B,
and in Fig. C and
in Fig. E and F represent
,
,
ethylbenzene conversion, acetophenone yield
and hydroperoxide yield using Co nanoclusters
(Co 93 μg/ml, red curve), CoCl2 salt (Co 85 μg/
ml, blue curve), the diluted Co clusters solution
with cis-cyclooctene (Co 9.3 μg/ml, orange
curve and Co 93 ng/ml, purple curve), and Co
nanoclusters (Co, 93 μg/ml) with the scavenger
addition at 25 h (black curve), respectively.
Table 1
Ethylbenzene conversion and acetophenone yield using different catalyst solution.
Catalyst
Ethylbenzene Conversion (%) after 50 hrs
Acetophenone Yield (%) after 50 hrs
a Co clusters in cyclooctene oxygenate
b Auto-oxidation filtrate
c Correction (a -b)
d Co clusters in cyclooctene oxygenate (Radical scavenger)
e Correction (d -1/2b)
50
13
36
31
24
31
6
25
25
22
the ethylbenzene conversion still increases slightly after the scavenger
input. Furthermore, the yield of acetophenone continued to increase,
resulting in a higher selectivity (around 80%) after 50 h than in the
absence of the radical quencher (Fig. 3A), even after taking into account
the different conversions. These results indicate that another oxidation
pathway different from phenylethyl radical may occur. In Table 1, the
activity of Co clusters catalyst (a) and radical scavenger addition (d)
were compared. Auto-oxidation filtrate (b) was used to correct the Co
clusters performance (c and e). The phenylethyl radical oxidation was
considered to carry out by the organics filtrate (b) [35]. Thus, after the
radical scavenger addition, the contribution from the phenylethyl ra-
dical (b) is ignored and the initial 25 h’ reaction in (d) is corrected by
half of 50 h’ reaction in (b). It was observed that acetophenone yield is
not largely influenced by the radical scavenger but similar in (c) and
(e). This suggests that ethylbenzene oxidation might not only result
from the phenylethyl radicals but also from direct catalysis by Co
clusters (a). The interesting product of acetophenone could be mainly
produced from the direct oxidation on Co clusters surface with the
dimer oxide stabilizer.
The reaction mechanism involving in situ formed Co clusters is
proposed in Fig. 1B. The Co clusters are proposed to be responsible for
the very important step of oxygen gas activation [54–58], possibly
forming a superoxide through the electron donation from the metal
clusters to oxygen gas [35,59–63], and meanwhile π bond of the or-
ganic reactants can coordinate with the metal surface. The activated
oxygen species is essential for α-CeH bond dissociation [35,64] and
5

Z. Wang, et al.
MolecularCatalysis470(2019)1–7
directly oxidizes ethylbenzene to 1-phenylethanol. Further, ConOO*
abstracted H% from 1-phenyethanol to form acetophenone and the ac-
tive oxygen on the clusters is consumed. This mechanism is different
from the phenylethyl radical chain reaction involved with 1-pheny-
lethylhydroperoxide, but similar to those on Ce1-xMnxO2, Co-N-C/
MnOx-CeO2 and Co-N-C-/SiO2 [65–67]. The mass transportation of
above process was carried out fluently through the bridge of the formed
cyclooctene dime oxide.
This understanding of in situ formed Co nanoclusters in aerobic
oxidation of ethylbenzene can also be expanded in another α-CeH
oxidation of cyclohexene using atmospheric pressure O2 (Fig. S8). Two
main products of 2-cyclohexene-1-ol and 2-cyclohexene-1-one were
observed with the total selectivity more than 90%. Initially, 2-cyclo-
hexene-1-ol is generated more than 2-cyclohexene-1-one. After 4 h, 2-
cyclohexene-1-one yield increases faster than 2-cyclohexene-1-ol. When
the reaction stops at 10 h, cyclohexene conversion and 2-cyclohexene-
1-one yield reach around 70% and 40%, respectively. High TON and
TOF of 2-cyclohexene-1-one of around 7 × 104 and 7 × 103 h−1 were
achieved at 333 K. The mechanism investigation by the free radicals
trap suggested that oxygen gas activated on Co nanoclusters surface
oxidizes the coordinated cyclohexene into 2-cyclohexene-1-one directly
[68].
Among the literatures about O2 activation [27,39,63], nanomater-
ials were introduced due to the structure sensitivity and quantum size
effect. Coma et al. found O2 is activated efficiently in the thiophenol
oxidation by small Au nanoparticles (≤ 1 nm) [27]. Huang et al. re-
ported that O2 activation is promoted by the formed surface plasmon
resonances on gold and silver nanoparticles [39]. Salisbury et al. re-
ported that low temperature activation of molecular oxygen is corre-
lated to the electron affinity of gold clusters which is influenced by the
nanocluster size [63]. Therefore, we used oxygen gas to induce the
sensitive structure of the cobalt nanoclusters. The formed nanoclusters
around 0.9 nm stabilized by the dimer oxide possesses the electron
donation, which facilitates the key step of oxygen gas activation in
aerobic oxidation of ethylbenzene or cyclohexene through donating the
electron to π* orbit of O2 [44].
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.mcat.2019.03.016.
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