Heterogeneous Cobalt-Catalyzed C?C Bond Cleavage in Alcohols to Carbonyl Compounds

Article abstract of DOI:10.1002/cctc.202100950

Cobalt oxide hydrate (Co(OH)_x) supported on metal oxides were prepared. The oxidative cleavage of C(OH)?C bond in alcohol was catalyzed by Co(OH)_x/metal oxides using molecular oxygen as the oxidant, and the corresponding product was obtained with high selectivity. The composition and characteristics of the catalysts were analyzed by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), high-angle ring dark-field scanning TEM (HAADF-STEM), N₂ physico-adsorption characterization, inductively coupled plasma optical emission spectrometry (ICP-OES), Fourier transform infrared spectrum (FTIR), X-ray photoelectron spectroscopy (XPS) and NH₃-temperature-programmed desorption (NH₃-TPD). Additionally, a reasonable reaction mechanism was proposed based on the characterization results and a series of controlled experiments. Notably, this heterogeneous catalytic system did not require any additives and had good recycling performance.

Full text of DOI:10.1002/cctc.202100950

The European Society Journal for Catalysis
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Accepted Article
Title: Heterogeneous Cobalt-Catalyzed C-C Bond Cleavage in
Alcohols to Carbonyl Compounds
Authors: Jia-nan Teng, Rui Zhu, Xinglong Li, and Yao Fu
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01/2020

10.1002/cctc.202100950
ChemCatChem
FULL PAPER
Heterogeneous Cobalt-Catalyzed C-C Bond Cleavage in Alcohols
to Carbonyl Compounds
Jia-nan Teng,[a] Rui Zhu,[a] Xinglong Li,[a] Yao Fu*[a]
[a]
J. Teng, R. Zhu, X. Li, Prof. Dr. Y. Fu
Anhui Province Key Laboratory of Biomass Clean Energy, CAS Key Laboratory of Urban Pollutant Conversion, University of Science and Technology of
China, Hefei 230026.
Institute of Energy, Hefei Comprehensive National Science Center, Hefei 230031.
fuyao@ustc.edu.cn
Abstract: The cobalt oxide hydrate (Co(OH)x) supported on metal
oxides were prepared. And the oxidative cleavage of C(OH)-C bond
in alcohol was catalyzed by Co(OH)x/metal oxides using molecular
oxygen as the oxidant, and the corresponding product was obtained
with high selectivity. The composition and characteristics of the
catalysts were analyzed by X-ray diffraction spectrum (XRD), high
resolution transmission electron microscopy (HRTEM), high-angle
ring dark-field scanning TEM (HAADF-STEM), N2 physico-
adsorption characterization, inductively coupled plasma optical
emission spectrometry (ICP-OES), Fourier transform infrared
spectrum (FTIR), X-ray photoelectron spectroscopy (XPS) and NH3-
temperature-programmed desorption (NH3-TPD). Additionally, a
reasonable reaction mechanism was proposed based on the
characterization results and a series of controlled experiments.
Notably, this heterogeneous catalytic system did not require any
additives and had good recycling performance.
catalyzed C-C bond oxidation cleavage studies with oxygen as
oxidant have been developed.[28-33] Among them, the problems
of low substrate concentration and low conversion efficiency
often exist in photocatalytic oxidation cleavage reaction, which
has no value for large-scale application at present. Therefore,
thermally catalyzed oxidative cleavage of C-C bond is still the
mainstream research.
Using transition metal-based homogeneous catalytic systems
for the oxidative cleavage of the C(OH)-C bond in alcohols has
been studied extensively. Among them, tert-butyl alcohol metal
potassium salt or sodium salt[34,35], copper-based complex[17,24]
,
silver salt[12], etc., can catalyze the oxidative cleavage reaction
of aliphatic and/or (heteroaryl) primary or secondary alcohols
under atmospheric or pressurized conditions to obtain acids or
esters with high selectivity. However, all of these systems need
excessive alkali or other additives (molecular sieves) to promote
the reaction. Chen et al.[36] used "PdCl2+CuCl" to catalyze the
continuous cleavage of C-C bond to obtain aldehydes with high
selectivity. However, the catalytic activity for the oxidative
cleavage of inactive long-chain alkyl alcohols was low or even
had no catalytic effect. Han et al.[37] reported the NO2 catalytic
system for the oxidative cleavage of C(OH)-C bond in aromatic
secondary alcohols under high temperature and pressure.
NaNO3 provided both NO2 and alkaline conditions. In general,
the significant disadvantages of these transition metal-based
homogeneous catalytic systems are that they cannot be recycled
and the reaction conditions are harsh. Due to the poor tolerance
of the catalyst to water, a non-aqueous system or a high
concentration reaction system is required. In addition, precious
metals (such as palladium and ruthenium) based catalysts also
have the problem of high cost (Scheme 1).[27] In contrast,
heterogeneous catalysts are easy to be separate and recycled,
but there are a few studies on the oxidation cleavage of C(OH)-
C bond using heterogeneous catalysts. Recently, Anastas et
al.[38] used layered sodium manganese mixed oxides (Na-Mn
LMO) to catalyze oxidation or cleavage of 1,2-diols to aldehydes
or ketones with high selectivity. However, the substrate was
limited to aromatic ring adjacent diols, there was no cleavage of
aliphatic or aliphatic adjacent diols. Gao et al.[27] prepared
atomically dispersed cobalt (3.8ꢀwt% Co) on N-doped carbon
catalyst for the oxidative cleavage of aromatic 1,2-diols to esters,
ketones or aldehydes. However, the potassium carbonate was
necessary, otherwise the reaction cannot be occurred. Dai et
al.[26] demonstrated Co-NC catalyst system for the continuous
cleavage of C-C bond in primary and secondary alcohol to
corresponding ester with high selectivity. And the catalytic
Introduction
With the rapid consumption of non-renewable resources and
the increasingly prominent problem of environmental
deterioration, it is urgent for human to construct the sustainable
development system of resources and energy. The development
and effective use of renewable resources are the key points. As
we all known, most of biomass resources are composed of C-C
and C-O bonds. And selective cleavage and functionalization of
tension-free C-C bonds[1-6] are of great importance in the fields
of organic chemistry, biodegradation and the petroleum
industry.[7-16] In particular, carbohydrate, chitin, lignocellulosic
and their platform molecules have a large number of C(OH)-C
bonds, whose cleavage and functionalization can degrade
biomass and convert it into sustainable chemicals. In addition,
effective treatment of polymer contaminants can also be
realized.[17-25] However, the bond energy of C-C bond is larger
than C-O bond. The energy barriers of most cleavage reactions
are high, so the conditions are harsh.[24,26] Traditionally, people
used stoichiometric quantities or even excessive oxidants for
oxidative cleavage of C-C bond, such as chromium oxidants,
sodium
periodate,
manganese
dioxide,
potassium
permanganate, lead tetraacetate. Although the effect is ideal, it
will bring a large number of environmental pollutants, which does
not meet the requirements of green chemistry. Hence, more and
more research began to use molecular oxygen as a clean
oxidant.[27] Based on this, a series of photocatalytic and thermally
1
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10.1002/cctc.202100950
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Table 1. Optimization of the Reaction Conditions. [a]
Yield[b] [%]
Conv.[b]
[%]
Entry
Catalyst
2a
n.d.
n.d.
n.d.
trace
n.d.
n.d.
n.d.
3a
50
4a
5
5a
32
n.d.
14
6
1
2
Co(OH)x/Al2O3
Co(OH)x/TiO2
Co(OH)x/CeO2
Co(OH)x/MnO2
Co(OH)x/MgO
Co(OH)x/ZnO
Co(OH)x/ZnO
99
< 5
99
20
99
99
99
n.d.
74
trace
8
3
4
n.d.
80
< 5
< 5
trace
5
5
17
10
11
6
85
7[c]
71
Scheme 1. Oxidative cleavage of C-C bond in alcohol.
[a] Reaction conditions: diphenylethanol (1a) (0.05 mmol), catalyst (HMF/Co
molar ratio=11.78), GBL (1 mL), O2 balloon, 150°C, 12 h. [b] Determined by
GC analysis using biphenyl as internal standard. [c] Air condition.
system was suitable for various functionality aromatic, aliphatic,
heterocyclic, allyl, propargylic primary and secondary alcohols as
well as inactive long chain alkyl aryl alcohols, but high
temperature and pressure and excessive alkali were necessary
(Scheme 1). In conclusion, it is still significative but challenging
to design an economical and sustainable heterogeneous catalyst
for the selective catalytic cleavage of C(OH)-C bonds in a variety
of alcohols under mild conditions.
Herein, we prepared cobalt oxide hydrate (Co(OH)x) supported
on metal oxides catalyst, and used them for oxidative cleavage
of the C-C bond in alcohols in one pot (Scheme 1). We used the
Co(OH)x/ZnO to catalyze the oxidative cleavage of the C(OH)-C
bond in the alcohol with clean molecular oxygen as oxidant
without any additives. We screened the effects of different
catalysts on the yield and selectivity of the reaction, and
optimized the reaction conditions such as reaction temperature
and solvent. The catalytic system was suitable for a wide range
of substrates, including 1,2-diols substituted by aromatic and
aliphatic, α-hydroxy ketones, α-hydroxy acids and lignin β-O-4
model compounds. Based on the characterization results of the
catalyst and a series of controlled experiments, we proposed a
reasonable mechanism of C-C bond cleavage. In addition, the
heterogeneous catalytic system had good recycling performance,
and could be recycled and reused for several times.
and benzoyl were the main by-products with yields of 1% and
10%, respectively. We speculated that the alkalinity of ZnO might
be strong, which reduced the redox potential of the oxidative
cleavage reaction of C-C bond. Thus, the reaction was promoted.
When the O2 balloon was replaced by air, the yields of the target
products reduced slightly (Table 1, entry 7).
In order to clarify the properties of cobalt supported on metal
oxides, CoCl2 was treated by alkaline (SI, Experimental
procedures) without carrier to produced Co(OH)x, and we
characterized its various properties. First, the composition and
structure of Co(OH)x and a series of catalysts were analyzed by
XRD spectrum (Figure S1). In order to clarify the properties of
cobalt supported on metal oxides, CoCl2 was treated by alkaline
(SI, Experimental procedures) without carrier to produced
Co(OH)x, and we characterized its various properties. First, the
composition and structure of Co(OH)x and a series of catalysts
were analyzed by XRD spectrum (Figure S1). As shown in the
XRD pattern of Co(OH)x (Figure S1g), the diffraction peaks
located at 2θ degrees of 20.5°, 36.5°, 39.0°, 46.0°, 50.5°, 65.5°,
69.0°, 77.5°, 78.5° were assigned to the (111), (010), (110), (112),
(221), (-110), (120), (-111) and (020) lattice planes of CoO(OH)
respectively (JCPDS No.72-2280).[39] The diffraction peaks
located at 2θ degrees of 19.05°, 32.47°, 37.92°, 51.36°, 57.91°,
59.57°, 61.53°, 69.51°, 71.79° were assigned to the (001), (100),
(101), (102), (110), (003), (111), (103) and (112) lattice planes of
β-Co(OH)2 respectively (JCPDS No.30-0443).[40] Both cobalt(II)
and cobalt(III) existed in the Co(OH)x indicated that the initially
formed Co(OH)2 was partially oxidized to CoO(OH) by oxygen in
air at room temperature. The diffraction peaks of the XRD
spectra pattern of Co(OH)x/ZnO were sharp, indicating that there
were ordered atomic plane in the grain interior and the
crystallinity was good (Figure S1d). Compared with the standard
spectra, the XRD spectra was in good agreement with the
hexagonal ZnO (JCPDS No.80-0074). The diffraction peaks
located at 2θ degrees of 31.732°, 34.364°, 36.207°, 47.469°,
56.526°, 62.754°, 66.292°, 67.851°, 68.995° and 76.848° were
assigned to the (100), (002), (101), (102), (110), (103), (200),
Results and Discussion
The catalytic activity of a series of catalysts for the oxidative
cleavage of C(OH)-C bond in alcohols was studied using
diphenylethanol (1a) as the model substrate. The model reaction
was carried out using γ-butyrolactone (GBL) as the solvent at
150°C with an oxygen balloon. First, the catalytic activity of
Co(OH)x supported on different metal oxides for the reaction
were investigated (Table 1, entry 1-6). The results showed that
the catalytic activity of the catalysts varied greatly.
Co(OH)x/CeO2, Co(OH)x/MgO and Co(OH)x/ZnO had better
catalytic activity. Co(OH)x/ZnO afforded the best performance,
and the yield of benzoic acid was 85%. 2-Phenylacetophenone
2
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Co(OH)x
δCo-O
Co(OH)x/ZnO
Figure 1. (a) TEM; (b) HRTEM; (c) HAADF-STEM image and elemental
ZnO
mapping of Co(OH)x/ZnO.
δO-H
δO-H
v
O-H
(112), (201) and (202) lattice planes of ZnO respectively. No
characteristic peaks of Co(OH)x were observed in the XRD
spectra pattern of all catalysts, because the content of Co(OH)x
was too low and evenly distributed, thus most diffraction peaks
were covered.
δZn-O
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-1)
As shown in Table 1, Co(OH)x/CeO2, Co(OH)x/MgO and
Co(OH)x/ZnO had better catalytic activity, and Co(OH)x/ZnO
afforded the best performance. To further investigate the
reasons, we investigated the morphologies and compositions of
these three catalysts by HRTEM, HAADF-STEM and elemental
mapping.
Figure 2. FTIR profiles of Co(OH)x, ZnO and Co(OH)x/ZnO.
The specific surface area and pore structure of a series of
catalysts were determined by the N2 adsorption-desorption
experiments at 77.2 K (Figure S4). The adsorption isotherm
curves of catalysts showed type IV isotherms with H4 hysteresis
loops between P/P0 0.5-1.0, and there were no obvious
saturation adsorption platforms, which were characteristic of
mesoporous structure. Thus, BET specific surface area, pore
size and pore volume were obtained (Table S1). The BET
specific surface area, pore diameter and pore volume of
Co(OH)x/ZnO were 10.7 m2 g-1, 20.4 nm and 0.06 cm3 g-1
respectively.
The actual cobalt loading on these catalysts were measured
by ICP-OES (Table 2), in which the actual loading of Co on ZnO
(2.5 wt%) was consistent with the theoretical value (2.5 wt%).
The actual loadings of Co on Al2O3 (2.3 wt%) and CeO2 (2.0 wt%)
were slightly lower than the theoretical value. As for TiO2 (1.2
wt%), MnO2 (1.5 wt%) and MgO (1.8 wt%), these values were
further less than 2.5 wt%, so their activities were relatively poor.
In addition, these results were consistent with the TEM results
(Figure 1 and Figure S3).
The characteristic bonds of Co(OH)x/ZnO were further
analyzed by FTIR spectrum. The spectra of Co(OH)x, ZnO and
Co(OH)x/ZnO (Figure 2) all exhibited stretching/bending vibration
absorption peaks of O-H bond, which proved that they were all
oxide hydrates. Among them, the intensive peaks at 3437 cm-1,
1625 cm-1 and 1400 cm-1 were the stretching vibration absorption
peaks of hydroxyl O-H in crystal water or hydroxide, the bending
vibration absorption peaks of O-H bond and the characteristic
peaks of bridged hydroxyl groups, respectively. The reason for
the existence of hydroxyl groups on ZnO was that it absorbed
water in air at room temperature. And most of the water were
disintegrated into hydroxyl groups. Macroscopically, it was
believed that oxides were converted into hydroxides. Meanwhile,
the peaks at 496 cm-1 and 438 cm-1 were the characteristic
absorption peaks of Zn-O bond, and the peak at 583 cm-1 was
the characteristic absorption peak of Co-O bond. These three
characteristic peaks were observed on the spectra of
Co(OH)x/ZnO, while the intensity of the characteristic peaks of
Co-O was very weak, because the loading of Co(OH)x was small
and evenly dispersed.
TEM image of Co(OH)x/ZnO showed that cotton-like Co(OH)x
was coated on hexagonal ZnO, and there were highly ordered
crystal faces on the ZnO grains (Figure 1a). The distance
between adjacent lattice fringes was about 0.235 nm,
corresponding to the (101) crystal face of ZnO (Figure 1b), which
was consistent with the analysis results of XRD (Figure S1d).
Energy dispersive X-ray (EDX) spectra showed the presence of
Zn, O, and Co (Figure S2). The elemental mapping analysis
confirmed that Co element uniformly dispersed on ZnO, which
may be a reason for its good catalytic performance (Figure 1c).
Based on previous report[41], during the synthesis of catalysts,
higher concentration of OH− can limit the aggregation of CoCl2
molecules, thus making the Co(OH)x loaded on metal oxides
relatively dispersed. By contrast, Co elements of Co(OH)x/CeO2
and Co(OH)x/MgO were relatively concentrated, indicating that
Co(OH)x aggregated in some regions (Figure S3d-e). Co(OH)x
supported on Al2O3 and MnO2 aggregated to a large extent as
well, while Co(OH)x was rarely seen on TiO2 (Figure S3a-c). In
addition, the size of ZnO was relatively larger than other carriers,
and it was one of the possible reasons for the uniform dispersion
of Co(OH)x.
Table 2. Properties of catalysts.
Entry
Catalyst
Co (wt%) [a]
Co3+/Co2+ [b]
1
2
3
4
5
6
Co(OH)x/Al2O3
Co(OH)x/TiO2
Co(OH)x/CeO2
Co(OH)x/ZnO
Co(OH)x/MnO2
Co(OH)x/MgO
2.3
1.2
2.0
2.5
1.5
1.8
1.7
1.0
2.0
3.0
1.4
2.3
[a] Determined by ICP-OES. [b] Determined by XPS spectra.
3
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The elemental composition and surface chemical state of the
catalyst were obtained by XPS analysis (Figure 3 and Figure S5).
Co and O existed on the surface of Co(OH)x (Figure S5a-b). The
Co 2p spectrum of Co(OH)x had two main peaks at 780 eV and
795 eV, and its spin orbits split into 15 eV, corresponding to Co
2p3/2 and Co 2p1/2, respectively. Furthermore, the
corresponding satellite peaks (written as Sat) were at 790 eV and
805 eV, which were characteristic of cobalt oxide (Figure S9a).
The deconvolution of Co 2p3/2 peak showed that both Co2+
(781.4 eV) and Co3+ (780.1 eV) existed, and the relative ratio of
Co3+/Co2+ was 1. The XPS spectra of Co(OH)x/ZnO indicated the
presence of Zn (34.8%), O (42.9%) and Co (5.3%) on the surface
(Figure 3a), which was consistent with the EDX spectroscopy
(Figure S2). Similar to Co(OH)x, there were low-energy band at
779.9 eV (Co 2p3/2) and high-energy band at 794.9 eV (Co
2p1/2), and its spin orbits split into 15 eV, the corresponding
satellite peaks were at 789.6 eV and 804.5 eV (Figure 3b). The
deconvolution of Co 2p3/2 peak showed that Co2+ (781.7eV) and
Co3+ (779.9 eV) existed, and the relative ratio of Co3+/Co2+ was
3. The oxidation of Co3+ is higher than that of Co2+, which may
be one of the reasons for the better catalytic activity of
Co(OH)x/ZnO. The XPS spectra and parameters of other
catalysts were shown below (Figure 3c-g). The deconvolution
results showed that the order of Co3+/Co2+ was basically
consistent with the activity of the catalyst (Table 2).
(a)
Co(OH)x/ZnO
1200
1000
800
600
400
200
0
Binding energy/eV
Co 2p3/2
(b)
(c)
Co(OH)x/Al2O3
Co 2p
Co(OH)x/TiO2
Co 2p
Co 2p3/2
Co2+
Co3+ Sat
Co3+
Co3+
Co2+
Co3+ Sat
Co2+ Sat
Co 2p1/2
Co 2p1/2
Sat
Sat
815 810 805 800 795 790 785 780 775 770
815 810 805 800 795 790 785 780 775 770
Binding energy/eV
Binding energy/eV
(e)
Co(OH)x/ZnO
Co 2p
Co 2p3/2
Co3+
(d)
Co(OH)x/CeO2
Co 2p
Co 2p3/2
Co3+
Co 2p1/2
Co 2p1/2
Co2+
Co2+
Co2+ Sat
Co2+ Sat
Co3+ Sat
Co3+ Sat
Sat
We checked the H2-TPR profiles of catalysts to study the
metal-support interaction and the oxidizing ability of Co(OH)x
(Figure S6). As shown in figure 4, there were small hydrogen
desorption peaks in the low temperature region of the H2-TPR
curves of all catalysts, which may be due to the adsorption of
hydrogen at low temperature and then H2 desorbed when the
temperature rose. In the hydrogen-containing atmosphere, the
process of the reduction of CoO(OH) is generally divided into
three steps (Formula 1-3). First, CoO(OH) (Co3+) is reduced to
Co3O4 (Co3+/Co2+), and then Co3O4 is reduced to CoO (Co2+) as
the temperature rises further. Finally, CoO (Co2+) is reduced to
Co0. The reduction of Co3O4 includes the last two steps (Formula
2-3), and the return reduction of CoO only has one step (Formula
3). In addition, as for the same reduction process, the positions
of reduction peaks in the H2-TPR of CoO(OH), Co3O4 and CoO
moved to the high temperature region in turn.[42] As mentioned
before, the existence of hydrogen desorption peak in the lower
temperature region in the H2-TPR of these catalysts may cover
part of reduction peak in the low temperature region
(CoO(OH)→Co3O4). In addition, the tail peak in the temperature
region above 500°C is usually the reduction peak of the carrier.
There is a certain interaction between cobalt species and carrier,
and their respective reduction peaks may overlap, which even
inhibits the reduction of cobalt species to promote the reduction
of the carrier. In general, the greater the H2 consumption, the
greater the reduction degree of the carrier.[43]
815 810 805 800 795 790 785 780 775 770
815 810 805 800 795 790 785 780 775 770
Binding energy/eV
Binding energy/eV
Co 2p3/2
(f)
Co(OH)x/MnO2
Co 2p
(g)
Co(OH)x/MgO
Co 2p
Co 2p3/2
Co3+
Co3+
Co2+
Co3+ Sat
Co 2p1/2
Co2+
Sat
Co 2p1/2
Co2+ Sat
Co3+ Sat
Co2+ Sat
Sat
815
810
805
800
795
790
785
780
775
815 810 805 800 795 790 785 780 775 770
Binding energy/eV
Binding energy/eV
Figure 3. XPS spectra of the catalysts: (a) survey spectra of Co(OH)x/ZnO; (b)
Co 2p of Co(OH)x/Al2O3; (c) Co 2p of Co(OH)x/TiO2; (d) Co 2p of
Co(OH)x/CeO2; (e) Co 2p of Co(OH)x/ZnO; (f) Co 2p of Co(OH)x/MnO2; (g) Co
2p of Co(OH)x/MgO.
The order of total hydrogen consumption of catalysts was
Co(OH)x/MnO2>>Co(OH)x/CeO2≈Co(OH)x/ZnO>Co(OH)x/TiO2>
Co(OH)x/Al2O3>Co(OH)x/MgO (Figure S6). H2 consumption of
Co(OH)x/MnO2 was significantly larger than that of others, and
four reduction peaks occurred in the H2-TPR profiles suggested
that Co(OH)x and the carrier MnO2 were reduced at the same
time. The corresponding temperatures of the first two reduction
peaks were high, indicating that Co(OH)x has poor reducibility at
low temperature (Figure 4e). The H2-TPR curves of
Co(OH)x/CeO2 and Co(OH)x/ZnO (Figure 4c and 4d) mainly
showed two reduction single peaks below 500°C , corresponding
to the process of Co3+→Co2+ and Co2+→Co0 respectively. The
reduction peak was in the lower temperature region and its
intensity was larger, which indicated that the catalyst has good
low temperature reduction and there were more cobalt species.
6CoO(OH) + H2 → 2Co3O4 + 4H2O
Co3O4 + H2 →3CoO + H2O
CoO + H2 → Co + H2O
(Formula 1)
(Formula 2)
(Formula 3)
4
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(a)
(b)
(a)
(b)
Co(OH)x/Al2O3
Co(OH)x/TiO2
Co(OH)x/Al2O3
Co(OH)x/TiO2
Co2+→Co0
Co3+→Co2+
Co2+→Co0
Co3+→Co2+
100
150
200
250
300
350
400
450
500
Temperature/℃350
100
200
300
400
500
600
700
800
800
800
50
100 150 200 250 300
400 450 500
100
200
300
400
500
600
700
800
800
800
Temperature (℃)
Temperature (℃)
Co(OH)x/ZnO
Temperature/℃
(c)
(d)
Co(OH)x/CeO2
Co(OH)x/ZnO
Co2+→Co0
(c)
(d)
Co(OH)x/CeO2
Co2+→Co0
Co3+→Co2+
Co3+→Co2+
100
150
200
250
300
350
400
450
500
100
150
200
250
300
350
400
450
500
100
200
300
400
500
600
700
100
200
300
400
500
600
700
Temperature/℃
Temperature/℃
Temperature (℃)
Temperature (℃)
(e)
Co(OH)x/MnO2
(f)
Co(OH)x/MgO
(e)
Co(OH)x/MnO2
(f)
Co(OH)x/MgO
Co2+→Co0
Co3+→Co2+
Co2+→Co0
Co3+→Co2+
100
150
200
250
300
350
400
450
500
100
150
200
250
300
350
400
450
500
100
200
300
400
500
600
700
100
200
300
400
500
600
700
Temperature/℃
Temperature/℃
Temperature (℃)
Temperature (℃)
Physical
absorption
Weak
acid sites
Medium strong
acid site
Strong
acid sites
Co2+→Co0
Co3+→Co2+
Decomposition of carrier
Figure 4. H2-TPR profiles of catalysts: (a) Co(OH)x/Al2O3, (b)
Co(OH)x/TiO2, (c) Co(OH)x/CeO2, (d) Co(OH)x/ZnO, (e) Co(OH)x/MnO2,
(f) Co(OH)x/MgO.
Figure 5. NH3-TPD profiles of the catalysts: (a) Co(OH)x/Al2O3; (b)
Co(OH)x/TiO2; (c) Co(OH)x/CeO2; (d) Co(OH)x/ZnO; (e) Co(OH)x/MnO2; (f)
Co(OH)x/MgO.
There were also two overlapping reduction peaks of
Co(OH)x/Al2O3 and Co(OH)x/MgO (Figure 4a and 4f) in the lower
temperature region, indicating that the lower temperature
reduction capacity of Co(OH)x were also good, but the lower
reduction peak intensity indicated that the content of Co(OH)x
were less. The reduction peak of Co(OH)x/TiO2 (Figure 4b)
moved to the high temperature region, indicating that valence
state of most cobalt was +2, which was consistent with the
analysis result of the XPS (Figure 3c and Entry 2 of Table 2).
Based on the H2-TPR results above, the trends of the initial
temperature and intensity of H2 consumption of these catalysts
were basically consistent with the order of catalytic activity.
The concentration and distribution of acid sites on the surface
of these catalysts were measured by NH3-TPD (Figure 5). It has
been reported that the alkaline system can reduce the redox
potential and promote the oxidation reaction. The desorption
curve of ammonia could be divided into four zones as below
150°C, 150-250°C, 250-400°C and 400-500°C, which
correspond to the physical adsorption zone, weak acid zone,
medium acid zone and strong acid zone in sequence.
Co(OH)x/Al2O3 and Co(OH)x/TiO2 had large physical adsorption
peaks, which were due to their large BET specific surface area
(Table S1). In addition, the total acid amount of Co(OH)x/Al2O3
and Co(OH)x/TiO2 were obviously higher than Co(OH)x/CeO2,
Co(OH)x/ZnO, Co(OH)x/MnO2 and Co(OH)x/MgO, and the order
of alkalinity probably reversed. The higher total acid amount of
Co(OH)x/Al2O3 and Co(OH)x/TiO2 could be derived from their
higher BET specific surface area that allowed to exposed more
accessible acid site on the surface of catalyst. According to
previous reports, the acid system was not conducive to oxidation
reaction, and the above results were consistent with this
conclusion.
Based on above results, we used Co(OH)x/ZnO as catalyst to
study the influence of reaction temperature on the oxidative
cleavage reaction of C(OH)-C bond (Figure 6). The results
showed that the effect of temperature on the reaction was
significant. The reaction did not work at 120°C. When the
temperature was raised to 130°C, the conversion of the substrate
reached 66%, and the substrate was almost completely
transformed when the temperature raised to 140°C, which
indicated that the C(OH)-C bond cleavage required a large
amount of energy. In addition, in the range of 130-150°C, with
the increase of temperature, the main product gradually changed
from 2-Phenylacetophenone to benzoic acid, while the yield of
benzaldehyde was always very low even zero, and the yield of
benzoyl was basically unchanged. When the temperature was
higher than 150°C, the yield of benzoic acid only increased
slightly.
The effect of solvent on the reaction was studied (Figure 7).
The conversion of the substrate in the polar aprotic solvent
DMSO was very low, and only produced benzoic acid and 2-
Phenylacetophenone with 4% yield. The reason was that the
sulfoxide group of DMSO may be oxidized into sulfone, which
was in competition with the target reaction. The conversion of the
substrate in DMA (aprotic polar solvent) and homopolytoluene
5
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10.1002/cctc.202100950
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(non-polar solvent) were higher, but the carbon yields were
unbalanced. In the case of DMA, the amide groups in the
structure might affect the coordination of metals. For alkyl-
substituted aromatic compound triethylbenzene, the poor
solubility for the substrate may lead to low reactivity. In addition,
the yield of benzaldehyde was considerable, indicating that the
existence of side reaction was not conducive to the further
oxidation of benzaldehyde into benzoic acid. The reaction result
of substrate in non-polar solvent GBL was best. The hydrogen
bond formed between GBL and alcohol resulted in the reduction
of the bond energy of carbonyl group (C=O) in GBL and (C(OH)-
C) in substrate, which was conducive to the oxidative cleavage
of C(OH)-C. It has been reported that the nucleophilic interaction
between carbonyl and hydroxyl ions makes C-C bond was more
prone to cleave.[41] In addition, the good solubility of oxygen in
non-polar solvents also contributed to the reaction.[44]
2a
3a
5a
4a
Conversion
100
80
60
40
20
0
100
80
60
40
20
0
1
2
3
4
5
6
7
8
9
10 11 12
Time/h
2a
4a
100
80
60
40
20
0
3a
5a
100
80
60
40
20
0
Figure 8. Products evolution in conversion of 1a. Reaction conditions:
diphenylethanol (1a) (0.05 mmol), Co(OH)x/ZnO (10 mg), GBL (1 mL), O2
balloon, 150°C.
Conversion
In order to further clarify the variation of product distribution
over the reaction time, we conducted kinetic experiment (Figure
8). The substrate completely transformed within 3 h. With the
reaction time increased, the yield of benzoic acid continued to
increase. The yield of 2-Phenylacetophenone decreased
continuously. Benzil increased slightly in the first 6 h, and then
decreased to basically unchanged. The yield of benzaldehyde
was always close to zero. This result evidenced that 2-
Phenylacetophenone, benzil and benzaldehyde might be the
intermediates of the reaction, and gradually convert to benzoic
acid. As for benzil, its yield did not decrease significantly, we
speculated that partial benzil conversed quickly to
benzaldehyde/ benzoic acid, as soon as it was formed.
120
130
140
150
160
170
Temperature/℃
Figure 6. Effect of the temperature. Reaction conditions: diphenylethanol (1a)
(0.05 mmol), Co(OH)x/ZnO (10 mg), GBL (1 mL), O2 balloon, 12 h.
The most attractive merit of the heterogeneous catalyst is easy
to be separated from the reaction system and can be reused.
Using diphenylethanol (1a) as the substrate, the heterogeneous
and recyclable properties of Co(OH)x/ZnO were verified by
experiments. The catalyst was separated by centrifugation after
the initial reaction, and the cobalt species in the reaction solution
were analyzed by ICP-OES. The result showed that the loading
cobalt of Co(OH)x/ZnO decreased from 2.5 to 2.45 wt% after
each cycle. The value of the decrease of cobalt loading is
negligible in heterogeneous catalysis. To further examine the
catalytic system was heterogeneous, we carried out a hot-
filtration test under the general condition (1a 0.05 mmol, GBL 1
mL, Co(OH)x/ZnO 10 mg, O2 balloon, 150°C) (Scheme S1). We
filtered the catalyst from the reaction solution after 45 min, and
detected the cobalt content in the reaction solution by ICP-OES.
It was calculated that the cobalt content in the filtrate was 1.3 ×
10-5 mg, which was equivalent that the cobalt loading on the
Co(OH)x/ZnO reduced by 0.0052 wt%. After the catalyst was
filtered out and continued to react for 12 h, the reaction did not
completely stop and the distribution of products varied slightly.
To test whether homogeneous cobalt has catalytic capacity, we
used homogeneous cobalt Co(Cl)2 as catalyst to carry out the
oxidative cleavage reaction of 1a (1a 0.05 mmol, GBL 1 mL,
Co(Cl)2 10 mg, O2 balloon, 150°C, 12 h) (Scheme S2). The
reaction result was very poor even though the amount of cobalt
was much larger than that of Co(OH)x/ZnO. It can be seen that
the catalytic capacity of homogeneous cobalt was much worse
100
80
60
40
20
0
100
80
60
40
20
0
2a
4a
3a
5a
Conversion
DMA
DMSO
triethylbenzene
GBL
Solvent
Figure 7. Effect of the solvent. Reaction conditions: diphenylethanol (1a) (0.05
mmol), Co(OH)x/ZnO (10 mg), solvent (1 mL), O2 balloon, 150°C, 12 h.
6
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10.1002/cctc.202100950
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than that of supported-cobalt Co(OH)x. This further indicated that
the properties of the supported-cobalt Co(OH)x and its
synergistic effect with the carrier ZnO further enhanced the
catalytic oxidation capacity of the catalyst. The catalyst could be
reused by simple treatment with aqueous solution of sodium
hydroxide. Recycle experiment showed that the conversion of
the substrate only decreased nearly 10%, but the selectivity of
products essentially unchanged (Figure 9). Furthermore, the
composition and structure of the regenerated Co(OH)x/ZnO
catalyst was characterized with TEM, FTIR and XPS (Figure 10).
The TEM images of the regenerated Co(OH)x/ZnO indicated that
the Co(OH)x was still uniformly dispersed on the ZnO. The FTIR
and XPS spectrums of the regenerated Co(OH)x/ZnO was similar
to fresh Co(OH)x/ZnO. These results demonstrated that the high
activity of the Co(OH)x/ZnO was stable and reusable, and the
catalytic process was truly heterogeneous.
Table 3. Substrate Scope of C(OH)-C Bond Cleavage. [a]
Entry
Substrate
Product
Yield[b]
1
85%[c]
3a
3a
1a
1b
2
3
87%
90%
2c
3a
3a
4a
5a
Conversion
100
80
60
40
20
0
100
80
60
40
20
0
1c
1d
4
5
81%
82%
2c
2f
1e
1f
6
7
68% [c]
83%
41%
99%
88%
99%
0
1
2
3
Catalyst recycle
Figure 9. (a) Recycling of the Co(OH)x/ZnO catalyst in the oxidative cleavage
of diphenylethanone. Reaction conditions: diphenylethanol (1a) (0.05 mmol),
Co(OH)x/ZnO (10 mg), GBL (1 mL), O2 balloon, 12 h.
3a
2h
3a
3a
3a
1g
1h
(a)
Co(OH)x/ZnO (fresh)
Co(OH)x/ZnO (regenerated)
8
9
1i
1j
500 nm
200 nm
10
11
Co 2p3/2
Co3+
(b)
(c)
Co 2p
Co(OH)x/ZnO (fresh)
Co2+ Sat
Co 2p1/2
Co2+
Co3+ Sat
Sat
Co(OH)x/ZnO (fresh)
Co 2p3/2
Co3+
Co(OH)x/ZnO (regenerated)
Co2+ Sat
Co2+
Co3+ Sat
Co 2p1/2
1k
Sat
Co(OH)x/ZnO (regenerated)
v
O-H
3000
δO-H
δZn-O
1000 500
4000
3500
2500
2000
1500
810
805
800
795
790
785
780
775
12
72%
Wavenumber (cm-1
)
Binding energy/eV
1l
2l
Figure 10. (a) TEM images, (b) FTIR profiles, (c) XPS spectra (Co 2p) of fresh
and regenerated Co(OH)x/ZnO.
[a] Reaction conditions: substrate (0.05 mmol), Co(OH)x/ZnO(10 mg), GBL (1
mL), O2 balloon, 150°C, 12 h. [b] The results were isolated yields. [c] The yield
was detected by GC.
7
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10.1002/cctc.202100950
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Under optimized reaction conditions, the range of alcohols was
further expanded (Table 3). The C(OH)-C bond of various
alcohols were cleaved successfully in this catalytic system. The
activity of the aromatic substituted 1,2-diols 1b, 1c, 1d and 1e
(benzopinacol) were high and obtained cleavage products
whose yields more than 80%. 1,2-diol substituted by aliphicyclic
group 1f gave corresponding the cleavage products with 68%
yield. The yield of benzoic acid obtained by α-hydroxy ketone 1g
was also more than 80%, but 1h which contained thiophene ring
rather than benzene ring only gave 41% corresponding acid. The
reactivity of α-hydroxy acid 1i was high, while α-carbonyl acid 1j
had a bit poor activity. 1k and 1l are the lignin β‑O‑4 model
compounds, 1k was almost quantitatively converted to benzoic
acid, and the 1l containing methoxy group on the left benzene
ring was completely converted, but only 72% of 4-
methoxybenzoic acid was obtained. It seems that the electron
properties of substituents on the benzene ring had a great effect
on the reactivity of C-C bond cleavage. Substrates containing the
electron-donating group (methoxy group) on the benzene ring
were less reactive. It was reasonable because previous reports
showed that substrate with electron-withdrawing groups could be
converted into corresponding acids in higher yields compared
with those bearing electron-donating groups.[26,27] Furthermore,
previous studies showed that oxidation reaction used the cobalt-
based catalyst often occurred oxidative coupling or overoxidation
of phenolic products.[45]
Conclusion
In summary, we used the Co(OH)x/metal oxides to catalyze the
oxidative cleavage of the C(OH)-C bond in alcohol with clean
molecular oxygen as oxidant in absence of any additives.
Characterizations of catalysts including XRD, HRTEM, HAADF-
STEM, N2 physico-adsorption characterization, ICP-OES, FTIR,
XPS and NH3-TPD, and only a small amount of metal leached
after reaction. Importantly, this work provided a possible catalytic
system for the conversion of biomass resources into high value-
added chemicals.
Experimental Section
Synthesis of catalyst
Preparation of the supported Co(OH)x catalysts: The supported
Co(OH)x catalysts were prepared by the reaction of metallic oxide with
CoCl2 in aqueous medium followed by the treatment with NaOH. The
powder metallic oxide (800 mg) pretreated at 550°C for 3 h was
vigorously stirred with 60 mL of an aqueous solution of CoCl2 (5.7 mM) at
room temperature. After 15 min, the pH of the solution was adjusted to
13.2 by addition of an aqueous solution of NaOH (1.0 M) and the resulting
slurry was stirred for 24 h. The solid was then filtered off, washed with a
large amount of water (18.0 MΩcm), and dried in vacuo at 100°C to afford
supported ruthenium hydroxide catalysts. The cobalt contents in
Co(OH)x/Al2O3,
Co(OH)x/TiO2,
Co(OH)x/CeO2,
Co(OH)x/MnO2,
Oxidative cleavage reaction of C(OH)-C bond in alcohols using
cobalt-based catalyst was promising. A series of controlled
experiments were carried out in order to understand the reaction
mechanism (Figure S7). Benzaldehyde, 2-Phenylacetophenone
and benzoin were all completely transformed in the catalytic
system, indicating that all three might be intermediates (Figure
S7). The above experimental results showed that 85% benzoic
acid was generated by bond breaking of 1a under standard
conditions, while benzaldehyde was completely converted to
benzoic acid, indicating that 1a did not directly produce
benzaldehyde or benzoic acid by Cα-Cβ bond cleavage, but other
reaction intermediates existed. Previous studies pointed out that
ketone was the first intermediate formed in the oxidation process
of benzyl alcohol[45], and it was easy to cleave because of its high
chemical activity and the lack of hydrogen bond formation[24]. 2-
Phenylacetophenone was converted into 86% of benzoic acid
and 13% of benzoyl. Combined with kinetic experiments, it was
considered that 2-Phenylacetophenone was the intermediate in
the oxidation cleavage of C(OH)-C reaction. 76% benzoic acid
was produced by benzoyl, which indicated it may be an
intermediate in this reaction route. In addition, we speculated that
the catalyst activated (oxidizes) the Cβ-H bond of 1a.
Based on the results of the controlled experiments and
previous work, we deduced a possible process of the oxidative
cleavage reaction of 1a catalyzed by Co(OH)x/ZnO, and detailed
discussions were given in the Supporting Information (Scheme
S3). The similarly possible reaction pathway of Co(OH)x/ZnO-
catalyzed the oxidative cleavage reaction of 1,2-diols, α-hydroxy
acid, α-carbonyl acid and lignin β‑O‑4 model compounds and
detailed discussions were given in the Supporting Information
(Scheme S4-S6).
Co(OH)x/MgO, and Co(OH)x/ZnO were 2.5wt%. The preparation of
cobalt hydroxide supported by V2O5 catalyst was unsuccessful, because
V2O5 supports were somewhat soluble in the aqueous solution of NaOH.
Preparation of Co(OH)x: The pH of an aqueous solution of CoCl2 (5.7
mM) was adjusted to 13.2 by addition of an aqueous solution of NaOH
(1.0 M) and the resulting solution was stirred at room temperature for 24
h. The solid was then filtered off, washed with a large amount of water,
and dried in vacuo to afford Co(OH)x.
General procedure for oxidative cleavage reaction
In a typical experiment, 0.05 mmol of substrate, 10 mg of Co(OH)x/ZnO
and 1 mL of γ-butyrolactone were added to a 10 mL Shrek tube, and an
O2 balloon was inserted into the tube. The mixture reacted at 150°C for
12 h for a certain time, during which the mixture was continuously stirred.
After the reaction, the product was analyzed qualitatively and
quantitatively.
Catalyst characterization
Catalysts were characterized by XRD, HRTEM, HAADF-STEM, BET
surface area analysis, ICP-OES, FTIR, XPS, and NH3-TPD. Relevant
measurements and data are provided in part I of the Supporting
Information.
Acknowledgements
This work was supported by the National Key R&D Program of
China (2018YFB1501604), the Strategic Priority Research
Program of the CAS (XDA21060101) and the National Natural
Science Foundation of China (21875239, 21732006, 51821006,
51961135104). The authors thank the Anhui Kemi Machinery
Technology Co., Ltd. for equipment that benefited our ability to
conduct this study.
8
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Entry for the Table of Contents
The catalyst of cobalt oxide hydrate (Co(OH)x) supported on metal oxides was prepared. And the oxidative cleavage of C(OH)-C bond
in alcohol was catalysed by Co(OH)x/metal oxides using molecular oxygen as the oxidant, and the corresponding product was obtained
with high selectivity.
10
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