Design and synthesis of a highly efficient heterogeneous MnCo2O4 oxide catalyst for alcohol oxidation: DFT insight into the synergistic effect between oxygen deficiencies and bimetal species

Article abstract of DOI:10.1039/c8cy02248h

The synergistic effect in multi-active site catalysts is difficult to monitor, and the effect of their intrinsic mechanism on their catalytic performance is important but very difficult to understand owing to the large quantities of active species. Based on the results of density functional theory, herein we report the design and synthesis of an oxygen vacancy-abundant spinel-structured MnCo₂O₄ oxide as a highly efficient catalyst for alcohol oxidation, which highlights the importance of the synergistic effect between oxygen deficiencies and bimetal species.

Full text of DOI:10.1039/c8cy02248h

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Design and synthesis of a highly efficient
heterogeneous MnCo O oxide catalyst for
2
4
Cite this: DOI: 10.1039/c8cy02248h
alcohol oxidation: DFT insight into the synergistic
effect between oxygen deficiencies and bimetal
species†‡
Dandan Li,§ Fei Ruan,§ Yangxin Jin, Qingping Ke, *ab Yali Cao, Hao Wang,
ab
ab
ab
a
a
a
a
b
Tingting Wang, Yujun Song and Ping Cui*
Received 30th October 2018,
Accepted 8th December 2018
The synergistic effect in multi-active site catalysts is difficult to monitor, and the effect of their intrinsic mecha-
nism on their catalytic performance is important but very difficult to understand owing to the large quantities of
active species. Based on the results of density functional theory, herein we report the design and synthesis of
DOI: 10.1039/c8cy02248h
2 4
an oxygen vacancy-abundant spinel-structured MnCo O oxide as a highly efficient catalyst for alcohol oxida-
rsc.li/catalysis
tion, which highlights the importance of the synergistic effect between oxygen deficiencies and bimetal species.
However, in most cases, additives such as (2,2,6,6-
tetramethylpiperidin-1-yl)oxyl (TEMPO) and/or explosive oxi-
1
. Introduction
Selective oxidations of alcohols to aldehydes/ketones are im-
portant reactions in organic transformations. Aldehydes and
dizing agents are essential for improving the activity and se-
lectivity of this oxidation reaction. Simultaneous enhance-
ment of the catalytic activity and selectivity in the oxidation of
alcohols, using recyclable earth-abundant metal catalysts
without additives, is therefore desirable.
1
ketones, which are traditionally obtained by selective oxida-
tion of alcohols in the presence of stoichiometric amounts of
explosive oxidizing agents and strong mineral acids, are es-
2
sential building blocks in organic synthesis. To meet the re-
3 4
Earth-abundant metal-based oxides, such as Co O and
quirements of green chemistry, homogeneous catalysts based
MnO , have been widely used as catalysts in alcohol oxida-
2
1
e,3
4
on noble metals, including palladium,
gold and plati-
tions due to their recyclable nature. Compared with simple
metal oxides, bimetal oxides may exhibit higher activity due
to the synergistic effect between the different metal active
5
num, have recently been developed for alcohol-oxidation cat-
alytic systems that are free of mineral acids and/or explosive
oxidizing agents. Because of the high costs and limited re-
serves of noble metals, non-noble metals, such as cobalt,
manganese and copper, are preferable, and have also been in-
7
7b
species. For example, Huang and co-workers
reported
MnCo catalysts as active catalytic components in the con-
2 4
O
densation of benzyl alcohol and aniline to synthesize imine.
They found that redox couples with similar MnIJIII)/MnIJII) and
CoIJIII)/CoIJII) ratios had the highest activity, probably because
they gave the best synergistic effect.
6
vestigated in this oxidation reaction. The corresponding
heterogeneous catalysts have attracted extensive attention due
to the salient advantages of ease of separation and reuse.
In addition to the synergistic effect between the metal ac-
tive species, defects are always mentioned in the literature
when the metal oxides are doped with heteroatoms. Recently,
oxygen vacancies (OVs) produced over bi-metal oxides as typi-
cal important defects were often mentioned in the oxygen
a
College of Chemistry and Chemical Engineering, Anhui University of Technology,
Hudong Road 59#, Maanshan, Anhui 243002, P. R. China.
E-mail: kqp1982@163.com, 20160065@ahut.edu.cn
b
Anhui Province Key Laboratory of Coal Clean Conversion and High Valued
Utilization, Anhui University of Technology, Hudong Road 59#, Maanshan, Anhui
8
evolution reaction but rarely reported in organic transforma-
2
43002, P. R. China. E-mail: mhgcp@126.com
tions. It is well known that adsorption on a heterogeneous
catalyst surface is commonly an essential step for obtaining
catalytic activity. In this case, using MnCo O as a bi-metal
†
Q. Ke was supported by the Anhui Provincial Science Fund for Excellent Young
Scholars (gxyqZD2018034). This work was supported by the National Science
Foundation of China (grant numbers 21476001, 21776002, 21306142). The au-
thors acknowledge Shenzhen HUASUAN Technology Co., Ltd for providing the-
ory computation support.
2
4
oxide example, we performed DFT calculations to elucidate
the effect of OVs on the adsorption process of benzyl alcohol
molecules (Fig. 1). It is found that the adsorption on the
MnCo O surface (1.13 eV, Fig. 1b) is energetically about 0.34
‡
Electronic supplementary information (ESI) available: Experimental section,
and NMR. See DOI: 10.1039/c8cy02248h
These authors are contributed equally to this work.
§
2
4
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Fig. 1 Side view of benzyl alcohol molecules adsorbed on Co
3
O
4
(a), MnCo
2
O
4
(b) and defective MnCo
2
O
4
surfaces (c). Colour code: white is
hydrogen, grey is carbon, red is oxygen, blue is cobalt, ice blue (yellow dotted frame) is manganese and transparent purple (yellow dotted frame) is
oxygen vacancy.
eV more stable than that on the Co O surface (0.77 eV,
cies was also studied by control experiments and DFT
calculations.
3
4
Fig. 1a), which indicated the presence of the synergistic effect
between the Co and Mn species. On the MnCo surface,
2 4
O
when OVs are present, the adsorption (2.96 eV, Fig. 1c) of the 2. Experimental
benzyl alcohol molecules is further enhanced. This result
2
.1. Materials synthesis
should be due to the fact that the octahedrally coordinated
3
+
Bimetal oxides were prepared by a hydrothermal method using
urea as a precipitant. As a typical example for the preparation of
pineapple-like spinel MnCo O nanorods (Co/Mn = 2 : 1), a homo-
geneous mixture was prepared by vigorously stirring cobaltIJII) ni-
trate hexahydrate (20.0 mmol), manganeseIJII) acetate tetrahydrate
Co center is more active towards the adsorption of the –OH
group near an OV on the MnCo surface, to achieve a coor-
dination number of six, than on the MnCo surface with-
out an OV, highlighting the importance of the OV.
Herein, MnCo spinel-structured catalysts are first put
2 4
O
2
4
2 4
O
2 4
O
(
20.0 mmol) and ethylene glycol (80.0 mL) at room temperature
for 20 min, then urea (80 mmol) was added into the mixture un-
der vigorous stirring to form MnCo precursors. The precursors
forward as ideal materials for investigating the synergistic ef-
fect between oxygen deficiencies and bimetal species in alco-
hol oxidations. The DFT calculations indicate that the oxygen
2 4
O
were stirred for 30 min at room temperature and transferred into
a stainless steel autoclave for another 5 h at 150 °C. After filtra-
tion, a pink powder was dried at 80 °C for 10 h. Then, the final
black catalysts of ∼2 g were obtained by calcination under an air
atmosphere at 350 °C. As a comparison, the final black catalyst
obtained by calcination under an oxygen atmosphere at 350 °C
was denoted as T-MnCo O . Before calcining, the sample was
deficiencies in MnCo
the adsorption stability and lowering the reaction energy.
MnCo catalysts with a high density of oxygen vacancies ex-
hibit excellent aldehyde selectivity (selectivities greater than
9.9% for all examples including aromatic, substituted aro-
2 4
O are of great importance in improving
2 4
O
9
matic and heterocyclic) and good to excellent yields under
mild conditions (60 °C, air). In addition, the mechanism for al-
cohol oxidation over MnCo O with different oxygen deficien-
2
4
treated in an oxygen atmosphere for 30 min with a flux of 30 mL
2
4
−1
min . Metal oxides (Co
3 4 3 4
O and Mn O ) were prepared by the
same method without adding the second metal salts.
2.2. Catalytic activity test
A typical procedure for oxidation of alcohols was as follows.
A mixture of 4-methylbenzyl alcohol (1 mmol), catalyst (150
Fig. 2 XRD (a) patterns of various bi-metal oxides, O 1s XPS spectra (b
and c), SEM (d) and TEM (e and f) images of the MnCo sample. The
EDS mapping images (e–j) of the MnCo sample shown in different
2 4
O
2
O
4
Fig. 3
2 3 4 3 4 2 4
O -TPD profiles of the synthesized Co O , Mn O , MnCo O ,
colors: blue-Co–K, green-Mn–K and red-O–K.
2 4
and T-MnCo O .
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Table 1 Oxidations of 4-methyl benzyl alcohol over the MnCo
Paper
2 4
O catalyst and other represented base-free catalytic systems
c
Entry
Catalyst
Temp./time
Conv. (%)
Sel (1a) (%)
Yield (%)
TON
1
2
3
4
5
6
7
8
9
—
60 °C/6 h
60 °C/6 h
60 °C/6 h
60 °C/6 h
60 °C/4 h
60 °C/6 h
60 °C/6 h
60 °C/6 h
60 °C/6 h
60 °C/6 h
60 °C/6 h
60 °C/6 h
60 °C/6 h
60 °C/6 h
60 °C/6 h
60 °C/6 h
130 °C/18 h
130 °C/5 h
120 °C/4 h
0
1.2
0.5
16.1
93.7
>99.9
79.4
3.6
0
0
1.2
0.5
16.1
93.7
>99.9
79.4
3.6
0.2
6.9
25.6
1.96
0.21
0.08
0.85
0.20
0
FeCo
MoCo
CuCo
MnCo
MnCo
T-MnCo
CoO
MnO
Co
Mn
MnO
CoIJNO
MnIJOAc)
PdO
CeO
Co Mn
Co-NG-750
2
O
4
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
—
0.01
0.01
0.18
1.56
1.11
0.88
0.04
∼0
2
O
4
2
O
4
2
O
O
4
2
4
a
2 4
O
0.2
6.9
10
11
12
13
14
15
16
17
18
19
3
O
4
0.08
0.28
0.02
∼0
3
O
4
25.6
1.96
0.21
0.08
0.85
0.20
—
2
3
)
2
2
∼0
0.01
∼0
2
2
3
O
8
76.0 (ref. 11)
92.4 (ref. 12)
76.6 (ref. 13)
0.42
1.85
1.92
94.8
79.4
97.5
96.5
b
CoAlIJOH)x/graphite oxide
a
Reaction conditions: catalyst (0.15 g), reactant (4-methyl benzyl alcohol, 1 mmol, 123 μL), toluene (2 mL), open to air. Catalyst was prepared
b
by calcination under an oxygen atmosphere. The reaction performed under 1 bar oxygen atmosphere, with DMF (5 mL) as the solvent.
c
−1
Turnover number given in molalcohol gCatal is based on the conversion at 4–18 h.
Fig. 4 Co 2p (a–c) and Mn 2p (d–f) XPS spectra of the MnCo
2
O
4
sample, where a and d refer to the raw MnCo
2 4
O sample; b and e refer to the
used MnCo sample under an air atmosphere; c and f refer to the used MnCo O
2
O
4
2 4
sample under a nitrogen atmosphere.
2
.3. Catalyst reusability test
The procedure for oxidation of 4-methylbenzyl alcohol (1
mmol) is the same as that in section 2.2. The MnCo cata-
mg), and toluene (2 mL) was added into a Schlenk flask. The
mixture was stirred at 60 °C for 6 h under an air atmosphere.
After reaction, the product was taken out of the mixture and
analyzed using a gas chromatograph (GC, Agilent 7890B,
USA) equipped with a flame ionization detector and an HP-5
column (30 m × 0.25 mm). The product yields are reported
using both GC yields. The products were then characterized
2 4
O
lyst (200 mg) was selected to evaluate the recyclability of the
CMO-n catalysts. The used catalyst was separated from the
1
¶
4-Methylbenzaldehyde, white oil, H NMR (400 MHz, CDCl
3
) δ 9.88 (s, 1H),
13
7
.70 (d, J = 8.1 Hz, 2H), 7.25 (d, J = 7.9 Hz, 2H), 2.36 (s, 3H). C NMR (101
1
13
by H and C NMR spectroscopy.¶
3
MHz, CDCl ) δ 191.97 (s), 145.54 (s), 134.20 (s), 129.77 (d, J = 12.0 Hz), 21.83 (s).
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Fig. 7 Recyclability test of the MnCo
-methyl benzyl alcohol to 4-methyl benzyl aldehyde (a). O
spectra of MnCo and used MnCo catalysts (b).
2
O
4
catalyst for oxidation of
4
2
-TPD
2
O
4
2 4
O
Fig. 5 Reaction energy profile for the two-step dehydrogenation pro-
cess of benzyl alcohol; the red line is on the intact MnCo
2
O
4
surface
and the black line on the defective MnCo
lows Fig. 1.
2
O
4
surface. Colour code fol-
reaction mixture by filtration, then the sample was washed
with a certain quantity of ethanol, dried at 100 °C for 6 h and
calcined at 500 °C for 4 h.
2.4. Characterization
Unless mentioned otherwise, all manipulations were
performed in an open environment. The products were char-
1
13
acterized by using H (400 MHz) and C NMR (101 MHz)
spectroscopy using CDCl3 as the solvent and tetra-
methylsilane as the internal standard. Mass spectra (MS)
were measured on a Shimadzu GCMS-QP2010 Plus spectro-
meter (Shimadzu, Kyoto, Japan). The conversion of the reac-
tant and the selectivity to the product were obtained using an
Agilent 7890B chromatograph (Agilent, USA). The mesopore
volumes and Brunauer–Emmett–Teller (BET) surface areas
were determined by using a surface area and porosity ana-
lyzer (ASAP 2020, Micromeritics Inc., USA). SEM images were
obtained with a field emission scanning electron microscope
(
NanoSEM 430, FEI, USA) operated at 15 kV. All the samples
Fig. 6 A plausible mechanism for alcohol oxidation over the MnCo
catalyst.
2
O
4
were coated with a thin layer of Au to prevent charging before
scanning. Transmission electron microscopy (TEM) images of
Table 2 Oxidation of alcohols to aldehydes
Entry
1
Product
Selectivity (%)
Yield (%)
81.7
>99.9
2
3
>99.9
>99.9
>99.9
54.6
4
5
>99.9
>99.9
77.5
96.5
2 4
Reaction conditions: MnCo O (0.15 g), reactant (1 mmol), toluene (2 mL), open to air, 6 h.
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the samples were taken on a JEM-2100 microscope, and the
electron beam accelerating voltage was 200 kV (JEOL LTD., Ja-
pan). X-ray photoelectron spectroscopy (XPS) spectra were
measured with a PHI-5000C ESCA system (Perkin-Elmer,
USA) with Al Kα radiation (hν = 1486.6 eV) for the X-ray
source; the binding energies were calibrated using C 1s as a
reference set at 284.6 eV. Temperature programmed oxida-
duced excellent yield (>99.9%) of the desired product (1a) in
toluene at 60 °C under an open atmosphere for 6 h (Table 1,
2 4
entry 6). Notably, the MnCo O catalyst gave the best catalytic
performance; it was better than those reported for
1
1
Co Mn O , Co-NG-750 (ref. 12) and Co/C-N700 (ref. 13) cata-
2
3 8
lysts (Table 1, entries 17–19) under additive-free conditions.
Meanwhile, the catalytic performance of MnCo O for this re-
2
4
tion (O -TPD) was carried out on an automated chemisorp-
tion analyzer (Quantachrome Instruments, USA).
action is much better than the corresponding metal oxides
and metal salts (Table 1, entries 8–14), confirming the exis-
tence of the synergistic effect between Co and Mn species for
promoting alcohol oxidation.
2
3
. Results and discussion
It should be noted that there was a significant difference
in the catalytic performance of MnCo O rich and poor in
3
.1. Structural and morphological characterization
2
4
A series of bimetal oxide catalysts were synthesized using
urea as a precipitant. To shed light on the role that OVs play
in alcohol oxidation, we prepared bi-metal oxides such as
OVs; the desired yield of 99.9% over MnCo O (Table 1, entry
2
4
6), rich in OVs, is obviously higher than that of 79.4% over
T-MnCo O poor in OVs (Table 1, entry 7). This result high-
2
4
2 4
MnCo O , rich or poor in OVs, by calcining the correspond-
lights the important role of OVs in spinel MnCo O oxide in
2
4
ing precipitant in an air or oxygen atmosphere, respectively.
The powder XRD patterns in Fig. 2a show that the synthe-
sized Co-based bimetal oxide catalysts are spinel-type oxides;
the corresponding heteroatom oxides are not observed for all
Co-based spinel oxides. In the O 1s XPS spectra, the three
peaks at 529.9, 530.6 and 531.6 eV were fitted according to
alcohol oxidation catalysis.
3
.3. Role of Co and Mn species
A significant difference in the desired product yields over
MnCo O was obtained when the reaction was performed un-
2
4
9
der air (yield >99.9%) or nitrogen (yield = 36.3%). We used
previous research. For MnCo O , the sample rich in OVs
2
4
XPS to probe the oxidation states of Co, Mn and O of the
2 4
(MnCo O , cal.Ovac/cal.OLatt = 1.75) was obtained through cal-
MnCo O catalysts before and after reaction under an air or
cination in an air atmosphere (Fig. 2b), while that poor in
OVs (T-MnCo O , cal.Ovac/cal.OLatt = 1.43) was achieved by cal-
2 4
nitrogen atmosphere (Fig. 4). The oxidation state of Co was
2
4
3
+
2+
estimated as a ratio of Co /Co = 1.7 in both the raw
Fig. 4a) and used MnCo O (air or nitrogen, Fig. 4b or c).
cination in an oxygen atmosphere (Fig. 2c), according to the
O 1s XPS data. The SEM images show that the as-prepared
MnCo O possesses pineapple-like morphologies (Fig. 2d).
(
2
4
2 4
Therefore, the Co species in the MnCo O catalyst should act
2
4
as reactant adsorbents but not as active sites. Meanwhile, the
oxidation state of Mn persisted at a ratio of Mn /Mn = 2.3
2 4
The TEM images show that MnCo O possesses abundant
4+
3+
mesoporous structures (Fig. 2e) and its spinel type structure
is also confirmed by TEM (Fig. 2f) based on the determined
fringe separations. Different colors are clearly observed in the
EDS mapping images: blue-Co–K, green-Mn–K and red-O–K.
The as-prepared MnCo O , rich or poor in OVs, was also
4
+
3+
(
=
Fig. 4d and e). In contrast, the reduction of Mn (Mn /Mn
1.8, Fig. 4f) was observed when MnCo was subjected to
reaction under a nitrogen atmosphere. There was an increase
2 4
O
3
+
4+
in the concentration of Mn due to the reduction of Mn to
Mn by oxidation of alcohol to aldehyde, although this cata-
lytic cycle was not completed owing to the absence of oxygen
2
4
3
+
2 2
confirmed by O -TPD experiments (Fig. 3). The O -TPD exper-
iments were carried out to investigate the oxygen species over
the synthesized Co and Mn oxides. Three oxygen desorption
(
the catalytic cycle will be provided in Fig. 6). Oxygen from
1
4
1
0
air played an important role in the composite oxides and
also the MnCo O catalyst to promote alcohol oxidation,
peaks were identified at temperatures of 80–120 °C, 120–
50 °C and above 450 °C, which are assigned to molecular ox-
ygen species adsorbed on oxygen vacancies (OVs), surface lat-
4
2 4
which should be adsorbed onto Mn species before reaction
and left as a water molecule after reacting with hydrogen
from alcohol. The results based on the DFT calculations
1
0
tice oxygen or surface adsorbed oxygen ions, and surface
and bulk phase lattice oxygen. As shown in Fig. 3, the OV de-
composition of the MnCo O sample was larger and broader
(
Fig. 1), catalytic activity tests (Table 1) and the above discus-
2
4
sion clearly stated the synergistic effect of Co and Mn species
for promoting alcohol oxidation.
(
in the range of 80–120 °C) than those of Mn
3 4
O and
T-MnCo O samples.
2
4
3
.2. Catalytic performance
3.4. Proposed mechanism
Next, we examined the catalytic performance of the as-
prepared bi-metal oxides for the oxidation of 4-methyl benzyl
alcohol as a model substrate (Table 1). The reaction does not
proceed without loading any catalyst. The desired product
DFT calculations were firstly employed to investigate the role
of OVs in alcohol oxidation. Fig. 5 shows the reaction energy
profile for benzyl alcohol oxidation on the intact MnCo O
2
4
surface (CMO) and defective MnCo O surface (D-CMO). As
2
4
(
1a) was observed over almost all the as-prepared bi-metal ox-
shown in the energy profile, the alcohol is easily adsorbed on
D-CMO compared with CMO (II). Once the molecule is
adsorbed on the surface, the hydrogen from –OH is bonded
ides (Table 1, entries 2–6). Among them, MnCo O was found
to be the best choice for alcohol oxidation catalysis, and pro-
2
4
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to oxygen defects by a hydrogen bond (III). Then, the first
step dehydrogenation process initiates the whole reaction
since the catalytic performance was maintained after eight cy-
cles (Fig. 7a). Before and after the reaction, the OV in the
(
IV): that is –OH is transferred to the neighboring O2c site. A
MnCo
terns (Fig. 7b).
2 4 2
O catalyst shows no obvious difference in O -TPD pat-
1
5
previous study showed that H on the O2c site can easily
transfer to the neighboring site and expose an active site.
The second hydrogen from the –CH
gen defects by a hydrogen bond (V). Then, the second step
dehydrogenation process occurs from the –CH – group (VI).
All of the adsorption and dehydrogenation processes are eas-
ily completed on O-CMO according to DFT calculations
2
– group is bonded to oxy- Conclusions
2 4
In summary, by taking MnCo O spinel oxides as an example,
2
we have clearly identified the importance of oxygen vacancies
in alcohol oxidation catalysts besides the synergistic effect
between (CoIJIII)/CoIJII) and MnIJIII)/MnIJIV)); these were con-
firmed by theoretical and experimental studies. The MnCo O
(Fig. 5), which indicates that the OV is of great importance in
2
4
improving the adsorption stability and lowering the reaction
energy.
Based on the above analysis and previous references,
plausible mechanism for alcohol oxidation over the MnCo
catalyst is proposed in Fig. 6. Firstly, oxygen from air is
adsorbed and activated on Mn sites (Fig. 6, a). Then, alcohol
catalyst with abundant oxygen vacancies (cal.Ovac/cal.OLatt
=
1
1
1.75) was prepared by calcination in air and showed excellent
aldehyde selectivity (>99.9%) and good-to-excellent yields for
alcohol oxidation under mild conditions (60 °C, additive-free
and air as an oxidant). Tuning the catalytic performance of
MnCo O by oxygen deficiencies and bi-metal redox couples
a
2 4
O
2
4
2 4
is diffused on MnCo O and its –OH group is adsorbed by
3
+
offers atomic-level insights into the surface oxygen–metal
species interactions toward the rational design of high-
performance spinel oxide catalysts.
Co sites (Fig. 6, b). The hydrogen of the –OH group is
bonded to oxygen vacancies through the hydrogen bond ef-
fect. This hydrogen-bond effect could decrease the O–H bond
activation barrier from 0.32 eV to 0.24 eV (Fig. 5, III), and the
Conflicts of interest
7 6
same effect over the Fe–FeO/PtIJ111) interface and Ni O /
1
6
AuIJ111) was also obtained. The first-step dehydrogenation
occurs from the –OH group and transfers to the Mn–O group.
This process leads to the abstraction of a proton and the for-
mation of an unstable metal-alkoxide species. The metal-
alkoxide species undergoes the second-step dehydrogenation
There are no conflicts to declare.
Notes and references
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Huang, H. Sun, M. Haruta, A. Wang, B. Qiao, J. Li and T.
Zhang, Angew. Chem., Int. Ed., 2018, 57, 7795; (b) R. V.
Jagadeesh, T. Stemmler, A.-E. Surkus, M. Bauer, M.-M. Pohl,
J. Radnik, K. Junge, H. Junge, A. Brückner and M. Beller,
Nat. Protoc., 2015, 10, 916; (c) G.-J. t. Brink, I. W. C. E.
Arends and R. A. Sheldon, Science, 2000, 287, 1636–1639; (d)
Y. Tu, M. Meng, Z. Sun, L. Zhang, T. Ding and T. Zhang,
Fuel Process. Technol., 2012, 93, 78–84; (e) D. I. Enache, J. K.
Edwards, P. Landon, B. Solsona-Espriu, A. F. Carley, A. A.
Herzing, M. Watanabe, C. J. Kiely, D. W. Knight and G. J.
Hutchings, Science, 2006, 311, 362–365; ( f ) S. Dabral, J. G.
Hernández, P. C. J. Kamer and C. Bolm, ChemSusChem,
(
Fig. 6, from b to c). Finally, benzaldehyde is obtained and
desorbed from the MnCo surface (Fig. 6, d), and the cata-
lytic cycle is completed by desorption of the water molecule
from the MnCo catalyst (Fig. 6, e).
2 4
O
2 4
O
3
.5. General applicability of the MnCo
Finally, sub-scopes and limitations were evaluated over the
MnCo catalyst. Functionalized alcohols underwent aerial
2 4
O catalyst
2 4
O
oxidation to the corresponding aldehydes (Table 2). Good to
excellent yields (54.6% ≥ 99.9%) were obtained for alcohols
3
containing electron-donating (–CH ) and strong electron-
withdrawing (–NO ) groups. Notably, good aldehyde yields
2
2
017, 10, 2707–2713; (g) P. Xin, J. Li, Y. Xiong, X. Wu, J.
(77.5%) were obtained from alcohol containing a sulfur-
Dong, W. Chen, Y. Wang, L. Gu, J. Luo, H. Rong, C. Chen,
Q. Peng, D. Wang and Y. Li, Angew. Chem., 2018, 130,
containing heterocycle (Table 2, entry 4) over the MnCo cat-
2 4
O
alyst. The formation of the corresponding products from alco-
hols with heterocyclic products was more difficult over the
4
732–4736; (h) H. Yu, S. Ru, G. Dai, Y. Zhai, H. Lin, S. Han
and Y. Wei, Angew. Chem., 2017, 129, 3925–3929.
reported Co
and even with palladium-based coordination complexes.
In addition to giving good to excellent aldehyde yields,
MnCo catalyzed the selective oxidation of 4-(methylthio)-
benzyl alcohol (Table 2, entry 5) to the corresponding
-(methylthio) benzaldehyde in excellent yield (96.5%), which
is industrially used as a raw material to produce sulindac.
2 3 8
Mn O (ref. 11) and Co/C-N700 (ref. 12) catalysts,
2
3
I. E. Markó, P. R. Giles, M. Tsukazaki, I. Chellé-Regnaut,
C. J. Urch and S. M. Brown, J. Am. Chem. Soc., 1997, 119,
17
1
2661–12662.
2 4
O
(a) D. R. Jensen, M. J. Schultz, J. A. Mueller and M. S.
Sigman, Angew. Chem., Int. Ed., 2003, 42, 3810–3813; (b) D.
Wang, A. B. Weinstein, P. B. White and S. S. Stahl, Chem.
Rev., 2018, 118, 2636–2679.
4
4
(a) A. Abad, P. ConcepciÓn, A. Corma and H. GarcÍa, Angew.
Chem., Int. Ed., 2005, 44, 4066–4069; (b) T. Ishida, M.
Nagaoka, T. Akita and M. Haruta, Chem. - Eur. J., 2008, 14,
8456–8460.
3
.6. Stability of the MnCo O catalyst
2 4
Further investigations on the recyclability of MnCo O verify
that the MnCo O catalyst is recyclable in alcohol oxidation,
2
4
2
4
Catal. Sci. Technol.
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