Oxidative cleavage of β-O-4 bonds in lignin model compounds with a single-atom Co catalyst

Article abstract of DOI:10.1039/c9gc00293f

Single-atom catalysts are emerging as primary catalysts for many reactions due to their 100% utilization of active metal centers leading to high catalytic efficiencies. Herein, we report the use of a single-atom Co catalyst for the oxidative cleavage of the β-O-4 bonds of lignin model compounds at a low oxygen pressure. Under the optimized reaction conditions, the conversion of 2-(2-methoxyphenoxy)-1-phenylethanol up to 95% with high selectivities was achieved with a variety of substrates investigated. The reusability of the Co catalyst with a high catalytic efficiency indicates its potential application in the oxidative cleavage of C-O bonds.

Full text of DOI:10.1039/c9gc00293f

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DOI: 10.1039/C9GC00293F
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ARTICLE
Oxidative Cleavage of β-O-4 Bonds in Lignin Model Compounds
with a Single-atom Co Catalyst
Received 00th January 20xx,
Accepted 00th January 20xx
Sijie Liu,a,b Lichen Bai, Antoine P. van Muyden, Zhangjun Huang, Xinjiang Cui, Zhaofu Fei,
c
a
a
a
a
b
c
a
Xuehui Li,* Xile Hu* and Paul J. Dyson*
DOI: 10.1039/x0xx00000x
Single-atom catalysts are emerging as the primary catalysts for many reactions due to their 100% utilization of active metal
centers leading to high catalytic efficiencies. Herein, we report the use of a single-atom Co catalyst for the oxidative
cleavage of the β-O-4 bonds of lignin model compounds at a low oxygen pressure. Under the optimized reaction
conditions conversion of 2-(2-methoxyphenoxy)-1-phenylethanol up to 95 % with high selectivities were achieved with a
variety of substrates investigated. The reusability of the Co catalyst with the high catalytic efficiency indicate its potential
application in the oxidative cleavage of C-O bonds.
www.rsc.org/
example, the catalytic transformation of 2-phenoxy-1-
phenylethanol to phenol, acetophenone and methyl benzoate was
Introduction
successfully achieved using Pd/CeO
2
as the catalyst in the presence
As the second largest component in lignocellulose, lignin is an
attractive renewable source of aromatic building blocks for the
chemical and associated industries. A number of approaches
18
of O
2
in MeOH. Recently, a two-step process has been developed
for selective oxidation of β-O-4 lignin model compounds based on
1
-3
19
VOSO
4
and copper catalysts using O
2
.
Ionic liquids also promote
4
,5
fast pyrolysis,6 hydrolysis,
5,7-8
including hydrogenolysis,
and
20
the aerobic oxidation of phenoxyacetophone, according to the
report 2-phenoxyacetophone is converted to phenol and benzoic
acid in 1-octyl-3-methylimidazolium acetate under a mild condition
9
enzymolysis have been used to obtain aromatic products from
lignin. Oxidative pathways are also appealing because they provide
access to different products such as dicarboxylic acids and
o
20
(
2
110 C, 1.5 MPa O ). Despite these impressive achievements
1
0
10
quinines, which represent key chemical building blocks. In
towards the catalytic oxidation of β-O-4 bonds, highly efficient
catalysts based on earth abundant metals that employ sustainable
oxidants could be advantageous.
1
1
oxidative reactions, the commonly used oxidants are chlorine,
hydrogen peroxide, ozone and oxygen.
oxidants, oxygen is considered to be the most sustainable since only
water is generated as a by-product.
1
2
13
14,15
Among these
1
0
Cleavage of β-O-4 type bonds in discrete molecular
compounds is frequently studied as the first step in the
development of new catalysts for the valorization of lignin. Because
it is the most abundant linkage and has a lower bond dissociation
energy than the C-C (β-5 and β-β) linkages present in lignin (Fig. 1),
1
4, 16-17
or as a direct route in the downstream processing/upgrading
1
,10
of β-O-4 linked dimeric compounds generated from lignin.
Indeed, many papers describe the catalytic cleavage of the β-O-4
linkage in lignin model compounds/lignin-derived dimers. For
1
,4
Figure 1 Representative structure of a lignin fragment with β-O-4 linkage.
a. Institute of Chemical Sciences and Engineering (ISIC), École Polytechnique
Fedérale de Lausanne (EPFL), 1015 Lausanne, Switzerland. E-mail:
paul.dyson@epfl.ch, Tel: +41 (0)21 693 98 54
A class of catalysts that are currently attracting considerable
attention are single-atom catalysts (SACs) which combine the
b.
School of Chemistry and Chemical Engineering, State Key Laboratory of Pulp and
single-site nature of homogeneous catalysis with the robustness of
Paper Engineering, South China University of Technology, Guangzhou,
Guangdong, 510640, China.
Laboratory of Inorganic Synthesis and Catalysis, Institute of Chemical Sciences
and Engineering, Ecole Polytechnique Fédérale de Lausanne (EPFL), EPFL-ISIC-
LSCI, Lausanne, CH 1015 Switzerland.
21-24
a heterogeneous catalyst support.
In general, SACs comprise
c.
mono-dispersed metal atoms stabilized by a specific support, such
as defect-rich metal oxides and heteroatom-doped carbon-based
2
1
materials. These catalysts are attractive because they exhibit close
to 100% atomic efficiency and possess well-defined active sites,
which potentially leads to high selectivities and facilitates
Electronic Supplementary Information (ESI) available: [ICP data, XRD and TEM of
other catalysts, control experiments, figures of effect on reaction time,
characterization of recycled catalyst and the NMR of the synthesized lignin model
compounds]. See DOI: 10.1039/x0xx00000x
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mechanistic understanding.21,22 In addition, SACs are comparatively
inexpensive and practical since the metal loading is much lower MgO and Co nanoparticles are not present as the material gives rise
XRD analysis of the Co-N-C catalyst confirms that crystalline
View Article Online
DOI: 10.1039/C9GC00293F
o
than that used in immobilized nanoparticle catalysts and bulk to only broad peaks (2θ=28 ) corresponding to amorphous carbon
2
2
materials.
(Fig. 3a). To gain further information on the composition and
Despite the rapid development of SACs, to the best of our oxidative state of the catalyst, XPS was performed and confirms the
knowledge, their use in the oxidative cleavage of C-O bonds has not presence of C, N, O and Co in the Co-N-C catalyst (Fig. 3b). The C 1s
been reported. Herein, we describe the application of a single atom spectrum shown in Fig. 3c, can be deconvoluted into three peaks
Co catalyst (Co-N-C) in the catalytic (oxidative) cleavage of β-O-4 and were assigned to the C in C=C double bonds (284.3 eV), C in
bonds with the expectation that the Co-N-C catalyst might show C=N double bonds (286.2 eV) and C in carboxylic groups (290.3 eV).
2
5-27
31-32
similar activity to noble metal (Pt, Pd) SACs,
but at a lower cost.
The N 1s spectrum was deconvoluted into three peaks, which
In addition, the Co-N-C might be more stable than other non-noble were assigned to pyridinic (398.4 eV), graphitic (401.2 eV) and N-
2
8-29
and hence any associated toxicity with Co oxide (402.8 eV) structures (Fig. 3d).25,26 The O 1s spectrum
metal SACs (Ni, Cu),
should be minimal. Compared to the other homogeneous and the indicates that the presence of O in the catalyst mainly involves C-O
traditional heterogeneous catalysts, the Co-N-C catalyst displayed single bonds, with a broad peak in 531.2 eV (Fig. 3e). There are also
superior activity and stability in the oxidative cleavage of model C=O bonds in the catalyst, which has a broad peak in 533.2eV (Fig.
compounds containing β-O-4 bonds.
3e). The Co 2p XPS is composed of two main peaks, i.e. Co 2p3/2 and
Co 2p1/2 at 780.7 and 796.3 eV, respectively (Fig. 3f). Combined with
two satellite peaks at 785.1 and 801.3 eV (Fig. 3f, blue line), these
3
3
Results and discussion
features imply that the Co is in the 2+ oxidation state.
Catalyst characterization
The Co-N-C catalyst was prepared by the pyrolysis of Mg(OH)
2
and
the
Co(phen)
2
(OAc)
2
(phen=1,10-phenanthroline)
complex
according to a literature procedure (see Experimental for full
details) which, according to the TEM bright field and dark field
images (Fig. 2a and b), has a porous graphic carbon structure absent
of any nanoparticles. EDX mapping was recorded in HAADF-STEM
mode corresponding to the dark field image (Fig. 2b), which showed
that the material mainly consists of homogeneously dispersed C, N,
O and Co (Fig. 2c) atoms. Spherical aberration corrected HAADF-
STEM (Fig. 2d) was used to observe the atomic resolution imaging
of Co atoms (although very small clusters cannot be ruled out). The
single Co atoms can be distinguished as well-dispersed bright dots
in the HAADF-STEM image (Fig. 2d, in the red cycle), indicative of
single-atoms. Co loadings of 3.1 to 3.4wt% was obtained by ICP-AES
2
5, 30
(Table S1), which is in accordance with previous studies.
a
b
Figure 3 XRD pattern (a) and XPS survey spectrum (b) of the Co-N-C catalyst, and
the detailed spectrum for C 1s (c), N 1s (d), O 1s (e) and Co 2p (f).
1
0 nm
100 nm
c
d
C
N
The SEM image of the Co-N-C catalyst (Fig. S1) reveals that the
material is highly porous, possibly as a consequence of etching with
2
-1
nitric acid, with a specific surface area of 322 m g determined by
Brunauer-Emmett-Teller (BET) analysis. Although several classes of
SACs have been reported, the BET analysis of these materials has
not been routinely studied. From the available reports in which the
porosity of the SACs was analysed, values range from 220 to 680
1
nm
O
Co
1
00 nm
2
-1 25,26
m g .
Oxidative cleavage of lignin model compound
The oxidative cleavage of 2-(2-methoxyphenoxy)-1-phenylethanol
Figure 2 Representative bright filed (a) and dark filed (b) TEM image, EDX
mapping images (c) and atomic resolution HAADF-STEM image (d) of the (MPP-ol) in the presence of the Co-N-C catalyst under air was
Co-N-C catalyst.
optimized as the reaction can afford guaiacol (G) and/or benzoic
2
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acid (BA) and also methyl benzoate (BM) when the reaction is increases from 78 to 95%, and the yield of G and BM increases to 59
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DOI: 10.1039/C9GC00293F
conducted in different solvents. MeOH, MeCN and H
2
O are widely and 83%, respectively (Table 1, entries 2 and 17) under the same
used as solvents with oxone or O
of lignin model compounds.
2
as oxidant in oxidative cleavage condition. Note that a conversion of 63% is obtained in the
In this work, those three solvents presence of NaOH in the absence of the Co-N-C catalyst (Table 1,
1
8-19, 34
were investigated as well using air oxygen as oxidant. Firstly, no entry 18). The positive influence of NaOH was confirmed in the
reaction was observed in the absence of catalyst in MeOH (Table 1, control experiment involving the conversion BA to BM (Table S2,
entry 1). The influence of the solvent was initially investigated in entries 5, 6), which is significantly improved in the presence of the
the presence of the Co-N-C catalyst (Table 1, entries 2-7) and MeOH base. A conversion of 37% using Co-N-C without NaOH (Table S2,
was found to give the highest conversion of MPP-ol of 78% under entry 2), and a conversion of 46% using NaOH without Co-N-C
the reaction conditions (Table 1, entry 2), with G, BM and BA (Table S2, entry 5) is boosted to 78% in the presence of both the
obtained in 45, 30 and 37%, respectively. Low conversions were catalyst and base (Table S2, entry 6). The effect of solid bases (MgO
3
8
observed in water and hexane (Table 1, entries 6-7), which is and CaO) was also investigated using MeOH as solvent and the
probably due to the low solubility of the substrate in these results are given in Table S3 (entries 2 and 3). The conversion of
3
5
solvents . The relatively higher conversion in MeOH than in other MPP-ol was 81 and 84%, respectively, compared with that using
solvents indicates that MeOH can react with the generated BA to NaOH (95%) under otherwise the same conditions (Table 1, entry
afford BM and promote the equilibrium to the desired products. To 17). The lower activity of the solid bases is probably due to less
confirm this hypothesis, the corresponding control experiments intimate interactions with the substrates.
from BA to BM in MeOH and acetonitrile were carried out in the
presence/absence of the catalyst. (Table S2). It was found that BA Table 1 Optimization of the reaction conditions. a
o
can be converted to BM in MeOH at high temperatures (150 C) with
OH
O
O
O
O
a moderate conversion of BA (32%) in the absence of catalyst (Table
S2, entry 1). Interestingly, the addition of the Co-N-C catalyst
improves the conversion only slightly (Table S2, entry 2). However,
O
Catalyst
HO
OH
O
+
+
Solvent 5ml
o
0.2 mmol
Air 1MPa, 150 C
BA
BM
G
3
when MeOH was replaced by CH CN, the formation of BM is not
observed and only trace quantities of benzamide are detected (by
GC-FID). (Table S2, entries 3-4). Thus, the MeOH is both the solvent
and a reagent in the reaction.
Single-atom catalysts based on other non-noble metals
including Mn, Ni and Cu were also prepared (see Figs. S2-S5 for
characterization) and evaluated in the reaction. However, these
catalysts are less efficient for the oxidative cleavage of MPP-ol than
the Co-N-C catalyst (Table 1, entries 8-10). The lower activity is
probably due to their poorer ability to bind and activate oxygen at a
Yield (%)b
Conv.
Entry
Catalyst
Solvent
(%)
G
BM
n.d.
30
BA
1
2
3
4
5
6
7
8
9
/
MeOH
MeOH
MeCN
DMF
2
n.d.
45
37
25
12
24
22
16
11
17
11
30
18
20
14
19
59
38
n.d.
37
n.d.
n.d.
n.d.
20
13
10
9
Co-N-C
Co-N-C
Co-N-C
Co-N-C
Co-N-C
Co-N-C
Mn-N-C
Ni-N-C
Cu-N-C
78
65
59
36
42
39
48
31
48
31
67
63
59
39
48
95
63
n.d.
n.d.
n.d.
n.d.
n.d.
13
DMSO
Hexane
2
H O
low O
experiments were also performed using homogeneous catalysts, i.e.
Co(OAc) ·4H O and Co(OAc) ·4H O with 1,10-phenanthroline, both
2
pressures (see Scheme 1 for a tentative mechanism). Control
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
8
2
2
2
2
1
0
11
14
16
35
14
11
6
of which result in lower conversions than the Co-N-C catalyst under
the same conditions (Table 1, entries 11-12). For comparison with
the Co-N-C, other commonly used oxidative heterogeneous
catalysts were also investigated. Co nanoparticles (Co NPs) were
synthesized from pure Co-containing metal-organic frameworks
according to the reported method3 and exhibited a worse activity
and selectivity than the Co-N-C catalyst (Table 1, entry 13 vs entry
1c
1
Co(OAc)
Co(OAc) /L
Co NPs
Co
CuO
CeO
2
12
2d
3e
4e
5e
6e
1
1
1
1
1
2
24
25
3
O
4
21
10
1
2
13
7
7f
8f
1
1
Co-N-C
/
83
9
2
). Note that Co nanoparticle impurities in the catalyst also weaken
the activity of Co-N-C catalyst (Fig. S6, Table S3, entry 1). Metal
oxide catalysts such as Co , CuO and CeO , which are usually used
47
9
L = 1,10-phenanthroline, n.d. = not detected.
3
O
4
2
a
Reaction conditions: catalyst (20 mg), substrate (48 mg, 0.2 mmol), solvent
5 ml), air (1 MPa), 150°C, 4 h. b Yields were determined by GC-FID.
1
0
as catalysts in oxidation of lignin model compounds , were also
investigated as catalysts for oxidative cleavage of MPP-ol using the
same catalytic conditions (Table 1, entries 14-16). Among those
(
c
O (12 mg). d Co(OAc)
Co(OAc) ·4H ·4H O (12 mg), 1,10-phenanthroline (12
2
2
2
2
mg). e Catalyst (20 mg). NaOH (0.2 mmol).
f
3 4
three catalysts, Co O shows the best activity which attains 59%
Influence of the reaction temperature and time
conversion of MPP-ol, but still less than Co-N-C (Table 1, entry 14 vs
entry 2). CuO has the worst activity in this system, which attains
only 39% conversion after 4h. Therefore, Co-N-C was selected to
the optimized catalyst for this reaction with MeOH as the solvent.
NaOH was added to improve the reaction as it induces the
esterification process and improves the cleavage of the β-O-4
In the absence of NaOH, cleavage of β-O-4 bond in MPP-ol is highly
temperature-dependent (Fig. 4a, black line). As the temperature is
raised from 110°C to 120°C, the conversion sharply increases from
4
8 to 69%, with the conversion increasing only slightly (to 78%) as
the temperature is raised from 120 to 150°C (Fig. 4a, black line).
Above 150°C the conversion decreases, presumably due to the
3
6,37
bond.
In the presence of NaOH (0.2 mmol) the conversion
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condensation of phenolic oligomers.39 In contrast, the Co-N-C indicating that elevated temperatures promote the cleVaievwaAgreticolefOβn-liOne-
catalyst is less sensitive to temperature in the presence of NaOH 4 bond as well as esterification of the BDAOI:w10it.h103M9/eCO9HG.C0In029th3eF
(Fig. 4a, red line), with a conversion of 88% observed at 110°C, presence of NaOH, however, the yield of BM increases considerably
which increases to the maximum value of 95% at 150°C.
at the expense of BA, due to the promotion of the esterification
under basic conditions.
The conversion of MPP-ol was studied as a function of time
with NaOH accelerating the reaction (Fig. S7a), as the base
facilitates the fast oxidation of MPP-ol to 2-(2-methoxyphenoxy)-1-
phenylethanone (MPP-one). The maximum yield of the three
products is observed after 4 h, with 59, 83 and 9% of G, BM and BA
obtained, respectively. The yield of BM in basic conditions reaches
stabilization within less than 1 h, further implying fast esterification
promoted by NaOH (Fig. S7b, c).
1
00
a
9
8
7
6
5
0
0
0
0
0
4
0
Table S4 lists other catalytic systems employed for the
oxidation of MPP-ol reported in recent years. Compared to these
Without NaOH
With NaOH
1
10
120
120
120
130
140
150
160
catalysts, our system shows several prominent advantages, such as
o
Temperature ( C)
18
higher activity than a Pd/CeO
2
catalyst at a lower temperature
o
o
(150 C compared to 185 C) and shorter reaction time (4 vs. 24 h).
3
4,41
Homogenous catalysts based on ruthenium and rhenium
exhibit
similar activity to the Co-N-C catalyst, but are considerably more
expensive, as are the ionic liquid-based systems employed in this
70
60
50
40
30
20
10
b
20,34
reaction.
Catalytic oxidation of other lignin model compounds
The oxidative cleavage of other compounds was evaluated
under optimized reaction conditions (Table 2). In general, the Co-N-
C catalyst exhibits good activity and yield for compounds containing
α
β-O-4 bonds with C -OH or β-O-4 ketone units (Table 2, entries 1-3),
Without NaOH
With NaOH
with conversions typically exceeding 80% (Table 2, entries 1-3). The
higher conversion obtained for 2-(2-methoxyphenoxy)-1-
phenylethanone (MPP-one) compared to MPP-ol indicates that
the MPP-ol is initially oxidized into MPP-one (see below, Scheme 1).
1
10
130
140
o
150
160
Temperature ( C)
However, with substrates that do not contain these units in the C
position, conversions are considerably lower, i.e. typically < 20%
α
90
80
70
60
50
40
30
20
10
c
(
Table 2, entry 4), as the β-O-4 alcohol is initially oxidized to β-O-4
1
9
ketone which then facilitates the cleavage of C-O bond.
BM without NaOH
BM with NaOH
BA without NaOH
BA with NaOH
Moreover, the Co-N-C catalyst does not efficiently cleave 4-O-5
Table 2, entry 5) and α-O-4 bonds (Table 2, entries 6, 7). It is known
(
that β-O-4 dimeric model compounds with more functional group
and polymeric substrates are structurally more close to true nature-
lignin. However, cleavage of the C-O bonds in these model
compounds usually requires more harsh conditions such as use of
4
2
43-44
oxygen (O
complicated dimeric model compounds 1-(3,4-dimethoxyphenyl)-2-
2-methoxyphenoxy)-propane-1,2-diol (DMPD) and guaiacylgly-
2
)
or hydrogen peroxide (H
2
O
2
).
Two more
1
10
130
140
o
150
160
Temperature ( C)
(
cerol-guaiacyl ether (GGE) were also studied (Table S5). For the
DMPD substrate, conversion cannot be detected at 150°C using air
as oxidant under otherwise same conditions used for MPP-ol (Table
S5, entry 1 vs Table 1, entry 17). By increasing the temperature, low
conversion can be observed (Table S5, entries 2-3). Similar
Figure 4 Effect of temperature on (a) 2-(2-methoxyphenoxy)-1-phenylethanol
MPP-ol) conversion, (b) guaiacol (G) yield, (c) methyl benzoate (BM) yield and
benzoic acid (BA) yield. Reaction conditions: Co-N-C (20 mg), MPP-ol (0.2 mmol),
MeOH (5 ml), air (1 MPa), 4 h.
(
The yield of
G gradually increases with temperature phenomena were observed when GGE substrate was used (Table S5,
irrespective of the NaOH additive (Fig. 4b). Control experiments entries 4-6). Only at high temperature (200°C) and by applying pure
employing G as the substance demonstrate that G is stable under oxygen, the conversion can be increased significantly for DMPD and
the reaction conditions (Table S3, entries 4 and 5). G is formed GGE to 65% and 48%, respectively (Table 2, entries 8-9). It is
through the cleavage of the C-O bond in β-O-4 model compound obvious that the use of high pressure oxygen is crucial. But even at
(Scheme 1), and higher temperatures favor cleavage of this C-O this harsher condition, the conversion of the dimers DMPD and GGE
bond. In the absence of NaOH, the yield of BM and BA increases are lower than that of the MPP-ol. The reduced efficiency for DMPD
slightly as the temperature is increased (Fig. 4c, black line),
4
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and GGE is probably due to the stronger steric hindrance in those
View Article Online
OH
O
O
O
O
O
two compounds, which usually hinder the β-O-4 cleavage in lignin.45
DOI: 10.1039/C9GC00293F
O
O
O
NaOH
O2
C-O
+
Table 2 Evaluation of the substrate scope of the Co-N-C catalyst.a
Cleavage
MPP-ol
MPP-one
G
Conv.
Entry Substrate
1
Products (% yield)b
O
O
O
O
(%)
O
O
O2
Oxidation
Co-N-C
99
93
82
O
O
8
6
1
91
86
51
O
OH
C-C
+
CO2 + H2O
OH Cleavage
H2O
O2
2
3
O
O
O
9
O
Co-N-C
O
+
H2O
Oxidation
CH3OH
Esterification
BA
BM
7
Scheme 1 Proposed reaction pathway of MPP-ol.
Recycling experiments
4
5
6
12
15
13
5
7
/
Recycling of the Co-N-C catalyst was studied under the optimized
reaction conditions (Fig. 5), demonstrating that it can be repeatedly
used with only a slight decrease in activity (the conversion
decreases from 95 to 92% over first four cycles and then to 82%
over a further four cycles). Moreover, the yield of each product
remains largely constant during the first four cycles. TEM images of
the catalyst after eight cycles are essentially the same as the
pristine Co-N-C catalyst (Fig. S8). However, the XPS surface content
of the recycled Co-N-C catalyst reveals that some cobalt has leached
from the surface (Table S6), although the oxidation states appear to
be unchanged (Fig. S9). Besides that, the catalyst was washed by
the MeOH (5×5 mL) after each run, thus the mechanical loss during
the separation also resulted in the deactivation of the catalyst.
1
1
9
6
7
14
65
/
1
1
8c
4
3
1
51
23
100
9c
48
80
60
40
20
0
3
a
Reaction conditions: Co-N-C (20 mg), NaOH (0.2 mmol), substrate (0.2
mmol), MeOH (5 ml), air (1 MPa), 150°C, 4 h. b Yields were obtained by GC-
FID. c Co-N-C (20 mg), NaOH (0.2 mmol), substrate (0.2 mmol), MeOH (5 ml),
o
O
2
(1MPa), 200 C, 10h, and the conversion was determined by HPLC.
Proposed mechanism
Based on the experimental data and previous reports36, 46-48, a
possible reaction pathway is proposed for the oxidative cleavage of
MPP-ol by the Co-N-C catalyst (Scheme 1). In the first step, the
0
1
2
3
4
5
6
7
Recycle number
Conversion
Guaiacol
Methyl Benzoate
Benzoic Acid
2
MPP-ol is oxidized to MPP-one in the presence of O and NaOH.
Then, C-O bond cleavage takes place to generate G and a radical
intermediate. In the presence of the Co-N-C catalyst, the radical
Figure 5 The recyclability Co-N-C catalyst. Reaction conditions: Co-N-C (20 mg),
NaOH (0.2 mmol), 2-(2-methoxyphenoxy)-1-phenylethanol (MPP-ol) (0.2 mmol),
intermediate can react further with oxygen to form MeOH (5 ml), air (1 MPa), 150°C, 4 h.
4
6-47
phenylglyoxal.
The C-C bond in phenylglyoxal can then be
1
8
cleaved to form BA, which subsequently reacts with methanol to
generate BM. To support this proposed mechanism, an additional
control experiment was conducted with phenylglyoxal, which is
converted exclusively to BM (Table S3, entry 6).
Conclusions
A single-atom Co-N-C catalyst was successfully applied in oxidative
cleavage of β-O-4 bond in MPP-ol and other substrates. Under the
optimize conditions (150°C, 4 h), 95% conversion of MPP-ol can be
attained and the yield of G and BM are 59 and 83%, respectively. By
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Journal Name
α
oxidation of the hydroxyl group in C , the β-O-4 bond becomes (XPS) was recorded on a PHI Versa Probe II scanning VXie-rwaAyrteiclleecOtnrloinne
easier to cleave and the combination of the Co-N-C catalyst with spectrometer with Al Kα X-ray sourced DtOhIe: 10c.u10rv3e9/Cf9itGtiCn0g02w93aFs
NaOH also accelerates the esterification of the in-situ generated BA performed by the PHI Multipak software. Inductively coupled
with MeOH. Furthermore, the Co-N-C catalyst shows good plasma-atomic emission spectroscopy (ICP-AES) was obtained by a
reusability and can be recovered readily using centrifugation and is NexIon 350 (Perkin Elmer) instrument. Before dissolving in ultra-
superior to other catalyst reported for this reaction.
pure nitric acid, all the samples were placed in muffle oven to
remove the carbon support and ensure all of the metal ions were
exposed by heated at 600 C for 6 h in air.
o
Experimental
Catalytic reaction
The catalytic reactions are performed according to reference
Materials and methods
1
8,19
methods.
In a typical reaction, the catalyst (20 mg),
2
-(2-methoxyphenoxy)-1-phenylethanol
methoxyphenoxy)-1-phenylethanone (MPP-one)
phenylethanol (PP-ol),
phenylethanol (DMP-ol) and 1-(benzyloxy)-2-methoxybenzene
(MPP-ol),
2-(2-
substrate (0.2 mmol) and MeOH (5 ml) were placed in an
autoclave and pressurized to 1 MPa with air. In some
reactions, NaOH (0.2 mmol) was added as a co-catalyst before
pressurizing with air. The autoclave was heated to the desired
temperature for the appropriate time and the contents of the
autoclave were stirred. After cooling to room temperature and
releasing the pressure, the catalyst was removed by
centrifuging (6000 rpm. 10 min.). The reaction products were
analysed on an Agilent 7890B gas chromatograph mass
spectrometer (GC-MS) equipped with an Agilent 7000C GC-MS
triple quad detector and a capillary column (Agilent,
,
2-phenoxy-1-
2-(2,6-dimethoxyphenoxy)-1-
1
9
(BMB) were synthesized according to literature methods and
the details can be found in SI. All of those synthesized
materials were characterized by NMR spectroscopy on a
Bruker 400 MHz instrument (Figs. S10-S14). 1-(3,4-
dimethoxyphenyl)-2-(2-methoxyphenoxy)-propane-1,2-diol (DMPD),
and guaiacylglycerol-guaiacyl ether (GGE), diphenyl ether and 1-
phenoxy-2-phenylethane were obtained from commercial
sources and used without further purification. The H NMR
spectra of DMPD and GGE are shown in Figs. S15 and S16.
Preparation of the Co-N-C catalyst
The catalyst was prepared using a modified literature method.25,26
1
3
0m×0.25mm×0.25μm) connected to a flame ionization
detector (FID). Dimethyl phthalate was used as the standard.
The NaOH co-catalyst was neutralized with aqueous HCl
(37wt%) prior to quantification by GC-MS.
Co(OAc)
mg, 1.5 mmol) were dissolved in ethanol (120 mL) and sonicated for
0 minutes. A homogenous solution containing the Co(phen) (OAc)
(4 g, 69 mmol) was
2 2
·4H O (125 mg, 0.5 mmol) and 1,10-phenanthroline (270
High performance liquid chromatography (HPLC) were
conducted with an Agilent 1200 Series instrument using a
reverse-phase Eclipse XDB-C18 column (4.6 × 150 mm, 5 μm,
Agilent) and a UV–vis 785A detector set at 260 nm. A mix of
water and MeOH (20:80m/m ratio) was used as eluent.
3
2
2
2
5
complex precursor was formed, Mg(OH)
2
added, and the resulting reaction mixture was sonicated for
another 30 min. The resulting suspension was heated at 60 C for 4
o
h and then the ethanol was removed under vacuum. The solid
material was ground into a powder using a ceramic mortar and then
Conflicts of interest
o
pyrolysed at 700 C under N
2
atmosphere for 2 h. The resulting
There are no conflicts to declare.
material was stirred in nitric acid (120 ml, 1M) for 2 h to remove the
MgO support and any other unsupported metal particles. Further
washing with deionized water (3×100 mL) was applied to remove
the acid. Other non-noble metal catalysts were synthesized using
Acknowledgements
the same method employing Mn(OAc)
Ni(OAc) ·4H O instead of Co(OAc) ·4H O.
Catalyst characterization
The morphology of the catalyst was determined on a JEOS JSM
700F field-emission scanning electron microscope (SEM).
Transmission electron microscopy (TEM) was performed on an FEI
Talos instrument operated at 200 kV high tension. Energy-
dispersive X-ray spectroscopy (EDX) mapping was used for
elemental characterization. For atomic resolution spherical
aberration corrected imaging, measurements were performed on
an FEI Titan Themis 60-300 operated at 200 kV with an aberration-
corrected electron probe in high angle annular dark field scanning
transmission electron microscopy (HAADF-STEM) mode. The surface
2
,
Cu(OAc)
2
·H
2
O
or
We thank the Swiss National Science Foundation (to A.v.M),
the European Research Council (no. 681292 to X.L.H), Natural
Science Foundation of China (21736003) and the Oversea
Study Program of Guangzhou Elite Project for financial
support. The Interdisciplinary Center for Electron Microscopy
at EPFL for assistance with electron microscopy and XPS, and
Dr. Wu Lan, Dr. Florent Héroguel and Prof. Jeremy Luterbacher
2
2
2
2
6
(EPFL) for help with the
N
2
adsorption-desorption
measurements.
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