Cobalt Single-Atom-Intercalated Molybdenum Disulfide for Sulfide Oxidation with Exceptional Chemoselectivity

Article abstract of DOI:10.1002/adma.201906437

The identification of chemoselective oxidation process en route to fine chemicals and specialty chemicals is a long-standing pursuit in chemical synthesis. A vertically structured, cobalt single atom-intercalated molybdenum disulfide catalyst (Co₁-in-MoS₂) is developed for the chemoselective transformation of sulfides to sulfone derivatives. The single-atom encapsulation alters the electron structure of catalyst owing to confinement effect and strong metal–substrate interaction, thus enhancing adsorption of sulfides and chemoselective oxidation at the edge sites of MoS₂ to achieve excellent yields of up to 99% for 34 examples. The synthetic scopes can be extended to sulfide-bearing alkenes, alkynes, aldehydes, ketones, boronic esters, and amines derivatives as a toolbox for the synthesis of high-value, multifunctional sulfones and late-stage functionalization of pharmaceuticals, e.g., Tamiflu. The synthetic utility of cobalt single atom-intercalated MoS₂, together with its reusability, scalability, and simplified purification process, renders it promising for industrial productions.

Full text of DOI:10.1002/adma.201906437

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Cobalt Single-Atom-Intercalated Molybdenum Disulfide
for Sulfide Oxidation with Exceptional Chemoselectivity
Zhongxin Chen, Cuibo Liu, Jia Liu, Jing Li, Shibo Xi, Xiao Chi, Haisen Xu, In-Hyeok Park,
Xinwen Peng, Xing Li, Wei Yu, Xiaowang Liu, Linxin Zhong, Kai Leng, Wei Huang,
Ming Joo Koh,* and Kian Ping Loh*
The production of high-value fine chemi-
The identification of chemoselective oxidation process en route to fine
chemicals and specialty chemicals is a long-standing pursuit in chemical
synthesis. A vertically structured, cobalt single atom-intercalated molybdenum
disulfide catalyst (Co1-in-MoS2) is developed for the chemoselective
transformation of sulfides to sulfone derivatives. The single-atom encapsulation
alters the electron structure of catalyst owing to confinement effect and
strong metal–substrate interaction, thus enhancing adsorption of sulfides
and chemoselective oxidation at the edge sites of MoS2 to achieve excellent
yields of up to 99% for 34 examples. The synthetic scopes can be extended
to sulfide-bearing alkenes, alkynes, aldehydes, ketones, boronic esters, and
amines derivatives as a toolbox for the synthesis of high-value, multifunctional
sulfones and late-stage functionalization of pharmaceuticals, e.g., Tamiflu.
The synthetic utility of cobalt single atom-intercalated MoS2, together with its
reusability, scalability, and simplified purification process, renders it promising
for industrial productions.
cals and specialty chemicals (e.g., phar-
maceuticals and agrochemicals) usually
involves multiple chemical conversion
steps, where a chemical reagent reacts
selectively with particular functional
groups (so-called chemoselectivity) while
leaving the (bio-)active functional groups
intact.[1] Given the importance of sulfone/
sulfoxide-containing reagents,[2] pharma-
ceuticals[3] and functional materials,[4]
selective oxidation of sulfides has become
an important strategy to access func-
tionalized sulfones/sulfoxides. Traditional
approaches using homogeneous catalysts
and/or oxidants (such as meta-chloroper-
oxybenzoic acid, mCPBA) suffer from side
reactions with oxidation-prone substrates
such as sulfide-bearing alkynes, alkenes,
ketones, aldehydes, carboxylic acids,
boronic acids and esters, and amines.[5] In the case of mCPBA
and homogeneous catalysts, the removal of unreacted oxidants,
side products and catalyst residuals from the reaction system
is tedious, hindering their applications in industrial process.[6]
Molybdenum catalyst has long been recognized as an outper-
former in sulfur conversions such as hydrodesulfurization[7,8]
and metal–sulfur batteries[9] owing to its preferred binding to
sulfur atoms in sulfur-containing molecules, thus allowing
catalytic modification of the sulfur containing sites. Selective
oxidation of sulfide has been attempted using homogeneous
molybdenum catalysts such as [Mo7O22(O2)2]6−,[10–12] but this
requires complex synthetic procedures, and suffers from cata-
lyst leaching during recovery of products. Motivated by the
benchmark performance of molybdenum disulfide (MoS2) in
hydrodesulfurization, as well as its earth abundance and low
cost, it is timely to investigate if it can be used as a catalyst in
the selective oxidation of sulfides.[13,14]
Owing to the presence of van der Waals gaps in 2D materials,
the intercalation of 2D materials has recently emerged as a pow-
erful tool to tune its optoelectronic properties. Charge transfer
interactions from intercalated atoms can modify electronic
order such as charge density waves and superconductivity.[15]
The intercalation of redox active or catalytic species can
also enhance the catalytic performance of 2D materials.[16]
Pt-intercalated MoS2 and graphene exhibit enhanced perfor-
mance for hydrogen evolution and glycerol oxidation reactions
due to the promoter effects of intercalants by charge transfer
Dr. Z. Chen, Prof. C. Liu, Dr. J. Liu, Dr. J. Li, Dr. X. Chi, Dr. H. Xu,
Dr. I.-H. Park, Dr. X. Li, W. Yu, Dr. K. Leng, Prof. M. J. Koh, Prof. K. P. Loh
Department of Chemistry and Centre for Advanced 2D Materials
(CA2DM)
National University of Singapore
3 Science Drive 3, Singapore 117543, Singapore
E-mail: chmkmj@nus.edu.sg; chmlohkp@nus.edu.sg
Prof. C. Liu
Department of Chemistry
Institute of Molecular Plus
School of Science
Tianjin University
Tianjin 300072, China
Dr. S. Xi
Institute of Chemical and Engineering Sciences
Agency for Science, Technology and Research (A*STAR)
1 Pesek Road, Jurong Island, Singapore 627833, Singapore
Prof. X. Peng, Prof. L. Zhong
State Key Laboratory of Pulp and Paper Engineering
South China University of Technology
Guangzhou 510641, China
Prof. X. Liu, Prof. W. Huang
Institute of Flexible Electronics
Northwestern Polytechnical University
Xi’an 710072, China
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adma.201906437.
DOI: 10.1002/adma.201906437
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Figure 1. Fabrication of Co1-in-MoS2 catalyst on carbon paper. A) Schematic of the electrochemical co-intercalation and subsequent annealing process
to convert the intercalants into single atom catalyst; B) typical CV patterns in various electrolytes containing CTAB and CoPc; C) SEM image showing
the vertical structure of MoS2 array; D) atomic-resolution STEM-HAADF image of Co1-in-MoS2; E) XRD patterns; F) Co K-edge XANES spectra;
and G) FT-EXAFS spectra of various catalysts. Dotted lines represent the fitting of EXAFS spectra; scale bar: C) 2 µm; D) 1 nm.
interactions.[17,18] The drawback of intercalated systems lies in
the mass diffusion limitation since the catalyst is encapsulated;
geometric constraint and confinement effect further affect reac-
tion kinetics and turnover number,[19,20] thus the morphology of
the catalyst has to be modified to increase its surface area and
activity.[16]
Herein, we report a straightforward oxidation strategy to
prepare multifunctional sulfones by a heterogeneous, single
atom-intercalated catalyst. The catalyst consists of MoS2
nanosheets grown vertically on carbon paper, thus ensuring
high surface area edge-exposed sites for catalysis. The edges
of the nanosheets are electrochemically intercalated with
Co-containing molecules, which are subsequently converted
to active Co catalyst following thermal annealing. The use
of Co1-in-MoS2 catalyst led to a benchmark performance of
>99% selectivity and yield for the conversion of functionalized
sulfides to sulfonated alkenes, alkynes, aldehydes, ketones,
amines, boronic acids and pinacol esters, outperforming the
state-of-the-art oxidation using mCPBA. Co1-in-MoS2 can easily
be recovered from reaction mixture, bypassing tedious purifica-
tion process in conventional sulfide oxidation.
The fabrication steps of Co single atom-intercalated
MoS2 catalyst on carbon paper (Co1-in-MoS2) is illustrated in
Figure 1A. Ultrathin MoS2 nanosheets were first grown on
hydrophilic carbon paper via low-temperature hydrothermal
reaction.[21] The grown structure is vertically aligned and its
edge sites are exposed, thus enabling a fast diffusion kinetics
of reactants.[18] Subsequent encapsulation of cobalt phth-
alocyanine (CoPc) in the layered spacing of MoS2 is achieved
by an electrochemical co-intercalation process, where a long-
chain quaternary ammonium molecule (cetyltrimethylammo-
nium bromide, CTAB) is chosen as a co-intercalant to expand
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the interlayer spacing for diffusional intercalation of CoPc.[22]
Successful intercalation is proven by the appearance of CTAB
intercalation peak at a concentration of 5 × 10−3 m CTAB in the
cyclic voltammograms (CV) in Figure 1B. The Co precursor can
be readily converted into Co single atoms by thermal annealing
in argon at 600 °C as indicated by the TGA curves in Figure S1
(Supporting Information). The encapsulation of Co was dem-
onstrated by a sample disruptive technique (time-of-flight sec-
ondary ion mass spectroscopy, ToF-SIMS) using MoS2 single
crystal in Figure S2 (Supporting Information),[17] where uni-
form Co+ signal was clearly seen in the horizontal and perpen-
dicular directions even after a long sputtering time.
Owing to strong metal–substrate interaction (SMSI) and con-
finement effect, the Co 2p XPS spectrum of Co1-in-MoS2
exhibits unique CoSx and satellite features not seen in pure
CoPc. This is consistent with the peak shift to lower energy
direction of Co1-in-MoS2 in the Co L2,3-edge XAS spectra.[27]
The oxidation reaction was implemented in acetonitrile
(CH3CN) by using sulfide 1a as model substrate, Co1-in-MoS2
(1 × 2 cm2) as catalyst and hydrogen peroxide (H2O2, 30% w/w,
2.5 equiv.) as oxidant (Figure 2A; Table S2, Supporting Informa-
tion). Full conversion of 1a and above 99% selectivity of sulfone
2a could be obtained within 20 min at 40 °C (Table S2, entry
2, Supporting Information). While vertically grown Co-doped
MoS2 and MoS2 on carbon paper gave a moderate yield
(75% and 70%, Table S2, entries 3 and 4, Supporting Informa-
tion), the use of commercial MoS2, exfoliated MoS2 and MoO3
powders gave poor results (Table S2, entries 8, 9, and 12, Sup-
porting Information). These results highlight the importance
of building vertical structure with a higher accessible surface
area and the promoter effect of Co single atom intercalants on
sulfide oxidation. As aforementioned, Co single atom encap-
sulation alters the electronic structure of MoS2, leading to
favorable adsorption of the substrates and enhanced reaction
activity. The promoter effect depends significantly on the nature
of intercalated metals. While Fe1, Ni1, and Cu1-in-MoS2 catalysts
gave slightly lower yields, Pt nanoparticles-in-MoS2 resulted in
much worse efficiency due to the rapid decomposition of H2O2
in the presence of noble metals.[17] The existence of defect or
edge also has a profound influence on catalytic activity, where
negligible conversion was observed for single crystal MoS2
(Table S2, entry 10, Supporting Information). Control experi-
ments reveal that both catalyst and oxidant were essential for
such oxidation reaction (Table S2, entries 1 and 18, Supporting
Information). Homogeneous Pd(PPh3)4 catalyst and heteroge-
neous 10% Pt/C catalysts gave negligible conversion of sulfide
(Table S2, entries 13–15, Supporting Information), proving the
importance of molybdenum for such transformation. Mean-
while, the oxidation reaction was completed within 10 min
at slightly elevated temperature or at room temperature. Pro-
longing reaction time to 60 min gives similar yields and selec-
tivities (Table S2, entries 16 and 17, Supporting Information).
As shown in Figure 2B, the Co1-in-MoS2 catalyst can easily be
recycled for five times without any decrease in reaction effi-
ciency and chemoselectivity, thus the catalyst is robust against
changes in the environment and has minimal leaching.[18] The
SEM images of recycled Co1-in-MoS2 catalyst in Figure S10
(Supporting Information) revealed no obvious difference to
fresh catalyst. Inductively coupled plasma optical emission
spectroscopy (ICP-OES) of the supernatant of reaction mixture
also confirms no metal leaching after reaction completion.
Chemoselective oxidation of functionalized sulfides to sul-
fones is challenging owing to side reactions caused by oxida-
tion-prone functional groups. To assess whether Co1-in-MoS2
shows good performance in chemoselective sulfide oxidation, a
wide range of sulfides were examined. As depicted in Figure 2C,
the most commonly occurring functionalities, including those
with potentially oxidizable functional groups such as benzyl
alcohol (2b), aldehyde (2c), ketone (2d–2g), amine (2h and 2i),
alkene (2m and 2n), alkyne (2o and 2p), pyridine (2j) and qui-
noline (2k) can be well tolerated to give corresponding products
The morphology of Co1-in-MoS2 catalyst has been charac-
terized by scanning electron microscopy (SEM). As shown
in Figure S3 (Supporting Information), ultrathin MoS2
nanosheets were grown vertically on the surface of each carbon
fiber with a typical thickness of 5 nm and width of ≈ 400 nm.
The morphology was retained after intercalation and annealing
(Figure 1C). Atomic resolution scanning transmission elec-
tron microscopy (STEM) images in Figure 1D and Figure S4
(Supporting Information) confirmed the uniform distribution
of individual Co atoms in-between MoS2 layers, which were
observed as bright spots overlapping with the Mo column in
the lattice structure of MoS2 and marked with white circles.[23]
Uniform distribution of Co could be seen throughout the MoS2
nanosheet without particle aggregation in the TEM/EDS map-
ping in Figures S5 and S6 (Supporting Information). Changes
in interlayer spacing of MoS2 was confirmed by XRD patterns
in Figure 1E and the cross-section STEM image in Figure S7
(Supporting Information). After electrochemical intercalation,
the {002} peak of the parent MoS2 at 14.2° became indiscern-
ible, and a broad and relatively weak reflection at 8.6° from the
intercalated species could be seen. The intercalated peak shifted
slightly to 9.1° after annealing due to thermal decomposition
of phthalocyanine skeleton and CTAB at high temperature. The
increase in layer spacing from 0.6 to 1.1 nm is beneficial for
edge-site promoted catalysis owing to a higher accessible sur-
face area. The atomic dispersion of Co was validated by X-ray
absorption near-edge structure (XANES) and extended X-ray
absorption fine structure (EXAFS) profiles in Figure 1F,G.
The Co K-edge XANES spectrum of CoPc-in-MoS2 was analo-
gous to that of CoPc in terms of peak position despite a much
lower white-line intensity and the absence of pre-edge signa-
ture due to the transfer of π-electron from CoPc to MoS2.[24,25]
The CoPc complex was then converted to single atom after
annealing, leading to an obvious shift in Co1-in-MoS2 to that
of CoSx species. In addition, a prominent Co–S peak at ≈1.6 Å
was observed in the Fourier transformed spectrum, which
was fitted with a coordination number of 4 in Figure S8 and
Table S1 (Supporting Information). No metallic Co–Co peak at
2.1 Å can be seen in Co1-in-MoS2, suggesting that all Co1 exist
as isolated atoms in-between MoS2 layers.[23]
The modification of electronic structure of MoS2 was con-
firmed by X-ray photoelectron spectroscopy (XPS), X-ray absorp-
tion spectroscopy (XAS) and Raman spectroscopy in Figure S9
(Supporting Information). High-resolution Mo 3d, S 2s XPS
ʹ
and Raman spectra suggested a 2H-1T -2H phase conversion
due to electron donation from CoPc and subsequent recovery
of the 2H phase after reducing CoPc to Co single atoms.[17,26]
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Figure 2. Synthesis of multifunctional sulfones by Co1-in-MoS2. A) Compared to other classes of catalysts, Co1-in-MoS2 exhibits superior activity and
selectivity in catalyzing the oxidation of sulfides to sulfones. Inset shows the model reaction to oxidize sulfide 1a; B) cycling stability of Co1-in-MoS2 for
sulfide 1a oxidation; C) a wide assortment of multi-functional sulfones containing oxidation-prone functional units can be accessed with Co1-in-MoS2
catalyst. Isolated yields of the pure sulfones were reported. The reaction was performed with 0.1 mmol of sulfide, 0.25 mmol of H2O2, 1 piece of Co1-
in-MoS2 catalyst (1 × 2 cm2) in 4 mL of CH3CN at 40 °C for 20 min, unless stated: a, 0.5 mmol of H2O2 at 60 °C for 1 h; b, 4 mL of CH3CN and 1 mL
of DMF, 1 h; c, 60 °C, 4h; d, 40 °C, 1.5 h.
in good to excellent yields. The wide functional group toler-
ance of Co1-in-MoS2 is not matched by existing methods using
H2O2 or mCPBA. Oxidation-prone functional groups undergo
side reactions such as oxidation of amine to oxime,[28] oxida-
tion of alkene or alkyne to epoxide,[29,30] Baeyer–Villiger oxi-
dation of ketone or aldehyde to ester,[31] Cope elimination of
tertiary amine alkene,[32] oxidation of N in pyridyl moiety to
N-oxide[33] and hydroxylation of arylboronic acid or pinacol
ester to phenol.[34] The chemoselectivity of Co1-in-MoS2 and
the best-performing homogeneous mCPBA reported in litera-
ture is compared in Table S3 (Supporting Information). Con-
gruent with earlier findings, the use of mCPBA gives a complex
reaction mixture due to the oxidation of both sulfide and the
oxidation-prone functional groups, posing a practical problem
to the synthesis of high-value chemicals containing sensitive
functional groups.[5] To further illustrate the functional group
tolerance of Co1-in-MoS2, multifunctional methyl 2-chloro-4-
(methylthio)benzoate, which is an important intermediate for
the FDA-approved anti-metastatic cancer drug Vismodegib,[35]
was synthesized in 93% yield (2s). Sulfones bearing 1-phenyl-
tetrazol scaffold (2t and 2u) was synthesized in 91% and 82%
yields, indicating that these can serve as good candidates in
nickel-catalyzed radical cross-coupling reactions.[36] It is also
possible to synthesize sulfones without conjugated section in
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the molecule (such as 2 g and 2n in Figure 2C and 2ag and
2ah in Figure S11 in the Supporting Information) that are dif-
ficult to be isolated by traditional methods. The sulfide oxida-
tion can be proceeded in excellent yields regardless of the
electronic attributes of –OMe, –CN, and –Br groups on aryl ring
(Figure S11, Supporting Information; 2w–2y). Furthermore, the
oxidation reactions occurred smoothly when replacing methyl
group with trifluoromethyl (–CF3), benzyl and other groups
bearing carboxylic acid, epoxide and azide moieties (Figure S11,
Supporting Information; 2z–2af). One pot synthesis of deuter-
ated sulfones with high deuterium ratio was achieved in alka-
line solution using D2O as deuteration reagent in Figure S13
(Supporting Information), revealing the universal applicability
of Co1-in-MoS2.
Owing to its high chemoselectivity and efficiency, most prod-
ucts can easily be isolated by simple removal of Co1-in-MoS2
catalyst on carbon paper. Small amounts of excess H2O2 will
be eventually converted into water, thus bypassing the tedious
separation or purification steps in conventional sulfide oxi-
dations. This is remarkably attractive for the synthesis of
sulfone-containing biomolecules.[2,6] The practical utility of
Co1-in-MoS2 catalyst was demonstrated by the gram-scale syn-
thesis of sulfone. Full oxidation of phenyl propargyl sulfide
(6.7 mmol, 1.0 g) using double usage of Co1-in-MoS2 catalyst with
portionwise addition of H2O2 was completed within 1 h at 40 °C
with ≈91% identical yield in Figure S14 (Supporting Information).
The scope of Co1-in-MoS2 catalyst was further highlighted by
controllable semioxidation to sulfoxides in Figure 3A. Eight rep-
resentative examples with aldehyde (3a), ketone (3b), boronic
acid (3c), alkene (3d), alkyne (3e), amine (3f), and heterocycles
(3g and 3h) were examined. Similar to the full oxidation of
sulfide to sulfone, all of the substrates can be chemoselectively
oxidized to sulfoxides with moderate to good yields (67–88%)
while leaving other functional groups intact. Notably, 4-(methyl-
thio)quinoline (1l) is a potential dye precursor, where its semi-
(3 g) and full oxidation (2l) products with characteristic green
and orange-red color can be obtained with desired amounts of
H2O2 in Figure S12 (Supporting Information).[37]
Finally, we explored the applicability of our oxidation pro-
tocol to the late-stage functionalization of Tamiflu (4a) in
Figure 3. Synthetic utility of Co1-in-MoS2 catalyst. A) Controllable semioxidation to sulfoxides containing useful functional units with Co1-in-MoS2 cata-
lyst. Isolated yields of the pure sulfones were reported. The reaction was performed with 0.1 mmol of sulfide, 0.11 mmol of H2O2, 1 piece of Co1-in-MoS2
catalyst (1 × 2 cm2) in 4 mL of CH3CN at 40 °C for 20 min, unless stated: a, 4 mL of CH3CN and 1 mL of DMF, 1 h; b, 0.25 mmol of H2O2 at 60 °C for
20 min; c, 60 °C, 4h; B) late-stage functionalization of Tamiflu using our Co1-in-MoS2 catalyst. Insets show single crystal structures of corresponding
semi- and full oxidation products. d, 0.25 mmol of H2O2 at 60 °C for 1 h; e, 0.5 mmol of H2O2 at 60 °C for 24 h.
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Figure 4. Promoter effect of Co single atoms and mechanistic insights. A) Proposed radical mediated pathway for sulfide oxidation; B) differential
charge density of the optimized adsorption configuration (side view) of the sulfide 1q at the edge of bilayered Co1-in-MoS2. Purple and brown region
represent the isosurface of the electron lost and gained. The color scheme used: dark cyan for Mo, yellow for S, blue for Co, gray for C, red for O, pink
for B and white for H; C) difference in adsorption energies for various adsorption configurations of sulfide 1q on Co1-in-MoS2; D) EPR experiments on
trapping DMPO-OH adduct (marked as *) in sulfide oxidation. Condition: 0.1 mmol of sulfide 1a, 1 piece of Co1-in-MoS2 (1 × 2 cm2), 0.25 mmol H2O2
and 0.5 mmol of DMPO in 4 mL CH3CN unless stated: 1) without H2O2 and catalyst; 2) without H2O2; 3) without sulfide 1a; and 4) standard conditions.
Figure 3B, which has an electron-deficient alkene that is prone
to traditional mCPBA oxidation.[38] Tamiflu was first subjected
to sulfurization by 2-(phenylthio)acetyl chloride in anhydrous
THF, followed by semi- or full oxidation with Co1-in-MoS2 to
give corresponding sulfoxide-modified and sulfonated Tamiflu
(4b and 4c) in 86% and 77% yields, respectively (see the Sup-
porting Information for details). Without using any purifica-
tion process, both functionalized pharmaceuticals can easily be
recrystallized to give ultrapure compounds (see single crystal
data in Figure 3B), which is a distinct advantage over homoge-
neous oxidation or cross-coupling reaction.
Sulfide oxidation by H2O2 involves a radical pathway and a
possible mechanism is proposed in Figure 4A. The preferred
adsorption of sulfide 1q at the edge of Co1-in-MoS2 is sup-
ported by density functional theory (DFT) calculations. The
enhanced absorption is attributed to the promoter effect of Co
is universal and not affected by the location of intercalated Co
single atoms (Figure S16, Supporting Information). The basal
plane of Co1-in-MoS2 is relatively inactive, suggesting the oxi-
dation reaction takes place at the active edge. Surface-absorbed
sulfide species (I) is then attacked by the ·OH radicals gener-
ated from the cleavage of H2O2 to give the intermediate (II),
which abstracts another ·OH radical with the leaving of a H2O
molecule to give semioxidation product sulfoxide (III). Similar
pathway is proposed for the subsequent oxidation of sulfoxide
to sulfone (VII), where at least two equivalents of H2O2 are
required to complete the catalytic cycle. As suggested by recent
reports, MoS2 serves as excellent promoter in catalytic decom-
position of H2O2 for advanced oxidation process involving
Fenton-like radical mechanism.[41,42] The existence of metallic
Co single atoms further boosts the catalytic efficiency of radical
generation.

single atoms, which weakens the sulfur molybdenum bonds
The radical pathway for sulfide oxidation is confirmed
by electron paramagnetic resonance (EPR) measurements
in Figure 4D.[43] The radicals generated in the presence of
H2O2 and catalyst were trapped with 5,5-dimethyl-1-pyrro-
line-N-oxide (DMPO) to form stable DMPO-OH adducts,
which show the fingerprint EPR signals (quartet peaks,
aN = aH = 14.9 G, marked by *).[41,44] However, hydroperoxyl
adducts (DMPO-OOH) was not detected, excluding the heter-
olysis of H2O2 in our Co1-in-MoS2 system. This is distinct from
conventional MoS2 and Co-doped MoS2 catalysts, where the
at the edge and induces a higher electron density of the nearby
Mo atoms (Figure 4B).[7,39,40] The intercalation of Co may also
introduce structural defects (such as S vacancy) due to its
lower coordination number (4) than Mo atoms (6). As shown
in Figure 4C, DFT studies show a much stronger adsorp-
tion of sulfide 1q in a vertical configuration on the sulfur site
over other competing sites (such as OH site in boronic acid
group), which is the basis for the excellent chemoselectivity
toward sulfide oxidation in our system. Such promoter effect
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loaded into a quartz tube mounted inside a tube furnace under Ar gas
and then heated at 600 °C for 2 h at 5 °C min−1.
existence of strongly oxidative hydroperoxyl radicals (marked
as Δ in Figure S17 in the Supporting Information) will lead
to fast catalyst degradations and poor catalytic conversions
due to the formation of nonoxidative H radicals. Upon the
addition of substrate to our system, no additional EPR peaks
could be observed in Figure 4D and Figure S18 (Supporting
Information), which is reasonable owing to poor resolution
of DMPO-S species.[45] Time-dependent EPR measurement in
Figure S19 (Supporting Information) also indicates a steady
generation of DMPO-OH adducts in the course of reaction
so that a small excess of H2O2 is sufficient to complete full
oxidation to sulfones. The radical pathway is further proved by
2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) quenching exper-
iment in Figure S20 (Supporting Information), where negligible
yield of 2a could be observed in the presence of TEMPO
molecule.
Highly efficient, chemoselective oxidation of sulfide was
achieved using Co1-in-MoS2 catalyst, which provides a conven-
ient way to access multifunctional sulfones. The good perfor-
mance of Co1-in-MoS2 is attributed to its enhanced binding with
the sulfur atoms in sulfur-containing molecules, coupled with
the strong promoter effect of cobalt single atoms. Oxidation-
prone sub-units such as aldehydes, ketones, alkenes, alkynes,
amines, boronic acid, and pinacol ester can be well tolerated as
building blocks to synthesize high-value chemicals. Oxidation
with Co1-in-MoS2 allows a controllable, stepwise radical pathway
to give semi/full oxidation products (sulfoxide/sulfone) under
stoichiometric amount of H2O2. As a proof of synthetic utility,
Tamiflu with electron-deficient alkene that is prone to oxidation
by conventional methods can be successfully transformed into
sulfoxide and sulfone-containing analogs. Therefore, the use of
Co1-in-MoS2 paves the way toward the rational design of het-
erogeneous catalysts for the production of high-value chemicals.
Sulfide Oxidation Using Co1-in-MoS2: Typically, 1 piece of Co1-in-MoS2
on carbon paper (≈1 × 2 cm2 with ≈1% Co loading from ICP-OES)
was loaded in a vial with 4 mL of CH3CN and stirred at 40 °C. Then,
0.1 mmol of sulfide and 0.25 mmol of 30% H2O2 (25 µL) were added
sequentially. The reaction mixture was stirred at 40 °C for 20 min for
the oxidation to sulfones. The usage of H2O2 was reduced to 0.11 mmol
(11 µL) for semioxidation to sulfoxides. After reaction, the clear solution
was taken out for further analysis and the catalyst was recovered by
simple washing with CH3CN, ethanol and acetone.
Material Characterization: The following equipment were used:
Electrochemistry (Ivium-n-Stat and Autolab PGSTAT30), STEM/EDS
(JEOL ARM200F equipped with ASCOR probe corrector, Oxford X-Max
100TLE, at 200 kV), SEM (JEOL JSM-6701F), TEM (JEM 2010F, 200 kV),
XPS (AXIS UltraDLD, monochromatic Al Kα), XRD (Bruker D8), ToF-
SIMS (ION-TOF SIMS5 with Bi+ and Cs+ beams using MoS2 single
crystal), TGA (Discovery 5500), UV–vis (Shimadzu UV-3600), Raman
(WITec Alpha 300R), XAS (SSLS, SINS beamline), HRMS (Bruker
MicroTOF-QII LCMS), NMR (Bruker AV400), single crystal XRD (Bruker
D8 venture), EPR (JEOL FA200), ICP-OES (Perkin Elmer Avio 500, ppm
level accuracy). XANES/EXAFS: 100 mg of sample was first ground
into fine powder using a mortar and pestle before being pressed into a
10 mm pellet. Measurements were carried out at Singapore Synchrotron
Light Source (SSLS), X-ray Absorption Fine structure for catalysis
(XAFCA) beamline.[46] Data analysis and simulation were carried out on
Athena, Artemis, and Hephaestus (Version 0.9.23).[47]
Data Availability: All data are available from the authors upon
reasonable request.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
Z.C., C.L., and J.L. contributed equally to this work. K.P.L. thanks
National Research Foundation, Singapore for NRF Investigator Award
“Graphene oxide a new class of catalytic, ionic and molecular sieving
materials” (award number NRF-NRF12015-01). M.J.K. thanks the
National University of Singapore start-up grant under the President’s
Assistant Professorship Scheme. The authors appreciate Poh Chong
Lim, Dr. Ming Lin at A*STAR, IMRE, for GIXRD measurements, and
Dr. Jiajia Zhang and Prof. Hongbin Lu at Fudan University for SAXS tests.
Experimental Section
Vertically Grown MoS2 Array on Carbon Paper: MoS2 nanosheets were
directly grown on O2-treated, hydrophilic carbon paper (Toray 60) by a
simple hydrothermal method.[21] In a typical synthesis, Na2MoO4·2H2O
(3 mmol, 726 mg) and thiourea (8 mmol, 609 mg) was dissolved in
50 mL of deionized water. After gentle stirring for 30 min, the solution
was then transferred to an 80 mL Teflon-lined stainless-steel autoclave
with a piece of carbon paper (2 × 5 cm2). The autoclave was sealed and
heated at 190 °C for 24 h in an oven and then cooled down to room
temperature naturally. Finally, the product was taken out, rinsed with
deionized water and ethanol several times and dried at 60 °C in air.
The loading for MoS2 nanosheet on carbon cloth was ≈5.0 mg cm−2 by
weight difference and ICP-OES.
Conflict of Interest
The authors declare no conflict of interest.
Synthesis of Co1-in-MoS2 Catalyst: The encapsulation of Co precursor
into MoS2 matrix was conducted by an electrochemical cointercalation
method using a standard two-electrode cell using a Pt plate as the
counter/reference electrode and as-grown MoS2 array on carbon paper
(2 × 5 cm2) as the working electrode. The electrolyte was prepared
by dispersing 28.6 mg of CoPc (1 × 10−3 m) and 91.1 mg of CTAB
(5 × 10−3 m) in 50 mL of 1-methyl-2-pyrrolidinone (NMP) with the aid of
30 min sonication. The electrochemical cointercalation was performed
by chronopotentiometry (CP) at a constant current density of 300 µA
with a cutoff potential of –4 V, where both CTAB and CoPc molecules
intercalated MoS2. The CoPc-modified electrode was rinsed by DMF,
ethanol, water and acetone for three times and dried at 60 °C. To convert
the intercalants into Co1-in-MoS2 catalyst, the modified electrode was
Author Contributions
Z.C. conceived the research, synthesized the materials, conducted
catalytic reactions, and wrote the draft with the collaboration with
C.L. and J.L. TEM, STEM characterization, and data analysis were
conducted by Z.C.; J.L. conducted Raman analysis and material
synthesis; S.X. performed the EXAFS and data processing; X.C., X.P.,
and L.Z. performed the XPS and XAS measurements; X.L. and
H.X. performed the SEM and XRD measurements. X.L. and W.H. assisted
in DFT calculations. I.-H. P., W.Y., and K.L. assisted with materials
characterization and data analysis. M.J.K. and K.P.L. supervised the
research. All authors discussed and commented on the manuscript.
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1906437 (7 of 8)
2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2019, 1906437

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©
Adv. Mater. 2019, 1906437
1906437 (8 of 8)
2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim