Seaweed-like 2D-2D Architecture of MoS2/rGO Composites for Enhanced Selective Aerobic Oxidative Coupling of Amines

Article abstract of DOI:10.1002/cctc.201900156

To provide a solution for pursuing high redox reactivity beyond the compromise between the numbers of sulfur vacancies and electron transfer ability, 2D-2D architecture of MoS₂/rGO composite was fabricated with 2D graphene-like transition metal

Full text of DOI:10.1002/cctc.201900156

Accepted Article
Title: Seaweed-like 2D-2D architecture of MoS2/rGO composites for
enhanced selective aerobic oxidative coupling of amines
Authors: Wang Mingming, Wu Haihong, Shen Chaoren, Luo Shuping,
Wang Dan, He Lin, Xia Chungu, and Gangli Zhu
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Seaweed-like 2D-2D architecture of MoS2/rGO composites for
enhanced selective aerobic oxidative coupling of amines
Mingming Wang#[a],[b], Haihong Wu# [b], Chaoren Shen[b], Shuping Luo*[a], Dan Wang[b], Lin He*[a],[b]
,
Chungu Xia[b] and GangliZhu*[b]
Abstract: To provide a solution for pursuing high redox reactivity
beyond the compromise between the numbers of sulfur vacancies
and electron transfer ability, 2D-2D architecture of MoS2/rGO
composite was fabricated with 2D graphene-like transition metal
dichalcogenides and graphene. By employing this material as
heterogeneous catalyst, imines, also known as Schiff base, was
synthesized by direct aerobic oxidative coupling of amines. Taking
advantage of few-layered MoS2 nanosheets and graphene, catalytic
performance was significantly enhanced, exhibiting both excellent
conversion and remarkable selectivity. Due to the good oil-absorbing
capability of this open porous 2D-2D architecture, the hydrolysis of
NH-imines by water was inhibited and thus aldehydes byproducts
were diminished. A second order kinetic behavior was observed in
the imines formation, indicating the bimolecular condensation might
be the rate determining step in such coupling reaction.
sheets materials are expected to own good redox abilities. To
the best of our knowledge, in contrast to hydrotreating
processes[12], the exploiting of 2D MoS2 sheets materials on
aerobic oxidation remain sparse[13]. For catalytic oxidization,
there is abundant room for 2D MoS2.
As for thermocatalysis, the catalytic activity largely depends on
morphology and orientation of MoS2 clusters[14]. It has been
recognized since early years[15] that the active sites are usually
located at edges and corners instead of basal planes due to the
strong Mo-S bonding in the basal planes. Therefore, bulk MoS2
has poor intrinsic activity. The stacking dependence of catalyst
was also studied[16] and showed that lateral edges are extremely
superior to middle edges for dibenzothiophene hydrogenation
because of steric hindrance. It was also confirmed both
experimentally[11a] and theoretically[11b] that for HER, sites on the
edges of 2D MoS2 layers are much more active than basal
surfaces. Overall, it is widely accepted that the coordinative
unsaturated sites of MoS2 are the active centers. Therefore,
exposing more edge sites and fabricating more defects would
produce more active sites. However, less free electrons resulted
from more active centers with sulfur vacancies would undermine
electron conductivity. Actually, redox reaction substantially
involves electron transfer, and can be reinforced by enhanced
electron transport if catalytic materials are conductive[7a, 11g, 11h]. It
Introduction
The graphene-like transition metal dichalcogenides (TMDCs),
emerging after the best-known graphene, have become
appealing materials due to the unique structures and interesting
physical and chemical properties[1 ]. In the TMDCs family, MoS2,
building up of bonded S-Mo-S layers through van der Waals
forces, has attracted widespread interests in research fields
such as lubrication[2], energy conversion and storage[3],
becomes
a compromise between the numbers of sulfur
vacancies and electron transferring ability when higher redox
reactivity is pursued. Since the graphene could provide
devices[4],
optoelectronics[5]
and
11g]
extraordinary electronic conductive channels[7a,
, 2D-2D
electroluminescent
photocatalysis[6]. The tunable band gap and semiconducting
properties also furnish MoS2 with impressive performance in
diverse electrocatalytic properties including HER[7], ORR[8],
OER[9] and NRR[10]. Outstanding achievements have been made
by delicately regulating the structure and properties of 2D MoS2
combination of MoS2 and rGO would be a reasonable option to
simultaneously increase active centers and sustain electron
transferring ability. Comparing with electrocatalysis, the
molecules diffusion should be more emphasized in
thermocatalysis in addition to electrons transfer. Thus, the 2D-
2D architecture could reduce agglomeration of exfoliated MoS2
layers and provide stable express channels for reaction.
Generally, there are two methodologies to fabricate 2D MoS2
structure, bottom up by liquid exfoliation[11f, 11g] and bottom up by.
solvothermal reactions[7a, 11h]. Graphene composites can be
tuned to have oleophilicity, hydrophilic or amphiphilic
properties[17], and catalytic performance can be improved by
materials [7a,7b, 8-11]
.
Inspired by the high performance in electrocatalysis, which is
substantially concerning the gain/loss of electrons, 2D MoS2
[a]
[b]
M. Wang, Prof. Dr. S. Luo
State Key Laboratory Breeding Base of Green Chemistry-Synthesis
Technology,
Zhejiang University of Technology,
Hangzhou 310014 (P. R. China)
E-mail: Luoshuping@zjut.edu.cn
Dr. H.Wu, Dr. C. Shen, D. Wang, Prof. Dr. L. He, Prof. Dr. C. Xia,
Prof. Dr. G. Zhu
adjusting wettability of catalyst [18]
.
In this work, aerobic oxidative coupling reaction of amines in
the presence of MoS2/rGO nanocomposite was chosen to
synthesize imines, which are intermediates widely used for
advanced synthesis and biologically active chemicals
preparation[19]. To avoid employing the traditional method by
condensation of amines and carbonyl compounds, especially
unstable aldehydes by using dehydrating agents as well as
Lewis acid catalysts[20], the heterogeneous catalyst was used in
imines synthesis by oxidative coupling reaction of amines with
environmentally friendly molecular oxygen serving as the
compromise between more active centers with sulfur vacancies
State Key Laboratory for Oxo Synthesis and Selective Oxidation,
Suzhou Research Institute of LICP
Lanzhou Institute of Chemical Physics (LICP), Chinese Academy of
Sciences
Lanzhou 730000 (P. R. China)
E-mail: helin@licp.cas.cn; zhugl@licp.cas.cn
These authors contributed equally to this work.
[#]
Supporting information for this article is given via a link at the end of
the document
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10.1002/cctc.201900156
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and high conductivity, seaweed-like architecture of MoS2/rGO
composite was constructed by 2D-2D combination. Taking
advantage of good oleophilicity of this architecture, hydrolysis of
NH-imines by water in the oxidation is minimized and the route
via aldehydes would not be preferred. At mild conditions, high
performance was achieved with high imines yield inhibiting
unstable aldehydes byproducts, which may continue to yield
undesired detrimental impurities. Kinetic study showed the
imines formation followed a second order kinetics, indicating the
bimolecular condensation might be the rate determining step in
the coupling reaction in this work.
seaweed-like 2D-2D architecture (Figure 1d-f). The MoS2/rGO-
73 was found to have disordered MoS2 few-layered nanosheets,
dispersing on rGO sheets. The open pores between the sheets
would provide the channels for express diffusion. The results are
consistent with the texture analysis. Morphology and orientation
of MoS2 clusters on supports are highly important for catalytic
activity[14]. Interestingly, HRTEM images (Figure 1f-g) also
showed that the parallel few-layered nanosheets were mainly
perpendicular to the rGO substrate. The layers in the MoS2
nanosheets stacked through interplanar weak binding by van der
Waals forces to form the (002) crystal plane along the C axis.
There were abundant inflated (002) planes with interspaces in
the range of 0.71-0.96 nm, besides the ordinary interplanar
distance of 0.62 nm. In a traditional supported MoS2 catalyst,
usually the basal-bonding model of MoS2 clusters (flakes up)
with the support surface could be observed[21]. The less favored
surface orientation of edge-bonding model would be expose
more planes with edges, thus high activity might be achieved.
To better understand the crystalline, XRD analysis of blank
rGO, MoS2/rGO composite, bare MoS2 and bulk MoS2 were
performed. As showed in Figure 2, the MoS2/rGO composite had
diffraction peaks at 2θ = 9.3o, 33.5o and 56.7o, which can be
attributed to the (002), (100) and (110) planes of MoS2.
Compared to bare MoS2, the intensity of the diffraction peak
around 9.3o and 14.7o of MoS2/rGO-73 decreased. This
indicates that the existence of rGO could inhibit the aggregation
of the MoS2 due to the interaction between the precursor of
MoS2 and abundant oxygenated functional groups on GO
support during the hydrothermal process. In addition, the (002)
peak at about 23.2o did not appeared in MoS2/rGO-73
composites, indicating that the π–π stacking of rGO sheets did
not occur in the 2D-2D architecture, probably caused by the fact
that the surface of rGO was anchored by MoS2 sheets and
regularity was destroyed. It should be noted that there was a
new peak at 2θ = 9.3o, besides the weak peak at 14.7 o, which
can be assigned to the (002) crystal plane in hexagonal MoS2
(2H- MoS2, JCPDS 37-1492). According to the Bragg equation
(2dsinθ = nλ, λ = 0.15418 nm for Cu Kalpha), the interplanar
Results and Discussion
The MoS2/rGO composites were prepared by hydrothermal
treatment. L-cysteine acted as both sulfur donor and reducing
agent during the preparation process, in which L-cysteine
decomposed and released H2S as sulfur source and reducing
agent. The MoS2/rGO composites were obtained through the in-
situ self-assembly of MoS2 on the surfaces of GO matrix and the
reduction of MoS2-decorated GO to rGO by L-cysteine. N2-
adsorption/desorption isotherms results (Figure 1a) showed that
the synthesized bare MoS2 sample exhibited higher adsorption
capacity than bulk MoS2. The capacity was further improved by
self-assembly with rGO to form MoS2/rGO composites. The
composites possess typical H3-type hysteresis loop of isotherm,
which can be attributed to the slit-shaped pores by stacking of
ultrathin sheets. The Brunauer-Emmett-Teller (BET) surface
areas of commercial bulk MoS2 and bare MoS2 sheets were 3.5
and 13.7 m2·g-1, respectively. Owing to the dispersion of MoS2
nanosheets on rGO layers, the MoS2/rGO-73 composites
possess much larger surface area of 29.1 m2·g-1.
The morphologies of bare MoS2 and MoS2/rGO-73 composite
were investigated (Figure 1b-g). The bare MoS2 sample without
GO doping has a morphology of urchins-like nanospheres with
diameter of ca. 200 nm, consisting of thin nanosheets (Figure 1b
and 1c). In contrast, The MoS2/rGO-73 composite has the
Figure 1. a) N2 adsorption/desorption isotherms; b,c) morphologies of bare MoS2 nanosheets; d,e) morphologies of MoS2/rGO-73; f,g) HRTEM images of
MoS2/rGO-73
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distance of MoS2 (002) planes in the composite should be
mainly around 0.95 nm, which is larger than 0.62 nm of ordinary
hexagonal MoS2. This might be caused by inflation of disordered
layers structure formed during the hydrothermal process.
Figure 3. Raman spectra of GO, MoS2/rGO composite, bare MoS2 and bulk
MoS2
Figure 2. XRD patterns of bare MoS2, rGO and MoS2/ rGO composites.
determine the composition of MoS2/rGO composite (Figure 4).
The XPS survey of the composite Figure 4a reveals that
MoS2/rGO consisted of Mo, S, C and small amount of O.
Quantification by using XPS showed the S/Mo ratio was 1.98,
agreeing with the stoichiometric atomic ratio of MoS2. The Mo 3d
XPS spectrum (Figure 4b) was fitted by Gaussian–Lorentzian
The stacking of 2D layers could have strong effect on the
electron-phonon coupling of different phonon modes, as evident
from previous Raman studies[22]. The blank GO, MoS2/rGO
composite, bare MoS2, and bulk MoS2 were characterized by
Raman spectroscopy. As shown in Figure 3, in the MoS2/rGO-73,
Raman peaks at 1350 and 1588 cm-1 for D and G bands could
be observed due to the doping of rGO. The D band corresponds
to scattering from local defects or disorders present in carbon,
curves using three pairs of peaks according to the literature[27]
.
In the MoS2/rGO-73 composite, the dominant doublet peaks at
228.7 and 231.9 eV, with about. 3.2 eV spin-orbit splitting,
belong to Mo4+3d5/2 and 3d3/2, respectively. The doublet peaks at
229.9 and 233.4 eV with a 3.5 eV spin-orbit splitting correspond
to the Mo5+ 3d5/2 and Mo5+ 3d3/2, respectively. The weak doublet
peaks at 232.7 and 235.8 eV can be assigned to Mo6+ with
about 3.1 eV spin–orbit splitting. The results reveal that there
were three Mo oxidation states in the MoS2, mainly Mo4+ (ca.
81%), part of Mo5+ (ca. 14%) and small amount of Mo6+ (ca. 5%).
The higher valence state Mo species indicate that there were
lower coordinated Mo sites due to the existing of sulfur
vacancies. Furthermore, comparing with bare MoS2, both the Mo
3d5/2 and 3d3/2 peaks of the MoS2/rGO-73 composite were
slightly shifted to lower binding energy by rGO doping, usually
explained as partial electrons transfer by interaction with
supports[28]. Sulfur species were determined from S 2p XPS
spectrum (Figure 4c). The main doublet peaks at the binding
and the
G bands originates from the in-plane tangential
stretching of the C-C bonds in the graphitic structure. The
intensity ratio value ID/IG for the MoS2/rGO composite was
slightly smaller than that for initial GO, suggesting the
delocalized π conjugation was partially restored during the
reduction process. In addition, the MoS2 exhibited characteristic
bands at 379 and 405 cm-1, corresponding to the first-order E1
2g
and A1 mode of the 2H-phase[22a]. The E1 mode can be
g
2g
associated to the in-plane opposite vibration of two S atoms with
respect to the Mo atom while the A1 mode originates from the
g
out-of-plane vibration of merely S atoms in the opposite
directions[22b, 23]. These results could confirm the formation of
2D-2D architecture of MoS2/rGO composite. The broadened
band for A1 mode with the larger full width at half maximum
g
energy of 161.5 and 162.8 eV belong to S2- 2p3/2 and S2- 2p1/2
,
(FWHM) than bulk MoS2 indicated that the MoS2 possessed the
few-layered structure in the composite[22a]
. Moreover, in
whereas the doublet peaks at 163.0 and 164.2 eV were
proposed to originate from the unsaturated sulfur S22- atoms on
the edge sites of MoS2[29]. Figure 4d showed that most oxygen-
containing functional groups of GO were eliminated during the
reduction process, with only small amount of C-O-C groups
remaining (286.2 eV), and no carbonyl (287.5 eV) and carboxyl
MoS2/rGO composite, the intensity of A1 mode was much
g
higher than that of E1 mode, indicating that the MoS2 sheets
2g
were edge-terminated[11h, 24], otherwise the E1 mode would be
2g
preferentially excited for terrace-terminated due to the
polarization dependence[25]. The edge termination structure may
enrich exposed edge sites and thus enhance the catalytic
groups (289.4 eV) could be observed in the XPS spectrum[30]
Therefore, the intrinsic oleophilicity of rGO sheets could be
restored when most oxygen containing groups was removed[17]
.
activity[15b,26]
.
X-ray photoelectron spectroscopy (XPS) was investigated to
.
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(benzylidene)benzylamine (results see in Table 1). It was found
that the activity of bare 2D MoS2 nanosheets was much higher
than that of bulk MoS2 due to the exposure of more active edge
sites. Obviously, the significant enhancement of catalysis was
achieved when MoS2/rGO 2D-2D composite was applied as
catalyst, exhibiting superior conversion of 99% and imine
selectivity of 99% with negligible amount of aldehydes (entry 3,
Table 1). The conversion by MoS2/rGO composite was 1.8 times
larger than the sum that by individual bare MoS2 and rGO,
indicated there was a synergistic interaction between MoS2
nanosheets and rGO in the oxidation of benzylamine (entries 2,
3, 7, Table 1).
Table 1. Performance comparison of catalysts in the oxidative coupling of
benzylamine to imine.a
Substrate
(mmol)
0.15
Conversion
Selectivity (%)
Entry
Catalyst
(%)
9
2
3
Figure 4. XPS patterns of MoS2/rGO-73 a) survey, b) Mo 3d, c) S 2p, and d) C
1s.
1
2
bulk MoS2
Bare MoS2
91
9
0.15
29
98
2
Owning to the removal of most oxygen containing groups and
abundant open slit-shaped pores by stacking of 2D sheets, the
MoS2/rGO composite may possess high oil absorption and
permeation abilities. The oil/water permeating and dispersion
behavior was studied (results see Figure 5). To reduce the
surface roughness for permeating test[31], samples were pressed
into flat tablets (ca. 1mm thickness) under 10 MPa pressure in
advance. Results showed that MoS2/rGO composite precipitated
at the bottom of the water solvent because of gravity. However,
in the n-octane and water mixture, MoS2/rGO composite
dispersed in the n-octane phase due to its good oleophilicity
(Figure 5a). The MoS2/rGO tablets also exhibited superior
absorbance of octane. n-Octane could spread and permeate
thoroughly within only 0.16s, while water droplet remained
staying on the substrate. The octane seems to have a passport
of super priority. This unique phenomenon probably was caused
by the relative strong oleophilicity of surface and good capillary
action due to open porous 2D-2D architecture[17a]. Because
water would be generated in the aerobic oxidative coupling
reaction of amines, the difference of transportation ability for
oil/water might inhibit the hydrolysis of NH-imines and minify
unstable aldehydes byproducts.
sheets
3
4
MoS2/rGO -73
MoS2/rGO -73
MoS2/rGO -73
MoS2/rGO -73-m
rGO
0.15
0.5
99
56
91
33
27
5
99
99
98
98
98
92
1
1
2
2
2
8
5b
6c
7
0.5
0.15
0.15
0.5
8
bulk MoS2
Bare MoS2
9
0.5
16
98
2
sheets
10
11
MoS2/rGO -56
MoS2/rGO -28
MoS2/rGO -17
MoS2/rGO -10
MoS2/rGO -5
rGO
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.15
54
50
45
39
27
15
19
90
99
98
99
98
98
98
99
96
98
99
2
1
2
2
2
1
4
2
1
12
13
14
15
16c
17d
18e
MoS2/rGO -73-m
MoS2/rGO -73
MoS2/rGO -73
aReaction conditions: benzylamine (0.15-0.5 mmol), catalyst (5 mg), n-octane
(2.5 mL), 120 oC, O2 balloon (1 atm), 3 h. . Conversion and selectivity were
determined on GC-FID with cyclohexane as an internal standard. breaction
time 20 h. cThe mechanically mixed sample of MoS2 and rGO (73 wt% MoS2),
dRecycled catalyst was used,20 h. e Reaction was conducted with 0.15 mmol
benzylamine in the hydrous conditions (6 equiv. H2O).
Figure 5. a) Catalyst dispersion state in water/n-octane solution; b) images of
spreading and permeating behavior of the water/n-octane on MoS2/rGO
substrates. n-Octane permeated quickly through the substrate within only 160
ms, and water remained staying on the substrate.
To confirm whether the architecture of 2D-2D combination was
necessary in this interaction, a simple mechanical mixture of
MoS2 nanosheets and rGO (denoted as MoS2/rGO-73-m) was
used to catalyze oxidative coupling of benzylamine (entries 6
and 16, Table 1), showing the conversion of only 33%. These
results showed better catalytic performance could be obtained
by intimate contact between the MoS2 sheets and rGO sheets,
The heterogeneous MoS2/rGO catalyst was applied in aerobic
oxidative coupling reaction of amines in solution of n-octane at
atmospheric pressure, and the major product was N-
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probably caused by better electron transferring ability, as
indicated by lower binding energy of Mo 3d peaks in the XPS
patterns. The similar trend also can be observed at a higher
substrate concentration with various MoS2 contents in the
catalyst composites (entries 8-16, Table 1). The catalysts also
can be recycled and have stable performance in water
containing substrates (entries 17-18, Table 1).
coupling step rather than the oxidation step was likely the rate
determining step in the oxidative coupling of benzylamine.
Meanwhile, the benzonitrile generation exhibited a different
kinetics behavior, with the rate being proportional to the
concentration of the benzylamine. Moreover, the Gibbs energy
of activation for production of benzonitrile was 66.2 kJ mol-1,
while that for production of N-(benzylidene) benzylamine was
only 40.1 kJ mol-1. There results suggested that the yield of N-
(benzylidene) benzylamine would be kinetically preferred under
reaction conditions in this work.
The major product formed during the aerobic oxidation of
benzylamine is N-(benzylidene)benzylamine, with only small
amount of benzonitrile produced over the MoS2/rGO catalysts.
Generally, two possible pathways have been proposed[20, 33-34]
for the aerobic oxidative coupling of primary amines. Both these
two pathways (Figure 7.) undergo the oxidative dehydrogenation
step of the amine to the NH-imines intermediate
(phenylmethanimine). In pathway A, hydrolysis of NH-imines
occurs to give the benzaldehyde due to the trace amount of
water generated from the oxidation step. It is noteworthy that, as
a
supplement, benzaldehyde can also be generated by
hydrolysis of the final product of N-benzylidenebenzylamine[33]
.
Then the final product of N-benzylidenebenzylamine could be
obtained by reaction of the resulting benzaldehyde and
benzylamine. Reaction following the pathway can be enhanced
by addition of water and yield considerable amount of aldehyde
byproducts[33-34,35]. In pathway B, this NH-imines intermediate
can directly react with another molecular of benzylamine to yield
aminal, which then releases ammonia to give the coupled N-
benzylidenebenzylamine. In this catalytic system with MoS2/rGO
catalysts, the direct bimolecular condensation (pathway B) may
be favored as no benzaldehyde was observed during the
reaction (see Figure S2 in SI). Considering the superior oil-
absorption ability of MoS2/rGO catalysts, it is likely reasonable
that the water produced in the oxidation would be quickly
substituted by the solvent of n-octane and expelled from the
Figure 6. Kinetic study of benzylamine oxidation on MoS2/rGO-73 catalysts.
Reaction conditions: substrate (benzylamine, ca. 0.2 mmol), catalyst
(MoS2/rGO-73, ca. 5 mg), solvent (n-octane, 10 mL), O2 balloon (1 atm), 95-
120 oC, 0-50 h.
Kinetic experiments were conducted isothermally at 95, 105,
and 120 oC, and kinetic models were developed to analyze data.
Eyring-Polanyi equation(see Eq. S1) basing on transition-state
theory was applied to obtain Gibbs energy ΔG‡ of activation[32]
.
Figure 7. Possible mechanism for the aerobic oxidative coupling reaction over
Figure 6 illustrated the kinetic behavior of oxidative coupling of
benzylamine by MoS2/rGO catalyst. As shown, the reaction
started smoothly without an induction period, and proceeded
with high selectivity towards N-(benzylidene)benzylamine. At
extended reaction times, the promoted formation of
benzaldehyde was not observed, which is quite different from
the case of well established CuO/CeO2 catalyst[33] It is
interesting that the generation of N-(benzylidene) benzylamine
followed the second order kinetics, that means, the reaction rate
was proportional to the square of the benzylamine
concentration. This found indicated that the bimolecular involved
MoS2/rGO catalyst.
catalyst surface. Therefore the hydrolysis related pathway A was
not preferred and high selectivity of aimed product could be
achieved[18, 36]. This was further supported by the deliberate
introduction/removal of water to/from the reaction system. No
change of kinetic behavior was observed and benzaldehyde was
not detected all through the reaction when 2 equiv. of water was
added to one of the two parallel reactions (Figure S2-S3 in SI).
And the results kept the same when water amount was enlarged
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to 6 equiv., which is far more than the amount that reaction
could yield. 3A molecular sieves, which had been pretreated
with calcination at 500 oC, was also used to carry out the parallel
reactions. There was no decrease of activity was found when ca.
5000 wt% 3A molecular sieves was added (Figure S4 in SI).
This also confirmed the water-resistance of the catalyst.
The 2D-2D architecture MoS2/rGO composites were prepared
through the in-situ self-assembly by hydrothermal process to
take the both advantages of abundant active edge centers of the
few-layered MoS2 nanosheets and high electron conductivity of
graphene. The existence of rGO reduced the aggregation of the
MoS2, leading to a dispersion of ultrathin MoS2 nanosheets and
larger surface area. The obtained MoS2/rGO-73 was found to
have edge-terminated, disordered MoS2 few-layered nanosheets,
mainly perpendicular to the rGO substrate. These composites
have the typical H3-type hysteresis loop of isotherm, which can
be assigned to open slit pores by 2D-2D construction. The
intrinsic oleophilicity of rGO sheets could be restored when most
oxygen containing groups was removed. Owning to intrinsic
oleophilicity and 2D-2D architecture, the MoS2/rGO composite
The applicability of MoS2/rGO-73 catalyst in the oxidative
coupling of amines to imines was investigated under the
optimized conditions (Table 2). Good conversion and selectivity
were achieved on most of substituted benzylamines bearing
electron-withdrawing or electron-donating groups (entries 2-11,
Table 2), as well as amines with aromatic heterocycles or fused
rings (entries 13-15, Table 2). The oxidation was favored by the
presence of electron-donating substituents (in the order of p-
OCH3 > p-CH3 > p-H > p-Cl >p-CF3). Due to the steric hindrance, possess much higher oil absorption ability than water.
the conversion of benzylamines with para-substituted was higher
than the corresponding meta- or ortho-substituted ones. The
halo-substituted benzylamines could be converted into
corresponding imines with high yield (entries 5, 6, Table 2),
which could be employed for further transformation and
functionality[37]. High yields of imines also can be achieved when
using hetero-cyclic amines containing oxygen and sulfur atoms,
which usually poison most metal catalysts (entries 12 and 13,
Table 2). The conversion was relative low for naphthenic and
aliphatic substrates (entries 15 and 16).
In the synthesis of imines by one-step oxidative coupling
reaction of amines with environmentally friendly molecular
oxygen serving as the terminal oxidant, the catalytic
performance was improved when this MoS2/rGO composite was
used, exhibiting superior conversion of 99% and imine selectivity
of 99% with negligible amount of aldehydes. In the oxidation of
benzylamine, the conversion achieved by MoS2/rGO composite
was much larger than the sum that by individual bare MoS2 and
rGO, indicated the synergistic interaction between 2D MoS2 and
rGO composites. Owning to the superior oil absorbance of this
open porous 2D-2D architecture of MoS2/rGO catalysts, the
direct bimolecular condensation, not involving hydrolysis of NH-
imines by water, may be favored and thus aldehydes byproducts
were minimized. The second order kinetic behavior of imines
generation suggesting that the direct bimolecular condensation
might be the rate determining step. The experimental Gibbs
energy of activation for N-(benzylidene) benzylamine formation
was 40.1 kJ mol-1 and that for benzonitrile formation was 66.2 kJ
mol-1.
Table 2. MoS2/rGO-73 catalyzed oxidation of different amines.a
Selectivityb(%)
Entry
1
R
Conversionb(%)
2
3
Ph
91
89
95
98
72
90
89
99
89
39
72
75
81
67
17
21
97
3
2
o-MeC6H4
m-Tol
99
96
96
99
99
99
99
97
99
99
99
99
99
96
97
1
4
4
1
1
1
1
3
1
1
1
1
1
4
3
3
Experimental Section
4
p-Tol
Materials: Sodium molybdate dihydrate (Na2MoO4·2H2O,
99.95% purity) and L-cysteine (98% purity) were purchased from
Sigma-Aldrich. 3A molecular sieves were purchased from
General Reagent and pretreated with calcination at 500 oC for 6
h. Deionized water (>18.2 MΩ/cm) was prepared by water
purification system (Millipore, Direct-Q3). Reagents including
graphene oxide (GO, Hummers method), bulk MoS2, n-octane,
n-dodecane, cyclohexane, benzylamine, α-methylbenzylamine,
hexahydrobenzylamine etc. were purchased with analytical
purity and used without further purification (details see section 1
in SI ).
5
o-ClC6H4
p-ClC6H4
o-MeOC6H4
p-MeOC6H4
m-FC6H4
o-CF3C6H4
p-CF3C6H4
2-Furyl
6
7
8
9
10
11
12
13
14
15
16
Preparation of MoS2/rGO composites, bare MoS2 and blank
rGO: MoS2/rGO composites were prepared by hydrothermal
treatment from Na2MoO4·2H2O, L-cysteine and GO. For
instance, Na2MoO4·2H2O (240 mg) and L-cysteine (500 mg)
were dissolved in 70 mL of deionized water, followed by addition
of GO (50 mg). After fully ultrasonic dispersion, the mixture was
transferred to a Teflon-lined stainless-steel autoclave to perform
2-Thiophenyl
2- Naphthyl
Cy
n-C7H15
a. Reaction condition: substrates (0.5 mmol), catalyst (5 mg), n-octane (2.5 mL),
120 oC, O2 (1 atm), 20 h. b Determined by GC-MS using cyclohexane as an
internal standard.
o
the hydrothermal process at 180 C for 24 h. Then the resulting
black solid was collected from the solution by centrifugation,
o
washed and dried in vacuum at 60 C for 12 h. The MoS2/rGO
composite thus was obtained. By regulating amounts of
Na2MoO4·2H2O and L-cysteine,
a
series of MoS2/rGO
Conclusions
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10.1002/cctc.201900156
ChemCatChem
FULL PAPER
composites with different MoS2 mass contents were prepared
and denoted as MoS2/rGO-X (mass content X%). Bare MoS2
and blank rGO were prepared using the same procedure in
absence of GO and Molybdenum source, respectively.
L.H.). Supports from the Suzhou Science and Technology
Bureau of Applied Foundation Research Project (SYG201628)
are also gratefully acknowledged.
Characterization: Morphology analysis was performed on field
emission scanning electron microscope (FE-SEM, Hitachi S-
4800, 15 kV) and transmission electron microscope (TEM, FEI
Tecnai F20, 200 kV). X-ray diffraction analysis was conducted on
powder X-ray diffractometer (Rigaku, SmartLab3kW) in the
range of 5-80° (2θ) operating at 40 kV and 30 mA. X-Ray
photoelectron spectroscopy (XPS) data were collected using a
Thermo Scientific K-Alpha XPS system (Thermo Fisher
Scientific, Thermo K-Alpha+) equipped with a micro-focused,
monochromatic Al Kα X-ray source (1486.6 eV). Raman spectra
of powder samples were collected on Raman microscope
(LabRAM HR800) using a laser excitation wavelength of 523
nm. The nitrogen adsorption and desorption isotherms were
collected using Micromeritics ASAP2020 surface area and
porosity analyzer at -196 oC. Automatic optical contact angle
measuring instrument (Dataphysics, OCA15) was applied to
characterize the catalyst materials. Before test, powdery
samples were pressed into flat disks under 10 MPa pressure in
a stainless steel mold.
Conflict of interest
The authors declare no conflict of interest.
Keywords: transition metal dichalcogenides • aerobic oxidative
coupling of amines • superior oil absorbance •2D-2D architecture
• molybdenum disulfide
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This research was financially supported by the Youth Innovation
Promotion Association CAS (2018453) and the National Natural
Science Foundation of China (21773270 for G.Z., 91645118 for
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Entry for the Table of Contents (Please choose one layout)
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In the deep forest of MoS2/rGO
seaweed, there are special fish
(amines) who breathe in air, exhaling
water and ammonia. They met,
excited, fell in love and coupled.
Mingming Wang#[a],[b], Haihong Wu# [b]
,
Chaoren Shen[b], Shuping Luo*[a], Dan
Wang[b], Lin He*[a],[b], Chungu Xia[b] and
Gangli Zhu*[b]
Page No. – Page No.
Seaweed-like 2D-2D architecture of
MoS2/rGO composites for enhanced
selective aerobic oxidative coupling
of amines
Layout 2:
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Author(s), Corresponding Author(s)*
((Insert TOC Graphic here; max. width: 11.5 cm; max. height: 2.5 cm))
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Title
Text for Table of Contents
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