Room-Temperature Activation of Molecular Oxygen Over a Metal-Free Triazine-Decorated sp2-Carbon Framework for Green Synthesis

Article abstract of DOI:10.1002/cctc.201801144

Additive-free activation of oxygen molecules under ambient conditions has been a great challenge for the green organic synthesis. To make it happen, the design of highly efficient catalyst is the key to make it happen. In this work, we report a simple method to prepare an atomic-scale carbocatalyst via decorating sp²-carbon framework with triazine (TA?G), which can activate molecular oxygen for highly efficient organic synthesis. Both theoretical and experimental results reveal that TA?G has a Fermi level lied in the middle of the oxygen 2p antibonding orbital of the absorbed O₂ to weaken the O?O bond for room-temperature and additive-free activation of oxygen molecules.

Full text of DOI:10.1002/cctc.201801144

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Accepted Article
Title: Room-temperature activation of molecular oxygen over a metal-
free triazine-decorated sp2-carbon framework for green synthesis
Authors: Li-Bing Lv, Shi-Ze Yang, Wei-Jie Feng, Wen-Yu Ke, Bing
Zhang, Zhi-Dong Jiang, Hong-Hui Wang, Juan Su, Xin-Hao
Li, and Jie-Sheng Chen
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Room-temperature activation of molecular oxygen over a metal-
free triazine-decorated sp2-carbon framework for green synthesis
Li-Bing Lv,+ [a] Shi-Ze Yang,+ [b] Wei-Jie Feng,[a] Wen-Yu Ke,[a] Bing Zhang,[a] Hong-Hui Wang,[a] Juan
Su,[a] Xin-Hao Li,*[a] Zhi-Dong Jiang, *[a] and Jie-Sheng Chen[a]
Dedication ((optional))
Abstract: Additive-free activation of oxygen molecules under ambient
conditions has been a great challenge for the green organic synthesis.
To make it happen, the design of highly efficient catalyst is the key to
make it happen. In this work, we report a simple method to prepare
an atomic-scale carbocatalyst via decorating sp2-carbon framework
with triazine (TA-G), which can activate molecular oxygen for highly
efficient organic synthesis. Both theoretical and experimental results
reveal that TA-G has a Fermi level lied in the middle of the oxygen 2p
antibonding orbital of the absorbed O2 to weaken the O-O bond for
room-temperature and additive-free activation of oxygen molecules.
nanocarbons, N-decorated nanocarbons complex could
principally elevate their affinity and thus depress the activation
energy of oxygen molecules even under mild conditions.
Developing efficient methods for precise control of specific
nitrogen-based chemical structure inside sp2-carbon lattice is thus
the key step to boost the real applications of nanocarbons as
effective catalysts for organic synthesis via a green manner.
Herein, we describe the design of triazine decorated graphenes
(TA-G) as an atomic-scale carbocatalyst to achieve room-
temperature activation of oxygen molecules for organic synthesis
(exemplified with oxidative coupling reaction of amine in this work).
Both experimental data and theoretical results demonstrated the
key role of the triazine rings, rather than separated nitrogen
dopants, in significantly enhancing the pre-adsorption of
molecular oxygen for desired selective oxidation reaction under
ambient conditions.
Introduction
The introduction of heteroatoms, e.g. nitrogen, boron or metal
elements, is essential to boost the intrinsic activity of sp2-carbon
framework for their potential applications in, not limited to,
catalysis and electrocatalysis.[1-4] N-doped nanocarbon as
effective electrocatalyst, the “nobility” of N-doped nanocarbons
has been well illustrated in pioneering work with improved redox
activity by introducing N dopants for oxygen reduction reaction
and hydrogen evolution reaction under a suitable work voltage.[5-
11] However, N-doped nanocarbon could only provide moderate to
good activity for organic synthesis, exemplified with selective
oxidation reaction using molecular oxygen, under critical
conditions and/or with the presence of scarifying initiators.
The ideal sustainable oxidation process requires a low energy-
consuming and zero-emission reaction using green oxidants (e.g.
oxygen gas) and reusable catalysts. Theoretical results indicate a
large room to modify the electronic structure and thus significantly
promote the affinity of adjacent carbon atoms to specific
molecules by introducing specific dimers, trimers or even more
complex structures of N dopants into the carbon network.[12-16] In
combination with the high chemical stability and reusability of
Results and Discussion
A nano-confinement method[17-20] was modified by carefully
optimizing the synthetic temperature to prepare the TA-G
samples (Figure 1a and S1). The mixture of glucose and
dicyandiamide was thermal-condensed at 600 °C into layered g-
C3N4 (Figure S1), which can act as a mechanical template to
confine carbonaceous intermediates in their interlayer space for
further carbonization into patched graphenes at elevated
temperatures. The g-C3N4 also acted as a chemical template to
release melem, triazine and other gaseous species at a
temperature higher than 750 °C (Figure S1) for further
hybridization with graphenes. The two dimensional structure of
the TA-G sample was directly observed by the scanning electron
microscopy (SEM) (Figure S1) and transmission electron
microscopy (TEM) (Figure S2) images. The mean thickness of
primary sheets in TA-G sample was estimated by the height
analysis of atomic force microscope (AFM) measurement (Figure
1b and S3) to be between 0.83 and 1.12 nm, from which infer the
presence of a hydrated graphene-like monolayer.[21] Indeed, the
single step (red lines in Figure 1b) of multisheet structures exhibits
a measured thickness of 0.42 nm, comparable to that of ideal
graphene (0.34 nm). As expected, the TA-G aerogel has a high
specific surface area (777.7 m2 g-1) according to the N2 adsorption
analysis results (Figure S4). The metal-free feature of TA-G
sample was confirmed by X-ray photoelectron spectroscopy
(XPS) results (Figure S5-8) with only C, N and O signals detected.
Corresponding powder X-ray diffraction (XRD) pattern (Figure S9)
exhibited rather broad diffraction peaks of graphite. The possibility
of a significant amount of g-C3N4 residual in the TA-G sample was
[a]
L.-B. Lv, W.-J. Feng, W.-Y. Ke, B. Zhang, H.-H. Wang, Dr. J. Su,
Prof. X.-H. Li, A.P. Zhi-Dong Jiang, Prof. J.-S. Chen
School of Chemistry and Chemical Engineering
Shanghai Jiao Tong University
Shanghai 200240 (P. R. China)
Fax: +86-21-54741297
E-mail: xinhaoli@sjtu.edu.cn; zdjiang@sjtu.edu.cn
Dr. S.-Z. Yang +
Materials Science and Technology Division,
Oak Ridge National Laboratory
Oak Ridge, Tennessee 37831-6201, United States
These author contributed equally to this work.
Supporting information for this article is given via a link at the end of
the document. ((Please delete this text if not appropriate))
[b]
+
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excluded by the Raman observation (Figure S10) without obvious
photoluminescence signal in the background. The triazine rings
could be observed in the high resolution scanning transmission
electron microscopy (HR-STEM) annular dark field (ADF) images
(Figure 1c, d and S13) with the schematic structure depicted in
Figure 1e. The element mapping high-angel annular dark field
(HAADF) image (FigureS12) revealed the homogeneous
distribution of nitrogen dopants in the carbon layers mainly in the
form of sp2-bonded N, as demonstrated by the electron energy
loss spectra (EELS) (Figure 1f).
position. The band gap of TA-G (Figure 2d) is estimated to be 0.91
eV based on the UPS results and Mott-Schottky plots (Figure 2c),
revealing an enhanced redox power by introducing triazine
subunits. The variation in the band gap of TA-G is a typical
indicator of the effect of nitrogen dopants and the triazine rings on
the modified electronic structure of the carbon framework.
Moreover, the lowered the VB (or HOMO) or elevated the CB (or
LUMO) could induce electron relocalization at the interface of the
substrate/oxygen molecules and TA-G and thus activate these
reactants for further oxidation reactions.
Figure 2. UPS spectra in the cutoff (Ecutoff) (a) and the onset (Ei) (b) energy
regions of TA-G. (c) Mott-Schottky plots for TA-G at different frequency. (d) The
conduction band (CB) and valence band (VB) position of TA-G, NG and Gr are
calculated based on the UPS and Mott-Schottky plots.
Figure 1. (a) The synthetic process of TA-G from thermal condensation of
glucose and dicyandiamide with g-C3N4 (upper left) formed at 600°C as
chemical and mechanical templates. (b) AFM image of TA-G deposited on
freshly cleaved mica and corresponding height analysis results. STEM-ADF (c-
d) images and corresponding schematic structure (e) of TA-G sample. EELS (f),
13C solid state NMR (g) and FTIR (h) spectra of TA-G and control samples.
Enlightened by the unique chemical and band structure of TA-G,
we initially examined its catalytic performance of activating
oxygen gas for organic synthesis under mild conditions. The
oxidative coupling reaction of amines is a typical model reaction
for evaluating the catalytic performance of various carbonaceous
catalysts.[24-26] We thus selected the oxidation of benzylamine as
a model reaction by using N’N-dimethylacetamide as the optimal
solvent (Table S1) and oxygen gas as the oxidant to test the
possibility of TA-G as carbocatalysts without the assistance of any
other additives. As shown in Table 1, oxygen gas could not be
activated to oxidize the benzyl amine without the involvement of
a catalyst at 60 °C. As the state-of-the-art metal-free catalyst, the
boron and nitrogen co-doped graphenes (BNHG) could give a
moderate conversion (40 %) and a good selectivity of 94% to n-
benzylidenebenzylamine. Moderate conversions were also
archieved by the control samples NG and Gr with more or less
nitrogen dopants. The TA-G catalyst offered the highest
conversion of benzylamine (93%) and excellent selectivity to
imine (>99%) among the used control sample (Table 1 and S3),
rather suggesting the important effect of the triazine units on
accelerating the activation of oxygen gas.
The presence of trianzine rings in TA-G sample was further
confirmed by the solid nuclear magnetic resonance (NMR) of 13C,
directly convincing the existence of triazine rings in TA-G with
typical signals the same with those of triazine subunits in g-C3N4
sample (Figure 1g).[22,23] The breathing mode of the triazine rings
with a typical fourier transform infrared spectroscopy (FTIR) peak
around 800 cm-1 (Figure 1h and S14) further demonstrated the
integration of triazine rings in the TA-G sample. Such a breathing
mode peak disappeared in the FTIR spectra of nitrogen doped
o
graphene (NG, sample obtained at 900 C) and graphene (Gr,
sample obtained at 900 oC) samples obtained at high
temperatures (>900 oC), suggesting the heat-induced
deconstruction of the triazine rings (Figure S14).
The introduction of trianzine subunits also lead to appropriate
band gap. The ultraviolet photoelectron spectroscopy (UPS)
(Figure 2a, b and S11) suggests a much lower valence band
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In a further set of experiments, we examined the general efficacy
and functional-group tolerance of TA-G under the typical reaction
condition for selective oxidation of substituted benzylamines
(Figure S16) with both electron-withdrawing (-Cl, -CF3) and
electron-donating (-CH3, -OCH3) functional groups. TA-G could
smoothly trigger the oxidative coupling of most substituted
benzylamines with excellent conversion and selectivity. The steric
effect decreased the conversion of 2, 5-dimethoxybenzylamine
towards corresponding imines. All these results suggested the
importance of the pre-adsorption of the substrate molecules on
the surface of TA-G for the mild and efficient oxidation reactions.
stimulate their chemical reactivity for various reactions. TA-G
could activate oxygen (1 bar) for the transformation of
benzylamine (Figure 3a), offering a conversion of 42% and a good
selectivity to imines (98%) in 24 h and nearly a complete
conversion with constant selectivity (97%) within 9 days. The turn
over frequency (TOF) value of TA-G (Figure 3b) was calculated
to be 0.202 h-1, whilst the TOF values of NG and Gr are negligible
under fixed conditions.
A
conversion of 64% for heterocyclic amines 2-
thiophenemethylamine over the TA-G catalyst with a high
selectivity (99%) further suggests the good compatibility of the
TA-G catalyst to sulphur heteroatoms, which are usually toxic
species to the metal-based catalysts.
Table 1. Study of reaction conditions.
Entry
Catalyst
T (°C)
60
Conv.(%)
Sel. (%)
-
1
--
--
2
BNHG
TA-G
NG
60
40
93
47
36
33
6
94
3
60
99
4
60
99
5
Gr
60
99
6
mpg-C3N4
g-C3N4
TA-G
TA-G
60
99
7
60
99
Figure 3. Room-temperature oxidation (a) of benzylamine over TA-G and
corresponding TOF values (b) as compared with NG and Gr catalysts. TOF was
calculated on the basis of the conversions (8%) at 4 h. Reaction conditions: 40
mg of TA-G, 5 mL of N’N-dimethylacetamide, 0.5 mmol benzylamine, 1 bar O2,
27 °C. The electronic partial density of states (PDOS) for theoretical models of
triazine- (c), graphitic nitrogen- (d) and pyridinic nitrogen- (e) decorated
graphenes and calculated adsorption energy (f) for oxygen molecule; black
area: the interaction between oxygen and catalyst; gray: the catalyst model; red:
the adsorbed oxygen molecule. For detailed calculation models please see
Figure S15. (g) O2-TPD results of TA-G, NG and Gr.
8
27
42
75
98
9a
60
59
Typical reaction conditions: 40 mg of catalyst, 5 mL of N’N-dimethylacetamide,
0.5 mmol benzylamine, 1 bar O2, 24 h. a 100 mol% acetic acid was added, the
only byproduct was N-acetylbenzylamine (selectivity 41%).
As the only sample containing an obvious amount of triazine rings,
the TA-G exhibited unexpected room-temperature activity to
trigger the selective oxidation of benzylamine using oxygen gas
without the assistance of additives or light irradiation. At a
temperature of 27 °C, In order to demonstrate remarkable activity
of TA-G catalyst to activate molecular oxygen for organic
synthesis, we investigated the electronic partial density of states
(PDOS) and adsorption energies of different calculation models
(Figure S15) on the basis of possible structures of nitrogen
heteroatoms in the TA-G sample. As expected, the TA-G sample
has the strongest interaction with O2 molecule and thus the
highest adsorption energy of oxygen (Figure 3f) to further
Indeed, there is a big difference in the band structures of different
C-N complex-based samples (TA-G, NG and Gr) according to the
UPS and Mott-Schottky plots (Figure 2 and S11), matching well
with the simulation results (Figure 3c-e and S15). Most
importantly, careful investigation on the PDOS plots of TA-G
(Figure 3c) showed a significant change in the position (black
arrows in Figure 3c-e) of fermi level of the absorbed O2 orbitals
(Figure 3c red) over different C-N complexes. The fermi level lies
in the middle of the oxygen 2p antibonding orbital of the triazine-
absorbed O2 (Figure 3c red), indicating an obvious weakening of
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