Grouping Effect of Single Nickel?N4 Sites in Nitrogen-Doped Carbon Boosts Hydrogen Transfer Coupling of Alcohols and Amines

Article abstract of DOI:10.1002/anie.201809858

As a new type of heterogeneous catalyst with “homogeneous-like” activity, single-site transition-metal materials are usually treated as integrated but separate active centers. A novel grouping effect is reported for single Ni?N₄ sites in nitrogen-doped carbon (Ni/NC), where an effective ligand-stabilized polycondensation method endows Ni/NC nanocatalysts with a high content of single-site Ni up to 9.5 wt %. The enhanced electron density at each single Ni?N₄ site promotes a highly efficient hydrogen transfer, which is exemplified by the coupling of benzyl alcohol and aniline into N-benzylaniline with a turnover frequency (TOF) value of 7.0 mol_{N-benzylaniline} mol_metal^?1 h^?1; this TOF outpaces that of reported stable non-noble-metal-based catalysts by a factor of 2.

Full text of DOI:10.1002/anie.201809858

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Accepted Article
Title: Grouping effect of single Ni-N4 sites in N-doped carbons boosts
hydrogen transfer coupling of alcohols and amines
Authors: Hui Su, Peng Gao, Meng-Ying Wang, Guang-Yao Zhai, Jun-
Jun Zhang, Tian-Jian Zhao, Juan Su, Markus Antonietti, Xin-
Hao Li, and Jie-Sheng Chen
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201809858
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10.1002/anie.201809858
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COMMUNICATION
Grouping effect of single Ni-N4 sites in N-doped carbon boosts
hydrogen transfer coupling of alcohols and amines
Hui Su,[a] Peng Gao,[a] Meng-Ying Wang,[a] Guang-Yao Zhai,[a] Jun-Jun Zhang,[a] Tian-Jian Zhao,[a]
Juan Su,[a] Markus Antonietti,[b] Xin-Hao Li*[a] and Jie-Sheng Chen*[a]
Abstract: As
a
new type of heterogeneous catalyst with
Herein we report the first realization of such a grouping effect
of single-atom metal centers in nitrogen-doped carbon,
significantly boosting the activation of H-species in the hydrogen
transfer coupling reaction of alcohols and amines. Dense arrays
of coordinated Ni atoms dispersed at the N-doped carbon (Ni/NC)
catalyst provide a relatively higher electron density at the single
Ni-N4 site and substantially decrease the binding energy of the H
radical to those sites, significantly accelerating the formation of
the target amine products.
“homogeneous-like” activity, single-site transition metal materials
have usually been treated as integrated but separate active centers.
Herein, we report a novel grouping effect of single Ni-N4 sites in N-
doped carbon (Ni/NC), where an effective ligand-stabilized
polycondensation method endows Ni/NC nanocatalysts with a high
content of single-site Ni up to 9.5 wt. %. The enhanced electron
density at each single Ni-N4 site promotes a highly efficient hydrogen
transfer, here exemplified by the coupling of benzyl alcohol and aniline
into N-benzyl aniline with a turnover frequency (TOF) value of 7.0 mol
N-benzyl aniline mol-1 metal h-1, outpacing that of stable non-noble-
metal-based catalysts in the literature by a factor of 2.
The hydrogen borrowing methodology involving the back-
and-forth transfer of hydrogen has achieved tremendous progress
in green chemistry, owing to high atom efficiency and clean
byproducts, such as H2O.[1] The field of organic chemistry
developed this sustainable strategy by coupling alcohols (as the
liquid hydrogen donor) to amines for the synthesis of a variety of
multiple or advanced amines in medicinal chemistry[2] for the
therapy of some serious diseases such as depressive disorder
and tuberculosis.[3] Great effort has been made in the design of
inexpensive and abundant non-noble-metal (e.g., Co, Mn, and
Fe) complex for the homogeneous N-alkylation of amines using
alcohols as the coupling partners.[4]
From the perspective of sustainable and ecofriendly
chemistry, a synthetic scheme that uses a stable, recyclable,
effective, and low-cost non-noble-metal heterogeneous catalyst
for N-alkylation under benign conditions is an eagerly sought-after
alternative to the industrial production of complex amines in the
future. The rational design of highly active metal centers in
heterogeneous catalysts always remains at the heart of the
development of sustainable catalysis.[5] Recently, engineering the
coordination environment of metal centers via introducing various
dopants into a support has turned out to be a promising design
principle for transition-metal (M=Fe, Co, Ni)-N-doped carbon (M-
N-C) materials,[6] where the M-N-C sites are usually treated as
separate active centers. However, the metal centers applied in
high density have to change the electronic structure of the carbon
support and thereby also the other metal centers surrounding
them. To analyze this question and potential catalytic option, a
powerful synthetic method is required to introduce high
concentrations of single metal sites into the carbon support.
Figure 1. (a) Proposed synthetic process for Ni/NC from ligand-stabilized Ni(II)
(ligands: terephthalic acid and triethylene diamine) confined on the surface of
polymeric carbon nitride. Carbon atoms are black, nitrogen dark yellow, oxygen
red and nickel dark blue. TEM (b), HAADF-STEM (c) and elemental-mapping
(d) images of Ni/NC. (e) XANES spectra at the Ni K-edge of Ni/NC and controls.
(f) Fourier transformed EXAFS spectrum (line) of Ni9.5/NC sample and
corresponding simulation curve (circles) on the basis of the Ni-N4 structure
(inset). (g) Fourier transformed EXAFS spectra of Nix/NC samples.
To introduce high concentrations of metal sites (exemplified
by Ni) into the carbon matrix, we developed a novel ligand-
stabilized polycondensation method to directly convert confined
metal cations on the surface of polymeric carbon nitride, which
was subsequently used as a directional 2D-template (denoted as
PCN) with the assistance of organic ligands, as depicted well by
scanning electron microscopy (SEM) and transmission electron
microscopy (TEM) (Figures 1a-b and S1). Further powder X-ray
diffraction (XRD) (Figure S2) and X-ray photoelectron
spectroscopy (XPS) analysis (Figure S3) results demonstrate the
successful incorporation of nickel metal center into carbon
framework via pro forma Ni(I)-N bonds (Figures S4-5), without the
formation of metal particles (Figure 1b). The bright spots in the
high-angle annular dark-field transmission electron microscopy
(HAADF-TEM) images (Figures 1c and S6) and the elemental-
mapping images (Figure 1d) directly present the well-dispersed
Ni sites with a high concentration along the whole carbon sheet.
[a]
H. Su, P. Gao, M. Y. Wang, G. Y. Zhai, J. J. Zhang, T. J. Zhao, J.
Su, Prof. X. H. Li, Prof. J. S. Chen
School of Chemistry and Chemical Engineering, Shanghai Jiao
Tong University
Shanghai 200240 (P. R. China)
E-mail: xinhaoli@sjtu.edu.cn
[b]
Prof. M. Antonietti
Department of Colloid Chemistry, Max-Planck Institute of Colloids
and Interfaces Institution
Wissenschaftspark Golm, 14424 Potsdam, Germany
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10.1002/anie.201809858
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The X-ray absorption near-edge structure (XANES) spectra
(Figures 1e and S7) of Ni/NC reveal a distorted square-planar
structure of the M-N4 unit for Ni sites with a typical shoulder peak
at 8340 eV. The strong Ni-N peaks of Fourier transform extended-
edge X-ray absorption fine structure (EXAFS) spectra at 1.45 Å
(Figures 1f-g and S8) match well with the fitting results of the
simulated model (Figure 1f inset and Table S1), directly
demonstrating the local Ni-N4 structure.[7]
The strong coordination effect of organic intermediates with
Ni species (Figure 1a left) sufficiently stabilizes the as-formed Ni
centers at 900 °C to yield highly dispersed Ni sites with a
concentration as high as 9.5 wt. %, as determined by inductively
coupled plasma (ICP) analysis (Figure S9). It should be noted that
XPS analysis revealed nearly 57 % of Ni sites (5.46 wt. %) to be
dispersed on the surface or subsurface of the carbon support,
thus surpassing by far the exposable Ni content values of state-
of-the-art single-site metal-based catalysts in the literature (Table
S2-3). All Nix/NC samples show a similar morphology, porosity
and C/N content (Figure S10-11 and Table S2), where x is the
measured metal weight percentage via ICP. Further increasing
the metal concentration of Ni up to 16.6 wt. % induced the
formation of Ni nanoparticles (Figure 1g). The fact that Ni
nanoparticles also form in the absence of terephthalic acid and
triethylene diamine as primary stabilizers even at a Ni content less
than 3 wt. %[8] underlines the key role of strong ligation by educts
and intermediates throughout the condensation process in
stabilizing a high concentration of Ni sites.
both the conversion (from 33 % to 99 %) and selectivity to N-
benzyl aniline (from 9 % to 92 %) (Figure 2a and Entry 4-5 of
Table S4). The Ni16.6/NC sample with Ni nanoparticles (Figures
1g, S2, and S10) gave a relatively lower activity (Entry 6 of Table
S4), suggesting single Ni-N4 sites as possible active centers for
the N-alkylation reaction. Ni9.5/NC was selected as the best
catalyst (Table S5-6) here for the subsequent characterizations.
The strong metal-support coordination effect between the Ni
centers and the N-doped carbon matrix ensures effective catalytic
activity of Ni9.5/NC even after four recycling runs (Figure S12)
without a detectable amount of Ni leached into the supernatant
(ICP, data not shown) or obvious textural changes (Figure S13-
14). The rich availability of exposed and stable Ni-N4 sites makes
it possible to realize complete conversion of aniline in such a
hydrogen transfer coupling reaction with a high substrate-to-metal
mole ratio of 185 in a solvent-free system (Figure 2b), reflecting
well the catalytic reactivity and sustainability of the single Ni-N4
sites. A turnover number (TON) of 170.6 was achieved over the
Ni9.5/NC catalyst, which is comparable to those of even noble-
metal-based nanocatalysts (Table S7-8).
Figure 2. (a) Yields of N-benzyl aniline over Nix/NC (x=4.8, 9.5, 16.6). Standard
conditions: 1 mmol aniline, 2 mmol benzyl alcohol, 10 mL of toluene, 100 mg of
Nix/NC catalyst, 160 °C, 4 h and 1 bar of argon gas. (b) Conversions of aniline
(Con.) and selectivity to N-benzyl aniline (Sel.) over the Ni9.5/NC catalyst at
different time intervals under solvent-free conditions. Reaction conditions: 30
mmol aniline, 60 mmol benzyl alcohol, 100 mg of Ni9.5/NC catalyst, 160 °C, 1
bar of argon gas.
Figure 3. Adsorption models of (a) NC and (c) Ni9.5/NC for the preadsorption
of benzalaniline and calculated adsorption energies (b). Yields of N-benzyl
aniline (red) and benzalaniline (green) over NC (d) and Ni9.5/NC (e).
Hydrogen transfer coupling of benzyl alcohol and aniline over Ni9.5/NC-H+ (f)
and SCN--poisoned Ni9.5/NC (g). Yields of N-benzyl aniline and benzalaniline
over Ni/NC for coupling reaction of benzaldehyde and aniline under
hydrogen (h) and argon (i) gas atmosphere. (g) Proposed reaction pathway
of aniline and benzyl alcohol over Ni/NC. Standard conditions: 1 mmol
aniline, 2 mmol benzyl alcohol, 10 mL of toluene, 100 mg of catalyst, 160 °C,
4 h and 1 bar of argon or hydrogen gas; (g) KSCN (20 mol %) was added;
(h-i) 2 mmol benzaldehyde was used to replace benzyl alcohol. Inset: (e) the
calculated structure of N-Ni couples of benzalaniline and Ni-N4 site.
The hydrogen transfer coupling of aniline and benzyl alcohol
into N-benzyl aniline (target product) as a model reaction was
used to estimate the performance of all Ni/NC catalysts and
control samples.[1b, 2b, 4b] Blank reaction without a catalyst (Entry 1
of Table S4) or using Ni(II) phthalocyanine (Ni(II)-Pc) or a
mechanical mixture of Ni(II)-Pc and nitrogen-doped carbon as
catalysts (Entry 2-3 of Table S4) could only give trace to moderate
conversions without the formation of detectable N-benzyl aniline.
In contrast, Ni/NC samples could trigger the formation of N-benzyl
aniline under standard conditions. Elevating the concentration of
Ni sites from Ni4.8/NC to Ni9.5/NC led to a significant increase in
The key role of the Ni-N4 group in boosting the selectivity
to N-benzyl aniline, addressing the critical problem of rather
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10.1002/anie.201809858
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low selectivity of the traditional oxide-supported Ni-based
catalysts,[9] has been examined via theoretical and
experimental methods (Figure 3). Density functional theory
(DFT) simulation results reveal the favorable preadsorption of
the benzalaniline molecule, the intermediate and main
byproduct obtained from the coupling reaction of the
dehydrogenated alcohol and aniline, on the surface of the
carbon framework (NC) in a coplanar fashion due to the strong
interaction (Figures 3a and S15). The further introduction of a
single Ni-N4 site into the carbon lattice results in a more
negative adsorption energy of benzalaniline on Ni9.5/NC
(Figure 3b) and a preferred orientation of the C=N double bond
of benzalaniline to the Ni-N4 site (Figures 3c and S15) to further
promote the hydrogenation process.
Bare NC could only trigger the dehydrogenation of benzyl
alcohol to benzaldehyde (Figure 3d and Entry 7 of Table S4)
for the successive coupling to benzalaniline (Figures 3i and
S16) due to its strong binding of aromatic molecules via -
charge transfer interactions.[10] At the earlier stage of the
coupling reactions over the Ni9.5/NC catalyst (Figure 3e),
benzalaniline is the main product, suggesting the hydrogen
transfer coupling process as the main path. Further
hydrogenation of benzalaniline proceeds smoothly over the
Ni9.5/NC catalyst (Figure 3e) at a much higher reaction rate,
giving the highest yield (94 %) of N-benzyl aniline within 7 h
under fixed conditions. The hydrogen transfer process from
carbon support to the single Ni-N4 sites was further confirmed
by the quenching experiments, with much lower yields of N-
benzyl aniline using protonated Ni9.5/NC (Ni/NC-H+) (Figure 3f
and Entry 8 of Table S4) with the same Ni centers (Figure S17
and Table S2) and poisoned Ni9.5/NC (Ni9.5/NC-SCN-) catalysts
to block the hydrogen transfer (Figure 3g and Entry 9 of Table
S4) and mass transfer paths to the single Ni-N4 sites,
respectively. The direct activation of hydrogen gas over the
Ni9.5/NC catalyst does not promote the formation of N-benzyl
aniline under otherwise fixed conditions (Figure 3h and Entry
10 of Table S4), directly indicating the key importance of the
synergistic effect between the Ni-N4 sites and the carbon
support to trigger the whole hydrogen borrowing process as
depicted in Figure 3j.[11]
The enhanced electron enrichment of the Ni-atom group
in Ni9.5/NC slightly depressed the binding energies of hydrogen
radicals to the single Ni-N4 sites, promoting the transfer
process of the Ni-H bond for the further hydrogenation of
imines into amines (Figure 4f).[13] Indeed, the simulation results
(Figures 4g and S21) indicate a relatively lower density of anti-
bonding states (ψ*) of the adsorbed H-atom on Ni9.5/NC (red
line) and thus a lower affinity of the H radical for the Ni9.5/NC-
H sample, promoting the transfer process of the Ni-H bond for
the further hydrogenation of imines into amines.[13,14] As
expected, Ni9.5/NC with the largest group of atomically
dispersed Ni centers exhibits the highest turnover frequency
(7.0 h-1) among all Nix/NC samples and far exceeds the value
of air-stable non-noble-metal-based catalysts in the literature
(orange line in Figure 4h and Table S9-10) by more than a
factor of two.
We further investigated the details of the interaction and
electronic structure among single Ni-N4 sites in generating the
dependence of the hydrogen transfer process on Ni site
density. DFT simulations suggest that the first nickel site on the
support with its rather positive work function indeed transfers
electron density to the support. This grouping effect saturates
with higher Ni-site-density, and the Ni centers in the Ni9.5/NC
sample are relatively electron-rich (Figures 4a-b and S18), as
revealed by the upward shift of the pyridinic N 1s XPS peaks
(Figure 4d), downward shift of the typical Ni 2p XPS peaks
(Figure S19) and the strongest EPR signal of unpaired
electrons in this sample (Figure 4e). The electron enrichment
of the Ni9.5/NC and thus lowered work functions was doubly
confirmed by both the calculated values and the measured
work functions via ultraviolet photoelectron spectroscopy
(UPS) analysis (Figures 4c and S20). This is to say that the in-
built Ni heteroatoms indeed “communicate with each other” via
electrons through the conductive support.[12]
Figure 4. (a) Proposed structure (left) of Nix/NC (x= 4.8, 9.5) according to
the ICP and XPS analysis results and calculated electron density map
(middle and right). (c) Measured (spheres) work function () and calculated
Fermi level (squares) of Nix/NC samples. N 1s XPS (d) and EPR (e) spectra
of Nix/NC (x=0, 4.8, 9.5) samples. (f) DFT-calculated binding energy (Eb) of
a hydrogen radical adsorbed on Nix/NC. (g) Partial density of states (PDOS)
of an H atom adsorbed on Nix/NC. (h) TOF values for the synthesis of N-
benzyl aniline from aniline and benzyl alcohol over the Nix/NC catalysts and
the state-of-the-art Ni-based catalyst (orange line) in the literature.
In summary, we demonstrated that grouping and
electronic interaction of single Ni-N4 sites in N-doped carbon
modify the electronic structure of Ni/NC to largely prompt the
N-alkylation process of alcohols and amines via a hydrogen
transfer mechanism. A suitably dense arrangement of single
Ni-N4 sites embedded within the N-doped carbon framework
ensured high conversion and chemoselectivity towards
asymmetric aromatic aniline compounds. The application of
the grouping effect of single Ni-N4 sites in N-doped carbon
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Acknowledgements
National Natural Science Foundation of China (21722103,
21720102002 and 21673140), Shanghai Basic Research
Program (16JC1401600), SJTU-MPI partner group, Shanghai
Eastern Scholar Program and Shanghai Rising-Star Program
(16QA1402100), supported this work. The authors thank the
Shanghai Synchrotron Radiation Facility for providing beamtime
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COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
The grouping effect of single Ni-N4
sites was observed in an N-doped
carbon supported Ni catalyst, of
which the Ni content was endowed
by an effective ligand-stabilized
polycondensation method up to 9.5
wt. %. The enhanced electron
Hui Su, Peng Gao, Meng-Ying Wang,
Guang-Yao Zhai, Jun-Jun Zhang,
Tian-Jian Zhao, Juan Su, Markus
Antonietti, Xin-Hao Li* and Jie-Sheng
Chen*
Page No. – Page No.
density at each single Ni-N4 site by
such a collective grouping effect
decreases the binding energy of H
radicals to Ni, promoting a highly
efficient coupling of benzyl alcohol
and aniline into N-benzyl aniline.
Grouping effect of single Ni-N4 sites
in N-doped carbon boosts
hydrogen transfer coupling of
alcohols and amines
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