Key Parameters for the Synthesis of Active and Selective Nanostructured 3d Metal Catalysts Starting from Coordination Compounds – Case Study: Nickel Mediated Reductive Amination

Article abstract of DOI:10.1002/cctc.202100562

The design of nanostructured catalysts based on earth-abundant metals that mediate important reactions efficiently, selectively and with a broad scope is highly desirable. Unfortunately, the synthesis of such catalysts is poorly understood. We report here on highly active Ni catalysts for the reductive amination of ketones by ammonia employing hydrogen as a reducing agent. The key functions of the Ni-salen precursor complex during catalyst synthesis have been identified: (1) Ni-salen complexes sublime during catalyst synthesis, which allows molecular dispersion of the metal precursor on the support material. (2) The salen ligand forms a nitrogen-doped carbon shell by decomposition, which embeds and stabilizes the Ni nanoparticles on the γ-Al₂O₃ support. (3) Parameters, such as flow rate of the pyrolysis gas, determine the carbon supply for the embedding process of Ni nanoparticles.

Full text of DOI:10.1002/cctc.202100562

The European Society Journal for Catalysis
Supported by
Accepted Article
Title: Key Parameters for the Synthesis of Active and Selective
Nanostructured 3d Metal Catalysts Starting from Coordination
Compounds – Case Study: Nickel Mediated Reductive Amination
Authors: Rhett Kempe, Mara Klarner, Patricia Blach, Haiko
Wittkaemper, Niels de Jong, and Christian Papp
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To be cited as: ChemCatChem 10.1002/cctc.202100562
Link to VoR: https://doi.org/10.1002/cctc.202100562
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10.1002/cctc.202100562
ChemCatChem
FULL PAPER
Key Parameters for the Synthesis of Active and Selective
Nanostructured 3d Metal Catalysts Starting from Coordination
Compounds – Case Study: Nickel Mediated Reductive Amination
Mara Klarner,[a] Patricia Blach,[b,c] Haiko Wittkämper,[d] Niels de Jonge,[b,c] Christian Papp,[d] and Rhett
Kempe*[a]
[a]
M. Klarner, Prof. Dr. R. Kempe
Anorganische Chemie II – Catalyst Design, Sustainable Chemistry Centre
University of Bayreuth
Universitätsstraße 30, 95440 Bayreuth (Germany)
E-mail: kempe@uni-bayreuth.de
[b]
[c]
P. Blach, Prof. Dr. N. de Jonge
INM – Leibniz Institute for New Materials
Campus D2 2, 66123 Saarbrücken (Germany)
P. Blach, Prof. Dr. N. de Jonge
Department of Physics
Saarland University
Campus D2 2, 66123 Saarbrücken (Germany)
H. Wittkämper, Dr. C. Papp
[d]
Physical Chemistry II
Department of Chemistry and Pharmacy, 91058 Erlangen (Germany)
Supporting information for this article is given via a link at the end of the document.
Abstract: The design of nanostructured catalysts based on earth-
abundant metals that mediate important reactions efficiently,
selectively and with a broad scope is highly desirable. Unfortunately,
the synthesis of such catalysts is poorly understood. We report here
on highly active Ni catalysts for the reductive amination of ketones
by ammonia employing hydrogen as a reducing agent. The key
functions of the Ni-salen precursor complex during catalyst synthesis
have been identified: (1) Ni-salen complexes sublime during catalyst
synthesis, which allows molecular dispersion of the metal precursor
on the support material. (2) The salen ligand forms a nitrogen-doped
carbon shell by decomposition, which embeds and stabilizes the Ni
nanoparticles on the γ-Al2O3 support. (3) Parameters, such as the
flow rate of the pyrolysis gas, determine the carbon supply for the
embedding process of Ni nanoparticles.
primary amines with a broad scope and functional group
tolerance had been found until 2019. We then presented a
nanostructured Ni catalyst for the synthesis of primary amines
by reductive amination, using ammonia dissolved in water.[5]
Our catalyst had a broad scope and an exceptional tolerance
towards functional groups, operated at low temperature and
pressure, was highly active, reusable, and easy to handle. The
synthesis from
a specific Ni complex, namely, a Ni-salen
complex, and γ-Al2O3 was straightforward, and the ligand-metal
combination of this complex was crucial. Other interesting earth-
abundant metal catalysts, synthesized via the pyrolysis of salen
complexes, were reported by us[6] and the Beller group.[7,8]
The superior performance of catalyst systems based on salen
precursor complexes was demonstrated in all these publications.
Unfortunately, the role of salen complexes in the pyrolysis-based
catalyst synthesis is incompletely understood.
Introduction
Herein, we report on a highly active Ni catalyst for the reductive
amination of ketones by ammonia employing hydrogen as
reductant. This Ni/Al2O3 catalyst was obtained by varying the
nickel salen complex precursors used for the catalyst synthesis.
Thereby, the key functions of the Ni-salen precursor complex
during catalyst synthesis have been identified: (1) The volatility
of Ni-salen complexes allows the molecular dispersion of the
metal precursor on the support material, which enables an
optimal bottom-up approach for the preparation of catalytically
active metal sites. (2) Tailored decomposition of the carbon and
nitrogen containing salen ligand forms a nitrogen-doped carbon
shell that covers the catalytically active nickel nanoparticles,
thereby stabilizing them. (3) A specific set of parameters during
the pyrolysis step in catalyst generation determines the carbon
supply, which is crucial for the embedding process of
nanoparticles. A too high carbon supply particularly favors the
undesired formation of carbon nanotubes.
Reductive amination is a very important reaction because the
products, alkyl amines, are used intensively as fine and bulk
chemicals, pharmaceuticals, and agrochemicals.[1] More
specifically, primary amines are the starting material for the
production of other amines or N-heterocyclic compounds.[2] The
synthesis of primary amines via reductive amination is very
challenging, because overalkylation and other side reactions
must be avoided. The reductive amination for the synthesis of
primary amines with hydrogen as
introduced 100 years ago by Mignonac, employing
a
reducing agent was
Ni
a
catalyst.[3] The use of hydrogen as a potentially sustainable and
cost-effective reductant is particularly attractive, however, a
catalyst is required for its activation. Furthermore, inexpensive
ammonia is utilized as a nitrogen source. Despite the use of Ni
catalysts ever since,[4,5] no catalyst system for the synthesis of
1
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10.1002/cctc.202100562
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Results and Discussion
4 wt.% nickel) in acetonitrile. After removal of the solvent, the
samples were pyrolyzed in a nitrogen flow up to 700 °C, followed
by a reduction step in forming gas at 550 °C.
We developed a Ni complex library to better address the
question about the role and the mandatory nature of Ni-salen
complexes in the generation of highly active hydrogenation
catalysts. Based on this, alumina-supported catalysts were
prepared and investigated for their catalytic activity in the
reductive amination of acetophenone as a model reaction. The
Ni-salen ligand structures were selected by fine-tuning the steric
properties and carbon and nitrogen content by choosing
appropriate amine precursors and ring substituents. Starting
from the known Ni precursor C1 [5] three different aldehydes
and four different diamines, including aliphatic, N-heteroaromatic
und polyaromatic compounds, were combined. A total of six
complexes, C1 to C6, were synthesized, as outlined in Figure 1.
The corresponding Ni/Al2O3 composite catalysts Cat-1 to Cat-6
were generated along an established three-step protocol for
3d metal catalysts supported on commercial supports[5,6]: The
γ-Al2O3 support was wet impregnated with C1 to C6 (ideally
The catalytic activity of the Ni/Al2O3 catalysts Cat-1 to Cat-6 was
compared in the reductive amination of acetophenone as a
model reaction. The table in Figure 1 gives an overview of the
catalytic activity of the catalysts and the respective loading of
nickel nanoparticles. We discovered an obvious dependence of
the performance of Ni/Al2O3 catalysts on the Ni precursor
complex used. Cat-4 showed the highest activity with a yield of
60 % of 1-phenylethylamine, followed by Cat-3 (55 %). By
contrast, Cat-5 (2 %) and Cat-6 (9 %) were nearly inactive in the
reductive amination of acetophenone under these reaction
conditions. It was shown that the catalytic activity of the Ni/Al2O3
catalyst published,[5] Cat-1 (35 %), was surpassed by fine-
tuning the salen ligand, which, similar to Cat-2 (34 %), led to
only a moderate yield.
Figure 1. A library concept of 6 Ni-salen complex precursors: Ni-salen complexes C1 to C6 synthesized by fine-tuning the steric properties and carbon and
nitrogen content (left). The catalytic activity of Ni/Al2O3 catalysts Cat-1 to Cat-6 studied in the reductive amination of acetophenone depending on the Ni source
(right). In order to ensure comparability, the reaction conditions from Ref.[5] were adjusted so that no complete conversion of the reactant occurred:
1.2 mol% Ni/Al2O3 catalyst (0.006 mmol Ni, 0.35 mg Ni), 0.5 mmol acetophenone, 0.5 mL aq. NH3 (25 %, 6.7 mmol), 2 mL H2O, 80 °C, 0.5 MPa H2, 20 h. Yields
were determined by gas chromatography using n-dodecane as an internal standard. Ni content was analysed by ICP-OES.
Salen
complexes
are
known
to
sublime
without
atomic composition of the salen precursors (Supporting
Information, Table S2, Figure S2). Transmission electron
microscopy (TEM) analysis of Ni/Al2O3 materials (Supporting
Information, Figure S3) showed a subordinate dependence of
the Ni particle size on the precursor complex. The average Ni
particle size of the catalysts varied from 8.0 to 11.0 nm with
narrow size distribution. Only Ni salen complex C2 was
decomposed to larger Ni particles of 30 nm in diameter. The Ni
particle size did not alter due to catalysis, as exemplified for
Cat-4 (Supporting Information, Figure S4). Initial results did not
give a clear indication that the Ni particle size and/or the amount
of nickel, carbon and nitrogen in the Ni/Al2O3 catalyst is the
determining factor for a high catalytic activity. However, we
observed a qualitative effect of the precursor complex itself.
decomposition.[9] Consequently, the volatility of the Ni-salen
complexes was investigated by thermogravimetric analysis
(TGA) analogous to the catalyst preparation parameters
(Supporting Information, Figure S1). All Ni salen complexes C1
to C6 volatilize into the gas phase above 400 °C (Supporting
Information, Table S1). Intriguingly, the compounds C3 and C4
show significantly higher volatility than the other Ni-salen
complexes. Possibly, the attached di-tert-butyl substituted
aromatic rings and aliphatic backbones are beneficial.
Inductively coupled plasma optical emission spectroscopy
(ICP-OES) was used to analyze the amount of Ni in the Ni/Al2O3
catalysts. Depending on the volatility of the Ni-salen complex
used, the nickel loading of the Al2O3 support decreased due to
removal in the gas flow during catalyst preparation. Cat-3 and
Cat-4 showed the lowest Ni loadings of 2.8 wt.% and 2.6 wt.%,
respectively, while less volatile Ni precursors resulted in higher
Ni contents (Figure 1). In addition, the carbon and nitrogen
contents were determined by elemental analysis, reflecting the
Since there might be
a correlation between the high
hydrogenation activity of Cat-4 and the volatility of the Ni-salen
complex C4, this led to the hypothesis that volatility might play a
crucial role in the generation of catalytically active metal sites.
Therefore, we investigated the role of the Ni-salen complex
2
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10.1002/cctc.202100562
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during active catalyst formation exemplified by complex C4. The
following three key properties were identified:
pyrolysis. Considering the almost quantitative sublimation of the
pure C4 (92 %), this is a clear indication of an attractive
interaction of the Lewis-acidic Al2O3 surface and the Ni-salen
complex.
(1) Molecular dispersion of the metal precursor on the
support material. Firstly, we focused on the interaction of the
Ni-salen complex C4 with the Al2O3 support material during the
impregnation and pyrolysis steps in catalyst preparation. We
found by TEM analysis of wet impregnated C4/Al2O3 (Supporting
Information, Figure S6) that C4 crystallizes in needles several
micrometers long, starting from Al2O3 agglomerates as
nucleation centers once the solvent is removed. This method of
impregnation does not yield molecular dispersion of C4 on Al2O3
to produce small Ni nanoparticles directly during pyrolysis.
In addition, we investigated the interaction of the Ni-salen
complex C4 with the Al2O3 surface using diffuse reflectance
infrared Fourier transform spectroscopy (DRIFTS). As measured
in pure Al2O3, the vibrational band of acidic Al3-OH is centered at
3696 cm-1 and the broad signal at 1250-900 cm-1 originates from
Al-O (Figure 2b).[10] The DRIFTS spectrum shows a very broad
band in the hydroxyl spectral region (4000-2500 cm-1) since the
untreated Al2O3 sample is surface hydrated by physically
absorbed water molecules. The DRIFTS spectrum of Ni-salen
complex C4 shows signals in the fingerprint region for
wavenumbers lower than 1680 cm-1, with the characteristic
signal at 1625 cm-1 originating from the C=N imine stretching.
The bands between 2951 and 2843 cm-1 were assigned to C-H
stretching vibrations. When impregnated C4/Al2O3 was heated to
469 °C as during catalyst formation and then cooled to room
temperature, the band at 3696 cm-1 was no longer visible in the
spectrum. The lack of a characteristic Al3-OH band suggests a
surface-absorbed complex C4 interacting with acidic centers of
Al2O3. The TEM analysis of this material showed no crystals of
C4 (Supporting Information, Figure S6).
As exemplified for C4, Ni-salen complexes sublime during
Ni/Al2O3 catalyst generation with negligible decomposition. This
property allows the molecular dispersion of single complex
molecules on the Al2O3 surface from the gas phase. Since this
dispersion cannot be achieved by wet impregnation with C4, the
volatility of the complexes plays a key role in this bottom-up
approach to generate catalytically active metal sites.
(2) Formation of a nitrogen-doped carbon shell for the
stabilization of nickel nanoparticles. We recorded X-ray
photoelectron spectroscopy (XPS) survey spectra between
0-1100 eV, given in Figure S7. All expected lines for the Al2O3
support were identified, and in addition, carbon and minor traces
of nitrogen were found. Small Ni signals were detected in the
Ni 2p region (Figure 3b), but further Ni lines were not identified
due to their small photoemission cross section and their partial
overlap with Al lines. The Ni/Al2O3 catalyst shows a combination
of metallic Ni0 signals located at 852.6 eV and a broad signal
located at around 854.6 eV which we assigned to oxidized Ni2+.
Contributions of Ni3+ are also possible due to the width of the
signal. The 6 eV satellite of Ni is found as a broad feature
between 858.6 and 865 eV.[11] The intensity ratio of Ni0:Ni2+ is
approximately 1.5:2. The C 1s signal observed at 284.7 eV is
close to what is typically observed for graphitic carbon.[12] The
binding energy of the N 1s signal observed is at around
399.2 eV, possibly a remnant of the N-containing precursor
molecule and its decomposition products (Supporting
Information, Figure S7).
Figure 2. (a) TGA analysis (heating ramp 10 K/min, in N2 flow) of L4 (dashed),
C4 (black) and impregnated C4/Al2O3 (grey) demonstrating the volatility of
salen compounds. (b) DRIFTS analysis of Al2O3 (black), C4 (red) and
C4/Al2O3 (blue, heated to Tsubl 469 °C of C4) confirming the interaction of the
Ni-salen complex C4 with the Al2O3 support. The Al3-OH band is shown in the
inset.
High-angle annular dark-field scanning TEM (HAADF-STEM)
analysis of Ni/Al2O3 confirmed the homogeneous distribution of
Ni nanoparticles with an average size of 8.0 nm on the Al2O3
support material (Figure 3a, c). Energy dispersive X-ray (EDX)
elemental maps for nickel and carbon recorded in the same
image section illustrate the embedding of Ni nanoparticles in a
carbon matrix covering the entire support material (Figure 3d, e).
Electron energy loss spectroscopy (EELS) was used to study
the near environment of a single nickel nanoparticle. The carbon
Comparative TGA analysis (Figure 2a) of salen ligand L4, Ni
salen complex C4 and impregnated C4/Al2O3 showed that the
salen ligand is already volatile, being thermally stabilized by the
chelating coordination of nickel. C4 itself sublimes at
a
temperature of 469 °C without decomposing, confirmed by mass
spectrometry of the residue (Supporting Information, Figure S5).
As briefly discussed above, a Ni loading of 2.6 wt.% implies that
about 65 % of C4 was decomposed on the Al2O3 support during
3
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10.1002/cctc.202100562
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and the nitrogen component cover the Ni particle and the
surrounding support material (Figure 3f). We conclude, from
XPS and HAADF-STEM investigations that the decomposition of
the salen ligand on γ-Al2O3 during catalyst generation provides a
defined N-doped carbon shell that stabilizes the Ni nanoparticles
on the support material.
In addition, the pyrolysis parameters for heating rate and gas
flow were varied during the catalyst synthesis. Changing the
heating ramp by ±5 K/min resulted in consistent catalytic activity
of the Ni/Al2O3 catalysts, as the sublimation properties of C4
were not significantly different within this chosen range
(Supporting Information, Figure S12). By contrast, changing the
N2 flow has a major impact on the catalytic activity. Reducing the
nitrogen flow by 75 % causes the catalytic activity to collapse
(Figure 4a), while increasing the gas flow by 25 % to the
maximum gas flow of the device shows no effect. The TEM
analysis of the less active catalyst material (Figure 4b) revealed
a broader Ni particle size distribution and a slightly larger mean
diameter of 12.5 nm than for the standard Ni/Al2O3 catalyst. The
elemental composition of this material also showed higher mass
fractions of nickel, carbon, and nitrogen (3.3 wt.% Ni, 8.1 wt.% C,
0.5 wt.% N) compared to Cat-4 (2.6 wt.% Ni, 4.6 wt.% C,
0.3 wt.% N). This led us to conclude that the volatility of the
Ni-salen complex in combination with judiciously chosen
pyrolysis parameters regulates the carbon supply during catalyst
preparation.
Figure 4. (a) The variation of the heating ramp (standard 10 K/min ± 5 K/min)
and N2 gas flow (1.25x and 0.25x standard flow) during Ni/Al2O3 catalyst
synthesis identified the flow as a critical parameter during the pyrolysis step.
Reaction conditions: 1.2 mol% Ni/Al2O3 catalyst (0.006 mmol Ni, 0.35 mg Ni),
0.5 mmol acetophenone, 0.5 mL aq. NH3 (25 %, 6.7 mmol), 2 mL H2O, 80 °C,
0.5 MPa H2, 20 h. Yields were determined by gas chromatography using
n-dodecane as an internal standard. (b) TEM analysis and the size distribution
of nickel particles are shown for Ni/Al2O3 synthesized under reduced N2 flow.
Figure 3. Characterization of Ni/Al2O3: (a) TEM image of the Ni/Al2O3 catalyst
shows that the Al2O3 support is covered with homogeneously distributed Ni
nanoparticles. The size distribution of Ni particles is given in the inset. (b) XPS
analysis of the Ni 2p3/2 region revealed minor traces of Ni within the composite
material. The Ni0 nanoparticles are surface oxidized to Ni2+/Ni3+ species.
(c) HAADF-STEM analysis of the Ni/Al2O3 catalyst and (d, e) representative
EDX element maps of nickel (red) and carbon (blue). (f) Overlapped EELS
element maps of nickel (red), carbon (blue) and nitrogen (green)
demonstrating the embedding of a Ni particle in a N-doped carbon layer.
Conclusion
In conclusion, key parameters for the synthesis of active and
selective nanostructured 3d metal catalysts starting from a
coordination compound were found, which may enable a more
rational design of such catalysts in the future.
(3) Determination of the carbon supply during the pyrolysis
step. Impregnated C4/Al2O3 material was sealed in a quartz
glass ampoule in inert atmosphere and treated according to the
standard catalyst synthesis process to suppress the sublimation
of Ni-salen complex. Carbon nanotubes with a diameter of
30-40 nm grew in this confined gas space, starting from Ni
particles of the same size (Supporting Information, Figure S11).
The growth of carbon nanotubes initiated by 3d metal particles is
well established.[13] We assume that excess carbon, in the form
of the salen ligand and its volatile decomposition products, could
not be removed due to the absence of a gas flow, thereby
favoring the formation of the carbon nanotubes.
Acknowledgements
This work was supported by the German Research Foundation
(DFG SFB 840, B1). The authors also acknowledge the support
of the Bavarian Polymer Institute (University of Bayreuth,
KeyLab Electron and Optical Microscopy). We thank T. Feller
(TGA measurements) and B. Brunner (ICP-OES measurements).
The help of H. Kurz, J. Timm and R. Marschall (DRIFTS
measurements) is gratefully acknowledged. Furthermore, we
thank the Elite Network Bavaria for financial and other support,
4
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Keywords: reductive amination • Ni catalyst • coordination
compounds • N-doped carbon • sustainable catalysis
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10.1002/cctc.202100562
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The Ni/Al2O3 catalyst is highly active in the reductive amination of ketones using ammonia and hydrogen as reducing agents. The key
role of Ni-salen precursor complexes in the catalyst synthesis was identified: Molecular dispersion on the support gives rise to Ni
nanoparticles upon decomposition of the complex, which are stabilized by a defined N-doped carbon layer.
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