Co-Catalyzed Synthesis of Primary Amines via Reductive Amination employing Hydrogen under very mild Conditions

Article abstract of DOI:10.1002/cssc.202100553

Nanostructured and reusable 3d-metal catalysts that operate with high activity and selectivity in important chemical reactions are highly desirable. Here, a cobalt catalyst was developed for the synthesis of primary amines via reductive amination employing hydrogen as the reducing agent and easy-to-handle ammonia, dissolved in water, as the nitrogen source. The catalyst operates under very mild conditions (1.5 mol% catalyst loading, 50 °C and 10 bar H₂ pressure) and outperforms commercially available noble metal catalysts (Pd, Pt, Ru, Rh, Ir). A broad scope and a very good functional group tolerance were observed. The key for the high activity seemed to be the used support: an N-doped amorphous carbon material with small and turbostratically disordered graphitic domains, which is microporous with a bimodal size distribution and with basic NH functionalities in the pores.

Full text of DOI:10.1002/cssc.202100553

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Very Important Paper
Co-Catalyzed Synthesis of Primary Amines via Reductive
Amination employing Hydrogen under very mild
Conditions
Matthias Elfinger,[a] Timon Schönauer,[a] Sabrina L. J. Thomä,[b] Robert Stäglich,[c]
Markus Drechsler,[d] Mirijam Zobel,[b] Jürgen Senker,[c] and Rhett Kempe*[a]
Nanostructured and reusable 3d-metal catalysts that operate
with high activity and selectivity in important chemical
reactions are highly desirable. Here, a cobalt catalyst was
developed for the synthesis of primary amines via reductive
amination employing hydrogen as the reducing agent and
easy-to-handle ammonia, dissolved in water, as the nitrogen
source. The catalyst operates under very mild conditions
outperforms commercially available noble metal catalysts (Pd,
Pt, Ru, Rh, Ir). A broad scope and a very good functional group
tolerance were observed. The key for the high activity seemed
to be the used support: an N-doped amorphous carbon
material with small and turbostratically disordered graphitic
domains, which is microporous with a bimodal size distribution
and with basic NH functionalities in the pores.
°
(1.5 mol% catalyst loading, 50 C and 10 bar H2 pressure) and
Introduction
replacement of rare elements in key technologies is very
important too. Catalysis, a key technology of our century, is often
based on rare noble metals such as Pd, Pt, Rh, and Ir. Earth-
abundant metal catalysts have played an important role with
respect to the development of reductive amination in its early
days,[4] in particular Raney Ni. Note that Raney Ni is difficult to
handle and its reusability is limited. Very recently, nanostructured
and reusable Co catalysts have been introduced for the general
synthesis of primary amines from ammonia and ketones or
aldehydes and hydrogen (Scheme 1).[5] Unfortunately, these
catalysts operate under rather harsh conditions. A general
problem with 3d-metal catalysts is their significantly lower
activity in many reactions in comparison to noble metal catalysts.
Thus, reusable or nanostructured catalysts based on abundantly
available elements that operate under very mild conditions and
Reductive amination, the reaction of an aldehyde or a ketone
and ammonia or an amine in the presence of a reducing agent,
is a very important chemical transformation intensively inves-
tigated and used in industry and academia.[1] The use of
inexpensive and potentially green hydrogen as the reducing
agent in the presence of a catalyst is especially attractive.[1] The
products of reductive aminations, alkyl amines, are very
important bulk and fine chemicals and/or biologically active
compounds such as pharmaceuticals or agro chemicals.[2]
Especially challenging and important is the synthesis of primary
amines. The reaction is challenging since over-alkylation and
other unwanted side reactions have to be avoided, and it is
crucial since primary amines are important starting materials to
synthesize other amines or N-heterocyclic compounds.[3] The
[a] M. Elfinger, T. Schönauer, Prof. Dr. R. Kempe
Inorganic Chemistry II – Catalyst design, Sustainable Chemistry Centre
University of Bayreuth
95440 Bayreuth (Germany)
E-mail: kempe@uni-bayreuth.de
[b] S. L. J. Thomä, Prof. Dr. M. Zobel
Solid State Chemistry – Mesostructured Materials
University of Bayreuth
95440 Bayreuth (Germany)
[c] R. Stäglich, Prof. Dr. J. Senker
Inorganic Chemistry III and North Bavarian NMR center
University of Bayreuth
95440 Bayreuth (Germany)
[d] Dr. M. Drechsler
Bavarian Polymer Institute (BPI), Keylab “Electron and Optical Microscopy”
University of Bayreuth
95440 Bayreuth (Germany)
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cssc.202100553
© 2021 The Authors. ChemSusChem published by Wiley-VCH GmbH. This is
an open access article under the terms of the Creative Commons Attribution
License, which permits use, distribution and reproduction in any medium,
provided the original work is properly cited.
Scheme 1. Reusable and nanostructured Co catalysts for the reductive
amination of ketones and aldehydes to primary amines using hydrogen as
the reducing agent.
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are competitive with noble metal catalysts are highly desirable.
We have recently introduced Ni[6] and Fe[7] catalysts and report
here on a highly active and reusable Co catalyst for the selective
synthesis of primary amines via reductive amination employing
hydrogen as the reducing agent. Our catalyst operates under
°
very mild conditions (1.5 mol% catalyst loading, 50 C and 10 bar
H2 pressure) and outperforms commercially available noble metal
catalysts (Pd, Pt, Ru, Rh, Ir), and ammonia dissolved in water,
which is easy to handle and inexpensive, is used. We observe a
broad scope with good functional group tolerance that
encompasses aryl-alkyl, diaryl, dialkyl ketones and aldehydes,
which can be converted into the corresponding primary amines.
Furthermore, the amination of biologically active compounds has
been demonstrated. The key for the high activity seems to be
the used N-SiC support. It is an N-doped (�7 wt%) amorphous
°
carbon material processed at 1000 C having graphitic domains
smaller than 2 nm with a turbostratic disorder and widened
average interlayer distances. The material is microporous with a
bimodal pore size distribution, which look as if to be of a
chemically different origin. The larger of them seem to contain
accessible basic NH functionalities most likely formed via
hydrolysis of CÀ NÀ Si bonds during NaOH treatment. Besides the
above-mentioned two Co catalysts for the synthesis of primary
amines from ketones or aldehydes and ammonia, other Co
catalyst systems for reductive amination reactions employing
hydrogen as the reducing agent have been introduced.[8]
Results and Discussion
Catalyst synthesis and characterization
First, the N-SiC support was synthesized following published
procedures by crosslinking the commercially available polycarbo-
silane precursor SMP 10 and acrylonitrile in dimethylformamide,
using azobis-isobutyronitrile as an initiator.[9] The greenbody was
°
pyrolyzed at 1000 C under inert atmosphere and then treated
°
with 1 m NaOH at 85 C overnight (see the Supporting
Information 2.1). To obtain the active catalyst, the NÀ SiC support
was impregnated with Co(NO3)2 ·6H2O dissolved in water,
°
°
pyrolyzed (700 C, N2) and reduced (550 C, N2/H2). Inductively
coupled plasma optical emission spectroscopy (ICP-OES) analysis
of the catalyst revealed a cobalt content of 2.24 wt% (see the
Supporting Information 3.1). The specific surface area could be
determined by argon physisorption measurements (Figure 1C)
and showed a decline in surface area from 545 m2 gÀ 1 of the
support material to 491 m2 gÀ 1 of the catalyst. A homogeneous
distribution of the cobalt nanoparticles over the N-SiC support
(Figure 1B) and an average particle size of 5.9 nm could be
determined via transmission electron microscopy (TEM). In
addition, X-ray photoelectron spectroscopy (XPS) showed the
presence of metallic cobalt and cobalt oxi(hydroxide) species as
well as different binding modes of nitrogen within the support
material at the surface of the catalyst material (see the
Supporting Information Figure S2). Scanning electron microscopy
(SEM) in combination with energy dispersive X-ray spectroscopy
(EDX) confirmed the homogeneous distribution of the cobalt
Figure 1. Synthesis and characterization of the Co catalyst: (A) Synthesis of
the crosslinked material, followed by pyrolysis and NaOH treatment to
obtain the support material; wet impregnation, pyrolysis and reduction led
to the active Co catalyst. (B) TEM analysis suggested the presence of
homogeneously distributed Co nanoparticles with an average particle size of
5.9 nm. High-resolution (HR)TEM micrograph with atomic resolution of a Co
nanoparticle surface. (C) Surface characterization and pore size distribution
of the catalyst via argon physisorption measurement [calculation model: Ar
at 87 K on carbon (cylindr. pores, NLDFT equilibrium model)]. The specific
surface showed a slight decline from 545 m2 gÀ 1 of the support material to
491 m2 gÀ 1 of the catalyst. (D) The differential-PDF of (Co/N-SiC)-(N-SiC) (blue)
was fitted with a two-phase fit (red) containing a Co fcc (green) and a CoO
fcc (pink) phase. Remaining distance correlations in the difference curve
(grey) exist for the short-range order up to 5 Å.
nanoparticles over the N-SiC support (see the Supporting
Information Figure S3). Pair distribution function (PDF) studies
(Figure 1D) indicate the presence of Co fcc and CoO fcc
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nanoparticles. The Co domain size was refined to about 9 nm in
comparison to the CoO domain size with about 3 nm (see the
Supporting Information Table S3). An average particle diameter
of 5.9 nm with a distribution from 3–9 nm was determined by
TEM (Figure 1B). This suggests that separate Co and CoO
nanoparticles exist rather than Co and CoO domains within one
particle.
TEM investigation of these materials revealed inhomogeneously
distributed or barely present nanoparticles (see the Supporting
Information Figure S4). A highly active Ni/Al2O3 catalyst intro-
duced by our group in 2019 also showed no activity when
applied under the conditions mentioned above.[6] For this Ni-
catalyst, a Ni complex stabilized by a N,O-ligand called salen
was used as a metal precursor and the C- and N-atoms of the
ligand provide an N-doped carbon matrix in which the Ni
nanoparticles are embedded and, so, fixed at the Al2O3 support.
It might be that the very mild reaction conditions make it very
challenging for other supports in combination with inexpensive
simple metal precursor Co nitrate. When comparing the activity
and selectivity of well-known and commercially available noble
metal catalysts to our cobalt system, it can be shown that the
latter is outperforming all noble catalysts applying the same
reaction conditions. Ru/C and Rh/C are inactive regarding the
formation of the primary amine 1. Other metals (Pd, Pt, and Ir)
form the desired product of about 50% and simultaneously
side products based on acetophenone hydrogenation mainly 1-
phenylethanol (2) and 1-phenyl-cyclohexylalcohol (3) (Table 2).
When compared to the literature where the selective reductive
amination of ketones to branched primary amines using
ammonia is addressed, all papers except one report harsher
conditions in terms of temperature and hydrogen pressure than
we have used for our cobalt catalyst system.[10] In 2020, Wang
and co-workers reported on palladium nanoparticles for the
reductive amination of 16 aromatic ketones with aqueous
ammonia under milder conditions.[11] However, extremely high
catalyst loadings of 37 mol% Pd were used.
Optimization of the reaction conditions
In order to determine the optimized reaction conditions, the
reductive amination of acetophenone to 1-phenylethylamine
was chosen as benchmark reaction. The solvent screening
revealed that a concentration of ammonia in water of 32%
leads to the best yield of amine product since the high
concentration accelerates the imine formation prior to the
reduction step. The optimally adjusted reaction parameters
could be defined as 3.5 mL aqueous ammonia (32%), 10 bar H2,
°
50 C and 1.5 mol% cobalt. Since water is our preferred solvent,
ammonia gas could also be used, which would form ammonia
dissolved in water then. We further investigated the influence
of varying metal loadings of the catalyst (Table 1). The experi-
ments revealed that the catalyst with theoretically 3 wt% cobalt
showed the best activity in the reductive amination. Lowering
°
the pyrolysis temperature to 600 C led to
a
significant
°
reduction of yield. Applying a higher temperature (800 C) led
°
to better results than 600 C but could not match the superior
°
activity of the catalyst that was pyrolyzed at 700 C. Pure N-SiC
support as well as cobalt nitrate on various commercially
available supports showed no conversion of acetophenone.
Substrate scope
Since the optimized reaction conditions have been predefined,
we were interested in the substrate scope of our highly
active/selective catalyst system. The product yields refer to the
isolated corresponding hydrochloride salts of the synthesized
amines.
Table 1. Cobalt catalyst comparison.[a]
Entry Metal source Support
Pyrolysis
temperature [ C]
Yield=
Conversion [%]
°
1
Co-nitrate
(3 wt% Co)
Co-nitrate
Co-nitrate
Co-nitrate
(2 wt% Co)
Co-nitrate
(4 wt% Co)
Co-nitrate
(5 wt% Co)
Co-nitrate
Co-nitrate
Co-nitrate
Co-nitrate
–
N-SiC
700
99
Table 2. Reductive amination with noble metal catalysts.[a]
2
3
4
N-SiC
N-SiC
N-SiC
600
800
700
3
87
21
5
6
N-SiC
N-SiC
700
700
91
32
Entry
Catalyst
Yield 1
[%]
Yield 2
[%]
Yield 3
[%]
Other products
[%]
7
8
9
10
11
12[b]
activated C 700
Al2O3
TiO2
SiO2
N-SiC
0
0
0
0
0
0
700
700
700
700
700
13[b]
14[c]
15[d]
16[e]
17[f]
Pd/C
Pt/C
Ru/C
Rh/C
Ir/C
46
48
3
0
49
53
32
58
0
–
–
6
48
–
19
32
51
20
Ni salen
Al2O3
30
–
complex
°
[a] Reaction conditions: 0.5 mmol acetophenone, 50 C, 20 h, 10 bar H2,
3.5 mL aq. NH3-32%. Yields were determined by GC using n-dodecane as
an internal standard. [b] 2.0 mol% Pd (10.0 wt% Pd). [c] 2.0 mol% Pt (5.0 wt
% Pt). [d] 2.0 mol% Ru (5.0 wt% Ru). [e] 2.0 mol% Rh (5.0 wt% Rh).
[f] 2.0 mol% Ir (1.0 wt% Ir).
[a] Reaction conditions: 1.5 mol% Co (2.24 wt% Co, 0.0075 mmol Co,
0.44 mg Co), 0.5 mmol acetophenone, 50 C, 20 h, 10 bar H2, 3.5 mL aq.
NH3-32%. Yields were determined by GC using n-dodecane as an internal
standard. [b] 2.0 mol% Ni (4.0 wt% Ni).
°
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First, we investigated the reductive amination of aryl-alkyl
benzophenone (Scheme 2, product 15) showed that aryl-aryl
ketones can also be converted with slightly harsher conditions,
which are also needed for the sterically demanding benzyl-4-
fluorophenyl ketone (product 18). The amination of a N-
heterocyclic ketone (Scheme 2, product 16) proceeded smoothly
with an isolated yield of 91%. The corresponding amine of 1-
indanon could be acquired with a 62% yield (product 17). For
the reductive amination of benzylic aldehydes, higher temper-
atures were needed to suppress the self-coupling reaction of the
primary amine product with unconverted substrate. Therefore
ketones with various substituents. Substrates with short, moder-
ate and long aliphatic moieties (Scheme 2, products 1–3) could
be isolated in very good yields of 84–99%. The conversion of
ketones with electron-withdrawing substituents, such as halo-
gens, also worked out well with yields of over 90% for the para-
substituted derivatives (Scheme 2, products 4–6). The sterically
more demanding meta-chloroacetophenone (Scheme 2, product
7) could also be converted with an isolated yield of 90%. The
heavily electron-withdrawing trifluoromethyl group (Scheme 2,
°
°
product 8) required higher reaction temperatures (60 C) to be
80 C and a catalyst loading of 1.1 mol% were used. Benzalde-
converted into its corresponding primary amine with an excellent
yield of 97%. Minor electron-donating substituents, such as
methyl groups, could be tolerated very well in meta- and para-
positions of the aromatic ring (Scheme 2, products 9 and 10). For
the ortho-substituted acetophenone, we had to increase the
hyde (Scheme 2, product 19), as well as its substituted derivates
could be converted in very good yields. Electron-donating
substituents such as methyl and methoxy groups (Scheme 2,
products 20 and 21) were tolerated very well with isolated yields
of the corresponding amines up to 88%. In addition, the
conversion of substituted benzaldehydes containing electron-
withdrawing moieties such as halogens was also accomplished
with very good yields up to 91% (Scheme 2, products 22 and
23).
°
temperature to 60 C to obtain 91% of isolated product
(Scheme 2, product 11). If the electron-donating property of the
substituents was increased (e.g., methoxy groups), meta- and
para-substituted ketones (Scheme 2, products 12 and 13), as well
as dimethoxy acetophenone (product 14) could be converted
smoothly into the corresponding primary amines without any
need to increase the temperature. The reductive amination of
In addition, we investigated the reductive amination of
purely aliphatic ketones. Substrates with long and short
branched and linear aliphatic chains as well as cyclic ketones
could be aminated in moderate to very good yields (36–93%,
Scheme 3). Aliphatic ketones with low molecular weight could
only be isolated in moderate yields due to their low boiling
points, which caused significant losses of product during the
workup procedure.
Next, we became interested in the reductive amination of
biologically active molecules under very mild conditions. The
selective conversion of nabumetone, 4- and 2-meth-
oxyphenylacetone to their corresponding primary amines could
be accomplished in isolated yields of up to 90% (Scheme 4). The
reductive amination of steroid molecules was rather challenging,
°
due to the poor water solubility of this substance class at 50 C.
Scheme 2. Reductive amination of aryl substituted ketones and aldehydes to
primary amines. [a] Reaction conditions for ketones: 1.5 mol% Co (2.24 wt%
°
Co, 0.0075 mmol Co, 0.44 mg Co), 0.5 mmol ketone, 50 C, 20 h, 10 bar H2,
Scheme 3. Reductive amination of purely aliphatic ketones to primary
amines. Reaction conditions: 1.5 mol% Co (2.24 wt% Co, 0.0075 mmol Co,
°
3.5 mL aq. NH3-32%. [b] 60 C. [c] Reaction conditions for aldehydes:
°
°
1.1 mol% Co, 0.5 mmol aldehyde, 80 C, 20 h, 10 bar H2, 3.5 mL aq. NH3-32%.
0.44 mg Co), 0.5 mmol ketone, 50 C, 20 h, 10 bar H2, 3.5 mL aq. NH3-32%.
Isolated yields of the converted hydrochloride salts.
Isolated yields of the converted hydrochloride salts.
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Scheme 4. Reductive amination of biologically active molecules to primary
amines. Reaction conditions:1.5 mol% Co (2.24 wt% Co, 0.0075 mmol Co,
°
0.44 mg Co), 0.5 mmol ketone, 50 C, 20 h, 10 bar H2, 3.5 mL aq. NH3-32%;
°
Isolated yields of the converted hydrochloride salts. [a] 60 C; [b] 3 mol% Co,
°
65 C, 3.2 mL aq. NH3-32%, 0.3 mL EtOH.
°
By applying 65 C and 3 mol% Co as well as adding 0.3 mL of
ethanol to enhance the solubility, 54% of Stanolone-NH2 could
be obtained. Attempts to aminated examples carrying an ester,
an amide, or a nitro group and a keto acid failed under the given
conditions.
Reusability and upscaling
Finally, we examined the reusability and upscaling of our cobalt
catalyst system. Five consecutive runs without any loss of activity
were performed to confirm the recyclability (see the Supporting
Information Figure S1), and the reductive amination of 10 mmol
acetophenone, our model substrate, could be accomplished with
95% yield (see the Supporting Information 2.4.3).
Nature of the N-SiC support
The key for the high activity seems to be the N-SiC support,
which we analyzed in detail employing solid-state NMR spectro-
scopy (ssNMR), synchrotron radiation-based powder X-ray dif-
fraction (PXRD)/PDF, Raman spectroscopy, XPS, and acid-base
titration. Key results of the NMR spectroscopic and PXRD/PDF
investigations are listed in Figure 2. The starting point of the
support synthesis is a copolymerization of acrylonitrile with the
commercially available polycarbosilane SMP 10 followed by
Figure 2. NMR and PXRD/PDF investigations. (A) 29Si SP MAS NMR spectra of
the N-SiC support prior (blue) and after (brown) NaOH treatment (activation).
(B) Ultrafast 1H MAS NMR spectra; SP excitation (top) and nitrogen selective
1H-14N HMQC excitation (brown). Both spectra are deconvoluted with the
same set of normal components (blue lines). The resulting lineshapes are
depicted by pointed lines beneath the experimental spectra (C) Variable-
temperature 129Xe NMR spectra using hyperpolarized 129Xe within the limiting
shift region. The inset depicts the intensity trend for the observed two
resonances as function of temperature. (D) PXRD data of N-SiC support
material (blue) with its fit for the crystalline phase fraction (yellow) of 17%
on the big amorphous background (green).
°
pyrolysis up to 1000 C. The pyrolyzed material is then treated
with a 1 m NaOH solution at 85 C for 16 h. 13C cross polarization
°
(CP) and 29Si single pulse magic angle spinning (SP MAS) NMR
spectra (see the Supporting Information Figure S12 and Fig-
ure 2A) suggest that the pyrolyzed material consists of graphitic
domains and silicon rich regions with mixed SiCxO4-x and SiNxO4-x
environments.[9,12] The NaOH treatment leads to
a nearly
complete removal of the silicon-rich domains. In this process, the
specific surface area increases from 6 to 545 m2 gÀ 1 [Ar sorption,
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Brunauer-Emmett-Teller (BET) model, see the Supporting Infor-
mation Figure S5]. The pore size distribution below 2.3 nm
supports a domain structure of the pyrolyzed material, which is
turned into a predominantly microporous material by the NaOH
titration revealed the presence of 0.42 mmol accessible basic N
atom functionalities per gram N-SiC material after activation.
In summary, our N-SiC support is an N doped (�7 wt%)
°
amorphous carbon material (processed at 1000 C) having
1
treatment. Besides adsorbed mobile water, the H NMR spectra
graphitic and nitrogen doped domains smaller than 2 nm with
a turbostratic disorder and widened interlayer distances. The
material is predominantly microporous with a bimodal size
distribution, which might be chemically different. The larger
one of them seems to contain the accessible basic NH
functionalities most likely formed via hydrolysis of CÀ NÀ Si
bonds during NaOH treatment.
obtained with ultrafast MAS and SP (black) as well as nitrogen-
selective 1H-14N heteronuclear multiple quantum coherence
(HMQC) excitation (brown) (Figure 2B) prove the presence of a
substantial amount of NHx (x=1, 2) functionalities.[13] The latter is
also supported by the 15N{1H} CP MAS NMR spectra (see the
Supporting Information Figure S14) and the downfield flank
within the 13C MAS NMR spectrum supportive for carbon atoms
in local CNy(NHx) environments also observed for graphitic
carbon nitrides.[14] The N1s XPS spectra (see the Supporting Conclusion
Information Figure S2D) are in agreement with this analysis.
Hyperpolarized 129Xe NMR (see Supporting Information Figur-
es S15, S16), and especially the line shape analysis in the high-
temperature regime (Figure 2C) reveal that the adsorbed signal
consists of two different resonances with a markedly different
depolarization behavior. The intensity of the highfield signal
increases with decreasing temperature, which is expected for
diamagnetic systems due to the larger Xe uptake.[15] In contrast,
the intensity of the downfield signal decreases, demonstrating
that depolarization dominates as typical for graphite.[16] We thus
assign the two 129Xe resonances to domains consisting of
graphite and a diamagnetic component (e.g., a carbon nitride
like region). In the low loading limit at high temperatures, usually
smaller pores are preferentially populated. The depolarization
behavior of the adsorbed hyperpolarized 129Xe (Figure 2C) thus
suggests that the smaller pores occur predominantly in the
We report on a highly active 3d-metal catalyst for the selective
synthesis of primary amines via reductive amination employing
hydrogen, a key reaction in industry and academia. Our catalyst
is nanostructured and reusable, has a large scope, and outper-
forms noble metal catalysts. The key for the high activity seems
to be our support: an amorphous N-doped carbon material with
small graphitic domains and with pores that carry NH-
functionalities. 88% of the basic NH-functionalities are still
present or available in the final cobalt catalyst. The accessible
NH-functionalities can bind cobalt species during catalyst syn-
thesis or even catalysis and might catalyze the crucial
condensation step. Related materials might be suitable for a
more rational design of highly active 3d-metal catalysts.
graphitic regions. Considering the bimodal pore size distribution Experimental Section
with nearly the same pore volume for both pore size ranges (see
Supporting Information Figure S5 and Figure 1C [catalyst]), it is
suggestive that the region with the larger pores contains the
observed NHx functionalities. A fit of the PXRD data of the N-SiC
support material after NaOH treatment (blue) resulted in a
crystalline phase fraction of about 17% (total fit yellow, Figure 2D
and Supporting Information Figure S9) on a big amorphous
background (green). A widened average interlayer distance of
3.57 Å was found by fitting the (002) reflex in PXRD data,
alongside a smaller fraction of stronger expanded stacks with an
average interlayer distance of 4.52 Å in comparison to hexagonal
graphite with an interlayer distance of 3.354 Å (see the
Supporting Information 3.8). The multinuclear NMR and Raman
spectra (see Figure 2 and Supporting Information Figure S6) are
in accordance with a dominantly amorphous graphitic material.
The PDF of the N-SiC support (see the Supporting Information
Figure S10) indicates hexagonal graphite structure. Below 5 Å, all
distances match in-plane carbon–carbon distances in hexagonal
carbon rings. Since the PDF decayed to 0 beyond about 20 Å, it
can be concluded that the structural coherence of the N-SiC
support is about 2 nm. Rather high atomic displacement
parameters u33 along the c-axis corroborate turbostratic disorder
in PDF refinements. HRTEM based analysis of the graphitic
domains (see the Supporting Information Figure S8) is in
accordance and revealed a mean size of the graphitic domains of
1.6 nm and a widened interlayer distance of 3.6 Å. Acid-base
General considerations
All air- and moisture sensitive reactions were performed under dry
argon or nitrogen atmosphere using standard Schlenk and glove-
box techniques. All dried solvents were obtained from a solvent
purification system (activated alumina cartridges) or purchased
from Acros. Deuterated solvents were dried via molecular sieves. All
chemicals were acquired from commercial sources with purity over
95% and used without further purification. The precursor SMP-10
was purchased from Starfire Systems, New York, USA. Pyrolysis of
the support material was carried out under nitrogen atmosphere in
a high temperature furnace (Gero, Berlin, Germany). Pyrolysis and
reduction of the catalyst were performed under nitrogen or forming
gas (90:10) atmosphere in a ChemBET Pulsar TPR/TPD instrument
from Quantachrome. TEM was carried out by using a Variant LEO
9220 (200 kV) and a JEOL JEM 2200FS (200 kV) device. For the
sample preparation, the samples were suspended in chloroform
and sonicated for 5 min.
Pore characterizations were carried out via nitrogen sorption
measurements using a 3P Micro 100 Surface Area and Pore Size
Analyzer device. The pore size distribution was computed via DFT
°
calculations (calculation model: Ar at À 186.15 C on cylindrical
pore, MDFT equilibrium model). The specific surface area was
calculated by using p/p0 values from 0.005–0.1 (BET).
NMR measurements were performed using a Varian INOVA 300
(300 MHz for 1H, 75 MHz for 13C) and a Bruker Avance III HD 500
(500 MHz for 1H, 125 MHz for 13C) instrument at 296 K.
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doi.org/10.1002/cssc.202100553
ChemSusChem
The hydrogenation experiments were carried out with Parr Instru-
ment stainless-steel autoclavesN-MT5 250 mL equipped with heat-
ing mantles and temperature controller.
measurements. We acknowledge the ERSF for providing beamtime
at ID31 as well as local support from Jakub Drnec. Open access
funding enabled and organized by Projekt DEAL.
PXRD data was collected at 65 keV (0.1907 Å) at ID31 at the
European Synchrotron Radiation Facility (ESRF) with a CdTe Dectris
Pilatus X 2 M detector. NIST cerium oxide standard was used for
distance calibration and determination of instrumental resolution
(Qdamp =0.016; Qbroad =0.008). Fits to the PXRD data were conducted
with the multipeak fitting option within the software Igor Pro 8.
Conflict of Interest
The authors declare no conflict of interest.
129Xe, 29Si, 15N and 13C solid-state NMR measurements were
performed on a Bruker Avance II NMR spectrometer operating at
an external field of 7.05 T corresponding to Larmor frequencies of
83.4, 59.6, 30.4 and 75.5 MHz for the 129Xe, 29Si, 15N and 13C,
respectively.
Keywords: catalysis · cobalt · ketones · primary amines ·
reductive amination
Variable-temperature measurements of hyperpolarized (hp) 129Xe
spectra were performed using a wideline 5 mm double-resonance
Bruker probe. Hyperpolarized Xenon was supplied by a homebuilt
polarizer with a gas composition of 1% v/v Xe with natural
abundance of the 129Xe isotope, 6% v/v N2 as fluorescence
quenching gas and He as buffer gas at a system pressure of 4×
105 Pa. 129Xe wideline SP NMR spectra were recorded in steps of
10 K with an equilibration time of 3 min before starting the
acquisition. The pulse length for the π/2 pulse was set to 3.25 μs
with a nutation frequency of approximately 77 kHz. Two scans were
accumulated for each temperature with a temperature dependent
recycle delay between 1 s at room temperature and 120 s at 130 K.
The chemical shift was referenced to gaseous xenon at zero
pressure (0 ppm).
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°
°
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Acknowledgements
Manuscript received: March 17, 2021
Revised manuscript received: April 1, 2021
Accepted manuscript online: April 7, 2021
Version of record online: April 7, 2021
R.K. (KE 756/34-1) and J.S. (SFB 840, C1) thank the DFG for
financial support. We thank F. Baier for XPS analysis, Dr. H.
Schmalz for Raman measurements and Dr. C. Denner for SEM/EDX
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© 2021 The Authors. ChemSusChem published by Wiley-VCH GmbH
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These are not the final page numbers!

FULL PAPERS
Reductive amination: A nanostruc-
tured highly active cobalt catalyst is
employed for the reductive
M. Elfinger, T. Schönauer, S. L. J.
Thomä, R. Stäglich, Dr. M. Drechsler,
Prof. Dr. M. Zobel, Prof. Dr. J. Senker,
Prof. Dr. R. Kempe*
amination of ketones and aldehydes
by ammonia and H2 under very mild
conditions. A broad substrate scope
and very good functional group
tolerance are observed. Detailed
analysis of the catalyst’s chemical
and physical nature provides key
reasons for its extraordinary activity,
outperforming noble metal catalysts.
1 – 8
Co-Catalyzed Synthesis of Primary
Amines via Reductive Amination
employing Hydrogen under very
mild Conditions