A State-of-the-Art Heterogeneous Catalyst for Efficient and General Nitrile Hydrogenation

Article abstract of DOI:10.1002/chem.202001866

Cobalt-doped hybrid materials consisting of metal oxides and carbon derived from chitin were prepared, characterized and tested for industrially relevant nitrile hydrogenations. The optimal catalyst supported onto MgO showed, after pyrolysis at 700 °C, magnesium oxide nanocubes decorated with carbon-enveloped Co nanoparticles. This special structure allows for the selective hydrogenation of diverse and demanding nitriles to the corresponding primary amines under mild conditions (e.g. 70 °C, 20 bar H₂). The advantage of this novel catalytic material is showcased for industrially important substrates, including adipodinitrile, picolinonitrile, and fatty acid nitriles. Notably, the developed system outperformed all other tested commercial catalysts, for example, Raney Nickel and even noble-metal-based systems in these transformations.

Full text of DOI:10.1002/chem.202001866

Accepted Article
Accepted Article
Title: A State-of-the-Art Heterogeneous Catalyst for Efficient and
General Nitrile Hydrogenation
Authors: Dario Formenti, Rita Mocci, Hanan Atia, Sarim Dastgir,
Muhammad Anwar, Stephan Bachmann, Michelangelo
Scalone, Kathrin Junge, and Matthias Beller
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.202001866
Link to VoR: https://doi.org/10.1002/chem.202001866
01/2020

10.1002/chem.202001866
Chemistry - A European Journal
RESEARCH ARTICLE
A State-of-the-Art Heterogeneous Catalyst for Efficient and
General Nitrile Hydrogenation
Dario Formenti,[a] Rita Mocci,[a, c] Hanan Atia,[a] Sarim Dastgir,[b] Muhammad Anwar,[b] Stephan
Bachmann,[d] Michelangelo Scalone,[d] Kathrin Junge,[a] and Matthias Beller* [a]
[a]
[b]
[c]
[d]
Dr. Dario Formenti, Dr. Rita Mocci, Dr. Hanan Atia, Dr. Kathrin Junge, Prof. Dr. Matthias Beller
Leibniz-Institut für Katalyse e.V. an der Universität Rostock
Albert-Einstein-Strasse 29a, 18059 Rostock, Germany
E-mail: matthias.beller@catalysis.de
Dr. Muhammad Anwar, Dr. Sarim Dastgir
Qatar Environment and Energy Research Institute (QEERI)
Hamad Bin Khalifa University (HBKU)
34110 Qatar Foundation, Doha, Qatar
Dr. Rita Mocci
Dipartimento di Scienze Chimiche e Geologiche
Universitá degli Studi di Cagliari
SS 554 bivio per Sestu, 09042 Monserrato, Italy
Dr. Stephan Bachmann, Dr. Michelangelo Scalone
Department of Process Chemistry and Catalysis, F. Hoffmann-La Roche Ltd.
Grenzacherstrasse 124, 4070 Basel, Switzerland
Supporting information for this article is given via a link at the end of the document.
Abstract: Cobalt-doped hybrid materials consisting of metal oxides
and carbon derived from chitin were prepared, characterized and
tested for industrially relevant nitrile hydrogenations. The optimal
catalyst supported onto MgO showed, after pyrolysis at 700 °C,
magnesium oxide nanocubes decorated with carbon-enveloped Co
nanoparticles. This special structure allows for the selective
hydrogenation of diverse and demanding nitriles to the corresponding
primary amines under mild conditions (e.g. 70 °C, 20 bar H2). The
advantage of this novel catalytic material is showcased for industrially
important substrates, including adipodinitrile, picolinonitrile, and fatty
acid nitriles. Notably, the developed system outperformed all other
tested commercial catalysts, e.g. Raney Nickel and even noble metal
based systems in these transformations.
drawback of this approach lays in the relatively high cost of this
nitrogen-containing molecules that could limit the applicability on
a large scale. Thus, the synthesis of (doped)-carbonaceous
materials starting from sustainable resources is of increasing
interest.[2] In this respect, abundantly available cellulose, chitin,
and lignin constitute “ideal” candidates for new catalyst
developments. Specifically, chitin is the second most abundant
biopolymer on Earth and represents a biocompatible nitrogen-
containing precursor directly available from crustacean shells. It
shows an irrelevant toxicity and hazardousness making its
environmental impact negligible.[3]
Recently, our group prepared new materials by pyrolysis of Co
salts and chitosan, which is obtained by deacetylation of chitin
using large amount of aqueous sodium hydroxide. Interesting
activities have been observed in the selective hydrogenation of
nitroarenes[4] and olefins,[5] and hydrodehalogenation of aryl and
alkyl halides.[6] In continuation with this topic, herein we report
unique nanostructured catalysts, whereby cobalt is supported on
metal oxide-carbon composites derived from chitin. The optimal
material outperforms representative catalysts for industrially
relevant hydrogenation of nitriles under the investigated
conditions.
Introduction
Catalysis in all its aspects plays a fundamental role in the
development of green chemical transformations. So far, most of
the efforts focused on the design of sustainable processes using
renewable chemical platforms, benign solvents, and/or earth-
abundant catalysts under mild and safe conditions with the aim of
minimizing wastes and energy consumption. Nevertheless, less
attention has been paid to the catalyst development using earth-
abundant and renewable starting materials. In the last decade,
transition-metals nanoparticles or single metal atoms (mainly
based on Fe, Co, Ni) supported onto nitrogen-doped modified
carbon materials have been disclosed as very successful catalytic
materials for various selective organic transformations.[1] They are
typically prepared by the pyrolysis of well-defined metal
complexes or chelates employing nitrogen donor ligands, metal-
organic frameworks or coordination polymers as precursors. A
Results and Discussion
In recent years, nanostructured carbon-metal oxide composite
materials gained attention mainly for energy-related
applications.[7] Notably, such materials have been scarcely
explored in chemical catalysis.[8] In this respect, we envisioned
the exploitation of new biomass-derived carbon-metal/metal oxide
composites for sustainable redox transformations. Inspired by the
1
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10.1002/chem.202001866
Chemistry - A European Journal
RESEARCH ARTICLE
preparation of chitosan-derived metal nanoparticles,[4-6]
selection of potential catalytic materials was prepared.
a
outcome. Without support, the resulting materials (Co-N-C) were
not selective in the desired transformation, even if ammonia was
added, which is known to suppress the undesired secondary
imine 2aa formation (see Scheme S1 for general reaction
pathway).[10] Apart from Fe3O4-supported materials – which were
completely inactive – addition of SiO2, Al2O3, MgO or TiO2 in
general improved the selectivity. In order to distinguish more
appropriately, the catalysts showing best performances (2a yield
≥90%, green color-code) were studied under milder conditions
(80°C, 20 bar H2). Here, all catalysts based on MgO outperformed
the other materials and preserved good activity. In particular, Co-
N-C@MgO-700 (treated at 700 °C) still showed complete
conversion accompanied with a very high selectivity for benzyl
amine 2a even at 20 bar H2 pressure (see Figure S20, Figure S21,
and Figure S22). To investigate the reaction in more detail,
different solvents have been employed and the proper ammonia
concentration was determined (Figure S23 and Figure S24,
respectively). In general, iPrOH in the presence of 1M ammonia
allowed for the best performance in terms of activity and
selectivity. Obviously, no-catalytic behavior was ruled out in the
absence of catalysts or hydrogen (Entries 2-3, Table S5). Notably,
the formation of the composite material is crucial for product
formation, as neither the unpyrolysed material nor the one
prepared without the addition of chitin (Entries 4-5, Table S5)
showed activity. As expected, doped-carbon/MgO composites
assembled without the addition of Co were inactive (Entries 6-10,
Table S5). All these experiments pointed out the importance of
the synergistic combination of cobalt, carbon matrix and metal
oxide. Systematic variation of the individual components of the
carbon-magnesium oxide composite by changing the ratio
between chitin and MgO from 1:3 to 3:1 followed by thermal
treatment at 600, 700, and 800 °C (Figure S25) revealed optimal
results of the catalyst prepared using a 1:1 mass ratio and
pyrolysed at 700 °C (Co-N-C@MgO-700). Composition/time chart
for this optimal catalyst (Figure S26) showed complete conversion
(>99%) and excellent selectivity (98%) at comparably low
temperature (80°C) over 24 h. With regard to selectivity, it is
interesting to notice that N-benzylidenebenzylamine 2aa is
formed as intermediate during the reaction.[14,15] Accordingly, if
2aa was subjected to the standard reaction conditions, 2a is
formed with full conversion and almost complete selectivity (Entry
1, Table S6).
Pyrolysis Temperature [°C]
600
700
800
89
32
9
>99
>99
57
>99
>99
>99
>99
>99
71
Co-N-C
68
67
77
93
91
87
80
<1
59
87
Co-N-C@SiO2
Co-N-C@γ-Al2O3
Co-N-C@MgO
Co-N-C@ZrO2
Co-N-C@TiO2
Co-N-C@Fe3O4
67
traces
68
91
94
91
>99
>99
>99
>99
>99
3
93
96
>99
4
89
<1
3
<1
yield. ≤ 70 %
70 < yield. < 90
yield. ≥ 90 %
Figure 1. Catalytic hydrogenation of benzonitrile (1a) to benzylamine (2a) and
N-benzylidenebenzylamine (2aa). The numbers in each cell represent the
conversion of 1a (left) and the selectivity in 2a (right). Color-code is based on
2a yield. Reaction conditions: 0.5 mmol PhCN, 15 mg catalyst (1 mol% Co), 2
mL iPrOH, 200 μL aq. NH3 (from a 25 % aqueous solution), 120 °C, 40 bar H2,
24 h. Conversion and selectivities were determined using GC (n-hexadecane
as internal standard).
As depicted in Figure 1A, after in-situ generation of
a
Co(OAc)2·4H2O/chitin mixture, different metal oxides were added
and stirred for 24 h. Afterwards, the solvent was removed and the
obtained solids were pyrolysed at three different temperatures
(600 °C, 700 °C and 800 °C with a temperature ramp of 25 °C·min-
1) under Ar atmosphere for 2 h (for detailed preparation protocol
the reader is referred to the Supporting Information).[9] The
resulting materials possess a cobalt content of around 2 wt%.
(Table S1). The activity of all these heterogeneous catalysts was
evaluated in the benchmark hydrogenation of benzonitrile 1a to
benzylamine 2a. In general, nitrile hydrogenation represents an
important transformation in the bulk and fine chemical industry,
since it furnishes primary amines that constitute key building
blocks for the preparation of active pharmaceutical ingredients
(APIs), life-science molecules, polymers, agrochemicals and dyes.
Conventionally, such transformations under industrial conditions
are conducted mostly applying skeletal catalysts such as
Raney®-Ni and Raney®-Co or supported noble-metals such as
Pd, Pd, Ru, Rh and Ir.[10] The increasing scarcity and price of the
latter and the hazardous nature of the former create interest
discovering less expensive and more sustainable catalysts. So far,
the cobalt-catalyzed direct hydrogenation of nitriles has been
scarcely explored and the corresponding catalysts were prepared
using more expensive and special starting materials (see Scheme
S2 for a summary of the previous works).[11-14]
Next, we compared the performance of Co-N-C@MgO-700 with
state-of-the art systems, such as currently employed commercial
heterogeneous catalysts, and related materials prepared from
biopolymers (chitosan and cellulose). As depicted in Figure S27,
the latter materials and previously reported cobalt nanoparticles
on various supports showed inferior results. Similarly, our catalyst
proved to be superior, even if compared with standard industrial
catalysts including Raney-Ni as well as noble metal systems.
Despite the good activities of noble-metal based catalysts in terms
of conversion, selectivity in the desired primary amine was low or
negligible due to the formation of various side products. In
particular, in the case of Pt/C, Raney-Ni and Ru/C the main side
products
were
N-benzylidenebenzylamine
2aa
and
dibenzylamine 2aaa. When Pd/C and Pd(OH)x/C were employed,
along with 2aa and 2aaa, cyclohexylmethanamine and
bis(cyclohexylmethyl)amine were detected by GC-MS indicating
partial reduction of the arene ring.
The results of the preliminary tests are summarized in Figure 1B.
As shown by color-code, it is immediately clear that both the
pyrolysis temperature and the support are crucial for the reaction
2
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Having an optimal material in hand, its reusability was examined.
Gratifyingly, no loss of activity was detected in six additional runs
(Figure S28). In agreement with this observation, cobalt leaching
(both for active or inactive Co species) was ruled out after the
sixth run by measuring the metal content in the liquid phase and
performing Maitlis' hot filtration test (Table S7).
Exploring the near-surface composition of the materials by means
of XPS, the Co2p core level spectrum of Co-N-C@MgO-T is
characterized by two distinct components, Co2p3/2 and Co2p1/2
with peaks at around 781 eV and 798 eV, respectively (Figure 2C).
Each of them showed a shake-up satellite located about 7 eV
above the primary emission peak and a spin-orbit separation (∆E
Co2p1/2-Co2p3/2) of 15.3 eV. This is in agreement with the
presence of Co in +2 oxidation state.[17] Contrariwise, Co-N-C-700
and Co@MgO-700 exhibited different peaks (Figure S5C). In
particular, Co-N-C-700 displayed a signal at 778.2 eV which is
typical for cobalt in zero oxidation state, while Co@MgO-700
showed signals at higher binding energies (782.3 eV and 796.8
eV for Co2p3/2 and Co2p1/2, respectively) indicating oxidized Co
species.[18] The material prepared with chitosan (Co-N-
C(CH)@MgO-700) exhibited a similar pattern to Co-N-C@MgO-
700, whereas Co-N-C(CE)@MgO-700 prepared from cellulose
displayed peaks slightly shifted towards lower binding energies
indicating
a
possible reduction of Co(II) (Figure S5B).
Deconvolution of the N1s region for Co-N-C@MgO-T revealed
the presence of two signals at 398 eV (pyridinic nitrogen or Co─
Nx functionalities) and 400.8 – 401.2 eV (pyrrolic or graphitic N
configurations) for each catalyst (Figure S6 and Figure 2D).[19]
Notably, a decreasing of the intensity of the peak centered at 398
eV and an increasing and broadening of the one centered at 401
eV can be detected with the increasing of the pyrolysis
temperature, which is in agreement with the structural evolution
of pyridinic to graphitic N functionalities in doped carbonaceous
materials upon heating.[20] Similar distribution of the N1s peaks
have been detected in Co-N-C-700 and N-C@MgO-700 (Figure
S6B). Analysis of the Mg1s core revealed the presence of three
deconvolution peaks (around 1303, 1304.5 and 1305.5 eV,
respectively) for each material that can be ascribed to the
presence of Mg(II) in the bulk (Mg2+ O2-), on the surface (-O-
Mg(OH)-O-), and as MgCO3 (Figure S7).[21] The presence of the
latter two species is not surprising since magnesium oxide
possess surface chemisorbed OH groups, which are retained also
at high temperature (up to 1200 °C).[22] In addition, pyrolysis of
chitin partially produces CO2 and water giving rise to the partial
formation of MgCO3 and Mg(OH)2. Carbon C1s core showed,
along with the reference carbon peak at 284.8 eV (ascribed to C
─C bonds), the presence of other C─X or C=X (X = N, O)
functionalities, which signals are located at higher binding
energies (Figure S8). The C1s XPS patterns are very similar for
all the characterized catalysts, except for the unpyrolysed
material (Co(OAc)2-Chitin-MgO) that showed numerous peaks,
which reflect the presence of chitin carbon atoms in different
electronic environments. Finally, regarding the O1s region,
deconvolution generated three peaks for all the analyzed
materials (Figure S9); however, a rigorous assignment was not
possible. It should be anyway noted that the treatment of MgO
with water or CO2-enriched atmospheric air led to the formation of
three O1s peaks after deconvolution (530 eV, 532 eV, and 533
eV), which were ascribed to bulk magnesium oxide, superficial
Figure 1. N2 adsorption-desorption isotherms (A), TPR-H2 patterns (B), Co2p
XPS (C) and N1s XPS (D) of selected catalytic materials.
Because of their activity and selectivity in the desired
transformation, especially materials based on MgO were
characterized by means of X-ray powder diffraction (XRPD), X-
ray photoelectron spectroscopy (XPS, ESCA), temperature
programmed reduction (TPR-H2) and transmission electron
microscopy (TEM). Chemical composition analyses (CHN) of Co-
N-C@MgO pyrolysed at different temperatures revealed constant
values for all the elements except for nitrogen which showed a
decreasing trend with the increase of the pyrolysis temperature
(Table S1 and Table S2). Specific BET areas (SBET) were
determined by N2 adsorption/desorption isotherms (Figure 2A and
Figure S1). All the catalysts prepared with chitin displayed a type
IV isotherm (according to IUPAC classification) typical for
mesoporous materials. In contrast, Co@MgO-700 showed a type
II isotherm indicating a non-porous material. As expected, the
SBET depends on the pyrolysis temperature and a gradual
decreasing is noticed with the increase of the pyrolysis
temperature (Table S4 and Figure S3). In X-ray powder
diffractograms the presence of MgO reflections (periclase) at 2θ
values around 37, 43, 63, 74, and 78° is dominant (Figure
S4A).(16) In all the materials belonging to Co-N-C@MgO-T, MgO
crystallite size (calculated via Scherrer equation) is around 41 nm,
which is very close to the one calculate for the non-pyrolysed
materials (42 nm). Apart from these signals, Co-N-C@MgO-800
showed a small diffraction peak at around 2θ of 44° ascribed to
Co(0).[17] An analogous situation is observed for Co-N-
C(CE)@MgO-700, Co-N-C(CH)@MgO-700 and Co@MgO-700
(Figure S4B and S4C). In contrast, the material prepared without
support (Co-N-C-700, Figure S4C) showed the exclusive
presence of metallic Co (2θ values of around 44, 52 and 76°) and
graphitic carbon (2θ value of 26°). Here, no crystalline cobalt
oxides diffraction lines (Co3O4 or CoO) were observed via XRPD,
which is in sharp contrast to previously known cobalt catalysts.[4,6]
magnesium
hydrates,
and
magnesium
carbonates,
respectively.[23] The reducibility of the catalysts has been
elucidated using temperature-programmed reduction technique
(TPR-H2). As depicted in Figure 2B, Co-N-C@MgO-T catalysts
showed a main signal at around 475 °C, which is typical for the
reduction of cobalt oxides in N-doped graphene matrix.[24] On the
contrary, Co@MgO-700 showed two peaks at around 350 °C and
475 °C indicating the presence of two reducible Co species.
3
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According to the literature, these can be assigned to the two-steps
reduction of supported Co3O4 (Co3O4→CoO→Co), which is in
accordance with XPS (vide supra).[25] Surprisingly, Co@MgO-800
showed a totally different pattern: A unique peak centered at
600 °C was detected indicating the presence of difficulty reducible
Co species. These can be attributed to Mg/Co spinels (MgCo2O4)
or MgO/CoxOy solid solutions.[26] Interestingly, it should be
noticed that Co-N-C@MgO-700 showed a shoulder in the 600-
650 °C region that became a defined peak in Co-N-C@MgO-800
indicating the partial formation of spinels and/or mixed oxides in
the Co-N-C@MgO-T materials, too. The decreasing of reducibility
of the latter samples has been also confirmed by the reduced
amount of consumed H2 in the TPR experiments (Figure S10).
ability of MgO to homogeneously disperse metal species.[27]
Moreover, moving from pyrolysis temperature from 700 °C to
800 °C a general increasing of the particle size is observed
(agglomeration). It is important to further notice that all the Co NPs
are embedded within various layers of ordered graphenic-like
carbon structures. The catalyst Co-N-C-700 showed also the
presence of Co NPs on carbon (Figure S15). However, the
dispersion is not homogeneous as in the case of Co NPs located
on MgO and a variety of Co particles ranging between 10-100 nm
NPs are clearly visible (Figure S15B and S15I). The material
prepared without chitin, Co@MgO-700 (not active in the desired
transformation) displayed a totally different composition (Figure
S16). Variously shaped (cubes, rectangles, finger-like) MgO
structures were present and no visible Co NPs could be detected.
However, TEM mapping shows a uniform dispersion of Co
species on the magnesium oxide support (compare Figure S16F
and S16G). This corroborates the postulated formation of a
Mg/Co spinels (MgCo2O4) or MgO/CoOx solid solutions on the
base of the XRPD, TPR-H2 and XPS results (vide supra).
Finally, the catalysts prepared using cellulose (Co-N-
C(CE)@MgO-700) and chitosan (Co-N-C(CH)@MgO-700) as
carbon precursors were analyzed (Figure S17 and FigureS18). In
both cases, mapping showed that variously-sized Co NPs are
preferentially located on the carbon phase leaving the MgO phase
almost unloaded. In addition, agglomeration of the particles is
detected. Both the latter features could be the reason for the
decreased activity of Co-N-C(CE)@MgO-700 and Co-N-
C(CH)@MgO-700. On the base of the previous we could assume
that the proposed spinels or solid solution play a minor or
negligible role in the catalytic event. On the other hand, particle
size and their location play a significant role. As a result, catalyst
Co-N-C@MgO-700 (which shows the best catalytic activity)
shows both small Co NPs which are preferentially located on MgO.
Lastly, the catalyst recovered after five recycling runs has been
also characterized. TEM analysis still showed the presence of Co
diffused Co species both on carbon matrix and MgO phase.
Additionally, as shown in Figure S19, XPS analysis revealed a
very similar peak distribution if compared with the fresh material
with small differences detected only for the O1s region (Figure
S19).
Figure 1. TEM images of Co-N-C@MgO-600 (A, B, C), Co-N-C@MgO-700 (D,
E, F) and Co-N-C@MgO-800 (G, H, I).
To further elucidate structural, electronic, and morphological
properties of various prepared materials, transmission electron
microscopy was performed. TEM investigations of the cobalt-free
N-C@MgO-700 sample showed the formation of variously-sized
MgO nanocubes (size of around 100 nm) bringing carbonaceous
structures regularly distributed on them (Figure S11). Similarly,
catalyst Co-N-C@MgO-600 showed both the presence of MgO
nanocubes and extended carbonaceous structures (see Figure
S12). Various Co NPs (10-20 nm size) were found to be well
distributed on the latter structures and a negligible amount of them
were present on or in the proximity of the MgO (Figure 3 and
Figure S12). Additionally, some agglomerated Co phases were
also present on the edges of the carbon phase (Figure S12K).
Moving to Co-N-C@MgO-700 and Co-N-C@MgO-800, Co NPs
were located both on the carbon matrix and on the MgO cubes
(Figure S3D-F and G-I), which in this case have a size up to 100
nm. Interestingly, TEM mapping clearly shows that the Co NPs on
the latter phase are smaller than the one present on the former,
reaching down to 10 nm (compare Figure S13H with Figure S13K,
and Figure S14H with Figure S14K). This can be ascribed to the
4
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Figure 1. Hydrogenation of aryl and heteroaryl nitriles: Reaction scope.
Reaction conditions: 0.5 mmol PhCN, 15 mg catalyst (1 mol% Co), 2 mL iPrOH,
200 μL aq. NH3 (from a 25 % aqueous solution), 80 °C, 20 bar H2, 24 h.
[a]Reaction was conducted at 100 °C. [b]Reaction was conducted at 110 °C
using 30 mg of catalysts (2 mol% Co). [c]Reaction was conducted at 70°C using
45 mg of catalyst (3 mol% Co). Products 2f, 2p, 2r, 2x and 2o were obtained as
hydrochloric salts. Products 2s, 2z, 2u were isolated as primary amines.
Remaining product yields were determined using GC (n-hexadecane as internal
standard).
catalyst loading in the presence of Et3N as base afforded the
desired primary amine with a selectivity of 70% (Table 1, entry 2).
With other metal catalysts based on Pd, Pt or Ru much lower
selectivity and mainly over-hydrogenated products were obtained
(Table 1, entries 3-6).
With a suitable catalyst and conditions in hand, scope and
limitations of the described protocol were explored starting with
substituted aryl nitriles (Scheme 2). Substrates bearing electron-
donating (1b, 1c, 1e) or electron-withdrawing (1d) groups in para-
position were smoothly converted to the corresponding benzyl
amines at 80°C or 100 °C in very high yields. Except for 4-
iodobenzonitrile (1j), all other halide-containing nitriles (1g, 1h, 1i)
did not pose any selectivity problems and furnished the desired
products in >98% yield. In case of ortho-substitution good product
yields (2k, 2l, 2m, 2n) were achieved at slightly higher reaction
temperature and/or catalyst loading. Dinitriles are common
starting materials for the preparation of important building blocks
Table 1. Hydrogenation of picolinonitrile 1ah.[a]
Entry
Catalyst amount
[mg] (Co mol%)
Temp.
[°C]
H2
[bar]
1ah
Conv.
[%]
2ah
Sel.
[%]
2aah
Sel.
[%]
1
15 (1)
80
20
40
40
40
40
40
26
22
70
<1
<1
31
49
70
25
<1
<1
<1
<1
for
polymer
production.
Hence,
1,3-
and
1,4-
2[b]
3[b]
4[b]
5[b]
6[b]
45 (3)
130
130
130
130
130
>99
>99
>99
>99
>99
phenylenedimenthanamines (2o and 2p) were produced following
our protocol. Interestingly, more functionalized substrates
containing carboxylic acid derivatives such as esters (1p),
phthalides (1r) or amides (1s, 1u) including biologically relevant
β-lactam scaffold (1x) were converted into the corresponding aryl
amines maintaining the integrity of the residual moieties.
Furthermore, amino groups are tolerated to give the
corresponding diamino compounds (2m, 2t). In addition,
piperonylonitrile (1y) and 4-benzyloxybenzonitrile (1z) yielded the
corresponding products in almost quantitative yield. Surprisingly,
no cleavage of the C─O bond was observed, despite the lability
of the benzyl ether moiety in the presence of hydrogen and metal
catalysts. Except for thiophene,[11] a selection of heteroaromatic
nitriles based on pyridine (1ab, 1ac), pyrrole (1ad), indole (1ae),
and furan (1af) were hydrogenated to the corresponding primary
amines in good yields, too. The sluggish reactivity showed by 1ag
was imputed by the poisoning effect of the sulfur atom. In fact, the
hydrogenation benzonitrile 1a conducted in the presence of
thiophene is completely inhibited (Table S8). While most of the
catalytic experiments (vide supra) have been performed under
mild conditions, it is possible to lower the reaction temperature
further on, which might be useful for pharmaceutical applications.
In fact, mild operative temperatures are not only relevant in terms
of energy saving but also due to safety aspects in the scale-up
process. As exemplified by reactions of 1n and 1y, very good
product yields were obtained even below 80 °C. Limitations to the
reported protocol were detected in the case of nitriles containing
C=C double bonds, nitro group and ketones. In these cases
mixtures, the nitrile group is reduced along with the second
functionality giving rise to a mixture of products (Scheme S3).
To demonstrate the advantages of this novel catalyst system the
hydrogenation of picolinonitrile (1ah) (2-pyridinecarbonitrile) was
investigated. Noteworthy, the selective reduction of this particular
substrate fails even with noble-metal catalysts such as Pd.[28] The
specific problem in this hydrogenation is explained by preferential
hydrolysis to picolinamide (2aah) due to an anchimeric assistance
of the heterocyclic nitrogen atom adjacent to the nitrile group.[29]
While poor conversion and selectivity were obtained at 80 °C
(Table 1, entry 1), reaction at higher temperature and increased
10% Pd/C
10% Pd(OH)x/C
10% Pt/C
5% Ru/C
Reaction conditions: 0.5 mmol picolinonitrile 1ah, 2 mL iPrOH, 200 μL aq. NH3
(from a 25 % aqueous solution), 24 h. Conversion and selectivities were
determined using GC (n-hexadecane as internal standard). [b] 373 μL Et3N and
140 μL H2O were used instead of NH3. Complete mass balance is given by the
formation of (E)-1-(pyridin-2-yl)-N-(pyridin-2-ylmethyl)methanimine that has
been detected by GC-MS.
Figure 1. Hydrogenation of aliphatic nitriles: Reaction scope. Reaction
conditions: 0.5 mmol substrate, 30 mg catalyst (2 mol% Co), 2 mL iPrOH, 200
μL aq. NH3 (from a 25 % aqueous solution), 110 °C, 20 bar H2, 24 h. Product
2ak was obtained as hydrochloric salt. Products 2ao, 2aq, 2ar and 2as were
isolated as primary amines. Remaining product yields were determined using
GC (n-hexadecane as internal standard).
Next, we investigated the reactivity of aliphatic nitriles (Scheme
3), which require harsher conditions. Nevertheless, benzyl
cyanides (1ai and 1aj), other aliphatic (1ak, 1al) and aliphatic
(1am, 1an, 1ao) nitriles were smoothly hydrogenated affording
the corresponding amines in good yields. Furthermore, we
5
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10.1002/chem.202001866
Chemistry - A European Journal
RESEARCH ARTICLE
applied our protocol for the synthesis of industrially relevant
aliphatic amines such as fatty amines and 1,6-diaminohexane.
Longer chain alkyl amines are applied in adhesive formulations,
softeners, emulsifiers and flotation agents (corresponding
salts).[30] These so-called fatty amines are usually produced on
large scale by the so-called Nitrile Process in the presence of
Raney-Ni, Raney-Co or copper-chromite catalysts.[31] Gratefully,
Co-N-C@MgO-700 successfully catalyzed this transformations at
110°C and 20 bar hydrogen giving the products in excellent yield.
It should be mentioned here that the products were obtained
straightforward in a pure form after simple filtration over Celite for
catalyst removal and consecutive solvent evaporation.
nitriles to primary amines in high yields and often under very mild
conditions (> 40 examples). Comparison of Co-N-C@MgO-700 to
noble metal-based catalysts for selected examples demonstrates
the improved performance of this novel material. Characterization
reveals the importance of Co NPs both on the carbon matrix and
on the MgO cubes.
Acknowledgements
Dr. Veronica Papa (LIKAT, Rostock), Dr. Basudev Sahoo (ICIQ,
Tarragona & LIKAT, Rostock), Dr. Pavel Ryabchuk (LIKAT,
Rostock), Dr. Kathiravan Murugesan (LIKAT, Rostock) and Dr.
Jacob Schneekönig (LIKAT, Rostock) are gratefully
acknowledged for helpful discussions. Analytic department of
LIKAT (especially Mrs. Astrid Leheman and Mr. Reinhard Eckelt
for elemental analysis and BET measurements, respectively) are
gratefully acknowledged for technical support. QEERI Core Lab
and collaborators (Dr. Said Mansour, Dr. Akshath R. Shetty,
Janarthanan Ponraj, Mujaheed Pasha and Yahya Zakaria) are
greatly acknowledged for analytical support and helpful
discussions. Finally, D. F. would like to thank the whole
Nachhaltige Redoxreaktionen Arbeitsgruppe (Sustainable Redox
Reaction Group) in LIKAT for the continuous support and the
pleasant working environment. This work has been supported by
the state of Mecklenburg Vorpommern, F. Hoffmann-La Roche
AG and Qatar National Research Fund (Grant Number NPRP9-
212-1-042).
Table 1. Hydrogenation of adiponitrile 1at.
Entry
Catalyst amount
[mg] (Co mol%)
Temp.
[°C]
1at
Conv.
[%]
2at
Sel.
[%]
2aat
Sel.
[%]
1
2
3
15 (1)
45 (3)
30 (2)
80
47
<1
95
98
95
<1
<1
80
>99
>99
110
Reaction conditions: 0.5 mmol adiponitrile 1at, 2 mL iPrOH, 200 μL aq. NH3
(from a 25 % aqueous solution), 20 bar H2, 24 h. Conversion and selectivities
were determined using GC (n-hexadecane as internal standard).
Keywords: Cobalt • Magnesium Oxide • Hydrogenation • Amine
• Heterogeneous Catalysis
The reaction of adiponitrile (1at) to hexamethylenediamine, which
is used as monomer for the production of Nylon 6.6 and other
related polymers, represents the largest nitrile hydrogenation in
industry. The current process uses Raney-Ni (low pressure
process) or promoted cobalt, iron oxides (high pressure process)
as catalysts and the desired product is obtained with a selectivity
of ca. 90%.[32] Interestingly, under our standard conditions
azepane (2aaat) is formed in high selectivity (95%). However, by
increasing either the temperature and/or the catalyst loading
(Table 2, entries 2 and 3) the desired product is obtained in high
conversion and selectivity (>99% and 98%, respectively).
Finally, the utility of our developed protocol was also tested in
some gram-scale reactions (scaled-up by a factor of 25). As
depicted in Scheme S4, the obtained yields were comparable to
the ones obtained in 0.5 mmol scale.
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7
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Chemistry - A European Journal
RESEARCH ARTICLE
Entry for the Table of Contents
A nanostructured catalytic material composed by cobalt nanoparticles, magnesium oxide and a biowaste-derived carbon matrix is able
to efficiently mediate the hydrogenation of nitriles. Worth of note are not only the mild operative conditions but also the remarkable
selectivity towards primary amines. This allows the preparation of a large array of molecules in high yields even under scaled-up
conditions.
8
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