Superior activity and selectivity of heterogenized cobalt catalysts for hydrogenation of nitroarenes

Article abstract of DOI:10.1039/c8cy01634h

The development of improved catalysts for highly selective hydrogenation of nitroarenes is described. For this purpose Co nanoparticles were supported on ordered mesoporous carbon CMK-3 and characterized in detail. The optimal CMK-3-CoPc catalyst exhibits excellent hydrogenation activity for several (hetero)aromatic nitro compounds and yielded the corresponding anilines under mild conditions (40 °C, 20 bar H₂).

Full text of DOI:10.1039/c8cy01634h

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Superior activity and selectivity of heterogenized
cobalt catalysts for hydrogenation of nitroarenes†
Cite this: Catal. Sci. Technol., 2019,
9, 157
Wu Li, ‡a Jens Artz, ‡b Cornelia Broicher,b Kathrin Junge,a Heinrich Hartmann,c
a
Astrid Besmehn, c Regina Palkovits
b and Matthias Beller
*
*
Received 7th August 2018,
Accepted 22nd November 2018
The development of improved catalysts for highly selective hydrogenation of nitroarenes is described. For
this purpose Co nanoparticles were supported on ordered mesoporous carbon CMK-3 and characterized
in detail. The optimal CMK-3-CoPc catalyst exhibits excellent hydrogenation activity for several (hetero)aromatic
nitro compounds and yielded the corresponding anilines under mild conditions (40 °C, 20 bar H2).
DOI: 10.1039/c8cy01634h
rsc.li/catalysis
General drawbacks of most of these transformations are
the relatively high temperatures (>100 °C) and/or pres-
Introduction
Catalytic hydrogenation of nitroarenes, either in vapor or liq-
uid phase, is a valuable method to synthesize intermediates
for dyes, pigments, optical brighteners, pharmaceuticals, ag-
rochemicals and monomers for polyurethanes.1 In general,
these processes are highly exothermic (>550 kJ mol−1 in liq-
uid phase); thus, there is an increasing interest performing
such hydrogenations at lower temperatures and pressures
even for large-scale production.2 Obviously, for functionalized
substrates, the use of milder reaction conditions is also ad-
vantageous with respect to selectivity and to avoid undesired
side-reactions.3
In the past two decades, the use of supported metal
nanoparticles (NPs) emerged as an innovative tool in catal-
ysis.4 As an example, precious metal-based nanoparticles,
such as Pt, Pd, Rh, Ru and Au, are nowadays routinely
used for hydrogenation of nitroarenes.1f,5 Similar to
molecularly-defined complexes, the activity and selectivity
of such supported nanoparticles can be fine-tuned by
means of their chemical surrounding. It was shown that
carbonizing organometallic complexes enables strong
metal–support interactions, thereby influencing the
electron density and steric environment of the active
sites.6 Following this concept, earth-abundant metals em-
bedded in nitrogen-doped carbon materials have been
employed in selective reductions of nitroarenes, nitriles,
aldehydes, and ketones as well as oxidation reactions.7–9
sures (50 bar H2) required. Hence, hydrogenations of
nitroarenes under milder conditions using earth-abundant
metal catalysts remain a challenging task in this field. To
improve their performance, we had the idea to increase
the accessibility of the active sites by further tuning the
structural properties of the solid support, e.g. porosity and
specific surface area.10 More specifically, using meso-
porous materials with ultrahigh surface areas, large pore
volumes and tunable pore sizes and shapes should consti-
tute promising supports for improved catalysis.11
Here, we report a facile synthesis of tailor-made porous
and carbonaceous Co-phthalocyanine (CoPc) based nano-
particles. As the optimal structure type, ordered mesoporous
carbon (CMK-3-type) materials were attempted,12 derived
from ordered mesoporous silica (SBA-15) as template.13 The
optimal CMK-3-CoPc catalyst showed high activity for the hy-
drogenation of a broad range of (hetero)aromatic nitroarenes
under mild conditions.
a Leibniz-Institut für Katalyse e.V. an der Universität Rostock, Albert-Einstein
Straße 29a, Rostock, 18059, Germany. E-mail: Matthias.Beller@catalysis.de
b Institut für Technische und Makromolekulare Chemie, RWTH Aachen University,
Worringerweg 2, 52074 Aachen, Germany. E-mail: Palkovits@itmc.rwth-aachen.de
c Forschungszentrum Jülich GmbH, Wilhelm-Johnen-Straße, 5248 Jülich, Germany
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8cy01634h
Scheme 1 Schematic illustration of the synthesis of CMK-3 replica by
using SBA-15 mesoporous silica as a hard-template: (a) impregnation
of the carbon and metal precursor (CoPc) into the pores of the SBA-15
hard-template via grinding (GR), wet impregnation (WI) or incipient
wetness impregnation (IWI), followed by (b) carbonization under nitro-
gen atmosphere at 990 °C for 1 h and subsequent template removal
with 5 M sodium hydroxide solution.
‡ These two authors contributed equally.
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Fig. 3 STEM analysis of different regions within CMK-3-CoPc-WI.
plate) with cobalt phthalocyanine (CoPc) (see ESI2.2.1–
2.2.3†). The resulting material was dried at 100 °C and 350
°C for 1 h and subsequently carbonized at 990 °C for 1 h un-
der nitrogen atmosphere. To remove the silica template from
the silica Co phthalocyanine composite, the resulting black
powder was treated with 5 M NaOH solution at 100 °C for 24
h. Centrifugation of the solution, decantation, and washing
with distilled water was repeated ten times to ensure com-
plete elimination of the silica. The obtained ordered meso-
porous carbon was finally dried (100 °C) and is denoted as
CMK-3-CoPc (Scheme 1). ICP-OES of this material revealed
that <5 wt% of Si remained within the porous carbon net-
work. For comparison, template-free CoPc was pyrolyzed un-
der identical conditions in order to investigate structure–ac-
tivity correlations. The detailed syntheses of the materials are
available in the ESI.†
Fig. 1 N2-physisorption isotherms (top) and corresponding pore size
distributions calculated via DFT (bottom) for the samples template-free
carbonized CoPc (black), CMK-3-CoPc-GR (green), CMK-3-CoPc-WI
(light blue and reproduced, blue), CMK-3-CoPc-IWI (magenta) as well
as CMK-3-CoPc-WI-(recyc.) (red) after recycling.
Table 1 Physical properties of CoPc catalysts: specific surface area (SBET
)
as determined via BET method, total pore volume (VPtotal) determined at
p/p0 = 0.98, average pore size distribution (DP) and metal content deter-
mined via ICP-OES (CoICP
)
SBET
VPtotal
DP
CoICP
Entry Catalyst
[m2 g−1
]
[cm3 g−1
]
[nm] [wt%]
Thermogravimetric analysis (TGA) indicates a high ther-
mal stability (>350 °C) of the structured CMK-3-CoPc (Fig.
S1†). N2-physisorption measurements (Fig. 1, Table 1) reveal
no pronounced porosity and only a marginal specific surface
area (SBET) for the template-free carbonized CoPc material. In
contrast, all templated CMK-3-CoPc catalysts exhibit in-
creased porosities ranging from 0.49–0.62 cm3 g−1 at surface
areas in-between 342 and 522 m2 g−1 (Table 1, entries 3–6).
Thus, all preparation techniques for structured CMK-3-CoPc
catalysts lead to comparable structural properties, although
mesoporosity appears to decline slightly in the order of incip-
ient wetness (IWI) to wet impregnation (WI) and further to
physical grinding (GR). This might be assigned to incomplete
filling of the pore system of SBA-15, leading to less developed
structures during carbonization. ICP-OES analysis confirms
cobalt loadings in the range of 5.8 to 7.0 wt.% within all
structured CMK-3-CoPc systems. This value is slightly lower
compared to the cobalt content of untreated CoPc (8.9 wt%,
Table 1, entry 1) and might be attributed to partial loss dur-
ing SBA-15 template removal (5 M NaOH, 100 °C, 24 h).
The rather harsh treatment, however, leads to the assump-
tion, that any residual Co-species are well embedded and
thus strongly incorporated within the carbon structure, po-
tentially preventing leaching and subsequent catalyst deacti-
vation. A slight loss in Co content is observable for the
template-free carbonized CoPc structure (Table 1, entry 2),
which might be assigned to volatile Co-species formed under
1
2
3
4
5
6
7
CoPc (as received)
CoPc (template free)
CMK-3-CoPc-GR
CMK-3-CoPc-IWI
CMK-3-CoPc-WI
CMK-3-CoPc-WI-rep.
—
—
—
8.9
7.8
6.1
7.0
6.1
5.9
5.8
116
342
522
466
442
0.10
0.49
0.62
0.61
0.56
1.07
5.0
3.0
4.4
4.4
3.8
4.0
CMK-3-CoPc-WI-recyc. 931
Fig. 2 XRD analysis of template-free carbonized CoPc (black), CMK-
3-CoPc-IWI (magenta) and CMK-3-CoPc-WI (light blue and
reproduced, blue) in comparison to Co3O4.
Results and discussion
At the start of our investigations, ordered mesoporous carbon
materials (CMK-3) were prepared by adopting a procedure de-
scribed by Ryoo et al.12 For this purpose, we applied different
methods to impregnate mesoporous silica SBA-15 (hard-tem-
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Table 2 Model reaction: variation of catalysts
Entry
Catalyst
T (°C)
p (bar)
Conv. (%)
Yielda (%)
1
2
3
4
5
6
7
8
9
CMK-3-CoPc-WI
CoPc (template-free)
CoIJOAc)2 carbonized (template-free)
CMK-3-CoPc-Gr
CMK-3-CoPc-IWI
CMK-3-CoPc-WI
CMK-3-CoPc-WI
Pd/C
40
40
40
40
40
40
40
40
RT
20
20
20
20
20
20
10
20
20
100
68
20
73
99
83
65
81
48
>99
67
20
72
97
81b
63
80
48
CMK-3-CoPc-WI
a
Reaction conditions: 0.5 mmol nitrobenzene, 6.2 mol% [Co] (30 mg), 1.5 mL 2-methyltetrahydrofuran and 0.5 mL H2O. GC conversions and
b
selectivities using n-dodecane as an internal standard; 16 h.
Table 3 Selective hydrogenation of (hetero)aromatic nitro compounds
92%
99%
98%
98%
99%
96%
90%
94%
98%
99%
98%
90%
99%
>99%
>99%
98%
98%
>99%
Reaction conditions: 0.5 mmol nitroarene, 6.2 mol% [Co] (30 mg), 1.5 mL 2-methyltetrahydrofuran and 0.5 mL H2O, 20 bar H2, 40 °C, 24 h.
Isolated yields.
synthesis conditions. Reproducibility for wet impregnation as
a facile preparation technique was also studied, and compa-
rable structure as well as cobalt contents were confirmed
(Table 1, entry 5 vs. 6).
Powder XRD patterns of the template-free and structured
materials mainly exhibit the C(002) reflections characteristic
for graphitic carbon, while no clear reflections for Co0 nano-
particles are observed (Fig. 2). In contrast, nanoparticle for-
mation is clearly confirmed by STEM imaging of the CMK-3-
CoPc-WI sample (Fig. 3). HAADF-STEM measurements reveal
both finely distributed Co-species throughout the whole
CMK-3 material as well as nanoparticles varying broadly in
size from approximately 5 to 20 nm. All cobalt nanoparticles
are embedded within a carbon shell, varying slightly in layer
thickness. Furthermore, the typical ordered porous CMK-3
structure becomes evident. XPS analysis (see supporting in-
formation, curve fitting was performed according to Biesinger
et al.14) of template-free CoPc and CMK-3-CoPc-WI reveals Co,
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Table 4 Selective hydrogenation of substituted nitroarenes
>99%
>99%
94%
95%
98%
>99%
Reaction conditions: 0.5 mmol nitroarene, 6.2 mol% [Co] (30 mg), 1.5 mL 2-methyltetrahydrofuran and 0.5 mL H2O, 20 bar H2, 40 °C, 24 hours.
Isolated yields.
O and N species in the presence of a pronounced graphitic
sp2-hybridized C 1s signal, which conforms to the results
from XRD analysis. The O 1s signal can mostly be assigned
to aromatic and aliphatic C–O bonding as well as adsorbed
water and O2 on the catalyst surface. All samples consist
mostly of graphitic and pyridinic N species, while only small
amounts of oxidized N is present. In addition, the unstruc-
tured CoPc (template-free) contains more pyridinic than gra-
phitic N and reveals a significant amount of CoO species,
which only occur in trace quantities for the structured CMK-
3-CoPc-WI catalysts. From the Co 2p3/2 spectra, CoO is con-
firmed as the major species compared to metallic Co(0) in
unstructured CoPc, while structured CMK-3-CoPc-WI further-
more contains large amounts of CoIJOH)2. A precise statement
regarding the exact nature of the catalytically active site can
therefore not be made at this point and will be part of future
investigations.
non-structured catalyst CoPc yielded the product in only 67%
(Table 2, entry 2), while simple CoIJOAc)2 carbonized under
the same conditions but in the absence of phthalocyanine
showed much lower activity with only 20% yield of aniline
(Table 2, entry 3). Apparently, both structuring and N-doping
of the material has a significant effect on its catalytic perfor-
mance. Thus, using CMK-3-CoPc-GR synthesized via grinding
and subsequent carbonization gave somewhat lower yield of
aniline in comparison to CMK-3-CoPc-WI (Table 2, entry 4 vs. 1).
On the other hand, 97% yield of aniline was achieved using
CMK-3-CoPc-IWI prepared via incipient wetness impregnation
prior to carbonization (Table 2, entry 5). Comparing all three
CMK-3-CoPc catalysts prepared via different synthesis proce-
dures, there seems to be no clear correlation between specific
surface area, porosity and catalytic activity. The most pro-
nounced difference in activity is observed for the catalyst CMK-3-
CoPc-GR prepared by physical grinding.
Initial catalytic tests were conducted with nitrobenzene
(1a) as industrially relevant substrate in the solvent mixture
Since all three structured catalysts exhibit comparable spe-
cific surface areas and pore volumes, its comparably low ac-
tivity can be correlated to the small number of mesopores
within CMK-3-CoPc-GR. Wet impregnation methods seem to
be more suitable to introduce the precursor into the pore sys-
tem of SBA-15 and replicate it, yielding a partially meso-
porous material. The template-free and thus unstructured
CoPc showed a similar structure–activity effect. Even though
the template-free CoPc already provides a specific surface
area of 150 m2 g−1, the obtained material is fully microporous
with a total pore volume of only 0.12 cm3 g−1. Thus, we con-
clude that transport pores in the mesoporous regime are ben-
eficial for the overall catalytic performance due to enhanced
accessibility of the active sites. Interestingly, the model reac-
tion proceeded well already at 10 bar H2 pressure
(Table 2, entry 7) and even at room temperature; albeit the
conversion was lower (Table 2, entry 9). This latter example
constitutes one of the rare examples of nitro hydrogenation
with an earth-abundant metal catalyst at ambient tempera-
ture. Comparing the activity of CMK-3-CoPc-WI with a bench-
mark Pd/C (10 wt% Pd) catalyst under the standard condi-
tions, our system outperforms the noble metal catalyst
2-methyltetrahydrofuran/H2O.
In
order
to
identify
outperforming catalysts, we conducted experiments at low
temperature (40 °C) and pressure (20 bar hydrogen). To
our delight, the desired aniline (2a) is obtained in >99%
yield at full conversion in the presence of CMK-3-CoPc-WI
(Table 2, entry 1). For comparison, the template-free and
Fig. 4 Recycling experiments using CMK-3-CoPc-WI. Yield of aniline
was determined by GC using n-dodecane as internal standard.
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(Table 2, entry 1 vs. entry 8). Next, we investigated the sub-
strate scope and long-term stability of the best catalyst
system.
nation. Finally, it should also be mentioned that this stable
and practical cobalt material can be conveniently handled un-
der air without pronounced deactivation.
CMK-3-CoPc-WI catalyzes the hydrogenation of a wide
range of substrates containing diverse substituent groups un-
der mild conditions (Table 3). For example, nitroarenes
containing electron-donating groups, e.g. methyl (1b),
methoxy (1e), ethoxy (1f), and amino (1i) gave high yields
(>90%) of the desired products. A similar behavior is ob-
served with arenes substituted with electron-withdrawing
groups such as carbamoyl (1l) and trifluoromethyl (1g). A va-
riety of industrially relevant anilines such as toluidines (2b),
chloro- (2c), and fluoroanilines (2j), were prepared in excel-
lent yields (90–99%), too. Notably, no dehalogenation prod-
ucts were detected for fluoro, chloro and bromo-substituted
substrates (2c, 2d, 2k, 2n and 2o). In general, nitrogen- and/
or sulfur-containing heteroaromatic amines are valuable
building blocks for the preparation of agrochemicals and
pharmaceuticals. Therefore, different nitroheteroarenes 1q,
1r and 1s were subjected to the standard hydrogenation con-
ditions, and all three substrates were reduced in excellent
yields (98–99%). In order to study the chemoselectivity of the
present system in more detail, we explored the hydrogenation
of nitroarenes bearing other reducible groups (Table 4). Grat-
ifyingly, the CMK-3-CoPc-WI catalyst hydrogenated the nitro
group selectively in the presence of cyano, ketone, and
alkenyl groups (2t, 2u and 2v). Most interestingly, the catalyst
is able to reduce nitro groups in the presence of alkenes, with
full conversion and without any detectable reduction of the
unsaturated unit (2w–2y).
Conclusion
In summary, we have prepared a highly active Co-catalyst via
structuring of cobalt phthalocyanine upon impregnation into
SBA-15 and subsequent carbonization at 990 °C. Hard-
templating enabled the synthesis of well-structured CMK-3-
CoPc catalysts with significantly enhanced activity due to im-
proved accessibility of the active sites. The type of impregna-
tion influences the resulting catalyst performance signifi-
cantly. In this respect, wet impregnation led to the best
results, which is attributed to the pronounced content of
mesopores derived from improved SBA-15 replication, thus
facilitating accessibility of the active sites within the catalyst.
Applying the optimal CMK-3-CoPc-WI catalyst, a wide range
of (hetero)aromatic nitro compounds has been hydrogenated
to the corresponding anilines under very mild conditions in
excellent yields. Notably, the catalyst showed high
chemoselectivity and sensitive groups such as halogens and
reducible functional moieties, e.g. nitriles, ketones, and CC
double bonds, are well tolerated.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
Both stability as well as reusability are important parame-
ters for the application of heterogeneous catalysts. Hence, we
explore the recyclability of CMK-3-CoPc-WI using nitroben-
zene as the substrate. As depicted in Fig. 4, after five succes-
sive cycles the desired product aniline was obtained in 89%
yield. No significant cobalt leaching could be identified (ICP-
OES analysis, see ESI†). In addition, the Co content within the
material remains unchanged (Table 1, entry 5 versus 7).
Interestingly, N2-physisorption reveals an increase in both
specific surface area and total pore volume upon recycling.
There is no clear indication, where this restructuring derives
from; however, the catalytic performance is only slightly af-
fected. From STEM images of the recycled catalyst, no drastic
change in the materials morphology can be confirmed, as the
CMK-3 structure is preserved over five successive runs and no
obvious change in nanoparticle size is observable, either (Fig.
S5†). We assume that the embedment of the active sites
within the structured support as well as the inherent forma-
tion of a carbon shell provides a stabilizing effect during ca-
talysis and therefore preserves the nanoparticles from further
agglomeration, leaching and subsequent deactivation. In addi-
tion, XPS analysis after the reaction reveals that Co-species
are only slightly oxidized further, mostly to CoIJOH)2 species,
which is to be expected in the presence of water under reac-
tion conditions. These results demonstrate that our cobalt
catalyst is generally stable during the course of nitro hydroge-
We thank S. Buchholz for her excellent analytical support as
well as the State of Mecklenburg-Western Pomerania for fi-
nancial support. Regina Palkovits and Cornelia Broicher ac-
knowledge the Federal Ministry of Education and Research
(BMBF) for funding part of this work with the MANGAN re-
search cluster (FKZ 03SF0508).
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