Cobalt-Nanoparticles Catalyzed Efficient and Selective Hydrogenation of Aromatic Hydrocarbons

Article abstract of DOI:10.1021/acscatal.9b02193

The development of inexpensive and practical catalysts for arene hydrogenations is key for future valorizations of this general feedstock. Here, we report the development of cobalt nanoparticles supported on silica as selective and general catalysts for such reactions. The specific nanoparticles were prepared by assembling cobalt-pyromellitic acid-piperazine coordination polymer on commercial silica and subsequent pyrolysis. Applying the optimal nanocatalyst, industrial bulk, substituted, and functionalized arenes as well as polycyclic aromatic hydrocarbons are selectively hydrogenated to obtain cyclohexane-based compounds under industrially viable and scalable conditions. The applicability of this hydrogenation methodology is presented for the storage of H₂ in liquid organic hydrogen carriers.

Full text of DOI:10.1021/acscatal.9b02193

Research Article
pubs.acs.org/acscatalysis
Cite This: ACS Catal. 2019, 9, 8581−8591
Cobalt-Nanoparticles Catalyzed Efficient and Selective
Hydrogenation of Aromatic Hydrocarbons
Kathiravan Murugesan,† Thirusangumurugan Senthamarai,† Ahmad S. Alshammari,‡
Rashid M. Altamimi,‡ Carsten Kreyenschulte,† Marga-Martina Pohl,† Henrik Lund,†
Rajenahally V. Jagadeesh,*,† and Matthias Beller*,†
†Leibniz-Institut fu
̈
r Katalyse e.V. an der Universitat Rostock, Albert-Einstein Str. 29a, Rostock D-18059, Germany
̈
‡King Abdulaziz City for Science and Technology, P.O. Box 6086, Riyadh 11442, Saudi Arabia
S
* Supporting Information
ABSTRACT: The development of inexpensive and practical catalysts for arene
hydrogenations is key for future valorizations of this general feedstock. Here, we
report the development of cobalt nanoparticles supported on silica as selective
and general catalysts for such reactions. The specific nanoparticles were
prepared by assembling cobalt-pyromellitic acid-piperazine coordination
polymer on commercial silica and subsequent pyrolysis. Applying the optimal
nanocatalyst, industrial bulk, substituted, and functionalized arenes as well as
polycyclic aromatic hydrocarbons are selectively hydrogenated to obtain
cyclohexane-based compounds under industrially viable and scalable conditions.
The applicability of this hydrogenation methodology is presented for the
storage of H2 in liquid organic hydrogen carriers.
KEYWORDS: arenes, cyclohexanes, co-nanoparticles, hydrogenations, catalysis
base metal catalysts are highly important and continue to
attract significant interest.
INTRODUCTION
■
Aromatic hydrocarbons represent an essential feedstock for the
chemical industry and their applications range from life
sciences to bulk and fine chemicals as well as polymers and
fuels.1,2 In fact, the basic substrates such as benzene, toluene,
and xylene are produced on >100 million tons per annum.3,4
Moreover, condensed arenes are available from oil refining on
large scale.4 Because of their importance and abundance,
synthetic organic chemistry in the past century and even today
focused to a large extent on their functionalization.1,2,5,6 In
general, hydrogenations of arenes constitute green and 100%
atom-efficient processes for the synthesis of cyclohexane-based
products,1,2,7−45 which are also used in industries, drug
development, material sciences, and energy sector.7−14 Because
of the high stability of the benzene ring, the majority of
catalysts for this transformation are based on precious
metals.15−37 In fact, the state-of-the-art systems constitute
Ru15−25 and Rh.15−19,26−35 For the advancement of sustainable
arene hydrogenation, the development of more earth abundant
metal catalysts is of central importance. In this respect, so far,
nickel38−43 including Raney nickel,44 as well as few other
Co38,42−48-based catalysts are known for the hydrogenation of
benzene and other simple substrates. However, in general,
these catalysts exhibit poor selectivity and low stability for
recycling and have not been applied for diverse com-
pounds.38−48 Thus, cost-efficient and chemo-, regio-, and
stereo-selective hydrogenation of structurally challenging
aromatic hydrocarbons in the presence of heterogeneous
In recent years, supported nanostructured materials had
tremendous impact on the chemical industry and the
innovation of chemical processes.49−62 Key for the implemen-
tation of these materials is the preparation of active
nanoparticles on appropriate supports using inexpensive
precursors and applying convenient, easy to follow up, and
reproducible methods.49−62 In this regard, pyrolysis of (in situ
generated) organometallic complexes,51,58−62 metal organic
frameworks (MOFs),52,63−67 and coordination polymers
(CPs)67−74 allows for the synthesis of unique catalytic
materials. Notably, metal containing CPs can be easily
engineered by variation of metal ions and organic linkers.67−74
Following this concept, the selection of suitable linkers for the
preparation of active nanoparticles with the precise size and
shape is most important. Advantageously, a plethora of
carboxylic acid- and nitrogen-based linkers are available to
prepare different CPs. Here, we describe the preparation of
cobalt-based CPs using inexpensive pyromellitic acid (PMA)
and piperazine (PZ) linkers and their use as convenient
precursor and structure directing template. Supporting these
CPs on commercial silica (AEROSIL) and subsequent
pyrolysis creates reusable cobalt nanoparticles for the hydro-
Received: May 27, 2019
Revised: July 15, 2019
© XXXX American Chemical Society
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Figure 1. Synthesis of supported cobalt nanoparticles using CPs as precursors and structure controlling templates.
Figure 2. Hydrogenation of aniline to cyclohexylamine: activity of cobalt catalysts. Reaction conditions: 1 mmol aniline, 120 mg catalyst (9 mol %
Co), 50 bar H2, 4 mL t-BuOH, 135 °C, 24 h, yields were determined by GC using n-hexadecane standard. *Catalyst was prepared using polymeric
material reported in ref 74.
genation of industrially important and structurally diverse
arenes.
Initially, all of the prepared materials were tested for the
hydrogenation of industrially important aniline to cyclohexyl-
amine as a benchmark reaction (Figure 2). Cyclohexylamine is
used as a valuable intermediate for pharmaceuticals (e.g.,
mucolytics, analgesics, bronchodilators) and herbicides.
Furthermore, it is the precursor for accelerators for vulcan-
ization and an effective corrosion inhibitor. First, we tested Co-
TPA-DBCO@SiO2-800 material prepared using TPA and
DABCO linkers, which were used previously for the
preparation of cobalt-based MOFs and nanocatalysts.52,66
Interestingly, this catalyst showed some activity (13%).
Variation of the nitrogen linker from DABCO to PZ gave
Co-TPA-PZ@SiO2-800 with increased product yield (20%).
Further preparation of CPs using different benzene carboxylic
acid linkers and PZ resulted in more active materials. Among
these, Co-CPs based on PMA and piparazine (PZ) linkers
produced the best catalyst (Co-PMA-PZ@SiO2-800), which
led to 90% cyclohexylamine. The reported cobalt-CP74 using
CoCl2·6H2O, PMA and PZ linkers with a molar ratio of 1:1:3
was also prepared and immobilized on SiO2. Pyrolysis of this
RESULTS AND DISCUSSION
■
Preparation of Cobalt-Based Nanocatalysts. First, we
generated different cobalt-based CPs in situ in N,N-
dimethylformamide (DMF) combining Co(NO3)2·6H2O
with different carboxylic acids such as terephthalic acid
(TPA; 1,4-benzenedicarboxylic acid; 1), trimesic acid (1,3,5-
benzenetricarboxylic acid; TMA; 2), PMA (1,2,4,5-benzenete-
tracarboxylic acid; PMA; 3), and nitrogen linker, for example,
1,4-diazabicyclo[2.2.2]octane (triethylendiamine; DABCO; 4)
and PZ (diethylenediamine; PZ; 5) (Figure 1).
The resulting materials were assembled on different supports
(SiO2, Al2O3, ZSM-5 TiO2, carbon, and MgO) and then
pyrolyzed under argon to obtain the respective nanomaterials
(see the Experimental Section for detailed preparation).
Hereafter, these materials are labeled as Co-acid linker-amine
linker@support-x, where x denotes the pyrolysis temperature.
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material at 800 °C under argon produced a much less active
catalyst for the formation of cyclohexylamine (5%). To
produce an active catalyst, 3 equivalents of each of PMA and
PZ linkers with respect to cobalt are required. The X-ray
powder diffraction (XRD) pattern of cobalt-PMA-PZ CP is
given in Figure S1. Comparison with materials made from
single linkers, Co-PMA@SiO2-800 and Co-PZ@SiO2-800,
showed activity (52%) for the former one, whereas the latter
is almost inactive. Similarly, cobalt nitrate@SiO2-800, the
nonpyrolyzed Co-PMA-PZ@SiO2 and corresponding Co-
PMA-PZ CP were not active too. In addition to the different
linkers, the effect of the pyrolysis temperature was also studied.
The material pyrolyzed at 400 °C (Co-PMA-PZ@SiO2-400)
again is completely inactive, whereas pyrolysis at 600 °C and
1000 °C (Co-PMA-PZ@SiO2-600; Co-PMA-PZ@SiO2-1000)
showed little activity (25 and 8%, respectively). Further, to
know the effect of supports, different supported materials
prepared using Co-PMA-PZ CP were also tested. Among these
Al2O3, TiO2, and zeolite-supported catalysts showed some
activity (10−28%). However, carbon- and MgO-supported
ones are completely inactive.
either Ni or Pt catalysts at >160 °C,75,76 is conveniently
formed in 95% yield at 135 °C and 50 bar H2 (Scheme 1;
product 8). Similarly, methylcyclohexane used as energy fuel is
obtained in 95% (product 10). Apart starting from aniline,
cyclohexylamine can be prepared directly from nitrobenzene in
82% yield (Scheme 1; product 7). It is worthwhile to mention
here that cyclohexylamine is industrially produced by the
hydrogenation of aniline (see benchmark reaction), which is
obtained from nitrobenzene. In this respect, the production of
cyclohexylamine directly from nitrobenzene has obvious
advantages with respect to step- and cost-economy. The
preference of this catalyst for chemoselective arene hydro-
genation is showcased using functionalized substrates contain-
ing amine, ether, amide, ester, and pyrazole groups (Scheme 1;
products 13−22). In all these cases, the benzene ring was
selectively reduced by tolerating these functional groups.
Hydrogenation of Multiring Aromatic Hydrocarbons. In
case of aromatic hydrocarbons with multiple rings, the
reduction of one in the presence of others is generally more
challenging and most of the known catalysts are less selective
for such substrates.15−48 However, using Co-PMA-PZ@SiO2-
800 catalyst regioselective hydrogenation of one ring in case of
naphthalenes, biphenyl, diphenyl methane, fluorine and
xanthene molecules took place at 135 °C (Scheme 2; products
Catalysis and Synthetic Applications. Hydrogenation
of Bulk and Functionalized Arenes. With the successful
catalyst Co-PMA-PZ@SiO2-800 for the benchmark process of
aniline to cyclohexylamine in hand, we carried out its general
applicability for the hydrogenation of various aromatic
hydrocarbons. As shown in Schemes 1−4, this catalyst allows
Scheme 2. Hydrogenation of Aromatic Hydrocarbons with
a
Multiple Rings Using Co-PMA-PZ@SiO2-800
Scheme 1. Co-PMA-PZ@SiO2-800 Catalyzed
Hydrogenation of Bulk and Functionalized Arenes to
a
Obtain Value-Added Cyclohexanes
a
Reaction conditions: 1 mmol substrate, 120 mg catalyst (9 mol %
Co), 50 bar H2, 4 mL t-BuOH, 135 °C, 24 h, yields were determined
by gas chromatography (GC) using n-hexadecane standard.
b
c
Nitrobenzene was used as substrate to obtain cyclohexylamine. At
a
Reaction conditions: 1 mmol substrate, 120 mg catalyst (9 mol %
d
e
150 °C for 48 h. At 150 °C for 34 h. For 40 h and isolated yields.
b
Co), 50 bar H2, 4 mL t-BuOH, 135 °C, isolated yields. In 4 mL i-
c
PrOH and GC yield using n-hexadecane. In 4 mL t-BuOH and 100
d
μL H2O. 150 °C.
for the selective hydrogenation of diverse functionalized and
substituted arenes, including annulated compounds to cyclic
aliphatic hydrocarbons. Several bulk chemicals including
benzene, toluene, and phenol were hydrogenated to produce
industrially important products in excellent yields (Scheme 1
products 8−11). As an example, cyclohexane is used for the
production of KA oil (mixture of cyclohexanone and
cyclohexanol), adipic acid, and caprolactam. This product,
which is currently produced (>5 million tons/annum) using
23−35). Notably, some of these partially hydrogenated
derivatives are of increasing interest as intermediates for
energy technologies. As an example, cyclohexylbenzene (CHB;
product 26) is used as electrolyte additive in lithium ion
batteries. Interestingly, at slightly elevated temperature (150
°C) we achieved full hydrogenation in selected molecules. As a
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a
Scheme 3. Diastereoselective Hydrogenation of Substituted Phenols and Arenes
a
Reaction conditions: 1 mmol substrate, 120 mg catalyst (9 mol % Co), 50 bar H2, 4 mL t-BuOH, 135 °C, 24 h, isolated yields. Cis−trans isomers
b
c
d
selectivity was determined by NMR spectral analysis. At 150 °C. In 4 mL i-PrOH. 1 g substrate, 9 mol % catalyst, 15 mL t-BuOH, 150 °C, 48 h,
isolated yields. Isomeric ratio in case of 2-isopropyl-5-methylcyclohexanol was determined by GC.
result, 9 bicyclic aliphatic hydrocarbons were obtained in up to
92% yields (Scheme 2; products 36−44).
products 29, 38,40). On the other hand, 2-tert-butylphenol
gave the corresponding cis product with 93% selectivity
(Scheme 3; product 46). To understand this observation,
isomerization experiments of both cis- and trans-2-methyl-
cyclohexanol were performed under the typical hydrogenation
conditions. However, no isomerization occurred. Similar to the
1,2-disubstituted phenols, the related 1,4-derivatives yield
preferentially the thermodynamically more stable trans isomers
(Scheme 3; products 30, 44, 49−54). An exception is the
formation of 27, which is explained by the stronger
coordination of the second arene ring to the catalyst surface,
which leads to the cis-product. In case of 1,3-disubstituted
phenols, the cis products were preferentially formed, which is
also explained by their increased thermodynamic stability
(Scheme 3; products 47−48). To showcase the selective
hydrogenation of disubstituted anilines and biaryls, N-phenyl-
toluidine and 4,4′-dimethylbiphenyl, respectively, were also
reduced in high yields (Scheme 3; products 44 and 27). As an
example for the reduction of trisubstituted arenes, the
hydrogenation of thymol (2-isopropyl-5-methylphenol; 55)
was investigated, which is industrially important for the
synthesis of menthol isomers.78 At 150 °C, full conversion of
thymol was observed using our novel base metal catalyst
yielding 71% of ( )-isomenthol.78
Diastereoselective Hydrogenation Process. A major
challenge for any new arene hydrogenation catalyst to be
implemented in synthetic chemistry is the diastereoselective
hydrogenation of multiple substituted derivatives.20,23,26,35,77
In general, such reactions offer a unique possibility to
synthesize platform molecules for new pharmaceuticals,
agrochemicals, and fine chemical applications. Unfortunately,
relatively, a few catalysts have been applied for such
transformations.20,23,26,37,77 Among these, notable works are
based on Rh26 and Ru20 for the preparation fluorinated
cycloalkanes and substituted cyclohexanols, respectively.
To evaluate the potential of our Co-PMA-PZ@SiO2-800
system in this context, the hydrogenation of a series of
substituted phenols for the preparation of cyclohexanols was
performed. Apart from being interesting intermediates, the
present hydroxyl group can be easily further functionalized. As
shown in Scheme 3, various di- and trisubstituted phenols
underwent hydrogenation to give the corresponding diaster-
eoisomers in good to excellent yields (Scheme 3). 1,2-
Disubstituted phenols with methyl, cyclohexyl, and benzyl
groups lead preferentially to the thermodynamically more
stable trans products in up to 95% selectivity (Scheme 3;
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a
Scheme 4. Application of Cobalt-Catalyzed Hydrogenation Process for the Storage of H2 in LOHCs
a
b
Reaction conditions: 2 mmol substrate, 240 mg catalyst (9 mol % Co), 50 bar H2, 8 mL t-BuOH, 150 °C, 48 h. At 135 °C, 24 h.
a
Scheme 5. Representing the Synthetic Utility of Cobalt-Catalyzed Hydrogenation Process for Gram-Scale Reactions
a
Reaction conditions: 1−25 g substrate, 4.5 mol % catalyst, t-BuOH solvent (10 v for each 1 g substrate), 135 °C. *With 9 mol % catalyst. †In i-
PrOH solvent.
Applicability of Cobalt Nanoparticles for the Storage of
H2 in LOHC. Catalytic hydrogenation reactions constitute a key
element for the transformation of renewable energy (wind, PV,
etc.) into chemical energy carriers. In this context, the storage
of hydrogen in liquid organic hydrogen carriers (LOHCs) has
attracted significant interest in recent years.79−81 In general,
different arenes such as benzene, methylbenzene, diphenyl-
methane, bipyridine, and dibenzyltoluene are considered as
LOHCs. For example, the latter compound is readily available
and technically applied as heat transfer oil (MARLOTHERM
SH, MSH).79−81 Regarding the known catalysts for this
transformation, commercially available Ru/Al2O3 was used for
the hydrogenation of MSH (45% storage of hydrogen at 150
°C).81 Using our cobalt catalyst at 150 °C, hydrogenation of
MSH yielded three different isomers with overall hydrogen
storage of 65% (Scheme 4). In addition, other selected
molecules reacted smoothly and resulted in up to 99%
hydrogen storage (Scheme 4).
significant loss of activity and in all cases, the yield of
cyclohexanol was >90% (Figure 3). In addition to recycling, we
also tested the stability of the Co-PMA-PZ@SiO2-800 catalyst
by performing recycling of the catalyst at reduced reaction time
(Figure S22). At 10 h of reaction time, the yield of
cyclohexanol in each run was rapidly decreased (Figure S22).
These studies showed that the catalyst is not stable and
deactivation occurred. Hence, 24 h of the reaction time is
Synthetic Utility Co-Catalyzed Arenes Hydrogenation
Protocol. After having demonstrated several applications of
this novel Co-PMA-PZ@SiO2-800 material, reaction upscaling
and catalyst recycling were showcased. Consequently, several
arenes were hydrogenated to obtain corresponding cyclo-
hexanes in up to 25 g (Scheme 5). Notably, all of these
reactions were carried out using lower catalyst loading (4.5 mol
% Co).
Figure 3. Catalyst recycling for the hydrogenation of phenol to
cyclohexanol. Reaction conditions: 10 g phenol (106 mmol), 6.35 g
catalyst (4.5 mol %), 100 mL t-BuOH, 135 °C, 24 h.
Recycling and reusability of the catalyst was performed using
phenol hydrogenation. It was recycled up to 4 times without
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required to obtain >90% cyclohexanol in up to 5th run (Figure
3)
Characterization of Co-Based Nanocatalysts. In order to
identify the structural features and to understand the varying
catalytic activities of these novel cobalt-based materials, we
characterized the active catalyst (Co-PMA-PZ@SiO2-800), less
active (Co-TPA-PZ@SiO2-800, Co-PMA@SiO2-800), and
completely inactive materials (Co-PZ@SiO2-800) using X-ray
powder diffraction (XRD), Brunauer−Emmett−Teller (BET),
scanning transmission electron microscopy (STEM), electron
energy loss spectroscopy (EELS), and energy-dispersive X-ray
spectroscopy (EDXS) and X-ray photoelectron spectroscopy-
(XPS). XRD analysis revealed that the most active catalyst
contains metallic cobalt particles along with some Co3O4
(Figure S2). In case of less active materials, a mixture of
metallic Co and oxidic cobalt (Co3O4 and CoO) particles were
present (Figures S3 and S4). However, in case of the inactive
material, the presence of mainly Co3O4 particles was observed
(Figure S5). Another nonactive material, cobalt nitrate@SiO2-
800 also contained mainly metal oxide particles (Figure S6).
BET measurements revealed the importance of the surface area
for the catalyst activity which increased in the order of 42 (Co-
PZ@SiO2-800), 77 (Co-PMA@SiO2-800) to 155 m2/g (Co-
PMA-PZ@SiO2-800) (Figures S8−S10).
The STEM analysis of the most active material revealed the
formation of cobalt nanoparticles with sizes between 5 and 20
nm of different compositions. Contrast in high-angle annular
dark field (HAADF) micrographs shows the presence of
probably metallic Co particles tightly covered in graphitic
carbon and particles of Co oxide (Figure 4A) more loosely
attached to carbon and the support. Unfortunately, during
EDXS and EELS measurements, the morphology of the Co-
containing particles changed somewhat under the extended
influence of the electron beam and formation of particles
similar to such Co oxide crystallites was observed. Therefore,
EELS data cannot be used to determine the exact nature of the
Co species, for example, by fingerprinting the fine structure of
the Co-L edge in the EELS data (Figures S12 and S13A).82
Additionally, there is a carbon phase containing N attached to
the SiO2 support as seen in the EELS elemental maps. High-
resolution HAADF images of a free hanging carbon structure
indicate the presence of even heavier atoms in this phase;
however, the density was too low for EDXS or EELS analysis
(Figure S14). In case of the less active material prepared using
TPA and PZ linkers (Co-TPA-PZ@SiO2-800), metallic cobalt
particles are surrounded by graphitic shells and Co oxide
particles show little contact to carbon structures when attached
to the silica support (Figures 4B and S13B). The Co-PMA@
SiO2-800 material contained generally small (<15 nm) with a
few large polycrystalline particles as exceptions (Figures 4C
and S13C). Some of these particles are encapsulated within
large well-ordered graphitic carbon others without carbon shell
and probably of a Co oxide variant are also located directly on
silica. In case of the inactive sample (Co-PZ@SiO2-800), large
particles up to 85 nm, which are completely encapsulated with
in graphitic shells, are found (Figures 4D and S13D). In
addition, some Co/Co3O4 particles in core shell structures
were also observed, however, with no clear connection to
carbon structures.
Figure 4. STEM-HAADF and -ABF images of silica supported cobalt
catalysts. Left column shows HAADF images of the general structure,
the middle column shows details of Co oxide particles with ABF
images as an inset to highlight possible carbon structures, and the
right column shows ABF images with details of Co particles, with
tightly packed graphitic covering of metallic Co particles where these
metallic particles are present, (a) Co-PMA-PZ@SiO2-800, (b) Co-
TPA-PZ@SiO2-800, (c) Co-PMA@SiO2-800, and (d) Co-PZ@SiO2-
800.
most active catalyst, (Co-PMA-PZ@SiO2-800) the deconvo-
lution of Co 2p peaks shows the presence of two types of Co-
species (Figure S15a; left image). The peak at 778.55 eV
represents metallic cobalt (Co 2p3/2), while the peak at 780.51
eV denotes Co2+ (2p3/2) species (Figure S15a; left image). The
corresponding Co 2p1/2 peaks are observed at 794.43 and
796.45 eV. Co 2p3/2 and Co 2p1/2 peak energy splitting was
found to be 15.88 eV for Co(0) and 15.90 eV for Co2+.83 The
catalyst Co-PMA@SiO2-800 also contains mixed oxidation
states for Co-species (Figure S15b; left image). A peak at
778.49 eV represents the presence of Co 2p3/2 in the metallic
state, whereas a peak at 779.92 eV can be assigned to Co2+
species. Although, a peak for Co3+ could not be deconvoluted,
however, the presence of strong satellite peaks at 786.43 and
802.20 eV are indicative for multiple oxidized Co species, as
satellite peaks arise due to spin−spin interactions of different
Co species.83,84 The Co-PZ@SiO2-800 material also shows the
presence of two cobalt species (Figure S15c; left image);
however, the absence of a peak around 778.5 eV indicates that
metallic Co is not present.85 The peaks present at 779.91 and
781.52 eV can be assigned to 2p3/2 Co2+ and Co3+ species. The
corresponding 2p1/2 peaks are present at 793.52 and 795.45 eV
for both these species (Figure S15c; left image). In these three
catalysts, surface concentration and states of N vary due to the
use of different ligands in the pyrolysis process. Clearly, the
presence of different linkers affects the graphitization process
and the occurrence of N at the surface.86 Nevertheless, in all
these samples, N 1s spectra are complicated due to the low
concentrations of N on the surface and also its interaction with
cobalt, carbon, and SiO2.86−88 For Co-PMA-PZ@SiO2-800,
Further XPS analysis for Co-PMA-PZ@SiO2-800, Co-
PMA@SiO2-800, and Co-PZ@SiO2-800 samples has been
conducted to understand in more detail the states of cobalt and
nitrogen at the surface of the catalysts (Figure S15). In the
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which was prepared using Co(NO3)2, PMA, and PZ, N 1s
spectra could be deconvoluted to three N 1s peaks within the
catalyst structure (Figure S15a; right image). The binding
energies centered at 399.35, 401.36, and 403.88 eV can be
assigned to pyridinic N, pyrrolic N, and graphitic N,
respectively.86,87 In contrast, the sample Co-PMA@SiO2-800,
which was prepared form Co(NO3)2, PMA, contains only two
types of N (Figure S15b; right image). Deconvolution results
fitting of two N-peaks centered at binding energies of 399.4
and 400.1 eV. Compared to Co-PMA-PZ@SiO2-800, in this
case, graphitic N is absent at the surface. The presence of N in
this sample obviously comes from cobalt nitrate. The material
Co-PZ@SiO2-800 prepared by Co(NO3)2 and PZ also
contains two types of N 1s peaks centered at 401.21 and
404.05 eV (Figure S15c; right image). In this case, pyridinic N
is absent. Further to know the phases of cobalt, in situ XRD
analysis has been performed (Figures S16−S21). If the catalyst
is decomposed, the formation of CoO has to be observed at
800 °C under a helium (He) flow of 10 mL/min. However, the
catalyst cooled down, and the product of the thermal treatment
was investigated ex situ and observed only Co3O4. According
to this result, the experiments were performed by flushing with
He (10 mL/min) for approximately 25 min. Then, the gas flow
was stopped, and the temperature program was started. After
heating to 800 °C (25 °C/min heating rate), the diffraction
data were collected over a period of 2 h (each 10 min) and the
cooling of the sample were monitored by the collection of
diffraction data each 100 °C step. In this experiment, the
formation of metallic Co can be stated. Then, the metallic Co
is oxidized to Co3O4 during the cooling process. An additional
experiment was also carried out to check the amount of O2
during heating, isothermal, and cooling procedure by the
analysis of the gas passed through an empty capillary. The
value of the O2 concentration was constant over the whole
experiment (heating to 800 °C, 30 min isothermal, cooling to
25 °C, heating rate: 25 °C/min).
this hydrogenation methodology is demonstrated by catalyst
recycling and gram-scale synthesis as well as for the storage of
H2 in LOHC.
EXPERIMENTAL SECTION
■
General Consideration. All substrates were obtained
commercially from various chemical companies. Cobalt(II)
nitrate hexahydrate (cat no. 139267-100G), PZ (ReagentPlus,
≥99%; cat no. P45907) 1,4-diazabicyclo[2.2.2]octane
(DABCO; ReagentPlus, ≥99%; cat no. D27802-25G), TPA
(cat no. 185361; 98%), and titanium(IV) oxide (TiO2; cat no.
718467; ≥99.5%) were obtained from Sigma-Aldrich. PMA
(cat no. B23099; 96%), 1,3,5-benzenetricarboxylic acid (TMA;
cat no. A15947; 98%), zeolite ZSM-5 ammonium (ZSM-5; cat
no. 45879), and magnesium oxide (MgO; cat no. 43196;
99.95%) were obtained from Alfa Aesar. t-Butanol (t-buOH,
code-390690025; 99.8%) was obtained from Across chemicals.
Silica (AEROSIL OX 50) was obtained from Evonik. Carbon
powder, VULCAN XC72R with code XVC72R and CAS no.
1333-86-4 was obtained from Cabot Corporation Prod. The
pyrolysis experiments were carried out in a Nytech Qex oven.
Before using, the purity of the substrate has been checked.
STEM measurements were performed at 200 kV with
aberration-corrected JEM-ARM200F (JEOL, Corrector:
CEOS). The microscope was equipped with a JED-2300
(JEOL) energy-dispersive X-ray-spectrometer and Enfinium
ER (GATAN) electron energy-loss spectrometer with dual
EELS for chemical analysis. HAADF and annular bright field
(ABF) detectors were used for general imaging; the annular
dark field detector was used for position control for EELS
acquisition. Dual EELS was done at a camera length of 4 cm,
an illumination semi angle of 27.8 mrad, and a filter entrance
aperture semi angle of 41.3 mrad using the low loss region for
compensation of energy shifts during acquisition. The solid
samples were deposed without any pretreatment on a holey
carbon supported Cu-grid (mesh 300) and transferred to the
microscope.
All of these characterization studies revealed that in the most
active material (Co-PMA-PZ@SiO2-800) different types cobalt
nanoparticles are present. Metallic Co particles tend to be
covered in graphitic shells possibly isolating them, whereas a
large fraction of Co oxide particles connects to the carbon
matrix containing nitrogen and possibly highly disperse Co.
The nitrogen content indicates that PMA with PZ as the linker
forms these structures which in accordance with the BET
results seems to enhance the active surface area of the catalyst
strongly when compared to the pure support.
XRD powder pattern were recorded on a PANalytical X’Pert
diffractometer equipped with a XCelerator detector using
automatic divergence slits and Cu Kα1/α2 radiation (40 kV,
40 mA; λ = 0.15406, 0.154443 nm). Cu β-radiation was
excluded using a nickel filter foil. The measurements were
performed in 0.0167° steps and 100 s of data collecting time
per step. The samples were mounted on silicon zero
background holders. The obtained intensities were converted
from automatic to fixed divergence slits (0.25°) for further
analysis. Peak positions and the profile were fitted with
Pseudo-Voigt function using the HighScore Plus software
package (PANalytical). Phase identification was done by using
the PDF-2 database of the International Center of Diffraction
Data (ICDD).
BET surface areas of the catalysts were determined on a
NOVA 4200e instrument by N2-physisorption at −196 °C.
Prior to the measurements, the known amount of the catalyst
was evacuated for 2 h at 220 °C to remove physically adsorbed
water.
In situ XRD studies were preformed on a Stoe Stadi P
equipped with a Stoe ht2-in situ oven and a Mythen 1K
detector in Debye−Scherrer geometry using a Mo-X-ray tube
[50 kV, 40 mA, Kα1: 0.70930 Å, Ge(111)]. Temperature
correction was applied by measurement of well-known phase
transitions of standard materials (AgNO3, KClO4, Ag2SO4,
SiO2, K2SO4, K2CrO4, WO3, BaCO3). Gas dosage was done via
CONCLUSIONS
■
In conclusion, we have developed efficient cobalt-based
nanocatalysts for general and selective hydrogenation of
aromatic hydrocarbons to produce the corresponding aliphatic
products, which serve as essential compounds in chemical and
life science industries as well as for energy and material
sciences. Key to success for this hydrogenating process is the
graphitic shell-encapsulated cobalt nanoparticles prepared by
the template synthesis of cobalt−PMA−PZ CP on commercial
silica and subsequent pyrolysis under an inert atmosphere.
This cobalt-based protocol proceeds for the hydrogenation of
industrial bulk, substituted, and functionalized arenes as well as
polycyclic aromatic hydrocarbons under industrially viable and
scalable conditions. In addition, this catalyst system has been
applied for the diastereoselective hydrogenation of multiple
substituted phenols and arenes. The utility and applicability of
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a Bronkhorst mass flow controller. The sample to investigate
was grinded, pressed to pellets by a pressure of 10 tons,
crushed, and sieved to fraction of 100−150 μm. The fraction
was filled to a quartz glass capillary (approx. 2 mm outer
diameter, 1 mm inner diameter, opened on both sides) to a
height of approx. 8 mm and fixed by quartz glass wool at both
sides. The loading of the catalyst precursor was increased to 20
wt % to obtain data of reliable quality after the decomposition
of the precursor. The capillary was mounted into the in situ
chamber, and the capillary was flushed with He (10 mL min−1)
for 20 min. The gas flow was switched off before starting the
heating. The sample was heated to 800 °C (25 °C min−1), and
the temperature was kept constant for 2 h while recording
diffraction data every 10 min (stationary measurement, 300 s
of acquisition time). The cooling process (25 °C min−1) was
monitored each 100 °C step.
block preheated at 150 °C and stirred for 1 h after fixing a
reflux condenser to the round-bottom flask. Then, the reflux
condenser was removed, and the reaction mixture was allowed
to stand without stirring for 1−2 h at 150 °C. A pink colored
cobalt-CP was formed and filtered through filter paper and
washed 3−4 times with hot DMF (15−20 mL each time). The
washed material was dried at 140−150 °C in an oven for 15 h
and then finally dried under high vacuum at room temperature
for 4−5 h.
General Procedure for the Hydrogenation of Arenes.
A magnetic stirring bar and 1.0 mmol of the corresponding
arene were transferred to a glass vial (8 mL), and 3 mL of the
solvent (t-BuOH) was added. Then, 120 mg of the catalyst
(Co-PMA-PZ@SiO2-800; 9 mol % Co) was added, and the
vial was fitted with a septum, cap, and needle. The reaction
vials (eight vials with different substrates at a time) were placed
into a 300 mL autoclave. The autoclave was flushed with 30
bar hydrogen twice, and then, it was pressurized with 50 bar of
hydrogen. The autoclave was placed into an aluminum block
preheated at 145 °C (placed 30 min before counting the
reaction time in order to attain reaction temperature), and the
reactions were stirred for the required time. During the
reaction, the inside temperature of the autoclave was measured
to be 135 °C, and this temperature was used as the reaction
temperature. After completion of the reactions, the autoclave
was cooled to room temperature. The remaining hydrogen was
discharged, and the vials containing the reaction products were
removed from the autoclave. The solid catalyst was filtered off
and washed thoroughly with ethyl acetate. The reaction
products were analyzed by GC−MS. The corresponding
products were purified by column chromatography (silica; n-
hexane−ethyl acetate mixture) and characterized by NMR and
GC−MS analysis.
Procedure for the Yields Determined by GC for Selected
Compounds. After completion of the reactions, n-hexadecane
(100 μL) was added as the standard to the reaction vials, and
the reaction products were diluted with ethyl acetate followed
by filtration using plug of silica and then analyzed by GC.
Procedure for Gram Scale Reactions. To a Teflon or
glass fitted 300 mL or 1.0 L autoclave, a magnetic stirring bar
and the corresponding arene were transferred and 20−250 mL
of t-BuOH was added. After adding the required amount of
catalyst (Co-PMA-PZ@SiO2-800; 4.5 mol % Co; 60 mg for
each 1.0 mmol substrate), the autoclave was flushed with 40
bar hydrogen twice, and then, it was pressurized with 40 bar
hydrogen. The autoclave was placed into an aluminum block
preheated at 145 °C (placed 30 min before counting the
reaction time in ordered to attain reaction temperature), and
the reaction was stirred for the required time. During the
reaction, the inside temperature of the autoclave was measured
to be 135 °C, and this temperature was used as the reaction
temperature. After completion of the reaction, the autoclave
was cooled to room temperature. The remaining hydrogen was
discharged, and the reaction products were removed from the
autoclave. The solid catalyst was filtered off and washed
thoroughly with ethyl acetate. The reaction products were
analyzed by GC−MS, and the corresponding products were
purified by column chromatography (silica; n-hexane−ethyl
acetate mixture) and characterized by NMR and GC−MS
spectral analysis.
All catalytic experiments were carried out in 300 and 100
mL autoclaves (PARR Instrument Company). In order to
avoid unspecific reactions, all catalytic reactions were carried
out either in glass vials, which were placed inside the autoclave
or glass/Teflon vessel-fitted autoclaves.
GC and GC−mass spectrometry (MS) were recorded on
Agilent 6890N instrument. GC conversion and yields were
determined by GC-FID, HP6890 with a FID detector, column
HP530 m × 250 mm × 0.25 μm.
1H and 13C NMR data were recorded on Bruker ARX 300
and Bruker ARX 400 spectrometers using DMSO-d6, CD3OD
and CDCl3 solvents.
HRMS data were recorded on a mass spectrometer MAT 95
XP (Thermo Electron), 70 eV.
Procedure for the Catalyst Preparation. Preparation of
Co-PMA-PZ@SiO2 Template (3 g Preparation) and Pyrolysis
To Obtain Nanomaterial. In a 50 mL round-bottom flask,
cobalt(II) nitrate hexahydrate (444.5 mg; 1.5 mmol) and PZ
(395.2 mg; 4.5 mmol) were stirred for 5 min in 15 mL of DMF
at 150 °C. To this mixture, the already dissolved (by heating)
PMA (1.16 g; 4.5 mmol) in 10 mL of DMF was added. Then,
the round-bottom flask containing the reaction mixture was
placed into an aluminum block preheated at 150 °C and stirred
for 20−30 min after fixing a reflux condenser to the round-
bottom flask. Then, 1.2 g of silica (AEROSIL OX 50) was
added followed by the addition of 15 mL of DMF, and the
reaction mixture was stirred again at 150 °C for 4−5 h. Then,
the reflux condenser was removed, and the round-bottom flask
containing the reaction product was allowed to stand without
stirring and closing for 20 h at 150 °C in order to slowly
evaporate DMF and to grow the cobalt-CP template on
carbon. After the evaporation of the solvent and ensuring the
complete drying, the material was cooled to room temperature
and grinded to give a fine powder. The powdered material was
pyrolyzed at the defined temperature (400, 600, 800, or 1000
°C) for 2 h under an argon atmosphere and then cooled to
room temperature after pyrolysis.
Elemental analysis (wt %): Co-PMA-PZ@SiO2-800: Co =
4.6%, C = 16.98%, H = 0.10%, Si = 32.59% N = 0.81%.
The same procedure has been applied for the preparation of
other supported materials.
Preparation of Co-PMA-PZ CP. In a 50 mL round-bottom
flask, cobalt(II) nitrate hexahydrate (444.5 mg) and PZ (395.2
mg) were stirred for 5 min in 15 mL DMF at 150 °C, and to
this mixture the already dissolved PMA (by heating) (1.16 g)
in 10 mL DMF was added. Then, the round-bottom flask
containing a reaction mixture was placed into an aluminum
Procedure for Catalyst Recycling. A magnetic stirring
bar and 106 mmol of phenol were transferred to a 300 mL
autoclave, and then, 70 mL t-BuOH was added. After adding
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required time. After the completion of the reaction, the
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for determining the yield of cyclohexanol. The separated
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and used without further purification or reactivation for the
next run.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.9b02193.
■
S
(15) Dyson, P. J. Arene hydrogenation by homogeneous catalysts:
fact or fiction? Dalton Trans. 2003, 2964−2974.
(16) Qi, S.-C.; Wei, X.-Y.; Zong, Z.-M.; Wang, Y.-K. Application of
supported metallic catalysts in catalytic hydrogenation of arenes. RSC
Adv. 2013, 3, 14219−14232.
Additional materials characterization data and NMR
data and spectra (PDF)
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in Science of Synthesis. Stereosel. Synth. 2011, 1, 257−294.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: jagadeesh.rajenahally@catalysis.de (R.V.J.).
*E-mail: matthias.beller@catalysis.de (M.B.).
́
(18) Gual, A.; Godard, C.; Castillon, S.; Claver, C. Soluble
■
transition-metal nanoparticles-catalysed hydrogenation of arenes.
Dalton Trans. 2010, 39, 11499−11512.
(19) Bianchini, C.; Meli, A.; Vizza, F. Hydrogenation of Arenes and
Heteroaromatics, From Handbook of Homogeneous Hydrogenation; de
Vries, J. G., Elsevier, C. J., Eds.; Wiley-VCH 2007.
ORCID
Kathiravan Murugesan: 0000-0003-1678-7728
Carsten Kreyenschulte: 0000-0002-2371-4909
Rajenahally V. Jagadeesh: 0000-0001-6079-0962
Matthias Beller: 0000-0001-5709-0965
(20) Maegawa, T.; Akashi, A.; Yaguchi, K.; Iwasaki, Y.; Shigetsura,
M.; Monguchi, Y.; Sajiki, H. Efficient and Practical Arene Hydro-
genation by Heterogeneous Catalysts under Mild Conditions.
Chem.Eur. J. 2009, 15, 6953−6963.
(21) Cui, X.; Surkus, A.-E.; Junge, K.; Topf, C.; Radnik, J.;
Kreyenschulte, C.; Beller, M. Highly selective hydrogenation of arenes
using nanostructured ruthenium catalysts modified with a carbon−
nitrogen matrix. Nat. Commun. 2016, 7, 11326.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
(22) Dwivedi, A. D.; Rai, R. K.; Gupta, K.; Singh, S. K. Catalytic
Hydrogenation of Arenes in Water Over In Situ Generated
Ruthenium Nanoparticles Immobilized on Carbon. ChemCatChem
2017, 9, 1930−1938.
(23) Li, H.; Wang, Y.; Lai, Z.; Huang, K.-W. Selective Catalytic
Hydrogenation of Arenols by a Well-Defined Complex of Ruthenium
and Phosphorus-Nitrogen PN3-Pincer Ligand Containing a Phenan-
throline Backbone. ACS Catal. 2017, 7, 4446−4450.
(24) Morioka, Y.; Matsuoka, A.; Binder, K.; Knappett, B. R.;
Wheatley, A. E. H.; Naka, H. Selective hydrogenation of arenes to
cyclohexanes in water catalyzed by chitin-supported ruthenium
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(26) Wiesenfeldt, M. P.; Nairoukh, Z.; Li, W.; Glorius, F.
Hydrogenation of fluoroarenes: Direct access to all-cis-(multi)-
fluorinated cycloalkanes. Science 2017, 357, 908−912.
■
We gratefully acknowledge the support of the European
Research Council (ERC), the Federal Ministry of Education
and Research (BMBF), the State of Mecklenburg-Vorpom-
mern, and King Abdulaziz City for Science and Technology
(KACST). We thank the analytical staff of the Leibniz-Institute
for Catalysis, Rostock, for their excellent service. All data are
available in the Supporting Information.
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