Tuning the Catalytic Performance of Cobalt Nanoparticles by Tungsten Doping for Efficient and Selective Hydrogenation of Quinolines under Mild Conditions

Article abstract of DOI:10.1021/acscatal.1c01561

Non-noble bimetallic CoW nanoparticles (NPs) partially embedded in a carbon matrix (CoW@C) have been prepared by a facile hydrothermal carbon-coating methodology followed by pyrolysis under an inert atmosphere. The bimetallic NPs, constituted by a multishell core-shell structure with a metallic Co core, a W-enriched shell involving Co7W6 alloyed structures, and small WO3 patches partially covering the surface of these NPs, have been established as excellent catalysts for the selective hydrogenation of quinolines to their corresponding 1,2,3,4-tetrahydroquinolines under mild conditions of pressure and temperature. It has been found that this bimetallic catalyst displays superior catalytic performance toward the formation of the target products than the monometallic Co@C, which can be attributed to the presence of the CoW alloyed structures.

Full text of DOI:10.1021/acscatal.1c01561

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Tuning the Catalytic Performance of Cobalt Nanoparticles by
Tungsten Doping for Efficient and Selective Hydrogenation of
Quinolines under Mild Conditions
Marta Puche, Lichen Liu, Patricia Concepción, Iván Sorribes,* and Avelino Corma*
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ABSTRACT: Non-noble bimetallic CoW nanoparticles (NPs)
partially embedded in a carbon matrix (CoW@C) have been
prepared by a facile hydrothermal carbon-coating methodology
followed by pyrolysis under an inert atmosphere. The bimetallic
NPs, constituted by a multishell core−shell structure with a
metallic Co core, a W-enriched shell involving Co7W6 alloyed
structures, and small WO3 patches partially covering the surface of
these NPs, have been established as excellent catalysts for the
selective hydrogenation of quinolines to their corresponding
1,2,3,4-tetrahydroquinolines under mild conditions of pressure
and temperature. It has been found that this bimetallic catalyst
displays superior catalytic performance toward the formation of the
target products than the monometallic Co@C, which can be
attributed to the presence of the CoW alloyed structures.
KEYWORDS: non-noble metal catalysts, CoW bimetallic alloys, heterogeneous catalysis, selective hydrogenation, quinolines
Pt,38−41 Rh,38,42−50 Ru,41,51−64 Ir,38,65,66 and Au36,67).
However, the high and volatile price associated with the low
INTRODUCTION
■
Catalytic hydrogenation by non-noble metal nanoparticles
availability of precious metals has boosted the interest to
(NPs) has attracted increasing interest in the research
laboratories and for industrial applications.1−4 Indeed, from
exploit cheap and earth-abundant metals for the development
of novel catalysts in recent times, although, to date, to a limited
the perspective of sustainability, it has become an essential
extent for the hydrogenation of quinolines. The earliest work
dealt with the use of the traditional Raney-Ni catalyst.1,68−70 In
2015, Beller and co-workers prepared a cobalt-based catalyst
consisting of N-graphene-modified cobalt oxide/cobalt NPs by
pyrolysis of a nonvolatile phenanthroline-ligated complex on
alumina (Co3O4−Co/NGr@α-Al2O3), and successfully ap-
plied for the hydrogenation of N-heteroarenes including
quinoline compounds.71 Later, the groups of Wang72 and
Li73 developed other related N-doped graphene-coated cobalt
(oxide) NPs avoiding the use of sophisticated ligands and
investigated their catalytic performance for the title reaction.
In 2018, we hydrothermally prepared cobalt−molybdenum−
sulfide (Co−Mo−S) catalysts with tunable phase composition
that display an optimal and broad-spectrum performance for
methodology for reductive transformations of chemical
substances.5 Since activity and selectivity of metal NPs can
be tuned by controlling their size, shape, composition, and
support interactions, hydrogenation reactions can be per-
formed avoiding or minimizing the formation of byprod-
ucts.6−21 Fundamentally, this is particularly advantageous for
the preparation of 1,2,3,4-tetrahydroquinolines, which are
important building blocks broadly present in many bioactive
compounds including natural products, agrochemicals, syn-
thetic drugs, and lead compounds.22−24 These industrially
valuable scaffolds can be conveniently synthesized by partial
hydrogenation of readily available quinoline derivatives, a
synthetic methodology with high atom-economy if the
formation of completely saturated products, as well as
hydrogenation of other co-existing reducible functionalities,
are avoided.25,26 In addition to selectivity issues, this
hydrogenative transformation also involves other scientific
and technological challenges, such as high reaction energy
barriers to break aromaticity and prevent catalyst poison-
ing.27,28
Received: April 6, 2021
Revised: June 9, 2021
Published: June 18, 2021
Impressive achievements have been reached in this area by
applying catalysts based on noble metal NPs (e.g., Pd,29−37
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a
Scheme 1. Schematic Representation of the Synthesis of Bimetallic CoW NPs Partially Coated by Carbon Layers (CoW@C)
a
Different types of atoms are indicated by different colors: cobalt (gray), tungsten (yellow), and oxygen (red).
the chemo- and regioselective hydrogenation of quinoline
derivatives.74 Shortly after, it was demonstrated that a wide
range of functionalized 1,2,3,4-tetrahydroquinolines can also
be accessed by applying N-doped carbon-modified iron-based
catalysts, which resulted from pyrolysis of a carbon-
impregnated composite obtained from an iron salt and N-
aryliminopyridines as ligands.75 More recently, MOF-derived
N-coordinated cobalt nanocrystals,76 Co−Nx sites previously
liberated from N-doped carbon nanotubes (N-CNTs) under
laser irradiation by the liquid technique,77 in situ generated Co
NPs by hydrolysis of NaBH4,78 copper oxide NPs supported
on alumina,79 and bimetallic CoCu oxide NPs,80 have also
been applied for the hydrogenation of various quinolines.
Meanwhile, this hydrogenative transformation catalyzed by
non-precious metal NPs-based catalysts (Cu,81 Ni,82 and
Co83,84) has also been investigated from the perspective of H2-
storage systems based on liquid organic hydrogen carriers.
It is also noteworthy that other reductive catalytic protocols
catalyzed by non-noble metal-based heterogeneous catalysts,
including transfer hydrogenation (with formic acid85−87 or
ammonia borane88−90), photocatalytic hydrogenation,91 and
sequential dearomative hydroboration/hydrogenation92 have
been proposed for the synthesis of 1,2,3,4-tetrahydroquino-
lines. However, the practical simplicity and higher atom
efficiency make the straightforward hydrogenation with
molecular hydrogen more advantageous.
In spite of the great success achieved for the hydrogenation
of quinoline derivatives catalyzed by non-noble metals, more
efficient catalysts are required for practical applications. A
constant target for substituting precious by non-noble metals
has been to achieve high activities similar to those of the
formers under mild reaction conditions while maintaining
good stability. In contrast, the non-noble NPs-based catalysts
reported to date for the hydrogenation of quinolines require
demanding conditions of H2 pressure, temperature, and/or
long reaction times; therefore, more advanced high-perform-
ance catalysts are required.
involving metal-like electronic states where N-containing
compounds tend to be adsorbed and activated.94−96 Interest-
ingly, the promoting role of Co and Ni has also been observed
in sulfur-free Mo- and W-based oxide catalysts applied for
hydroconversion reactions of hydrocarbons, which have the
advantage of avoiding deactivation by desulfurization under
hydrogenative conditions. With high controversy, there is a
strong tendency to attribute this synergy effect to the
formation of β-Co(Ni)W(Mo)O4 species that promote a
better electronic interaction between the metals (W, Mo) and
the promoters (Co, Ni), thus favoring the reductive and acid
properties of the mixed oxide catalysts.97−100
Inspired by industry, in this work, we have developed a
series of Co- and W-based bimetallic materials (CoW@C) and
have applied them for the selective hydrogenation of quinoline
(1a) to afford the partially hydrogenated product 1,2,3,4-
tetrahydroquinoline (2a). We have seen that the activity of Co
NPs is enhanced by the presence of surface CoW alloyed
species, while the presence of metal oxides decreases their
catalytic performance. Notably, under otherwise the same
reaction conditions, the most active catalyst CoW@C-0.05
displays higher catalytic activity (determined by comparison of
the initial reaction rates normalized to the mass of metal
weights) than previously reported Co-based catalysts. Com-
parative H2−D2 exchange experiments suggest that a homolytic
rather than a heterolytic dissociation of H2 takes place on the
metallic surface, which is proposed to be the active site where
1a is also adsorbed. The surface reaction between both the
activated reactants is the rate-determining step in the
hydrogenation process, as revealed by kinetic experiments
performed with the catalyst CoW@C-0.05. Furthermore, we
have demonstrated that the use of this catalyst allows the
preparation of a broad range of 1,2,3,4-tetrahydroquinolines
under mild conditions of both temperature and pressure, thus
constituting a real alternative to precious metal-based catalysts
for practical applications.
RESULTS AND DISCUSSION
The hydrogenation of N-heteroarene compounds, including
quinoline derivatives, is a key reaction step that takes place
before C−N bond breaking in hydro-denitrogenation (HDN)
treatments, routinely applied in the refining industry for
removing nitrogen heteroatoms from crude feedstocks.
Bimetallic sulfides, constituted by a combination of group VI
metals (either Mo or W) with group VIII metals (such as Co
and Ni), supported on acidic substrates are the most
commonly used catalysts in the industry for hydrotreatment
processes.93 It is generally accepted that in these systems Co or
Ni atoms decorate the edge position of MoS2 or WS2 forming
Co(Ni)−W(Mo)−S structures that display “brim” sites
■
Synthesis and Characterization of CoW@C Materials.
In recent years, our group has been engaged in research
devoted to the preparation of Co NPs and their application as
catalysts for selective hydrogenation of fine chemicals for
substituting noble metals with non-noble ones. In 2016, we
reported monodispersed Co NPs coated with carbon layers,
prepared by thermal decomposition of a Co−EDTA complex,
that displayed higher catalytic activity than previously reported
heterogeneous non-noble metal and the state-of-the-art Au
catalysts for the hydrogenation of substituted nitroarenes into
the corresponding anilines under mild conditions.101 Next, we
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prepared improved catalysts based on Co NPs by a simple
carbon coating process consisting of a hydrothermal treatment
with glucose as a carbon source, followed by pyrolysis.102,103
The resultant NPs stabilized by a few layers of carbons (Co@
C) show enhanced catalytic activity for the hydrogenation of
nitroarenes to the corresponding anilines and for the
hydrogenation of levulinic acid to γ-valerolactone. Particularly
worth mentioning is the fact that the carbon-coating method
used for the preparation of these non-noble metal-based
nanoparticulate materials also enables the synthesis of
bimetallic NPs. Indeed, CoNi@C NPs were prepared with
significantly higher catalytic activity than the monometallic
ones (Co@C NPs), while maintaining high selectivity for the
chemoselective hydrogenation of nitroarenes.102
In this work, we developed a hydrothermal carbon-coating
methodology for the preparation of bimetallic CoW@C
materials (Scheme 1). First, bimetallic oxides were obtained
by slowly dripping an aqueous solution of sodium carbonate
into an ethylene glycol mixture of cobalt(II) acetate and
different amounts of sodium tungstate at 165 °C. The resultant
solids were coated by glucose through a hydrothermal
treatment in an autoclave at 175 °C and subsequently
pyrolyzed under an inert atmosphere at 600 °C (see the
Experimental Section for the extended preparation details). At
such a high temperature, according to the so-called
carbochemical reduction,104 the bimetallic oxides are expected
to be reduced by carbon from glucose, while some of the
carbon is oxidized to CO2. Importantly, the rest of the carbon
will tend to be graphitized into thin layers, resulting in the
partial covering of the nanoparticles. According to our previous
studies, the role of the carbon layers is to protect the non-
noble metal NPs from agglomeration and from deep oxidation
by air, as well as to promote the in situ reduction of oxide
species on the surface of the NPs under reaction
conditions.101−103,105 The resultant bimetallic catalysts were
denoted CoW@C-X, where X indicates the W/Co mole ratio
used in the catalyst preparation. For comparison, monometallic
Co@C NPs were also synthesized following the same
preparation methodology.
the material with the lowest content of tungsten, named
CoW@C-0.05, is dominated by the presence of diffraction
peaks at 2θ values of 44.2, 51.5, and 75.8° corresponding to the
(111), (200) and (220) planes, respectively, of the cubic Co
(face-centered cubic (fcc) type, PDF code: 96-900-8467)
phase. In addition, other weak diffraction peaks at 41.6 and
47.5°, characteristic of the (100) and (101) planes,
respectively, of the hexagonal Co (hexagonal close-packed
(hcp) type, PDF code 96-900-8493) phase, are also slightly
visible. As a result of the low amount of tungsten doping, no
additional peaks associated with tungsten-containing species
were detected, which also suggests their high dispersion. In
contrast, in materials with a higher content of tungsten, other
diffraction peaks become noticeable besides the ones
associated with metallic cobalt. All additional diffraction
peaks of the XRD pattern of CoW@C-0.25 correspond to
the mixed metal oxide CoWO4 (PDF code 00-015-0867), with
the exception of a peak at a 2θ value of 47.3°, which could be
attributed to the (004) plane of the triclinic WO3 phase (PDF
code 00-020-1323). Further increase of the tungstate salt in the
catalyst preparation produced the formation of the segregated
phase of WO3 to a higher extent, as revealed by the XRD
pattern of the bimetallic material CoW@C-0.50.
The morphology and elemental distribution of the bimetallic
CoW@C and the monometallic Co@C materials were
investigated by high-resolution TEM (HRTEM) and EDS
elemental mapping (Figure 2; see also Figures S1−S4). CoW@
C-0.05 is a nanoparticulate material with particle sizes ranging
from ca. 10 to ca. 40 nm (Figure 2a−d). The NPs are
embedded in a carbon matrix derived from the thermal
decomposition of glucose. The crystal lattice fringes with a
spacing of 0.215 nm corresponding to the (100) and (111)
planes of the hexagonal Co phase can be inferred in the core of
these NPs. Interestingly, other interlayer distances of 0.231 and
0.202 nm, similar to those of the (110) and (0111) planes,
respectively, of the alloyed structure Co7W6, could also be
found on the upper surface of these NPs. Moreover, different
lattice fringes with spacings of 0.272 and 0.180 nm, which
could be attributed to the (022), and either to the (114) or the
̅ ̅
(114) planes, respectively, of a segregated WO3 phase were
X-ray diffraction (XRD) patterns of the prepared bimetallic
CoW@C materials are shown in Figure 1. The XRD pattern of
also detected as small patches on the surface of these NPs.
These results suggest that CoW@C-0.05 is composed of
bimetallic NPs, partially embedded in a carbon matrix, that
display a multishell core−shell structure with a metallic Co
core, a W-enriched shell involving Co7W6 alloyed structures,
and small WO3 patches partially covering the surface of these
NPs. This is consistent with the EDS elemental mapping
(Figure 2h) and, more precisely, with the quantitative EDS
analyses from selected regions of these NPs (Figure S1) that
confirm their bimetallic distribution with a major concen-
tration of W species on the surface.
The HRTEM micrographs of the material CoW@C-0.25
revealed a more heterogeneous size distribution of bigger (up
to 80 nm) and more agglomerated NPs of different nature, all
of them embedded in a carbon matrix (Figures 2e,f and S2). In
addition to bimetallic CoW NPs, WO3 NPs of ca. 10 nm size,
and the mixed metal oxide CoWO4 in the form of larger NPs
could also be detected. In contrast, the material with the
highest content of tungsten, CoW@C-0.50, displayed a
different morphology, where NPs, although present, were
hardly found (Figures 2g and S3). This material is mainly
composed of interlaced phases that display characteristic
lattice-fringe spacings of 0.264 and 0.202 nm, respectively,
Figure 1. XRD diffraction patterns of the monometallic Co@C and
bimetallic CoW@C materials.
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Figure 2. HRTEM images of catalysts (a−d) CoW@C-0.05, (e, f) CoW@C-0.25, and (g) CoW@C-0.50. (h) High-angle annular dark-field
scanning transmission electron microscopy (HAADF-STEM) image of the catalyst CoW@C-0.05 and elemental mapping of C, Co, and W.
Figure 3. Co 2p3/2 (a) and W 4f (b) regions of the XPS spectra of the in-situ reduced catalysts CoW@C and Co@C.
associated with the (2
of 0.455 and 0.247 nm associated with the (001) and (200)
planes, respectively, of CoWO4.
̅
00) and (132) planes of WO3, as well as
0.05) is slightly shifted to lower temperatures, thus indicating
that their presence influences the cobalt oxide reduction.
Moreover, both catalysts display a broad H2-uptake peak
centered at temperatures around 550−625 °C, which is
typically associated with the gasification of carbon and the
reduction of surface oxygenated groups present on the carbon
surface.107−109 In addition, for the catalyst CoW@C-0.05, a
broad shoulder attributed to the reduction of tungsten oxide
species can also be distinguished at higher temperatures
between 700 and 800 °C.110 The H2-TPR profile of the
catalyst CoW@C-0.25 is dominated by the presence of a peak
associated with the reduction of Co2+ species (at ∼330 °C)
together with the broad peaks related to carbon and the
reduction of tungsten oxide species. In contrast, the highest H2
consumption for the catalyst CoW@C-0.50 takes place in the
Further evidence on the different nature of the non-metallic
phase composition for catalysts CoW@C synthesized with
different W/Co molar ratios was obtained by the temperature-
programmed reduction under hydrogen (H2-temperature-
programmed reduction (H2-TPR); see Figure S5). Reduction
profiles for catalysts Co@C and CoW@C-0.05 do not differ
significantly and show two different H2-uptake peaks, the first
one at ∼180 to 200 °C and a second one that finalizes around
400−500 °C. These reduction events are associated with the
sequential reduction of Co3O4 (a phase also detected by
Raman spectroscopy, as shown in Figure S6) to CoO, and to
metallic Co, respectively.106 Interestingly, the reduction profile
of the catalyst containing tungsten doping species (CoW@C-
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Table 1. XPS Binding Energy (BE) Values and Surface Concentration for the In Situ Reduced Catalysts CoW@C and Co@C
c
Co 2p3/2 (BE (eV))
W 4f7/2 (BE (eV))
W4+ (%)
c
sample
Co@C
Co0 (%)
a
Con+ (%)
W0 (%)
W6+ (%)
Co/W/O/C
a
b
778.2 780.9 ; 783.2 (74.8%) 779.1; 783.2 (25.2%)
14.5:--:15.2:70.4
8.2:2.9:29.2:59.7
5.7:4.1:29.2:61.0
2.0:3.5:20.0:74.4
a
a
b
CoW@C-0.05 778.2 780.8 ; 783.1 (90.1%) 779.7; 785.0 (9.9%)
CoW@C-0.25 778.2 780.9 ; 783.1 (13.5%) 779.7; 785.0 781.4; 787.6 (86.5%)
CoW@C-0.50 778.2 780.9 ; 783.1 (60.1%) 780.1; 785.0 781.8; 786.1 (39.9%)
32.2 (41.1%) 34.4 (43.3%) 36.0 (15.6%)
a
a
b
b
36.0 (100%)
36.0 (100%)
a
a
b
b
a
b
c
Plasmon loss peaks of cobalt. Satellite peak of oxidized cobalt. Surface atomic ratio.
Figure 4. (a) Catalytic performances of CoW@C and Co@C for the hydrogenation of quinoline (1a). (b) Yields of 1a and 1,2,3,4-
tetrahydroquinoline (2a) vs time for the catalyst CoW@C-0.05.
reduction zone between 700 and 800 °C due to the presence
of the tungsten oxide species to a higher extent.
together with two plasmon loss peaks at ∼3.0 and 5.0 eV above
the main peak is observed. Other components at higher
binding energy (BE) associated with the oxidized cobalt
species are also present (779.7−786.2 eV) in combination with
the associated satellite peaks, which were also included in the
deconvolution and fitting in spite of the fact that their
identification was difficult because of their overlapping with
other peaks. More specifically, the peak at BE of 779.7 eV
corresponds to the Co3+ (i.e., Co3O4) species and it is
observed as a minority phase. In addition, the peak at BE of
781.4 eV has been related to the highly ionic Co2+ type
species111 that can be correlated to CoWO4, which appears in
catalysts CoW@C-0.25 and CoW@C-0.50, in good agreement
with the XRD and HRTEM characterization. The presence of
oxidized and metallic cobalt species in all materials is also
confirmed in the CoL3VV auger spectra (Figure S7), where
the main peak at 773 eV is associated with metallic cobalt,
while the shoulder at 767.0 eV corresponds to the oxidized
cobalt species.112 However, from the Co XPS BE, it is difficult
to differentiate between the metallic cobalt and CoW alloyed
species, whose BE appears in both cases at around 778.2 eV.113
Regarding the tungsten species, the XPS W 4f core level
spectra (Figure 3b and Table 1) of both CoW@C-0.25 and
CoW@C-0.50 materials exclusively show two peaks with BE of
36.0 and 38.1 eV, which are associated with the characteristic
spin−orbit splitting of 4f7/2 and 4f5/2, respectively, and denote
the presence of W6+ species, indicating that the tungsten
species were in the forms WO3 and CoWO4 on the surface of
these catalysts, while no peaks associated to metallic tungsten
were observed. In contrast, the W 4f core level spectrum of
Catalytic Performance of Bimetallic CoW@C Materi-
als for the Hydrogenation of Quinoline. The hydro-
genation of quinoline (1a) to its partially reduced product
1,2,3,4-tetrahydroquinoline (2a) was used as a model reaction
to compare the catalytic activity of the bimetallic CoW@C and
the monometallic Co@C materials. Initial hydrogenation
experiments were performed at 100 °C, 8 bar of H2, and
using toluene as a solvent. Based on the H2-TPR results, and
according to our previous work on related Co NPs in which
the surface of the metal NPs was partially oxidized by contact
with air,101,102 a catalyst pre-activation treatment (170 °C, 2 h,
10 bar H2) in the same autoclave was performed before the
catalytic reaction. In the absence of this pre-activation
treatment, an induction period associated with the in-situ
reduction of the oxidized surface under catalytic reaction
conditions was observed (Figure S8), thus suggesting that the
presence of metallic species on the surface is crucial for the
reaction to be accomplished.
The surface composition and oxidation state of the in-situ
reduced catalysts were investigated by X-ray photoelectron
spectroscopy (XPS). The XPS of Co 2p3/2 are shown in Figure
3a and their associated components are included in Table 1.
From the surface atomic ratio, it is observed that the content of
metal species on the surface decreases by increasing the
tungsten loading in the catalyst preparation due to the
formation of metal oxides to a higher extent and/or to a
more efficient embedding of these species in the carbon matrix.
In all materials, a peak at 778.2 eV due to metallic cobalt
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CoW@C-0.05 after deconvolution and fitting also displays two
additional doublet peaks at a lower BE due to the presence of
W4+ (34.4 and 38.1 eV) and W0 (32.2 and 36.0 eV) species.114
Interestingly, the BE ascribed to W0 is slightly shifted to higher
BE (32.2 eV) compared to the literature data (31.6 eV), which
can tentatively be ascribed to the formation of CoW alloyed
species.
The catalytic performance of the bimetallic CoW@C and
the monometallic Co@C materials for the hydrogenation of 1a
is shown in Figure 4. The catalyst CoW@C-0.05 containing
the surface CoW alloyed species displayed higher activity
(higher than twice the initial reaction rate) than the
monometallic catalyst Co@C, while a further increase of the
tungsten content in the catalyst preparation led to detrimental
results. This sharp decrease in the catalytic activity observed
for catalysts CoW@C-0.25 and CoW@C-0.50 can be ascribed
to the formation of both the bimetallic CoWO4 and the
monometallic WO3 oxides as separate phases, which displayed
a negligible conversion of 1a when they were pre-activated and
used as catalysts under otherwise the same conditions (Figure
S9). When the tungsten doping content was further reduced in
the catalyst preparation, the obtained catalyst (CoW@C-
0.025) also displayed a slightly lower activity than the catalyst
CoW@C-0.05 (Figure S9).
Figure 5. Temperature-programmed desorption (TPD) mass
spectrometry experiments on CoW@C and Co@C.
In terms of regioselectivity, CoW@C-0.05 afforded a
quantitative yield (99%) of 2a after full conversion of 1a,
with no formation of other byproducts even after longer
reaction times (Figure 4b). In contrast, in the presence of the
catalyst with lower tungsten doping content (CoW@C-0.025)
and the monometallic Co@C material, traces (<2 and <5%,
respectively) of 5,6,7,8-tetrahydroquinoline (3a) as byproduct
were also formed by the hydrogenation of the N-free aromatic
ring of 1a (Figure S9). Assuming that the electronic structure
of Co is modified by charge transfer with the tungsten doping
metal,10,115 a feasible reason of full selectivity for the catalyst
CoW@C-0.05 could be ascribed to the most acidic (i.e.,
electronegative) nature of the tungsten species, which could
hinder to a higher extent the flat adsorption of 1a on the
catalyst surface through the higher interaction with the
electron-donating N-heteroaromatic ring.96,116−118 To get
further insights on this assumption, the interaction strengths
of 1a with catalysts CoW@C-0.05 and Co@C were
investigated by analyzing their desorption profiles at increasing
temperatures in the temperature-programmed desorption
(TPD) setup coupled with a mass spectrometer analyzer
(see the Experimental Section for more details). As shown in
Figure 5, while the interaction strengths of the monometallic
catalyst Co@C are characterized by desorption temperatures
of 108 and 125 °C, the TPD-mass spectra of the catalyst
CoW@C-0.05 also display higher desorption temperatures of
142 and 164 °C, thus revealing that quinoline (1a) interacts
stronger on the surface of the bimetallic catalyst CoW@C-0.05
than on the monometallic catalyst.
Figure 6. H2−D2 exchange experiments on the pre-activated catalysts
CoW@C-0.05 and Co@C.
decrease of the ion current for the HD mass signal during
the H2−D2 exchange experiments over the bimetallic CoW@
C-0.05 catalyst was achieved.5,119 This result suggests that a
homolytic dissociation of H2 is taking place on the surface of
both catalysts rather than a heterolytic cleavage.
Kinetic Study. A comparison of the catalytic activities of
both CoW@C-0.05 and Co@C catalysts with the results
obtained from the H2−D2 exchange and TPD-mass spectrom-
etry experiments reveals that the most active catalyst (i.e.,
CoW@C-0.05) displays lower H2 dissociation activity and a
stronger interaction with quinoline (1a), thus demonstrating
that the activation of H2 cannot be the controlling step of the
process, and suggests that it could be controlled by the
adsorption/activation of 1a. Therefore, to get further insights
into the rate-determining step of the reaction, kinetics
experiments for the hydrogenation of 1a on CoW@C-0.05
were performed by measuring the initial reaction rates at
conversion levels below 20% at different H2 pressures and
different concentrations of 1a, while keeping one of these two
reactants constant. As shown in Figure 7, in both cases, the
initial reaction rate experiences a fast increase up to a
maximum, followed by a decrease with increasing the H2
pressure or the concentration of 1a. According to the kinetic
models developed by Hougen−Watson/Langmuir−Hinshel-
Next, H2−D2 exchange experiments were performed over
the pre-activated catalysts CoW@C-0.05 and Co@C to
investigate the influence of the tungsten doping metal on the
catalytic performance of these nanoparticles for H2 activation.
As shown in Figure 6, the catalyst CoW@C-0.05 displays a
considerably lower H2 dissociation rate than the monometallic
Co@C. This significant difference can be explained on the
basis of higher electronegativity of tungsten, which hinders the
donation of d-electrons to the σ* antibonding orbital of H2 to
weaken the H−H bond, and consequently, a significant
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Figure 7. Kinetic studies of the hydrogenation of quinoline (1a) in the presence of CoW@C-0.05. (a) Initial reaction rate at different H2 pressures
and 0.53 mmol of 1a. (b) Initial reaction rate at different concentrations of 1a at 8 bar H2. Reaction conditions: 10 mg CoW@C-0.05, 3 mL of
toluene, 0.3 mmol of dodecane as an internal standard, 100 °C. The catalyst was pre-activated at 170 °C with 10 bar H2 for 2 h, and then cooled to
room temperature before injecting the reaction mixture to the batch reactor.
wood for reaction mechanisms in heterogeneous cataly-
sis,103,120−122 the observed evolution trends for the initial
reaction rates indicate that the rate-determining step of the
reaction is not H2 dissociation, or adsorption, or activation of
1a, but a hydrogenation step involving a surface reaction
between the activated H2 (metal hydrides) and the adsorbed
1a. These results can be described by the kinetic rate equation
surface, which are formed to a higher extent by the reduction
of the oxidized surface under a harsher pre-activation
treatment.
Optimization of Reaction Conditions for the Hydro-
genation of Quinoline. Further optimization of the reaction
conditions was carried out with the most active catalyst
CoW@C-0.05. No loss of selectivity was achieved by using
solvents other than toluene but it led to a lower reactivity
(Table 2, entries 1−6). Remarkably, CoW@C-0.05 is also
k·kQ ·kH ·CQ ·PH
2
2
γ =
1 + KQ ·CQ
+
KH ·PH
2
2
Table 2. CoW@C-0.05 Catalyzed Hydrogenation of
a
Quinoline (1a)
where CQ is the concentration of 1a, PH is the H2 pressure, kQ,
2
kH , and k are the kinetic constants for H2 dissociation, 1a
2
adsorption, and surface reaction, respectively, and KH and KQ
2
are the equilibrium adsorption constants for H2 and 1a.
Notably, taking into account this result and the fact that
differences between the bimetallic CoW@C-0.05 and the
monometallic Co@C catalysts mainly arise from the presence
of CoW surface alloyed species (and non-active tungsten
oxides), the superior activity of the catalyst CoW@C-0.05 can
be ascribed to these alloy species, which promote a better
interaction (i.e., more efficient d-orbital hybridization)
between both metals (Co and W), thus likely favoring the
controlling surface reaction to be more efficiently accom-
plished.
On the other hand, the parabolic trend observed in the
kinetic experiments also suggests that both reactants (H2 and
1a) share the same type of active sites on the catalyst surface.
To further clarify whether the decrease of reactivity at high
hydrogen pressure (>12 bar) is due to a competitive
adsorption of reactants or involves an irreversible variation of
the catalyst surface, a control experiment was performed.
Instead of the common catalyst pre-activation treatment (at 10
bar H2), the catalyst CoW@C-0.05 was pre-activated at 22 bar
H2 (pressure at which its catalytic activity for the hydro-
genation of 1a is considerably lower) and used for the
hydrogenation of 1a under standard conditions at 8 bar H2.
Interestingly, a higher reaction rate was achieved when
compared with that obtained through the common catalyst
pre-activation treatment (Figure S10). This result not only
confirms that a competitive adsorption/activation of H2 and 1a
takes place on the same active sites, but also further suggests
that the active sites are the metallic species on the catalyst
b
b
entry
solvent
conversion (%)
yield 2a (%)
1
2
3
4
5
6
1,4-dioxane
n-hexane
2-propanol
THF
MeOH
toluene
93
86
85
75
47
88
83
84
75
43
99
88
>99
92
c
7
toluene
a
Reaction conditions: 1a (0.53 mmol), catalyst (20 mg), solvent (3
mL), 6 h. Determined by gas chromatography (GC) using dodecane
as an internal standard. Catalyst (40 mg), 60 °C, 24 h.
b
c
active even at a lower reaction temperature (60 °C; Table 2,
entry 7). At this point, it is also worth mentioning that this
catalyst allows performing the hydrogenation reaction of
quinoline (1a) to 1,2,3,4-tetrahydroquinoline (2a) under
milder conditions than any of the non-noble metal-based
catalysts reported to date. Typically, these kinds of catalysts are
only active at a relatively high temperature (>120 °C) and/or
at least 20 bar of H2 pressure (see Table S1). In contrast,
CoW@C-0.05 catalyzes this reaction at milder conditions of
both temperature and H2 pressure (<100 °C and <8 bar H2,
respectively). Under otherwise the same reaction conditions as
that used for CoW@C-0.05, other previously reported Co-
based catalysts showed lower catalytic activity, which has been
determined by comparison of the initial reaction rates
normalized to the mass of metal weights (see Figure S11).
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Reusability of the Catalyst CoW@C-0.05. Next, we
studied the catalyst recyclability for the model reaction. An
important aspect of the catalyst CoW@C-0.05 is its para-
magnetic character, which facilitates the catalyst separation
from the liquid phase with the help of a magnetic bar. As
shown in Figure 8, CoW@C-0.05 was reused for five runs
performed the hydrogenation of structurally diverse quinolines
under the same mild reaction conditions used previously
(Table 3). Quinaldine and 8-methylquinoline were smoothly
hydrogenated affording the corresponding 1,2,3,4-tetrahydro-
quinolines in exceptional yield (Table 3, entries 1 and 2,
respectively). No noticeable influence on the catalytic activity
was observed when quinoline was substituted at the 2-position
with a more sterically hindered phenyl group (Table 3, entry
3). The electron-rich 6-metoxy-1,2,3,4-tetrahydroquinoline
could be easily prepared in excellent yield (92%) by the
hydrogenation of the corresponding quinoline precursor
(Table 3, entry 4). Fluoro-substituted quinolines either alone
or accompanied with methyl or methoxy groups are also
suitable candidates to accomplish the target selective hydro-
genation reaction. In fact, the electron-deficient 6-fluoro-
1,2,3,4-tetrahydroquinoline was obtained in 88% isolated yield
(Table 3, entry 5), and the disubstituted quinolines were
successfully converted to their corresponding 1,2,3,4-tetrahy-
droquinoline congeners in excellent yields (Table 3, entries 6−
8). Furthermore, the more sterically hindered 8-chloro-
quinaldine was hydrogenated into its pyridine-moiety hydro-
genated product in 90% isolated yield (Table 3, entry 9). It is
important to mention that no dehalogenated byproducts were
detected in all of the above-described reactions. Interestingly,
hydrogenation of quinolines bearing a redox-sensitive ester
group to their 1,2,3,4-tetrahydroquinoline derivatives could
also be accomplished in excellent yields (85 and 94%) without
reduction of the ester group (Table 3, entries 10 and 11,
respectively).
Figure 8. Recycling of CoW@C-0.05 for the hydrogenation of
quinoline (1a) to 1,2,3,4-tetrahydroquinoline (2a). Reaction con-
ditions: 1a (0.53 mmol), CoW@C-0.05 (20 mg), toluene (3 mL), 6 h
(run 1−3), 6.5 h (run 4), and 10 h (run 5).
achieving excellent yield of the desired product 2a after full
conversion of 1a. It is noticeable that no metal leaching
occurred in the reaction medium, as revealed by inductively
coupled plasma-mass spectrometry (ICP-MS) analysis of the
filtrates. The stability of this catalyst was further corroborated
by XRD and TEM characterization. No additional diffraction
peaks associated to oxide species were observed in the XRD
pattern of the recycled catalyst (Figure S12). As in the fresh
catalyst, the recycled one is formed by monodispersed NPs on
thin carbon films, which still covered the NPs (Figure 9a−c).
These results confirm the key protecting role of the carbon
layers that confer good stability to the NPs, thus avoiding them
from agglomeration and overoxidation, and therefore, allowing
for a prolonged use. Furthermore, the distribution of both
metals and carbon is still maintained, as revealed by HRTEM
and STEM-HADDF elemental mapping (Figure 9c,d,
respectively).
CONCLUSIONS
■
We have prepared bimetallic CoW@C materials by a facile
hydrothermal carbon-coating methodology, followed by
pyrolysis under an inert atmosphere, and applied them for
the hydrogenation of quinoline (1a) to its partially hydro-
genated product 1,2,3,4-tetrahydroquinoline (2a). Depending
on the W/Co mole ratio used in their preparation, the
synthesized materials comprise metallic Co species, including
CoW alloyed structures and metal oxide phases (CoWO4 and
WO3), all of them partially embedded in a carbon matrix. The
higher the W/Co mole ratio, the higher segregation of oxide
phases and lower the catalytic performance. In particular, the
most active catalyst CoW@C-0.05 is composed of bimetallic
NPs that display a multishell core−shell structure with a
metallic Co core, a W-enriched shell involving Co7W6 alloyed
structures, and small WO3 patches partially covering the
surface of these NPs. Interestingly, this catalyst displays higher
Scope of the Catalyst CoW@C-0.05. Finally, we focused
on the scope of the catalyst CoW@C-0.05; for that, we
Figure 9. (a−c) HRTEM micrographs and (d) HAADF-STEM image and elemental mapping of Co, W and C of the recycled catalyst CoW@C-
0.05 after the fifth run.
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a
Table 3. CoW@C-0.05 Catalyzed Hydrogenation of Substituted Quinolines
a
b
c
Reaction conditions: substrate (0.53 mmol), catalyst (20 mg), toluene (3 mL). Determined by GC using dodecane as an internal standard. Yield
d
e
f
of the isolated product. 7 h. 7.5 h. 8.5 h.
catalytic activity and selectivity than the monometallic Co@C
catalyst. The superior catalytic performance of the catalyst
CoW@C-0.05 can be attributed to the presence of the CoW
alloyed species, which significantly influences its reactivity as a
catalyst for the regioselective hydrogenation of 1a to its
partially hydrogenated product 2a. More specifically, the
alloyed phase promotes a better interaction of both metals
(Co and W) that likely boosts the controlling surface reaction
between the homolytically activated H2 species and the
adsorbed 1a to be more efficiently accomplished. Moreover,
it has been demonstrated that the tungsten doping metals
modify the Lewis acidic properties of the Co NPs, favoring the
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interaction of 1a with the metallic catalyst surface (where H2 is
also activated) to avoid the hydrogenation of the N-free
aromatic ring, thus enhancing the regioselectivity of the
catalytic process.
CCD detector. The laser power on the sample was ∼10 to 25
mW and a total of 20 acquisitions were taken for each spectra.
The TPD experiments were carried out in a home-made
flow reactor connected to a Balzer mass spectrometer. Prior to
TPD experiments, the samples were impregnated with
quinoline (1a), following the subsequent procedure: 100 mg
of samples were in situ reduced in a three-neck glass flask at
200 °C for 2 h in a H2 flow. Afterward, the samples were
cooled in H2 to room temperature, and then, flushed with a N2
flow for 10 min. Next, 0.2 mL of the (1a: MeOH = 1:3)
mixture was added using a syringe under a N2 atmosphere.
After stirring for 10 min, the samples were dried under vacuum
and heated at 60 °C for 1 h. For the TPD experiment, 70 mg of
the impregnated sample was exposed to an Ar flow of 20 mL/
min, and after 10 min stabilization at room temperature, the
temperature was increased to 200 °C at a heating rate of 2 °C/
min. Mass spectra were collected in a multi-ion detection
mode (MID) following the fragmentation peaks: m/z = 130,
129, 102,103, 76, 78, and 79 uma. For discussion, the m/z = 78
uma fragmentation peak was used.
H2−D2 exchange experiments were performed in a flow
reactor. The reaction products (H2, HD, D2) were analyzed
with a mass spectrometer (Omnistar, Balzers). The Co@C and
CoW@C samples were pre-activated at 200 °C for 2 h with a
temperature increasing rate of 10 °C/min from room
temperature to 200 °C.
The samples for HR-TEM were ultrasonically dispersed in
CH2Cl2 and transferred into carbon-coated copper grids. HR-
TEM images were recorded using a JEOL JEM2100F
microscope operating at 200 kV. The spatial distribution of
Co@C and CoW@C samples were determined using an
energy-dispersive X-ray analysis (EDXA) system (Oxford
Instruments) attached to a JEOL JEM2100F electronic
microscope.
The GC yields were determined by a GC-flame ionization
detection (GC-FID) using dodecane as an internal standard.
GC-FID analyses were performed on a Bruker 430-GC System
equipped with a 25 m capillary column of 5% phenyl-
methylsilicone. Mass determination was carried out on a GC-
Mass Agilent 6890 Network equipped with the same column as
the GC and a mass selective detector. 1H NMR and 13C NMR
spectra of the isolated products were recorded on a Bruker AV
300 spectrometer.
Catalytic Experiments. Hydrogenation experiments were
carried out in a 12 mL stainless steel autoclave equipped with a
Teflon liner, a pressure controller, and a cannula ending with
an open/off valve that allows for taking out samples during the
reaction. The Teflon vessel containing a stirring bar was
charged with 20 mg of catalyst CoW@C (or Co@C) and
introduced into the stainless steel autoclave. After sealing, the
autoclave was purged by flushing three times with 10 bar H2,
pressurized again, and kept at 170 °C for 2 h. Then, the
autoclave was cooled to room temperature and carefully
depressurized to 1.5−2 bar H2. Without opening the autoclave,
a mixture of the quinoline substrate (0.53 mmol), dodecane as
an internal standard (0.3 mmol) and toluene as a solvent (3
mL) were added though the incorporated cannula. Next, the
H2 was increased to 8 bar, and the autoclave was seated into an
aluminum block located on a heating plate previously set at
100 °C and 1100 rpm of stirring speed. To follow the reaction,
aliquots (20 μL) were taken out from the reaction mixture for
GC and GC-mass analysis at different reaction times. For
catalyst recycling experiments, after the completion of the
In view of the catalyst recyclability experiments, the catalyst
CoW@C-0.05 has demonstrated to exhibit good stability
under the reaction conditions, likely because of the protection
of the thin carbon layers, which avoid from agglomeration and
overoxidation. In the presence of this catalyst, a variety of
functionalized quinolines, even bearing other sensitive groups
such as halogens and esters, have been successfully hydro-
genated to the corresponding 1,2,3,4-tetrahydroquinolines in
excellent yields. It is worth mentioning the mild reaction
conditions under which these selective hydrogenations take
place, what makes think that this work may pave the way for
designing non-noble bimetallic NPs for heterogeneous catalytic
hydrogenation reactions as substitutes for precious metal-based
catalysts.
EXPERIMENTAL SECTION
■
Synthesis of Catalysts CoW@C and Co@C. The
bimetallic catalysts CoW@C with different W/Co mole ratios
(0.025, 0.05, 0.25, and 0.50) were prepared by adapting a
carbon-coating methodology previously described in our
recent work.102,103,105 To 100 mL of a homogeneous ethylene
glycol solution of Co(OAc)2 (4.94 g) and NaWO4·2H2O
(0.23, 0.41, 2.31, or 4.62 g, respectively) at 165 °C, an aqueous
solution of Na2CO3 (4.24 g, 5.62 g, 6.75 g, respectively, in 160
mL) was added drop by drop for ca. 1.5−2 h under stirring
conditions. After addition, the mixture was aged at this
temperature for one more hour before cooling to room
temperature. Then, the formed solid was filtered, generously
washed with acetone, until a dry powder was obtained. Next,
one fraction of this dried solid (0.45 g) was dispersed in 20 mL
of an aqueous solution of glucose (0.36 g) by ultrasonic
treatment, transferred to a 35 mL stainless steel autoclave
equipped with a Teflon liner, and reacted at 175 °C under
static conditions for 18 h. After cooling to room temperature,
the solid material was collected by filtration, washed with
distilled water and acetone, and dried at 60 °C. In a final step,
this solid was pyrolyzed under a N2 atmosphere at 600 °C for 2
h with a ramp rate of 10 °C/min.
The monometallic catalyst Co@C was prepared following
the procedure described for the bimetallic CoW@C catalysts
but without the addition of the NaWO4·2H2O salt.
Catalyst Characterization. XRD analysis was carried out
with a Philips X’PERT diffractometer using Cu Kα at 1.54178
Å radiation.
X-ray photoelectron spectra were collected using a SPECS
spectrometer with a 150-MCD-9 detector and using a
nonmonochromatic Al Kα (1486.6 eV) X-ray source. Spectra
were recorded using an analyzer pass energy of 30 eV, an X-ray
power of 100 W, and under an operating pressure of 10−9
mbar. During data processing of the XPS spectra, binding
energy (BE) values were referenced to the C 1s peak (284.7
eV). Spectra treatment was performed using CASA software.
The samples have been in situ pre-activated in H2 (10 bar) at
170 °C for 2 h, in a high-pressure catalytic reactor (HPCR)
connected to the XPS equipment and transferred under
vacuum for analysis.
Raman spectra were obtained from solid samples using an
excitation wavelength of 785 nm in a Renishaw Raman
spectrometer equipped with an Olympus microscope and a
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reaction, the autoclave was cooled to room temperature and
depressurized. Because of the paramagnetic nature of these
catalysts, they could be easily recovered with the stirring bar,
thoroughly washed with fresh toluene, dried at ambient
conditions, and used for the next run. When isolated yields
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acscatal.1c01561.
■
sı
Extended data on the characterization of catalysts
CoW@C and Co@C; additional experimental results
and procedures; and characterization data of isolated
products (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
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C. J.; Hutchings, G. J. Designing Bimetallic Catalysts for a Green and
Sustainable Future. Chem. Soc. Rev. 2012, 41, 8099−8139.
Iván Sorribes − Instituto de Tecnología Química, Universitat
̀
̀
Politecnica de Valencia-Consejo Superior de Investigaciones
Científicas, 46022 Valencia, Spain; orcid.org/0000-0002-
3721-9335; Email: ivsorter@itq.upv.es
̌
́
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Supported Metal Nanoparticles. Nat. Mater. 2013, 12, 34−39.
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J.; Rabeah, J.; Huan, H.; Schuenemann, V.; Brueckner, A.; Beller, M.
Nanoscale Fe2O3-Based Catalysts for Selective Hydrogenation of
Nitroarenes to Anilines. Science 2013, 342, 1073−1076.
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M.; Brueckner, A.; Beller, M. Heterogenized Cobalt Oxide Catalysts
for Nitroarene Reduction by Pyrolysis of Molecularly Defined
Complexes. Nat. Chem. 2013, 5, 537−543.
Avelino Corma − Instituto de Tecnología Química, Universitat
̀
̀
Politecnica de Valencia-Consejo Superior de Investigaciones
Científicas, 46022 Valencia, Spain; orcid.org/0000-0002-
2232-3527; Email: acorma@itq.upv.es
Authors
Marta Puche − Instituto de Tecnología Química, Universitat
̀
̀
Politecnica de Valencia-Consejo Superior de Investigaciones
Científicas, 46022 Valencia, Spain
Lichen Liu − Instituto de Tecnología Química, Universitat
̀
̀
(14) Serna, P.; Corma, A. Transforming Nano Metal Nonselective
Particulates into Chemoselective Catalysts for Hydrogenation of
Substituted Nitrobenzenes. ACS Catal. 2015, 5, 7114−7121.
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Nanoscale Intimacy in Bifunctional Catalysts for Selective Conversion
of Hydrocarbons. Nature 2015, 528, 245−248.
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Developments in the Synthesis of Supported Catalysts. Chem. Rev.
2015, 115, 6687−6718.
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Nanoparticles Catalyze a General Synthesis of Amines. Science
2017, 358, 326.
Politecnica de Valencia-Consejo Superior de Investigaciones
Científicas, 46022 Valencia, Spain; Present
Address: Department of Chemistry, Tsinghua University,
100084 Beijing, China.; orcid.org/0000-0001-5067-
0481
Patricia Concepción − Instituto de Tecnología Química,
̀
̀
Universitat Politecnica de Valencia-Consejo Superior de
Investigaciones Científicas, 46022 Valencia, Spain;
orcid.org/0000-0003-2058-3103
Complete contact information is available at:
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(18) Wei, H.; Ren, Y.; Wang, A.; Liu, X.; Liu, X.; Zhang, L.; Miao, S.;
Li, L.; Liu, J.; Wang, J.; Wang, G.; Su, D.; Zhang, T. Remarkable
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(19) Liu, L.; Corma, A. Metal Catalysts for Heterogeneous Catalysis:
From Single Atoms to Nanoclusters and Nanoparticles. Chem. Rev.
2018, 118, 4981−5079.
Financial support from the Spanish MINECO (Severo Ochoa
excellence program SEV-2016-0683) and the European
Research Council (grant ERC-AdG-2014-671093, SynCat-
Match) is gratefully acknowledged. I.S. acknowledges funding
by the Generalitat Valenciana (SEJI/2020/018). The authors
also thank the Electron Microscopy Service of the UPV for
TEM and STEM facilities.
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Electrocatalysts and Photocatalysts. Nat. Rev. Chem. 2021, 5, 256−
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https://doi.org/10.1021/acscatal.1c01561
ACS Catal. 2021, 11, 8197−8210