Nanolayered Cobalt-Molybdenum Sulfides as Highly Chemo- and Regioselective Catalysts for the Hydrogenation of Quinoline Derivatives

Article abstract of DOI:10.1021/acscatal.7b04260

Herein, a general protocol for the preparation of a broad range of valuable N-heterocyclic products by hydrogenation of quinolines and related N-heteroarenes is described. Interestingly, the catalytic hydrogenation of the N-heteroarene ring is chemoselectively performed when other facile reducible functional groups, including alkenes, ketones, cyanides, carboxylic acids, esters, and amides, are present. The key to successful catalysis relies on the use of a nanolayered cobalt-molybdenum sulfide catalyst hydrothermally synthesized from earth-abundant metal precursors. This heterogeneous system displays a tunable composition of phases that allows for catalyst regeneration. Its catalytic activity depends on the composition of the mixed phase of cobalt sulfides, being higher with the presence of Co₃S₄, and could also be associated with the presence of transient Co-Mo-S structures that mainly vanish after the first catalytic run.

Full text of DOI:10.1021/acscatal.7b04260

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Nanolayered Cobalt-Molybdenum Sulfides as Highly Chemo- and
Regioselective Catalysts for the Hydrogenation of Quinoline Derivatives
Ivan Sorribes, Lichen Liu, Antonio Doménech-Carbó, and Avelino Corma
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b04260 • Publication Date (Web): 11 Apr 2018
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Nanolayered CobaltꢀMolybdenum Sulfides as
Highly Chemoꢀ and Regioselective Catalysts
for the Hydrogenation of Quinoline Derivatives
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Iván Sorribes,† Lichen Liu, † Antonio Doménech-Carbó,‡ and Avelino Corma*,†
†Instituto de Tecnología Química, Universitat Politècnica de ValènciaꢀConsejo Superior
de Investigaciones Científicas, Avenida de los Naranjos s/n, 46022 Valencia, Spain.
‡Departament de Química Analítica. Universitat de València, Dr. Moliner, 50, 46100
Burjassot (Valencia), Spain.
ABSTRACT: Herein, a general protocol for the preparation of a broad range of valuable
Nꢀheterocyclic products by hydrogenation of quinolines and related Nꢀheteroarenes is
described. Interestingly, the catalytic hydrogenation of the Nꢀheteroarene ring is
chemoselectively performed when other facile reducible functional groups, including
alkenes, ketones, cyanides, carboxylic acids, esters and amides, are present. The key to
successful catalysis relies on the use of a nanolayered cobaltꢀmolybdenum sulfide
catalyst hydrothermally synthesized from earth abundant metal precursors. This
heterogeneous system displays a tunable composition of phases that allows for catalyst
regeneration. Its catalytic activity depends on the composition of the mixed phase of
cobalt sulfides, being higher with the presence of Co3S4, and could also be associated to
the presence of transient CoꢀMoꢀS structures that are mainly vanished after the first
catalytic run.
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KEYWORDS: Chemoꢀ/Regioselective hydrogenation, Heterogeneous catalysis,
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Quinolines, Nꢀheterocycles, CoꢀMoꢀS catalysts
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INTRODUCTION
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Catalytic hydrogenation of Nꢀheteroarenes is a fundamentally important reaction in the
petrochemical industry and represents a promising method for organic synthesis.1,2 In
particular, the hydrogenation of quinoline derivatives has drawn the attention of the
synthetic chemists because of its practical simplicity and high atom efficiency for the
production of 1,2,3,4ꢀtetrahydroquinolines, a core structural motif present in many
alkaloids and bioactive molecules used as drugs and agrochemicals.3 However,
hydrogenation of these Nꢀheteroarenes involves the challenging tasks of breaking
aromaticity as well as overcoming catalyst poisoning by strong interaction with starting
materials and their hydrogenated products.2c,4 Nonetheless, different homogeneous
systems based on transition metals, such as Ir,5 Ru,5a,6 Rh,5a,7 Mo,8 Os,5a,9 Fe10 and Co11,
have been applied for the hydrogenation of quinolines. Particularly worth mentioning
since it represents a suitable route to valuable Nꢀheterocyclic compounds, is a wellꢀ
defined homogeneous cobalt catalyst that allows the chemoselective hydrogenation of
Nꢀheteroarenes functionalized with other sensitive reducible groups (Scheme 1a).11b,12
However, despite the good reactivity and selectivity exhibited by some of these
homogeneous catalysts, the use of airꢀsensitive and/or expensive ligands, additives as
well as the inherent recyclability issues hinder their industrial scale application. In
contrast, heterogeneous catalytic systems offer a convenient alternative since catalysts
can be easily recovered and reused. Different heterogeneous catalysts based on precious
metals including Pd,13 Pt,14 Rh,14a,15 Ru,16 Ir14a,17 or Au13h,18 have been used for the
hydrogenation of quinolines and its derivatives. In general, they are highly active but
are not selective displaying low functional group tolerance. In fact, only a handful of
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these precious metalꢀbased heterogeneous catalysts allow for the chemoselective
hydrogenation of quinolines in the presence of other reducible functionalities.
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Scheme 1. Chemoselective hydrogenation of quinolines bearing other reducible
functional groups
In 2012, Cao and coꢀworkers described that Au nanoparticles supported on high
surface area (HSA) TiO2 can selectively hydrogenate the Nꢀheteroarene ring of
quinolines bearing olefinic and ketone groups (Scheme 1b).18ꢀ19 In 2013, the
chemoselectivity was expanded to the presence of a carboxylic acid group using a Pt
nanowireꢀtype catalyst (Scheme 1c).14b More recently, Rh nanoparticles immobilized in
a Lewis acid ionic liquid,15i as well as Ru and bimetallic AuPd nanoparticles supported
16k
on nanoporous 12CaO·7Al2O3 or CeO2 nanorods,13h respectively, have also been
applied for the preparation of 1,2,3,4ꢀtetrahydroquinolines functionalized with
aldehydes or esters groups by hydrogenation of their parent quinolines (Scheme 1dꢀf).
Despite their selectivity, it is today an important issue to replace precious metals by
earthꢀabundant metalꢀbased catalysts, and this is also the case for the hydrogenation of
Nꢀheteroarenes.
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Nowadays, heterogeneous approaches using nonꢀnoble metals for the synthesis of
1,2,3,4ꢀtetrahydroquinolines by hydrogenation of quinolines are scarce. They involve
the use of traditional Raney®ꢀNi2b,20 or cobaltꢀbased catalysts.21 In addition, these
catalytic systems present a limited substrate scope, especially for quinolines
functionalized with other reducible functional groups. A catalyst based on Nꢀgrapheneꢀ
modified cobalt nanoparticles supported on alumina, was applied for the chemoselective
hydrogenation of a quinoline bearing an olefinic group (Scheme 1g).21a,22 However, this
selectivity was only demonstrated at moderate conversion (61%), reached after long
reaction time (48 h). Furthermore, the selectivity when other functional groups were
present was not reported. Hence, heterogeneous catalysts based on nonꢀnoble metals
with superior activity and chemoselectivity for quinolines hydrogenation are still highly
desirable.
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Herein, we describe the hydrothermal preparation of nanolayered mixed cobaltꢀ
molybdenum sulfide (CoꢀMoꢀS) materials. We demonstrate that their application as
catalysts for quinolines hydrogenation allows for the straightforward and general
preparation of 1,2,3,4ꢀtetrahydroquinolines with high tolerance to other sensitive
moieties (Scheme 1h). Furthermore, we show that the nonꢀnoble metalꢀbased catalytic
system presented here is also applicable for the regioselective hydrogenation of other
heteroaromatic nitrogen compounds.
RESULTS AND DISCUSSION
Preparation and Characterization of Catalysts
Hydrogenation of the heteroaromatic ring of Nꢀheteroarene compounds constitutes a key
step of the overall reaction schemes involved in the hydrodenitrogenation (HDN)
processes, routinely applied in petroleum refineries for removing nitrogen heteroatoms
from crude feedꢀstocks.23 Promoted molybdenum sulfides, specifically by nickel, are the
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most commonly used catalysts for HDN processes, while molybdenum disulfide cobaltꢀ
promoted materials are more widespread used for hydrodesulfurization (HDS).1,24 It is
wellꢀestablished that in these materials the CoꢀMoꢀS (or NiꢀMoꢀS) structures, formed by
adsorption of cobalt (or Ni) on the edge positions of MoS2 layers, plays a crucial role in
their catalytic activity.25,26 Recently, Mavrikakis and coꢀworkers using density
functional theory (DFT) have shown that nitrogenꢀcontaining compounds, including
quinolines, tend to be strongly adsorbed and activated on the “brim” sites of the CoꢀMoꢀ
S structure edges due to the presence of metalꢀlike electronic states.25k,25n,27 This
background suggests that cobaltꢀmolybdenum sulfideꢀbased catalysts could also be of
interest for the efficient hydrogenation of Nꢀheteroarenes. Nevertheless, so far, there are
not general catalytic protocols for the hydrogenation of quinolines to 1,2,3,4ꢀ
tetrahydroquinolines by using these type of catalysts.24a,28
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Quite recently, we reported the hydrothermal preparation of nanolayered
molybdenum disulfide cobaltꢀpromoted materials and their application as catalysts for
the chemoselective hydrogenation of nitroarenes.29 All prepared CoꢀMoꢀSꢀX catalysts
contained, as determined by ICP analysis, either lower or the same cobalt content with
respect to molybdenum (X = Co/(Mo + Co) mole ratio ≤ 0.5). In contrast, in the present
work we have adapted this synthetic methodology to obtain CoꢀMoꢀSꢀX unsupported
materials in which the content of cobalt is higher than that molybdenum (X = Co/(Mo +
Co) mole ratio > 0.5). In this way, the preparation was performed at 180 ºC in an
autoclave containing ammonium molybdate, sulfur, different amounts of cobalt(II)
acetate, and an aqueous solution of hydrazine. In addition, the catalyst CoꢀMoꢀSꢀ0.39
that contains more molybdenum than cobalt, molybdenum disulfide and two different
molybdenumꢀfree cobalt sulfide materials were also prepared (see reference 29 and the
Supporting Information for characterization and preparation details). It should be noted
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that the catalyst CoꢀMoꢀSꢀ0.39 displayed high activity and selectivity for the
chemoselective hydrogenation of nitroarenes.29
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Xꢀray diffraction (XRD) patterns of the prepared materials with different cobalt
content are shown in Figure 1. All of them are dominated by the presence of diffraction
peaks associated to cobalt sulfide species. Catalysts CoꢀMoꢀSꢀ0.58 and CoꢀMoꢀSꢀ0.66
exhibit similar diffraction patterns with peaks at 2θ values of 28º, 32º, 36º, 40º, 46º, 55º,
58º, 60º, 63º, 75º, 77º, 79º, 86º corresponding to the (111), (200), (210), (211), (220),
(311), (222), (230), (321), (331), (420), (421) and (422) planes of CoS2 (PDF Card 00ꢀ
041ꢀ1471). Notably, most of these peaks are very sharp, indicating that CoS2 is present
in the form of large particles. In addition, the broad diffraction peaks at 14º, 33º, 39º,
58º characteristic of the (002), (101), (103) and (110) basal planes of MoS2 with very
low crystallinity can also be detected. When a higher amount of the cobalt salt was used
in the catalyst preparation, the obtained material CoꢀMoꢀSꢀ0.83 is constituted by
different cobalt sulfide phases. Together with a significant decrease of CoS2 diffraction
peaks, the XRD pattern shows the presence of peaks at 2θ values of 32⁰, 38⁰, 47⁰, 50⁰,
55⁰ which can be indexed to the (311), (400), (422), (333) and (440) planes of the cubic
phase of Co3S4 (PDF card 00ꢀ011ꢀ0121). These XRD peaks are much broader than
those of CoS2 in catalysts CoꢀMoꢀSꢀ0.58 and CoꢀMoꢀSꢀ0.66, indicating that the surface
area of the Co3S4 phase is larger than that of CoS2. Further increase of the initial cobalt
salt amount leads to the formation of the Co9S8 phase in the material CoꢀMoꢀSꢀ0.91. It
should be noted that the increase of Co content also affects the crystalline structure of
the MoS2 phase. By comparing the XRD patterns in Figure 1, it can be seen that the
intensity of the diffraction peak associated to the (002) plane, that is representative for
the stacking of MoS2 layers, undergoes a significant decrease in materials CoꢀMoꢀSꢀ
0.83 and CoꢀMoꢀSꢀ0.91, thus revealing that MoS2 is present with a limited number of
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stacked atomic layers. In all prepared catalysts, XRD peaks of the ternary CoꢀMoꢀS
structure are indistinguishable from those of MoS2 because the CoꢀMoꢀS structure
consists of MoS2 decorated at the edges by cobalt atoms. Therefore, the XRD is totally
determined by the MoS2 part.30
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Figure 1. Xꢀray diffraction patterns of mixed cobaltꢀmolybdenum sulfide (CoꢀMoꢀS)
catalysts
Microstructure details of prepared CoꢀMoꢀS catalysts were analyzed by high–
resolution transmission electron microscopy (HRTEM) and the results are shown in
Figure 2. Catalysts CoꢀMoꢀSꢀ0.58 and CoꢀMoꢀSꢀ0.66 exhibit the characteristic group of
parallel fringes with an average number of stacked layers of 5ꢀ8 and with an interlayer
distance of 0.63ꢀ0.64 nm, which correspond to the (002) plane of MoS2 (Figure 2aꢀd).31
Free CoS2 particles of different sizes (Table S1 in the Supporting Information), that
displays the characteristic latticeꢀfringes spacing of 0.27 and 0.25 nm, associated with
the (200) and (210) planes respectively, are widely found in both catalysts. However,
catalyst CoꢀMoꢀSꢀ0.66 displays more interlaced MoS2 and CoS2 phases assuring the
strong interaction between them, and thus promoting a presumably higher formation of
active CoꢀMoꢀS structures. This is consistent with the metal distributions determined by
Energyꢀdispersive Xꢀray (EDX) elemental mapping (Figures S1ꢀS2 in the Supporting
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Information). HRTEM micrographs of catalyst CoꢀMoꢀSꢀ0.83 display the three
obviously different lattice fringes with spacing of 0.63ꢀ0.64, 0.27 and 0.24 nm
corresponding to (002), (200) and (400) planes of MoS2, CoS2 and Co3S4, respectively
(Figure 2eꢀg). In contrast, the interlayer distances of 0.30 and 0.25 nm characteristic of
(311) and (400) planes of Co9S8 phase are detected in catalyst CoꢀMoꢀSꢀ0.91 (Figure
2h). Notably, MoS2 is hardly found as isolated layers, which is in concordance with the
XRD analysis that revealed a depressed layer stacking with the increase of cobalt
content. Component distributions of catalysts CoꢀMoꢀSꢀ0.83 and CoꢀMoꢀSꢀ0.91 were
determined by EDX elemental mapping reveling that these materials are mostly
constituted by cobalt and sulfur with low content of molybdenum but homogeneously
distributed on cobalt sulfide phases (Figure 3 and Figure S3 in the Supporting
Information).
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Figure 2. HRTEM micrographs of (a,b) CoꢀMoꢀSꢀ0.58, (c,d) CoꢀMoꢀSꢀ0.66, (eꢀg) Coꢀ
MoꢀSꢀ0.83, (h) CoꢀMoꢀSꢀ0.91
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Figure 3. (a,b) Highꢀangle annular darkꢀfield scanning transmission electron
microscopy (HAADFꢀSTEM) images of CoꢀMoꢀSꢀ0.83. (cꢀh) EDX elemental mapping
of cobalt, molybdenum and sulfur
Further characterization of catalysts was carried out by nitrogen physisorption studies.
The specific surfaces areas (SA), total pore volumes (PV) and pore size (PS) of catalysts
containing different Co/(Mo + Co) mole ratios are summarized in Table 1. The SA and
PV range from 80 to 44 m2/g and 0.18 to 0.07 cm3/g, respectively. Catalyst CoꢀMoꢀSꢀ
0.58 presents the highest values, but a progressive decrease in both SA and PV is
achieved upon increasing the Co/(Mo + Co) mole ratio. This decrease can be attributed
to the lower MoS2 content and the concomitant formation of cobalt sulfide phases with
lower surface area but higher pore size.
Table 1. BET surface area, pore volume and pore diameter of CoꢀMoꢀS catalysts
Surface Area
(m2/g)
79.4
Pore Volume
(cm3/g)
0.18
Pore Diameter
Catalyst
(nm)
9.2
CoꢀMoꢀSꢀ0.58ꢀ180
CoꢀMoꢀSꢀ0.66ꢀ180
CoꢀMoꢀSꢀ0.83ꢀ180
CoꢀMoꢀSꢀ0.91ꢀ180
63.4
0.15
9.6
48.9
0.07
10.3
13.2
44.4
0.08
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Catalytic Results
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The hydrogenation of quinoline (1a) to the corresponding 1,2,3,4ꢀtetrahydroquinoline
(2a) was investigated as the benchmark system to compare the catalytic activity of the
prepared cobaltꢀmolybdenum sulfide materials. Initial hydrogenation experiments were
performed in a batch reactor using toluene as solvent, under 12 bar of H2, at 150 ºC and
1000 rpm of stirring speed to avoid limitation by external diffusion (Figure S6 in the
Supporting Information). As shown in Figure 4, the catalyst CoꢀMoꢀSꢀ0.83, mainly
constituted by a mixture of MoS2, CoS2 and Co3S4 phases, proved to be the most active
system achieving after 6 h almost full conversion of 1a with excellent regioselectivity to
the desired hydrogenated product 2a. In the presence of catalyst CoꢀMoꢀSꢀ0.91, which
contains a different cobalt sulfide phase of Co9S8, 1a undergoes lower conversion, thus
revealing that the cobalt phase composition of the catalyst exerts a significant effect on
its catalytic performance. Different activity is also observed for catalysts CoꢀMoꢀSꢀ0.39,
CoꢀMoꢀSꢀ0.58 and CoꢀMoꢀSꢀ0.66, all of them formed by MoS2 and CoS2 as major
phases. Apart from their different metal mole ratio, these catalysts present significant
differences on the metal distribution, as it has been inferred by electron microscopy
characterization (Figure 2, Figures S1ꢀS2 in the Supporting Information and reference
29). Catalyst CoꢀMoꢀSꢀ0.58, with more separated CoS2 phase, displays the lowest
catalytic activity. However, conversion of 1a is enhanced in catalysts CoꢀMoꢀSꢀ0.39
and CoꢀMoꢀSꢀ0.66 with metal sulfides more homogeneously distributed, as a result of
the generation of CoꢀMoꢀS active structures to a larger extent.25 It is noteworthy that the
use of MoS2 or cobalt sulfides as catalysts leads to lower conversions of 1a than all
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prepared CoꢀMoꢀS catalysts. Nevertheless, an excellent regioselectivity of the
Nꢀ
heteroarene ring hydrogenation is achieved in all prepared materials with no detection
of reaction intermediates or overꢀhydrogenated products (Figure S7 in the Supporting
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Information). As in the case of the soꢀcalled Nebula catalyst,24e,32 comparison of the
catalytic activity per unit volume of catalyst between these unsupported CoꢀMoꢀS
catalysts and a conventional supported one reveals that our prepared unsupported
catalysts display a larger population of active sites per unit volume (Figure S8 in the
Supporting Information). Thus, despite of their higher cost, they allow for a higher
throughput.
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Figure 4. (a) Catalytic performance of CoꢀMoꢀS catalysts for the hydrogenation of
quinoline (1a) to 1,2,3,4ꢀtetrahydroquinoline (2a). (b) Yield of 1a and 2a versus
reaction time for catalyst CoꢀMoꢀSꢀ0.83.
Further optimization experiments were carried out with the most active catalyst (Coꢀ
MoꢀSꢀ0.83). First, the influence of the solvent was investigated (Table 2, entries 1ꢀ8). In
general, the use of other solvents different than toluene led to lower reactivity. At longer
reaction time (10 h), excellent conversions of 1a were also achieved by using protic
solvents, such as ethanol and methanol (Table 2, entries 1ꢀ2). Interestingly, in the
absence of any solvent 2a is obtained in excellent yield in 2h (Table 2, entry 8). The
catalytic hydrogenation of 1a could also be achieved at lower temperature affording 2a
in a quantitative yield at 120 ºC or 87% yield at 100 ºC with after longer reaction time
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(Table 2, entries 9 and 10). When hydrogen pressure was halved, 1a was almost fully
converted affording 2a in 90% yield (Table 2, entry 11).
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9
Table 2. Regioselective hydrogenation of quinoline (1a) catalyzed by CoꢀMoꢀSꢀ0.83a
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Entry Solvent
Conversion (%)b
Yield (%)b
1
EtOH
94
84
91
83
2
MeOH
1,4ꢀdioxane
CH3CN
THF
3
60
20
21
67
>99
99
99
87
92
50
4
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6
nꢀBu2O
Toluene
ꢀ
58
7
98
8c
9e
10f
11g
98d
99 (92)
87
Toluene
Toluene
Toluene
90
aReaction conditions: 1a (0.25 mmol), catalyst (6.5 mg), solvent (1.6 mL). Determined by GC using
b
c
dodecane as an internal standard; yield of isolated product on a 10 mmol scale in parentheses. 2 h.
dAverage value for 6 catalytic reactions: (97.8 ± 1.1). e120 ºC, 12 h. f100 ºC, 18 h. g120 ºC, 12 h, 6 bar H2.
Figure 5. Catalyst recycling for the hydrogenation of quinoline (1a) to 1,2,3,4ꢀ
tetrahydroquinoline (2a) before and after reactivation
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With the above optimized conditions in hand (Table 2, entry 9), the stability and
recyclability of catalyst CoꢀMoꢀSꢀ0.83 was investigated. As shown in Figure 5, the
conversion of 1a decreased gradually with the reaction cycles, being halved for the third
run. Nevertheless, excellent yields of the hydrogenated product 2a could be obtained
again even after the sixth run by increasing reaction temperature (up to 150 ºC) and
reaction times.
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XRD and HRTEM characterization of the catalyst after different catalytic cycles
revealed a continuing evolution of the mixed phase of cobalt sulfides (Figures 6 and 7;
see also Figures S9ꢀS12 in the Supporting information). In the fresh catalyst, both CoS2
and Co3S4 are present (see Figures 1 and 2). After the first run, the Co3S4 phase is
present in catalyst CoꢀMoꢀSꢀ0.83ꢀR1 in a higher extent as result of the transformation of
some CoS2. With two additional reaction cycles, a new phase of Co9S8 can also be
detected in the reused catalyst CoꢀMoꢀSꢀ0.83ꢀR3 indicating that the mixed phase of
cobalt sulfides is further transformed. After the sixth run, the mixed phase of cobalt
sulfides in catalyst CoꢀMoꢀSꢀ0.83ꢀR6 is mainly constituted by Co9S8. These results and
the presence of H2S in the gasꢀphase (detected by GC) after catalytic reaction suggest
that catalyst undergoes desulfurization during the catalytic hydrogenation process. The
extent of stability of the cobalt sulfide phases is in line with the previously reported
thermodynamic data that predict poor stability of the CoS2 and Co3S4 phases under pure
hydrogen atmosphere (or low content of H2S) at the working reaction temperatures.33
However, while the hydrogenation of 1a was poisoned with the presence of H2S in feed,
the addition of sulfur to the reaction mixture slowed down but did not avoid the catalyst
deactivation (see Figure S13 in the Supporting Information).
Interestingly, the hydrothermal treatment of the reused catalyst CoꢀMoꢀSꢀ0.83ꢀR6
with an excess of sulfur powder in the presence of an aqueous solution of hydrazine
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allowed the successful catalyst reꢀsulfurization and the recovering of most of the
activity (see the Supporting Information for further details). This process leads to a new
phase transformation affording a reactivated catalyst (CoꢀMoꢀSꢀ0.83ꢀReact) mainly
formed by Co3S4 and CoS2 as mixed phase of cobalt sulfides. This reactivated catalyst
shows a similar diffraction pattern to the catalyst after one catalytic cycle (CoꢀMoꢀSꢀ
0.83ꢀR1), and both catalysts (CoꢀMoꢀSꢀ0.83ꢀReact and CoꢀMoꢀSꢀ0.83ꢀR1) display
similar catalytic activity for the hydrogenation of 1a under otherwise the same reaction
conditions (Figure 5). These results demonstrate that catalytic activity of this cobaltꢀ
molybdenum sulfide material relies on the cobalt phase composition, being higher with
the presence of Co3S4 phase but lower when increasing the relative content of Co9S8
phase. Accordingly to the comparative catalytic study (see Figure 4 and Figure S7 in the
Supporting Information), catalytic performance of CoꢀMoꢀS catalysts is diminished
when increasing the relative content of the separated CoS2 phase, and therefore the high
activity of catalyst CoꢀMoꢀSꢀ0.83 could be associated to the presence of the active
Co3S4 phase. However, the reused catalyst CoꢀMoꢀSꢀ0.83ꢀR1 containing Co3S4 phase
in a higher extent is less active than the fresh catalyst (CoꢀMoꢀSꢀ0.83), and therefore the
catalytic activity should not be only ascribed to this active phase.
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Figure 6. XRD patterns of reused and reactivated CoꢀMoꢀS catalysts
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Figure 7. HRTEM micrographs of (a,b) CoꢀMoꢀSꢀ0.83ꢀR1, (c,d) CoꢀMoꢀSꢀ0.83ꢀR3,
(e,f) CoꢀMoꢀSꢀ0.83ꢀR6, (g,h) CoꢀMoꢀSꢀ0.83ꢀReact
To get more clues on the origin of the catalytic activity and therefore on the catalyst
deactivation, an electrochemical study of the fresh, reused and reactivated catalysts was
performed applying the voltammetry of immobilized particles (VIMP) methodology.
This technique, developed by Scholz et al.34 have provided valuable analytical
information on a variety of sparingly soluble solids upon their attachment as microꢀ or
submicrosamples to inert electrodes in contact with suitable electrolytes.35 The cyclic
voltammogram of the fresh catalyst CoꢀMoꢀSꢀ0.83 displays cathodic signals (C1)
between 0.0 and ꢀ0.5 V (vs Ag/AgCl), preceding the prominent current peak associated
to the hydrogen evolution reaction (HER),36 and a main anodic signal (A1) at +0.95 V
(Figure S14 in the Supporting Information). These signals can be attributed to protonꢀ
assisted solidꢀstate reduction and sulfideꢀcentered oxidation processes, respectively.
Reused catalysts show similar cyclic voltammograms, but significant differences could
be detected using a particularly sensitive technique, such as the square wave
voltammetry. As shown in Figure 8A, the signal A1 for the fresh catalyst presents
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significant peak splitting with components at +1.02, +0.92 and +0.72 V preceding a
weak peak at +1.2 V, this last one being associated to the oxidation of MoS2 and CoS2
(Figure S15 in the Supporting Information). However, in the cyclic voltammograms of
the reused catalysts only the central component at +0.92 V of the split signal is
apparently retained whereas the components at +1.02 and +0.72 V have entirely or
almost vanished. Unfortunately, no clear reappearance of these last two signals is
achieved under the hydrothermal regeneration process. These results indicate that both
signals correspond to active species that are consumed during the catalytic cycles,
mainly in the first run, being likely associated to transient CoꢀMoꢀS structures.
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Figure 8. Square wave voltammograms of (a,f) CoꢀMoꢀSꢀ0.83, (b,g) CoꢀMoꢀSꢀ0.83ꢀR1,
(c,h) CoꢀMoꢀSꢀ0.83ꢀR3, (d,i) CoꢀMoꢀSꢀ0.83ꢀR6 and (e,j) CoꢀMoꢀSꢀ0.83ꢀReac catalystꢀ
modified graphite electrodes immersed into 0.10 M H2SO4 aqueous electrolyte.
Potential scan initiated at (A) 0.0 V in the positive direction or (B) +0.80 V in the
negative direction; potential step increment 4 mV; square wave amplitude 25 mV;
frequency 5 Hz.
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The square wave voltammetry of the cathodic region (C1) also provided differences
between the fresh and reused catalysts. As shown in Figure 8B, broad cathodic waves at
+0.40 and ꢀ0.28 V are superimposed to peak signals at +0.32, +0.17, ꢀ0.06 and ꢀ0.40 V
in the fresh catalyst. By comparison with the voltammogram of MoS2 (Figure S16 in the
Supporting Information), signals marked with asterisks at +0.32, +0.17, ꢀ0.06 V can be
tentatively assigned to Mo(IV)ꢀcentered reduction processes to Mo(III) and Mo(II)
sulfides successively. Interestingly, these signals are clearly diminished in the reused
catalysts, thus denoting that the MoS2 phase plays a role in the catalytic hydrogenation
of 1a and are partially consumed during the reaction. To our delight, the MoS2 phase
was found to be regenerated under the hydrothermal catalyst reactivation, as revealed by
the enhancement of the signals at +0.32, +0.17, ꢀ0.06 V in catalysts CoꢀMoꢀSꢀ0.83ꢀ
React.
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On the basis of all the above results we can conclude that the catalytic activity of
these materials does not only depends on the composition of the mixed phase of cobalt
sulfides, but also is associated with the presence of active, but poorly stable, CoꢀMoꢀS
structures that are mainly vanished after the first catalytic run.
Reaction Scope
The scope of the chemoꢀ and regioselective hydrogenation protocol in the presence of
catalyst CoꢀMoꢀSꢀ0.83 was broadly investigated for the hydrogenation of a large
number of quinolines substituted with different functional groups (Table 3). Notably,
the catalytic process is completely regioselective towards the hydrogenation of the Nꢀ
heteroarene ring to afford in all tested substrates the corresponding 1,2,3,4ꢀ
tetrahydroquinolines, without formation of the corresponding 5,6,7,8ꢀtetrahydroꢀ or
decahydroꢀderivatives. Quinolines 1b-1e bearing a methyl group either on the benzene
or the heteroarene ring reacted smoothly, affording the corresponding products in
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excellent yields (Table 3, entries 1ꢀ4). When the methyl group is located at the 4ꢀ
position, higher temperature (150 ºC) was required to get full conversion (Table 3, entry
3). In contrast, the more sterically hindered phenyl group at the 2ꢀposition had no
significant influence on the activity of this catalytic system (Table 3, entry 5). Halogenꢀ
substituted quinolines were also readily transformed leading to the corresponding
halogenated 1,2,3,4ꢀtetrahydroquinolines 2g-2l in high isolated yields (Table 3, entries
6ꢀ11). In this respect, the only exception is the hydrogenation of 7ꢀbromoquinoline (1h)
that required higher temperature (150 ºC), higher catalyst loading and longer reaction
time (24 h) to be fully converted (Table 3, entry 7). Under similar reaction conditions,
6ꢀmethoxyquinoline (1m) and 8ꢀaminoquinaldine (1n) were successfully hydrogenated
to the desired products 2m and 2n in 90% and 85% isolated yield, respectively (Table 3,
entries 12 and 13). Interestingly, this catalytic protocol was also applicable for the
preparation, in excellent yield, of 2ꢀ(2ꢀ(benzo[d][1,3]ꢀdioxolꢀ5ꢀyl)ethyl)ꢀ1,2,3,4ꢀ
tetrahydroquinoline (2o), a direct precursor of the pharmacologically active natural
product (±)ꢀgalipinine (Table 3, entry 14; see also Scheme S1 in the Supporting
Information).37
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Table 3. Chemoꢀ and regioselective hydrogenation of substituted quinolines catalyzed
by CoꢀMoꢀSꢀ0.83.a
Entry Substrate
1
Product
Conv. (%)b Yield (%)c
>99
>99
96
90
1b
1c
2b
2c
2
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3d
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>99
>99
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94
96
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1f
2f
6
>99
>99
>99
98
50
83
1g
1h
2g
7e,f,g
2h
8h
1i
2i
2j
9
>99
>99
92
91
1j
10i,j
1k
2k
>99
95
11
1l
2l
12f,k
96
98
90
85
1m
2m
13f,k
N
NH2
1n
2n
14f,l
>99
98
1o
2o
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15l,m
16n
95
96
74
82
1p
2p
1q
2q
9
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17o,p
97
99
77
87
1r
2r
18e,q,r
1s
2s
19o,s
>99
>99
>99
74
97
96
1t
2t
20
1u
1v
1w
2u
21o
2v
22f,k
98
78
2w
aReaction conditions: substrate (0.25 mmol), catalyst (6.5 mg), toluene (1.6 mL). Determined by GC
b
c
d
e
f
using dodecane as an internal standard. yield of isolated product. 15 h, 150 ºC. 18 h, 150 ºC. Catalyst
(13.1 mg). g2a (15%) as byꢀproduct. hTraces (< 2%) of 1,2,3,4ꢀTetrahydroquinaldine as byꢀproduct. 18 h.
i
jTraces (< 5%) of 1,2,3,4ꢀTetrahydroquinaldine as byꢀproduct. 24 h, 150 ºC. 24 h. Please, see Scheme
k
l
m
S2 in the Supporting Information. n24 h, 110 ºC, 30 bar H2; Traces (< 5%) of product with reduced double
o
p
q
bond as byꢀproduct. 10 h, 150 ºC. 3ꢀethylꢀ1,2,3,4ꢀtetrahydroquinoline (15%) as byꢀproduct. Catalyst
r
(9.8 mg). 6ꢀmethylꢀ1,2,3,4ꢀTetrahydroquinoline (6%) and traces (< 2%) of 6ꢀmethylquinoline as byꢀ
products. sTraces (< 5%) of 2a as byꢀproduct.
The functional group tolerance of this catalytic protocol was further studied by
exploring other reducible substituents. Gratifyingly, high chemoselectivity to the Nꢀ
heteroarene ring hydrogenation was achieved in the presence of sensitive alkene groups
that were wellꢀretained in the final hydrogenated products (Table 3, entries 15 and 16).
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It should be noted that this selectivity is reached even at high conversions of substrates.
Interestingly, the ketone sensitive group was also well tolerated under the hydrogenative
reaction environment (Table 3, entry 17). Furthermore, 1,2,3,4ꢀtetrahydroquinolines
furnished with carboxylic acid derivative groups, including cyanides, acids, esters and
amides (2sꢀ2w) were also afforded in good to excellent isolated yields from their
corresponding functionalized quinolines (Table 3, entries 18ꢀ22). To the best of our
knowledge, the chemoselectivity in the presence of cyanide or amide functional groups
have not been reported with homoꢀ or heterogeneous catalysis.
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Finally, we have further investigated the applicability of this catalytic protocol for the
hydrogenation of other structurally related Nꢀheteroarenes. As shown in Scheme 2,
benzo[h]quinoline (1x) and 3ꢀmethylbenzo[f]quinoline (1y) were almost fully converted
by hydrogenation of the Nꢀheteroarene ring providing the corresponding products 2x
and 2y in 81% and 92% isolated yields, respectively. Interestingly, for 1,10ꢀ
phenanthroline (1z) only one Nꢀheteroarene ring was hydrogenated obtaining 1,2,3,4ꢀ
tetrahydroꢀ1,10ꢀphenanthroline (2z) in moderate yield.
Scheme 2. Regioselective hydrogenation of Nꢀheteroarenes catalyzed by CoꢀMoꢀSꢀ
0.83. Reaction conditions: substrate (0.25 mmol), catalyst (13.1 mg), toluene (1.6 mL).
a20 bar H2.
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In summary, we have hydrothermally prepared a series of unsupported nanolayered
cobaltꢀmolybdenum sulfide basedꢀmaterials with different chemical compositions that
display abundance active sites per unit volume. All prepared catalysts exhibit good
catalytic performance for the regioselective hydrogenation of the Nꢀheteroarene ring of
the quinoline. The most active catalyst (CoꢀMoꢀSꢀ0.83) presents a tunable phase
composition constituted by MoS2 and a mixed phase of cobalt sulfides, including CoS2
and Co3S4. The electrochemical characterization has allowed us to detect transient
species, likely associated to CoꢀMoꢀS structures, which are mainly vanished after the
first catalytic run. The superior activity of catalyst CoꢀMoꢀSꢀ0.83 could be attributed
not only to the presence of these unstable CoꢀMoꢀS structures, but also to the
composition of the mixed phase of cobalt sulfides, being higher with the presence of
Co3S4 phase but lower when increasing the relative content of Co9S8 phase. A catalyst
phase transformation has been observed under successive catalyst reuses as a
consequence of the vanishing of the ternary CoꢀMoꢀS structures as well as to the
catalyst desulfurization under hydrogenative conditions, thus leading to a variation of its
catalytic performance. Recovering of most of the catalytic activity has been successfully
accomplished by a facile hydrothermal reꢀsulfurization process that allows for
regeneration of the mixed phase of cobalt sulfides and MoS2, but no the ternary CoꢀMoꢀ
S structures.
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Application of this nonꢀnoble metalꢀbased heterogeneous catalyst has allowed for the
efficient hydrogenation of functionalized quinolines and some other related Nꢀ
heteroarenes to the corresponding tetrahydroderivatives. Notably, the catalyst displays
high chemoselectivity for the hydrogenation of the Nꢀheteroarene ring of the quinolines
in the presence of other easily reducible groups, such as alkenes, ketones, cyanides,
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carboxylic acids, esters and amides. This impressive chemoselectivity makes this
catalyst unique in comparison with other heterogeneous catalytic protocols reported to
date and extend the tool box of synthetic strategies for valuable Nꢀheterocyclic
compounds.
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EXPERIMENTAL SECTION
Synthesis of Co-Mo-S Catalysts
The hydrothermal synthesis of unsupported CoꢀMoꢀS catalysts was accomplished
following the previously reported methodology used for the preparation of catalysts
with lower or the same content of cobalt with respect to molybdenum (Co/(Mo + Co)
mole ratio ≤ 0.5),29 but different loading of Co(OAc)2·4H2O (471.6, 592.5, 1100.4 and
1650.7 mg) was used to obtain unsupported CoꢀMoꢀS catalysts with Co/(Mo+Co) mole
ratio > 0.5 (0.58, 0.66, 0.83, 0.91, respectively).
Catalysts Characterization
Xꢀray diffraction (XRD) patterns, electron microscopy (HRTEM and HAADFꢀSTEM)
images and textural properties (BET surface area, pore volume and pore diameter) of
prepared CoꢀMoꢀS catalysts were obtained as previously reported.29
Voltammetric measurements were performed in a conventional electrochemical cell
using graphite bar (2 mm diameter) working electrodes, a Pt mesh auxiliary electrode
and a Ag/AgCl (3 M NaCl) reference electrode using a CH I660 potentiostat. For
voltammetry of immobilized particles (VIMP) experiments, the graphite electrode was
modified by rubbing its lower end over ca. 1 mg of the CoꢀMoꢀS material, previously
grinded and finely distributed on the plane surface of an agate mortar. Then, the
modifiedꢀgraphite bar was rinsed with water to remove the illꢀadhered particles and its
lower end was immersed into the 0.10 M H2SO4 aqueous electrolyte solution in the
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electrochemical cell. This solution was optionally deaerated by bubbling Ar during 10
min. The voltammetric profiles of all measured materials were maintained in series of at
least 10 successive measurements on the modified electrode.
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Hydrogenation of Quinoline Derivatives
The hydrogenation of quinoline derivatives was carried out in a 300 mL autoclave.
Inside the autoclave, a 8 mL glass vial sealed with a perforated septum with a syringe
needle, and containing a stirring bar, the CoꢀMoꢀS catalyst (6.5 mg), the quinoline
derivative (0.25 mmol), dodecane (50 µL) as an internal standard and toluene (1.6 mL),
was set in an alloy plate. Once the autoclave was tightly closed and purged 3 times with
hydrogen up to 30 bar, it was pressurized to 12 bar and seated into an aluminum block
located on a heating plate, which was set at 120 °C and 750 rpm of stirring speed.
After reaction time, the autoclave was cooled down in an ice bath, and carefully
depressurized. A sample of the reaction crude diluted with ethyl acetate was taken to be
analyzed by GC. Reproducibility of these experiments was ensured by performing each
reaction at least twice. The isolated yields of the tetrahydroderivative products were
determine as follows: Catalytic reactions were carried in the absence of dodecane as an
internal standard. After reaction time, ethyl acetate was added to the reaction mixture,
which was filtered over celite and taken to dryness under reduced pressure. Prior to
characterization, purification by silica gel chromatography was also accomplished for
some of the products (see Supporting Information).
AUTHOR INFORMATION
Corresponding Author
*acorma@itq.upv.es
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Notes
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The authors declare no competing financial interest.
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ASSOCIATED CONTENT
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The Supporting Information is available free of charge via the Internet at
http://pubs.acs.org.
Extended morphological characterization of CoꢀMoꢀS catalysts, details on
preparation and characterization (XRD and HRTEM) of molybdenumꢀfree
cobalt sulfide catalysts CoꢀSꢀ1 and CoꢀSꢀ2, general procedure for the kinetic
experiments, initial reaction rate depending on the stirring speed, additional
reaction profiles for CoꢀMoꢀS catalysts, comparison of catalytic activity per unit
volume of catalyst, general procedure for the catalyst recycling experiments and
catalyst reactivation, morphological characterization of reused and reactivated
CoꢀMoꢀS catalysts, catalyst recycling in the presence of H2S or sulfur,
electrochemical characterization of reused and reactivated CoꢀMoꢀS catalysts,
experimental details for the catalytic hydrogenation of quinolines 1o, 1p and
synthesis of 1q, characterization data and experimental details of isolated
tetrahydroderivative products
ACKNOWLEDGMENT
Financial support by the Spanish GovernmentꢀMINECO through program ‘‘Severo
Ochoa” (SEVꢀ2016ꢀ0683) is gratefully acknowledged. I.S. also thanks the Spanish
GovernmentꢀMINECO for a “Formación Postdoctoral” fellowship and ViceꢀRectorate
for Research, Innovation and Transfer of the Universitat Politècnica de València (UPV)
for a postdoctoral fellowship. The authors also thank the Microscopy Service of UPV
for kind help with TEM and STEM measurements.
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