General and Chemoselective Copper Oxide Catalysts for Hydrogenation Reactions

Article abstract of DOI:10.1021/acscatal.8b04807

Copper oxide catalysts have been prepared by pyrolysis of copper acetate on aluminum oxide. The material resulting from pyrolysis at 800 °C allows for catalytic hydrogenations at low temperature of a variety of unsaturated compounds such as quinolines, alkynes, ketones, imines, and polycyclic aromatic hydrocarbons as well as nitroarenes with good activity and selectivity.

Full text of DOI:10.1021/acscatal.8b04807

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Article
General and Chemoselective Copper Oxide
Catalysts for Hydrogenation Reactions
Wu Li, Xinjiang Cui, Kathrin Junge, Annette#Enrica Surkus, Carsten
Robert Kreyenschulte, Stephan Bartling, and Matthias Beller
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b04807 • Publication Date (Web): 29 Mar 2019
Downloaded from http://pubs.acs.org on March 29, 2019
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General and Chemoselective Copper Oxide
Catalysts for Hydrogenation Reactions
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Wu Li, Xinjiang Cui, Kathrin Junge, Annette-Enrica Surkus, Carsten Kreyenschulte,
Stephan Bartling, and Matthias Beller*
Leibniz−Institut für Katalyse e.V. an der Universität Rostock, Albert−Einstein Straße
29a, 18059 Rostock, Germany
ABSTRACT. Copper oxide catalysts have been prepared by pyrolysis of copper
acetate on aluminum oxide. The material resulting from pyrolysis at 800 °C allows for
catalytic hydrogenations at low temperature of a variety of unsaturated compounds
such as quinolines, alkynes, ketones, imines, polycyclic aromatic hydrocarbons, as
well as nitroarenes with good to good activity and selectivity.
KEYWORDS. Copper, heterogeneous catalysis, Hydrogenation, Semihydrogenation,
Aluminum oxide.
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Introduction
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Catalytic hydrogenations are arguably the most useful and widely applicable methodologies for
the reduction of chemical substances, and have found numerous applications both in academic
laboratories and industrial processes.1 Today, most of the large scale hydrogenations make use
of heterogeneous catalysts containing different metals. In general, materials based on precious
metals such as platinum, palladium, rhodium, and ruthenium show good to excellent activities,
selectivities, and even broad substrate scope.2 However, for a number of applications, these
metals are too expensive. Obviously, non-precious metal catalysts have substantial cost
advantages, but they are often less active than their noble metal counter parts. A frequently used
example includes Raney nickel and derivatives, which have been developed as economical
alternatives.3 Notably, with respect to toxicity, precious metals as well as nickel are known or
suspected human carcinogens or show other poisonousness effects.4 For example, their use in
food, agrochemical, pharmaceutical as well as cosmetic applications is strictly regulated and
those metals must be removed completely from the final products, resulting in additional
expenses.5 Therefore, using abundant metals with low or minimal safety concern, especially
iron, zinc or copper for hydrogenation reactions is most desirable.6 In this respect, in recent
years significant attention focused on the development of iron-based hydrogenation catalysts
and important progress has been achieved.7 Somewhat surprisingly, copper-based catalysts
were much less considered, although they are well-established in industry and the most
prominent example is the ternary system Cu-ZnO-Al2O3 used in methanol synthesis from gas
mixtures (H2/CO2/CO).8 This process has been extensively studied due to its high industrial
relevance since its first use in 1966. Furthermore, modified copper catalysts have been used for
the hydrogenation of C=O bonds,9 the hydrogenation/dehydroxylation of biomass (e.g.
fructose, sorbitol, glycerol or furfural) to valuable diols,10 photocatalytic H2 production,11 and
methanol steam reforming.12 However, supported copper catalysts for hydrogenation of other
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unsaturated functional groups are much less explored.13 Notably, most copper-catalyzed
hydrogenations are performed under relatively drastic conditions (temperatures >200 °C),
which obviously limits their applications. To conclude a general Cu catalyst which works for
several classes of substrates under milder conditions is basically unknown and would be
desirable.
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To date, the synthesis of supported metal nanoparticles (NPs) has become a useful tool for the
development of new catalysts for advanced organic synthesis.14 In the last decade, apart from
precious metals, a large focus was given to more earth-abundant metals, specifically cobalt,
nickel and iron, which were successfully employed in several redox reactions.7, 15 In this respect,
we became interested in the synthesis of copper-based materials for broad-scope catalytic
hydrogenation reactions.
Results and Discussion
Following our previous work,7, 16 Cu(OAc)2 with or without additional ligands was immobilized
on different supports and pyrolyzed at 600-1000 °C. All the resulting materials were tested in
the hydrogenation of quinoline, which is of considerable interest in the fields of petrochemicals,
specialty chemicals, and bio-active compounds, under comparably mild conditions (80 °C, 40
bar hydrogen, 2 mL toluene, 24 h).17
As shown in Table 1, using a copper catalyst prepared in the presence of 1,10- phenanthroline
as ligand and carbon (Vulcan XC 72r), unfortunately no conversion to the desired product
1,2,3,4-tetrahydroquinoline (1b) was observed (Table 1, entry 1). Surprisingly, without any
ligand the carbon-supported catalyst gave 1b in 15% (Table 1, entry 2). Next, different supports
such as γ-Al2O3, α-Al2O3, MgO, CeO2 and Al2O3 (activated, neutral, Brockmann Activity I)
were used for the preparation of the catalysts. While most of the supports did not create active
catalysts, the Al2O3–based catalysts gave 1b in 8-33% yield (Table 1, entries 3-7). Testing the
best catalyst system in the presence of different solvents (1,4-dioxane, isopropanol,
cyclohexane, and 2-methyltetrahydrofuran) unfortunately gave no improvement (Table 1,
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entries 8-11). Similarly, materials pyrolyzed at 500, 600, 900, and 1000 °C provided lower
yields or did not exhibit any catalytic activity for quinoline hydrogenation (Table 1, entries
12−15). However, modifying the temperature ramp of the pyrolysis procedure from 25 °C/min
to 10 °C/min led to significantly improved yield (94%) of 1b (Table 1, entry 16). Finally,
slowing down the pyrolysis procedure to 5 °C/min gave a quantitative yield of the desired
product (Table 1, entry 17). Interestingly, the system is also active at 60 °C (Table 1, entry 18).
Comparing the activity of our catalytic system with Cu/ZnO, a benchmark copper based
methanol synthesis catalyst and with Raney nickel, the new material outperforms these
commercially available catalysts (Table 1, entries 17 versus 19 and 20).
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Table 1. Model reaction: Hydrogenation of quinoline.a
[Cu] Cat.
solvent
H2 (40 bar), T (°C)
N
N
H
24 h
1a
1b
Entry Catalyst
Pyrolyzed T (°C)
Solvent
toluene
toluene
toluene
toluene
toluene
toluene
toluene
T (°C)
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
Yield (%)a
1
Cu-Phen/Carbon
800
800
800
800
800
800
800
800
800
800
800
1000
900
600
500
0
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30
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2
Cu/Carbon
Cu/γ-Al2O3
Cu/α-Al2O3
Cu/MgO
3
4
5
trace
0
6
Cu/CeO2
Cu/Al2O3
Cu/Al2O3
Cu/Al2O3
Cu/Al2O3
Cu/Al2O3
Cu/Al2O3
Cu/Al2O3
Cu/Al2O3
Cu/Al2O3
7
33
0
8
1,4-dioxane
isopropanol
cyclohexane
2-MeTHF
toluene
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8
10
11
12
13
14
15
12
10
0
toluene
trace
31
27
toluene
toluene
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Cu/Al2O3
800b
800c
800c
toluene
toluene
toluene
toluene
toluene
80
80
60
80
80
94
>99
57
0
Cu/Al2O3
Cu/Al2O3
CuO/ZnOd
Raney nickele
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aReaction conditions: 0.5 mmol quinoline, 40 mg catalyst (2.5 mol% Cu), p bar H2, 2 mL solvent, T °C, 24 h. GC yield using
n-dodecan as internal standard. bThe furnace was heated to 800 °C at a rate of 10 °C per minute. cThe furnace was heated to
800 °C at a rate of 5 °C per minute. dCopper based methanol synthesis catalyst (1.6 mg, 2.5 mol%). e0.74 mg (2.5 mol%).
Based on the catalyst optimization, different substituted quinolines were tested under the
standard conditions (2.5 mol% Cu/Al2O3, 2.0 mL toluene, 40 bar H2, 24 h). As shown in
Table 2. Hydrogenation of various quinolines and isoquinoline.a
Cu/Al2O3
toluene
R2
R2
R1
R1
H2 (40 bar), 80 °C
24 h
N
N
H
a
b
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Me
Me
Me
5
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N
Me
N
H
N
H
N
H
H
N
H
b
8
9
1b
2b
3b
4b
5b
98%
92%
96%
92%
F
92%
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MeO
Cl
N
H
N
N
N
H
N
H
N
H
H
H
Me
6b
OEt
8b
F
b
b
b
b
c
7b
9b
10b
11b
>99%
98%
98%
>99%
93%
98%
Me
MeO
F
N
H
Ph
N
H
Me
F
N
H
Me
N
H
Me
N
H
Me
13b
14b
16b
94%
98%
99%
12b
15b
90%
90%
F
Cl
NH
Cl
N
H
Me
N
H
Me
N
H
N
Me
H
Cl
OMe
b
b
b
c
17b >
18b
19b
20b
21b
92%
99%
>99%
>99%
>99%
aReaction conditions: 0.5 mmol substrates, 40 mg catalyst (2.5 mol% Cu), 40 bar H2, 2 mL toluene, 80 °C, 24 h.
b100 oC, 50 bar H2. c140 °C, 50 bar H2. Isolated yields.
Table 2, quinolines containing electron-donating as well as electron-withdrawing groups are
compatible with the catalytic system and the hydrogenated products (2b-20b) were isolated in
excellent yields. Notably, groups substituted at the different positions on the aromatic ring had
no significant influence on the product yields. For example, the methyl group in 2b, 3b, 4b, 5b,
6b, 13b and methoxy, ethoxy, and phenyl substituents in 7b, 8b and 12b as well as halogens in
9b, 10b, and 11b gave over 90% of the corresponding product. Di-substituted quinolines were
also converted to 1,2,3,4-tetrahydroquinolines (13b-20b) in 90->99% yields. In addition,
isoquinoline (21a) was hydrogenated selectively, albeit at higher temperature and hydrogen
pressure.
Obviously, stability and recyclability are important parameters for the application of any
heterogeneous catalyst, which are crucial performance metrics for efficient and cost-effective
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processes. Hence, the heterogenized Cu/Al2O3 catalyst was recycled up to seven times. As
depicted in Figure 1, no significant loss of activity was observed and 89% of 1b was still
achieved after the seventh run. ICP measurements proved small amounts of copper leaching
after first run (1.08 ppm) and second to sixth run (2.00 ppm) (Table S1, see Supporting
Information).
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Standard conditions
N
N
H
1a
1b
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Recycling run
Figure 1. Recycling experiments using Cu/Al2O3 catalyst. Yields were determined by GC
using n-dodecan as internal standard.
To demonstrate the general reactivity of this copper catalyst, several hydrogenations of other
functional groups including alkynes, olefins, ketones, imines, nitriles, polycyclic aromatic
hydrocarbons, as well as nitroarenes were investigated. For this purpose, we explored the
semihydrogenation of several internal aryl alkynes using the Cu/Al2O3 catalyst under similar
conditions at 120 °C (Table 3). Notably, the resulting Z-alkene scaffold is widely found in food,
medical as well as cosmetic applications.18 Until to date, Lindlar’s catalyst is the most
prominent material for industrial semihydrogenation of alkynes to generate Z-alkenes.19 In
general, good selectivity and excellent yields for the Z-isomer formation was achieved for
diarylalkynes (22b, 23b, 25b, 26b, 28b and 29b). Interestingly, substrate 24a bearing a
Table 3. Copper-catalyzed semihydrogenation of various internal alkynes.a
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Cu/Al2O3
toluene
R2
R2
R1
H2 (50 bar), 120 °C
24 h
R1
a
b
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O
Et
OMe
Et
OMe
>99%
Me
Me
26b
90%
22b
23b
24b
25b
93%
94%
>99% (Z/E = 26/1)
n-C4H9
nBu
27b
OMe
96% (Z/E = 10/1)
Si(CH3)3
F
b
b
28b
29b
30b
67%
98%
78%
31b
50%
aReaction conditions: 0.5 mmol substrates, 40 mg catalyst (2.5 mol% Cu), 50 bar H2, 2 mL toluene, 120 °C, 24 h.
Isolated yields. bYield determined by GC using n-dodecan as the standard.
reducible carbonyl group is also compatible with these conditions. In addition to internal
alkynes, terminal alkynes such as ethynylbenzene (30a) and 1-ethynyl-4-fluorobenzene (31a)
gave the corresponding olefins with full conversion under standard conditions. Since no
ethylbenzenes were detected, we propose poly- and/or oligomerization of the substrates as side
reactions. However, using water as solvent instead of toluene, styrene was reduced to
ethylbenzene quantitatively, demonstrating the possibility for olefin hydrogenation at different
conditions.
Hence, we performed the hydrogenation of C=O, C=N, and other C=C double bonds by using
EtOH and H2O as the solvents to explore the general applicability of the catalytic system further
on (Table 4). Gratifyingly, all substrates were converted into the desired products with ≥90%
yields. For example, the polycyclic aromatic hydrocarbon anthracene was easily hydrogenated
in 98% yield to give 37b. In addition, aromatic ketones were hydrogenated to benzyl alcohols
(32b-35b) in high yields. In the case of N-benzylideneaniline, hydrogenation reaction also
proceeded smoothly to 36b. When using benzonitrile as substrate in water (50 bar H2, 120 °C,
24), even nitrile reduction to the corresponding dibenzylamine took place with full conversion.
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Table 4. Copper-catalyzed hydrogenation of ketones, imine and anthracene.a
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7
8
9
R2
R2
Cu/Al2O3
EtOH/H2O
X
X
R1
R1
H2 (50 bar), 120 °C
24 h
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X = O, N, C
b
OH
OH
OH
OH
Cl
N
H
Me
b
32b
33b
34b
35b
36b
37b
98%
96%
90%
94%
94%
96%
aReaction conditions: 0.5 mmol substrates, 40 mg catalyst (2.5 mol% Cu), 50 bar H2, EtOH/H2O (1.5 mL/0.5 mL),
120 °C, 48 h. Isolated yields. b83 mg catalyst (5.0 mol%), toluene (2.0 mL), 140 °C, 48 h.
To finish the substrate scope, we investigated the hydrogenation of nitroarenes, a reaction
frequently used in the industrial production of dyes, pigments, pharmaceuticals, agrochemicals
and monomers for polyurethanes. As demonstrated in Table 5, the Cu/Al2O3 can be used for the
reduction of a wide range of substrates containing diverse substituents. Using toluene as
solvent, no dehalogenation was detected for chloro-substituted substrates (40b and 47b).
Interestingly, under these conditions the catalyst is also able to reduce the nitro group in the
presence of reducible groups such as cyano and ketone, which showed reactivity in a different
solvent system vide supra. Gratifyingly, full conversion was obtained without any detectable
reduction of the unsaturated units (41b, 42b and 44b).
Table 5. Hydrogenation of various aromatic nitro compounds.a
Cu/Al2O3
toluene
NH2
NO2
R1
R1
140 °C, H2 (50 bar)
48 h
b
a
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NH2
NH2
NH2
NH2
NH2
Me
Cl
Me
NC
O
b
38b
39b
40b
41b
42b
92%
79%
76%
72%
NH2
82%
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NH2
NH2
NH2
Cl
Cl
NH2
O
Ph
Ph
Ph
N
H
O
Cl
OH
46b
43b
44b
45b
47b
80%
66%
92%
96%
74%
aReaction conditions: 0.5 mmol substrates, 40 mg catalyst (2.5 mol% Cu), 50 bar H2, toluene 2.0 mL, 140 °C, 48 h.
bGC yield sing n-dodecan as internal standard.
Catalyst characterization
The presented optimal catalyst was prepared using Cu(OAc)2 as the precursor and Al2O3 as the
support (see Supporting Information). The Cu composition of the most active system
(Cu/Al2O3) is 2.0 wt%, which is determined by elemental analysis. To elucidate the structure
and morphology of the material and to get more insight into its chemical composition, scanning
transmission electron microscopy (STEM) measurements have been performed. As can be seen
from Figures 2a-b, the fresh catalyst contains mainly rather large particles of polycrystalline Cu
containing particles. In agreement with this observation, STEM-HAADF image and the
corresponding EDX spectra (Figure S1a) confirm that particles of similar morphology albeit
different sizes contain Cu as indicated by the Cu signal. General morphology of these particles
suggests the particles to be of Cu oxide, which could be confirmed by EDX analysis of a free
hanging Cu containing particle (Figure S1b).
To obtain further information about the valence states of this copper catalyst, X-ray
photoelectron spectroscopy (XPS) measurements were performed. The Cu2p region (Figure
S4) contains two main peaks (2p3/2 and 2p1/2) and a satellite featured around 944 eV
characteristic for Cu2+. The main peaks can be fitted with two components suggesting the
presence of Cu1+ and Cu2+ according to their binding energies at 932.4 ± 0.3 eV and 934.3 ±
0.3 eV respectively, after pyrolysis at 800 °C (Figure S4a). To distinguish between Cu1+ and
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metallic Cu the Cu LMM region is shown in Figure S5a, revealing a broad peak at a binding
energy of 570 eV characteristic for Cu1+. No indications for metallic Cu at 568 eV are visible,40
thus supporting the peak assignment in the Cu2p spectra. In the recycling experiments after one
catalytic run the double peak structure of the Cu2p region was still visible while all peaks are
shifted by about 2.5 eV to higher binding energies (Figure S4b). This shift correlates well
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a
b
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e
f
d
Figure 2. HAADF-STEM images of fresh Cu/Al2O3, (a) showing an overview and (b) giving
a more detailed part of the Cu oxide on the Al2O3 (a, b). HAADF-STEM image of Cu/Al2O3
catalyst after 1 run (c, d) and after 7 runs (e, f) of the model reaction.
with the observed morphological changes to smaller Cu particles in STEM measurements and
is known for Copper nanostructures.20 The peak deconvolution reveals a dominating Cu1+ state
which is also supported by the even smaller satellite peak of Cu2+ and the changes observable
in the Cu LMM region (Figure S5).21
The STEM images of the copper catalyst after the first run and the seventh run are shown in
Figure 2c-f. Interestingly, most of the original large copper oxide particles are gone after the
first run. This reformation process seems to happen during the first run resulting in highly
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distributed copper atoms, clusters and small particles (Figure 2c-d). STEM-HAADF image and
the corresponding EDX spectra of the once used Cu/Al2O3 catalyst shows the remnants of a
larger Cu containing particle (Figure S2) indicated by the strong Cu signal, while the spectrum
of a different area shows a much lower Cu signal, which can be attributed to a highly dispersed
copper phase (Figure S2). The size and distribution of the copper phase is very different
compared to the fresh one, although some of the large Cu oxide particles still exist. After the
seventh run, a finely distributed Cu phase down to small clusters, even single atoms and
particles showing crystal structures with sizes up to about 10 nm are observed (Figure 2e-f).
STEM-HAADF image and the corresponding EDX spectra of the marked regions (Figure S3)
of the seven times used Cu/Al2O3 catalyst show only a small Cu signal besides the Al2O3 signal
indicating the highly distributed phase.
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Conclusion
This work demonstrates for the first time that several classes of unsaturated compounds can be
hydrogenated with one specific copper catalyst at comparably low temperature. For example,
quinolines, alkynes, aromatic ketones, imines, a polycyclic aromatic hydrocarbon, as well as
nitroarenes are reduced with high chemoselectivity (under different conditions) using the same
catalyst. In case of diarylalkynes excellent selectivity for the Z-isomer formation was achieved.
Upscaling and further applications using this heterogeneous copper catalyst are currently under
way in our laboratory.
ASSOCIATED CONTENT
The Supporting Information is available free of charge on the ACS Publications website at
http://pubs.acs.org Experimental details, catalyst and recycled catalyst as well as organic
compounds characterization, and NMR spectra (PDF) are provided in the Supporting
Information.
AUTHOR INFORMATION
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Corresponding Author
*Matthias.Beller@catalysis.de
ORCID
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Matthias Beller: 0000-0001-5709-0965
Author Contributions
The manuscript was written through contributions of all authors. All authors have given
approval to the final version of the manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
We thank Dr. Susann Buchholz and Reinhard Eckelt for the excellent analytical support. We
thank Karin Struve for ICP measurement. We thank the analytical team of LIKAT for their kind
support. We gratefully acknowledge the support from the Federal Ministry of Education and
Research (BMBF) and the State of Mecklenburg-Vorpommern.
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Table of Contents
Ar
R
Ar
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N
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H
O
OH
Me
NH2
H2
+
Ar
Me
NO2
Heterogeneous
Cu catalyst
Ar
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