Transfer hydrogenation of unsaturated bonds in the absence of base additives catalyzed by a cobalt-based heterogeneous catalyst

Article abstract of DOI:10.1039/c4cc08946d

A novel non-noble Co@C-N catalytic system has been developed for catalytic transfer hydrogenation reactions. Co@C-N was found to be highly active and selective in the hydrogenation of a variety of unsaturated bonds with isopropanol in the absence of base additives.

Full text of DOI:10.1039/c4cc08946d

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Transfer Hydrogenation of Unsaturated Bonds in the
Absence of Base Additives Catalyzed by a Cobalt-
Based Heterogeneous Catalyst
Jilan Long, Ying Zhou and Yingwei Li*
Multiple advantages of the proposed catalytic system include a
remarkable versatility not only for C=O hydrogenation (including
ketones and aldehydes) but also for C=C, C≡N, and N=O bonds),
an environmental-benign protocol (reactions conducted in a neutral
environment under mild reaction conditions) and the use of a
magnetically separable non-noble metal catalyst in a simple and safe
reaction setup (in the absence of molecular hydrogen, under easy
operation and facile separation owing to the magnetic nature of the
Co system).
A
novel non-noble Co@C-N catalytic system has been
developed for catalytic transfer hydrogenation reactions.
Co@C-N was found to be highly active and selective in the
hydrogenation of a variety of unsaturated bonds with
isopropanol in the absence of base additives.
Transition metal-catalyzed transfer hydrogenation protocols are
convenient and alternative green chemical methods to traditional
energy-consuming hydrogenation processes, able to provide high
atom efficiency and generating advantageous economics.1 The use of
hydrogen donor reagents (e.g. alcohols) in transfer hydrogenation
reactions can avoid the use of autoclaves and high-pressure
hydrogen, being highly relevant for industrial applications.2
Various noble metals (e.g., ruthenium, iridium, and rhodium) have
been typically employed in transfer hydrogenation reactions of a
number of chemical functionalities including ketones, aldehydes,
nitrocompounds and imines.3 However, most of these systems are
largely based on homogeneous organometallic complexes which
additionally required the addition of base additives to provide good
product yields .3a-3j
Noble metal free systems have recently emerged as promising
alternative in transfer hydrogenation reactions,4 including highly
effective and soluble Fe,5 Cu,6 Zn,7 and Co8 complexes. Similarly,
the majority of the reported catalytic systems required base
additives5b-5l or/and homogeneous phosphorus-containing ligands.5a-
Figure 1. Powder XRD patterns of the Co@C-N samples: (a) Co@C-N-
500-15h, (b) Co@C-N-600-15h, (c) Co@C-N-700-15h, (d) Co@C-N-800-
15h, (e) Co@C-N-900-15h, and (f) Co@C-N-900-15h after reaction.
5f,6
Significant improvements are therefore desirable to advance in
the development of more sustainable, environmentally friendly, and
economic catalytic systems in transfer hydrogenation processes.
In continuation with recent research endeavours from the group,
here we report the design of non-noble metal-based heterogeneous
catalysts for transfer hydrogenation reactions of various unsaturated
bonds catalyzed in the absence of any base additives. The catalysts
were prepared using a Co-containing metal-organic framework
(MOF) as sacrificial template and the reactions were conducted
using i-PrOH as hydrogen donating solvent. The choice of the Co-
MOF as template relates to the presence of triethylenediamine basic
sites (even after simple thermolysis) which could be favorable for
transfer hydrogenation reactions. To the best of our knowledge, there
are currently no reports on the use of MOF-derived metal/carbon
nano-composites for transfer hydrogenation.
The powder X-ray Diffraction (XRD) patterns of Co-MOF (Fig.
S1) matched well with those reported in literature,9 confirming the
structure of the as-synthesized MOF material in this work (see the SI
for the preparation of the MOF). Thermogravimetric Analysis (TGA)
curves (Fig. S2) showed that the Co-MOF was stable at temperatures
around 400 °C.10
Co@C-N-X-T was prepared after Co-MOF pyrolysis (here X and
T represent thermolysis temperature and time, respectively). XRD
patterns of Co@C-N-X-T materials (Fig. 1 and Fig. S3) exhibited
five diffraction peaks at around 44.3°, 51.6°, 76.1°, 92.3°, and 97.9°,
respectively, characteristics of metallic Co (JCPDS No. 15-0806).
Co diffraction peaks showed improved intensity for materials
prepared at higher pyrolysis temperatures with longer times owing to
the higher crystallization degree of Co.
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As shown by the N2 adsorption/desorption isotherms and pore
distribution, micropores were only present in Co-MOF (Fig. S4 and
Table S1). After thermolysis, the observation of hysteresis loops at
higher P/P0 indicated the presence of mesopores in Co@C-N-X-T
materials (Figs. S5-S6, and Table S1). Co and N contents in Co@C-
N-X-T samples were ca. 37-41% and 1.5%, respectively (Table S2).
Figure 5. CO2-TPD profile for (a) Co@C-N-700-15h, (b) Co@C-N-800-
15h, and (c) Co@C-N-900-15h.
The basicity of Co@C-N was determined by Temperature
Programmed Desorption (TPD) of CO2, with results depicted in Fig.
5. As the pyrolysis temperature increased, the number of strong basic
sites increased with a simultaneously enhanced basicity. Four peaks
could be observed (550 °C, 450 °C, 250 °C, and 150 °C), for Co@C-
N-900, which corresponded to the desorption of CO2 adsorbed on
basic sites with different basicity. These basic sites could be related
to nitrogen-containing species derived from the triethylenediamine
moiety in Co-MOF during thermolysis.
After characterisation, the catalytic activities of the Co@C-N
materials were subsequently tested in the transfer hydrogenation of
acetophenone to phenethanol as model process. Reactions were
performed at 80 °C and N2 atmosphere using isopropanol as both
reductant and solvent under base-free conditions. Blank runs
(without catalyst) showed no reactivity (Table S3, entry 1). To our
delight, as-synthesized Co@C-N materials were highly active for
this transformation. Generally, materials prepared under longer
thermolysis times and higher temperatures exhibited higher activity
and selectivity (Table S3, entries 2-11). Ethyl benzene was the main
side product produced in this reaction. Co@C-N-900-15h was found
to be the best catalyst among investigated materials in terms of
conversion and selectivity to phenethanol (Table S3, entry 6). The
amount of isopropanol influenced the activity and selectivity of the
reaction (Table S3, entries 12 and 13); i.e., decreasing the quantity of
isopropanol to 1 mL resulted in a reduced selectivity for phenethanol
while lower conversions were obtained at increasing isopropanol
quantities. The reaction also proceeded well at a low catalyst loading
(1 mol%), giving 72% conversion with 98% selectivity within 96 h
(Table S3, entry 14).
Figure 2. SEM image (a), and elemental mapping of Co@C-N-900-15h: C
(b), N (c), Co (d).
The surface morphology of Co@C-N-X-T was subsequently
investigated by SEM (Scanning Electron Microscope). As shown in
Figs. S7-S8, all Co@C-N exhibited similar particle shapes to those
of parent Co-MOF. Moreover, some mesopores could be observed in
Co@C-N, with an increasingly rough surface at increasing pyrolysis
temperature and time. Elemental mapping (Fig. 2) pointed out a
highly uniform distribution of Co, C, and N on the surface of
Co@C-N materials.
Transmission Electron Microscopy (TEM) micrographs (Fig. 3
and Fig. S9) revealed Co nanoparticles homogeneously dispersed in
Co@C-N materials. Co nanoparticles obtained at higher thermolysis
temperatures exhibited relatively larger sizes. The mean particle
sizes ranged from 6 nm to 17 nm as the pyrolysis temperature was
increased from 500 oC to 900 oC.
To elucidate the origin of catalytic activity, the metal was
removed from Co@C-N-900-15h using aqua regia treatment for 2
days. A reaction run with the metal-free C-N composite gave no
conversion, demonstrating the requirement of a metal to catalyze the
transfer hydrogenation (Table S3, entry 15). On the other hand, a
Co/C material was also essentially not active in this transformation
(Table S3, entry 16), which may be related to the lack of basic sites
on the carbon support.5i,5l,11 These results suggest the importance of
both a metal (Co) and basic sites in Co@C-N materials to achieve a
high catalytic efficiency in transfer hydrogenation.
Figure 3. TEM images of Co@C-N-900-15h (a, b), EDS (c), and the
corresponding size distribution of Co nanoparticles (d).
We further investigated the reusability of Co@C-N-900-15h in the
transfer hydrogenation of acetophenone to phenethanol with
isopropanol. Results included in Fig. S10 revealed no appreciable
loss of conversion or selectivity up to four reaction runs. Powder
XRD characterization proved that the structure of catalyst was
mostly preserved (Fig. 1), indicating the stability of the material
under the investigated conditions.
The scope of the protocol for the transfer hydrogenation of a range
of various unsaturated bonds was subsequently investigated under
base-free conditions. Firstly, phenyl ketones and cyclohexyl ketones
Figure 4. XPS spectra of Co@C-N-900-15h (a) and Co-MOF (b).
X-ray Photoelectron Spectroscopy (XPS) spectra of Co-MOF and
Co@C-N-900-15h are shown in Fig. 4. Co@C-N-900-15h exhibited
two Co 2p peaks at 778.9 eV and 793.8 eV, which were shifted to
lower binding energies (2.4 eV and 3.3 eV, respectively) as
compared to those of Co-MOF, indicating that Co(II) in Co-MOF
was converted to metallic Co after thermolysis.
2 | J. Name., 2012, 00, 1-3
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were investigated, which were shown to undergo the transfer
hydrogenation with isopropanol to give the corresponding products
in quantitative yields (Table 1, entries 2-7). Notably, highly
sterically hindered ketones, such as t-BuC(O)CH3, could also be
hydrogenated in excellent yields after 60 h reaction (Table 1, entry
8). Aldehydes also worked well, although a slightly longer time
and/or higher temperature were required to complete the reactions
(Table 1, entries 9-17). In general, electron-rich benzaldehydes
exhibited higher activity than electron-deficient ones. For the
transfer hydrogenation of benzaldehydes with an electron-
withdrawing group such as 4-(trifluoromethyl)benzaldehyde, 4-
fluorobenzaldehyde or 4-bromobenzaldehyde, a slight increase in
reaction temperature was needed (Table 1, entries 13-15). 1-
Naphthalene formaldehyde could also be smoothly hydrogenated in
93% yield (Table 1, entry 16). Aliphatic aldehydes are rarely
employed as substrates in transfer hydrogenation owing to the aldol
condensation reaction. However, aliphatic aldehydes also underwent
transfer hydrogenation under the investigated conditions, affording
the desired products in quantitative yields (Table 1, entry 17).
18c
19c
50
46
40
>99
>99
>99
90
>99
>99
20b,c
21b,c
22b,c
23d
46
24
80
>99
>99
80
>99
90
>99
24b
50
>99
>99
>99
>99
>99
25b
26d
27d
28
50
60
50
20
20
>99
95
98
>99
>99
>99
>99
Table 1. Transfer hydrogenation of various substrates with
isopropanol a
29
a
Reaction conditions: substrate (0.5 mmol), Co@C-N-900-15h (Co 10
d
Time
(h)
Con.
(%)
b
c
Entry
Substrate
Sel. (%)
mol%), isopropanol( 2 mL), 80 °C. 100 °C. Isopropanol (1 mL).
Catalyst (Co 20 mol%), 150 °C.
1
2
3
4
17
12
30
30
99
95
As compared to traditional pathway of nitriles hydrogenation
(high molecular hydrogen pressures, ca. 1-5 MPa),12 transfer
hydrogenation represents an attractive route to amines owing to the
relatively mild reaction conditions which avoided the use of any high
pressure equipment.2 However, the transfer hydrogenation of nitriles
is particularly difficult because of easy formation of the
corresponding methyl group.13 Particularly for aliphatic nitriles
(e.g.n-pentanenitrile and phenylacetonitrile), moderate yields or even
no corresponding products could be detected.14 Nevertheless, m-
tolunitrile, o-tolunitrile, 1-naphthonitrile, and even aliphatic nitriles
smoothly underwent transfer hydrogenation under mild reaction
conditions, with yields over 90% for the desired products (Table 1,
entries 18-22).
Transfer hydrogenation also provides an excellent method for the
hydrogenation of nitro compounds as compared to hydrogenations
using molecular hydrogen under high pressure.15 In the present
catalytic system, the transfer hydrogenation of nitrobenzene and
derivates with isopropanol were conducted at ambient pressure at
120-150 °C, giving the corresponding amines in good to excellent
yields (Table 1, entries 23-25). Transfer hydrogenation of
halogenated nitrobenzene was more challenging due to
dehalogenation.16 However, in our system the reactions of
halogenated nitrobenzenes such as p-chloronitrobenzene and p-
iodonitrobenzene were found to be promoted under the investigated
reaction conditions to afford the desired products in excellent yields
with >99% selectivity (Table 1, entries 26-27).
Furthermore, we also investigated the transfer hydrogenation of
C=C bonds. Excellent conversions and selectivities were obtained
for cycloalkenes without the detection of ring opening by-products
under the investigated conditions (Table 1, entries 28-29). The
reaction results summarized in Table 1 demonstrated the general
applicability of this catalytic system in the transfer hydrogenation of
various unsaturated chemicals including aldehydes, ketones, nitro-
compounds, nitriles, and alkenes.
Two different reaction mechanisms have been proposed for the
transfer hydrogenation in the presence of base additives.17 Similarly,
both of them suggest that the presence of bases may facilitate the
formation of metal hydrides for the transition-metal catalyzed
>99
>99
>99
>99
>99
>99
5b
24
>99
>99
6b
7b
24
24
>99
>99
>99
>99
>99
8b
9
60
34
95
>99
>99
10
11
40
45
>99
>99
>99
>99
12
13b
14b
40
45
35
>99
>99
>99
>99
>99
>99
15b
35
>99
>99
16b
17
36
30
>99
>99
93
>99
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transfer hydrogenation reactions.18 As demonstrated by CO2-TPD
(Fig. 5), basic sites were present in as-synthesized Co@C-N
materials. Therefore, we speculated that the basic sites on Co@C-N
might play a similar role to that of base additives to facilitate the
transfer of protons to Co nanoparticles to form Co metal hydride
species, thus promoting the production of the desired hydrogenation
products.
In summary, we have developed a novel non-noble Co@C-N
system for catalytic transfer hydrogenation reactions. The
heterogeneous Co@C-N catalysts, prepared using a Co-containing
MOF as sacrificial template, were proved to be highly active and
selective in the hydrogenation of a variety of unsaturated bonds with
isopropanol in the absence of any base additives. Moreover, the
catalysts are magnetically separable (Fig. S11) and reusable under
the investigated conditions. The combination of high efficiency,
versatility and recyclability as well as mild reaction conditions (in
the absence of bases and gaseous hydrogen) makes this system an
attractive alternative pathway for various hydrogenation processes.
We thank the National Natural Science Foundation of China
(21322606 and 21436005), the Doctoral Fund of the MOE of China
(20120172110012), the Fundamental Research Funds for the Central
Universities (2013ZG0001), and Guangdong NSF (S2011020002397
and 10351064101000000) for financial support.
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Notes and references
School of Chemistry and Chemical Engineering, South China University
of Technology, Guangzhou 510640 (China), E-mail: liyw@scut.edu.cn
† Electronic Supplementary Information (ESI) available: experimental
details, catalysts characterization, and additional reaction results. See
DOI: 10.1039/c000000x/
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