Transfer hydrogenation of ketones by ceria-supported Ni catalysts

Article abstract of DOI:10.1039/c2gc35836k

Ni-loaded CeO₂, prepared by H₂-reduction of NiO-loaded CeO₂, was found to be an effective and recyclable catalyst for the transfer hydrogenation of aliphatic and aromatic ketones by 2-propanol to the corresponding alcohols under base free conditions with low catalyst loading (1-3 mol%).

Full text of DOI:10.1039/c2gc35836k

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COMMUNICATION
Transfer hydrogenation of ketones by ceria-supported Ni catalysts†
Katsuya Shimura and Ken-ichi Shimizu*
Received 30th May 2012, Accepted 31st August 2012
DOI: 10.1039/c2gc35836k
of 2-octanone (Table 1). NiO-loaded precursors (Ni = 5 wt%)
Ni-loaded CeO2, prepared by H2-reduction of NiO-loaded
CeO2, was found to be an effective and recyclable catalyst for
the transfer hydrogenation of aliphatic and aromatic ketones
by 2-propanol to the corresponding alcohols under base free
conditions with low catalyst loading (1–3 mol%).
were prepared by evaporation to dryness of the slurry composed
of an aqueous solution of Ni nitrate and support, followed by
calcination in air at 350 °C for 4 h. Before the reaction the pre-
cursors were reduced in a flow of H2 at 500 °C for 0.5 h,
followed by cooling in H2 to room temperature. Then, the reac-
tion mixture was injected into the pre-reduced catalyst inside the
glass tube through a septum inlet, and the mixture was stirred
and heated at 85 °C (reflux conditions) in a N2 atmosphere. The
initial formation rate of 2-octanol (V0) listed in Table 1 was
measured under the condition where the conversion of 2-octa-
none was below 40%. Among the various Ni(5 wt%)-loaded
MOx catalysts (MOx = CeO2, Al2O3, ZrO2, SiO2, SiO2–Al2O3
and TiO2), Ni/CeO2 showed a one to four orders higher activity
than the other catalysts. Commercially available Ni compounds
(Ni powder, RANEY® Ni and NiO) and CeO2 showed negli-
gible activity. The activity of the Ni/CeO2 catalyst was higher
than those of CeO2-supported noble metal (Pd, Ru) and non-
noble metal (Cu, Co) catalysts. Next, we optimized the prep-
aration conditions of Ni/CeO2 catalysts. First, we investigated
the effect of pre-treatment conditions of Ni/CeO2 (results not
shown). Without the pre-reduction treatment, the NiO/CeO2 cata-
lyst showed no activity. Although the catalyst pre-reduced at
The reduction of carbonyl compounds to the corresponding alco-
hols is an important transformation in organic synthesis. Among
various methods such as the use of stoichiometric reducing
reagents and hazardous molecular hydrogen (H2), transfer hydro-
genation1 has attracted much attention, because the hydrogen
donor (e.g. 2-propanol) is cheap and easy to handle, and no elab-
orate setups (e.g. high pressure reactors) are required. Homo-
geneous noble metal catalysts such as Ru2 and Ir3 complexes are
known to be effective for this reaction, but they suffer from pro-
blems such as difficulty in the recovery and reuse of expensive
catalysts, and the necessity of co-catalysts (base and ligand).
Recently, noble metal (Pt,4 Pd,5 Ru,6 and Au7)-based hetero-
geneous catalysts have been reported to act as effective and re-
usable catalysts for this reaction. However, these systems have
problems such as necessity of basic additives (except for sup-
ported Ru(OH)x catalysts6a,d) and high cost. As cost-effective
and eco-friendly alternatives, heterogeneous catalysts such as
non-noble metal catalysts (Ni,8–10 Co11 and Cu12), solid bases
(hydrotalcites13a and K3PO4 13b), and Sn- or Zr-zeolite14 cata-
lysts were developed. However, they suffer from drawbacks such
as high catalyst loading (10–20 mol%),8–10b high reaction temp-
erature (150 °C),10g special reaction methods (microwave
heating),10h,12a or excess amounts of a co-catalyst (1 equiv.
KOH).10b–f,11 Although a few systems (Ni nanoparticles,9 Cu-
zeolite,12b solid bases13 and zeolites14) succeeded in the reaction
of activated (aromatic and α,β-unsaturated) carbonyl compounds
under co-catalyst free conditions, these systems were not gener-
ally effective for less reactive aliphatic carbonyl compounds. We
report herein that Ni/CeO2 catalyst, prepared from inexpensive
commercial materials, acts as a recyclable catalyst for the transfer
hydrogenation of ketones including less reactive aliphatic
ketones under additive-free conditions with low catalyst loading
(1–3 mol%).
Table 1 Transfer hydrogenation of 2-octanone
Catalyst
Vo a/mmol h−1 g−1
Ni/CeO2
Ni/Al2O3
Ni/ZrO2
Ni/SiO2
Ni/SiO2–Al2O3
Ni/TiO2
Ni powder
RANEY® Ni
NiO
20.8
1.7
1.5
0.020
0.015
0.009
0
0.10
0
First, we compared the activity of various Ni catalysts, bare
CeO2 and metal-loaded CeO2 samples for transfer hydrogenation
CeO2
0.070
5.1
3.8
0.80
0.70
Pd/CeO2
Ru/CeO2
Cu/CeO2
Co/CeO2
Catalysis Research Center, Hokkaido University, N-21, W-10, Sapporo
001-0021, Japan. E-mail: kshimizu@cat.hokudai.ac.jp;
Fax: +81-11-706-9163
†Electronic supplementary information (ESI) available. See DOI:
10.1039/c2gc35836k
a Initial formation rate of 2-octanol per weight of the catalyst.
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Table 2 Transfer hydrogenation of ketones by Ni/CeO2
500 °C (Ni/CeO2) showed high activity, the catalyst was comple-
tely deactivated when it was exposed to air at room temperature.
From the optimization tests of H2-reduction temperature
(400–600 °C), the reduction temperature of 500 °C was found to
be the best (not shown). The effect of the Ni loading (3–10 wt%)
showed that the activity monotonically decreased with the
loading (Fig. S1†). From these results, the Ni/CeO2 (Ni = 3 wt%)
catalyst pre-reduced at 500 °C was the best catalyst.
Entry
1
Substrate
Conv./%
85
Yield/%
85
Using the optimized catalyst, we examined the scope and
limitation for transfer hydrogenation of ketones by 2-propanol
(Table 2). Aromatic (entries 1–4) and aliphatic (entries 5–12)
ketones were reduced to the corresponding alcohols in good to
excellent yields (70–98%). The present system is especially
effective for the reaction of aliphatic ketones. For example, the
reaction of 2-adamantanone at 80 °C for 24 h resulted in a 98%
yield of 2-adamantanol. Norbornanone was selectively converted
to exo-norbornanol. The turnover number (TON) of 65 is more
than four times higher than those of the recently reported hetero-
geneous non-noble metal catalysts: Cu-zeolite (16)12b and Ni
nanoparticles (4).9a We performed catalyst recycle tests for the
reaction of 2-octanone (entry 9). After the reaction, the catalyst
separated from the reaction mixture by centrifugation was
washed with acetone, followed by calcination in air at 300 °C for
1 h, and by reducing in H2 at 500 °C for 0.5 h.15 The recovered
catalyst was reused at least four times without any indication of
catalyst deactivation. The total TON for the five successive reac-
tions is 464, which is more than 5 times higher than those of the
previously reported heterogeneous catalysts: Ru(OH)x/TiO2
(95),6a Ni nanoparticles (9),10a Zr0.8Ni0.2O2 (7),10c and
RANEY® Ni (0.2).8c ICP analysis of the solution after the reac-
tion showed that the amount of Ni species dissolved in the solu-
tion was below the detection limit. These results show that
Ni/CeO2 is a highly effective and recyclable catalyst for transfer
hydrogenation of ketones. To the best of our knowledge, this is
the first successful example of the heterogeneous non-noble
metal catalyst for the transfer hydrogenation of aliphatic ketones
under base free conditions with low catalyst loading
(1–3 mol%).
2
3
4
97
96
96
97
94
96
5a
6a
100
72
98
70
7b
98
86
98
85
8a
9c
92,d 93,e 93,f
93,g 93h
92,d 93,e 93,f
93,g 93h
Finally, we studied the structure–activity relationship. The
structure of Ni species on Ni/CeO2 was studied by Ni K-edge
X-ray absorption near-edge structures (XANES) and extended
X-ray absorption fine structures (EXAFS) shown in Fig. S2.†
Fitting analysis of XANES showed that more than 90% of Ni
species on the catalyst are metallic Ni. The EXAFS of Ni/CeO2
consisted of one Ni–Ni shell at a bond distance of 2.47 Å due to
metallic Ni (Table S1†). The Ni–Ni coordination number (7.5)
was smaller than that of Ni foil (12). These results suggest that
small Ni metal particles are the dominant Ni species on Ni/
CeO2. Exposure of Ni/CeO2 to air at room temperature resulted
in the appearance of the EXAFS feature due to the Ni–O bond
and an increase in the fraction of NiO species from 0.07 to 0.40.
Combined with the catalytic result that air-exposure of Ni/CeO2
leads to catalyst deactivation, these results indicate that the
surface metallic Ni species, as catalytically active species, are re-
oxidized by O2 to form inactive NiO species. The Ni–Ni coordi-
nation number increases from 7.5 to 8.9 with an increase in the
Ni loading from 5 wt% to 10 wt%, indicating that the size of Ni
metal particles increases with the loading. Considering the result
that the catalytic activity decreased with Ni loading (Fig. S1†), it
10a
11a
12
86
90
91
85
90
90
a T = 80 °C. b T = 80 °C, catalyst = 1.5 mol%. c T = 80 °C, catalyst =
1 mol%. d First run. e Second run. f Third run. g Fourth run. h Fifth run.
is shown that Ni metal with smaller particle size shows higher
activity. From these results, it is concluded that metallic Ni
species on small Ni particles is the active species. In Fig. 1 the
catalytic activity is plotted as a function of the number of surface
basic sites of Ni/MOx catalysts estimated by CO2-TPD
(Fig. S3†). There is a tendency that the activity increases with
the number of desorbed CO2 (number of surface basic sites), and
Ni/CeO2 as the most basic catalyst showed the highest activity.
The in situ IR spectrum of the adsorption complexes formed by
2984 | Green Chem., 2012, 14, 2983–2985
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Fig. 1 Rate for transfer hydrogenation of 2-octanone by Ni/MOx vs.
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ðCH3Þ2CHOH þ Ce–O ! ðCH3Þ2CH–O–Ce þ Ce–OH
In conclusion, we have developed Ni/CeO2-catalyzed hetero-
geneous transfer hydrogenation of aliphatic and aromatic ketones
under base free conditions as a clean, versatile, and economic
method for the reduction of ketones. The metallic Ni species on
the surface of small Ni particles is the active species and the
reaction was promoted by basic sites on the CeO2 support.
This work was supported by a “Grant for Advanced Industrial
Technology Development” in 2011 from the New Energy and
Industrial Technology Development Organization (NEDO) of
Japan. The X-ray absorption experiment was performed with the
approval of the Japan Synchrotron Radiation Research Institute
(Proposal No. 2010B1447).
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This journal is © The Royal Society of Chemistry 2012
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