Copper nanoparticles on hydrotalcite as a heterogeneous catalyst for oxidant-free dehydrogenation of alcohols

Article abstract of DOI:10.1039/b809012b

We have developed a highly efficient heterogeneous catalytic system using hydrotalcite-supported Cu nanoparticles (Cu/HT) that can successfully promote the oxidant-free dehydrogenation of various alcohols under liquid-phase conditions. The Royal Society of Chemistry.

Full text of DOI:10.1039/b809012b

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Copper nanoparticles on hydrotalcite as a heterogeneous catalyst
for oxidant-free dehydrogenation of alcoholsw
Takato Mitsudome,a Yusuke Mikami,a Kaori Ebata,a Tomoo Mizugaki,a
Koichiro Jitsukawaa and Kiyotomi Kaneda*ab
Received (in Cambridge, UK) 28th May 2008, Accepted 8th July 2008
First published as an Advance Article on the web 12th August 2008
DOI: 10.1039/b809012b
We have developed a highly efficient heterogeneous catalytic
system using hydrotalcite-supported Cu nanoparticles (Cu/HT)
that can successfully promote the oxidant-free dehydrogenation
of various alcohols under liquid-phase conditions.
compound (CuII/HT) as a blue powder. This CuII/HT was
dried in vacuo at 180 1C for 0.5 h and then reduced with 1 atm
of molecular hydrogen (H2) at 180 1C for 0.5 h, which afforded
HT-supported Cu catalyst (Cu/HT) as a black powder. The
formation of a Cu(0) species was confirmed (Fig. 1S, ESIw)
from the Cu K-edge X-ray absorption fine-structure spectrum
(XAFS). Cu/MgO, Cu/Al2O3, Cu/SiO2, Cu/TiO2 and
Cu/Hydroxyapatite (HAP) were also prepared by similar
procedures.
Alcohol oxidation is a key reaction in both academic and
industrial synthetic chemistry. Recently, oxidant-free alcohol
dehydrogenation1 has received considerable attention because
it has the advantages of being able to prevent overoxidation of
aldehyde functionalities and also of eliminating the formation
of water as a by-product, which could lead to catalyst deac-
tivation and the hydrolysis of ester and ether moieties in the
substrate. We recently developed Ag nanoparticles on hydro-
talcite (Ag/HT)2,3 as a heterogeneous and reusable catalyst for
the efficient oxidant-free dehydrogenation of various alcohols.
The activity of Ag/HT is comparable to those of previously
reported catalysts such as Pd,4 Pt,5 Ru6 and Au,7 which have
achieved high performance for various alcohols using mole-
cular oxygen as the oxidant. From ecological and practical
points of view, the development of a reusable and less
expensive base-metal catalyst is highly desirable.
The Cu loading of Cu/HT is 4.6 wt% from elemental
analysis. The mean diameter and standard variation of the
Cu particles were, respectively estimated to be 7.5 and 4.3 nm
from transmission electron microscopy (Fig. 2S, ESIw).
Initially, the oxidation of cyclooctanol to cyclooctanone
using Cu/HT was examined as a model reaction. The dehy-
drogenation occurred under Ar atmosphere at 130 1C for 3 h,
giving cyclooctanone in 99% yield together with the genera-
tion of H2 (Table 1, entry 1). An almost equimolar amount of
H2 to cyclooctanone was confirmed during the course of the
dehydrogenation. The use of other inorganic materials sup-
porting Cu such as Cu/MgO, Cu/Al2O3, Cu/HAP, Cu/TiO2,
and Cu/SiO2 also gave high yields of cyclooctanone (entries
2–6). Further screening of the above Cu catalysts using
2-octanol revealed that HT is the best support (entries 7–12).
We next investigated the catalytic activity of Cu/HT in the
dehydrogenation of various alcohols as summarized in Table 2.
Aliphatic and benzylic secondary alcohols were smoothly oxi-
Herein, we reveal that Cu nanoparticles grafted on hydro-
talcite (Cu/HT) exhibit high catalytic activity for the dehy-
drogenation of various alcohols, including less reactive
cyclohexanols, without using any oxidant additives under
liquid phase conditions. This Cu/HT catalyst has the advan-
tages of eliminating the need to use high temperatures,8
additives,9 and high catalyst loading10 that have plagued
previously reported Cu catalysts. Moreover, Cu/HT is reus-
able while maintaining both high activity and selectivity.
The HT (1.0 g) was synthesized by the co-precipitation
method according to literature procedures.11 The HT (1.0 g)
was added to a solution of 0.8 mmol of copper(II) trifluor-
omethanesulfonate in 50 mL of deionized water, then the pH
was adjusted to 8.0 with a solution of ammonia (25 wt%).
After stirring the mixture for 1 h at 25 1C, the obtained solid
was filtered, washed with deionized water, and dried in vacuo
overnight at 25 1C, yielding the HT-supported copper(II)
Table 1 Dehydrogenation of alcohols using various Cu catalystsa
Entry
Substrate
Catalystb
t/h Conv.c (%)
Sel.c (%)
1
2
3
4
5
6
Cu/HT
3
6
499
83
89
82
86
499
499
93
99
99
Cu/MgO
Cu/Al2O3
Cu/HAP
Cu/SiO2
Cu/TiO2
82
499
7
8
9
Cu/HT
97
59
96
43
56
42
99
93
93
93
98
88
Cu/MgO
Cu/Al2O3
Cu/HAP
Cu/SiO2
Cu/TiO2
a Department of Materials Engineering Science, Graduate School of
Engineering Science, Osaka University, 1-3 Machikaneyama,
Toyonaka, Osaka 560-8531, Japan.
10
11
12
a
E-mail: kaneda@cheng.es.osaka-u.ac.jp; Fax: +81-6-6850-6260;
Tel: +81-6-06-6850-6260
Reaction conditions: alcohol (1.0 mmol), Cu catalyst (Cu: 7.3 mol%),
p-xylene (5 mL), Ar. b Cu/HT (4.6 wt%), Cu/MgO (5.0 wt%), Cu/Al2O3
(4.9 wt%), Cu/SiO2 (4.7 wt%), Cu/TiO2 (4.9 wt%), Cu/HAP
b Research Center for Solar Energy Chemistry, Osaka University,
1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan
w Electronic supplementary information (ESI) available: XAFS and
TEM analysis. See DOI: 10.1039/b809012b
c
(5.3 wt%). Determined by GC using an internal standard technique.
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Table 2 Alcohol dehydrogenation catalyzed by Cu/HTa
Conv.b Sel.b
Table 2 (continued )
Conv.b Sel.b
Entry Substrate
26
Product
T/1C
t/h
(%)
(%)
Entry Substrate
Product
T/1C
t/h
(%)
(%)
130
12
19
47
1
130
3
499
499
a
Reaction conditions: alcohol (1.0 mmol), Cu/HT (7.3 mol%),
b
p-xylene (5 mL), Ar. Determined by GC using an internal standard
2
3
1st reuse
2nd reuse
130
130
3
3
99
99
94
96
c
technique. Aldol product was formed as a by-product. Alcohol
d
e
(0.5 mmol), Cu/HT (14 mol%). Cu/HAP (14 mol%). Mesitylene
f
g
h
i
4
130
4
499
93
(5 mL). Alcohol (0.20 mmol). Alcohol (0.3 mmol). Benzyl benzo-
ate was formed as a by-product.
5
6
130
140
3
99
99
33c
94
18
dized to give the corresponding ketones in high yields, and the
Cu/HT was also effective for a bulky alcohol and a heterocyclic
alcohol including a nitrogen atom (entries 20 and 23). Unfortu-
nately, the dehydrogenation of primary alcohols gave the
corresponding aldehydes in moderate yields (entries 24 and
26). In the case of benzyl alcohol, the yields of benzaldehyde
were not improved, despite prolonging the reaction time, due to
the formation of an ester of benzyl benzoate (entry 25). It is
notable that alicyclic alcohols showed high reactivity for the
dehydrogenation (entries 1–20) except for cyclopentanol where
an aldol adduct of 2-cyclopentylcyclopentanone was formed as
a by-product caused by the basic sites of HT (entry 5). Further,
cyclohexanol, which is known to be a less reactive substrate
even for the oxidation by precious metals such as Pd and Ru
under molecular oxygen, was smoothly converted to give
cyclohexanone in high yield (entry 6). Moreover, Cu/HT was
7d
8e
140
140
8
86
95
80
24
499
9d
10e
140
150
5
84
82
90
95
24
11d
12e
140
150
5
72
83
93
12
499
13d
14e
140
150
8
94
93
88
24
499
broadly applicable to reactions with
a wide range of
15d
16e
160
150
6
96
98
72
85
cyclohexanol derivatives and only small amounts of aldol
condensation products were generated. The use of neutral
HAP supporting Cu (Cu/HAP) instead of Cu/HT achieved
high selectivity for ketones while suppressing the formation of
the aldol products (entries 8, 10, 12, 14, 16, 18).
24
17d
18e
150
150
9
9
99
95
97
98
This Cu/HT catalyst could also function in alcohol dehy-
drogenations under neat conditions (Scheme 1). A 100 mmol-
scale dehydrogenation of cyclooctanol without solvent gave an
85% yield of cyclooctanone. The turnover number (TON)
reached 1164 after 3 h [turnover frequency (TOF): 388 hÀ1].
These TON and TOF values are remarkably higher than those
reported for other heterogeneous Cu catalyst systems with or
without hydrogen acceptors.12
19f
150
24
99
94
20fg
150
17
84
96
21
22
130
130
6
3
97
98
99
99
After about 50% conversion of cyclooctanol, Cu/HT was
filtered off at the reaction temperature, and further stirring of
the filtrate under similar reaction conditions did not afford any
dehydrogenation products. No Cu ions in the filtrate were
detected by inductively coupled plasma (ICP) analysis (detec-
tion limit: 0.054 ppm). These results reveal that the present
23
130
17
84
96
24
130
130
9
60
85
92
25h
13
69i
Scheme 1 A 100 mmol-scale dehydrogenation of cyclooctanol under
neat conditions.
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alcohol dehydrogenation clearly took place on the Cu nano-
particles located on the HT surface. Furthermore, Cu/HT
could be recycled twice in the dehydrogenation of cyclooctanol
with retention of its activity and selectivity, giving 93% yield
of cyclooctanone on the 1st reuse, and 95% on the 2nd reuse.
In conclusion, we have shown that Cu nanoparticles grafted
on HT are highly active heterogeneous catalysts for acceptor-
free dehydrogenation of various alcohols including less reactive
cyclohexanol derivatives. This inexpensive base-metal catalyst
was easily reusable and effective under neat conditions.
This work was carried out with partial support from the
Grant-in-Aid for Scientific Research from the Ministry of
Education, Culture, Sports, Science, and Technology of
Japan, and from the Grant-in-Aid for Scientific Research on
Priority Areas (No. 18065016, ‘‘Chemistry of Concerto Cata-
lysis’’) from the Ministry of Education, Culture, Sports,
Science, and Technology of Japan. TEM experiments were
carried out by using a facility in the Research Center for
Ultrahigh Voltage Electron Microscopy, Osaka University.
We thank Dr. Uruga and Dr. Tanida (Spring-8) for XAFS
measurements. One of the authors (Y. M.) expresses his special
thanks to The Global Center of Excellence Program ‘‘Global
Education and Research Center for Bio-Environmental
Chemistry’’ of Osaka University.
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4806 | Chem. Commun., 2008, 4804–4806