Oxidant-free alcohol dehydrogenation using a reusable hydrotalcite- supported silver nanoparticle catalyst

Article abstract of DOI:10.1002/anie.200703161

Additive free: Hydrotalcite-supported silver nanoparticles act as a highly efficient heterogeneous catalyst for the dehydrogenation of diverse alcohols (see picture) in the absence of any acceptors such as molecular oxygen, hydrogen peroxide, or unsaturated organic compounds. The solid Ag catalyst was readily reusable without any loss of activity and selectivity. (Figure Presented).

Full text of DOI:10.1002/anie.200703161

Communications
DOI: 10.1002/anie.200703161
Heterogeneous Catalysis
Oxidant-Free Alcohol Dehydrogenation Using a Reusable
Hydrotalcite-Supported Silver Nanoparticle Catalyst**
Takato Mitsudome, Yusuke Mikami, Hisashi Funai, Tomoo Mizugaki, Koichiro Jitsukawa, and
Kiyotomi Kaneda*
The dehydrogenation of alcohols to carbonyl compounds is
one of the most important transformations in synthetic
tedious purification of products from the aqueous reaction
mixture; 2) it is tolerant toward alcohols having O -sensitive
2
[1]
chemistry. Recently, much attention has been paid to the
functional groups; 3) it produces H , which is an attractive
2
[
2]
[3]
development of homogeneous and heterogeneous cata-
lysts for the highly efficient oxidation of alcohols using
feedstock for energy generation; and 4) it suppresses over-
oxidation of the substrate to carboxylic acids. Despite
industrial success in the gas-phase dehydrogenation of
molecular oxygen (O ) as the oxidant because of the necessity
2
[5]
of replacing stoichiometric reactions involving toxic oxidants
such as chromium and manganese salts with environmentally
alcohols, few examples of milder liquid-phase catalyst
systems for the acceptor-free dehydrogenation of alcohols
have appeared to date, and these systems have often suffered
[
4]
benign catalytic oxidation systems [Eq. (1)]. Alternative
[6]
from low activity, limited substrate scope, and reusability.
Hydrotalcites (HTs) are inorganic, layered materials that
have attracted attention because of their potential not only
[
7]
[8]
[9]
for adsorption and drug delivery but also as catalysts.
Recently, we reported new strategies for the design of solid
catalysts by utilizing modified HTs for various organic
reactions such as the epoxidation of olefins using hydrogen
[
9e]
[9a]
peroxide, the oxidation of alcohols, and carbon–carbon
[9c]
bond formation.
acceptor-free dehydrogenation of alcohols using hetero-
We now describe the highly effective
[10]
designs can be conceived for advanced, more atom-efficient
catalyst systems, which do not use molecular oxygen or air as
the oxidant for the above oxidation [Eq. (2)]. Our develop-
geneous hydrotalcite-supported silver nanoparticles (Ag/
HT). This Ag/HT catalyst system showed remarkably high
catalytic activity for a wide range of alcohols. Moreover, the
catalyst could be readily reused without any loss of activity
and selectivity.
ment of such a promising O -free methodology is particularly
2
interesting both from a practical and an environmental point
of view because 1) it eliminates the formation of H O, a by-
product which often deactivates catalysts and necessitates
Hydrotalcite (HT, Mg Al (OH) CO ) was synthesized
according to literature procedures. To prepare the catalyst
.0 g of the HT was added to 10 mL of a 5.0 mm aqueous
2
6
2
16
11]
3
[
1
solution of AgNO , after which the heterogeneous mixture
was stirred for 1 h in air at 38C. The resulting slurry was
filtered, washed thoroughly with deionized water, and dried
3
[
*] Dr. T. Mitsudome, Y. Mikami, H. Funai, Dr. T. Mizugaki,
Prof. Dr. K. Jitsukawa, Prof. Dr. K. Kaneda
Department of Materials Engineering Science
Graduate School of Engineering Science
Osaka University
in vacuo at room temperature to yield a white powder. The
I
HT-supported Ag ions were reduced with H at 1108C for
2
1-3, Machikaneyama, Toyonaka, Osaka 560-8531 (Japan)
3
0 minutes to give Ag/HT before it was used in the
Fax: (+81)6-6850-6260
dehydrogenation. The positions of the peaks in the X-ray
diffraction (XRD) spectrum of Ag/HT after the above
pretreatment were similar to those of the parent HT. The
fraction of Ag atoms in the Ag/HT composite was determined
by elemental analysis to be 0.005 wt%. The transmission
electron microscopy image of the Ag/HT showed Ag nano-
E-mail: kaneda@cheng.es.osaka-u.ac.jp
Homepage: http://www.cheng.es.osaka-u.ac.jp/kanedalabo/
index_eng.html
Prof. Dr. K. Kaneda
Research Center for Solar Energy Chemistry
Osaka University
[
12]
1-3, Machikaneyama, Toyonaka, Osaka 560-8531 (Japan)
particles with a mean diameter of 3.3 nm.
[
**] This study was supported by a Grant-in-Aid for Scientific Research
from the Ministry of Education, Culture, Sports, Science, and
Technology of Japan (18360389). This work was also supported by a
Grant-in-Aid for Scientific Research on Priority Areas (no. 18065016,
When a mixture of 1-phenylethanol and Ag/HT in p-
xylene was heated at 1308C in an Ar atmosphere, quantitative
dehydrogenation of 1-phenylethanol occurred within 16 h to
afford acetophenone. The turnover frequency (TOF) and
“
Chemistry of Concerto Catalysis”) from the Ministry of Education,
À1
turnover number (TON) reached up to 1375 h and 22000,
Culture, Sports, Science, and Technology (Japan). Some of the
experiments were carried out at a facility in the Research Center for
Ultrahigh Voltage Electron Microscopy, Osaka University.
respectively (Table 1, entry 1). The reaction rate could be
accelerated under a stream of Ar, and the TOF significantly
À1
[13]
increased to 2000 h (entry 2). The mole ratio of H to
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
2
acetophenone generated in the dehydrogenation of 1-phenyl-
1
38
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2008, 47, 138 –141

Angewandte
Chemie
[
a]
Table 1: Dehydrogenation of various alcohols catalyzed by Ag/HT.
mixture was hot-filtered at 50%
conversion of the alcohol. Further
treatment of the solid-free solution
did not give any products. More-
over, no silver leached into the
filtrate, as confirmed by an induc-
tively coupled plasma analysis
Entry
Substrate
Product
t [h]
Conv. [%]
Sel. [%]
TON
1
2
16
11
>99
>99
>99
>99
22000
22000
[
b]
3
4
5
6
reuse 1
reuse 2
reuse 3
reuse 4
16
16
16
16
99
96
99
99
>99
>99
>99
>99
21800
21200
21800
21800
(detection limit: 0.007 ppm). It
thus became clear that dehydro-
genation took place only on the
surface of the Ag/HT catalyst.
7
8
12
15
>99
>99
99
98
22000
21800
The scope of the dehydrogen-
ation reaction using the Ag/HT
catalyst was explored next. The
results shown in Table 1 demon-
strate that this methodology can be
employed for the synthesis of var-
ious carbonyl compounds, includ-
ing benzylic and secondary ali-
phatic alcohols (entries 1–14). In
the case of a-cyclopropylbenzyl
alcohol, the dehydrogenation of
the alcohol moiety occurred with-
out cleavage of the cyclopropyl
ring (entry 7). The oxidation of
benzyl alcohol by the use of Ag/
HT showed moderate selectivity,
and benzyl benzoate was obtained
[
c]
9
10
14
>99
90
99
5900
1
0
1
98
21600
[
d]
d]
1
40
>99
98
11000
[
1
2
3
31
18
84
97
99
97
9200
1
21300
1
1
1
4
5
6
24
72
18
59
17
99
99
13000
600
[
c,d]
[13]
>99
>99
22000
as
a side product (entry 9).
Allylic alcohol was also tolerated,
thereby affording the correspond-
ing a,b-unsaturated carbonyl com-
pound (entry 16). Notably, this
dehydrogenation could also be car-
ried out using less reactive alicyclic
alcohols, such as cyclopentanol and
cyclohexanol (entries 17–20), as
[
d,e]
d,e]
1
1
1
2
7
8
9
0
22
21
15
24
>99
>99
>99
93
99
99
1100
1100
[
>99
>99
22000
20500
well as bulky alcohols, despite
their steric hindrance (entry 21).
Moreover, this oxygen-free dehy-
[
d,e]
2
1
96
83
>99
900
drogenation system was applicable
to heterocyclic alcohols containing
nitrogen atoms, and gave high
yields of the products (entries 22
and 23). Unfortunately, primary
[
c,d]
c,d]
2
2
3
24
30
99
98
99
3300
3300
[
2
>99
aliphatic alcohols such as 1-octanol
were not dehydrogenated effi-
ciently under the present condi-
tions (entry 15).
[
a] Reaction conditions: Ag/HT 0.10 g (Ag: 45 nmol), substrate (1 mmol), p-xylene (5 mL), 1308C, Ar
atmosphere. [b] Under a flow of Ar. [c] Substrate (0.3 mmol). [d] Ag/HT (0.20 g). [e] Substrate
0.1 mmol).
(
Furthermore, although various
catalyst systems have often been
ethanol was about 1:1, thus showing that H was generated
reported to suffer from the problem of decreasing reusability,
the Ag/HTrecovered after its first use in the dehydrogenation
of 1-phenylethanol was successfully used in four subsequent
reactions with no reduction in the reaction rate: the total
TON based on Ag was greater than 100000 (entries 3–6). This
value is significantly higher than the reported TONs at high
conversion for other oxidant-free catalyst systems, for exam-
2
quantitatively during the dehydrogenation. The use of other
heterogeneous catalysts such as Ag/SiO or Ag/TiO instead
2
2
of Ag/HT yielded only trace amounts of acetophenone. The
AgNO precursor and the parent HT displayed almost no
3
[14]
catalytic activity under similar reaction conditions.
To confirm whether the dehydrogenation of 1-phenyl-
ethanol proceeded on the solid catalyst surface, the reaction
[6a]
[6b]
ple, Ru/Al O (TON = 20), Ru/AlO(OH) (333), [Cp*Ir-
2
3
Angew. Chem. Int. Ed. 2008, 47, 138 –141
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
139

Communications
[
6c]
(
(
1
2
PyOH)] (Cp* = C Me ; 700),
[Ru(m-OCOC F OCO)-
In conclusion, we have created a highly atom-efficient and
reusable Ag/HT catalyst system for the heterogeneous
dehydrogenation of alcohols under oxidant- and additive-
free conditions. The Ag/HT afforded high yields of the
corresponding carbonyl compounds with coproduction of
equivalent molar amounts of H2.
5
5
2
4
CO)(H O)(binap)] (binap = 2,2’-bis(diphenylphosphanyl)-
2 2
[
6d]
,1’-binaphthyl; 186),
,6-bis-(di-tert-butylphosphinomethyl)pyridine; 160).
We compared Ag/HT, Ru/HT, and Pd/HT for the
and [(PNP)Ru(H)Cl(N )] (PNP =
2
[
6e]
oxidation of cinnamyl alcohol in an Ar atmosphere. Of
these, Ag/HT showed the highest chemoselectivity for the
dehydrogenation of cinnamyl alcohol to cinnamaldehyde
without hydrogen transfer or isomerization (Figure 1). More-
over, when 1-phenylethanol was oxidized by the Ag/HT
Received: July 6, 2007
Revised: September 26, 2007
Published online: November 23, 2007
Keywords: alcohols · dehydrogenation ·
.
heterogeneous catalysis · nanoparticles · silver
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1
40
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Angew. Chem. Int. Ed. 2008, 47, 138 –141

Angewandte
Chemie
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À1
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2
3
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141