Gold nanoparticles on hydrotalcites as efficient catalysts for oxidant-free dehydrogenation of alcohols

Article abstract of DOI:10.1039/b923047e

Hydrotalcite-supported gold nanoparticles with sizes of less than 5 nm are highly efficient and reusable catalysts for the oxidant-free dehydrogenation of alcohols to carbonyl compounds and hydrogen. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/b923047e

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Gold nanoparticles on hydrotalcites as efficient catalysts for
oxidant-free dehydrogenation of alcoholsw
Wenhao Fang, Qinghong Zhang,* Jing Chen, Weiping Deng and Ye Wang*
Received (in Cambridge, UK) 3rd November 2009, Accepted 6th January 2010
First published as an Advance Article on the web 27th January 2010
DOI: 10.1039/b923047e
Hydrotalcite-supported gold nanoparticles with sizes of less than
5 nm are highly efficient and reusable catalysts for the oxidant-
free dehydrogenation of alcohols to carbonyl compounds and
hydrogen.
dehydrogenation reactions although a few studies have shown
that supported Au could catalyse the decomposition of
ammonia or formic acid or the dehydrogenation of propane
in the presence of H2 and O2.1b,e,8,9 Herein, we demonstrate
that the supported Au nanoparticles are highly active and
selective catalysts for the oxidant-free dehydrogenation of
various alcohols. We have clarified that the size of Au nano-
particles plays a pivotal role in this dehydrogenation catalysis.
Table 1 shows the catalytic performances of Au catalysts
loaded on various supports, which have been prepared by an
impregnation method, followed by H2 reduction at 250 1C (see
the ESIw for experimental details), for the dehydrogenation of
benzyl alcohol in the absence of any oxidant or hydrogen
acceptor at 120 1C. Among these supported Au catalysts, the
Au/hydrotalcite (Au/HT) exhibited the highest activity and
selectivity. Au/Al2O3 possessed relatively higher activity but
slightly lower selectivity, while Au/MgO possessed relatively
higher selectivity but lower activity. Other supported Au
catalysts were much less active for the dehydrogenation of
benzyl alcohol (conversion o10%). We have also compared
the catalytic performance of the Au/HT with those of the
Ag/HT and Cu/HT, because the latter two catalysts have been
reported to be efficient for the dehydrogenation of alcohols.5
Under the same reaction conditions, the Au/HT exhibits a
better performance than the Ag/HT and Cu/HT (Table 1).
Highly dispersed gold species have been demonstrated to be
active catalysts in many hydrogenation and oxidation
reactions such as hydrogenation of alkenes and aldehydes,
CO oxidation, propylene epoxidation, hydrogen peroxide
synthesis, and aerobic oxidation of alcohols.1 For some
reactions, supported gold catalysts even show performances
superior to those of platinum group metal catalysts. It is
widely accepted that the size of gold plays a dominating role
in catalysis although many other factors such as the chemical
state of gold (metallic, cationic or anionic), the morphology of
gold particles, and the interaction between gold and a support
may also influence the catalytic behaviours. Gold catalysis has
become an exciting research area, showing magic changes of
catalytic behaviours with size of particles.
On the other hand, the dehydrogenation of alcohols to
carbonyl compounds is one of the most essential transforma-
tions in organic synthesis. A number of heterogeneous
catalysts including supported Au nanoparticles have been
developed for the selective oxidation of alcohols by O2 to
replace the stoichiometric reactions using dichromate or
permanganate oxidants.2,3 From the viewpoints of safety
and avoiding possible over-oxidation of products, the oxidant-
free dehydrogenation of alcohols to aldehydes or ketones and
H2 is a more desirable route. A few platinum group catalysts,
especially Ru complexes and supported Ru catalysts, could
catalyse the dehydrogenation of alcohols in the absence of any
oxidants or hydrogen acceptors.4 Recently, it has been
disclosed that supported Ag and Cu nanoparticles can also
catalyse the oxidant-free dehydrogenation of alcohols with
good efficiency.5,6 To date, only one report has mentioned
the Au-catalysed oxidant-free dehydrogenation of benzyl
alcohol to benzaldehyde, but toluene was formed with a high
selectivity (B47%) over the catalyst (Au–Pd/TiO2) with a high
activity and the Au/TiO2 alone gave a very low activity.7
Moreover, there is little knowledge about the Au-catalysed
Table 1 Catalytic performances of Au catalysts loaded on various
supports (Au loading, 0.1 wt%) and HT-supported Ag and Cu
catalysts (Ag and Cu loadings, 0.1 wt%) for the oxidant-free
dehydrogenation of benzyl alcohola
Mean size
of metal
Selectivity/%
particlesb/nm Conv./%
Aldehyde Toluene Benzene
Catalyst
Au/Cab-O-Sil 9.0
0.5
0.4
4.6
1.5
1.4
5.3
4.6
4.3
37
59
61
14
51
47
53
95
90
88
97
97
84
499
96
499
85
40
29
13
18
Au/SBA-15
Au/CNTc
Au/TiO2
Au/ZrO2
Au/La2O3
Au/CeO2
Au/HAPe
Au/MgO
Au/Al2O3
Au/HT
4.3
8.2
5.8
18
4.8
6.5
9.3
1.8
2.4
1.9
0.4
2.9
0.3
14
0.7
3.2
2.5
0.8
0.9
1.0
0.2
0.5
0
n.d.d
8.7
28
11
5.1
8.0
n.d.
n.d.
State Key Laboratory of Physical Chemistry of Solid Surfaces,
National Engineering Laboratory for Green Chemical Productions of
Alcohols, Ethers and Esters, College of Chemistry and Chemical
Engineering, Xiamen University, Xiamen 361005, China.
E-mail: zhangqh@xmu.edu.cn, wangye@xmu.edu.cn;
Fax: +86-592-2183047; Tel: +86-592-2186156
w Electronic supplementary information (ESI) available: Experimental
details, TEM micrographs and particle size distributions for supported
Au catalysts and recycling uses of the Au/HT. See DOI: 10.1039/b923047e
Cu/HT
Ag/HT
1.0
0.2
499
0.3
a
Reaction conditions: catalyst (prepared by impregnation), 0.20 g;
T = 120 1C; benzyl alcohol, 1.0 mmol; p-xylene, 5.0 cm3; Ar,
3 cm3 minÀ1; time, 6 h. See Fig. S1 in the ESIw for TEM micrographs
and particle size distributions. CNT denotes carbon nanotube. Not
e
detected. HAP denotes hydroxyapatite.
b
c
d
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Recently, Shimizu et al.6 reported that the nature of the
support played a key role in the Ag-catalysed dehydrogenation
of 4-methylbenzyl alcohol and they proposed that the
acid–base bifunctional support (e.g., Al2O3) was superior to
the basic or the acidic supports. Our present results also
suggest that the choice of a proper support is important for
obtaining high activity and selectivity for the Au-catalysed
oxidant-free dehydrogenation of benzyl alcohol. Future
studies are needed to clarify the functions of the support in
our Au-catalysed dehydrogenation of alcohols.
It can be expected that the size of Au nanoparticles may
significantly affect the catalytic performances. We have
performed TEM measurements for some typical catalysts
listed in Table 1. The mean size of Au nanoparticles for the
Au/HT was 8.0 nm, which was larger than those for the
Au/SBA-15, Au/TiO2 and Au/Al2O3, and was similar to those
for the Au/Cab-O-Sil, Au/CNT and Au/CeO2 (Table 1). By
using a deposition–precipitation (DP) method (see the ESIw
for details), we have succeeded in preparing Au/HT catalysts
with smaller Au nanoparticles. TEM observations for the
0.06 wt% Au/HT prepared by the DP method showed that
the Au nanoparticles were 1.5–4.5 nm in size and the mean size
of Au was calculated to be 2.8 nm by counting B200 Au
nanoparticles (Fig. 1). This catalyst gave a significantly higher
benzyl alcohol conversion (94%) under the reaction conditions
of Table 1. We have investigated the catalytic performances of
the Au/HT catalysts prepared by the DP method with different
Au loadings. HTs without Au could not catalyse the
dehydrogenation of benzyl alcohol, suggesting that Au was
the active phase. The variation of Au loadings between
0.03 and 12 wt% provided variable benzyl alcohol conversions
between 34 and 499%, but consistently high benzaldehyde
selectivity (499%). We observed a unique dependence of
benzyl alcohol conversion on the Au loading (Fig. 2). The
conversion first increased to 94% as the Au loading rose to
0.06 wt%, and then underwent a decrease with an increase in
Au loadings from 0.06 to 0.26 wt%. However, further
increases in Au loadings caused increases in benzyl alcohol
conversions again, and the catalyst with an Au loading of
12 wt% afforded a conversion of 499%.
Fig. 2 Dependence of benzyl alcohol conversion on Au loadings for
the Au/HT catalysts prepared by the DP method. Reaction conditions:
catalyst, 0.20 g; T = 120 1C; benzyl alcohol, 1.0 mmol; p-xylene,
5.0 cm3; Ar, 3 cm3 minÀ1; time, 6 h.
Fig.
3 TEM micrographs for the 0.26 and 12 wt% Au/HT
catalysts prepared by the DP method. (a) 0.26 wt% Au/HT,
(b) 12 wt% Au/HT.
exist over the 0.26 wt% Au/HT catalyst (Fig. 3a), indicating the
aggregation of Au nanoparticles over this sample. The mean size
of Au particles was calculated to be 13.6 nm. On the other hand,
the 12 wt% Au/HT catalyst exhibited a bimodal size distri-
bution; smaller Au nanoparticles (r4 nm) could also be
observed besides the larger Au particles (Fig. 3b). The mean
size of the smaller Au nanoparticles was 2.8 nm. We still cannot
make a straightforward interpretation of this phenomenon.
Combining this unique observation with the catalytic behaviours
shown in Fig. 2, we propose that the smaller Au nanoparticles
(o5 nm) are more active than the larger Au particles (45 nm).
We have performed detailed studies for the 0.06 wt%
Au/HT catalyst prepared by the DP method, which possesses
only smaller Au nanoparticles (1.5–4.5 nm, Fig. 1) and affords
a higher benzyl alcohol conversion. Fig. 4 shows the time
course for benzyl alcohol conversion. Benzaldehyde selectivity
was always 498%. Benzyl alcohol conversion increased
almost linearly with time and reached 490% after 6 h of
reaction. Accompanying the conversion of benzyl alcohol to
benzaldehyde, H2 was formed, and the formation of H2
also increased almost linearly with the reaction time.
The calculation revealed that the molar ratio of H2 to benz-
aldehyde produced in 6 h was about 0.95 : 1, showing that H2
To understand the unique dependence of catalytic
performances on Au loadings, we performed further TEM
measurements for the samples with Au loadings of 0.26 and
12 wt%. It is of interest that only larger Au particles (45 nm)
Fig. 1 TEM micrograph and Au particle size distribution for the
0.06 wt% Au/HT catalyst prepared by the DP method.
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the dehydrogenation of benzyl alcohol. No Au can be detected
in the filtrate after the reaction. The recycling uses of this
sample showed no significant decreases in benzyl alcohol
conversion and benzaldehyde selectivity (see Fig. S2, ESIw).
Thus, the present Au/HT catalyst could be used repeatedly.
We have further examined the catalytic behaviours of the
0.06 wt% Au/HT catalyst prepared by the DP method for the
oxidant-free dehydrogenation of various alcohols. The results
in Table 2 show that the present catalyst is effective for
the dehydrogenation of benzylic alcohols with different
substituents (entries 1–7), alicyclic alcohols (entries 8 and 9)
and heterocyclic alcohols containing
a nitrogen atom
(entry 10). The present catalyst could also catalyse the
dehydrogenation of less active linear aliphatic alcohols
although the efficiency was lower (entries 11 and 12).
In summary, we have demonstrated that the HT-supported
Au nanoparticles can efficiently catalyse the oxidant-free
dehydrogenation of alcohols to the corresponding carbonyl
compounds. The size of the Au nanoparticles plays a key role
in the dehydrogenation reaction. The present Au/HT catalyst
can be used repeatedly, and can be applied to the synthesis of
various carbonyl compounds.
Fig. 4 Time course for the dehydrogenation of benzyl alcohol over
the 0.06 wt% Au/HT catalyst prepared by the DP method. (a) Benzyl
alcohol conversion, (b) benzaldehyde selectivity, (c) H2 produced.
Reaction conditions: catalyst, 0.20 g; T = 120 1C; benzyl alcohol,
1.0 mmol; p-xylene, 5.0 cm3; N2, 10 cm3 minÀ1
.
Table 2 Catalytic behaviours of the 0.06 wt% Au/HT catalyst
prepared by the DP method for dehydrogenation of various alcoholsa
This work was supported by the NSFC (Nos. 20625310,
20773099 and 20873110), the National Basic Research
Program of China (Nos. 2010CB732303 and 2005CB221408)
and the Key Scientific Project of Fujian Province of China
(2009HZ0002-1).
Carbonyl
compound
Time/h Conversion/% selectivity/%
Entry Substrate
1
Notes and references
9
9
499
499
499
97
499
499
499
499
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was generated stoichiometrically during the dehydrogenation
of benzyl alcohol.
We confirmed that no leaching of Au from the 0.06 wt%
Au/HT catalyst prepared by the DP method occurred during
ꢀc
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Chem. Commun., 2010, 46, 1547–1549 | 1549