Promoting gold nanocatalysts in solvent-free selective aerobic oxidation of alcohols

Article abstract of DOI:10.1039/b706864f

A trace amount of metal carbonate, acetate or borate significantly boosts gold nanocatalysts in selective aerobic oxidation of alcohols under mild solvent-free conditions. The Royal Society of Chemistry.

Full text of DOI:10.1039/b706864f

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Promoting gold nanocatalysts in solvent-free selective aerobic oxidation
of alcohols{
Nanfeng Zheng and Galen D. Stucky*
Received (in Berkeley, CA, USA) 8th May 2007, Accepted 22nd June 2007
First published as an Advance Article on the web 10th July 2007
DOI: 10.1039/b706864f
however, when a small amount of K2CO3/Na2CO3 (see below) is
added (Table 1, entries 2 and 3). For example, the addition of
K2CO3 increases the conversion of benzyl alcohol from 2.5 to
76.6% with benzaldehyde selectivity of 50.9%.
A trace amount of metal carbonate, acetate or borate
significantly boosts gold nanocatalysts in selective aerobic
oxidation of alcohols under mild solvent-free conditions.
Supported gold nanoparticles have been recently extensively
studied as catalysts for a wide range of oxidation reactions
including low-temperature CO oxidation,1–4 alkene epoxidation,5,6
aldehyde oxidation,7–9 and aerobic oxidation of alcohols in both
gas- and liquid-phase under relatively mild conditions.9–23 Among
noble metal nanoparticle catalysts used in the liquid-phase aerobic
oxidation of alcohols, properly sized supported gold nanoparticles
are the most selective catalysts, the least prone to leaching due to
over-oxidation of the metal active sites, and not easily poisoned
through ligand chelating.11,20,23
Carbonates (e.g., K2CO3, Na2CO3) are widely applied as weak
bases in many organic syntheses, particularly in reactions involving
proton extraction. In most of these syntheses, a stoichiometric
excess of carbonate is required to achieve high reaction efficiencies.
In the gold-nanoparticle-catalyzed oxidation reactions where we
use a carbonate, the molar ratios of carbonate/alcohol were,
however, much lower. The promotional effect is effective even if a
trace amount of carbonate, with a K2CO3/alcohol molar ratio of
1 6 1023, is used. Increasing the amount of K2CO3 does not
linearly increase the alcohol conversion (Fig. 1).
The application of supported gold nanoparticles in liquid-phase
alcohol oxidation under mild conditions typically requires an
aqueous alkaline reaction medium (e.g., excess NaOH, K2CO3),
which results in carboxylate products.11–13 The catalytic activity by
oxide-supported gold nanoparticles under solvent-free conditions
at temperatures less than 100 uC has been demonstrated but is still
limited.14–16 Furthermore, the catalytic performance of oxide-
supported gold nanoparticles is very sensitive to the size of the gold
nanoparticles and also the physical and chemical nature of the
metal oxide support.15–18
Fig. 2 shows the aerobic oxidation of benzyl alcohol as a
function of time with the additions of K2CO3 promoter. The
promotional effect by carbonates is maximized at the beginning of
promoter additions (i.e., at 0 and 75 h) and decays with reaction
time. The first addition of K2CO3 (1 mol% to alcohol) leads to
alcohol conversion up to y38% within 2 h. Since the produced
benzaldehyde can be further oxidized to form benzoic acid by-
product which reacts with K2CO3, a decay of reaction rate is
observed. However, a further increase in the alcohol conversion is
observed when the second K2CO3 addition is applied. In addition,
it is worth noting that aldehyde is still the key product for the
reactions promoted by a small amount of carbonates.
Here we report a methodology to promote oxide-supported
gold nanocatalysts in solvent-free selective aerobic oxidation of
alcohols. The significant promotional effect is achieved by a
catalytic amount of low-cost promoters such as metal carbonates,
acetates or borate, which is applicable to all oxide-supported gold
nanoparticles. A turnover-frequency (TOF) as high as 25030 h21
has been obtained in the aerobic selective oxidation of alcohols
under solvent-free conditions at 100 uC. By choosing appropriate
promoters, product selectivity and alcohol conversion have been
simultaneously improved.
Without the promoter, the catalytic performance of oxide-
supported gold nanoparticles is highly dependent on the metal
oxide support. To study the effect of metal-oxide supports with the
use of K2CO3, we deposited the same-sized (6.3 nm) gold
nanoparticles on different metal oxides ranging from semiconduct-
ing (i.e., TiO2, ZnO), acidic (i.e., SiO2, zeolite), to basic (i.e., Co3O4,
ZnO, MgO) oxides. With the help of a small amount of promoter
(e.g., K2CO3), all of the prepared oxide-supported gold nanopar-
ticles efficiently catalyze the oxidation of alcohols under mild
conditions (Table 1, entries 10–15).
Oxide-supported gold nanoparticle catalysts were prepared by
the general strategy that we have recently developed.22 In the
absence of a base (e.g., K2CO3, Na2CO3), these oxide-supported
gold nanoparticles do not efficiently catalyze the selective oxidation
of alcohols under mild solvent-free conditions (Table 1, entry 1),
which is consistent with previous reports.14–16 At 100 uC and 2 atm
O2, the conversion of benzyl alcohol (10 mL) is only 2.5%. A
dramatic improvement in the alcohol conversion is observed,
Recently, Hutchings and co-workers demonstrated that Au/Pd-
TiO2 catalysts exhibit very high activity of alcohol oxidation under
mild solvent-less conditions.14 For solvent-free oxidation of benzyl
alcohol, the reported Au/Pd-TiO2 catalyst gives a TOF of 6440 h21
(measured from the first 0.5 h of reaction) at 100 uC and 2 atm O2.
With the use of low-cost promoting agent (i.e., K2CO3) instead of
Pd and under similar catalysis conditions, we have found that
oxide-supported pure gold nanoparticles (e.g., Au–TiO2) can
catalyze the oxidation of alcohols even more efficiently (Table 1,
entry 16) with a TOF of 7851 h21 (based on a 5 h reaction) at
100 uC. It should be pointed out that a TOF of 25030 h21 is
Department of Chemistry and Biochemistry, University of California,
Santa Barbara, California, 93106, USA. E-mail: stucky@chem.ucsb.edu;
Fax: (+1) 805-893-4120; Tel: (+1) 805-893-4872
{ Electronic supplementary information (ESI) available: Experimental
section, gold particle size distribution before and after catalysis. See DOI:
10.1039/b706864f
3862 | Chem. Commun., 2007, 3862–3864
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Table 1 Selected catalytic results of selective oxidation of alcohols by using supported gold nanoparticles as catalysts
Au/alcoholc
Promoter/
alcohol
Conversion
(%)
Selectivityd
(%)
Entrya
Alcohol
Catalystb
(6 1026
)
Promoter
TOFe/h21
1
2
3
4
5
6
7
8
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Ethanol
2.5% on TiO2
2.5% on TiO2
2.5% on TiO2
2.5% on TiO2
2.5% on TiO2
2.5% on TiO2
2.5% on TiO2
2.5% on TiO2
2.5% on TiO2
2.5% on TiO2
2.5% on ZnO
2.5% on Zeolite
2.5% on SiO2
2.5% on Co3O4
2.5% on MgO
2.5% on TiO2
2.5% on TiO2
2.5% on TiO2
2.5% on TiO2
2.5% on TiO2
2.5% on TiO2
2.5% on TiO2
2.5% on TiO2
262
262
262
131
131
131
131
131
131
131
131
131
131
131
131
2.6
None
K2CO3
Na2CO3
NaOH
NEt3
NaCH3COO
KCH3COO
Na2B4O7
K2B4O7
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Co(CH3COO)2
CoCl2
K2CO3
NA
0.10
0.10
0.02
0.02
0.01
0.01
0.005
0.005
0.01
0.01
0.01
0.01
0.01
0.01
0.002
0.01
0.01
0.1
2.5
76.6
75.3
9.3
.99.9
50.9
51.6
58.1
90.5
74.6
72.9
80.4
82.1
55.0
81.5
65.8
59.3
55.7
58.0
78.7
93.8
19
584
574
142
137
648
620
584
604
534
581
485
512
1003
723
7851
352
29
8.7
42.5
40.7
38.3
39.6
35.0
38.1
31.8
33.6
65.8
47.4
10.3
46.2
3.8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
a
f
262
262
74
95
116
97
.99.9
.99.9
.99.9
.99.9
.99.9
.99.9
b
38.0
35.1
58.9
15.3
64.8
1026
739
1014
315
643
1-Propanol
1-Butanol
2-Propanol
2-Octanol
K2CO3
K2CO3
K2CO3
K2CO3
0.1
0.1
0.1
0.1
202
Catalysis conditions: solvent-free reactions; 100 uC for entries 1–18, and 80 uC for entries 19–23; 2 atm oxygen; 5 h. All are based on 6.3 nm
c
d
gold nanoparticles. Calculated for all gold atoms, both on the surface and in the core of the nanoparticles. Selectivity to benzaldehyde for
entries 1–18, ethyl acetate for entry 19, propyl propionate for entry 20, butyl butanoate for entry 21, acetone for entry 22, and 2-octanone for
entry 23. TOF calculations were based on the analysis at the end of 5 h reactions. CBV-600 Zeolite from Zeolyst International.
e
f
obtained for our reaction when the calculation is based on the 0.5 h
reaction that the Hutchings group used.14 In contrast to the more
complicated Au/Pd–TiO2 catalysts which are inactive for the
oxidation of 2-octanol, pure supported gold nanoparticles are
highly active with the help of a small amount of K2CO3 (Table 1,
entry 23).
(i.e., triethylamine) bases (Table 1, entries 4 and 5) does not lead to
the promotional effect as prominent as that from carbonates,
acetates and borates. The result suggests that basicity is necessary
and the nature of applied bases is critical in promoting alcohol
conversion.
For the oxidation of secondary alcohols by supported gold
nanoparticles, the only products are ketones (Table 1, entries 22
and 23). The catalytic oxidation of primary alcohols produces the
corresponding aldehydes or/and esters. The selectivity of the
primary alcohol oxidation depends largely on the oxidative
stability of produced aldehydes. For example, esters are the only
products from the oxidation of ethanol, 1-propanol, 1-butanol at
In addition to alkaline carbonates, we found that incorporation
of a small amount of acetate (i.e., NaCH3COO, KCH3COO,
Table 1, entries 6–7) and borate (i.e., Na2B4O7?10H2O,
K2B4O7?10H2O, Table 1, entries 8 and 9) also significantly
promote the reactions under mild conditions. In comparison, the
substitution of carbonates with stronger (i.e., NaOH) or organic
Fig. 2 The catalytic kinetics of benzyl alcohol oxidation with the
additions of K2CO3. Catalysis conditions: 0.250 g of 2.5% 6.3 nm Au on
TiO2; 20 mL benzyl alcohol; 2 atm O2; 0.280 g K2CO3 were added at 0 and
75 h.
Fig. 1 The effect of amount of K2CO3 on the gold-catalyzed aerobic
oxidation of benzyl alcohol. Catalysis conditions: 0.200 g of 2.5% 6.3 nm
Au on TiO2; 20 mL benzyl alcohol; 2 atm O2; 5 h.
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alcohol gives both benzaldehyde and benzyl benzoate. For
example, the use of NaCH3COO gives a benzaldehyde selectivity
of 74.6% (Table 1, entry 6). With the use of Co(CH3COO)2?4H2O
instead of NaCH3COO, however, a much higher selectivity to
aldehyde (93.8%) has been achieved together with the high
efficiency (TOF = 353 h21) at 100 uC (Table 1, entry 17). Such
an improvement in both alcohol conversion and product selectivity
is not achievable by CH3COO2 or Co2+ alone (Table 1, entries 6
and 18).
In conclusion, we have discovered that low-cost and non-toxic
promoters dramatically boost the catalytic activity of gold
nanocatalysts in the selective oxidation of alcohols by using
oxygen as the ultimate oxidant under mild solvent-free conditions.
The promoted gold catalysis is an environmentally and economic-
ally viable process for producing a wide range of useful chemicals
from alcohols. Further spectroscopic studies are needed for a
complete elucidation of the role of promotors in the oxidative
process.
This work was supported in part by the NSF under Award No.
DMR-02-33728 and the NASA University Research Engineering
and Technology Institute on Bio Inspired Materials (BiMat) under
Award No. NCC-1-02037. It made use of the MRL central
facilities supported by the MRSEC Program of the National
Science Foundation under Award No. DMR0-05-20415.
Notes and references
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3864 | Chem. Commun., 2007, 3862–3864
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