Table 2 Catalytic results of the oxidation of secondary amines to
imines for supported Au nanoparticles (1 wt% Au/C) and Au powder
and selectivities were the same as when the reactions were run
separately. This is consistent with the scheme proposed above.
In summary, we have demonstrated that Au in the form of
both very large particles and 16 nm nanoparticles is an active
a
catalysts using O
2 2
or NO
Catalyst
Oxid. Substrate
Product
Conversion (%)
b
epoxidation catalyst when NO is used as the oxidant; under
2
1 wt% Au/C O
Au powder
2
91.1
83.1
b
the same conditions O2 is ineffective. This new route is of
interest for application in a wide range of Au-catalyzed
selective oxidation reactions both in the liquid phase, and in
O
2
b
c
c
1 wt% Au/C O
Au powder
2
19.6 ; 30.8
23.1 ; 24.7
b
O
2
2 2
the gas phase. In contrast, both NO and O are effective for
b
1 wt% Au/C NO
Au powder NO
2
2
98.9
86.1
the Au-catalyzed oxydehydrogenation of secondary amines,
with the former being significantly more active. The active
oxygen-containing surface species must be different in these
two different classes of reaction and we propose schemes that
are consistent with observation: oxygen adatoms, deposited by
NO , are responsible for alkene epoxidation whereas NO (a)
b
b
c
c
1 wt% Au/C NO
Au powder NO
2
2
63.7 ; 72.2
75.8 ; 63.2
b
a
2
2
Reaction conditions: 100 mg of 1 wt% Au/C or 500 mg Au powder,
amine (0.1 mmol), solvent (5 mL), 100 1C, 24 h, under 0.5 bar
overpressure of gas. Toluene as solvent. DMF as solvent.
(
and O (a)) drive oxidative dehydrogenation of secondary
2
b
c
amines by ‘‘cleaning off’’ H adatoms.
MT & OPHV acknowledge financial support from the UK
Engineering and Physical Sciences Research Council and
King’s College, Cambridge, respectively.
surface areas, and confirming the inert nature of the support.
Again, we show that there is no chemical difference between
1
000 nm and 16 nm Au particles.
Table 2 also shows that NO
Notes and references
2
is an even more effective
oxidant than O
2
in this reaction, especially when account is
w Supported 1 wt% Au/C catalysts were prepared using a microemul-
16,17
taken of the fact that the NO partial pressure was B50 times
lower than the O partial pressure. For example, the conver-
2
2
sion technique. Au nanoparticles were prepared in water–oil
microemulsions with n-heptane as the continuous oil domain. An
aqueous solution of the Au precursor hydrogen tetrachloroaurate
trihydrate (0.1 M HAuCl ) was added to a mixture of the oil and
4
22.93 wt% surfactant, polyethylene glycol dodecyl ether (Brij 30,
Fluka). The volume of aqueous solution added was calculated to
sion of 1,2,3,4-tetrahydroisoquinoline catalyzed by Au powder
increased from 23% to 76% when 1.5 bar O was replaced by
2
0
.03 bar NO
no major effect on overall performance (Table 2).
The fact that both O and NO are effective oxidants for
amine oxidative dehydrogenation whereas only NO is effec-
2
. Use of DMF as solvent in place of toluene had
result in a water-to-surfactant molar ratio of 8 (n
complexes were reduced by adding hydrazine (Au : N
w
/n
s
¼ 8). The Au
2
H , 1 : 8). The
4
2
2
carbon support (C black, 50% compressed, 499.9%, Alfa Aesar, 75
2
ꢀ1
2
m g ) was added to the particle suspension under vigorous stirring,
after which acetone was added slowly to break the microemulsion and
allow the particles to deposit on the support. The mixture was then
stirred, filtered, and washed with acetone and absolute ethanol to
remove the surfactant. The resulting catalyst was dried in air at 85 1C
for 24 h. HRTEM studies were performed in a JEOL JEM-3011
electron microscope operating at 300 kV. Samples were ultrasonically
dispersed in acetone prior to deposition on carbon-coated Cu grids.
tive for styrene epoxidation clearly demonstrates that the
active oxygen-containing species is different in the two cases.
Our findings may be rationalized in terms of the following
surface mechanism. Note that H-abstraction from adsorbed
5
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organic molecules by gold surfaces has long been known.
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molar (0.1 mmol) quantities of alkene and secondary amine
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318 | Chem. Commun., 2008, 2316–2318
This journal is ꢁc The Royal Society of Chemistry 2008