C O M M U N I C A T I O N S
Scheme 1. Two Reaction Pathways for Oxidation of Alcohols
Supporting Information Available: Experimental details, STEM
images, and EDS analyses. This material is available free of charge
References
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At this stage, it is not clear why the Au-Pt clusters contain an
∼1:1 ratio of gold to platinum and the Au-Pd clusters contain an
∼4:1-3:1 ratio of gold to palladium in alloy clusters that were
formed by our standard preparation method (in both cases 1:1 Au/
Pd or 1:1 Au/Pd was combined).
We further prepared PI-CB/Au-Pd catalysts with different Au/
Pd ratios (Table 3). It was found that in all cases the ratios of gold
to palladium in clusters determined by EDS were larger than those
determined by ICP. This means that palladium exists not only in
clusters but also in the polymer backbone.16 For catalyst activity,
PI-CB/Au showed almost no selectivity for the ester or carboxylic
acid (entry 1). The PI-CB/Au-Pd catalyst with a high Au ratio
gave a similar result (entry 2). When a PI-CB/Au-Pd catalyst with
a 2:1 or 1:1 ratio of Au/Pd was used, the desired ester was obtained
in high selectivity (entries 3, 4). On the other hand, when the ratio
of palladium was larger (Au/Pd ) 1:2 or 1:3), the ester selectivity
was significantly decreased (entries 5, 6). PI-CB/Pd showed much
lower reactivity and gave the carboxylic acid and the aldehyde
(entry 7). It should be noted that the PI-CB/Au-Pd catalyst with
a 1:1 ratio of Au/Pd gave the best ester selectivity.
Table 3. Effect of Ratio of Gold and Palladium for Direct Oxidative
Ester Formation
(9) Review: Ley, S. V.; Baxendale, I. R.; Bream, R. N.; Jackson, P. S.; Leach,
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yieldc (%)
Aua
Pda
ratio in
(mmol/g) (mmol/g) clustersb ester acid aldehyde alcohol
entry
catalyst
1
2
3
4
5
6
7
Au
Au-Pd(3:1) 0.1522 0.06655 7:1
Au-Pd(2:1) 0.1802 0.1137 6:1-5:1 62 29 trace
0.2178
-
-
43 55 n.d.
n.d.
n.d.
8
13
2
41 53
4
Au-Pd(1:1) 0.1933 0.2167 4:1-3:1 78
7
n.d.
5
2
Au-Pd(1:2) 0.1400 0.2856 2:1-1:1 45 47
Au-Pd(1:3) 0.1070 0.3265 2:1-1:2 46 51
n.d.
n.d.
Pd
-
0.2534
-
7
42 50
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J. Catal. 2007, 251, 332. (b) Nielsen, I. S.; Taarning, E.; Egeblad, K.;
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(16) Details are shown in the Supporting Information.
(17) Reactivity depended on the solvent system. See Supporting Information.
(18) Order of electronegativity of gold, platinum, and palladium is gold (2.54)
> platinum (2.28) > palladium (2.20) by Pauling’s scale: (a) Pauling, L.
J. Am. Chem. Soc. 1932, 54, 3570. (b) Allred, A. L. J. Inorg. Nucl. Chem.
1961, 17, 215.
a Determined by ICP. b Au-Pd ratio in clusters determined by EDS.
c Detemined by GC. n.d.: not detected.
In summary, we have developed novel carbon-stabilized polymer-
incarcerated gold-based bimetallic nanocluster catalysts (PI-CB/
Au-Pt, PI-CB/Au-Pd). The catalysts can be applied for the
aerobic oxidation of alcohols and direct oxidative ester formation
under very mild conditions, such as atmospheric oxygen and
ambient temperatures. While PI-CB/Au-Pt catalysts provided
aldehydes/carboxylic acids, PI-CB/Au-Pd catalysts gave esters.
The STEM and EDS analyses suggested formation of Au-Pt
clusters containing an ∼1:1 ratio of gold to platinum, and the
Au-Pd clusters contain an ∼4:1-3:1 ratio of gold to palladium
in the alloys, respectively. This structure difference is considered
to be a key for selective formation of aldehydes/carboxylic acids
and esters. Further investigation to clarify the role of carbon, the effect
of bimetallic clusters, and the reaction mechanisms is ongoing.
(19) Platinum (1.44) > gold (1.42) > palladium (1.35) by the Allred-Rochow
scale: (a) Allred, A. L.; Rochow, E. G. J. Inorg. Nucl. Chem. 1958, 5,
264. (b) Little, E. J.; Jones, M. M. J. Chem. Educ. 1960, 37, 231.
(20) Gold (5.8 eV) > platinum (5.6 eV) > palladium (4.44 eV) by Pearson’s
absolute electronegativity: Parr, R. G.; Pearson, R. G. J. Am. Chem. Soc.
1983, 105, 7512.
Acknowledgment. This work was partially supported by JSPS,
Global COE Program, The University of Tokyo, MEXT, Japan,
and NEDO. Special thanks to Mr. Noriaki Kuramitsu (The
University of Tokyo) for STEM and EDS analyses.
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