same shift direction compared with the monometallic Au/TiO2
and Ag/TiO2. The negative shift of the Au 4f BE suggests that a
negative charge is deposited on Au clusters, while considering the
negative shift of Ag 3d BE, according to the anomalous spectral
shift reported for several Ag compounds, this spectral behavior
suggests that the Ag species becomes more cationic. Details can
be found in Fig. S2 and S3 in the ESI.† In conclusion, these
XPS results indicate that Ag donates electrons to Au sites. We
assume that alloying of Ag with Au would give gold a slightly
greater tendency to obtain electrons while the silver has a greater
tendency to lose electrons, indicating the presence of a synergistic
effect between Au and Ag.
The catalytic activity for the benzyl alcohol oxidation of the
AuAg/TiO2 bimetallic catalysts prepared in the present experi-
ments with different Au/Ag ratios was investigated (Fig. 4). It
is found that after 10 h there was no benzyl alcohol detected,
meaning the benzyl alcohol was completely transformed. Thus,
the conversions of benzyl alcohol are all 100%. The yield of
bimetallic catalysts changed dramatically with the variation in
Au/Ag ratios. And the catalyst with Au/Ag ratio of 1 : 3 had
the highest yield from benzyl alcohol to sodium benzoate which
reached 82%. Furthermore, all bimetallic catalysts had higher
performance than monometallic ones. The Au and Ag bimetallic
system shows strongly synergistic effect with better catalytic
performance.
Fig. 5 Recyclability of 1Au3Ag/TiO2 for sodium benzoate and benzoic
acid synthesis. (Reaction condition: same as Fig. 4.)
In summary, we have prepared a novel efficient AuAg/TiO2
bimetallic catalyst with high stability for one-pot solvent-free
synthesis of sodium benzoate and benzoic acid from the green
oxidation of benzyl alcohol using air as the oxidant under
ambient pressure without any wastes emission. The low toxicity
of titania and gold, combined with the all aqueous mild synthesis
conditions made the reported synthesis route a promising one
for green chemistry applications. According to our studies, the
AuAg/TiO2 bimetallic catalysts showed strong synergistic effect
with high catalytic activity. The best-performing Au–Ag alloy
catalyst was the one with a nominal Au/Ag ratio of 1 : 3.
And even after being reused for 6 times, the 1Au3Ag/TiO2
bimetallic catalyst still remained high catalytic activity (the
yield >75%), which opens new possibilities for environmentally
benign oxidative catalysis.
Acknowledgements
Financially supported by the Major State Basic Resource Devel-
opment Program (Grant No. 2003CB615807), NSFC (Project
20973042), the Research Fund for the Doctoral Program of
Higher Education (20090071110011) and the Science and Tech-
nology Commission of Shanghai Municipality (08DZ2270500).
Fig.
4
Catalytic activity test of benzyl alcohol oxidation over
AuAg/TiO2 with different Au/Ag molar ratios. (Reaction conditions:
temperature 200 ◦C, reaction time 10 h, 1.08 g benzyl alcohol, 0.5 g
NaOH, 0.1 g catalyst.)
Notes and references
1 H. D. Holtz and L. E. Gardner, (Philips Petroleum Co.), US Patent,
4088823, 1978.
2 D. A. Bulushev, F. Rainone and L. Kiwi-Minsker, Catal. Today,
Moreover, the as-prepared catalysts can be conveniently
recycled with high stability. The best 1Au3Ag/TiO2 bimetallic
catalyst can be easily recovered and could be reused for more
than six successive reactions without significant loss of the
catalytic activity (Fig. 5). And the desired product with high
purity and good yield can be conveniently obtained via simple
steps: after the reaction, the mixture of solid product and catalyst
were dissolved in deionized water and then the catalyst was easily
recovered by centrifugation. Sodium benzoate and benzoic acid
can be easily separated out by adjusting the pH of solution with
diluted HCl aqueous solution from 8.0 to 2.0 spontaneously.
The as-obtained white product (sodium benzoate) was obtained
by simple evaporation of excess water and crystallization. While
the as-obtained benzoic acid was collected by filtration, washed
three times with deionized water, and dried in air.
2004, 96, 195–203.
3 M. J. Earle and S. P. Katdare, World Patent, 2002030862, 2002.
4 A. Worayingyong, A. Nitharach and Y. Poo-arporn, ScienceAsia,
2004, 30, 341.
5 U. Armbruster, A. Martin, S. Wilhelm and S. Mothes, Chem. Ing.
Tech., 2002, 74, 1450–1454.
6 G. C. Bond, C. Louis and D. T. Thompson, Catalysis, Gold London:
Imperial College Press, 2006, p. 217.
7 D. Y. Cha and G. Parravan, J. Catal., 1970, 18, 200–211.
8 L. Guczi, G. Peto, A. Beck, K. Frey, O. Geszti, G. Molnar and C.
Daroczi, J. Am. Chem. Soc., 2003, 125, 4332–4337.
9 A. Ueda and M. Haruta, Appl. Catal., B, 1998, 18, 115–121.
10 V. R. Choudhary, A. Dhar, P. Jana, R. Jha and B. S. Uphade, Green
Chem., 2005, 7, 768–770.
11 M. Haruta, T. Kobayashi, H. Sano and N. Yamada, Chem. Lett.,
1987, 16, 405–408.
12 J. Huang, W. L. Dai, H. X. Li and K. Fan, J. Catal., 2007, 252, 69–
76.
1646 | Green Chem., 2011, 13, 1644–1647
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