Selective Liquid Phase Oxidation of Alkene with Dioxygen on Tunabe Modified Gold
Nanocatalysts
Hadi Salari, Mohammad Reza Gholami
Sharif University of Technology, Azadi Ave., Tehran, Iran
salari@mehr.sharif.ir
Abstract
Oxidation is an important method for the synthesis of various chemical intermediates and compounds. In
recent years there has been an increasing interest in nano-gold systems as particle size is known to
have a great influence on the catalytic properties of gold. Supported gold nano-cluster catalysts exhibit
catalytic activities that are radically different from bulk gold. Among the various metals, gold has some
rather peculiar properties; high resistance to corrosion and oxidation, highest ductility and malleability
amongst metals and very high electron negativity. Supported gold nanoparticles have been extensively
studied as catalysts for a wide range of oxidation reactions including low-temperature CO oxidation,
alkene epoxidation, and aerobic oxidation of alcohols in gas and liquid phases under relatively mild
conditions [1-5]. In the case of selective oxidation of alkenes catalyzed by supported gold catalysts,
Kiely et al. [1] and subsequently Lambert et al. [2] have shown that gold nanoparticles supported on a
range of supports are active for the oxidation of Cyclohexene, Cyclooctene and Styrene. It is known that
gold nanoparticle size associated with the gold-support interactions is one of the key issues to create
higher catalytic activities. Recently, Lambert et al. [2] have found a sharp size threshold (2 nm) in
catalytic activity for the selective oxidation of styrene by dioxygen, although the conversion and also the
epoxide selectivity need further improving. Very small gold entities (1.4 nm) derived from 55-atom gold
clusters and supported on inert materials were efficient and robust catalysts for the reaction. Above that
particle size the supported gold catalysts became completely inactive. Several methods are actually
available to obtain supported gold nanoparticles: coprecipitation, deposition–precipitation, chemical
vapour deposition, impregnation of phosphine complexes or clusters and the recently introduced
immobilisation of gold sols. However, the choice of the preparation method is closely connected to the
type of employed supporting material, the nature of which also possibly influences the reactivity of the
catalyst.
Gold particle size (Figure 1) was considered as the most important factor that affects the activity of gold
catalysts. Haruta and coworkers [4] have reported that the turnover frequency increased with a
decrease in the mean diameter of gold particles, it is due to the contact structure giving the longest
perimeter distance of gold-support interface, which was responsible for active site.
In this wok, we report that Au/TiO2 and Au/Al2O3 prepared by homogeneous deposition-precipitation
(HDP) (using urea as the precipitation agent) method is an active and selective catalyst for the liquidphase oxidation of Cyclohexene to different products which is shown in Figure 2. The obtained catalysts
were modified with some Amino acids and Ionic Liquids.
The Cyclohexene oxidation reaction over the catalysts was carried out at 2 atm pressure of Oxygen gas
by contacting 0.03 gr catalyst with 0.7 ml Cyclohexene in 15 ml solvent in a stirred batch reactor
(capacity: 50 cm 3), under reflux (at 80–82 0C) and vigorously stirring for a period of 24 h. The catalyst
was separated from the reaction mixture by filtration. The reaction products and unconverted reactants
were analysed by GC with FID using HP5 column and He as carrier gas. The reaction performed in
Hexane, 1,4-Dimethylbenzene, 1,2-Dimethylbenzene, Tolouene, Chlorobenzene and some Ionic Liquids
as new classes of friendly environmentally solvents. The results showed that selectivity and productivity
of reaction are influenced drastically by solvents systems. Gold catalysts have significant potential for
selective oxidation of alkenes. The productivity of reaction increases with Au loading percent increasing.
High selectivity of Cyclohexene demonstrates that oxidation of C=C double band is occurring with these
catalysts.
References
[1] Mathew D. et al., Nature, 437 (2005) 1132-1135.
[2] Mark Turner, et al., Nature, 454 (2008) 981-983.
[3] T.V. Choudhary and D.W. Goodman, Topics in Catalysis, 21 (2002) 25-34
[4] M. Date, et al., Catal Today, 72 (2002) 89–94.
[5] An-Fei An, An-Hui Lu, Qiang Sun, Jie Wang, Wen-Cui Li, Gold Bull, 44 (2011) 217–222
Figures
Figure 1. TEM images of supported Au catalysts.
Figure 2. Cyclohexene oxidation reaction with molecular oxygen using Au catalysts.