Supported gold nanoparticles have been shown to catalyze alkene epoxidations in the liquid phase in the presence of catalytic amounts of tert-butylhydroperoxide (TBHP). The Au/TiO₂ reference catalyst from the World Gold Council in particular exhibits high selectivity for the aerobic epoxidation of stilbene.¹ However, the reference catalyst is poorly dispersed within the apolar reaction medium. Improvement in the overall efficiency of the reaction is thus expected by optimizing mass transfer of the catalytic system. Here, we present the design of a new family of nanosized hybrid titanias that allows to stabilize small gold particles and enhance dispersion of the resulting catalytic powder in the reaction mixture.

First, we have synthesized new well-defined citrate-modified titanium precursors and characterized them by molecular spectroscopies. We have then used them to prepare crystalline surface-modified titania nanoparticles in an original one step, efficient, low temperature (100°C) and aqueous sol-gel synthetic methodology.² It leads to high and precise control of the organic loadings and a very good chemical stability of the hybrid titania powders, as shown by TG-DTA-MS, BET, XRD, FTIR characterization. These materials are then reacted with aqueous solutions of HAuCl₄.3H₂O or home-made Au(en)₂Cl₃ at 100°C for 3 h, in the presence of tannic acid, PVA / NaBH₄, or NaOH / NH₄OH. Catalytic evaluation is carried out with trans-stilbene (1 mmol), solvent (methylcyclohexane, 20 mL), TBHP (0.05 mmol) and gold catalyst (2 mmol Au) in air (atmospheric pressure) at 80°C for 24 h.

The role of citrate content in gold deposition, particle growth (TEM), catalytic conversions and selectivities (HPLC) will be discussed. We will also show how the resulting catalytic powders get dispersed within the apolar liquid reaction medium and increase turnover numbers.

References

1)   P. Lignier et al., Chem. Commun. (2007) 186.

2)   R. Rahal et al., Eur. J. Inorg. Chem. (2008) 980.

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1. Facile one-pot aqueous hybrid Inorganic/Organic TiO₂ nanoparticles synthesis by chemical design