Dependence of the Heck coupling in aqueous microemulsion by supported palladium acetate on the surfactant and on the hydrophobicity of the support

Heck coupling of aqueous microemulsions of bromobenzene and styrene by palladium acetate entrapped within hydrophobicitized silica sol-gel is associated with the formation of supported Pd(0) nanoparticles that catalyze the process. The efficiency of the coupling strongly depends on the hydrophobicity of the sol-gel matrix. In a series of experiments in which the support was prepared by co-condensation of tetramethoxysilane [Si(OMe) ₄] and ca. 5% of either alkyl or aryl alkoxysilanes [R _n(OR′) _4-n] the activity of the immobilized catalyst increased along with the increasing length of the hydrophobic alkyl chain R in the alkylalkoxysilane as well as with the increase of n. Hydrophobic aryl groups are superior to alkyl ones but electron-withdrawing substituents on the aryl moieties lower the reactivity. The rate of the coupling process depends also on the nature of the surfactant used to solubilize the reactants. The order of rates is anionic < non-ionic < cationic surfactants. The metallic nanoparticles can be recycled and reused for at least 4-6 further catalytic runs without any loss of catalytic activity.