Sol–gel supported palladium catalysts are investigated for the Heck coupling reaction between styrene and iodo-/bromobenzene to trans-stilbenes in o/w-microemulsions as alternative reaction medium. High conversions and selectivities are obtained with these catalysts and they show better catalytic performance than their commercial analogs Pd@SiO2 or Pd/C. The influence of the catalyst structure on the activity is investigated in detail showing mass transport limitations that can be optimized by the palladium loading. The catalyst is recyclable >6 times with negligible palladium leaching into the solution. Because of the good recyclability under retention of activity and selectivity, the influence of transport limitations is suppressed and the total catalyst efficiency is increased to more than 2.

Tandem catalytic systems containing one or two sol-gel immobilized catalysts were successfully applied in the synthesis of trans-stilbene oxide and 1,2-diphenylethane. The catalysts were prepared from palladium and/or manganese precursors in the presence of orthosilicates using a sol-gel method and could be reused several times successfully.

In the course of our studies toward the development of new heterogeneous conditions for better controlling regioselectivity in organic reactions, we investigated the application of sol–gel immobilized organometallic catalyst for regioselective hydroaminomethylation of vinylarenes with aniline or nitroarene derivatives in an aqueous microemulsion. By immobilization of 6 mol % [Rh(cod)Cl]₂ within a hydrophobic silica sol–gel matrix we were able to perform efficient hydroaminomethylation under mild conditions and isolate 2-arylpropylamines with high regioselectivity. The regioselectivity of the reaction was found to be mainly dependent on the hydrophobicity of the catalyst support. It is also significantly affected by the electronic nature of the substrates, by the reaction temperature, and by syngas pressure. The heterogenized catalyst can be reused for several times.

In the course of our attempts to develop sustainable conditions for the catalytic organic reactions by replacement of the traditional but environmentally disfavored organic solvents by water, we studied the cyclotrimerizations of alkynes, in aqueous microemulsions. The catalyst for these reactions was the rhodium-trichloride encaged within silica sol–gel. In acidic aqueous media, alkynes can undergo addition of water to form their corresponding ketones. In order to eliminate completely the formation of ketones, relatively low reaction temperatures are required. At higher reaction temperatures, however, buffered microemulsion media is preferable. Cyclotrimerization of alkynes proved to be dependent on the reaction temperatures, the electronic nature of the substrates, the electronic nature of the surfactant and on the hydrophobicity of the sol–gel support. During the cyclotrimerization reaction, the rhodium complex was turned to Rh(0) nanoparticles characterized by TEM measurements. These investigations may be regarded as model studies for the conversion of alkynes into substituted benzenes in water.

In the course of our attempts to replace harmful solvents in organic processes by environmentally favored media, we investigated the use of aqueous microemulsions for catalytic decarbonylation of different kinds of aldehydes. The aldehydes were solubilized in the microemulsions with the aid of the cationic surfactant, cetyltrimethylammonium bromide. The aldehydes were transferred into CO-free products by sol–gel entrapped catalysts. The best results were obtained in the presence of nanoparticles of Pd(0). The heterogenized catalyst could usually be recycled 7–8 times without loss of catalytic activity. At relatively low temperatures (140°C) the decarbonylation proceeds stepwise. Initially a mixture of saturated and unsaturated products is formed. At 180°C however, fast hydrogenation of the unsaturated compounds takes place. These investigations may be regarded as model studies for the conversion of biomass derived intermediates to fuels and chemicals.

Terminal aromatic alkynes are converted rapidly into ketones in a regioselective manner by treatment of their microemulsions with 0.33?M mineral acid between 80 and 140?°C. Internal and aliphatic acetylenes are likewise hydrated, but require longer reaction periods. The products are easily isolated from the reaction mixtures by phase separation. Replacement of H2O by D2O leads to the formation of trideuteriomethyl ketones.

In the course of our attempts to replace harmful organic solvents used in organic processes by environmentally favored media, we investigated the transfer hydrogenation of various, unsaturated substrates by cyclohexene and similar water-insoluble hydrogen donors. The catalyst in these reactions was Pd(0) in form of nanoparticles, stabilized by hydrophobic silica sol–gel which can be reused at least 4–6 times without significant loss of activity. The process was shown to depend on the electronic nature of the surfactants. Marlipal® 24/70 gave in most experiments the best results. Except for the hydrolysable 1-chloromethylnaphthalene, where chlorine can be substituted by a hydroxyl group, the aqueous medium does not take part in the catalytic process.

Two methods for selective hydroformylation of vinylarenes in aqueous media are described. One method relies on the application of [Rh(cod)Cl]2 and a tertiary phosphane entrapped within an ionic liquid-confined silica sol–gel support. The second method utilizes the same rhodium compound, encaged within ionic-liquid-free hydrophobicized sol–gel. Both methods are best carried out at 50°C in aqueous emulsions or microemulsions that consist of the substrate, a surfactant, a co-surfactant and >89% water. The optimal H2/CO ratio is between 1 and 1.1. Both methods allow the reuse of the heterogenized catalyst for several runs. While the regioselectivity and the yield are hardly affected by the electronic nature of the substrates, they are significantly dependent on the reaction temperature, on the surfactant employed, and on the hydrophobicity of the support of the catalyst. Despite the use of H2 in the reactions, no transformation of the organometallic catalyst into metallic nanoparticles could be detected.