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Projekt Druckansicht

Catalytic C-C-coupling and related reactions with hydrophobic substrates in surfactant based aqueous media

Fachliche Zuordnung Technische Chemie
Förderung Förderung von 2009 bis 2016
Projektkennung Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 126176829
 
Erstellungsjahr 2016

Zusammenfassung der Projektergebnisse

The general goal of the trilateral project was the development of a new sustainable method for applied organic synthesis. The major aims were (a) the replacement of conventional harmful organic solvents and (b) the recycling of the applied catalyst. The emulsion (or microemulsion)-solid state-transport (EST) method combines both aims in one approach as it uses surfactant-based aqueous reaction media and a heterogeneous sol-gel catalyst. The EST method was successfully applied to manifold reactions, e.g. Heck coupling, isomerization, disproportionation, hydrogenation decarbonylation, and cyclotrimerization. The catalytic activity is a complex function of surface polarity of the sol-gel catalyst and applied surfactant. We found that hydrophobic surfaces, obtained by the addition of hydrophobic silica precursors during the synthesis, are beneficial for reactions with hydrophobic reactants (and vice versa) as they support the mass-transfer to the active center inside the pores. The impact of the surfactant is difficult to predict as it is involved in the solubilization of the reactants in the bulk phase through the formation of micelles and the transport of the reactants from the bulk phase to the catalyst via an adsorbed surfactant layer. For optimized reaction conditions, a surfactant screening is mandatory. The immobilization of a homogenous catalyst, e.g. Pd(OAc)2 or RhCl3, by the sol-gel support led to active and stable catalysts that could be recycled several times, but the transformation of the homogenous complex into metallic nanoparticles during EST conditions was also observed for different reactions. In order to evaluate the performance of the sol-gel catalysts in comparison to a homogenous catalyst complex which cannot be recycled, we introduced the productivity P as an indicator. For the calculation of P we considered different steps, e.g. immobilization, intercalation, pore diffusion, and recycling. Although, the sol-gel catalysts show significant pore-diffusion limitation, their stability and recyclability allow for higher productivity after several recycling steps. For the Heck coupling reaction we obtained a P value of 2 after 6 runs. As the sol-gel catalyst can be easily separated from the product, it offers the possibility for tandem reactions. We performed Heck/hydrogenation and Heck/epoxidation tandem reactions, without isolation of the Heck coupling intermediates. For the Heck/hydrogenation tandem reaction we obtained good results. The hydrogenation of the Heck coupling product was comparable with the hydrogenation of commercial trans-stilbene. In the case of the Heck/epoxidation tandem reaction, the results were insufficient. For the expoxidation reaction with Mn(Acac)2@PhSiO2, we observed strong catalyst leaching and the yields were lower in comparison to commercial Manganese catalysts. Based on the results of the EST method, we proposed a concept for reaction and product/catalyst isolation for a single sol-gel catalyzed reaction that can be extended in the case of tandem reactions.

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