Soluble and supported carbonylation catalysts derived from rhodium-phosphonate-phosphane complexes

Abstract

Phosphonate-phosphanes form both chelate and open-chain complexes with rhodium, which can easily be converted into each other. This property has been used in the development of new and efficient carbonylation catalysts. During the catalytic cycle, the phosphane group is assumed to be coordinated to the transition metal, while the phosphoryl-oxygen of the phosphonate group is supposed to change between a free and a coordinated state, thus vacating or occupying a coordination site. IR studies support the hemilabile behaviour of surface Rh-complexes under the reaction conditions. Activation enthalpies, obtained for hemilabile rhodium catalysts in homogeneous methanol carbonylation, increased significantly with growing distance between phosphonate and phosphane group. Attempts of preparing stable rhodium complex catalysts adsorbed on silica or alumina for slurry-phase or vapour-phase carbonylations failed. Activated carbon has been shown to be a suitable support for hemilabile rhodium complexes, but normal diffusion of reactants begins to limit the carbonylation rate over the supported catalysts.