In Situ Formation of Wilkinson-Type Hydroformylation Catalysts: Insights into the Structure, Stability, and Kinetics of Triphenylphosphine- and Xantphos-Modified Rh/SiO₂

An investigation has been carried out to identify the effects of catalyst preparation on the activity, selectivity, and stability of phosphine-modified rhodium/silica catalysts (Rh/SiO2) for propene hydroformylation. High selectivity to aldehydes was achieved, without the formation of propane or butanol. Catalyst activity and selectivity was found to depend strongly on the nature and concentration of the phosphine ligands and the amount of rhodium dispersed on the silica support. Screening of different ligands showed that a bidentate xantphos (X) ligand was ∼2-fold more active than the monodentate phosphine ligand (PPh3) screened at a ligand-to-rhodium ratio of 15:1. Investigation of the effects of reaction temperature, reactant partial pressures, and phosphine-to-rhodium ratio indicates that the kinetics of propene hydroformylation over X-promoted Rh/SiO2 is nearly identical to those for sulfoxantphos-modified rhodium-containing supported ionic liquid phase (SX-Rh SILP) catalysts. In-situ FTIR and solid-state 31P MAS NMR characterization provide evidence for the formation of HRh(CO)n(PPh3)4–n species on PPh3-modified Rh/SiO2, and HRh(CO)2(X) species on xantphos-modified Rh/SiO2. The high catalytic activity observed over rhodium-containing silica catalysts is attributed to formation of Rh(I)(CO)2 species by the process of corrosive chemisorption of Rh nanoparticles by CO and the subsequent ligation of phosphine ligands to the dicarbonyl species. Evidence is also presented suggesting that the active form of the catalyst resides on the surface of the Rh nanoparticles.