Study of the influence of PPh3 used as capping ligand or as reaction modifier for hydroformylation reaction involving Rh NPs as precatalyst
Résumé
The presence of capping ligands on the surface of metal nanoparticles (NPs) has been recognized to be a key factor for the control of their size and shape as well as for their stabilization, but their influence on the NP behavior in catalysis is far from being well-understood. Here, we investigated the performance of Rh NPs stabilized by tetraoctylammonium bromide (Rh-TOAB NPs) when applied as precatalyst for hydroformylation reaction. An ehnancement of the catalytic activity was clearly observed after adding monophosphines as a reaction modifier. More interestingly, a much higher effect was observed when a phosphine was directly used as capping ligand for the synthesis of the Rh particles, i.e. in the absence of TOAB. Indeed, triphenylphosphine-stabilized Rh NPs (Rh-PPh3 NPs) were found to be a unique precursor for hydroformylation active catalytic species, leading to higher activity attributed to the existence of a strong phosphine-rhodium interaction in this case. The Rh-PPh3 NPs appeared less affected when adding poisoning ligands in catalytic reaction than the Rh-TOAB NPs PPh3 system which deactivates in the presence of CS2. In addition, a large excess (20 equiv. per Rh atom) of PPh3 was necessary to activate the Rh-TOAB NPs, similarly to the amount typically used in homogeneous catalytic systems. In the contrary, one equiv. of PPh3 (per Rh atom) was enough to prepare Rh-PPh3 NPs, which are more active and more stable against CS2 poisoning. All together our results evidence that a direct interaction of PPh3 ligand at the surface of Rh NPs plays an important role on their behavior in hydroformylation catalysis.