From poison to promoter: investigation of supported rhodium sulphides as heterogeneous hydroformylation catalysts

Abstract

Herein we report the use of supported nanoparticles of crystalline rhodium sulphides as active heterogeneous catalysts for the hydroformylation of alkenes showing an excellent selectivity towards the aldehyde products. It was found that supported RhxSy (x=17, y=15 or x=2, y=3 with 1 and 10 wt.% Rh) nanoparticles greatly outperform pure Rh nanoparticles in terms of activity, with Rh17S15/SiO2 being the superior catalyst candidate. The TOF for the hydroformylation of styrene, determined for the lead system Rh17S15/SiO2 (1 wt.% Rh), is with 2620 h-1 significantly higher than TOF values reported for rhodium phosphide-based systems. As the presence of S in the Rh sulphides introduces structural diversity in the bulk as well as the range of potential catalytic surfaces, we enumerate all possible terminations, use local environment descriptors combined with unsupervised Machine learning techniques and DFT to draw structure performance relationships. We find that the presence of S on the surface and in close proximity to the catalytic site had a profound impact on the chemical behavior of the catalyst, thus unlocking the tunability of the surface catalytic properties. This allows for the affinity towards the substrate to be modulated to a high degree ranging from S rich Rh-top like sites of low; to Rh rich, and in particular for Rh17S15, (Rh4) sites with adsorption energies rivaling those of pristine Rh and improved spatial resolution.