Studying the Recycling and Deactivation of Rh/Biphephos Complexes in the Isomerization–Hydroformylation Tandem Reaction

Rh/Biphephos is a widely used homogeneous catalyst for the hydroformylation of internal olefins and mixtures of olefins due to its rapid isomerization and high n:iso selectivity. In this work, the recycling of the catalyst via distillation in high-boiling propylene carbonate as a catalyst phase solvent in the hydroformylation of internal olefins was investigated. In an initial recycling experiment, the catalyst was recycled 6 times before the Biphephos ligand was fully decomposed via oxidation. Deactivation studies using NMR and atmospheric pressure chemical ionization mass spectrometry (APCI MS) showed the sensitivity of the phosphite ligand toward water, oxygen originating from air, and nonanal. After purification of the ligand via crystallization, the ligand barely showed any sensitivity toward these contaminants, indicating an enhancement of decomposition by the impurities in the ligand. Oxidation of the ligand still occurred under the reaction conditions due to the formation of hydroperoxides of the substrate with oxygen traces. With these learnings, the improved stability of the catalyst was demonstrated in a new recycling experiment with less than 20% of Biphephos having decomposed after 10 runs.