Rhodium Complexes in the Low-Temperature CO2 Hydrogenation: A Relationship Between the Organophosphorus Ligand Nature and Process Selectivity

Abstract

Direct catalytic hydrogenation of CO₂ into methanol and methyl formate (MF) under formic acid (FA) synthesis conditions is an exclusive property of catalysts based on rhodium complexes. In their presence, the CO₂ conversion occurs at room temperature without the special addition of alcohols to the reaction medium. In this work, the transformation of CO₂ and H₂ into FA, methanol, and MF under low-temperature conditions over rhodium chloride and different composition rhodium compounds (RhCl (PPh₃)₃, HRh (PPh₃)₄, (acac)Rh (CO)₂, and [RhCl (CO)₂]₂) modified with organophosphorus ligands (triphenylphosphine, triphenylphosphite, diphenylphosphonite, and oligoarylene phenylphosphonites) has been investigated. The in situ FTIR method confirmed the hydride nature of the active catalyst complex and the CO₂ incorporation into the Rh-H bond during the CO₂ hydrogenation at room temperature. The dependence of the process selectivity on the donor–acceptor properties of the ligand, which affects the Rh-H electronic state, has been revealed. Whereas catalytic systems with the σ-electron donor modifier (triphenylphosphine) synthesize FA with 100% selectivity, systems with the π-electron acceptor ligand (CO) ensure methanol selectivity above 97%. Catalysts modified with mixed donor–acceptor characteristics (phosphites and phosphonites) are favorable for the MF production. Catalysts with oligoarylene phenylphosphonites provide MF selectivity of 80%–87%, with TON_MF reaching 750–980.