Rh‐Catalyzed Hydrogenation of CO2 to Formic Acid in DMSO‐based Reaction Media: Solved and Unsolved Challenges for Process Development

Abstract

Process concepts have been conceived and evaluated for the amine‐free homogeneous catalyzed hydrogenation of CO2 to formic acid (FA). Base‐free DMSO‐mediated production of FA has been shown to avoid the formation of stable intermediates and presumably the energy‐intensive FA recovery strategies. Here, we address the challenges in the development of an overall process: from catalyst immobilization to the FA isolation. The immobilization of the homogeneous catalyst was achieved using a multiphasic approach (n‐heptane/DMSO) ensuring high retention of the catalyst (>99%) and allowing facile separation of the catalyst‐free product phase. We show that the strong molecular interactions between DMSO and FA on the one hand shift the equilibrium towards the product side, on the other hand, lead to the formation of an azeotrope preventing a simple isolation step by distillation. Thus, we devised an isolation strategy based on the use of co‐solvents and computed the energy demands. Acetic acid was identified as best co‐solvent and its compatibility with the catalyst system was experimentally verified. Overall, the outlined process involving DMSO and acetic acid as co‐solvent has a computed energy demand on a par with state‐of‐the art amine‐based processes. However, the insufficient chemical stability of DMSO poses major limitations on processes based on this solvent.