Process development of the continuously operated synthesis of N,N- dimethylformamide based on carbon dioxide

The development of a process concept for the carbon dioxide based synthesis of N,N-dimethylformamide (DMF) is presented in this paper. Investigations were performed with a ruthenium catalyst comprising RuCl3 hydrate as precursor and BISBI (2,2′-bis(diphenylphosphinomethyl)-1,1′-biphenyl) as ligand. A previously developed recycling concept where the catalyst complex was immobilized in 2-ethylhexan-1-ol was transferred from lab-scale into a continuously operated miniplant in order to evaluate stability, selectivity and recyclability of the catalyst. The results confirmed a high robustness of the reaction system and showed carbon monoxide as only byproduct in small ppm concentrations. The gas dosage had a major impact on the catalyst activity. While an oversaturation with carbon dioxide lead to only 14% yield of DMF, a stoichiometric dosage of carbon dioxide to amine lead up to a yield of 43% DMF at an even shorter residence time of 3.5 h. The main reason for this effect could be located on the equilibrium formation of ammonium salts with carbonates and/or carbamates, which reduced the basicity of the reaction system. The whole setup showed a stable activity of over 95 h without catalyst refreshment. Investigations towards the product isolation by distillation showed that all substrate and solvent streams are recyclable and no product decomposition occurred. A ruthenium containing precipitate could be obtained in the purification process that still showed up to 80% of the catalytic activity compared to fresh precursor.