Direct CO2 Capture and Reduction to High-End Chemicals with Tetraalkylammonium Borohydrides

We demonstrate the ability of tetraalkylammonium borohydrides to capture large amounts of CO2, even at low CO2 concentrations, and reduce it to formate under ambient conditions. These materials show CO2 absorption capacities up to 30 mmolCO2 g(-1) at room temperature and 1 bar CO2. Every BH4- anion can react with three CO2 molecules to form triformatoborohydride (HB(OCHO)(3)). The thermodynamics and kinetics of the reaction were monitored by a magnetic suspension balance (MSB). Direct CO2 capture and reduction from air was achieved with tetraethyl, -propyl, and -butylammonium borohydride. The alkyl chain length played an important role in the kinetics and thermodynamics of the reaction, especially in CO2 diffusivity (crystallinity and free-volume), activation energy (charge-transfer dependent on the alkyl chain), and hydrophobicity. Adding HCl gave formic acid and the corresponding chloride ammonium salt, which can be recycled. In addition, transfer of formate was achieved for the N-formylation of an amine.