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

Abstract

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 mmol <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mrow></mml:mrow> <mml:mrow><mml:mi>CO</mml:mi> <mml:msub><mml:mrow></mml:mrow> <mml:mn>2</mml:mn></mml:msub> </mml:mrow> </mml:msub> </mml:math> 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.