Production of formic acid from CO₂ reduction by means of potassium borohydride at ambient conditions

The present study provides an efficient process for the high-yield production of formic acid (24%) by reduction of carbon dioxide (CO₂) with potassium borohydride at ambient conditions. The effects of reaction temperature, CO₂ pressure and borohydride concentration have been investigated. For a 0.5M borohydride solution, 0.15mol/L of formic acid were produced at room temperature and ambient pressure with yields increasing at higher pressures. A time-resolved in situ ¹H and ¹¹B nuclear magnetic resonance (NMR) technique was firstly developed to monitor the elementary reaction processes under real working conditions. Direct evidence is given for the formation of H₂, HD and a hydroxyborohydride intermediate (BH₃OH^-) formed during borohydride decomposition indicating that the source of the hydrogen gas comes from both the borohydride anion and water, while borohydride works as a water-splitting reagent. Consequently, a reaction mechanism involved in both borohydride hydrolysis and CO₂ reduction has been established.