Reduction of CO₂ with KBH₄ in solvent-free conditions

Metal hydrides have been commonly used as reducing agents in organic and inorganic chemistry. Until today, the capability of potassium borohydride (KBH₄) to reduce aldehydes and ketones to alcohols has been known for its advantage of high stability on air and in alkaline solutions. Conversion of CO₂ to formates and boron methoxide compounds by metal borohydrides has been recently studied. In this work we investigated the solid–gas non-catalytic reaction between KBH₄ and CO₂ under both mechanochemical and thermal-induced conditions with the simultaneous formation of potassium formylhydroborates, K[H_xB(OCHO)_{4 − x}] (x = 1–3), as main products. The first crystal structure of a product of solid–gas metal borohydride – CO₂ reaction, potassium triformylhydroborate, K[HB(OCHO)₃], obtained mechanochemically, was elucidated. The evolution of the reaction between solid KBH₄ and CO₂ was monitored by a combination of thermogravimetric analysis coupled with mass spectrometry and infrared spectroscopy from room temperature to 500 °C, revealing the generation of hydrogen, methanol and carbon monoxide in a three-step mass increase reaction. Variable temperature in situ synchrotron X-ray powder diffraction under CO₂ pressure revealed the formation of a new crystalline intermediate phase with an unidentified composition but crystallizing in a monoclinic space group, and KBO₂ during the second and third steps, respectively. Gas chromatography of evolving species under CO₂ flow revealed for the first time the formation of methanol and methane in water-free conditions.