Unveiling the Direct Air Capture and Transformation to Formate Under Mild Conditions.

Abstract

Addressing the challenges in the inherent direct air capture and conversion (DACC) is of special interest. This study focuses on the development and optimization of hydroxide-based absorbents for CO₂ capture and subsequent hydrogenation to formate. The research built upon previous work with hydroxide-based systems, which successfully captured atmospheric CO₂ but left open questions regarding the effects of solvents and cations on capture efficiency. A systematic experimental and theoretical investigation is conducted to evaluate a series of organic cations (tetrabutyl ammonium and phosphonium) and different solvents (DMSO, H₂O, MeOH) in the CO₂ capture and further hydrogenation to formate. The findings demonstrate a CO₂ capture efficiency of up to 1.0 mol CO₂ per mol of TBA⁺ (tetrabutylammonium) in DMSO/water, with water boosting the overall sorption process. Molecular dynamics simulations revealed that solvent composition controls solvation and aggregation behavior. Additionally, the hydrogenation of the captured bicarbonate is optimized using Ru-based complexes. Ru₃(CO)₁₂ achieved a formate yield of up to >99% under mild hydrogen pressure (5 bar), smaller pressure reported so far for DACC. Density of functional theory calculations provided key insights into the reaction pathway. These insights contribute to the advancement of efficient DACC technologies by reducing H₂ pressure requirements while maximizing CO₂-to-formate conversion.