Synergetic effects on the capture and release of CO2 using guanidine and amidine superbases

Abstract

The capture of CO₂ from air is of utmost importance, not only to reduce its impact on climate change but also for its utilisation as a tremendous, renewable source of C1 building blocks for sustainable chemical synthesis. Novel and known superbase structures are compared in a new selection of solvents for CO₂ capture and release. Bicyclic amidine and guanidine superbases with 6–5, 6–6 and 6–7 configurations and many methylated analogues are investigated. As reported here, identified superbase/solvent combinations offer a highly efficient, reversible, and kinetically favourable CO₂ capture process from air. The two most beneficial superbase/solvent synergic combinations identified are 1,5,7-triazabicyclo[4.3.0]non-6-ene (TBN) in butyl acetate and 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) in acetonitrile. They reach saturation after 15 min with pure CO₂ and after 24 hours under open-air conditions and release CO₂ with a CO₂/superbase molar ratio of 0.41 and 0.25, respectively. Due to the favourable thermodynamics of the systems, quantitative CO₂ release for TBN and DBN occurs under mild conditions at 90 °C and 60 °C within 20 minutes. The required time for a complete absorption–desorption cycle for both TBN-butyl acetate and DBN-acetonitrile was only 48.5 and 38.5 minutes respectively. Superbase–solvent mixtures are recyclable and the system retains its initial CO₂ capturing capability after 5 cycles. As this apparently easy emerging system design allows the direct capture of CO₂ from air, it has potential for positive utilization on the global scale.