Thermochemistry of a Biomimetic and Rubisco-Inspired CO₂ Capture System from Air

In theoretical studies of chemical reactions the reaction thermochemistry is usually reported for the stoichiometric reaction at standard conditions (ΔG°, ΔH°, ΔS°). We describe the computation of the equilibrium concentrations of the CO₂-adducts for the general capture reaction CO₂ + Capture System ⇆ CO₂-adduct (GCR) and the rubisco-type capture reaction CO₂ + Capture System ⇆ CO₂-adduct + H₂O (RCR) with consideration of the reaction CO₂(g) ⇆ CO₂(aq) via Henry’s law. The resulting equations are evaluated and graphically illustrated as a function of atmospheric CO₂ concentration and as a function of temperature. The equations were applied to the thermochemistry of small molecule rubisco-model reactions and series of additional model reactions to illustrate the range of the Gibbs free enthalpy for the effective reversible capture and of the reaction entropy for economic CO₂ release at elevated temperature. A favorable capture of free enthalpy is of course a design necessity, but not all exergonic reactions are suitable CO₂ capture systems. Successful CO₂ capture systems must allow for effective release as well, and this feature is controlled by the reaction entropy. The principle of using a two-pronged capture system to ensure a large negative capture entropy is explained and highlighted in the graphical abstract. It is hoped that the presentation of the numerical examples provides useful guidelines for the design of more efficient capture systems.