CO2 capture from water using a copper/chromium-adenine supramolecularly assembled porous metal-organic material

A supramolecular metal-organic porous material [CrCu₆(μ-adeninato-кN3:кN9)₆(μ-OH)₆(μ-OH₂)₆](SO₄)_1.5 (Cu₆Cr) has been tested for CO₂ capture from water. The properties of this compound, insolubility in water, a flexible supramolecular structure, and protonable positions on the adeninato ligands, make it a potential candidate for this task. The experimental determination of CO₂ capture from water was performed using two techniques: magnetic sustentation, for first time, and gravimetric measurements. Both techniques verified the CO₂ capture providing complementary information. The magnetic sustentation technique measures the mass being incorporated into the porous material (which depends on the chemical form in which is being captured: physisorption, HCO₃^- or carbamate), whereas the gravimetric measurement quantifies the total mass of CO₂ being captured in the aqueous suspension of the Cu₆Cr particles regardless the chemical form in which this capture takes place. After 1 hour of CO₂ bubbling (300 mL·min⁻¹) into the aqueous suspension of Cu₆Cr particles, capture mass values of 22.3 % and 17.1 % are measured by magnetic sustentation and gravimetric techniques, respectively. This difference is because the CO₂ is captured as H₂CO₃ that reacts with the adeninato ligands to form adenine/HCO₃^- pairs. These values normalize to 5.9 HCO₃^- and 6.4 CO₂ molecules per Cu₆Cr entity, which are close to the theoretical value of 6, because of the six adeninato ligands per heptameric Cu₆Cr entity. CO₂ adsorption isotherms, adsorption/desorption kinetics and cycling stability are also reported. Kinetic studies provide a ΔH_ads = -19.4 kJ/mol, which is a significantly lower than other CO₂ adsorbents. However, the cycling stability needs to be improved.