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

Abstract

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 ΔHads = -19.4 kJ/mol, which is a significantly lower than other CO2 adsorbents. However, the cycling stability needs to be improved.

Keywords

• Porous metal-organic materials
• CO2 capture
• Adsorption from water
• Adsorption quantification techniques