The role of CO₂ on climate change is motivating the development of new technologies for CO₂ capture. In particular, adsorbents capable of direct air capture (DAC) of CO₂ are highly sought since this process is not constrained to point sources. In addition, CO₂ removal from confined environments such as NASA spacecraft requires highly selective and efficient adsorbents. Our group has been investigating metal-organic frameworks (MOFs) containing transition metal hydroxide (M–OH) groups as selective adsorbents for CO₂ DAC. For example, CFA-1-ZnOH (Zn(ZnOH)₄(bibta)₃, bibta^2– = 5,5’-bibenzotriazolate) adsorbs large amounts of CO₂ at very low partial pressures (>2 mmol g^-1 at ≤ 5 mbar) via a reversible M–OH + CO₂ = M–O₂COH chemisorption process and can be regenerated with relatively mild heating. Short intercluster Zn-Zn distances (~7.7 A) between the secondary building units (SBUs) of this MOF result in synergistic hydrogen-bonding interactions that lead to its outstanding low-pressure CO₂ adsorption performance.
This presentation will discuss the synthesis and characterization of a novel Zn benzotriazolate MOF, Zn₉(bbtCO)₆(O₂CCH₃)₆ (bbtCO²^– = bis(1H-benzo[d][1,2,3]triazolate-5-yl)methanone). The structure of this new framework has been determined using a combination of electron and X-ray diffraction analysis, revealing the presence of Zn₉SBUs and short intercluster Zn–Zn distances of ~6 Å. Postsynthetic OAc^–/OH^– ligand exchange yields Zn₉(bbtCO)₆(OH)₆, which shows good CO₂ adsorption at very low pressures. Diffuse reflectance infrared Fourier transform spectroscopy confirms the formation of zinc bicarbonate groups upon adsorption CO₂. Thermal swing adsorption/desorption experiments show that the MOF requires high temperatures for regeneration, indicating that the shortened intercluster Zn–Zn distance has a profound influence on CO₂ adsorption.

Casey Wade

Assistant Professor, Ohio State University

Appearances