(465d) Carbon Dioxide Capacity Retention on Elastic Layered Metal Organic Frameworks Subjected to Hydrothermal Cycling

Flexible MOFs, also known as soft porous crystals (SPCs), are a subset of MOFs that possess both a highly ordered network and structural transformability. In contrast with rigid MOFs, which retain their structure and their porosity irrespective of environmental factors, SPCs undergo structural transformations that are dependent on external stimuli such as temperature, mechanical pressure, or guest adsorption, on account of their bi-stable or multi-stable attributes. This facet of SPCs has led to the observation of previously unanticipated gas adsorption phenomena.

A subset of SPCs are the so-called elastic layered metal-organic frameworks (ELMs). ELMs are composed of metal vertex ions, connecting ligands, and charge-balancing counter-ions that are arranged into two-dimensional sheets that in turn assemble into three-dimensional stacked structures. These materials show a latent porosity for the adsorption of gas molecules above a specific pressure, termed the “gate pressure”, that results in an expansion of the interlayer spacing between the two-dimensional sheets and a corresponding jump in the adsorption isotherm that cannot be classified in accordance with the conventional IUPAC isotherm designations. The exotic adsorption characteristics of ELMs are not observed in more familiar commercial adsorbents such as activated carbons or zeolites. Nor are they found in the adsorption isotherms of MOFs with rigid pore structures. These unusual features confer upon ELMs potential advantages for CO₂ capture, inasmuch as they combine a high selectivity for separation of CO₂ from gas mixtures with a low energy requirement for adsorbent regeneration and CO₂ recovery.

Adsorption of carbon dioxide on elastic layered metal-organic frameworks (ELMs) was investigated during and after exposure to water. Two ELM variants, ELM-11 and ELM-12, were contacted with water vapor and the impact of cyclical exposure on the CO₂ capacity of the adsorbents was observed. ELM-11 was found to lose CO₂ capacity with each successive exposure to water, whereas ELM-12 retained CO₂ capacity through four exposure cycles. Density functional theory calculations were performed to interpret these observations. Changing the counter-ion from the simple tetrafluoroborate (BF₄^-) to the larger and more complex trifluoromethanesulfonate (CF₃SO₃^-) anion expands the number of potential binding sites for adsorbate molecules. While CO₂ directly competes with other adsorbates for binding sites in ELM-11, CO₂ does not directly compete with other adsorbates in ELM-12 due to its preference for direct interaction with both fluorine and oxygen atoms in CF₃SO₃^-.