Cooperative adsorbents are a class of responsive materials that exhibit phase change-like behavior upon adsorption. These materials offer several advantages over classical adsorbents for use in selective gas separations by featuring high working capacities while requiring only small temperature swings in potential temperature swing adsorption processes. However, rational design of such materials has remained elusive. In theory, a metal–organic framework containing coordinatively unsaturated metal sites can act as a selective and cooperative adsorbent if gas binding at one metal site triggers an electronic transformation that subsequently alters the binding properties at neighboring metal sites. We report a new class of materials that illustrates this concept through carbon monoxide binding at coordinatively unsaturated Fe(II) sites in a metal–organic framework. Functioning through a novel mechanism in which the spin states of neighboring iron sites simultaneously transition above a temperature-dependent threshold pressure to concertedly bind CO, these materials feature very large working capacities of 12 wt% with only 40 °C temperature swings. Additionally, due to the spin transition mechanism, these materials remain extremely selective for CO over H2, N2, and even typically strongly binding gases such as CO2, making them very industrially relevant. Importantly, due to the tunability of open metal sites in metal–organic frameworks, we envision that this electronic basis for cooperative adsorption can be applied to other materials for designing selective, cooperative adsorbents suitable for a wide variety of processes.

Cooperative, adsorbate-induced spin transitions for efficient carbon monoxide separations in an Fe(II) metal–organic framework

253rd National Meeting (2017)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.