(456b) Understanding Carbon Monoxide Binding and Interactions in M-MOF-74 (M = Mg, Mn, Ni, Zn)

Authors: 
Pandey, I. - Presenter, Texas Tech University
Howe, J., Texas Tech University
Chen, C. C., Texas Tech University
Lin, L. C., The Ohio State University
Yang, C. T., The Ohio State Universtity
Carbon monoxide (CO) is an industrially important gas used as a precursor in a wide variety of chemical syntheses. To use CO for these applications, it must often be separated from mixtures. Adsorptive separation of CO is attractive as an energy-efficient alternative to conventional cryogenic distillation technologies. Metal—organic frameworks (MOFs) can have high adsorption capacities and selectivities, making them promising for adsorptive separation applications. Especially in open metal site (OMS) MOFs, a class of MOFs that has drawn particular interest for adsorptive separations, CO can engage in CO-to-metal σ-donation and metal-to-CO π-backbonding with metal centers.[1] These electronic interactions can lead to several structural changes in adsorbents that affect adsorption energetics and thus separation capabilities of materials, and any models of separation of CO from other gases must account for these energetic effects in order to be chemically accurate. To characterize CO adsorption in MOFs that may exhibit both σ- and π-complexation behaviors, we have used density functional theory (DFT) to study OMS MOFs with a range of d-band occupations from among 3d transition metals. Binding as a function of distance was studied in a range of 2-5 Å for Mg (3d0), Mn (3d5), Ni (3d8), and Zn (3d10) with various structural constraints ranging from geometrically constrained adsorbent and adsorbate ions to fully optimized geometries to deconvolute the relative contributions of various structural effects to the adsorption energetics and binding distances observed. Our data indicate that the greatest structural effects during adsorption correlate to the greatest π-backbonding behaviors and commensurately result in the greatest qualitative changes to the binding curves observed for CO adsorption. For instance, we find that for Ni-MOF-74, the difference between geometrically constrained and geometrically optimized MOF and adsorbate in equilibrium binding distance is roughly 0.02 nm and the difference binding energy is roughly 9 kJ/mol, indicating the importance of these electronic interactions and concomitant geometric distortions to adsorption behavior. Our data are in excellent agreement with previously published experimental binding energetics [2] and geometries[3]. The insights built from this work make progress toward solving two longstanding research challenges within the MOF community: rational design of materials for separations through exploiting phenomena like the distortions and associated binding energetics highlighted in this work, and design of force-fields for capturing relevant behaviors for modeling of adsorption and separations.

References:

  1. Aubke, F., et al., “Carbon Monoxide as a σ-Donor Ligand in Coordination Chemistry.” Coordination Chemistry Reviews, 137, pp. 483–524 (1994).
  2. Bloch, Eric D., et al., “Reversible CO Binding Enables Tunable CO/H2 and CO/N2 Separations in Metal–Organic Frameworks with Exposed Divalent Metal Cations.” Journal of the American Chemical Society, 136 (30), pp. 10752–10761 (2014).
  3. Lee, Kyuho, et al., “Small-Molecule Adsorption in Open-Site Metal–Organic Frameworks: A Systematic Density Functional Theory Study for Rational Design.” Chemistry of Materials, 27 (3), pp. 668–678 (2015).