Exploitation of flexibility, responsivity and chemical selectivity in switchable pillared layer MOFs for specific gas adsorption and separation – a first-principles approach

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Switchable pillared layer metal-organic frameworks offer high selectivity towards adsorbed gas species. This selectivity can even be enhanced if in addition the flexibility, which is offered by DUT-8, is exploited, as demonstrated for the example of CH4 vs. CO2 adsorption by Kaskel and Brunner (S2, P1). Here, we will explore how flexibility and selectivity can be synergistically combined. The potential to tailor linkers, pillars and metal centers offers a rich diversity of materials that can undergo both significant structural changes, resulting in different pore volumes, and subtle rearrangements of the constituent building blocks, which can enable particular geometries with specific interactions towards target molecules. The interplay between flexibility and linker functionality is still rather unexplored to date. Four key questions will be tackled by T1: First, we aim to understand the role of the adsorbate (QP4) and of the long-range ordering (QP3) on the flexibility of DUT-8(M), M=Ni, Cu, Co, Zn. This will be carried out in close collaboration with S2, where structural data will be obtained, and with T2, supporting T1 with force field calculations of large super cells. Here, we will also develop a numerical model that allows to estimate the flexibility of DUT-8 depending on the thermodynamic conditions and the type of adsorbate gas species (QP1). Second, we will investigate the responsivity of DUT-8(M) towards binary gas mixtures (QP4, QF1), in particular for gases imposing significant differences in responsivity of the MOF. Third, in order to guide synthesis in S2, we will investigate if and how building block variation, including functionalization, will affect the flexibility of DUT-8 and expanded DUT-128 derivatives (QP1, QF1). Finally, we will elucidate the specific structure of adsorbed molecules with specific groups and understand their interaction with the framework (QF2). In collaboration with P1, NMR spectroscopy of various nuclei will guide us to understand the specific adsorbate-MOF interactions. For the lattice geometry, we will support P2, which uses EPR techniques to follow the hinge dynamics of the MOFs either by spin probes in the mixed metal paddle-wheels of the framework or by adsorbed spin-carrying probe molecules in the pore (QP2).