(Keynote) Electromagnetic Field Stimulated MOFs

Abstract

Electromagnetic, external stimuli can be used to switch Metal-Organic Framework (MOF) materials and to achieve control over the pores of these materials on different ways. We designed MOFs with switchable moieties in their side-chains, such as azobenzene, to be switchable under irradiation of UV/Vis light. Additionally, we developed devices to be able to measure the impact of electromagnetic fields in-situ. We focused on surface anchored MOF (SURMOF) thin films, deposited on asymmetric, porous α-Al2O3 supports as gas separation membrane layers. It is possible to control the gas transport properties of the materials using, but also using electric fields, and measure changes gas separation (permeability and selectivity) of binary mixtures in-situ, using the Wicke-Kallenbach technique. The light switchable systems were investigated on their CO2 separation performance: By switching the azobenzene between cis and trans isomers inside the pore windows, it was possible to switch dipolar moments, thus changing the adsorption properties towards the quadrupolar moment of CO2. Using electric fields of 500 V/mm we could show on the prominent example of ZIF-8, a zeolitic imidazolate framework, that it has a strong impact on the gas transport properties, especially looking at the molecular sieving. The mechanistic of the electric field influence on the ZIF is as follows: The electric field distorts the lattice of the MOF, shifting the Zn2+ ions, lowering the symmetry of the unit cell. This effect leads to polarized polymophs of the structure, sharpening the molecular sieving through a stiffening effect. The purification of Propylene/Propane, a highly desired separation for the plastics industry, could be increased through the electric field stimuli by 33%. References Knebel, A. et al. Defibrillation of soft porous metal-organic frameworks with electric fields. Science 358, 347–351; 10.1126/science.aal2456 (2017). Wang, Z. et al. Tunable molecular separation by nanoporous membranes. Nat. Commun. 7, 13872; 10.1038/ncomms13872 (2016). Müller, K. et al. Switching Thin Films of Azobenzene-Containing Metal-Organic Frameworks with Visible Light. Chem. Eur. J. 23, 5434–5438; 10.1002/chem.201700989 (2017). Knebel, A. et al. Azobenzene Guest Molecules as Light-Switchable CO 2 Valves in an Ultrathin UiO-67 Membrane. Chem. Mater. 29, 3111–3117; 10.1021/acs.chemmater.7b00147 (2017). Tan, N. Y. et al. Investigation of the terahertz vibrational modes of ZIF-8 and ZIF-90 with terahertz time-domain spectroscopy. Chem Commun 51, 16037–16040; 10.1039/c5cc06455d (2015). Figure 1