X-ray absorption spectroscopy as a useful tool for revealing the structuraldetails associated to the ZIF-8 flexibility upon gas adsorption.
Roberto Boada1,2
1

Chemistry Department, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain2

Diamond Light Source, Harwell Science and Innovation Campus, OX11 0DE, UK

Among the different families of metal organic frameworks (MOFs), zeolitic imidazolate frameworks(ZIFs)1 have received a great deal of attention due to their thermal and chemical stability.2 However, unlikezeolites with relatively rigid frameworks and apertures, the structure of ZIFs is rather flexible.3,4 Indeed, inthe case of the prototypical ZIF-8 (Zn(MeIM)2; MeIM = 2-methylimidazole), a change in the structure can betriggered by either mechanical pressure,5 or gas adsorption.6 This so called “gate opening” transition
involves the swing of the MeIM linkers increasing the pore window size which induces a sudden increase in
the gas uptake.6 We have studied the local structural changes associated with the ZIF-8 framework flexibility
upon nitrogen gas adsorption by in situ X-ray absorption spectroscopy (XAS) and high energy resolution
fluorescence detected X-ray absorption near edge structure (HERFD-XANES) spectroscopy.7 To elucidate thesource of the spectral changes in the XANES region of the absorption spectra observed along the gas
adsorption and through the transition from the closed to the open pore configuration, ab-initio calculations
have been performed. Our results demonstrate that the transition from the closed to the open poreconfigurations not only involves the rotation of the MeIM ligand, but a further bend of the Me group awayfrom the plane defined by the IM ring. Additionally, the contribution of the N2 molecules adsorbed in thecentre of the 4-ring window has been included in the scattering model to fully reproduce the main featuresof the X-ray absorption spectra in the open pore configuration.

References:
1. Park, K. S. et al. Exceptional chemical and thermal stability of zeolitic imidazolate frameworks. Proc. Natl. Acad.
Sci. U. S. A. 103, 10186–10191 (2006).
2. Eddaoudi, M., Sava, D. F., Eubank, J. F., Adil, K. & Guillerm, V. Zeolite-like metal–organic frameworks (ZMOFs):
design, synthesis, and properties. Chem. Soc. Rev. 44, 228–249 (2015).
3. Bux, H. et al. Zeolitic Imidazolate Framework Membrane with Molecular Sieving Properties by MicrowaveAssisted Solvothermal Synthesis. J. Am. Chem. Soc. 131, 16000–16001 (2009).
4. Ryder, M. R. et al. Identifying the role of terahertz vibrations in metal-organic frameworks: From gate-opening
phenomenon to shear-driven structural destabilization. Phys. Rev. Lett. 113, 1–6 (2014).
5. Moggach, S. A., Bennett, T. D. & Cheetham, A. K. The effect of pressure on ZIF-8: Increasing pore size withpressure and the formation of a high-pressure phase at 1.47 GPa. Angew. Chemie - Int. Ed. 48, 7087–7089 (2009).
6. Fairen-Jimenez, D. et al. Opening the gate: Framework flexibility in ZIF-8 explored by experiments andsimulations. J. Am. Chem. Soc. 133, 8900–8902 (2011).
7. Boada, R. et al. Unraveling the Molecular Details of the “Gate Opening” Phenomenon in ZIF-8 with X-ray
Absorption Spectroscopy. J. Phys. Chem. C 126, 5935–5943 (2022).