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Revisiting Metal−Organic Frameworks Porosimetry by Positron Annihilation: Metal Ion States and Positronium Parameters

Attallah, A. G.; Bon, V.; Maity, K.; Zaleski, R.; Hirschmann, E.; Kaskel, S.; Wagner, A.


Metal-organic frameworks (MOFs) stand as pivotal porous materials with exceptional surface areas, adaptability, and versatility. Positron Annihilation Lifetime Spectroscopy (PALS) is an indispensable tool for characterizing MOF porosity, especially micro- and mesopores in both open and closed states. Notably, PALS offers porosity insights independently of probe molecules, vital for detailed characterization without structural transformations. This study explores how metal ion states in MOFs affect PALS results. We find significant differences in measured porosity due to paramagnetic or oxidized metal ions compared to simulated values. By analyzing CPO-27(M) (M=Mg, Co, Ni), with identical pore dimensions, we observe distinct PALS data alterations based on metal ions. Paramagnetic Co and Ni ions hinder and quench positroni-um (Ps) formation, resulting in smaller measured pore volumes and sizes. Mg only quenches Ps, leading to underestimated pore sizes without volume distortion. This underscores metal ions' pivotal role in PALS outcomes, urging caution in interpreting MOF porosity.

Keywords: Metal-Organic Frameworks (MOFs); Positron Annihilation Lifetime Spectroscopy (PALS); Positronium quenching; Paramagnetic Effects; MOF Pore Analysis

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