Journal Article

PUBDB-2026-02022

http://join2-wiki.gsi.de/foswiki/pub/Main/Artwork/join2_logo100x88.png Open Metal Sites Govern Hydration Kinetics and Molecular Fluctuations in Metal–Organic Frameworks

Yildirim, A. ; Szymoniak, P. ; Haug, T. C. V. ; Fröba, M. ; Huber, P. (Corresponding author)Extern*DESY* ; Schönhals, A.

2026
Soc. Washington, DC

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Please use a persistent id in citations: doi:10.1021/acs.jpcc.6c02160

Abstract: Water under nanoscale confinement exhibits structural and dynamical states distinct from bulk behavior, yet the role of pore chemistry in governing these states remains insufficiently understood. Herein, the hydration kinetics and molecular-scale fluctuations of water confined within a comprehensive series of MOF-74 metal-organic frameworks incorporating Mg, Ni, Co, and mixed-metal compositions were investigated and compared with those observed in mesoporous MCM-41 silica. In situ impedance spectroscopy resolves a pronounced multistep hydration mechanism in MOF-74, initiated by rapid coordinative binding at open metal sites, followed by cluster formation and final capillary condensation. In contrast to MCM-41 silica, where capillary condensation dominates, the presence of coordinatively unsaturated metal centers fundamentally alters adsorption pathways. Mixed-metal MOF-74 exhibits accelerated and temporally broadened hydration, reflecting heterogeneous distributions of adsorption energies. Broadband dielectric spectroscopy reveals a distinct water-specific relaxation process (w-relaxation) attributed to fluctuations of water clusters interacting with metal nodes. Relaxation rates follow Arrhenius behavior with activation energies increasing in the order Mg < Ni < Co and further enhanced in mixed-metal systems. Remarkably, water confined within MOF-74 remains liquid-like down to 133 K without crystallization, whereas crystallization is observed in MCM-41 silica. All activation data of the MOF-74 systems obey a common Meyer–Neldel compensation relation, indicating cooperative molecular dynamics and suggesting a hindered glass-transition–like process of confined water clusters. These findings demonstrate that open metal sites and metal composition enable programmable control over hydration kinetics and collective water dynamics in microporous frameworks. MOF-74 thus provides a tunable model platform for engineering water-driven transport, catalysis, and energy-relevant processes at the nanoscale.

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Contributing Institute(s):

1. CIMMS-RA Center for integr. Multiscale M (CIMMS)

Research Program(s):

1. 632 - Materials – Quantum, Complex and Functional Materials (POF4-632) (POF4-632)
2. DFG project G:(GEPRIS)533771286 - EXC 3120: BlueMat: Wassergesteuerte Materialien (533771286) (533771286)

Experiment(s):

1. No specific instrument

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Database coverage:
; Clarivate Analytics Master Journal List ; Current Contents - Physical, Chemical and Earth Sciences ; Essential Science Indicators ; IF < 5 ; JCR ; SCOPUS ; Science Citation Index Expanded ; Web of Science Core Collection

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