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Assessing Guest‐Molecule Diffusion in Heterogeneous Powder Samples of Metal–Organic Frameworks through Pulsed‐Field‐Gradient (PFG) NMR Spectroscopy
Author(s) -
Thoma Roland,
Kärger Jörg,
de Sousa Amadeu Nader,
Nießing Sandra,
Janiak Christoph
Publication year - 2017
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201702586
Subject(s) - pulsed field gradient , diffusion , microporous material , porosity , nanometre , chemical physics , particle (ecology) , spectroscopy , chemistry , materials science , molecule , analytical chemistry (journal) , nanotechnology , organic chemistry , thermodynamics , physics , oceanography , quantum mechanics , composite material , geology
Investigation of guest diffusion in porous metal–organic frameworks (MOFs) is of major importance, because many porosity‐related properties of MOFs are influenced by diffusion effects. The diffusion of dimethyl sulfoxide (DMSO) in the MOF MIL‐53‐NH 2 (Al) was investigated through pulsed‐field‐gradient (PFG) NMR spectroscopy. The microporous material was synthesized in small crystallites (under 500 nm), which agglomerated in a large range of particle sizes (from hundreds of nanometers to tens of micrometers), giving a morphologically very heterogeneous sample. No special agglomeration pattern could be observed, which makes a PFG NMR investigation very challenging, yet it represents a realistic situation for the diffusion of guest molecules in porous materials. We were able to distinguish between two diffusion regimes existing in parallel with each other over the total range from 15 to 200 ms of observation times as accessible in the experiments: In the large crystal agglomerates (diameters above 20 μm), guest movement was found to be subdiffusive, with a time exponent κ =0.8 (rather than one as for normal diffusion). Guest diffusion in the remaining, smaller host particles followed the pattern of normal diffusion within a bed of spheres of impenetrable external surfaces, with a size distribution in good agreement with that of the material under study. Diffusion in a rather complex system could thus be referred to a two‐region model with new potentials for application to systems of intricate topology.