Postsynthetic tuning of hydrophilicity in pyrazolate MOFs to modulate water adsorption properties

Metal-organic frameworks (MOFs) have attracted interest as adsorbents in water-based adsorption heat pumps owing to their potential for increased water loading capacities and structural and functional tunability versustraditionally used materials such as zeolites and silica. Although pyrazolate-base...

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Bibliographic Details
Published in:Energy & environmental science 2013-07, Vol.6 (7), p.2172-2177
Main Authors: Wade, Casey R, Corrales-Sanchez, Tachmajal, Narayan, Tarun C, Dinca, Mircea
Format: Article
Language:English
Subjects:
Adsorbents
Adsorption
Channels
Ligands
Matlab
Relative humidity
Surface chemistry
Uptakes
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Summary:Metal-organic frameworks (MOFs) have attracted interest as adsorbents in water-based adsorption heat pumps owing to their potential for increased water loading capacities and structural and functional tunability versustraditionally used materials such as zeolites and silica. Although pyrazolate-based MOFs exhibit exceptional hydrolytic stability, the water adsorption characteristics of this class of frameworks have remained unexplored in this context. In this report, we describe the modular synthesis of novel dipyrazole ligands containing naphthalenediimide cores functionalized with -H (H sub(2)NDI-H), -NHEt (H sub(2)NDI-NHEt), or -SEt (H sub(2)NDI-SEt) groups. Reaction of these ligands with Zn(NO sub(3)) sub(2) afforded an isostructural series of MOFs, Zn(NDI-X), featuring infinite chains of tetrahedral Zn super(2+) ions bridged by pyrazolate groups and similar to 16 Aa-wide channels with functionalized naphthalenediimide linkers lining the channel surface. The Type V water adsorption isotherms measured for these materials show water uptake in the 40-50% relative humidity range, suggesting hydrophobic channel interiors. Postsynthetic oxidation of Zn(NDI-SEt) with dimethyldioxirane was used to generate ethyl sulfoxide and ethyl sulfone groups, thereby rendering the channels more hydrophilic, as evidenced by shifts in water uptake to the 30-40% relative humidity range. Such tunability in water adsorption characteristics may find utility in the design of new adsorbents for adsorption-based heat transfer processes. An original MATLAB script, MOF-FIT, which allows for visual modeling of breathing and other structural deformations in MOFs is also presented.
ISSN:1754-5692
1754-5706
DOI:10.1039/c3ee40876k