Fluorinated Nanosized Zeolitic-Imidazolate Frameworks as Potential Devices for Mechanical Energy Storage

Fluorination is one
of the most efficient and universal
strategies
to increase the hydrophobicity of materials and consequently their
water stability. Zeolitic-imidazolate frameworks (ZIFs), which have
limited stability in aqueous media and even lower stability when synthesized
on a nanometric scale, can greatly benefit from the incorporation
of fluorine atoms, not only to improve their stability but also to
provide additional properties. Herein, we report the preparation of
two different fluorinated ZIFs through a simple and scalable approach
by using mixed ligands [2-methylimidazole, as a common ligand, and
4-(4-fluorophenyl)-1H-imidazole (monofluorinated linker) or 2-methyl-5-(trifluoromethyl)-1H-imidazole
(trifluorinated linker) as a dopant], demonstrating
the high versatility of the synthetic method developed to incorporate
different fluorine-containing imidazole-based ligands. Second, we
demonstrate for the first time that these nanoscale
fluorinated ZIFs outperform the pristine ZIF-8 for water intrusion/extrusion,
i.e., for storing mechanical energy via forced intrusion of nonwetting
water due to the improved hydrophobicity and modified framework dynamics.
Moreover, we also show that by varying the nature of the F-imidazole
ligand, the performance of the resulting ZIFs, including the pressure
thresholds and stored/dissipated energy, can be finely tuned, thus
opening the path for the design of a library of fluorine-modified
ZIFs with unique behavior.