Flexible Nickel-Fumarate Metal–Organic Framework Exhibiting Reversible Single-Crystal-to-Single-Crystal Phase Transformation for Adsorptive Separation of Water–Organic Azeotropic Mixtures

Abstract

Separating water–organic solvent azeotropes remains a major challenge, rendering conventional distillation ineffective. While metal–organic frameworks (MOFs) show promise for adsorption-based separation, most rely on static, preengineered pores, limiting discrimination between similar molecules. Here, we report Ni-fum, a flexible MOF built from nickel ions and fumaric acid, that undergoes a reversible single-crystal-to-single-crystal transition and achieves highly selective water adsorption from azeotropic organic solvents via guest-induced structural adaptability. Thermal activation at 393 K under vacuum removes guest molecules and coordinated water, triggering relative displacement of adjacent one-dimensional (1D) Ni–O chains and forming interchain Ni–O–Ni linkages. This reorganizes 1D secondary building units (SBUs) into two-dimensional (2D) layered inorganic blocks, constricting pores along the a axis to yield the high-temperature phase, Ni-fum-HT. Crucially, this transformation is water-specific: only water fully restores Ni-fum-HT to its original room-temperature phase (Ni-fum-RT); ethanol, isopropanol, THF, and acetonitrile do not. Leveraging this selectivity, Ni-fum-HT exhibits high water uptake at low relative pressures but a negligible uptake of the tested organics, delivering excellent H2O/organic selectivity across a broad pressure range. This work reports the first reversible 1D-to-2D SBU single-crystal-to-single-crystal transformation, in which reversible Ni–O bond reorganization and water-triggered structural recovery underpin selective adsorption, providing an alternative structural paradigm for designing stimuli-responsive MOFs toward energy-efficient azeotropic dehydration.