Water vapour and gas induced phase transformations in an 8-fold interpenetrated diamondoid metal-organic framework

In this work, we report the synthesis, structural characterisation and sorption properties of an 8-fold interpenetrated diamondoid (dia) metal-organic framework (MOF) that is sustained by a new extended linker ligand, [Cd(Imibz)2], X-dia-2-Cd, HImibz or 2 = 4-((4-(1H-imidazol-1-yl)phenylimino)methyl) benzoic acid. X-dia-2-Cd was found to exhibit reversible single-crystal-to-single-crystal (SC-SC) transformations between four distinct phases: an as-synthesised (from N,N-dimethylformamide) widepore phase, X-dia-2-Cd-a; a narrow-pore phase, X-dia-2-Cd-b, formed upon exposure to water; a narrow-pore phase obtained by activation, X-dia-2-Cd-g; a medium-pore CO2-loaded phase X-dia-2Cd-d. While the space group remained constant in the four phases, the cell volumes and calculated void space ranged from 4988.7 A3 and 47% (X-dia-2-Cd-a), respectively, to 3200.8 A3 and 9.1% (X-dia-2-Cdg), respectively. X-dia-2-Cd- g also exhibited a water vapour-induced structural transformation to the water-loaded X-dia-2-Cd- b phase, resulting in an S-shaped sorption isotherm. The inflection point occurred at 18% RH with negligible hysteresis on the desorption profile. Water vapour temperaturehumidity swing cycling (60% RH, 300 K to 0% RH, 333 K) indicated hydrolytic stability of X-dia-2-Cd and working capacity was retained after 128 cycles of sorbent regeneration. CO2 (at 195 K) was also observed to induce a structural transformation in X-dia-2-Cd-g and in situ PXRD studies at 1 bar of CO2, 195 K revealed the formation of X-dia-2-Cd-d, which exhibited 31% larger unit cell volume than X-dia-2Cd-g.

Automated summary

In this study, a novel 8-fold interpenetrated diamondoid metal-organic framework (MOF) with reversible single-crystal-to-single-crystal transformations and variable pore sizes was synthesized and characterized. The framework exhibited different phases depending on the synthesis and activation conditions, with varying cell volumes and void space. The sorption properties of the MOF were also investigated, showing a water vapor-induced structural transformation and CO2-induced expansion of the unit cell volume. The MOF demonstrated hydrolytic stability and retained its working capacity after multiple cycles of sorbent regeneration.