Selective Carbon Dioxide Adsorption by Two Robust Microporous Coordination Polymers

In the present work we report the design, synthesis, crystal structure determination, and adsorption properties of two new cadmium-based porous coordination polymers, [Cd(pda)_0.5(spiro-4-py)_0.5(HCOO)]·2H₂O·DMF (compound 1, pda = p-phenylenediacetate, spiro-4-py = (2,2',7,7'-tetra(pyridin-4-yl)-9,9'-spirobi[fluorene], DMF = N,N'-dimethylformamide), and [Cd₂(pda)(spiro-4-py) (CH₃COO)₂]·DMA (compound 2, DMA = N,N'-dimethylacetamide) with similar structures. The coordination between cadmium and two organic linkers, pda and spiro-4-py, has yielded two-dimensional frameworks with rhombic openings. Stacking of these two-dimensional networks does not block the openings but rather results in permanent porosity with one-dimensional channels in the final structures. The permanent porosity of these compounds is confirmed by gas adsorption measurements. Compounds 1 and 2 have Brunauer-Emmett-Teller surface areas of 687 and 584 m²/g, respectively. Both compounds show favorable adsorption toward carbon dioxide over other light gases such as nitrogen, oxygen, and carbon monoxide. Ideal adsorbed solution theory is employed to predict the adsorption selectivity of binary gas mixtures. Though compounds 1 and 2 possess similar structures, differences are observed in their gas adsorption behaviors, which can be attributed to their different terminal ligands of formate and acetate, respectively. Strikingly, both compounds show exceptionally high stability in aqueous media with a wide pH range, a characteristic that is highly desirable for gas separation-related applications. The robustness of these structures suggests that the use of hydrophobic spiro-based multipyridine ligands can lead to water stable frameworks built on late-transition metals that are otherwise sensitive to moisture.