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.