Adsorbate Selectivity of Isoreticular Microporous Metal-Organic Frameworks with Similar Static Pore Dimensions

Adsorbate selectivity has been investigated using three isoreticular metal-organic frameworks (MOFs) {[ML], 1 (M = Cu2+) and 2 (M = Zn2+) where L = 5-(pyridin-3-ylethynyl)isophthalate; 3 (M = Cu2+) where L = [(pyridin-3-ylmethyl)amino] isophthalate} of similar "static aperture size" but of different framework flexibility, where the MOFs have the same two different types of cagelike pores, cage A and cage B. While cage A of the MOFs with sufficiently large aperture size compared with the dimensions of the adsorbates investigated does not show any adsorbate selectivity, cage B with an approximate size match between the adsorbates and the pore apertures shows size selectivity for the adsorbates. Although the static aperture size of cage B in 3 is smaller than those in 1 and 2, the order of the "effective aperture sizes" of the cage Bs of the activated MOFs, 1a-3a, is 2a >= 3a > 1a, which reflects the differing framework flexibility. The size selectivity of the MOFs for N-2 and Ar follows the more shape-dependent second minimum dimension (MIN-2) of the adsorbate rather than the widely used kinetic diameter (KD). However, the size selectivity of the MOFs for CO, CO2, and O-2 is neither based on the KD nor on the MIN-2. Not only the aperture size but also the functionality of the aperture-constituting group plays a role in the selective adsorption.