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 N2 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 O2 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.