Measuring Diffusion Kinetics with Zero-Length Chromatography in Isomorphous and Ultramicroporous Phosphonate Metal–Organic Frameworks

We report thermodynamic and kinetic gas separation studies in ultramicroporous phosphonate MOFs, previously reported Calgary Framework (CALF)-35 (Sr2+) as well as two new, fully characterized, isomorphous trivalent analogues (CALF-35 (La3+) and (Ce3+)). The trivalent materials exhibit dramatic improvements in stability, retaining their structure and porosity when exposed to both acidic and basic solutions (pH 1–11) compared with the divalent analogue. CALF-35 has 1-D pores with an undulating channel structure owing to transannular methyl groups that constrict the channel. While isomorphous, the three structures show subtle differences in the ultramicroporous channels. Trivalent analogues exhibit contracted pore apertures, resulting in pronounced differences in their ability to adsorb guest molecules. CALF-35 (Sr2+) has more nonpolar 1-D channels that exhibit high affinity for C2H4 and C2H6 over CO2. In contrast, in isomorphous CALF-35 (La3+), this selectivity is reversed with a higher CO2 uptake. To probe the relative kinetics of adsorption in these heavily constricted pore channels, diffusion coefficients for C2H6, C2H4, C2H2, CH4, CO2, and N2 for representative divalent CALF-35 (Sr2+) and trivalent CALF-35 (La3+) analogues were determined using zero-length chromatography.