Influence of Metal Substitution on the Pressure-Induced Phase Change in Flexible Zeolitic Imidazolate Frameworks

<p>Metal–organic frameworks that display step-shaped adsorption profiles arising from discrete pressure-induced phase changes are promising materials for applications in both high-capacity gas storage and energy-efficient gas separations. The thorough investigation of such materials through chemical diversification, gas adsorption measurements, and <i>in situ </i>structural characterization is therefore crucial for broadening their utility. We examine a series of isoreticular, flexible zeolitic imidazolate frameworks (ZIFs) of the type M(bim)<sub>2</sub> (SOD; M = Zn<sup> </sup>(ZIF-7), Co (ZIF-9), Cd (CdIF-13); bim<sup>–</sup> = benzimidazolate), and elucidate the effects of metal substitution on the pressure-responsive phase changes and the resulting CO<sub>2</sub> and CH<sub>4</sub> step positions, pre-step uptakes, and step capacities. Using ZIF-7 as a benchmark, we reexamine the poorly understood structural transition responsible for its adsorption steps and, through high-pressure adsorption measurements, verify that it displays a step in its CH<sub>4 </sub>adsorption isotherms. The ZIF-9 material is shown to undergo an analogous phase change, yielding adsorption steps for CO<sub>2</sub> and CH<sub>4</sub> with similar profiles and capacities to ZIF-7, but with shifted threshold pressures. Further, the Cd<sup>2+</sup> analogue CdIF-13 is reported here for the first time, and shown to display adsorption behavior distinct from both ZIF-7 and ZIF-9, with negligible pre-step adsorption, a ~50% increase in CO<sub>2</sub> and CH<sub>4</sub> capacity, and dramatically higher threshold adsorption pressures. Remarkably, a single-crystal-to-single-crystal phase change to a pore-gated phase is also achieved with CdIF-13, providing insight into the phase change that yields step-shaped adsorption in these flexible ZIFs. Finally, we show that the endothermic phase change of these frameworks provides intrinsic heat management during gas adsorption. </p>