Metal-organic frameworks (MOFs) and related flexible microporous structures are promising materials for molecule separation and storage due to their large surface area and tunable topology by chemical design. Unlike zeolite, MOFs often exhibit complex free-energy profiles, which can lead to occurrence of multiple stable and metastable polymorphs. They can undergo significant structural changes upon adsorption of guest molecules, external pressure and temperature changes. The challenge is to understand mechanistically the occurrence of structural transitions in flexible materials. Zeolitic imidazolate frameworks (ZIFs) are an important subgroup of metal-organic frameworks (MOFs) named after the resemblance of the Zn- imidazolate -Zn linkage to the Si-O-Si linkage found in zeolites and it has been reported that ZIFs often exhibit significant framework flexibility as a response to either pressure or temperature.

Among all ZIFs, ZIF-7 is fairly unique. Reyes et al discovered and others later confirmed that ZIF-7 undergoes a reversible “gate opening” upon adsorption of CO₂, C₂H₆, C₂H₈ and other guest molecules. The transition is from a low porosity, narrow pore structure to a high porosity, large pore (rhombohedral) structure. We’ve examined side-by-side the structural changes based on XRD and the CO₂ adsorption isotherms at different pressures, confirming that it is the highly reversible equilibrium based phase transition that drives the step change in adsorption isotherms. We’ve also discovered the unit cell of the lp phase can continue to expand to another steady large pore phase under the same rhombohedral symmetric upon adsorption of larger linear alcohols.

Interestingly, at high enough temperature (500 ^oC) under vacuum, the same phase transition will also occur without the presence of any guest molecules. That is driven by entropy only. We believe our results highlight general features of the polymorphic phase transitions in ZIFs and other structures, namely stabilization of more open structures by adsorption of guest molecules at low temperatures, and occurrence of more open structures at high temperatures for entropic reasons.

Peter Ravikovitch

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