Template‐Mediated Control over Polymorphism in the Vapor‐Assisted Formation of Zeolitic Imidazolate Framework Powders and Films

The landscape of possible polymorphs for some metal–organic frameworks (MOFs) can pose a challenge for controlling the outcome of their syntheses. Demonstrated here is the use of a template to control in the vapor‐assisted formation of zeolitic imidazolate framework (ZIF) powders and thin films. Introducing a small amount of either ethanol or dimethylformamide vapor during the reaction between ZnO and 4,5‐dichloroimidazole vapor results in the formation of the porous ZIF‐71 phase, whereas other conditions lead to the formation of the dense ZIF‐72 phase or amorphous materials. Time‐resolved in situ small‐angle X‐ray scattering reveals that the porous phase is metastable and can be transformed into its dense polymorph. This transformation is avoided through the introduction of template vapor. The porosity of the resulting ZIF powders and films was studied by N2 and Kr physisorption, as well as positron annihilation lifetime spectroscopy. The templating principle was demonstrated for other members of the ZIF family as well, including the ZIF‐7 series, ZIF‐8_Cl, and ZIF‐8_Br. A template‐mediated MOF‐CVD (metal—organic framework‐chemical vapor deposition) method was developed for the syntheses of MOFs in both powder and thin‐film forms. The time‐resolved in situ synchrotron small‐angle X‐ray scattering reveals that introducing template vapor into the MOF‐CVD process precludes transformation into a dense phase, thus resulting in the porous structure.