Evolution of Local Structure and Pore Architecture during Zeolitic Imidazolate Framework-L to Zeolitic Imidazolate Framework-8 Phase Transformation Investigated Using Raman, Extended X-ray Absorption, and Positron Annihilation Lifetime Spectroscopy

Zeolitic imidazolate framework-leaf(ZIF-L) is transformed to ZIF-8through a topotactic phase transformation. This phase transformationis driven by the removal of the hydrogen-bonded linker molecules from2D galleries of ZIF-L, leading to distortion in the hydrogen-bondedstructure. Investigation of crystallization kinetics of ZIF-L showsthat crystalline particles are formed within the first minute, andthe growth of leaf-type structures is finished in nearly 10 minutes.During ZIF-L to ZIF-8 transformation, leaf-like ZIF-L particles arecleaved and transformed to 3D ZIF-8 nanocrystals. The chemical bondingstructure investigated through Fourier transform infrared and Ramanspectroscopy shows shifting and narrowing of characteristic peaksof stretching modes corresponding to linker molecules and Zn-Nduring the transformation. It confirms that distortions occur in thehydrogen-bonded structure of the ZIF-L phase. X-ray absorption spectroscopyshows modifications in the local structure around the Zn atom dueto the reorientation of the fully bridged linker without affectingthe Zn-N bond distances in the basic ZnN4 tetrahedralunit of ZIF-L or ZIF-8. Pore architecture evolution during the phasetransformation is investigated by positron annihilation lifetime spectroscopy.During the transformation, the pores of ZIF-L are initially expandedwith the decrease in their number density due to the breakdown ofwalls formed by intercalated linker molecules. With the increase inthe ZIF-8 phase, the channel network of ZIF-8 sodalite topology alongwith inter-crystalline voids is gradually produced. A significantpore tuning of ZIF-L and ZIF-8 is achieved depending on the phasetransformation time, which can be utilized for the enhancement inthe gas separation efficiency of ZIF-8- or ZIF-L-based membranes.