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
Jaideep Mor, P. Utpalla, Ravi Kumar, Jitendra Bahadur, Sandeep Kumar Sharma
Abstract
Zeolitic imidazolate framework-leaf (ZIF-L) is transformed to ZIF-8 through a topotactic phase transformation. This phase transformation is driven by the removal of the hydrogen-bonded linker molecules from 2D galleries of ZIF-L, leading to distortion in the hydrogen-bonded structure. Investigation of crystallization kinetics of ZIF-L shows that crystalline particles are formed within the first minute, and the growth of leaf-type structures is finished in nearly 10 minutes. During ZIF-L to ZIF-8 transformation, leaf-like ZIF-L particles are cleaved and transformed to 3D ZIF-8 nanocrystals. The chemical bonding structure investigated through Fourier transform infrared and Raman spectroscopy shows shifting and narrowing of characteristic peaks of stretching modes corresponding to linker molecules and Zn–N during the transformation. It confirms that distortions occur in the hydrogen-bonded structure of the ZIF-L phase. X-ray absorption spectroscopy shows modifications in the local structure around the Zn atom due to the reorientation of the fully bridged linker without affecting the Zn–N bond distances in the basic ZnN 4 tetrahedral unit of ZIF-L or ZIF-8. Pore architecture evolution during the phase transformation is investigated by positron annihilation lifetime spectroscopy. During the transformation, the pores of ZIF-L are initially expanded with the decrease in their number density due to the breakdown of walls formed by intercalated linker molecules. With the increase in the ZIF-8 phase, the channel network of ZIF-8 sodalite topology along with inter-crystalline voids is gradually produced. A significant pore tuning of ZIF-L and ZIF-8 is achieved depending on the phase transformation time, which can be utilized for the enhancement in the gas separation efficiency of ZIF-8- or ZIF-L-based membranes.