Investigation of Compression-Induced Deformations in Local Structure and Pore Architecture of ZIF-8 Using FTIR, X-ray Absorption, and Positron Annihilation Spectroscopy

Abstract

Compression-induced (≤1.01 GPa) phase transformation, local deformations, and pore architecture modifications in zeolitic-imidazole framework (ZIF-8) powder have been investigated, ex situ, using X-ray diffraction, extended X-ray absorption fine structure, Fourier transform infrared, and positron annihilation lifetime spectroscopy. Compression-induced reduction in crystallinity leading to partial amorphization of ZIF-8 is observed to be an irreversible phenomenon. The amorphization of ZIF-8 was accompanied by local deformations in imidazolate ring bonding. The study has confirmed the retention of Zn–N tetrahedral arrangement in the amorphous state of the framework through X-ray absorption measurements. The phase transformation and local deformations lead to modifications in powder morphology as well as reduction in internal porosity of the framework. The triplet state of positronium (o-Ps) has been used to investigate the pressure-induced pore architecture modifications. At the lowest applied pressure (0.20 GPa), partial collapse of open free volume inside the crystalline ZIF-8 leading to reduction in internal porosity has been observed, while the crystal structure is maintained. On further increasing the pressure, amorphization of ZIF-8 is observed leading to cataclysmic modifications in the pore architecture. The amorphous phase of the framework possesses a continuous random network of open volumes having a broader size distribution with fewer interconnections as compared to the crystalline counterpart. The present study also indicates the evolution of a new crystalline topology of ZIF-8 at applied pressure of 1.01 GPa through complementary techniques.