Interactions of Zeolitic Imidazolate Framework ZIF-8 with Solvents: Implications for Stability in Polymer Composites

High Resolution Image Download MS PowerPoint Slide Zeolitic Imidazolate Frameworks (ZIFs) are a subclass of metal organic frameworks (MOFs) that have been investigated for applications in adsorption processes, gas separation, catalysis, drug delivery, and sensors due to their microporous structure. Additionally, ZIF-8 nanoparticles have been applied in polymer composites including membrane technology; however, fewer studies have considered the chemical stability of ZIF-8 in the presence of organic solvents, which are used to dissolve polymers. The aim of this work is to quantify the impact of water and NMP (common solvents for the synthesis of mixed matrix membranes) on the crystalline structure and microstructure of ZIF-8 over time. Two ZIF-8 particles were studied: 1) ZIF-8S, synthesized using methanol as solvent, with a particle size of 50 nm and a zeta potential (ZP) of −5.5 ± 1.1 mV, and 2) ZIF-8C, a commercial Basolite ZIF-8, with a particle size of 100–500 nm and a ZP of 9.1 ± 1.5 mV. After 24 h of hydrothermal treatment, ZIF-8C showed a minimal decrease in crystallinity, while ZIF-8S experienced a reduction of nearly 50% in crystallinity. Stirring ZIF-8S and ZIF-8C in NMP also resulted in measurable changes in the crystalline structure, particularly for ZIF-8C, which had a 43% reduction in surface area compared to ZIF-8S, which only had a 25% reduction. XPS revealed that ZIF-8C and ZIF-8S exhibit the characteristic surface chemistry of ZIF-8, with ZIF-8C showing a Zn:N ratio (0.24) consistent with a defect-free surface, while ZIF-8S presents a higher ratio (0.31) indicative of some surface defects. Fourier transform infrared spectroscopy (FTIR) provided evidence for the presence of imidazole in ZIF-8S and ZIF-8C samples after treatment, which indicate structural degradation. These results demonstrate the need to consider solvent effects on the stability and performance of ZIF-8, particularly when incorporating MOFs into polymer composites such as mixed matrix membranes.