Zeolitic imidazolate framework-8 coating in supercritical carbon dioxide for novel membranes design and the supercritical bridge phenomenon

Abstract

The study investigates Zeolitic imidazolate framework-8 (ZIF-8) coating with adipic dihydrazide (ADH) using supercritical carbon dioxide (scCO₂), with the aim of designing composite membranes that exhibit improved homogeneity and compatibility between the polymer and filler. Two high pressure techniques were applied to modify ZIF-8 particles, comprising supercritical solvent impregnation (SSI) that avoids the usage of any organic solvent, and supercritical assisted impregnation (SAI) in the presence of either water or DMF as a solvent for ADH. The effects of pure scCO₂ on ZIF-8 and its high-pressure hydrolysis were also investigated. The ZIF-8 modification was followed by FTIR, XRD, TGA, BET, and SEM methods. Cellulose acetate-based composite membranes were prepared using modified and pristine ZIF-8 and characterized by SEM/FIB and for hydrogen permeation. The results revealed that the coating of ZIF-8 by SAI and DMF as a solvent for ADH affected the structure of ZIF-8 least. Interestingly, despite the low solubility of ADH in scCO₂, the SSI was successful due to the high affinity of ADH toward ZIF-8, demonstrating a supercritical bridge phenomenon. However, the process affected the BET surface area, particle morphology, and porosity. Composite membranes prepared with a modified filler showed improved homogeneity and reduced hydrogen permeation, qualifying for further studies on the separation of gas mixtures containing hydrogen and carbon dioxide.