Stability of porous liquids based on ZIF-8 framework and ionic liquid/water mixtures: a spin-probe EPR study

Porous liquids (PL) emerge as a new class of materials for multifunctional applications, and type-III PLs consisting of metal-organic frameworks (MOFs) dispersed in liquids attract particular attention. They are easy to prepare and functionalize; however, the stability of such media is a significant challenge. In this work we employ Electron Paramagnetic Resonance (EPR) to investigate stability of PLs based on ZIF-8 framework micro/nanoparticles and mixtures of water with ionic liquid (IL) [Bmim]BF 4 . For this sake, stable nitroxide radical TEMPO was permanently encapsulated into ZIF-8, which allows monitoring the dissolution of MOF and solvent penetration into its pores vs. time using EPR. It has been found that most stable PLs are obtained with ZIF-8 dispersed in pure IL or pure water: these PLs remain intact up to 95 % in two days following their preparation. However, PLs prepared with IL/water mixtures suffered from drastically enhanced MOF dissolution rates due to their lower pH, resulting in much lower lifetimes. In all these cases, the penetration of liquid into pores was negligible, while the main factor affecting stability was the dissolution of MOF particles, which could be precisely followed by in situ EPR. Thus, this work provides general and effective tools to investigate stability of PLs using spin-probe EPR approach and reveals new rational strategies for adjusting type-III PL compositions toward their practical applications. • A series of porous liquids (PL) based on metal-organic framework ZIF-8 is studied. • A spin-probe electron paramagnetic resonance approach is proposed and efficiently applied. • High stability of several PL compositions with water and ionic liquid [Bmim]BF4 is evidenced on a timescale of days. • Optimal design principles of ZIF-8-based PLs are proposed.