Two structural transformation mechanisms of ZIF-8 induced by encapsulation of imidazolium-based aprotic ionic liquids

Abstract Metal–organic frameworks (MOFs) are three-dimensional crystalline materials composed of metal nodes and organic ligands, forming porous coordination networks with high surface areas and tunable internal cavities. These structural features enable MOFs to act as hosts for small guest molecules, such as ionic liquids (ILs), facilitating host–guest interactions that can modify physical properties and enhance performance in applications such as fuel cells and batteries. In this study, two imidazolium-based aprotic ILs with different anions, 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([Emim][TfO]) and 1-ethyl-3-methylimidazolium bis-(trifluoromethylsulfonyl)imide ([Emim][TFSI]), were successfully encapsulated within the pores of the MOF ZIF-8. The structural and vibrational impacts of these ILs on the ZIF-8 framework were systematically investigated using powder X-ray diffraction (PXRD) and Fourier-transform infrared spectroscopy (FT-IR). The results revealed two distinct mechanisms for the uptake of an IL, leading to an expansion of the MOF cage structure: [Emim][TfO]@ZIF-8 exhibited a phase transition characterized by the coexistence of two lattice constants, while [Emim][TFSI]@ZIF-8 maintained a single-phase structure with continuous lattice expansion upon increased loading. These behaviors are attributed to the swing motion of 2-methylimidazole linkers and the elongation of Zn–N bonds induced by internal pressure from the confined ILs. Additionally, confinement within the ZIF-8 cages led to the strengthening of –SO₃ and S–N–S bonds in the ILs, as evidenced by the blue-shifted and intensified IR vibrational modes. This study provides fundamental insights into the structural evolution and host–guest interactions in IL@MOF systems, offering guidance for tailoring MOF properties through ILs encapsulation.