Short- and long-term thermal stabilities of imidazolium-type ionic liquids on catalytic metal-oxide supports

Abstract

Ionic liquids (ILs) have attracted considerable interest because of their negligible vapor pressure, high thermal stability, strong solvating ability, and tunable physicochemical properties. They are widely used in catalysis, particularly in supported ionic liquid phase (SILP) catalysts and solid catalysts with ionic liquid layers (SCILL). In these systems, interactions between ILs and metal-oxide supports strongly influence thermal stability, making the stability limits of bulk ILs unsuitable for predicting catalyst performance. This study investigates the short- and long-term thermal stability of 34 imidazolium-based ionic liquids supported on SiO₂, γ-Al₂O₃, and MgO. Short-term stability was evaluated using thermogravimetric analysis (TGA), while long-term stability was assessed through prolonged isothermal treatments combined with FTIR spectroscopy and SEM/EDX analyses. The results demonstrate that thermal stability depends strongly on the acidity of the support, the cation–anion structure, alkyl chain length, and interactions between the ionic liquid and the metal-oxide surface. In general, thermal stability decreases as support acidity decreases, although several ionic liquids exhibit different trends due to their unique structural features. The findings provide guidance for selecting thermally stable ionic liquids for supported catalytic systems operating under different conditions.