Observation of a Reversible Order‐Order Transition in a Metal‐Organic Framework – Ionic Liquid Nanocomposite Phase‐Change Material

Abstract Metal‐organic framework (MOF) composite materials containing ionic liquids (ILs) have been proposed for a range of potential applications, including gas separation, ion conduction, and hybrid glass formation. Here, an order transition in an IL@MOF composite is discovered using CuBTC (copper benzene‐1,3,5‐tricarboxylate) and [EMIM][TFSI] (1‐ethyl‐3‐methylimidazolium bis(trifluoromethanesulfonyl)imide). This transition – absent for the bare MOF or IL – provides an extended super‐cooling range and latent heat at a capacity similar to that of soft paraffins, in the temperature range of ≈220 °C. Structural analysis and in situ monitoring indicate an electrostatic interaction between the IL molecules and the Cu paddle‐wheels, leading to a decrease in pore symmetry at low temperature. These interactions are reversibly released above the transition temperature, which reflects in a volume expansion of the MOF‐IL composite.