Structural Strengthening of Metal–Organic Frameworks Owing to the Confinement Effect of Ionic Liquids in the Nanopores

Abstract

Understanding the atmospheric structural stability and mechanical strength of ionic liquid-loaded metal–organic frameworks (IL-loaded MOFs) is indispensable considering their device applications. We investigated the confinement effect of IL [bmim+][TFSI–] (1-butyl-3-methylimidazolium bis(trifluoromethane sulfonyl)imide) in the nanopores of MOF Cu3(btc)2 (btc; 1,3,5-benzenetricarboxylic acid) using X-ray diffraction and nanoindentation analyses with single-crystal samples. The structural stability against moisture and mechanical strength of the Cu3(btc)2 framework were found to drastically improve by loading [bmim+][TFSI–] into the nanopores. Coulomb interaction between [TFSI–] anions and Cu ions, because of the spatial restriction of constituent ions for [bmim+][TFSI–] with the nanopores of Cu3(btc)2, provides a capping effect on the unsaturated Cu metal sites of Cu3(btc)2, preventing water molecules from adsorbing. Furthermore, the interaction increases the hardness of the Cu3(btc)2 framework owing to additional cross-linkages among the Cu metal sites by [TFSI–] anions. The observed synergy, which is controllable by exploiting the designability of ILs and MOFs, can further accelerate the material development of IL-loaded MOFs for device applications. Simultaneously, our findings indicate that the MOFs that have been avoided for use due to poor stability against moisture and external force should be revisited in the study of IL-loaded MOFs.