The influence of Metal-Organic Frameworks in the structure of ionic liquids near the interface
Karina SHIMIZU1, João AFONSO1, Adilson DE FREITAS1, José CANONGIA LOPES1
1Centro de Química Estrutural, Institute of Molecular Sciences, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais 1049 001 Lisboa, Portugal, Lisbon, Portugal
Type III porous liquids are defined as porous solid hosts dispersed in size-excluded liquids. These multiphase fluids have potential in processes such as gas storage, ion transport, energy storage, separation, and catalysis. In this work the interactions on Type III porous liquids were characterized using classical Molecular Dynamics (MD) simulations. For this purpose, the mobile phase consisted of Ionic Liquids, while the solid substrate comprised Metal-Organic Frameworks.
Ionic liquids are organic salts with melting temperatures below 100 °C, and have interesting properties such as high thermal stability, low vapor pressure and wide electrochemical window. Also, combined with their virtually unlimited chemical tunability, ionic liquids are promising candidates for applications in separation technology, catalysis, gas storage and many other fields. Metal-organic frameworks (MOFs) are porous crystalline materials with large pore volume and high surface area, with potential in processes such as gas storage, separation, and catalysis.
The ionic liquids chosen in this study were [P6 6 6 14][NTf2], [C14C1im][NTf2], [N14 1 1 2OH][NTf2] and [C14C1im]Cl representing different possible arrangements of cations and anions interacting with MOFs (Figure 1). The MOF used was the crystalline structure of ZIF-8, consisted of zeolitic imidazolate frameworks.
The MD analyses indicate that [P6 6 6 14][NTf2], [N14 1 1 2OH][NTf2] and [C14C1im][NTf2] are affected by the presence of the MOF, while [C14C1im]Cl show no deviation in comparison with its bulk phase results. Near the ZIF-8 surface the first three ionic liquids are settled in a chessboard-like pattern formed by the cation headgroups and anions. This pattern is less defined for [P6 6 6 14][NTf2], that has a more pronounced non-polar part attached to the cation. In addition, the Cl- anion changes radically the behaviour of [C14C1im]Cl polar part near the ZIF-8 interface.
These MD results provided a wealth of information about the interactions between ionic liquids and metal organic frameworks and their usability as porous liquids.