Determination of Water Loading of Supported Ionic Liquids by Microwave Analysis - A contribution for Operando Monitoring of Gas Drying by Adsorption

Abstract Ionic liquids (ILs) immobilized on solid porous supports are alternative materials for gas drying by fixed-bed adsorption: The IL 1-ethyl-3-methylimidazolium methanesulfonate has a high capacity for sorption of water vapor, and losses of the IL via evaporation are negligible because of the extremely low vapor pressure. Hence, ILs can be durably immobilized by coating of porous supports. Ionic liquids have a high electrical conductivity compared to supports such as silica. This can be utilized to monitor the water loading of supported IL by microwaves. A fixed-bed adsorber was placed in a cylindrical microwave resonator, and the water loading and the corresponding electrical signals were measured by a humidity sensor and a network analyzer, respectively. The partial pressure of steam (carrier gas N2) and the temperature were varied in a range relevant for gas drying (5 - 20 mbar; 40 - 90 °C). The results reveal that the microwave parameters, such as the resonance frequency shift, are suitable to determine accurately and operando the water loading of supported ionic liquids. Further analysis of the microwave parameters leads to the complex susceptibility, which in turn shows a linear dependence of the polarization effects on the electrical losses.