Studying ZIF-8 SURMOF Thin Films with a Langatate Crystal Microbalance: Single-Component Gas Adsorption Isotherms Measured at Elevated Temperatures and Pressures

Abstract

Characterizing the adsorption behavior of thin films at technical operation ranges is highly relevant in order to obtain appropriate experimental parameters, as well as to evaluate their possible integration in multiple applications. Nonetheless, gathering such experimental data is not a trivial task. In the case of metal-organic framework thin films, a particular interesting class of highly porous coatings, determination of adsorption isotherms at elevated temperatures and pressures is not commonly reported. Using a custom-designed langatate crystal microbalance following the principle for piezoelectric microbalances, but under harsher conditions, allowed us to present adsorption equilibrium measurements on a zeolitic imidazolate framework-8 (ZIF-8) surface-mounted metal-organic framework (SURMOF) thin film at temperatures between 35 and 100 °C and at a pressure range up to 10 bars. The layer-by-layer liquid-phase epitaxy approach was used to deposit ZIF-8 SURMOF thin films on langatate crystals. X-ray diffraction, infrared reflection absorption spectroscopy, and scanning electron microscopy combined with energy dispersive X-ray mapping were used to gather deeper information on the ZIF-8 thin film growth. Single-component isotherms were collected for different gases of practical interest, that is, CO₂, C₂H₆, C₂H₄, C₃H₈, and C₃H₆. The measurements were obtained at 35, 50, 75, and 100 °C at a gauge pressure range up to 10 bars. The collected data showed a clear preferential adsorption of ethane over ethylene, whereas only a slight difference was found between propane and propylene. Additionally, the experimental data allowed for the determination of adsorption equilibrium constants and saturation loadings of the ZIF-8 material as a thin SURMOF film, which is of great importance for its integration in multiple applications, such as sensor devices or in membrane-based separation.