Characterizing Kinetics of MOFs for adsorption cooling: IR LTJ experiments of Al-fumarate and CAU-10(Al)-H

High water uptake and excellent cyclic stability make the metal-organic frameworks (MOFs) Al-fumarate (Al-fum) [1] and CAU-10 [2] promising materials for water-based adsorption chillers. However, water uptake captures only equilibrium behavior, while process performance depends strongly on kinetics, i. e. the overall dynamics of heat and mass transfer processes [3]. In this work, we determine the kinetics of MOF-coatings for ad- and desorption. For both Al-fum and CAU-10, two coatings of different thicknesses are studied by small-scale Infrared Large-Temperature-Jump (IR-LTJ) experiments [4] for the temperature triple 10/30/80 °C. Monolayers of commercially available granular silica gels (SG123, Siogel) serve as benchmark. Kinetics are determined by characteristic time constants 𝜏 [5], (i) area-specific mean power 𝑃̅𝐴, and (ii) volume-specific mean power 𝑃̅𝑉 [6]. The chosen characteristic time as well as the chosen reference unit strongly affect the determined performance (Figure 1). Siogel provides the highest area-specific power 𝑃̅𝐴 for all time constants. CAU-10 and SG123 both perform similarly, and Al-fum falls behind for this temperature triple. The highest volume-specific power 𝑃̅𝑉 is provided by Siogel only for small time constants while CAU-10 performs better at intermediate time constants. The results highlight the importance of considering different time constants to assess the potential of MOF-coatings for adsorption chillers. High water uptake of MOFs is not sufficient to outperform granular silica gels. The overall dynamics of hHeat and mass transfer has toshould be considered when evaluating MOFs.