Adsorption Dynamics and Hydrothermal Stability of MOFs Aluminium Fumarate, MIL-160 (Al), and CAU-10-H, and Zeotype TiAPSO for Heat Transformation Applications

Metal-organic frameworks (MOFs) can be beneficial for heat transformation applications due to their potentially high water uptake and tunable working temperature levels. Although the hydrothermal stability has been assessed in some cases in terms of maximum water uptake and structural changes, there is no data on the impact of hydrothermal stress tests on adsorption dynamics. However, to maintain the designed heating or cooling power in the application, the hydrothermal stability in terms of both water uptake and adsorption dynamics is decisive. To close this gap, we present experimental data for the comprehensive evaluation of hydrothermal stability for three different MOFs and the commercially available zeotype TiAPSO. The hydrothermal stress test includes around 70,000 temperature swing cycles on aluminium sheets with a binder-based coating of different adsorbents. As a novelty of this study, adsorption dynamics are determined before and after the hydrothermal stress test using effective thermal resistances and the characteristic temperature difference. Our results show degradation in terms of a decrease in uptake around 5-10% after hydrothermal stress test for all samples. Under temperature boundary conditions relevant for the application, MIL-160(Al) shows even a drastic uptake reduction of around 35-45%. Except for CAU-10-H, none of the adsorbents show a degradation in terms of increased heat and mass transfer resistance. In case of CAU-10-H, the overall effective heat and mass transfer resistance increases by around 30-40% after the hydrothermal stress test. These results indicate that the hydrothermal stability of MOFs must be assessed in terms of both, uptake and adsorption dynamics, to ensure stable long-term performance in real-world devices.