Enhancing adsorption heat transformers with functionalized MOF-808: A statistical physics approach

Abstract

The recovery and upgrading of low-grade thermal energy has become an important challenge for improving industrial energy efficiency. Adsorption heat transformers (AdsHT) provide a viable solution by converting waste heat into higher-temperature heat streams exceeding 120 °C through reversible adsorption–desorption cycles. In this study, the thermodynamic performance of an AdsHT system was assessed using two post-synthetically modified MOF-808 materials, MOF-808NDS and MOF-808PDS, as adsorbents for water vapor. A statistical physics approach based on the grand canonical ensemble was applied to investigate the adsorption mechanism and to determine the key energetic and structural parameters governing the process. The modeling results reveal that water adsorption on both materials can be described by a monolayer model involving two distinct adsorption energy levels. Stereographic analysis suggests that water molecules interact with the adsorption sites mainly in a non-parallel orientation. The calculated adsorption energies, lower than 40 kJ mol⁻¹, indicate that the process is dominated by weak physical interactions such as hydrogen bonding and van der Waals forces. Furthermore, pore size distribution analysis confirms the microporous structure of the studied frameworks. The thermodynamic analysis of the adsorption cycle shows coefficients of performance (COP_GCSP) of 0.764 for MOF-808NDS and 0.826 for MOF-808PDS, highlighting the superior efficiency of MOF-808PDS due to its higher water uptake and favorable pore characteristics. These findings demonstrate that functionalized MOF-808 materials are promising candidates for enhancing the performance of adsorption heat transformer systems dedicated to sustainable thermal energy utilization.