Dalton Transactions PERSPECTIVE Cite this: Dalton Trans., 2020, 49, 10295 Received 18th May 2020, Accepted 6th July 2020 DOI: 10.1039/d0dt01784a rsc.li/dalton A practical guide to calculate the isosteric heat/ enthalpy of adsorption via adsorption isotherms in metal–organic frameworks, MOFs†‡ Alexander Nuhnen and Christoph Janiak * Porous materials such as MOFs are interesting candidates for gas separation and storage. An important parameter to gain deeper insights to the adsorption process of an adsorptive on an adsorbent is the isos- teric enthalpy of adsorption, ΔH ads which is defined as the heat to be released/required when an adsorp- tive binds to/detaches from the solid surface of an adsorbent. Two or three adsorption isotherms at different but close temperatures with ΔT ≤ 20 K for two and ΔT ≈ 10 K for three isotherms are the basis to derive the isosteric enthalpy of adsorption through the Clausius–Clapeyron approach or the virial ana- lysis. This Perspective presents the procedure of the common (dual-site) Freundlich–Langmuir fit/ Clausius–Clapeyron approach and the virial fit of the isotherms with usable Excel sheets and Origin files for the subsequent derivation of ΔH ads . Exemplary adsorption isotherms of CO 2 , SO 2 and H 2 at two temp- eratures on MOFs are analyzed. The detailed computational description and comparison of the Clausius– Clapeyron approach and the virial analysis to determine ΔH ads outlines the limitations of the two methods with respect to the available experimental data, especiallyat low pressure/low uptake values. It is empha- sized that no extrapolation beyond the experimental data range should be done. The quality of the impor- tant and underlying isotherm fits must be checked and ensured with logarithmic-scale n/p isotherm plots for the (dual-site) Freundlich–Langmuir fit in the low-pressure region and through low standard deviations for the coefficients in the virial analysis. Introduction Gas sorption for storage and separation is of continuous inter- est with porous materials, such as activated carbon, zeolites, silica gel, metal–organic frameworks (MOFs) etc. At the moment, MOFs, which are potentially porous three-dimensional coordination polymers, 1 appear to receive the highest attention for such gas sorption applications, as they feature a high func- tional and composition diversity. 2 MOFs are built from metal ions or metal clusters, connected by multidentate organic ligands and can be tuned in regards to their physiochemical properties depending on their organic and inorganic building blocks. 3 Over the last two decades plenty of studies about gas separation and storage in MOFs have been published. 4,5 Of interest for gas sorption with MOFs are, e.g., carbon capture and storage (CCS) technologies. 6,7 Furthermore, MOFs with high hydrogen uptake are envisioned for energy storage and as carrier in mobile applications, 8,9 in order to achieve a higher volumetric energy density, which is otherwise only accessible under high pressures (up to 700 bar) or cryogeni- cally (cooled to 20 K). By physisorption of supercritical gases, such as dihydrogen in a porous material a liquid-like adsor- bate phase (with higher than gas density) is formed. For practi- cal use at operating conditions of 1.5 to 30 bar at 298 K light- weight materials with high adsorption capacities and binding enthalpies of about −15 to −20 kJ mol −1 are required. 10,11 Higher H 2 binding enthalpies are achieved by chemisorption of H 2 as in metal hydrides or other materials which store hydrogen through chemical bond formation; there the release is slow and requires heating. 12 Besides, the sorption of CO 2 and H 2 , also CH 4 separation and storage draws interest 13 and an effective capture of harmful gases such as SO 2 and NO x is of growing importance. 14 In the chemical industry the separation of gas mixtures by pressure swing or thermal swing adsorption at a surface is an † In order not to interrupt the flow of reading we have added a glossary for sorp- tion specific terms as Appendix. ‡ Electronic supplementary information (ESI) available: Details for Clausius– Clapeyron and virial equation, Freundlich–Langmuir fit of n vs. p isotherms, MOF structures, SO 2 adsorption isotherms of NH 2 -MIL-125(Ti) at 273 K and 293 K, virial analysis for SO 2 isotherms of NH 2 -MIL-125(Ti) at 273 K and 293 K with a larger number of a i and b i fit parameters, enthalpy of adsorption for CO 2 on MIL-100(Cr), workable Excel and ORIGIN files with data sheets. See DOI: 10.1039/d0dt01784a Institut für Anorganische Chemie und Strukturchemie, Heinrich-Heine-Universität Düsseldorf, D-40204 Düsseldorf, Germany. E-mail: janiak@hhu.de This journal is © The Royal Society of Chemistry 2020 Dalton Trans. , 2020, 49, 10295–10307 | 10295