Thermodynamics of Hydrogen Adsorption on Metal-Organic Frameworks

Interaction between adsorbed hydrogen and the coordinatively unsaturated Mg2+ and Co2+ cationic centres in Mg-MOF-74 and Co-MOF-74, respectively, was studied by means of variable-temperature infrared (VTIR) spectroscopy. Perturbation of the H2 molecule by the cationic adsorbing centre renders the H-H stretching mode IR-active at 4088 and 4043 cm-1 for Mg-MOF-74 and Co-MOF-74, respectively. Simultaneous measurement of integrated IR absorbance and hydrogen equilibrium pressure for spectra taken over the temperature range of 79-95 K allowed standard adsorption enthalpy and entropy to be determined. Mg-MOF-74 showed δ0=-9.4 kJ mol-1 and δS0=-120 J mol-1 K-1, whereas for Co-MOF-74 the corresponding values of δH0=-11.2 kJ mol-1 and δS0=-130 J mol-1 K-1 were obtained. The observed positive correlation between standard adsorption enthalpy and entropy is discussed in the broader context of corresponding data for hydrogen adsorption on cation-exchanged zeolites, with a focus on the resulting implications for hydrogen storage and delivering. Variable-temperature infrared spectroscopy shows that hydrogen adsorption on open metal-cation centres in MOFs (stars) follows a positive enthalpy-entropy correlation similar to that displayed by zeolites (dots). Copyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.