Enthalpy–Entropy Correlation for Hydrogen Adsorption on MOFs: Variable‐Temperature FTIR Study of Hydrogen Adsorption on MIL‐100(Cr) and MIL‐101(Cr)

The thermodynamics of hydrogen interaction with the coordinatively unsaturated Cr III cationic sites in the metal–organic frameworks MIL‐100(Cr) and MIL‐101(Cr) was studied by variable‐temperature infrared (VTIR) spectroscopy. Simultaneous measurement of the equilibrium pressure of hydrogen and the integrated IR absorbance over the temperature range 79–105 K led to the determination of the corresponding values of standard adsorption enthalpy (Δ H 0 ) and entropy (Δ S 0 ). For MIL‐100(Cr) these values were Δ H 0 = –6.9 kJ mol –1 and Δ S 0 = –80 J mol –1 K –1 , whereas for MIL‐101(Cr) the values Δ H 0 = –9.5 kJ mol –1 and Δ S 0 = –112 J mol –1 K –1 were obtained. These thermodynamic quantities show a positive correlation between Δ H 0 and Δ S 0 , which is analyzed in the broader context of the corresponding data available for hydrogen adsorption on other two MOFs and also on several cation‐exchanged zeolites. The implications for hydrogen storage and delivery by using MOFs are discussed.