Metal–organic frameworks (MOFs) have become an important focus for the safe and economically applicable storage of hydrogen as a clean fuel. The adsorption enthalpy of molecular hydrogen on MOFs is often used as a measure of potential H₂ uptake, but the thermodynamics are not ruled by ΔH₀ alone: entropy must also be considered.

Carlos Otero Areán and co-workers at the University of the Balearic Islands, Palma de Mallorca, have determined not only the adsorption energy but also the corresponding entropy term from variable-temperature infrared spectroscopic studies on MOFs with open metal (Cr^III) cations.

In light of their results, they argue that the optimum value of ΔH₀ for storage/delivery cycles at ambient temperature (in the pressure range 30 to 1.5 bar) is likely to fall in the range –22 to –25 kJmol^–1, rather than the often quoted value of –15.1 kJmol^–1. As current MOFs have ΔH₀ values in the range –9 to –13 kJmol^–1, significant improvements still need to be made.

• Enthalpy–Entropy Correlation for Hydrogen Adsorption on MOFs: Variable-Temperature FTIR Study of Hydrogen Adsorption on MIL-100(Cr) and MIL-101(Cr)
  Gemma Turnes Palomino, Carlos Palomino Cabello, Carlos Otero Areán,
  Eur. J. Inorg. Chem. 2011.
  DOI: 10.1002/ejic.201001116