Thermodynamics of hydrogen adsorption on calcium-exchanged faujasite-type zeolites

A combination of variable-temperature infrared spectroscopy with volumetric gas adsorption measurements was used to study the thermodynamics of hydrogen adsorption, at a low temperature, on calcium-exchanged zeolites X and Y. Two adsorption regimes were considered: (i) localized adsorption of dihydrogen molecules on Ca2+ cation sites, and (ii) delocalized hydrogen adsorption following saturation of the Ca2+ adsorbing centres. For localized adsorption, the corresponding enthalpy change was found to be in the range of −12 to −15 kJ mol−1, while the isosteric heat of delocalized adsorption was found to be in the range of 4.5–5.5 kJ mol−1. These experimental results are discussed in the broader context of corresponding data for other alkaline zeolites, with a focus on correlation between adsorption enthalpy and entropy for the localized adsorption regime.