Thermodynamic studies on hydrogen adsorption on the zeolites Na-ZSM-5 and K-ZSM-5

Adsorption of dihydrogen onto the zeolites Na-ZSM-5 and K-ZSM-5 renders the fundamental H-H stretching mode infrared active. The corresponding infrared absorption bands were found at 4101 and 4112 cm−1 for H2/Na-ZSM-5 and H2/K-ZSM-5, respectively. Thermodynamic characterization of the adsorbed state was carried out by means of variable-temperature infrared spectroscopy; simultaneously measuring integrated band intensity, temperature and equilibrium pressure of the gas phase. For the H2/Na-ZSM-5 system, the standard adsorption enthalpy and entropy resulted to be ΔH° = −10.3 (±0.5) kJ mol−1 and ΔS° = −121 (±10) J mol−1 K−1. For H2/K-ZSM-5 corresponding values were −9.1 (±0.5) kJ mol−1 and −124 (±10) J mol−1 K−1, respectively.