Role of extraframework metal sites for hydrogen adsorption into the pores of a zeolite: FT-IR study

In order to compare the adsorptive properties of nanoporous zeolites containing extraframework cations of different nature, we have studied the interaction of H2 with Na-A, Ca-A, and Co,Na-A zeolites. Low temperature Fourier transform infrared (FT-IR) spectroscopy was used for the investigation, as this technique is highly sensitive and responsive to the nature of the gas/surface interaction and can in addition allow for the estimation of the adsorption enthalpy. In all cases the spectra of adsorbed H2 have complex structure due to ortho/para splitting as well as to surface structural disorder. Na+ and divalent Ca2+ were found to induce almost similar perturbation on H2 molecule, resulting in the shift of the H–H vibrational frequency of −86 cm−1 and −76 cm−1 respectively (as compared to the Raman frequency of gaseous H2). The enthalpy of adsorption, estimated by the Variable Temperature Infrared (VTIR) method, is −13 ± 1 kJ mol−1 for the strongest adsorptive sites in Na-A and Ca-A samples. In the case of Co,Na-A the shift of the H–H frequency due to the formation of H2⋯Co2+ complexes is larger (ca. −180 cm−1) suggesting that the interaction can involve some, although small, chemical contribution.