Estudio espectroscópico y termodinámico de la adsorción de hidrógeno en zeolitas

Abstract

Zeolites belonging to the structural type LTA and FAU (X and Y) were prepared (in their Na+ form) by standard hydrothermal methods. These zeolites were then exchanged with Mg2+ and Ca2+ ions, and characterized by powder X-ray diffraction, electron microscopy and FTIR spectroscopy. Variable-temperature FTIR spectroscopy was used to determine the standard adsorption enthalpy, ΔH0, and entropy, ΔS0, of hydrogen adsorption in the Mg2+ and Ca2+ ion-exchanged zeolites. The results obtained were in the range of -12 to -18.2 kJ mol-1 for ΔH0 and -114 to -136 J mol-1 K-1 for ΔS0. The observed differences of ΔH0 values clearly showed that both, the cation polarizing power and its location (which depends on the zeolite structure type) have a very significant effect on the hydrogen adsorption enthalpy. For the H2/Ca-X and H2/(Ca,Na)-Y systems, the thermodynamic study was extended to the determination of the isosteric heats of adsorption as a function of the adsorbed amount of H2. Two successive H2 adsorption processes, both of them complying with Langmuir type adsorption, were observed. First a localized adsorption of H2 molecules on the cationic sites of the zeolite, followed by a non-site specific adsorption in the zeolites micropores. Site-specific localized adsorption is ruled by ΔH0 values in the range of -12 to -15 kJ mol-1, while non-localized adsorption gave isosteric heats in the range of 4.5 to 5.5 kJ mol-1. For the H2/Ca-A system, DFT calculations showed that two types of cationic adsorption sites (S1 and S2) can occur. Nevertheless, only a single IR absorption band was observed, at 4083 cm-1. The most likely explanation is that either the site S2 is not actually occupied, or the corresponding H2 adsorption complex is highly symmetric. The combined analysis of the results obtained, as well as those available in the literature for hydrogen adsorption on other alkaline zeolites, showed that there is an approximate linear correlation between HH and ΔH0, the observed deviations could be explained in terms of either a small cation size (such as in the case of Li+) or an unusually low coordination number of the cation with oxygen atoms of the zeolite framework. Another relevant result found was the existence of a positive, and non-linear, correlation between ΔH0 and ΔS0; this correlation suggests that the optimum value of ΔH0 for hydrogen adsorption and delivery at ambient temperature is of about -22 to -25 kJ mol-1.