Thermodynamics of reversible gas adsorption on alkali-metal exchanged zeolites—the interplay of infrared spectroscopy and theoretical calculations

Abstract

Detailed understanding of weak solid-gas interactions giving rise to reversible gas adsorption on zeolites and related materials is relevant to both, fundamental studies on gas adsorption and potential improvement on a number of (adsorption based) technological processes. Combination of variable-temperature infrared spectroscopy with theoretical calculations constitutes a fruitful approach towards both of these aims. Such an approach is demonstrated here (mainly) by reviewing recent studies on hydrogen and carbon monoxide adsorption (at a low temperature) on alkali-metal exchanged ferrierite. However, the methodology discussed, which involves the interplay of experimental measurements and theoretical calculations at the periodic DFT level, should be equally valid for many other gas-solid systems. Specific aspects considered are the identification of gas adsorption complexes and thermodynamic studies related to standard adsorption enthalpy and entropy.

Computational and Variable‐Temperature Infrared Spectroscopic Studies on Carbon Monoxide Adsorption on Zeolite Ca‐A

“…[39,40] Briefly, under adsorption equilibrium at any given temperature, the integrated intensity (absorbance) of a characteristic IR absorption band should be proportional to the fractional coverage q of the adsorbed species giving rise to that band, and therefore this integrated intensity gives information on the activity of both the adsorbed species and the empty adsorption sites (1Àq). At the same time, the equilibrium pressure gives the activity of the gas phase.…”

“…Note that for CO adsorbed on zeolite Na-A (in which the concentration of cations is twice as large as in zeolite Ca-A) formation of adsorption complexes on single-cation sites was not found at all. [40] Another reason for formation of complexes on dual-cation sites in zeolite Ca-A is that the repulsion between CO molecules located in the same supercage is minimized when they move from single sites (where the CO molecule points to the void space in the supercage) to dual sites (where the CO molecule is closer to the supercage wall).…”

A review of the computational studies of proton- and metal-exchanged chabazites as media for molecular hydrogen storage performed with the CRYSTAL code

“…The feasibility of this method of storage is basically ruled by the interaction between H 2 and the host material; therefore, the understanding of this interaction is crucial to infer the potential of different materials as storage media. With this objective, several experimental studies on the H 2 interaction with diverse zeolites have been reported in the last years [10,[12][13][14][15][16][17][18][19][20][21][22][23][24][25], focused mainly on the estimation of adsorption enthalpies by means of variable-temperature infrared spectroscopy [26,27]. A summary of the data obtained in these experimental works is presented in Table 1, where it is worth noticing that beside the structural properties of the various studied zeolites (e.g.…”

Theoretical Chemistry of Zeolite Reactivity

“…The smaller the cation is, the stronger its polarizing ability and Lewis acidity are. As a result, similar to the cases of CO and H 2 adsorption on alkali-exchanged zeolites [79,84], one expects a decrease of the N 2 O 4 adsorption energies simultaneously with the increase of the ionic radius Table 11.2 Adsorption energies (kJ mol −1 ) of N 2 O 4 molecules at S II and S III cations a in the cage of alkaline-exchanged zeolites X [82] and Y [83].…”