The nature of cationic adsorption sites in alkaline zeolites—single, dual and multiple cation sites

Abstract

Gas adsorption on zeolites constitutes the base of many technological applications of these versatile porous materials. Quite often, especially when dealing with small molecules, individual extra-framework (exchangeable) cations are considered to be the adsorption site on which molecules coming from a gas phase form the corresponding adsorption complex. Nonetheless, while that can be the case in some instances, recent research work that combines variable temperature infrared spectroscopy with periodic DFT calculations showed that some types of adsorption sites involve two or more cations, which constitute dual and multiple cation sites, respectively. Adsorption complexes formed on these cationic adsorption sites differ in both structure and stability from those formed on a single cation alone. Examples concerning CO, CO(2) and H(2) adsorption on alkali and alkaline-earth metal exchanged zeolites are reviewed, with the double purpose of clarifying concepts and highlighting their relevance to practical use of zeolites in such fields as gas separation and purification, gas storage and heterogeneous catalysis.

CO 2 adsorption–desorption properties of zeolite beta prepared from OSDA-free synthesis

“…2-IE is presumably due to the predominant formation of dual cation sites, in which two neighboring Na cations complementary interact with one CO 2 molecule. The possibility of CO 2 interaction with dual cations was proposed by Nachtigall and Cejka [66,69,70], in which they controlled the homogeneity of Al distribution in Na-FER (model zeolite with Si/Al ¼ 15.7) by changing the type of organic structure-directing agents during the synthesis [66]. When Al sites were distributed homogeneously in the vicinity of Na cations, one Na cation can interact with an average of one CO 2 molecule where the isosteric heat of adsorption is in the range of 42e47 kJ mol À1 (depending on Al T-site location) [66].…”

MgO-modified mesoporous silicas impregnated by potassium carbonate for carbon dioxide adsorption

“…The weak interaction of CO 2 with surface of Al-SBA-15 silica was also reported [22]. In addition, the size of the pores is too large to influence the adsorption by the effect ''from the top'' as recently described by Nachtigall et al [39,40] and which is so important in the case of CO 2 adsorption in microporous zeolites. The CO 2 isotherms on modified samples exhibit nonlinear concave decreasing course typical for adsorption of CO 2 on inorganic materials (zeolites, mesoporous adsorbents, hydrotalcite-like compounds.)…”

A rapid microwave-assisted synthesis of a sodium–cadmium metal–organic framework having improved performance as a CO2 adsorbent for CCS

“…The smaller values found herein for the investigated (Na,Cd)-MOF suggest a weaker (local) interaction of the CO 2 molecule with the Na + cation. However, analysis of fine details cannot be done at this stage, because it was shown that for CO 2 adsorbed on both, MOFs and zeolites alike, at least one half of the gas-solid interaction energy comes from long range dispersion interactions, 25,55,56 and the actual value of H 0 depends on a complex interplay of electrostatic and dispersion forces. Adsorption selectivity, of carbon dioxide versus nitrogen, referred to post-combustion flue gas that (besides other minor components) usually contains CO 2 and N 2 at a partial pressure of about 0.15 and 0.75 bar, respectively, is commonly assessed from single-component gas adsorption isotherms (at 298 K) by using the selectivity factor, S, expressed as: 2,11 (4) where q i and p i are the molar fraction and partial pressure, respectively, of component i.…”