Sorption of the Inert Gases (Ar, Kr, and Xe) in Type A Zeolites

Abstract

RUSSELL IAN DERRAH and DOUGLAS MORRIS RUTHVEN. Can. J. Chem. 53,996 (1975). Theoretical potential profiles for Ar, Kr, and Xe occluded in an idealized 5A zeolite are calculated and used to evaluate the Henry's law equilibrium constants for these species. Calculations were made using the three common formulae to estimate the dispersion constants (Kirkwood-Mueller, Slater-Kirkwood, and London) and the theoretical Henry constants are compared with the values-obtained directly from experimental equilibrium isotherms. For both Kr and Xe the Slater-Kirkwood formula gives the best agreement with the experimental values while, for argon, the London formula is best. At higher concentrations the isotherms for Kr can be well represented by a simple Langrnuir expression with eleven sites per cavity while for Xe there appears to be a transition from localized sorption at low temperatures to a mobile adsorbed phase at higher temperatures. These conclusions are qualitatively consistent with the results of the theoretical potential calculations.RUSSELL IAN DERRAH et DOUGLAS MORRIS RUTWEN. Can. J. Chem. 53,996 (1975). On calcule les profils potentiels theoriques de Ar, Kr et Xe occlus dans une zeolithe 5A idealis&; on utilise les valeurs ainsi calcul&s pour kvaluer les constantes dY6quilibre de la loi d'Henry pour ces especes. On a fait des calculs utilisant les trois formules ordinaires pour estimer les contantes de dispersion (Kirkwood-Mueller, Slater-Kirkwood et London) et l'on compare les constantes theoriques #Henry avec les valeurs obtenues directement a partir des isothermes experimentales d'equilibre. Pour le Kr et le Xe, la formule de Slater-Kirkwood donne la meilleure relation avec les valeurs experimentales alors que pour I'argon, la formule de London est la meilleure. A des concentrations plus hautes, les isothermes de Kr peuvent aussi dtre presentes par une expression simple de Langmuir avec onze sites par cavite alors que pour Xe il semble qu'il existe une transition passant d'une sorption localisee a basse temperature a une phase adsorb& mobile a plus haute tempbature. Ces conclusions sont qualitativement en accord avec les rksultats des calculs potentiels thioriques.[Traduit par le journal]

Prediction of gas adsorption in 5a zeolites using Monte Carlo simulation

“…Most simulation studies of adsorption in zeolites have neglected electrostatic effects and used simple smoothed potentials to calculate the adsorbate-zeolite energy, although some investigators have used more complex potential models to estimate the heat of adsorption and Henry's Law constants (Kiselev and Du, 1981;Derrah and Ruthven, 1975). These more complex models are based on the site-site potential model, in which the surface is treated as a collection of overlapping spheres or sites.…”

“…The repulsion constant is determined by setting the equilibrium separation, re,, of an adsorbate-zeolite pair equal to the average of the van der Waals radii (Derrah and Ruthven, 1975): where urk is the pair interaction energy. The radii of the oxygen, sodium and calcium of the zeolite are those used by Derrah and Ruthven (1975).…”

“…The radii of the oxygen, sodium and calcium of the zeolite are those used by Derrah and Ruthven (1975). The van der Waals radii of the adsorbates are determined from the Lennard-Jones potential parameters:…”

“…To model the electrostatic interactions, the way in which the negative charge is distributed on the oxygen atoms in the framework must be estimated. Several previous inves-tigators have suggested distributing the negative charge evenly over all of the framework oxygen atoms (Derrah and Ruthven, 1975;Barrer, 1978). In the 5A structure, this even distribution leads to a -1/4 charge on each oxygen atom.…”

Noble Gas Immobilization in Zeolites

“…The a-cages are interconnected by 0.42 nm apertures. From these considerations it is apparent that gases like Ar, Kr and Xe, whose van der Waals diameter are all smaller than the apertures of the large a-cage of zeolite 5A, will readily be sorbed into the a-cage (but not the l3-cage) of this molecular sieve [10][11][12] and that an encapsulation cannot take place. It was thus surprising to find that large amounts of Ar, Kr and Xe can be immobilized in type A zeolites (3A, 4A and 5A) as well as in other zeolites such as mordenite (zeolon 900 Na) or chabazite (zeolon 500), having even larger pore openings, when the fixation temperature is raised up to 500 -650°C and the samples are activated until a certain residual amount of water is left in the zeolite framework.…”