Multilayered adsorption of gases and vapores on activated carbons, zeolites and macroporous adsorbents

Abstract

At present, a large number of equations have been proposed for quantitative description of adsorption isotherms for gases and vapors on solid adsorbents, which were obtained by the authors on the basis of various theoretical concepts. In particular, the equation of the theory of polymolecular adsorption of BET and its modification, proposed by GL Aranovich, and also the equations of the theory of volume filling of micropores, developed under MM Dubinin's direction, are widely used. When adsorbing gases and vapors on zeolites, it is also proposed to use equations obtained on the basis of stoichiometric and osmotic adsorption theories. It should be noted that the generally recognized equation of the adsorption isotherm, which is applicable for the quantitative description of all types of adsorption isotherms according to S. Brunauer's classification, does not yet exist for all types of adsorbents. In this paper, an attempt is made to propose such a theory and the equation of the adsorption isotherm corresponding to this theory. 

In accordance with the proposed concept of multilayer adsorption, several layers of adsorbent can be simultaneously adsorbed on the surface of the adsorbent. In the first adsorption layer, each adsorbent molecule interacts with (n) adsorption centers of the surface to form (n) «adsorbent-adsorbate» adsorption complexes. This equilibrium process is characterized by a constant of adsorption equilibrium (Kp), which enters into the three-constant equation of the adsorption isotherm. The third constant in the adsorption isotherm equation (am) characterizes the capacity of the adsorbent layer. The surface of the adsorbent with adsorptioncomplexes «adsorbent-adsorbate» is able to interact with molecules of the next adsorbent layer to form adsorption complexes «adsorbent/adsorbate-adsorbate». This equilibrium process is characterized by its equilibrium constant. Based on the range of van der Waals forces, due to which physical adsorption takes place, it can be assumed that the number of adsorption layers can not be more than three. The amount of adsorption
will be equal to the sum of the adsorption of all adsorption layers at a given equilibrium pressure of the adsorbent. The coefficients in the equation of the adsorption isotherm for each adsorption layer are uniquely determined from the experimental data by the method of successive approximations under the condition that the isotherm is linear in the Cartesian coordinate system. 

This paper shows the applicability of the proposed adsorption isotherm equation to describe the experimental adsorption isotherms on various types of adsorbents published in the literature. In particular, adsorption on microporous adsorbents - propane on nuksite at 333K, adsorption on macroporous adsorbents -
benzene on graphite black at 303K,  adsorption on zeolites - xenon on NaX at 170K. It is shown that the experimental isotherms are completely satisfactorily described by the proposed equation. In this case, the adsorption of propane on the nuxite occurs in one layer, benzene on graphitized soot in two layers, and xenon on NaX zeolite in three layers. It is also shown that upon adsorption on a macroporous adsorbent, capillary condensation of the adsorbent occurs. To calculate the amount of condensed adsorption, an equation similar to
the adsorption isotherm equation is proposed. The paper concludes that the proposed equation of the adsorption isotherm is applicable for describing all types of adsorption isotherms according to S. Brunauer classification in the entire range of equilibrium partial pressures of adsorbents. It is applicable for adsorption on micro-, macroporous adsorbents and zeolites. It is shown that the number of adsorption layers can be no more than three. Thus, the proposed concept of multilayer adsorption and the corresponding equation of the adsorption
isotherm make it possible to satisfactorily describe all types of experimental adsorption isotherms according to S. Brunauer's classification on all types of adsorbents in the entire range of equilibrium partial pressures of the adsorbent.