The Differences in the Adsorption Processes in Micro and Supermicropores

Abstract

A new approach to micropore adsorption has been developed, which makes use of Amagate equation of state for the adsorbed fluids. It has been found that this state equation, or its two-dimensional (2D) form, describes well the state properties of typical adsorbates confined in micropores as well as the state properties of 2D fluids. The reason is that the critical temperatures of both fluids are substantially lower than the critical temperature of the bulk fluid. Another situation has been found for fluids confined in supermicropores. In spite of the increasing number of the adsorbate molecules in the pore, the adsorbed fluid properties are here the more similar to those of the bulk fluid. The thermodynamic of small sytems in the field of adsorption forces enables to derive the equation of adsorption isotherms. This equation describes well the adsorption isotherms of vapours and gases on microporous adsorbents at different temperatures also in the cases, when the Dubinin's theory cannot be applied (zeolites), as well as the isotherms on nonporous solids. The adsorption isotherms of methane, propane, n-hexane, n-heptane and benzene on zeolite NaX, measured at different temperatures, as well as the adsorption isotherms of benzene on active carbons have been characterized by the derived equation. The parameters of this equation, the mean value of the potential energy ϕ of the adsorbed molecule and the perturbation change of entropy ΔS x + , related to the elimination of the translation degrees of freedom of the small systems localized in micropores, are in good agreement with the theoretically expected and evaluated values. From the approach described it follows, that adsorption equilibrium of vapours and gases on solids may be described by one temperature independent characteristic isotherm a=[In(p g /p g =0.5 ), where a is the amount adsorbed, p g and p g =0.5 are the equilibrium pressure and the same adsorptive pressure at the fraction of saturation of sorbent with sorbate 0.5, respectively. The values of the pressures p g 0.5 and its dependence on the temperature T, are controlled by the potential energy ϕ and the change of the entropy ΔS + .