Absolute Adsorption Isotherms for Ethylbenzene + n-Octane + Zeolite NaX System: Determination of the Activity Coefficients of the Adsorbed Phase

Abstract

Adsorption from solution of nonelectrolytes by microporous adsorbents such as zeolites is not only of practical interest. Because of the well-defined internal structure of zeolite, it can be a good model system which provides the possibility to measure absolute adsorption in a direct experiment. Such data may lead to a better understanding the properties of the liquid solution in the micropores. Whereas the excess adsorption isotherm can easily be measured, the exact determination of the absolute adsorption isotherm requires rather painstaking and time-consuming experiments. So the main problem is to convert excess adsorption data to the absolute adsorption isotherm.

Given this circumstance, we previously proposed a method to estimate the absolute adsorption isotherm of a binary solution on a microporous adsorbent. The method is based on the Dubinin-Radushkevich equation modified for adsorption from solutions. The equation gives the concentration dependence of absolute adsorption of solution and includes three non-fitting parameters: the absolute (limiting) adsorption values of the pure components, which are much easier to measure than the absolute adsorption isotherm of solution, and the value of the equilibrium concentration corresponding to the maximum excess adsorption.

The purpose of this work was to study the physicochemical characteristics of the adsorption phase based on the absolute adsorption data of the adsorption system ethylbenzene + n-octane + zeolite NaX. Excess adsorption isotherms were measured at temperatures 303.15, 338.15, and 363.15 K using the conventional static method. The limiting (absolute) adsorption values of pure components were measured using the pycnometric method. It is shown that the measured excess adsorption isotherms belong to type II according to the Schay and Nagy classification. This indicates strong adsorption of ethylbenzene in the entire concentration range. Absolute adsorption isotherms of the solution and the preferentially adsorbed component were calculated using the above method. These data made it possible to calculate for the adsorbed phase the excess volume of mixing, solution density and activity coefficients of the solution components. Based on the latter, the excess thermodynamic functions of mixing for the adsorption solution were calculated: free energy, entropy and enthalpy. The data obtained allow us to draw some conclusions about the adsorbed solution. The negative value of the excess volume of mixing indicates volume contraction. The adsorbed phase activity coefficients indicate a significant negative deviation from ideality, which means the dominance of intermolecular interaction between different molecules. This is also indicated by the negative value of the excess free energy of mixing.