The thermodynamics of the adsorption of xenon and argon on zeolite NaX at high pressures

Abstract

The heats of adsorption of xenon and argon in the region of the highly nonideal gas phase depend significantly on temperature. The partial molar isosteric heat capacity of the adsorbate increases sharply at high concentrations of the adsorbed molecules in the cavities. Most studies on the thermodynamic properties of adsorption systems at high pressures have been based on the analysis of adsorption isosteres (1.-3)o This approach, however, involves significant difficulties due to the necessity of the correct consideration of the nonideal nature of the equilibrium phase and the adsorption deformation of the adsorbent. Thus, the heat capacity of the adsorbate, which is a term proportional to the second deriva- tive of the initial adsorption isotherms, is found with about 30% error. Nevertheless, in previous work (2), we showed that the partial molar heat capacities of the adsorbate in microporous adsorbents at high occupancies and pressures have maxima. This finding apparent ~- ly indicates that the structure of the adsorbate is altered under certain conditions. A direct calorimetric study at high pressures would help elucidate the behavior of the ther- modynamic functions of the adsorbate. Work in this area has recently been undertaken (4-6). The thermodynamic properties of xenon and krypton adsorbed on zeolite NaX was calcu- lated using the adsorption isosteres (7, 8). In previous work (6), we directly measured the absolute heats of adsorption of Xe and Kr on zeolite NaX at 304.5 K and from I0 kPa to I0 MPa. These heats are called absolute since the uniform definition of adsorption as the complete content of the adsorbate in the adsorption field is used in the measurement of the adsorption itself. The measured heats of adsorption and the heats calculated from the iso- steres are in accord within experimental error. Curve I in Fig~ I shows the approximation of the dependence of the calorimetric heats of xenon adsorption in zeolite NaX on the adsorption at 304.5 K, curve 3 shows the dependence of the isosteric heat of adsorption at 165 K, and curves 2 and 4 show the analogous curves for krypton in zeolite NaXo Figure i implies that the heats of adsorption depend significantly on the temperature~ This finding was noted previously for these systems (7, 8). Although the curves for the heats of adsorption at 165 K for xenon and at 120 K for krypton in Fig. i were obtained from isosteres at pressures less than at- mospheric and the curves at 304.5 K were obtained calorimetrically , their comparison is still possible since the isosteric heats at low pressures are close to the calorimetric values. This circumstance permits us to determine the behavior of the other thermodynamic parameters of Xe and Kr adsorbed on zeolite NaX.