Thermodynamics of CO₂ adsorption on zeolite NaX in wide intervals of pressures and temperatures

Abstract

The dependences of the differential molar isosteric heat of adsorption and entropy of adsorption of CO₂ on zeolite NaX were determined in wide temperature (196–423 K) and pressure (0.1 Pa to 5.4 MPa) intervals. In the initial region of adsorption (a < 1 mmol g⁻¹), the differential molar heat of adsorption increases from 40 to 43 kJ mol⁻¹ and then decreases to 33 kJ mol⁻¹. The heat of adsorption remains virtually unchanged at 3 mmol g⁻¹< a < 6.5 mmol g⁻¹ and decreases sharply at high fillings of zeolite micropores (a > 7 mmol g⁻¹). The heat of adsorption was found to be temperature-dependent. The region with the constant heats shrinks with the temperature increase, and the heats begin to decrease at lower fillings of micro pores. The dependences of the change in the differential entropy of the adsorption system on the amount adsorbed were calculated at different temperatures. The specific features of the behavior of the thermodynamic functions of this adsorption system in the initial and medium region of fillings kre associated with interactions of adsorbate molecules with Na⁺ cations and walls of large cavities. For high fillings, an increase in repulsion forces between adsorbed molecules results in a sharp expansion of the adsorbent and a decrease in the heat of adsorption.