Significant Temperature Dependence of the Isosteric Heats of Adsorption of Gases in Zeolites Demonstrated by Experiments and Molecular Simulations

Heat of adsorption is an important factor in determining the utility of a porous material for gas separation and storage applications. Although theoretically the heat of adsorption can depend on temperature, it is common practice to assume that this dependence is so weak that it can be ignored. In this paper, we challenge this common wisdom. We simulated the adsorption isotherms and heats of adsorption of small molecules (CO2, CH4, and N-2) in reference siliceous (LTA, CHA, MFI) and cation-exchanged (LTA-4A, Na-LTA Si/Al = 2,5) zeolites and found very significant temperature dependence of the isosteric heat of adsorption for CO2 at low loadings for some systems. In cation-exchanged LTA zeolites, we found more than a IS kJ/mol decrease over a 300 K range (similar to 30% variation). We also found remarkable temperature dependence for CO2 in some siliceous zeolites with eight-membered-ring windows (e.g., ITQ-29). A weak temperature dependence was observed for CO2 on silica MFI and for CH4 and N-2 adsorption in all materials. Concurrent adsorption microcalorimetry measurements on cationic 4A and siliceous ITQ-29 (LTA) zeolites fully support the theoretical predictions. Our results demonstrate how the temperature dependence of the isosteric heat is related on the microscopic level to the redistribution of adsorption sites with changes in temperature. A wider implication of our findings is that many porous materials exhibit distinct populations of adsorption sites that can lead to significant temperature dependence of the isosteric heats of adsorption. Therefore, care should be exercised when reporting isosteric heats of adsorption on such materials. For some systems, the significant temperature dependence of the isosteric heats of adsorption may need to be accounted for in process design.