Thermodynamics of Carbon Dioxide Asdorption on the Protonic Zeolite H-ZSM-5

Adsorption of carbon dioxide on H-ZSM-5 zeolite (Si:Al= 11.5:1) was studied by means of variable-temperature FT-IR spectroscopy, in the temperature range of 310-365 K. The adsorbed CO2 molecules interact with the zeolite Bronsted-acid OH groups bringing about a characteristic red-shift of the O-H stretching band from 3610 cm(-1) to 3480 cm(-1). Simultaneously, the v(3) mode of adsorbed CO2 is observed at 2345 cm(-1). From the variation of integrated intensity of the IR absorption bands at both 3610 and 2345 cm(-1), upon changing temperature (and CO2 equilibrium pressure), the standard adsorption enthalpy of CO2 on H-ZSM-5 is Delta H-0=-31.2(+/- 1)kJ mol(-1) and the corresponding entropy change is Delta S-0 = -140(+/- 10) J mol(-1)K(-1). These results are discussed in the context of available data for carbon dioxide adsorption on other protonic, and also alkali-metal exchanged, zeolites.