Thermodynamics of carbon dioxide adsorption 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 CO(2) molecules interact with the zeolite Brønsted-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 nu(3) mode of adsorbed CO(2) 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 CO(2) equilibrium pressure), the standard adsorption enthalpy of CO(2) on H-ZSM-5 is DeltaH(0)=-31.2(+/-1) kJ mol(-1) and the corresponding entropy change is DeltaS(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.