Combined DFT/CC and IR spectroscopic studies on carbon dioxide adsorption on the zeolite H-FER

Adsorption of carbon dioxide on H-FER zeolite (Si: Al = 8: 1) was investigated by means of a combined methodology involving variable-temperature infrared spectroscopy and DFT/CC calculations on periodic zeolite models. The experimentally found value of adsorption enthalpy was Delta H-0 = -30 kJ mol(-1). According to calculations, adsorption complexes on isolated Si(OH) Al Bronsted acid sites (single sites) involve an adsorption enthalpy in the range of -33 to -36 kJ mol-1, about half of which is due to weak intermolecular interactions between CO2 and the zeolite framework. Calculations show clearly the significant role played by weak intermolecular interactions; adsorption enthalpies calculated with standard GGA type exchange-correlation functionals are about 13 kJ mol(-1) underestimated with respect to experimental results. Good agreement was also found between calculated and experimentally observed stretching frequencies for these complexes. Calculations revealed that CO2 adsorption complexes involving two neighbouring Bronsted acid sites (dual sites) can be formed, provided that the dual site has the required geometry. However, no clear evidence of CO2 adsorption complexes on dual sites was experimentally found.