Coupling of framework modes adsorbate vibrations for CO2 molecularly adsorbed on alkali ZSM-5 zeolites: Mid Far-infrared spectroscopy ab-initio modeling

A thorough vibrational characterization of CO2 molecules adsorbed at room temperature on alkali metal exchanged M-ZSM-5 zeolites (M=Li+, Na+, K+, and Cs+) has been obtained: All three fundamental modes (nu(1), nu(2), and nu(3)) have been measured in the mid-IR, together with combination bands of the nu(1) and nu(3) modes with modes at 18, 45, and 60 cm(-1). The nature of these low-lying modes is discussed, and it is proposed that these are framework vibrations. Ab initio calculations on molecular models mimicking the building blocks of zeolites revealed the systematic presence of such low-lying modes, and far-IR measurements using synchrotron radiation showed a couple of bands at 55-45 cm(-1) in the spectrum of the bare samples, which undergo a slight perturbation upon CO2 adsorption. The presence of combination modes between molecular and framework vibrations lends support to the concept already advanced [Bonelli , J. Phys. Chem. B 104, 10978 (2000)] that the CO2 molecule has a primary interaction with the cation and a secondary one with an adjacent oxygen anion; the latter not strong enough to lead to a carbonate species. The occurrence of similar combination bands for CO2 molecularly adsorbed on other systems is also discussed.