Coupling of framework modes and adsorbate vibrations for CO_2 molecularly adsorbed on alkali ZSM-5 zeolites: Mid- and far-infrared spectroscopy and ab initio modeling

摘要

A thorough vibrational characterization of CO_2 molecules adsorbed at room temperature on alkali metal exchanged M-ZSM-5 zeolties (M = Li~+, Na~+, K~+, and Cs~+) has been obtained: All three fundamental modes (v_1, v_2, and v_3) have been measured in the mid-IR, together with combination bands of the v_1 and v_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 CO_2 adsorption. The presence of combination modes between molecular and framework vibrations lends support to the concept already advanced [Bonelli et al., J. Phys. Chem. B 104, 10978 (2000)] that the CO_2 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 CO_2 molecularly adsorbed on other systems is also discussed.