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

Abstract

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 (ν1, ν2, and ν3) have been measured in the mid-IR, together with combination bands of the ν1 and ν3 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 et al., 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.

Structure and potential energy surface of K+·CX2

“…5, favorably occurs as a result of attraction forces between both sites. These results agree very well with those obtained by Lee et al [4] and with the results in references [16,17]. The potential energy surface (PES) was examined and constructed using the IRC test (see Fig.…”

Adsorbate Interactions in Metal Organic Frameworks Studied by Vibrational Spectroscopy

“…A shoulder in the vicinity of the main CO 2 asymmetric stretch mode can therefore be assigned to n 3 þ d-d combination mode. [24] 2.2.1. Vibrational Signature of CO 2 Adsorbed in MOFs…”

Isosteric heats of adsorption of carbon dioxide on zeolite MCM-22 modified by alkali metal cations

“…It has been clearly established that the extraframework cations compensating the negative charge of the faujasite framework, play a decisive role in adsorption on these materials. The calculated enthapies − H o of interaction of CO 2 molecule with bare alkali metal cations are generally much larger than the experimental ones due to the absence of the zeolite framework (Garrone et al 2002). Moreover, calculated enthalpies decrease quickly in the sequence Li + > Na + > K + > Cs + from 83.63 kJ/mol to 53.17, 32.40 and 20.50 kJ/mol, respectively.…”