Adsorption of CO on NaZSM-5 zeolite under moderate temperature and pressure conditions: An FTIR investigation

Abstract

Adsorption of CO on NaZSM-5 zeolite was investigated at temperatures in the range 300–470 K and at pressures of 5–500 Torr using FTIR spectroscopy. The effect of exchanging the charge balancing cation in NaZSM-5 with a proton or calcium was evaluated. Data were also collected on NaY, CaY and CaX zeolites for comparison. We detected the development of six distinct C–O stretching bands with maxima at around 2111, 2130, 2146, 2160, 2176 and 2194 cm^-1 during the adsorption of CO on NaZSM-5 zeolite at ambient temperatures. This was accompanied by the appearance of a prominent band at 2356 cm^-1 and weak shoulder bands at frequencies around 2336, 2340, 2370 and 2380 cm^-1 in the ν₃ region of CO₂. All the ν(CO) bands and also the bands in the ν₃ region of CO₂ exhibited similar behaviour as a function of adsorbate pressure, evacuation, rise in sample temperature, and the exchange of charge balancing cation. For instance, the intensity of all the C–O stretching bands showed a similar growth behaviour with increasing adsorbate pressure, though the extent of this growth was different for the individual IR bands. Similarly, these bands were removed simultaneously on evacuation. Furthermore, while all the vibrational bands in the ν(CO) region showed a uniform isotopic shift corresponding to a frequency ratio ν(¹³C/¹²C) of ca. 0.977 and ν(¹⁸O/¹⁶O) of 0.976 for the adsorption of ¹³C¹⁶O and ¹²C¹⁸O, respectively, the bands in the ν₃(CO₂) region showed a red shift ν(¹³C/¹²C) of 0.972 with ¹³CO and an isotopic shift corresponding to ¹⁶O¹²C¹⁸O on ¹²C¹⁸O adsorption. No shift in ν(OH) bands was observed after CO adsorption under the conditions of this study. The results thus indicate that the individual zeolitic surface sites e.g., the Al³⁺ sites, Bronsted acid sites or the charge balancing cations, may not participate directly in the bonding of CO molecules at room temperature or above. Instead, the cage effect of zeolites plays an important role. The data are interpreted to suggest the formation of weakly bonded clusters of CO and CO₂ molecules, occluded in the zeolitic cages and stabilized under the cationic field.