CO Adsorption as a Probe of Acid Sites and the Electric Field in Alkaline Earth Exchanged Zeolite Beta Using FT-IR and ab Initio Quantum Calculations

Fourier transform infrared (FT-IR) spectra were obtained for carbon monoxide adsorbed on a series of alkaline earth exchanged beta zeolites. A cation-dependent vibrational band was observed at 2184, 2190, 2195, and 2210 cm -1 for CO adsorbed on Ba-beta, Sr-beta, Ca-beta, and Mg-beta, respectively. Using the experimental CO vibrational frequencies, the calculated electric fields ranged from 7.3 V/nm (Ba-beta) to 12.9 V/nm (Mg-beta). A vibrational band at 2228 cm -1 was observed in all of the IR spectra and was assigned to CO adsorbed on Lewis acid sites. A shoulder observed at 2188 cm -1 in some of the infrared spectra was assigned to CO interacting with Bronsted acid sites. The FT-IR experimental studies were combined with ab initio calculations to understand the effect of the different alkaline earth cations on the observed vibrational frequencies. Model compounds M(OH) 2 −CO (where M = an alkaline earth cation) and Al(OH) 3 −CO were used in the calculations to approximate the zeolitic environment. The computational results reproduced the observed trends in the experimental vibrational frequencies and confirmed that the M−CO interaction is electrostatic in nature.