N₂ and CO molecules as probes of zeolite acidity: an infrared spectroscopy and density functional investigation

The interaction of N₂ with Brønsted acid centers of H-ZSM-5 zeolite has been investigated employing Fourier transform infrared spectroscopy and cluster model calculations based on a gradient corrected density functional method. A comparison is made with CO, which is widely used as a probe for surface acidity. It is shown that the computational approach is capable of almost quantitatively reproducing a number of sensitive parameters of the H-bonded dinitrogen and carbonyl complexes, like adsorption energy, adsorption-induced changes of the vibrational frequencies and of their intensities. According to a constraint space orbital variation analysis, the carbonyl and dinitrogen complexes mainly differ by the somewhat stronger σ donation ability of CO as compared to N₂. It is concluded that dinitrogen may serve as a convenient probe for the acidity of zeolites.