N-2 AND CO MOLECULES AS PROBES OF ZEOLITE ACIDITY - AN INFRARED-SPECTROSCOPY AND DENSITY-FUNCTIONAL INVESTIGATION

The interaction of N-2 With Bronsted acid centers of H-ZSM-5 zeolite h as 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 app roach is capable of almost quantitatively reproducing a number of sens itive parameters of the H-bonded dinitrogen and carbonyl complexes, li ke adsorption energy, adsorption-induced changes of the vibrational fr equencies and of their intensities. According to a constraint space or bital variation analysis, the carbonyl and dinitrogen complexes mainly differ by the somewhat stronger sigma donation ability of CO as compa red to N-2. It is concluded that dinitrogen may serve as a convenient probe for the acidity of zeolites.