Vibrational Analysis of Pyridine Adsorption on the Bronsted Acid Sites of Zeolites Based on Density Functional Cluster Calculations

Adsorption of pyridine on Bronsted acid sites of protonic zeolites has been studied using density functional calculations within the cluster approach. Two types of cluster models consisting of 3 (3T: 2Si, Al) and 12 ring (12T: 8Si, 4Al) tetrahedral atoms (with SUM ratio of 2) with dangling bonds saturated by either hydrogen atoms or hydroxyl groups have been considered to represent the zeolite acid sites. The B3LYP functional with the 6-31+G* basis set are used to calculate the geometrical structures, adsorption energies, and vibrational frequencies of the complexes formed upon interaction of pyridine on the zeolite clusters. The minimum energy pathways for proton transfer from the Bronsted acid site of zeolite (ZOH) to pyridine molecule are characterized. For all clusters considered, the ion-pair complexes (PYH+center dot center dot center dot OZ(-)) are the most stable, while the stability of the hydrogen-bonded complexes (PY center dot center dot center dot HOZ) depends on the size and nature of the terminating groups of the clusters. Since the proton-transfer barrier is found to be quite small, the hydrogen-bonded complex is expected to have a very short lifetime. The vibrational spectra of the adsorbed pyridine, in either hydrogen-bonded or in ion-pair complexes, are determined, analyzed, and compared to those of the free molecule. Contrary to the adsorption energy, the calculated frequencies and frequency shifts of the adsorbed pyridine are a little sensitive to the cluster models. The interactions responsible for the vibrational properties are essentially local. The 8a and 19b vibration modes of pyridine describing the CCN ring stretching and the admixture of CH bending with ring stretching, respectively, are the most affected ones upon adsorption. Except for the CCN ring stretching mode 8b of pyridine within the ion-pair complex, there is a good agreement not only for the absolute frequencies but also for the frequency shifts.