Theoretical study of pyridine and 4,4'-bipyridine adsorption on the Lewis acid sites of alumina surfaces based on ab initio and density functional cluster calculations

The interactions of pyridine and 4,4'-bipyridine with the Lewis acid sites of alumina surfaces are investigated using ab initio and density functional calculations. Four cluster models of different sizes and shapes are chosen to represent the Lewis acid sites: three hydrogenated clusters Al(OH)(3), Al4O9H6, and Al10O21H12 and one non-hydrogenated cluster Al4O6. The Hartree-Fock (HF) and B3LYP approaches with two basis sets 6-31G* and 6-31+G* are used to calculate the geometries, the electronic structures, the vibrational frequencies, and the adsorption energies of the complexes formed upon interaction of pyridine or 4,4'-bipyridine ligands on the cluster surfaces. Electronic structures are determined by the electrostatic potential (ESP) analysis of charges. Adsorption energies are calculated with corrections made for zero-point energies (ZPE) and basis set superposition error (BSSE). The ESP analysis of atomic charges reveals that the charge-transfer effects are more important in Lewis complexes formed with Al4O6 cluster than in those formed with hydrogenated clusters Al(OH)(3), Al4O9H6, and Al10O21H12. The significantly larger charge transferred from pyridine or 4,4'-bipyridine ligand to Al4O6 cluster should increase the adsorption energy of these complexes. Consequently, at all levels of calculation, the adsorption energies of pyridine and 4,4'-bipyridine complexed to Al4O6 cluster (similar to 46 kcal/mol), which compare very well to experiment, are strongly larger than those obtained for both pyridine and 4,4'-bipyridine ligands complexed to Al(OH)(3) (32 kcal/mol), Al4O9H6 (24 kcal/mol) and Al10O21H12 (25 kcal/mol) clusters. The corrected adsorption energy is found to be insensitive to basis set and electron correlation effects. It essentially depends on the ionic character of the cluster model rather than on its size. For 4,4'-bipyridine complexes, similar results to those obtained for pyridine are found, and the geometry and the amount of charge of the unbound pyridyl ring are unchanged upon complexation. The calculated vibrational frequencies and frequency shifts are little sensitive to the size and shape of the cluster model. The two ring stretching modes 8a and 19b of pyridine and 4,4'-bipyridine observed in the 1400-1600 cm(-1) region are the most affected modes upon adsorption, in good agreement with the available infrared and Raman data.