Theoretical study of toluene adsorbed on zeolites X and Y:: Calculation of 13C NMR parameters

The adsorption of toluene on basic faujasite-type zeolites with different Si/Al ratio and charge compensating interstitial alkali metal cations, M-X and M-Y (M = Li+, Na+, K+, Rb+, and Cs+) has been investigated with molecular mechanics and quantum chemical methods. The Monte Carlo docking method was used to find energetically favorable positions of the toluene molecule within the zeolitic void space. Subsequently, density functional calculations of geometries, Mulliken partial charges, adsorption energies, and C-13 NMR chemical shift parameters of clusters representing the adsorption site were performed. The polarization of the toluene carbon atoms is held to be a decisive step in the zeolite-catalyzed alkylation reaction and will therefore influence the outcome of this reaction in terms of the preference between ring and side-chain alkylation. Both the Lewis acidity of the alkali metal cations and, to a lesser extent, the basicity of the zeolite, which is governed by the nature of these interstitial cations and by the amount of aluminum atoms in the framework, simultaneously influence the electron distribution within the toluene molecule. Key geometric parameters, the Mulliken partial charges of the toluene carbon atoms, adsorption energies, and the chemical shift parameters are presented as a function both of the alkali cation and of the Al/Si ratio of the zeolite. As a general result, the C-13 NMR chemical shift parameters delta(11), delta(22), delta(33), delta(anis), and eta of the toluene carbon atoms are sensitive to the changes in composition mentioned above, while delta(iso) hardly shows any sensitivity. Our results are in accord with experimental findings and indicate that adsorption on a single alkali cation site in zeolite X or Y does not result in the activation of toluene for alkylation. We expect our results to be useful in improving and,interpreting data from MAS NMR experiments.