Density functional study of acidic hydroxyl groups in zeolites and their interaction with carbon monoxide

The interactions of carbon monoxide with the free hydroxyl and bridged hydroxyl groups in zeolites are investigated using density functional theory. The BLYP and Becke3LYP functionals due to Becke, Lee, Yang, and Parr are used to calculate geometries, vibrational frequencies, vibrational frequency shifts, and adsorption energies on complexation with carbon monoxide. Vibrational frequencies are calculated at the harmonic level and anharmonically, using Morse potentials. Adsorption energies are calculated with corrections made for zero-point energies and BSSE. The models used to represent the free and bridged hydroxyl groups are described exceptionally well by the density functional methods, especially Becke3LYP, which predicts almost identical geometric data to the MP2 level of theory. The CO molecule is found to interact preferentially via the carbon lone pair. The calculated vibrational frequencies, frequency shifts, and adsorption energies are found to be in excellent agreement with experimental data. Both density functionals, especially Becke3LYP, significantly improve on the results of Hartree-Fock and MP2 theory. Accounting for the anharmonicity of the O-H bond is found to be essential if one is to reproduce experimental vibrational shifts.