Quantum-Chemical Justification of the Zeolite Acid Strength Measurement by Infrared Spectroscopy

The quantitative measurement of the strength of Brønsted acid sites in zeolites would be very useful in catalyst development. One possible method is the measurement of the shift in the stretching frequency of Brønsted hydroxyls on adsorption of a weak base. An empirical correlation for deprotonation energy versus infrared shift has been suggested in the literature, but it cannot be directly experimentally tested. We have performed quantum-chemical calculations of the variation of the infrared shift with Brønsted acid strength. The three probe bases considered are as follows: ethene, carbon monoxide, and nitrogen. The calculations were carried out at the MP2 level with a basis set close to 6-31G** and with explicit consideration of anharmonicity of the OH stretching potential. The results show that the experimental correlation for carbon monoxide gives both absolute acid site deprotonation energies and differences between them to within 10%, supporting the quantitative use of this correlation.