QUANTUM-CHEMICAL JUSTIFICATION OF THE ZEOLITE ACID STRENGTH MEASUREMENT BY INFRARED-SPECTROSCOPY

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