Two-Dimensional correlation analysis to study Bronsted acid sites in zeolites
Résumé
The Brønsted acidity of a dealuminated H-Y zeolite toward CO is investigated by an experimental approach, using both infrared spectroscopy and chemometrics. 2D correlation is a powerful technique applicable to spectroscopy and requires only a series of varying analytical signal, like spectra induced by an external perturbation applied to the system of interest. In our case, perturbation is the introduction of small doses of CO in our catalytic cell, inducing a pressure change and a modification of adsorbed CO concentration. A set of spectra collected under this perturbation is then converted to 2D correlation spectra, which provide rich and useful information about changes in the spectra. The combination of the 2D correlation analysis of the IR spectra with 2D NMR in a series of progressively Na-exchanged HY zeolites allowed a full assignment of the spectral signals for OH groups in the large cavities of the structure, and the determination of the acid strength of all individual sites.