Quantification of brønsted acid sites in microporous catalysts by a combined FTIR and NH₃-TPD study

This article reports the use of combined techniques (thermal desorption and spectroscopic analysis) for the characterization of Brønsted sites in microporous solid catalysts. The specific aim was to provide a quantitative determination of sites with slightly different acidities in chabazite-related silicoaluminophosphate (SAPO) materials, which display three bridged hydroxyls, OH_A, OH_B, and OH_C, absorbing at 3630, 3614, and 3600 cm^-1, respectively. To this aim, different approaches were employed. First, the total concentration of Brønsted sites was calculated by classical NH₃-TPD experiments. Ammonia was preferred to pyridine due to the small kinetic diameter, allowing easy access and diffusion inside the small cavities of chabazite. Second, FTIR spectroscopy was employed to study the adsorption of NH₃ and CO. The Lambert - Beer equation, using extinction coefficients from the literature, was employed to estimate the total amount of Brønsted sites involved in the formation of NH₄ ⁺ ammonium ions or in CO/H⁺ adducts. The agreement among the data obtained by the three methods, which is excellent, is discussed critically. The fraction of each acid group in samples with different Brønsted site densities and strength distributions was determined with great accuracy. This finally allowed calculation of the extinction coefficients of the three hydroxyls of the H-SAPO-34 catalysts, which were ε_A = ε_B = 3.9 cm μmol^-1 and ε_C = 6.0 cm μmol^-1. It is proposed that these values are of general use for determining the distribution of acid sites of SAPOs and zeolites whose hydroxyls absorb in the same range of wavenumbers.