Direct Measurement of Zeolite Brønsted Acidity by FTIR Spectroscopy: Solid-State 1H MAS NMR Approach for Reliable Determination of the Integrated Molar Absorption Coefficients

Fourier transform infrared (FTIR) spectroscopy is broadly applied nowadays for probing the concentration and strength of acid sites in zeolite catalysts. Accuracy of direct determination of the quantity of different hydroxyl groups by FTIR method suffers from uncertainty of the integrated molar absorption coefficients, ε. The values of ε reported by different authors might differ by an order of magnitude. This paper provides an approach for reliable determination of the integrated molar absorption coefficients by combining 1H magic-angle spinning (MAS) NMR and FTIR spectroscopy techniques. The concentration of Brønsted acid sites for the series of H-ZSM-5 and H-ZSM-23 zeolite samples with different Si/Al ratio has been reliably established with 1H MAS NMR using methane and benzene as internal standards adsorbed on the studied samples. The data on the obtained concentration were further used to analyze same zeolite samples with FTIR spectroscopy and derive the information on the values of the integrated molar absorption coefficients. The coefficients ε have been reliably determined to be 3.06 ± 0.04 and 1.50 ± 0.06 cm μmol–1 for the IR bands at 3605–3615 and 3740–3747 cm–1, respectively. The ε values are similar for both H-ZSM-5 and H-ZSM-23 zeolites. It is also established that the ε values are constant with respect to the concentration of hydroxyl groups for H-ZSM-5 and H-ZSM-23 zeolites. The determined coefficients ε can be further used for reliable assessment of zeolite Brønsted acidity with the aid of the widely available and relatively simple methodology of FTIR spectroscopy.