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

Fourier transform infrared (FTIR) spectrosco-py 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, epsilon. The values of epsilon 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 H-1 magic-angle spinning (MAS) NMR and FTIR spectroscopy techniques. The concentration of Bronsted 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 H-1 MAS NMR using methane and benxene as internal standards adsorbed on the studied samples. The data on the obtained MAS NMR using methane and benzene as internal standards adsorbed on the 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 e have been reliably determined to be 3.06 +/- 0.04 and 1.50 +/- 0.06 cm mu mol(-1) for the IR bands at 3605-3615 and 3740-3747 cm(-1) respectively. The epsilon values are similar for both H- ZSM-5 and H-ZSM-23 zeolites. It is also established that the e values are constant with respect to the concentration of hydroxyl groups for H-ZSM-5 and H-ZSM-23 zeolites. The determined coefficients e can be further used for reliable assessment of zeolite Bronsted acidity with the aid of the widely available and relatively simple methodology of FTIR spectroscopy.