Probing the acid sites of zeolites with pyridine: Quantitative AGIR measurements of the molar absorption coefficients

• Published in: Journal of catalysis 2020-05, Vol.385, p.52-60
• Main Authors: Zholobenko, Vladimir, Freitas, Cátia, Jendrlin, Martin, Bazin, Philippe, Travert, Arnaud, Thibault-Starzyk, Frederic
• Format: Article
• Language: English
• Subjects: Acid sites; Catalysis; Chemical Sciences; FTIR; Molar absorption coefficients; or physical chemistry; Pyridine; Theoretical and; Zeolites
• ISSN: 0021-9517; 1090-2694
• DOI: 10.1016/j.jcat.2020.03.003

[Display omitted] •Integrated FTIR-gravimetry methodology for quantitative acidity measurements is established.•The validity of Beer-Lambert-Bouguer law for IR characterisation of solid acids is demonstrated.•The molar absorption coefficients of Py complexes with acid sites in zeolites are determined.•The key effects of experimental design on FTIR measurements are quantified. This study presents a detailed methodology, which combines high-precision thermogravimetry and FTIR spectroscopy, aiming to establish the most accurate and reliable means of measuring the molar absorption coefficients of adsorbed species. As the integrated molar absorption coefficients of Py complexes with Brønsted and Lewis acid sites, ɛ(Py-B) and ɛ(Py-L), are determined and the validity of the Beer-Lambert-Bouguer law for IR characterisation of solid acids is demonstrated, this work is setting a benchmark for the quantitative acidity measurements in zeolites and related materials. The following values of ɛ(Py-B) have been obtained at 150 °C (band at ~1545 cm−1): 1.09 ± 0.08 cm µmol−1 for ZSM-5; 1.12 ± 0.16 cm µmol−1 for BEA; 1.29 ± 0.04 cm µmol−1 for MOR and 1.54 ± 0.15 cm µmol−1 for FAU. The value of ɛ(Py-L) (band at ~1455 cm−1, which refers to different cations) measured at the same temperature is 1.71 ± 0.1 cm µmol−1. Values of ɛ(Py-B) depend on the zeolite structure, in contrast to that for ɛ(Py-L). Clear correlations are presented between the temperature of the FTIR measurements and ɛ values for Py complexes and other species, which decrease by ~10% as the temperature increases by 100 °C. In addition, the effects of key experimental procedures, instrumentation design and sample preparation are established and quantified.