Quantitative infra-red studies of Brønsted acid sites in zeolites: Case study of the zeolite mordenite

Nowadays, curve fitting of unresolved (or poorly resolved) IR bands is a routine tool for quantification of catalytically active Brønsted acid sites (BAS) in zeolite catalysts. A number of experimental studies demonstrated the effect of the local environment on the catalytic properties of BAS in mordenites (MOR); these studies are rooted in IR spectroscopy and curve fitting methods. In this paper we consider the existing IR-methods for BAS quantification in MOR zeolites and identify a number of shortcomings, possible experimental and computational problems. Our results show that fitting precision only cannot prove the validity of the curve fitting method and that consideration of the uncertainty of the results is required. Also, physical criteria, such as compliance with the Beer’s law and the ability of the method to describe the IR spectra of a wide range of zeolite samples, have to be checked. Using both the mathematical and physical criteria, we have proposed a 3-band curve fitting method of MOR IR spectra. Although only MOR zeolite was considered, our discussion and conclusions are relevant to studies of OH groups in other zeolites as well as in metal-organic frameworks.