Fourier self-deconvolution of the IR spectra as a tool for investigation of distinct functional groups in porous materials: Brønsted acid sites in zeolites

For many decades, IR and FT-IR spectroscopy has generated valuable information about different functional groups in zeolites, metal-organic frameworks (MOFs), and other porous materials. However, this technique cannot distinguish between functional groups in different local environments. Our study demonstrates that this limitation could be overcome by using Fourier self-deconvolution of infrared spectra (FSD-IR). We apply this method to study three acidic mordenite zeolites and show (i) that these zeolites contain six distinct Brønsted acid sites (BAS) as opposed to 2-4 different BAS previously considered in literature and (ii) that the relative amounts of these BAS are different in the three zeolites examined. We then analyze possible locations of six BAS in the mordenite structure and explain a number of conflicting results in literature. On this basis, we conclude that the FSD-IR method allows direct visualization and examination of distributions of distinct BAS in zeolites, thus providing a unique research opportunity, which no other method can provide. Given the similarities in the IR analysis of different functional groups in solids, we expect that the FSD-IR method will be also instrumental in the research into other porous materials, such as solid oxides and MOFs. The latter point is illustrated by FSD of the IR spectrum of hydroxyl groups in a sample of α-alumina.