Insights into Bronsted Acid Sites in the Zeolite Mordenite

The unique feature of the zeolite catalysts is the presence of catalytically active acidic hydroxyls, also known as Bronsted acid sites (BAS), in the zeolite micropores of molecular dimensions. The accessibility and catalytic properties of BAS depend on their local environment, and it is therefore important to know the exact locations of BAS and the number of BAS in these locations. This paper reports a detailed FT-IR investigation into BAS present in the acidic and partially Na-exchanged samples of industrially important mordenite (MOR) zeolite. Our results demonstrate the existence of (at least) six distinct BAS that can be visualized by six single bands in Fourier self-deconvolution traces of the IR spectra. The quantitative estimates for the amounts of these distinct BAS were obtained using the six-band deconvolution method developed in this work. These estimates show that in the purely acidic H-MOR sample about 25% of BAS are located in eight-membered ring (8-MR) channels (O1H and O9H hydroxyls), 13% of BAS are at the intersections between the side pockets and 12-MR channels (O5H hydroxyls), and 62% of BAS are located in 12-MR channels (39% correspond to O2H and/or O10H hydroxyls and the remaining 23% to O3H and O7H hydroxyls). These quantitative data demonstrate that the acid sites are distributed quite evenly between oxygen atoms in different crystallographic positions, thus revealing the complexity of the experimental identification of distinct BAS in mordenites and explaining the variety of the earlier suggestions regarding their positions in these zeolites.