Insights
into Brønsted Acid Sites in the Zeolite Mordenite
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Abstract
The
unique feature of the zeolite catalysts is the presence of catalytically
active acidic hydroxyls, also known as Brønsted 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 (O1–H and O9–H hydroxyls), ∼13% of BAS
are at the intersections between the side pockets and 12-MR channels
(O5–H hydroxyls), and ∼62% of BAS are located in 12-MR
channels (∼39% correspond to O2–H and/or O10–H
hydroxyls and the remaining 23% to O3–H and O7–H 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