Interaction of Probe Molecules with Bridging Hydroxyls
of Two-Dimensional Zeolites: A Surface Science Approach
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Abstract
Bridging
hydroxyls (Si–OH–Al) in zeolites are catalytically active
for a multitude of important
reactions, including the catalytic cracking of crude oil, oligomerization
of olefins, conversion of methanol to hydrocarbons, and the selective
catalytic reduction of NO<sub><i>x</i></sub>. The interaction
of probe molecules with bridging
hydroxyls was studied here on a novel two-dimensional zeolite model
system consisting of an aluminosilicate forming a planar sheet of
polygonal prisms, supported on a Ru(0001) surface. These bridging
hydroxyls are strong Brönsted acid sites and can interact with
both weak and strong bases. This interaction is studied here for two
weak bases (CO and C<sub>2</sub>H<sub>4</sub>) and two strong bases
(NH<sub>3</sub> and pyridine), by infrared reflection absorption spectroscopy,
in comparison with density functional theory calculations. Additionally,
ethene is the reactant in the simplest case of the olefin oligomerization
reaction which is also catalyzed by bridging hydroxyls, making the
study of this adsorbed precursor state particularly relevant. It is
found that weak bases interact weakly with the proton without breaking
the O–H bond, although they do strongly affect the O–H
stretching vibration. On the other hand, the strong bases, NH<sub>3</sub> and pyridine, abstract the proton to produce ammonium and
pyridinium ions. The comparison with the properties of three-dimensional
zeolites shows that this two-dimensional zeolite model system counts
with bridging hydroxyls with properties similar to those of the most
catalytically active zeolites, and it provides critical tools to achieve
a deeper understanding of structure–reactivity relations in
zeolites
