Interaction of probe molecules with bridging hydroxyls of two-dimensional zeolites: A surface science approach

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_x. 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 ₂H₄) and two strong bases (NH₃ 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, NH₃ 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.