Interaction of Probe Molecules with Bridging Hydroxyls of Two-Dimensional Zeolites: A Surface Science Approach

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_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 C₂H₄) 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

Removal of antibiotics and antibiotic resistance genes from domestic sewage by constructed wetlands: Optimization of wetland substrates and hydraulic loading

“…Microporous structures in zeolite can provide a high surface area for chemical sorption and microbial attachment (Hu et al, 2012a), while bridging hydroxyls are catalytically active for various chemical reactions (Boscoboinik et al, 2013). This could explain the best performance of the zeolite CWs in removing the antibiotics and ARGs as well as conventional pollutants from raw wastewater.…”

Exploring Zeolite Chemistry with the Tools of Surface Science: Challenges, Opportunities, and Limitations

“…The vast majority of these polygons are hexagonal prisms and thus we will refer to this structure as "2dH" (two-dimensional hexagonal), following the notation established previously. 19 The surface symmetry was determined by low energy electron diffraction (LEED), and Si and Al contents were determined by x-ray photoelectron spectroscopy. 17,20 While LEED shows long-range ordering (i.e.…”

“…Figure 6 shows a plot of  (O-H) shifts induced by CO and C 2 H 4 for a variety of zeolites and zeotypes taken from the literature and for our H-2dH films 19 . As it can be seen in this figure, the acidity of the model system is similar to, and even higher than some of the catalytically active zeolites used in the industry.…”

“…19 As an example of adsorption studies with more catalytically relevant molecules, we address here the interaction of an aluminosilicate film with methanol which is one of the reactants in the methylation of olefins. 35,36 Figure 7 shows a series of IRA-spectra after CD 3 OD adsorption at ~ 100 K followed by heating to the temperatures as indicated.…”

“…The Sauer group has provided a useful description of the influence of the curvature on the acidity of the bridging hydroxyl, as measured by the heat of adsorption calculated using the DFT+D approach, comparing H-2dH film with H-chabasite and separating the contribution of dispersion forces to the heat of adsorption. 19 It was found that, for weak bases CO and C 2 H 4 interacting with bridging hydroxyls, the heat of adsorption is higher for OH br in H-chabasite than for OH br in H-2dH. However, the case of adsorption on Hchabasite also shows much higher dispersion forces when compared to H-2dH, likely due to the van der Waals interactions of CO and C 2 H 4 with the pore walls in chabasite, which is greatly reduced in the planar case.…”