Surface Interactions of C₂ and C₃ Polyols with γ-Al₂O₃ and the Role of Coadsorbed Water

Abstract

The formation of surface species from two- and three-carbon polyols on γ-Al(2)O(3) in the presence and absence of coadsorbed water is investigated. Aqueous-phase adsorption isotherms indicate that competitive adsorption between water and polyol inhibits the uptake of the polyol molecules on γ-Al(2)O(3) and that the polyol with the most hydroxyl groups, glycerol, experienced the greatest uptake. Deuterium solid echo pulse NMR measurements support the fact that glycerol strongly interacts with γ-Al(2)O(3) in the presence of physisorbed water and that ethylene glycol interacts with γ-Al(2)O(3) only after the physisorbed water has been removed. In situ high-vacuum FT-IR analysis combined with DFT simulations demonstrate that glycerol readily forms a multidentate alkoxy species through its primary hydroxyl groups with coordinatively unsaturated Al atoms of γ-Al(2)O(3) in the presence of physisorbed water. This surface species exhibits a bridging alkoxy bond from one of its primary hydroxyl groups and a strong interaction with the remaining primary hydroxyl group. FT-IR analysis of 1,3-propanediol on γ-Al(2)O(3) also demonstrates the formation of a multidentate alkoxy species in the presence of coadsorbed water. In contrast, polyols with hydroxyl groups only on the one- and two-carbon atoms, ethylene glycol, and 1,2-propanediol do not form alkoxy bonds with the γ-Al(2)O(3) surface when coadsorbed water is present. These polyols will form alkoxy bonds to γ-Al(2)O(3) when coadsorbed water is removed, and these alkoxy species are removed when water is readsorbed on the sample. The formation of strongly bound, stable multidentate alkoxy species by ethylene glycol and 1,2-propanediol on γ-Al(2)O(3) is prevented by steric limitations of vicinal alcohol groups.

The Role of Brønsted and Water‐Tolerant Lewis Acid Sites in the Cascade Aqueous‐Phase Reaction of Triose to Lactic Acid

“…One should bear in mind though that the activity of water-tolerant Lewis acid sites will also be defined by the nature of the oxygenate molecule being converted. Copeland et al [65,66] studied the competitive adsorption of oxygenates with water on metal oxides and showed that the structure of the oxygenate molecule -carbon chain size and type, number and position of functional groups -can alter its interaction strength with the solid surface and its ability to replace water on surface sites. Water molecules (Lewis base) in the water-Lewis acid adducts can be displaced from the acid sites by a more basic oxygenate molecule, rendering an active Lewis acid site.…”

Water-assisted oxygen activation during selective oxidation reactions

“…Finally, a better mechanistic understanding of which oxygen species participate in desired and undesired oxidation pathways is required to improve selectivity and atom-efficiency. Besides catalytic oxidation reactions, water-mediated proton transfer has other farreaching implications, for instance, in catalytic hydrodeoxygenation of bio-oil [72], electrocatalysis [73][74][75], and any reaction using solid acid catalysts including zeolites [76,77]. Thus, we see numerous opportunities for innovative research on water-assisted catalysis in diverse areas that may ultimately lead to disruptive technologies.…”

Metal–Organic Framework Mediated Synthesis of Small‐Sized γ‐Alumina as a Highly Active Catalyst for the Dehydration of Glycerol to Acrolein

“…The conversion of glycerol at an initial reactionp eriod of 2h as af unction of temperature is displayed in Figure 4. [40] We therefore assign the high activities of M-Al 2 O 3 and N-Al 2 O 3 to the fact that they have higher acid-site densitiest han C-Al 2 O 3 .M -Al 2 O 3 exhibits ah igher weaka cid site density and al ower strong acid site density than N-Al 2 O 3 ,a nd this couldd ecreaset he coking rate. Their initial conversionsf ollow the same order as their stabilities, as shown in Figure 4, that is, M-Al 2 O 3 > N-Al 2 O 3 > C-Al 2 O 3 .F or the glycerol dehydration reaction, the catalytic activities of zeolite materials are governed mainly by pore-confinement effects and the mesopore size, and the hydrophilic hydroxy-containing surface of g-Al 2 O 3 is governed mainly by acid-site density.…”