Oxidant‐Free Dehydrogenation of Alcohols Heterogeneously Catalyzed by Cooperation of Silver Clusters and Acid–Base Sites on Alumina

Abstract

A gamma-alumina-supported silver cluster catalyst--Ag/Al(2)O(3)--has been shown to act as an efficient heterogeneous catalyst for oxidant-free alcohol dehydrogenation to carbonyl compounds at 373 K. The catalyst shows higher activity than conventional heterogeneous catalysts based on platinum group metals (PGMs) and can be recycled. A systematic study on the influence of the particle size and oxidation state of silver species, combined with characterization by Ag K-edge XAFS (X-ray absorption fine structure) has established that silver clusters of sizes below 1 nm are responsible for the higher specific rate. The reaction mechanism has been investigated by kinetic studies (Hammett correlation, kinetic isotope effect) and by in situ FTIR (kinetic isotope effect for hydride elimination reaction from surface alkoxide species), and the following mechanism is proposed: 1) reaction between the alcohol and a basic OH group on the alumina to yield alkoxide on alumina and an adsorbed water molecule, 2) C-H activation of the alkoxide species by the silver cluster to form a silver hydride species and a carbonyl compound, and 3) H(2) desorption promoted by an acid site in the alumina. The proposed mechanism provides fundamental reasons for the higher activities of silver clusters on acid-base bifunctional support (Al(2)O(3)) than on basic (MgO and CeO(2)) and acidic to neutral (SiO(2)) ones. This example demonstrates that catalysts analogous to those based on of platinum group metals can be designed with use of a less expensive d(10) element--silver--through optimization of metal particle size and the acid-base natures of inorganic supports.

Supported rhenium nanoparticle catalysts for acceptorless dehydrogenation of alcohols: structure–activity relationship and mechanistic studies

“…This conclusion is consistent with our previous conclusions on the valence state of the transition metals (Ag, 17 Co, 18 Ni, 19 Pt 20 )-loaded catalysts for the dehydrogenation of the alcohol.…”

“…[8][9][10][11][12][13][14][15][16][17][18][19][20] However, rational improvement of the catalytic efficiency of heterogeneous catalyst for this reaction is still difficult due to a lack of fundamental information on the structure-activity relationship and reaction mechanism. For example, controversial models are proposed on the oxidation states of transition metal catalysts for this reaction; metallic species are proposed to be effective for the supported Ag, 17 Co, 18 Ni 19 and Pt 20 catalysts, whereas Re 0 -core/ReO 2 -shell model is proposed as an active Re species of unsupported nanocrystalline catalyst. 16 As our continuous studies on the structural and mechanistic aspects of supported transition metal catalysts for this reaction, [17][18][19][20] we report herein the effects of oxidation state of Re and support materials on heterogeneous Re catalysis for the dehydrogenation of secondary alcohols.…”

Silver Cluster Catalysts for Green Organic Synthesis

“…Reaction mechanism and structure-activity relationship are presented to discuss a design concept of Ag catalysts. 46) Al2O3-supported Ag catalyst (5 wt% Ag) was prepared by impregnating γ-AlOOH, γ-Al2O3, or θ-Al2O3 with an aqueous solution of silver nitrate followed by evaporation to dryness at 120 ℃. Before each catalytic or spectroscopic experiment, the precursor was calcined in air at 600 ℃, followed by reduction by H2 at 300 ℃.…”

New Gateways to the Platinum Group Metal-Catalyzed Direct Deuterium-Labeling Method Utilizing Hydrogen as a Catalyst Activator

“…However, organic solvents have been required for the reported methods. [36][37][38][39][40][41][42][43] Although the dehydrogenation of alcohols using a platinum group metal on carbon as a catalyst in H 2 O can proceed as shown in Chart 5 to give the intermediary carbonyl products, the reverse hydrogenation by the in situ-generated H 2 immediately reproduces the mother alcohols. Therefore, the suppression of the reverse hydrogenation is important to achieve the dehydrogenation of alcohols into carbonyl products.…”