Surface‐Inspired Molecular Vanadium Oxide Catalysts for the Oxidative Dehydrogenation of Alcohols: Evidence for Metal Cooperation and Peroxide Intermediates

Abstract

On the basis that thiacalix[4]arene (H(4)T4A) complex (PPh(4) )(2) [H(2)T4A(VO(2))](2) (Ia) was found to be an adequate functional model for surface species occurring on vanadium oxide based catalysts and itself catalyses the oxidative dehydrogenation (ODH) of alcohols, an analogue containing 2,2'-thiobis(2,4-di-tert-butylphenolate), (S)L(2-), as ligand, namely, (PPh(4))(2)[(S)LVO(2)](2) (II) was investigated in the same context. Despite the apparent similarity of Ia and II, studies on II revealed several novel insights, which are also valuable in connection with surfaces of vanadia catalysts: 1) For Ia and II similar turnover numbers (TONs) were found for the ODH of activated alcohols, which indicates that the additional OH units inherent to Ia do not contribute particularly to the activity of this complex, for instance, through prebinding of the alcohol. 2) On dissolution II enters into an equilibrium with a monomeric form, which is the predominant species in solution; nevertheless, ODH proceeds exclusively at the dimeric form, and this stresses the need for cooperation of two vanadium centres. 3) By omitting O(2) from the system during the oxidation of 9-fluorenol, the reduced form of the catalyst could be isolated and fully characterised (including single-crystal X-ray analysis). The corresponding intermediate had been elusive in case of thiacalixarene system Ia. 4) Reoxidation was found to proceed via a peroxide intermediate that also oxidises one alcohol equivalent. As the peroxide can also perform mono- and dioxygenation of the thioether group in II, after a number of turnovers the active catalyst contains a sulfone group. The reduced form of this ultimate catalyst was also isolated and structurally characterised. Possible implications of 1)-4) for the function of heterogeneous vanadia catalysts are discussed.

A study of oxidizing centers in carbon nanotubes by solid-state NMR

“…CNTs also show great potential in the eld of catalysis. 22,23 Although the exact function of each surface species in an oxidation reaction is yet to be understood, it is generally recognized that oxidizing oxygen species remain closely related to active center and critical for catalyzing an oxidation reaction. However, when they are functionalized by certain chemical methods (for example, oxidized by concentrated HNO 3 or H 2 O 2 ), the sp 2 structure of the carbons is destroyed, and functional groups are created on the surface, thus making them ready to be used in a variety of chemical reactions.…”

Direct Proof for a Lower Reactivity of Monomeric vs. Dimeric Oxidovanadium Complexes in Alcohol Oxidation

“…For several years we have been interested in the elucidation of the reaction mechanisms of these catalysts, by mimicking their postulated active sites on a molecular level,2–10 that is, we used molecular model compounds capable of simulating either the structural and the spectroscopic properties or the reactivity of corresponding vanadium oxide surface species. In order to simulate the oxidic environments that oxidovanadium moieties experience on the surfaces of oxidic support materials we have successfully employed three different types of ligands: The deprotonated forms of incompletely condensed silsesquioxanes,2–4 calixarenes,5–8 and thiacalixarenes, and very recently, of thiobisphenols,9,10 which represent a section of a thiacalixarene 10,20. As part of these studies, the results of a detailed mechanistic work on the thiobisphenolate based oxidovanadium model system I (Scheme ) with respect to the ODH of alcohols were reported.…”

“…As part of these studies, the results of a detailed mechanistic work on the thiobisphenolate based oxidovanadium model system I (Scheme ) with respect to the ODH of alcohols were reported. The insights obtained regarding the catalytic cycle are summarized in Scheme 10…”

“…I is a dinuclear compound in the solid state but upon dissolution is present in an equilibrium between the dimeric form I D (as in the solid state) and a monomeric form ( I M ). The results of kinetic investigations suggested that only dimer I D is the catalytically active species in the ODH of alcohols 10. In the first and rate‐determining step of the catalytic cycle I D oxidizes one equivalent of alcohol (exemplified for 9‐fluorenol) to give the corresponding ketone as well as the vanadium(IV) intermediate II , that could be isolated.…”

“…The peroxide III is able to oxidize a second equivalent of alcohol but also mediates the internal sulfoxidation of the thioether function of the employed thiobisphenolate ligand. The latter process leads to a doubly S‐oxidized derivative of I as the actual catalytically active species for which the shown catalytic cycle stays valid 10. All the dimeric as well as peroxidic species occurring were isolated and/or their reactivity was examined directly.…”