Reactivity Concepts for Oxidation Catalysis: Spin and Stoichiometry Problems in Dioxygen Activation

Abstract

Es ist der Valenzwechsel einer Eisenverbindung, auf dem die katalytische Oxydation in der lebendigen Substanz beruht. Otto Warburg, Les Prix Nobel 1931 Essays in Contemporary Chemistry: From Molecular Structure towards Biology. Edited by Gerhard Quinkert and M. Volkan Kisakürek © Verlag Helvetica Chimica Acta, Postfach, CH-8042 Zürich, Switzerlland, 2001This reaction appears straightforward, and appropriate processing may permit the prevention of over-oxidation -that is, subsequent degradation of the product SO -as long as the rate for the oxidation of S is not much slower than that for SO. Furthermore, the use of dioxygen provides a formidable thermochemical driving force such that Reaction 5 is exothermic for almost every substrate.A third topic, not specific to the use of dioxygen as oxidizing agent, concerns selectivity and, in particular, the closely related aspect of over-oxidation. This problem is most severe in the partial oxidation of saturated hydrocarbons, because activation of alkanes is usually more difficult than that of any of the oxygenated products (e.g., alcohols and aldehydes), which are easily oxidized further. The reason is simply that C-H bonds of functionalized alkanes are generally weaker than those of the parent hydrocarbons. Moreover, as far as metal catalysis is concerned, the polar oxidation products can 132 ESSAYS IN CONTEMPORARY CHEMISTRY:

Strategies for the Direct Catalytic Valorization of Methane Using Heterogeneous Catalysis: Challenges and Opportunities

“…The formation of M n +2 O species as oxidants, via the heterolytic reduction mechanism, is the preferred path. In this case, the stoichiometric problem of how to generate the MO species to favor the oxygen atom transfer reaction is important (see Scheme ), and two different scenarios have to be considered: (1) producing 1 equiv of MO per O 2 molecule (having to scarify the other oxygen as, i.e., H 2 O) or (2) generating two neighboring MO species. Naturally, the second option is more attractive; however, the steric control for partial oxidation plays a key role.…”

Spin catalysts: A quantum trigger for chemical reactions

“…Historically, spin catalysis has been investigated in the paradigm of spin chemistry concurrently with chemically induced dynamic nuclear polarization and reactions in external magnetic fields, mainly for processes in solution, and investigated by electron paramagnetic resonance spectroscopy. The theory of such processes can be found in the works of Buchachenko, Minaev, and Schwarz [2][3][4][5]. Only brief excerpts are given here.…”