Selective oxidation mechanism

Transition metal oxides represent a prominent class of partial oxidation catalysts [1-3]. Nevertheless, materials belonging to this class are also active in catalytic combustion. Total oxidation processes for environmental protection are mostly carried out industriaUy on the much more expensive noble metal-based catalysts [4]. Total oxidation is directly related to partial oxidation, athough opposes to it. Thus, investigations on the mechanism of catalytic combustion by transition metal oxides can be useful both to avoid it in partial oxidation and to develop new cheaper materials for catalytic combustion processes. However, although some aspects of the selective oxidation mechanisms appear to be rather established, like the involvement of lattice catalyst oxygen (nucleophilic oxygen) in Mars-van Krevelen type redox cycles [5], others are still uncompletely clarified. Even less is known on the mechanism of total oxidation over transition metal oxides [1-4,6]. 

Many studies have been devoted to the clarification of the selective oxidation mechanism and the nature of the active sites. In the following, the known bismuth molybdate phases and their significance will first be briefly reviewed, followed by a discussion of the mechanism and kinetics. 

Based on these results, a general selective oxidation mechanism evolves (Scheme 7) (27). Initial hydrogen abstraction to form an allyl intermediate which is n-bonded to a coordinately-unsaturated Mo, the O-inserting site,... 

A comparison of this proposed selective oxidation mechanism (Scheme 11) with three other important mechanisms from the literature (Table VI) (27) shows that there is a considerable amount of discrepancy between the assignment of the role of the individual metallic components of the catalyst (72). Matsuura (25), based on low-temperature adsorption studies, attributes chemisorption and first hydrogen abstraction to Mo, while in Haber s... 

The majority of selective oxidation mechanisms can be divided into two fundamentally different types homolytic and heterolytic ones [15]. Homolytic mechanisms involve one-electron elementary steps, such as hydrogen atom transfer (HAT), single electron transfer (SET), addition of a radical species to aromatic nuclear, etc. Heterolytic mechanisms do not engage radical species and merge a range of two-electron processes, that is, oxygen atom transfer or hydride transfer. In this section, we discuss some fundamental features of the mechanisms relevant for the selective oxidation of aromatic rings. 

Alloy selection depends on several factors, including electrical properties, alloy melting range, wetting characteristics, resistance to oxidation, mechanical and thermomechanical properties, formation of intermetaUics, and ionic migration characteristics (26). These properties determine whether a particular solder joint can meet the mechanical, thermal, chemical, and electrical demands placed on it. 

Double bonds in a,/3-unsaturated keto steroids can be selectively oxidized with alkaline hydrogen peroxide to yield epoxy ketones. In contrast to the electrophilic addition mechanism of peracids, the mechanism of alkaline epoxidation involves nucleophilic attack of hydroperoxide ion on the con-... 

Our study was focused on the influence of reducing power on the selective oxidation of H2S over the various transition metal oxides, which would be proceeded by the redox mechanism [5,6]. The redox mechanism and the reducing power [7] in selective oxidation of H2S can be defined as follows ... 

In the case of selective oxidation catalysis, the use of spectroscopy has provided critical Information about surface and solid state mechanisms. As Is well known( ), some of the most effective catalysts for selective oxidation of olefins are those based on bismuth molybdates. The Industrial significance of these catalysts stems from their unique ability to oxidize propylene and ammonia to acrylonitrile at high selectivity. Several key features of the surface mechanism of this catalytic process have recently been descrlbed(3-A). However, an understanding of the solid state transformations which occur on the catalyst surface or within the catalyst bulk under reaction conditions can only be deduced Indirectly by traditional probe molecule approaches. Direct Insights Into catalyst dynamics require the use of techniques which can probe the solid directly, preferably under reaction conditions. We have, therefore, examined several catalytlcally Important surface and solid state processes of bismuth molybdate based catalysts using multiple spectroscopic techniques Including Raman and Infrared spectroscopies, x-ray and neutron diffraction, and photoelectron spectroscopy. 

Selective oxidation and ammoxldatlon of propylene over bismuth molybdate catalysts occur by a redox mechanism whereby lattice oxygen (or Isoelectronlc NH) Is Inserted Into an allyllc Intermediate, formed via or-H abstraction from the olefin. The resulting anion vacancies are eventually filled by lattice oxygen which originates from gaseous oxygen dlssoclatlvely chemisorbed at surface sites which are spatially and structurally distinct from the sites of olefin oxidation. Mechanistic details about the... 

The Holy Grail of catalysis has been to identify what Taylor described as the active site that is, that ensemble of atoms which is responsible for the surface reactions involved in catalytic turnover. With the advent of atomically resolving techniques such as scanning tunnelling microscopy it is now possible to identify reaction centres on planar surfaces. This gives a greater insight also into reaction kinetics and mechanisms in catalysis. In this paper two examples of such work are described, namely CO oxidation on a Rh(llO) crystal and methanol selective oxidation to formaldehyde on Cu(llO). 

In the cases of the selective oxidation reactions over metal oxide catalysts the so-called Mars-van Krevelen or redox mechanism [4], involving nucleophilic oxide ions 0 is widely accepted. A possible role of adsorbed electrophilic oxygen (molecularly adsorbed O2 and / or partially reduced oxygen species like C , or 0 ) in complete oxidation has been proposed by Haber (2]. However, Satterfield [1] queried whether surface chemisorbed oxygen plays any role in catalytic oxidation. 

These results being quite untypical for zeolites give rise to a number of fundamental questions i) what makes the zeolite to function as an active catalyst ii) what makes N2O to function as a selective oxidant iii) what is the reaction mechanism. We shall shortly discuss the situation with these issues because of their importance for our further consideration. 

Neumann and Khenkin28 review most of the various oxidation methods of dienes and polyenes and their mechanisms. They obviously emphasize the difference between non-conjugated and conjugated dienes and polyenes in selected oxidation reactions. 

Methods for indirect oxidation have also been developed. The combination of KF/ wCPBA in acetonitrile and water has been used to generate KOF CH3CN reagent, a mild and selective oxidant that reacts at 0 °C with no overoxidation [78]. This reagent functions by providing a fluorosulfonium ion intermediate, which is hydrolyzed in the presence of water to the desired sulfoxides. As a result of the indirect oxidation method, the typical stereoselectivity of mCPBA-type oxidations is not observed here. The KOFCH3CN oxidant is similar in scope and mechanism to 1-fluoropyridinium triflates, Selectfluor [302] and the more classical t-butyl hypochlorite [288]. 

Figure 1.8 TPSR spectra obtained after saturation of a Mo03/AI203 catalyst with methanol at room temperature [61], Seen here are mass spectrometry traces corresponding to methanol (mle = 28 and 32), formaldehyde (mle = 28 and 30), water (mle = 18), and dimethyl ether (mle = 45). These data were used to propose a mechanism for the selective oxidation of methanol on Mo03-based catalysts. (Reproduced with permission from Elsevier.)...

Table 7. Selected reactions of a C3H8 oxidation mechanism...

TABLE C7 Selected Reactions of a C3H8 Oxidation Mechanism 1... 

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