Other Selective Oxidation Reactions

Other Selective Oxidation Reactions 22.4.1. Selective oxidation of amines 

The selective oxidation of amines using oxygen is an important goal for green chemistry. Recently, several authors have reported that gold, either as supported nanoparticles or in powder form, can perform the selective oxidation of 

Nikolaas Dimitratos, Jose A. Lopez-Sanchez and Graham J. Hutchings 

Silanols are utilised for silicon-based polymeric materials and also find use as nucleophilic coupling partners in organic synthesis. Traditional synthetic methods utilise toxic reagents and are non-environmentally friendly, and other recently reported synthetic methods, in the absence of organic solvents, suffer the main drawback of the production of disiloxanes. Recent results by Kaneda et al. overcome this by using water as the solvent, with silver supported in hydroxyapatite with Mtde condensation to the disiloxanes. They show that the reaction can also be catalysed by homogeneous silver, although the supported nanoparticles were superior and reusable without any loss of activity or selectivity. 

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Having explored the potential of hydrotalcites for selective hydroxylation of phenol to dihydroxy benzenes, efforts are been continually made for exploiting these materials for other selective oxidation reactions. Functionalization of C-H in benzene to C-OH as phenol is one of the challenging reaction, which has been attempted over these catalysts. Encouraging conversion with good selectivity towards phenol is noted. Carpentier et ai [117J have studied... 

A review of the experimental results reported in the literature (up to November 2014) reveals that all the results reported seem to obey an empirical correlation given by C2 selectivity-F CH4 conversion <1 (see Figure 33.23). This strongly indicates that improved yields can only be achieved with novel reactor concepts. An important aspect of the oxidative coupling reaction (and many other selective oxidation reactions) that can be exploited to maximize the total C2 product yield is that the O2 reaction order of the preferred OCM reaction is lower than the O2 reaction order of the unselective (total) oxidation reactions. 

The oxidation of CH3OH to HCHO is considered as a probe reaction for other selective oxidation reactions such as butane to maleic anhydride, o-xylene to phthalic anhydride, and ODH of alkanes to alkenes. Consequently, the concepts developed for the selective oxidation of methanol over vanadium oxide catalysts can be easily transferred to other catalytic reactions. Weckhuysen and Keller [82] carried out methanol oxidation as a probe reaction over various V2O5/S oxides (S = HO2, Zr02, Nb205, Ce02, and AI2O3). The relative independence of turnover frequency (TOP) to vanadia loading on amorphous oxide supports indicated that the reaction was first order with respect to surface vanadium oxide site. 

Following the discovery of TS-1 [125], a titanium-substituted MFl, the use of zeolitic materials for oxidation increased significantly. The presence of the Ti atom in the framework of a zeolite structure provides a site-isolated Ti center, a situation not possible with other Ti-containing materials while also allowing shape-selective oxidations. The combination of the two effects gives highly active and selective oxidation reactions [126]. 

Mechanisms There is a derth of knowledge about the mechanisms operative in selective oxidation reactions. The only exceptions are the reactions of ethylene to ethylene oxide on supported silver catalysts and of propylene to acrolein on bismuth molybdate type catalysts. For the latter, it is well established through isotopic labeling experiments that a symmetric allyl radical is an intermediate in the reaction and that its formation is rate-determining. Many studies simply extrapolate the results substantiated for this case to other reactions. New ideas on mechanisms are presented by Oyama, et oL, Parmaliana, et aL, and Laszlo. 

On the other hand, it is generally accepted that the redox properties of the selective oxidation catalysts control the oxygen activation as well as the surface stabilization of the oxygen activated species and their reactivity (19), In particular, the stabilization of active oxygen forms requires the presence of reduced sites on the surface. In fact, the peculiar behaviour of Mo, V and Fe oxides in selective oxidation reactions is strictly linked with the stabilization of reduced states (19), This point has stimulated a growing interest in providing correlation between the degree of reduction (32) or the extent of reduced sites (20) and the reactivity in... 

The viability of one particular use of a membrane reactor for partial oxidation reactions has been studied through mathematical modeling. The partial oxidation of methane has been used as a model selective oxidation reaction, where the intermediate product is much more reactive than the reactant. Kinetic data for V205/Si02 catalysts for methane partial oxidation are available in the literature and have been used in the modeling. Values have been selected for the other key parameters which appear in the dimensionless form of the reactor design equations based upon the physical properties of commercially available membrane materials. This parametric study has identified which parameters are most important, and what the values of these parameters must be to realize a performance enhancement over a plug-flow reactor. 

Although spillover species may, in other cases, promote solid-state reactions leading to a more complicated change of activity or selectivity, we shall focus on the clearly positive effects they exert in many (probably most) of the selective oxidation reactions, by preventing phase reduction. 

A more long term goal is to explore the use of OMS and OL systems for other selective oxidations. Conventional manganese oxide reagents are typically used as oxidizing agents for stoichiometric oxidations on a laboratory scale. 51 Specific examples include oxidation of primary and secondary alcohols,10 transformation of vitamin A to retinene, l02 overoxidation of aldehydes to acids and esters, 03 etc. Radical intermediates and specific stereochemistries are important in these systems. We expect that these reactions can be carried out catalyticaiiy using our OMS and OL materials. 

Partly because gold catalysts are active under mild conditions, they have been investigated for other selective oxidation and for selective hydrogenation reactions. Details are given in Sect. 6.2.4. 

Substantial improvements in the performance of several processes of hydrocarbon selective oxidation can be achieved solely by developing new reactor configurations. An important step in this direction is exemplified by the circulating fluidized bed reactor, which over the years has been proposed for use in several selective oxidation reactions and has, finally, found application in w-butane selective oxidation to maleic anhydride. Although production at the plant (built in Spain) was later stopped, because it was uneconomic, it remains an interesting example that may find application in other reactions. 

Oxidative coupling of isobutene suffers from severe deep oxidation. As in many other partial oxidation reactions selectivity remains low, despite intensive optimization of catalysts and reaction conditions. Among various new reactor concepts, the separation of catalyst reduction and reoxidation is very promising (two step process). Reaction engineering investigations of the two step process have been done. The influence of reaction conditions and reversibility of reduction/reoxidation cycles have been investigated. Based on the reaction engineering results a first approach to a kinetic model of both reaction steps has been developed. 

On the other hand, the promoting effect of potasaum doping in selective oxidation reactions is well known. So, addition to metal oxide catalysts, such as VjOs/TiOj and MoOs/TiOj increases the selectivity to oxidehydrogenation (OXD) of propane [28]. It has been shown that potassium addition brings about a decrease in acidity, lowers sur ce potential (work fimcdon) and hinders the formation of electrophihc O spedes, vsdiich are the responsible for total combustion. Thus, the reduction of total converdon as well as the increase in the selectivity to OXD products when the potassium contort increases could be interpreted as due to the modifications of these properties induced by the presence of potasshim... 

The oxidation of various hydrocarbons such as n-octane, cyclohexane, toluene, xylenes and trimethyl benzenes over two vanadium silicate molecular sieves, one a medium pore VS-2 and the other, a novei, iarge pore V-NCL-1, in presence of aqueous HjOj has been studied. These reactions were carried out in batch reactors at 358-373 K using acetonitrile as the solvent. The activation of the primary carbon atoms in addition to the preferred secondary ones in n-octane oxidation and oxidation of the methyl substituents in addition to aromatic hydroxyiation of alkyl aromatics distinguish vanadium silicates from titanium silicates. The vanadium silicates are also very active in the secondary oxidation of alcohols to the respective carbonyl compounds. V-NCL-1 is active in the oxidation of bulkier hydrocarbons wherein the medium pore VS-2 shows negligible activity. Thus, vanadium silicate molecular sieves offer the advantage of catalysing selective oxidation reactions in a shape selective manner. 

Many methods have been developed for the oxidation of primary and secondary alcohols. Oxidation of secondary alcohols normally gives rise to ketone products, whereas primary alcohols form aldehydes or carboxylic acids, depending on the reagent and conditions. Selective oxidation reactions have been developed that give these different types of products, even in the presence of other sensitive functionality. This section will describe, in turn, the different reagents used for the formation of aldehydes and ketones, before discussing the formation of carboxylic acids. 

The Oppenauer oxidation of alcohols by ketones is a very selective oxidation reaction when the molecule to be oxidised contains other groups susceptible to oxidation. The opposite reaction, the Meerwein-Ponndorf-Verley reduction of ketones by alcohols is simply the reverse reaction. These conversions are catalysed by Lewis acids. These are typically metal tert-butoxides in solution,... 

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