Selective oxidative dehydrogenation mechanism

The role of adsorbed oxygen species in the mechanism of alkane transformation, on the contrary, is more questionable. The effect induced by the substitution of O2 with N2O and IR indications are in agreement with this interpretation, but, on the other hand, activated electrophilic oxygen species form on reduced sites, preferably in tetrahedral coordination (79). The partial reduction of tetrahedral V =0 with formation of tetrahedral v after propane oxidative dehydrogenation can be observed using UV-Visible diffuse reflectance, ESR and V-NMR spectroscopies. It is thus not possible to assign unequivocally the active species in propane selective activation to a tetrahedral V =0 species or to or V -0-0 species formed in the... 

The kinetics of the electrochemical oxidation of ammonia on platinum to dinitrogen in basic electrolytes has been extensively studied. In the widely supported mechanism originally suggested by Gerischer and Mauerer[ll], the active intermediate in the selective oxidation to N2 is a partly dehydrogenated ammonia adsorbate, NH2 ads or NHaatomic nitrogen adsorbate N ag, which is apparently formed at more positive potentials, is inactive toward N2 production at room temperature. Generally, only platinum and iridium electrodes exhibit steady-state N2 production at potentials at which no sur-... 

Pincer-ligated iridium complexes have been used as homogeneous catalysts for the dehydrogenation of aliphatic polyalkenes to give partially unsaturated polymers. The catalyst appears to be selective for dehydrogenation in branches as compared with the backbone of the polymer.56 The mechanism shown in Scheme 1 has been suggested for an [IrCl(cod)]2-catalysed oxidative esterification reaction of aliphatic aldehydes and olefinic alcohols.57... 

Modern variations include the in situ, and thus catalytic, use of this high-valent selective reagent, not only for alcohols but also for ethers (see later). Ru(VII) (perruthenate) in the compounds tetra-n-butylammonium perruthenate (TBAP) and tetra-n-propylammonium perruthenate (TPAP) has found wide application in alcohol oxidation. Ru-oxo complexes with valence states of IV to VI are key intermediates in, for example, the selective oxygen transfer to alkenes, leading to epoxides. On the other hand 16-electron Ru(II) complexes can be used to catalyse hydrogen transfer thus these are excellent catalysts for oxidative dehydrogenation of alcohols. A separate section is included to describe the different mechanisms in more detail. 

The heterobimetallic complexes [N(n-Bu)4] [Os(N)R2(/u.-0)2Cr02] catalyze the selective oxidation of alcohols with molecular oxygen. A mechanism in which alcohol coordinates to the osmium center and is oxidized by B-hydrogen elimination (see -Hydride Elimination) is consistent with the data. The hydroxide adduct of OSO4, [0s(0H)204], with ferric cyanide and other co-oxidants catalyzes the oxidative dehydrogenation of primary aromatic and aliphatic amines to nitriles, the oxidation of primary alcohols to carboxylic acids, and of secondary alcohols to ketones. Osmium derivatives such as OsCb catalyze the effective oxidation of saturated hydrocarbons in acetonitrile through a radical mechanism. ... 

The selective oxidation of propylene to acrolein was suggested to occur via a Mars-van Krevelen mechanism (i.e. reaction of the hydrocarbon with lattice oxygen), where in the first step bismuth dehydrogenates propene to form an allylic species. This has mainly been concluded from isotopic scrambling studies. The oxidation step itself is believed to occur via the mechanism shown in Figure 7.11. 

A homogeneous catalytic solution to the alcohol inhibition problem (see the discussion under Uncatalyzed chain reactions of the oxidation of alcohol intermediates, above) does not appear to have been found. However, the presence of a heterogeneous oxidative dehydrogenation catalyst has been reported to be effective in the direct oxidation of alcohols to carbonyls and acids [109, 110]. The mechanism probably involves preliminaiy heterogeneous (oxidative) dehydrogenation of carbinols to carbonyls. If the carbonyl is an aldehyde, it is readily converted to the acid. Platinum, palladium, ruthenium, rhodium, and iridium catalysts, supported on carbon, are reported to be active and selective catalysts for the purpose [109]. Promoters such as cobalt and cadmium have been reported to be effective additives. 

We shall first show that it is still far fiom clear which are the families of catalysts to be used for the various reactions mainly oxidative dehydrogenation or oxidation to oxygen-containing molecules of ethane, propane or isobutane. Much research is still necessary for understanding the mechanisms leading to high selectivity. In this context, we shall suggest that many concepts inherited from the development in selective oxidation and ammoxidation of olefins are probably of little use. 

No new formulations seem to have been proposed for propane oxidative dehydrogenation since 1993, the year of the work cited above, and a literature computer survey indicates only 7 articles which deal with mechanisms, one mentioning catalysts containing Bi, Mo, W, V and Ti [5], and 2 rather general patents. The literature is not richer for the oxidation of propane to acrolein, with mention of the formulation Ago.oi Bio.85 V0.54 M00.45 O4.0, giving a selectivity to acrolein of 32% at a propane conversion of 52% [6], quite comparable to results obtained in 1991 by other authors [7]. Similar remarks can be made for other oxidation reactions. 

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