Reoxidation of catalysts

Most oxidation reactions over oxide catalysts are well nnderstood in terms of the redox mechanism, for example, repeated rednction and oxidation of the surface layer or bulk of the oxide catalyst. In the first step, a metal oxide catalyst oxidizes reactant molecules, such as carbon monoxide to carbon dioxide (equation 1 reduction of catalyst). In the second step, the reduced catalyst is oxidized back to its initial state by oxygen molecules supplied by the gas phase (equation 2 reoxidation of catalyst). The catalytic oxidation (equation 3) proceeds by repetition of this redox cycle. 

The observed nonlinear dependence of the rate on [BuOH] is ascribed to a ratedetermining alcohol-independent reoxidation of catalyst. 

Fresh butane mixed with recycled gas encounters freshly oxidized catalyst at the bottom of the transport-bed reactor and is oxidized to maleic anhydride and CO during its passage up the reactor. Catalyst densities (80 160 kg/m ) in the transport-bed reactor are substantially lower than the catalyst density in a typical fluidized-bed reactor (480 640 kg/m ) (109). The gas flow pattern in the riser is nearly plug flow which avoids the negative effect of backmixing on reaction selectivity. Reduced catalyst is separated from the reaction products by cyclones and is further stripped of products and reactants in a separate stripping vessel. The reduced catalyst is reoxidized in a separate fluidized-bed oxidizer where the exothermic heat of reaction is removed by steam cods. The rate of reoxidation of the VPO catalyst is slower than the rate of oxidation of butane, and consequently residence times are longer in the oxidizer than in the transport-bed reactor. 

Ca.ta.lysis, The most important iadustrial use of a palladium catalyst is the Wacker process. The overall reaction, shown ia equations 7—9, iavolves oxidation of ethylene to acetaldehyde by Pd(II) followed by Cu(II)-cataly2ed reoxidation of the Pd(0) by oxygen (204). Regeneration of the catalyst can be carried out in situ or ia a separate reactor after removing acetaldehyde. The acetaldehyde must be distilled to remove chloriaated by-products. 

Partial oxidations over complex mixed metal oxides are far from ideal for singlecrystal like studies of catalyst structure and reaction mechanisms, although several detailed (and by no means unreasonable) catalytic cycles have been postulated. Successful catalysts are believed to have surfaces that react selectively vith adsorbed organic reactants at positions where oxygen of only limited reactivity is present. This results in the desired partially oxidized products and a reduced catalytic site, exposing oxygen deficiencies. Such sites are reoxidized by oxygen from the bulk that is supplied by gas-phase O2 activated at remote sites. 

Similar kinetic experiments were carried ont to compare the activities of the fully oxidized and fully reduced forms of BaCeo 95Pdoo502 95, Figure 27. lb. The activity of the reduced catalyst is much lower than that of the as-prepared (i.e., oxidized) catalyst, resulting in only 2% yield after 4 min. However, high activity is completely restored upon reoxidation of the reduced catalyst. 

The most effective way to improve the SCR performance is to increase the rate of reaction itself by the help of N02 [28,36,37], When equimolar amounts of NO and N02 are used, a very high DeNO, is observed ( fast-SCR reaction), as the very potent oxidizing agent N02 reoxidizes the catalyst much faster than oxygen [38] ... 

In agreement with the TPR results, the hydrogen chemisorption/pulse reoxidation data provided in Table 8.3 indicate that, indeed, the extents of reduction for the air calcined samples are -20% higher upon standard reduction at 350°C (compare 02 uptake values). Yet in spite of the higher extent of reduction, the H2 desorption amounts, which probe the active site densities (assume H Co = 1 1), indicate that the activated nitric oxide calcined samples have higher site densities on a per gram of catalyst basis. This is due to the much smaller crystallite that is formed. The estimated diameters of the activated air calcined samples are between 27 and 40 nm, while the H2-reduced nitric oxide calcined catalysts result in clusters between 10 and 20 nm, as measured by chemisorption/pulse reoxidation. 

Hilmen, A.M., Schanke, D., and Holmen, A. 1997. Reoxidation of supported cobalt Fischer-Tropsch catalysts. Stud. Surf. Sci. Catal. 107 237 -2. 

The transients shown in Figure 6 (see (9)) suggest that in the H2O/N2 phase, t O reacts with adsorbed CO to produce C0 and H2 ana that the Hj wavefront concentration is two-fold of the CO2 wavefront concentration. The overproduction of H on the wavefront seems to have been caused by the reoxidation of the catalyst by H 0. The reoxidation rate is therefore on the wavefront equal to the rate of the shift reaction, it is namely the limiting step in the global relaxation process. Furthermore, the fact that I CC is 2 1 on the wavefront suggests that presumably the shift con-... 

Wacker oxidation of l-alkenes. The Wacker oxygenation of 1-alkenes to methyl ketones involves air oxidation catalyzed by PdCl2 and CuCU, which is necessary for reoxidation of Pd(0) to Pd(II).1 This oxygenation is fairly sluggish and can result in chlorinated by-products. A new system is comprised of catalytic amounts of Pd(OAc)2, hydroquinone, and 1, used as the oxygen activator.2 The solvent is aqueous DMF, and a trace of HClOj is added to prevent precipitation of Pd(0). Oxygenation using this system of three catalysts effects Wacker oxidation of 1-alkenes in 2-8 hours and in 67-85% yield. 

The above considered reactions model the reductive half cycle of GO where a primary alcohol is oxidized to an aldehyde with concomitant reduction of a (phe-noxyl)copper(II) complex to the reduced (phenol)copper(I) species. In the first two cases, reoxidation of the reduced catalyst was achieved by an external oxidant such as tris(4-bromophenyl)aminium or an electrode but not dioxygen. 

Based on the experimental data and some speculations on detailed elementary steps taking place over the catalyst, one can propose the dynamic model. The model discriminates between adsorption of carbon monoxide on catalyst inert sites as well as on oxidized and reduced catalyst active sites. Apart from that, the diffusion of the subsurface species in the catalyst and the reoxidation of reduced catalyst sites by subsurface lattice oxygen species is considered in the model. The model allows us to calculate activation energies of all elementary steps considered, as well as the bulk... 

Scheme 30 shows the proposed reaction mechanism, which involves the formation of an acylpalladium species as the key intermediate, in tautomeric equilibrium with a cyclic 7r-allyl complex (in this and in the following Schemes, unreactive ligands are omitted for clarity). The main reason for the high activity of the Pdl42 -based catalyst in this process lies in the very efficient mechanism of reoxidation of Pd(0), which involves oxidation of HI by 02 to I2, followed by oxidative addition of the latter to Pd(0). It is worth nothing that under these conditions Pd(0) reoxidation occurs readily without need for Cu(II) or organic oxidants. 

The rate constants of elementary reactions (see Scheme 3) were estimated for the PVP-Cu,Mn catalyst. For example, the rate constant of electron-transfer (ke) and of catalyst reoxidation (ko) were determined by measuring the decrease and the increase in the d-d absorption of Cu(n). The ke value for Cu(n) ->-Cu(I) 14 min-1 was much larger than that for Mn(ni) - -Mn(n). ko were PVP-Mn (0.042 min-1 ) PVP-Cu,Mn (0.040)>PVP-Cu(0.013), respectively. Furthermore the following rapid redox reaction was regoc-nized. 

Figure 4. Catalytic activity of the pyridine-Cu catalyst in DMSO-benzene solvent (a) and activity of the PSP-Cu catalyst in DMSO (b) (O) oxidative polymerization rate of XOH (A) rate constant of electron transfer step (ke) (0) rate constant of catalyst reoxidation step...

Sharpless stoichiometric asymmetric dihydroxylation of alkenes (AD) was converted into a catalytic reaction several years later when it was combined with the procedure of Upjohn involving reoxidation of the metal catalyst with the use of N-oxides [24] (N-methylmorpholine N-oxide). Reported turnover numbers were in the order of 200 (but can be raised to 50,000) and the e.e. for /rara-stilbene exceeded 95% (after isolation 88%). When dihydriquinidine (vide infra) was used the opposite enantiomer was obtained, again showing that quinine and quinidine react like a pair of enantiomers, rather than diastereomers. 

The study of the mechanism of the fast SCR over V-W-Ti-0 catalysts was addressed first by Koebel and co-workers [65-68]. They suggested that (i) the reoxidation ofthe catalyst is rate determining at low temperature in the redox cycle of standard SCR catalyst, (ii) NO2 reoxidizes the catalyst faster than O2 the NO2-enhanced reoxidation of the catalyst was demonstrated by in situ Raman experiments, (hi) the reaction occurs via the nitrosamide intermediate in both standard and fast SCR and (iv) ammonium nitrate is considered an undesired side-product. 

It is worth mentioning that the same chemistry is involved when surface nitrite and nitrate are considered instead of gas-phase nitrous and nitric add. Indeed, surface nitrate has been suggested to take part in the reoxidation of the reduced catalyst sites, thus accounting for the higher rate of the fast SCR reaction compared with the standard SCR reaction [74]. 

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