Catalysts, activity reoxidation

Experience led to the introduction of catalysts based on nickel nitrate and oxalate, followed by lactate or formate as well as the original carbonates, all supported on infusorial earth, pmnice, or even charcoal to increase activity. Reduction procedures were foimd to be important in obtaining the highest catalyst activity. Reoxidation of nickel before use had to be avoided. Mixed nickel oxide and copper oxide reduced more easily than nickel oxide alone. ... 

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... 

Alumina-zeolite supported nickel catalysts were investigated for the conversion of n-hexane and dewaxing of diesel oil fraction. The method of nickel incorporation influenced the susceptibility of nickel to reduction and catalyst activity. Investigated also was the effect of reoxidation temperature on the accessibility of the metallic surface. Differences in the activities of the catalysts can be attributed to the morphology of the deposits. 

The catalytic cycle is illustrated in Scheme 1, the example used being the Cu complex catalyzed oxidation of 2,6-dimethyIphenol (3 ). In the first step, the substrate phenol coordinates to the Cu(II) complex and one electron transfers from the substrate to the Cu(II) ion. Then the activated substrate dissociates from the catalyst and the reduced Cu(I) catalyst is reoxidized to the original Cu(II) complex. Among these elementary reactions, the electron-transfer step is the most important process governing the catalytic behavior of a polymer-metal complex for the following reasons (i) The electron-transfer step is often the slowest... 

Unfortunately, the catalyst can also become deactivated during the calcination, by several processes. Bulk hexavalent chromium oxide, CrC>3, or chromic acid, is unstable at temperatures above approximately 200 °C and begins to decompose into the trivalent oxide Cr2C>3 [39,40,42], On the catalyst, it is only the esterification with silica that stabilizes chromium in the hexavalent form at temperatures up to 900 °C. However, the chromate or dichromate ester can be hydrolyzed by water vapor present in the air used for the catalyst activation, as shown in Scheme 53. When this happens at elevated temperatures, decomposition to Cr(III) occurs. In the presence of water vapor and traces of Cr(VI), large crystallites of a-chromia are formed [74,75,134,135,731-733], which can be very difficult to reoxidize and disperse. 

During the first days of synthesis a partial oxidation of the catalyst occurs, acoompanied by a slight decrease of activity. The synthesis temperatures however, are lower than the reduction temperature and the re -oxidation of the catalyst is less dangerous at synthesis conditions than insufficient preliminary reduction. After some time, the reoxidation process reaches a kind of equilibrium. The influence of a certain oxygen-content at these equilibria conditions on catalyst activity, is not identical with the influence of a similar oxygen content remaining in the catalyst at the end of the reduction process. The conditions of the reduction are not of the same importance for different kinds of catalysts. 

Analysis of the solid-state chemistry of the V-Sb-0 catalyst identified the important role cation vacancies play in catalyzing the propane ammoxidation reaction (129). Reduction of the catalyst with ammonia indicated that the catalyt-ically active site is associated with the cation vacancy in the rutile structure. This was based on the results of in situ UV Raman spectroscopy along wdth computational modeling to interpret the vibrational spectra. Specifically, the two-coordinated oxygen at the cation vacation is removed from the structure during the reduction treatment and is replenished when the catalyst is reoxidized wdth 02- The study further showed that the stability of the catalyst vmder ammoxidation conditions is enhanced by the introduction of titanium into the structure as a solid solution. Titanium is incorporated into the rutile structure of V—Sb—O as a solid solution of the following composition ... 

Buchwald described an oxypalladation reaction, followed by a C-H functionalization. This entirely intramolecular reaction is initiated through a 5-exo Wacker-type cychzation of 84. The resulting a-alkyl-paUadium intermediate M provides subsequent C-H activation at the neighboring arene, which allows a paUadium(II) intermediate N bearing a-alkyl and o-aryl substituents, respectively. Reductive elimination provides the C-C bond installation of 85 with the concomitant release of a paUadium(O) catalyst state. Reoxidation imder aerobic conditions, most probably through a palladium(II) -peroxo complex and protonolysis with the acetic add hber-ated in the previous steps, regenerates the original paUadium(II) diacetate catalyst. 

While the product of this reaction somewhat resembles a Nazarov cyclization, it is thought that the reaction proceeds via a different mechanistic pathway and depends on the nature of the palladium catalyst. Activation of the olefin by palladium, followed by cyclization gives palladium enolate 41. When Pd(OAc)2 is used as the catalyst, P-hydride elimination of 41 produces cyclopentenone 42, and molecular oxygen reoxidizes the palladium. On the other hand, when PdCl2(MeCN)2 is used as a catalyst, hydrolysis generates HCl which results in rapid protonation of 41, giving cyclopentenone 43 as the product. 

The absence of water thus avoids the two problems of hydrolysis of the surface chromates and water-induced sintering of the support, both of which lead to a catalyst with lower activity. Reoxidation of the chromium after carbon monoxide reduction gives a different active site precursor, which produces a catalyst which gives a broader molecular weight distribution in the resulting polymer. Catalysts pre-reduced with carbon monoxide are more active than those reduced with ethylene in the reactor. 

To illustrate the inner-sphere characteristics of the CH activation chemistry, an analogy can be made between CH activation by coordination of an alkane CH bond to a metal center and the known catalysis resulting from coordination of olefins via the CC double bond (note that the nature of the orbitals involved in bonding are quite different). It is well known that coordination of olefins to electrophilic metal centers can activate the olefin to nucleophilic attack and conversion to organometallic, M-C, intermediates. The M-C intermediates thus formed can then be more readily converted to functionalized products than the uncoordinated olefin. An important example of this in oxidation catalysis is the Wacker oxidation of ethylene to acetaldehyde. In this reaction, catalyzed by Pd(II) as shown in Fig. 7.14, ethylene is activated by coordination to the inner-sphere of an electrophilic Pd(II) center. This leads to attack by water and facile formation of an organometallic, palladium alkyl intermediate that is subsequently oxidized to acetaldehyde. The reduced catalyst is reoxidized by Cu(II) to complete the catalytic cycle. The Wacker reaction is very rapid and selective and it is possible to carry out the reaction is aqueous solvents. This is largely possible because of the favorable thermodynamics for coordination of olefins to transition metals that can be competitive with coordination to the water solvent. The reaction is very selective presumably because the bonds of the product (po-... 

From these results, it was believed that the decrease of the V2O5 reoxidation property, caused by the increase of the reducing power, leaded to the deterioration of the activity of V2O5 catalyst in the selective oxidation of H2S under the environment of coal-derived synthesis gas. And the decreased reoxidation property of the V2O5 was enhanced by increasing oxygen and/or water content. 

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. 

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