Oxidation heterogeneously catalyzed

M. Ehsasi et al. Steady and nonsteady rates of reaction in a heterogeneously catalyzed reaction Oxidation of CO on platinum, experiments and simulations. J Chem Phys 97 4949-4956, 1989. 

Ethylene oxide is produced by a heterogeneous catalyzed gas phase oxidation of ethylene with pure oxygen at temperatures of 240-290°C and 5-25 bar [63] ... 

As is well known chemical reactions are accelerated by increasing the temperature. This also applies to heterogeneously catalyzed reactions taking place on the surface of polar sorbents such as aluminium oxide or silica gel (Tables 2.1 and 2.2). Such reactions have also been reported on the moderately polar NH2 layers. Alperin et al. have described the activation of cellulose to yield specific information concerning the substances chromatographed [1]. 

Vibrational spectroscopic studies of heterogeneously catalyzed reactions refer to experiments with low area metals in ultra high vacuum (UHV) as well as experiments with high area, supported metal oxides over wide ranges of pressure, temperature and composition [1]. There is clearly a need for this experimental diversity. UHV studies lead to a better understanding of the fundamental structure and chemistry of the surface-adsorbate system. Supported metals and metal oxides are utilized in a variety of reactions. Their study leads to a better understanding of the chemistry, kinetics and mechanisms in the reaction. Unfortunately, the most widely used technique for determining adsorbate molecular structure in UHV,... 

In the present work, a Cu-13X zeolite sample was ceramized and used as a catalyst for the wet oxidation of phenol solutions and olive oil mill wastewaters (OOMW). The material showed good catalytic activity for the abatement of phenol and poly-phenols, excellent stability and no leaching of the active species. In this way a real heterogeneously catalyzed reaction was performed. Moreover, the catalyst was reused without special reactivation treatments for different consecutive reaction cycles. 

Spiro [27] has derived quantitative expressions for the catalytic effect of electron conducting catalysts on oxidation-reduction reactions in solution in which the catalyst assumes the Emp imposed on it by the interacting redox couples. When both partial reaction polarization curves in the region of Emp exhibit Tafel type kinetics, he determined that the catalytic rate of reaction will be proportional to the concentrations of the two reactants raised to fractional powers in many simple cases, the power is one. On the other hand, if the polarization curve of one of the reactants shows diffusion-controlled kinetics, the catalytic rate of reaction will be proportional to the concentration of that reactant alone. Electroless metal deposition systems, at least those that appear to obey the MPT model, may be considered to be a special case of the general class of heterogeneously catalyzed reactions treated by Spiro. 

B. Better tools available, but no consensus on mechanism or active site—1980 to 2006. Rhodes et al.291 published a comprehensive review on the heterogeneously catalyzed water-gas shift mechanism in 1995. Included in that discussion was the copper/zinc oxide/alumina system. The conclusion was that this system appears to be constructed of small metallic islands of copper resting on a zinc oxide alumina phase. Zinc oxide may exert some impact on catalytic activity, but it was suggested in the review that the contribution is small. It was indicated that strong evidence exists to support either a formate or a redox mechanism, and the authors even suggest the possibility that both mechanisms might occur, though insufficient data exist to determine which mechanism predominates. 

Abstract Polyfunctionality of carbohydrates and their low solubility in conventional organic solvents make rather complex their conversion to higher value added chemicals. Therefore, innovative processes are now strongly needed in order to increase the selectivity of these reactions. Here, we report an overview of the different heterogeneously-catalyzed processes described in the literature. In particular, hydrolysis, dehydration, oxidation, esterification, and etherification of carbohydrates are presented. We shall discuss the main structural parameters that need to be controlled and that permit the conversion of carbohydrates to bioproducts with good selectivity. The conversion of monosaccharides and disaccharides over solid catalysts, as well as recent advances in the heterogeneously-catalyzed conversion of cellulose, will be presented. 

Here we report an overview of the different heterogeneously-catalyzed pathways designed for the selective conversion of carbohydrates. On the basis of these results, we shall try to determine the key parameters allowing a better control of the reaction selectivity. Water being commonly used as solvent in carbohydrate chemistry, we will also discuss the stability of solid catalysts in the aqueous phase. In this review, heterogeneously-catalyzed hydrolysis, dehydration, oxidation, esterification, and etherification of monosaccharides and polysaccharides are reported. 

As depicted in Scheme 11, o-glucose can be converted to several oxidized products. In this section we will only discuss the heterogeneously-catalyzed oxidation of D-glucose to D-gluconic acid since this compound is one of the most valuable oxygenated derivatives obtained from glucose. Indeed, D-gluconic acid is now widely used as an intermediate in food and pharmaceutical industries [108]. 

Oxidation of carbohydrates is probably the most efficient heterogeneously-catalyzed process since very high yield and selectivity are reported for reactions over solid catalysts. Despite important recent advances, the conversion of carbohydrates to HMF still requires further investigations. For reactions, in water, the yield of HMF is still too low due to the significant formation of side products. Today, several procedures for the conversion of monosaccharides, disaccharides, oligosaccharides, and starch in water into high value added materials are available. With cellulose, its heterogeneously-catalyzed conversion into useful products... 

The oxidation of ethanol to acetic acid was among the first heterogeneous catalyzed reactions to be reported, but it has not attracted continued interest. During the 1990ies, however, 100% conversion of ethanol coupled with 100% selectivity to acetic acid was reported in a gas-phase reaction using molybdenum oxide catalytic systems on various supports, at temperatures below 250 Similarly, a tin oxide and molybdenum oxide catalyst was... 

The IR study performed in static controlled atmospheres in the IR cell allowed us to identify a number of adsorbed intermediate and secondary products, together with the main reaction products in oxidation and ammoxidation of toluene and the three xylene isomers. Surface reactions schemes are proposed that account for most of the mechanistic features of the heterogeneously-catalyzed industrial reactions. Our data support the following conclusions ... 

Vinyl acetate is produced by the oxidation of ethylene and acetic acid (4,5). Catalysts for the gas phase oxidation are made from palladium compounds with additional metal compounds on a porous support (6). Catalysts, preferably coated catalysts, can be used for many heterogeneously catalyzed reactions such as hydrogenations and oxidations. 

Kuhn and colleagues (797) investigated the kinetics of the heterogeneously catalyzed benzene oxidation at Pb/Pb02 electrodes in sulfuric acid. This reaction was worked out on a semitechnical scale for quinone/ hydroquinone production (792) ... 

The heterogeneously catalyzed Mn02-mediated oxidation of diacetone-L sorbose to diacetone-2keto-L sorbic acid, the latter being a precursor to vitamin C, at nickel anodes and based on the chemical oxidation of the substrate by NiOOH is of technical relevance. The limiting current density in 1 M KOH solution is under operation conditions only 10 A/cm2 leading to relatively poor space-time yields. Robertson and Ibl showed that acceptable space-time yields can by obtained by using thin layer cells of Swiss roll type (193, 194), which leads to an efficient compression of the cell width to fractions of a millimeter. 

A proper description of heterogeneously catalyzed oxidation reactions must treat several difficult problems simultaneously. First is the characterization of the solid surface in its reactive state. What oxygen species exist on this surface and what reactions does each species undergo What other sites for adsorption are present Second is the problem of reaction path. What steps are involved in the reaction What are the structures and relative energy contents of the intermediates Third is the problem of reaction velocity, a general and difficult problem in all chemistry. What are transition states, activation energies, and reaction probabilities for the various steps ... 

Heterogeneous Oxidation Reactions Catalyzed by Heteropoly Compounds... 

The subject of heterogeneously catalyzed selective oxidation has been reviewed many times. Under the keyword combination selective catalytic oxidation the ISI database reports about 5400 papers. Over 100 reviews on the topic have been published. In the present discussion, the subjects of methane activation and model studies of unselective CO oxidation, which represent large fields, are excluded. Homogeneously or biologically catalyzed selective oxidation, a combined field that is about 10-fold larger in scientific coverage, is also excluded from this chapter. 

Table 6.1 Selected examples of heterogeneously catalyzed H202- and 02-based oxidations by POMs. 

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