Active sites oxidation, alcohols

Step 3 of Figure 29.3 Alcohol Oxidation The /3-hydroxyacyl CoA from step 2 is oxidized to a /3-ketoacyl CoA in a reaction catalyzed by one of a family of L-3-hydroxyacyl-CoA dehydrogenases, which differ in substrate specificity according to the chain length of the acyl group. As in the oxidation of sn-glycerol 3-phosphate to dihydroxyacetone phosphate mentioned at the end of Section 29.2, this alcohol oxidation requires NAD+ as a coenzyme and yields reduced NADH/H+ as by-product. Deprotonation of the hydroxyl group is carried out by a histidine residue at the active site. 

The photocatalytic oxidation of alcohols constitutes a novel approach for the synthesis of aldehydes and acid from alcohols. Modification of Ti02 catalyst with Pt and Nafion could block the catalyst active sites for the oxidation of ethanol to CO2. Incorporation of Pt resulted in enhanced selectivity towards formate (HCOO ad)-Blocking of active sites by Nafion resulted in formation of significantly smaller amounts of intermediate species, CO2 and H2O, and accumulation of photogenerated electrons. The IR experimental teclmique has been extended to Attenuated Total Reflectance (ATR), enabling the study of liquid phase photocatalytic systems. 

If the two Oads species are not scavenged, then the reaction will stop. This is the case, for instance, of NO decomposition on Cu/ZSM-5 [25], Adsorbed oxygen species have to be scavenged either by an activated form of the initial HC reductant, such as QH O , (alcohol, aldehyde, etc.) or by the initial HC if their total oxidation is simultaneous with NO decomposition-reduction to N2. These oxygenates and/or HC suffer a total oxidation to C0/C02 and H20, regenerating the active site this is the principle of catalysis. Once the active site is recovered, the reaction continues to turn over. This is the catalytic cycle . 

Similar results have been obtained over alumina alone, in the presence of propene [27], The initial HC of the feed (propene) has to first transform to oxygenates (alcohol, aldehyde, etc.) - simultaneously to the NO decomposition (function 3) - to scavenge adsorbed oxygen species left by NO decomposition and regenerate the active sites of function 3. The mild oxidation of HC to oxygenates is the role of function 2 of the present model. 

Although the complete mechanism for each of the previously described reactions is not known, substantial details have been worked out. First, it is clear that Ti is incorporated into the framework of the silicalite structure. Too much Ti (more than about 2.5%) in the preparation steps forms nonframework TiOz crystallites, which decompose H202. Second, the rate enhancement due to methanol suggests a tight association at the Ti active site as shown in Fig. 6.8.37,38 This is supported by the fact that methanol oxidizes much more slowly than other alcohols.47 This tight coordination of methanol is proposed to increase the electrophilicity of the Ti-coordinated H202 and facilitate oxygen transfer to the alkene.31... 

Of particular interest are oxidations of unsaturated alcohols, for example, oxidation of cinnamyl alcohol to cinnamaldehyde,74,75 and special promoters have been added to increase selectivity (Fig. 6.13).75 Although the functions of these promoters are still not fully undestood, some authors attribute their increased selectivity to physical blocking of reaction sites. This blocking reduces the size of the active site ensemble and suppresses the tendency for alcohols to strongly adsorb and dissociate on Pt.75... 

Since the oxidative polymerization of phenols is the industrial process used to produce poly(phenyleneoxide)s (Scheme 4), the application of polymer catalysts may well be of interest. Furthermore, enzymic, oxidative polymerization of phenols is an important pathway in biosynthesis. For example, black pigment of animal kingdom "melanin" is the polymeric product of 2,6-dihydroxyindole which is the oxidative product of tyrosine, catalyzed by copper enzyme "tyrosinase". In plants "lignin" is the natural polymer of phenols, such as coniferyl alcohol 2 and sinapyl alcohol 3. Tyrosinase contains four Cu ions in cataly-tically active site which are considered to act cooperatively. These Cu ions are presumed to be surrounded by the non-polar apoprotein, and their reactivities in substitution and redox reactions are controlled by the environmental protein. 

Aramendia et al. (22) investigated three separate organic test reactions such as, 1-phenyl ethanol, 2-propanol, and 2-methyl-3-butyn-2-ol (MBOH) on acid-base oxide catalysts. They reached the same conclusions about the acid-base characteristics of the samples with each of the three reactions. However, they concluded that notwithstanding the greater complexity in the reactivity of MBOH, the fact that the different products could be unequivocally related to a given type of active site makes MBOH a preferred test reactant. Unfortunately, an important drawback of the decomposition of this alcohol is that these reactions suffer from a strong deactivation caused by the formation of heavy products by aldolization of the ketone (22) and polymerization of acetylene (95). The occurrence of this reaction can certainly complicate the comparison of basic catalysts that have different intrinsic rates of the test reaction and the reaction causing catalyst decay. 

Promotion and deactivation of unsupported and alumina-supported platinum catalysts were studied in the selective oxidation of 1-phenyl-ethanol to acetophenone, as a model reaction. The oxidation was performed with atmospheric air in an aqueous alkaline solution. The oxidation state of the catalyst was followed by measuring the open circuit potential of the slurry during reaction. It is proposed that the primary reason for deactivation is the destructive adsorption of alcohol substrate on the platinum surface at the very beginning of the reaction, leading to irreversibly adsorbed species. Over-oxidation of Pt active sites occurs after a substantial reduction in the number of free sites. Deactivation could be efficiently suppressed by partial blocking of surface platinum atoms with a submonolayer of bismuth promoter. At optimum Bi/Ptj ratio the yield increased from 18 to 99 %. 

There are numerous indications in the literature on catalyst deactivation attributed to over-oxidation of the catalyst (3-5). In the oxidative dehydrogenation of alcohols the surface M° sites are active and the rate of oxygen supply from the gas phase to the catalyst surface should be adjusted to that of the surface chemical reaction to avoid "oxygen poisoning". The other important reason for deactivation is the by-products formation and their strong adsorption on active sites. This type of... 

When the air flow was temporarily substituted by a nitrogen flow for 15-20 minutes in the reaction represented by Figure 5a, the rate of alcohol oxidation did not increase. These experiments also prove that the reason of catalyst deactivation is not the over-oxidation of Pt° active sites, but a partial coverage of active sites by impurities (chemical deactivation). 

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