Selectivity in oxidations

Another method to achieve selectivity in oxidative splitting of C C bonds to carbonyl groups is controlled epoxidation followed by periodate cleavage (J.P. Nagarkatti, 1973). 

P. Gancitano, R. Ciriminna, M. Luisa Testa, A. Fidalgo, L. M. Ilharco and M. Pagliaro, Enhancing selectivity in oxidation catalysis with sol-gel nanocomposites, Org. Biomol. Chem., 2005, 3, 2389. 

The oxidation of various alcohols by the poly(e-carbobenzoxy-L-lysine)-Cu complex was studied by Welch et aL127. The polymer catalyst showed selectivity in oxidation by virtually excluding alcohols of bulky structure such as diisopropyl and diisobutyl carbinol while admitting simple alcohols arch as n-butyl, iso-butyl, and sec-butyl. It was suggested from structural studies that the selectivity of the polymer catalyst resulted from the highly complex geometry of the molecular cage formed by the helix and the amino acid side chain around the coordinated Cu. The... 

TS-2 belonging to a different structure group exhibits a similar activity and similar high selectivity in oxidation of hexane.191 Branched and cyclic alkanes react much more slowly,186,187,189 indicating that the oxidation occurs preferentially inside the channels of the catalyst structure.189... 

Table I. Selectivity in oxidations using Fe /zeolites and hydrogen peroxide (22)...

Selectivity in oxidation catalysis has been reviewed for conventional catalysts used for the production of bulk chemicals and epoxidations. The point of activation of the substrate is identified as a key factor identifying three mechanistic features. These are (i) activation of the weakest C-H bond in a substrate, (ii) activation of the strongest C-H bond and (iii) electrophilic attack in olefins. Key features of each type of reaction are identified and new catalyst types needed to break through existing selectivity barriers are discussed. 

It seems that zeolite membranes may not be the best choice as catalytic contactors and oxygen distributors to enhance selectivity in oxidative dehydrogenation of propane. This may be due in part to the intrinsic catalytic activity of the zeolite material for this reaction, which probably exerts a nonselective contribution. Therefore, this specific application operates at conditions that do not make use of the most important properties, which are characteristic of zeolite membranes. More efficient approaches for selectivity enhancement can be obtained with zeolite membranes, as shown in the next section. 

Sodium periodate specifically oxidizes guaiacyl groups to quinones. Hydrogen and sodium peroxides in alkali are also somewhat selective in oxidizing lignin and destroy chromophoric groups such as quinones and carbonyl functions while also degrading only aromatic units with free phenolic hydroxyls to dibasic acids. 

Grasselli, R. K., Structures as Related to Activity and Selectivity in Oxidation and Ammoxidation Catalysis, presented at the Advances in Oxidation Chemistry Series, University of Manchester, U.K., April 17, 1975. 

Figure 16. Selectivity in oxidation of MEP over the 67 mole % catalyst and B.E. and FWHM of the V core line as a...

Highly ordered mesoporous mono- and bi-metallic incorporated catalysts with high porosity and surfece area are very active and selective in oxidation reaction of aromatic hydrocarbons such as styrene and benzene. Their framework composition can be modified by incorporation of various transition metals in the single or associated forms. The ordered hexagonal arrangement, the morphology, the surface area, and the catalytic activity and selectivity can be modified by method of the synthesis, nature and content of the metal and the molar ratio of the metals. Very active catalysts were obtained by incorporation of chromium and nickel in the hexagonal ordered structure. 

Conversion and selectivity in oxidation of alkylaromatic hydrocarbon with H2O2 over TS-1. 

General selectivity in oxidation catalysis involves activation of the reactant through rupture of a C-H bond (the kl route in scheme 1), r frereas diminishing selectivity is associated with rupture of any bond in the selective oxidation product (the k2 route in scheme 1). As a means of validating this hypothesis the upper selectivity limit, attained at a fixed conversion, in all 14 reactions used in this study was plotted against the function ... 

Figure 5 influence of the ratio of C-C to C-H bonds in ethane, propane and n-butane on the selectivity in oxidative dehydrogenation at 0, S and 10 % conversion. 

Selectivity in the coupling reaction is determined by selectivity in oxidative addition and normally the differences between halogens are dominant I > Br > Cl. Triflate is usually more reactive than bromide, but it may not always override other effects (see the examples below). When the halogens are the same, differences in positional reactivity come into play. The tendency is for selective oxidative addition to occur at the carbon of lowest electron density and this can be determined by C NMR spectroscopy. Other effects may be involved, such as chelation or steric hindrance, particularly when there is competition between two otherwise identical halogens. 

In addition to the effect on the product selectivity, it is seen in Table 3 that, for a given synthetic method, the activity (1-hexene and H202 conversion) as well as the selectivity of H202 increases as the A1 content of the zeolite decreases. These results show that the A1 content of zeolite Ti-Beta is one of the most important factors in determining its activity and selectivity in oxidation reactions, and the benefits that the new methods of synthesizing Ti-Beta with low A1 content can provide. 

Titanium silicate (TS-1) which has a structure similar to the zeolite ZSM-5 has been shown to catalyse a number of synthetically important oxidations with hydrogen peroxide under mild conditions.34 A useful feature of the TS-1 catalyst is its enhanced product selectivity in oxidation reactions, for example, cyclohexane is selectively oxidised to cyclohexanone inside the pores of TS-1. On the external surfaces where there is little steric control cyclohexane is oxidised to the dicarboxylic acid. Spinace and co-workers have shown that these external reactions can be prevented by the addition of an antioxidant such as 2,6-dwert-butyl-4-methylphenol (BHT) but which does not interfere with the internal reactions since it is too bulky to enter the pores of the TS-1.35... 

Table 5 Reactant shape selectivity in oxidation of alcanes...

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