Selectivity in Oxidation Catalysis

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. 

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. 

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 ... 

The oxidation of propene is at present the most extensively studied gas phase heterogeneous oxidation process. The selective production of acrolein over cuprous oxide has been known for a very long time. However, the discovery of bismuth molybdates as highly active and selective catalysts for the oxidation to acrolein, and particularly the ammoxidation to acrylonitrile, has opened a new era in oxidation catalysis. 

Selective oxidation of alkanes and benzene derivatives to alcohols and phenols, respectively, are among the most difficult reactions in oxidation catalysis. Therefore, the stoichiometric hydroxylation of alkenes and aromatics performed by a-oxygen at room temperature has aroused great interest as a potential way for developing new steady state catalytic processes for the preparation of these valuable products, similar to the hydroxylation of benzene to phenol. 

The discovery of titanium silicalite-1 (TS1) by Enichem scientists (20-22) and its commercial use as a catalyst for a variety of selective oxidations with aqueous hydrogen peroxide under mild conditions (Figure 1.3) constituted a major breakthrough in oxidation catalysis. 

Protic zeolites may also offer a new type of spacial constraint and the attendant shape selectivity, in acid catalysis conducted at low-enough temperatures when the reactive intermediate is an adduct complex formed between an olefin and the protonated oxide ion ( C NMR evidence). With a reaction intermediate "spirming top" linked to fiamewoik oxygen, a new type of spacial constraint is imposed on the reactant molecule. 

Several oxidation reactions (Section 5.5) have been selected. Selective oxidation of propene Section 5.5.1) is a key example of the use of oxides in oxidation catalysis. 

The results of the oxidation of C2—C5 olefins over copper(i) oxide, silver, and gold catalysts are summarized in Table 1. We have excluded data from studies where additives have been deliberately included in the catalyst, or process gas stream, in order to improve the performance. Where several studies have been carried out we have quoted the best selectively obtained. While copper(i) oxide and gold give unsaturated aldehydes as the major product of partial oxidation, silver gives the epoxide. Copperfii) oxide is not a selective catalyst for olefin oxidation. The difference in behaviour between copper(i) and copper(ii) oxides is in line with the general trend in oxide catalysis. The selective catalysts tend to be those with either a full or an empty tZ-shell, i.e. the oxides of Groups IVA, VA, and VIA, and IB, IIB, IVB, VB, and VIB. ... 

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. 

The novel zeolite UTD-1 with titanium in the framework (up to 3.5% by weight) is an effective catalyst for the oxidation of cyclohexane, cyclohexene and 2,6-di-tert-butyl phenol. The catalytic behavior is similar to that of other large pore zeolites and mesoporous molecular sieves modified with titanium which includes solvents effects. Additionally, Ti-UTD-1 allows the use of oxidants and substrates too large for the commercial TS-1 catalyst. We are currently evaluating further the role of solvent and oxidant in an effort to improve selectivity as well as expand the utility of this material in oxidation catalysis. 

The essential value of a selective oxidation catalyst can be represented by a simple selectivity-conversion plot, the best catalysts being those that give the highest selectivity at a given conversion [1], Other factors such as activity and deactivation are less iiH)ortant, because the former can always be boosted by increasing the W/F ratio or the temperature and generally deactivation phenomena in oxidation catalysis are not severe. 

---