Homogeneous catalyst deactivation

Transition metal-mediated phosphorus-carbon bond cleavage and its relevance to homogeneous catalyst deactivation. P. E. Gorrou, Chem. Rev., 1985,85,171 (109). 

An ideal system would enable continuous operation with separation under conditions similar to those of the reaction itself, as a result of which all the catalyst remains in its active state for all times. Use of SCCO2 as a reaction medium would not only allow for reaction and separation under similar conditions to reduce the problems of catalyst deactivation, but would also have a considerable effect on the plant design. Its total lack of flammabihty, contrary to conventional organic solvents, reduces safety problems and makes SCCO2 attractive for homogeneously catalyzed reactions, particularly for oxidation and epoxidation reactions [143]. 

A further example of ion-exchange of an organometallic complex onto a layered support has been provided by the anion exchange of a sulfonated ruthenium BINAP complex onto the external surface of layered double hydroxides [119]. Although achvihes and enantioselechvities for the hydrogenation of dimethyl itaconate were comparable to the homogeneous catalyst, and catalyst deactivation was not detected, with geraniol as substrate, catalyst deactivation was unavoidable. 

The route of catalyst deactivation via a cyclic metal impregnation and deactivation method has produced significant improvements in approaching realistic vanadium and nickel profiles over the catalyst particles. From electron microprobe analyses of Ni and V loaded catalyst it has been established that after pore volume saturation, Ni and V are rather homogeneously distributed over the catalyst. 

In the autoxidation of neat hydrocarbons, catalyst deactivation is often due to the formation of insoluble salts of the catalyst with certain carboxylic acids that are formed as secondary products. For example, in the cobalt stearate-catalyzed oxidation of cyclohexane, an insoluble precipitate of cobalt adipate is formed. 18fl c Separation of the rates of oxidation into macroscopic stages is not usually observed in acetic acid, which is a better solvent for metal complexes. Furthermore, carboxylate ligands may be destroyed by oxidative decarboxylation or by reaction with alkyl hydroperoxides. The result is often a precipitation of the catalyst as insoluble hydroxides or oxides. The latter are neutralized by acetic acid and the reactions remain homogeneous. 

Early efforts to model catalyst deactivation either utilized simplified models of the catalyst s porous structure, such as a bundle of nonintersecting parallel pores, or pseudo-homogeneous descriptions in terms of effective diffusivities and tortuosity... 

The packed bed reactors section of this volume presents topics of catalyst deactivation and radial flow reactors, along with numerical techniques for solving the differential mass and energy balances in packed bed reactors. The advantages and limitations of various models (e.g., pseudo-homogeneous vs. heterogeneous) used to describe packed bed reactors are also presented in this section. 

Recently it has become clear that tertiary phosphine-metal complexes are reactive and liable to undergo carbon-phosphorus bond scission. The reaction between the C— P " bond and the transition metal to which the tertiary phosphine is bound has profound implications on homogeneous catalysis, particularly on the mode of homogeneous catalyst deactivation in hydroformylation (Rh- and Co-catalyzed) and various other hydrogenation/dehydrogenation reactions, including asymmetric hydrogenation. 

Silica-supported homogeneous catalysts, especially phosphino-iridium compounds, appear more promising in the hydrogenation of a,y9-unsaturated aldehydes, provided that their productivity can be improved and catalyst deactivation is avoided so that recycling of these materials could be meaningful [35]. 

This contribution will address some issues which are not examined usually in meetings on catalyst deactivation. We will look at catalyst deactivation in a broader context, namely the modifications that solid catalysts (and probably also homogeneous ones ) continuously undergo while they are acting catalytically under reaction conditions. As underlined in our previous contributions, this is particularly relevant when solid-state reactions are considered, because solids have a sort of memory (1,2). Their transformations at a given moment are strongly influenced by the whole succession of conditions they have been subjected to. 

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