Catalysts, general deactivation

Most low-valence metal complexes are generally deactivated by air and sometimes also by water. Carbon monoxide, hydrogen cyanide, and PH3 frequently act as poisons for these catalysts. Poisoning by strongly co-ordinating molecules occurs by formation of catalytically inert complexes. An example is the poisoning of Wilkinson s catalyst for alkene hydrogenation ... 

Hence, P-C bond-cleavage followed by isomerization is responsible for the formation of side products. Furthermore, due to destabilization of the catalyst complex, deactivation occurs and palladium black is formed, which is a notorious disadvantage of Pd-phosphine catalysts in general. Catalyst decomposition and the formation of side products causes additional separation and catalyst recovery problems. These problems have been solved by the discovery of novel catalyst complexes, which are active and stable at temperatures of over 250 °C (Cornils and Herrmann, 1996). 

The problem with carbene formation is that they can displace the phosphine ligands attached to the catalyst and deactivate the catalyst. In general, the active catalyst is a palladium(O) compound and this low oxidation state is best stabilized by very bulky phosphines such as P(lBu)3 mentioned above. 

As described in the preceding sections, asymmetric amplification is generally a consequence of the formation of aggregates (i.e., dimers or oligomers that are homochiral or heterochiral) of a chiral catalyst. However, even a racemic catalyst can be used as a chiral catalyst with the aid of chiral additives (a simple model consisting of dimers is depicted in Scheme 9.17). If a chiral additive (R)-B is selectively associated with (S)-A in the racemic catalyst, the remaining (R)-A could operate as the chiral monomer catalyst (asymmetric deactivation). Conversely, the chiral additive (/ )-B can be selectively associated with (/ )-A in racemic catalyst to generate an active dimeric catalyst (asymmetric activation). 

In certain instances of poisoning, especially in the case of base metal catalysts, the deactivation can be simply explained by the formation of new bulk solid phases between the base metal and the poison. Examples are the formation of lead vanadates (14) in vanadia catalysts, or of sulfates in copper-chromite and other base metal catalysts (81). These catalyti-cally inactive phases are identifiable by X-ray diffraction. Often, the conditions under which deactivation occurs coincide with the conditions of stability of these inert phases. Thus, a base metal catalyst, deactivated as a sulfate, can be reactivated by bringing it to conditions where the sulfate becomes thermodynamically unstable (45). In noble metal catalysts the interaction is assumed to be, in general, confined to the surface, although bulk interactions have also been postulated. 

Polyterpene resins are related to the oldest reported polymerization, as they were first observed in 1789 by Bishop Watson by treatment of turpentine with sulfuric acid [92]. Commercial polyterpene resins are synthesized by cationic polymerization of /3- and a-pinenes extracted from turpentine, of rf,/-limonene (dipentene) derived from kraft-paper manufacture, and of d-limonene extracted from citrus peels as a by-product of juice industry [1,80,82,93]. The batch or continuous processes are similar for the three monomers. The solution polymerization is generally performed in mixed xylenes or high boiling aromatic solvent, at 30-55° C, with AlCl3-adventi-tious water initiation. The purified feedstream (72-95% purity, depending on monomer) is mixed in the reactor with solvent and powdered A1C13 (2—4 wt% with respect to monomer), and then stirred for 30-60 min. After completion of the reaction, the catalyst is deactivated by hydrolysis, and evolved HC1 is eliminated by alkaline aqueous washes. The organic solution is then dried, and the solvent is separated from the resin by distillation. 

Supported metal catalysis are employed in a variety of commercially important hydrocarbon conversion processes. Such catalysts consist, in general, of small metal crystallites (0.S to 5 nm diameter) dispersed on non-metallic oxide supports. One of the major ways in which a catalyst becomes deactivated is due to accumulation of carbonaceous deposits on its surface. Catalyst regeneration, or decoking, is normally achieved by gasification of the deposit in air at about 500°C. However, during this process a further problem is frequently encountered, which contributes to catalyst deactivation, namely particle sintering. Other factors which can contribute to catalyst deactivation include the influence of poisons such as sulfur, phosphorus, arsenic and... 

Catalyst developers experience woe in their quest to find a practical catalyst owing to the very general taidency of catalysts to deactivate during use onstream. To live with this phenomenon, a developer must know the time scale of the deactivation process in order to know what kind of reactor to use. If development schedules permit, the developer may even have the luxury of making a detailed study of the deactivation process in the laboratory or in a pilot plant to find conditions to increase the life of the catalyst on stream. 

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. 

The solvent-free reaction has some other advantages in fact, although the catalysts used in 1,2-dichloroethane are generally deactivated after one cycle, the solvent-free reaction shows a satisfactory catalyst reusability after washing and drying (80% resorcinol conversion in the second cycle). 

Deactivation of HDC catalysts generally has been ascribed to either interactions between hydrogen chloride (HCl) and the catalyst [7,8] or to coke formation [1-4,9]. To understand the cause(s) of deactivation for this specific reaction and catalyst, the various products observed in the effluent were hydrodechlorinated at 523 K. Results from this study showed that the Pt/ri-alumina catalyst deactivated rapidly during the HDC of 111 TCA and 11 DCA (saturated chlorocarbons containing multiple Cl atoms) and remained stable for the HDC of 11 DCE (unsaturated chlorocarbon), chloroethane (saturated chlorocarbon containing only one Cl atom), and ethylene (unsaturated hydrocarbon). Large quantities of coke were observed on the Pt/rj-alumina after the HDC of 111 TCA, while very little coke was observed on the used catalyst after the HDC of any of the other compoimds. From these experiments, a conceptual model was developed to explain the causes of deactivation, and the reaction sequences that take place with different reactants and catalysts [3,4]. The deactivation of... 

Although the reactions in [28] and [29] show the possibility of approaching the ideal situation, the number of processes where this can be realized is very small because, in general, reaction products are not sufEciendy volatile to be distilled from the reaction mixture at temperatures below the decomposition temperature of the catalyst. In most cases, this problem is circumvented by removing some of the reaction mixture and carrying out the separation ex situ by low-pressure distillation, phase separation, etc. However, this means that, at aU times, some of the catalyst is outside the reactor and is held under conditions for which it has not been optimized. This can lead to catalyst precipitation, deactivation, or decomposition. In extreme cases, attractive reactions have not been commercialized because the separation problem has not been solved. 

---