Catalysts heterogeneous, deactivation

While the first process represents a positive event because acetoxyaceto-phenones are convertible into HAPs and can give further intermolecular phenol acylation affording both ortho- and para-HAP, the second process produces ketene, which, being highly reactive, represents the most important source of coke responsible for heterogeneous catalyst deactivation. 

In these kinetic equations, [A], [B], [C], [P], [F] are the concentrations of, respectively, acetophenone, R-1-phenyl ethyl alcohol, S-1-phenyl ethyl alcohol, i -1-phenyl ethyl acetate, and ethyl benzene Ka, Kb, Kq, Kp, Kp are the adsorption constants for acetophenone, i -1-phenyl ethyl alcohol, S-1-phenyl ethyl alcohol, ethyl benzene, R-1-phenyl ethyl acetate on the solid catalyst, fe reaction rate constants of a particular step (fei and fe2 contain also hydrogen pressure dependence), f the rates of reactions. Qat and Cenzyme are the concentrations of the metal catalyst and the immobilised enzyme, respectively. Since heterogeneous catalysts deactivated in the presence of enzyme the generation rates for each compound should include the activity function (Jjeact ... 

Supported aqueous phase (SAP) catalysts (16) employ an aqueous film of TPPTS or similar ligand, deposited on a soHd support, eg, controlled pore glass. Whereas these supported catalysts overcome some of the principal limitations experienced using heterogeneous catalysts, including rhodium leaching and rapid catalyst deactivation, SAP catalysts have not found commercial appHcation as of this writing. 

Very few examples have been described for the non-covalent immobilization of chiral porphyrin complexes (Fig. 26). In the first case, the porphyrin-dichlororutheninm complex was encapsulated in silica, which was prepared around the complex by a sol-gel method [78], in an attempt to prevent deactivation observed in solution in the epoxidation of different alkenes with 2,6-dichloropyridine N-oxide. In fact, the heterogeneous catalyst is much more active, with TON up to 10 800 in the case of styrene compared to a maximum of 2190 in solution. Enantioselectivities were about the same imder both sets of conditions, with values aroimd 70% ee. 

The catalyst deactivates, but after four runs the conversion is still significantly higher (> 99% after 2 h) as compared with the uncatalyzed reaction. Moreover, the Z-selectivity in all four runs is higher than 80%, whereas in the uncatalyzed reaction, it is typically only 30% (Z). The fact that the solid powder can be used several times furthermore supports the fact that the reaction mechanism is heterogeneous. The reason for the deactivation is unknown. A disadvantage of the nanoparticles is the difficulty of separation. Thus, in some cases the particles form col-... 

The heterogeneous catalytic system iron phthalocyanine (7) immobilized on silica and tert-butyl hydroperoxide, TBHP, has been proposed for allylic oxidation reactions (10). This catalytic system has shown good activity in the oxidation of 2,3,6-trimethylphenol for the production of 1,4-trimethylbenzoquinone (yield > 80%), a vitamin E precursor (11), and in the oxidation of alkynes and propargylic alcohols to a,p-acetylenic ketones (yields > 60%) (12). A 43% yield of 2-cyclohexen-l-one was obtained (10) over the p-oxo dimeric form of iron tetrasulfophthalocyanine (7a) immobilized on silica using TBHP as oxidant and CH3CN as solvent however, the catalyst deactivated under reaction conditions. 

Catalyst deactivation refers to the loss of catalytic activity and/or product selectivity over time and is a result of a number of unwanted chemical and physical changes to the catalyst leading to a decrease in number of active sites on the catalyst surface. It is usually an inevitable and slow phenomenon, and occurs in almost all the heterogeneous catalytic systems.111 Three major categories of deactivation mechanisms are known and they are catalyst sintering, poisoning, and coke formation or catalyst fouling. They can occur either individually or in combination, but the net effect is always the removal of active sites from the catalyst surface. 

A major problem associated with the operation of heterogeneous catalysts is their gradual loss of activity with time. Inevitably, chemical and/or physical parameters affect the action of catalysts and progressively lead to its partial or total catalyst deactivation. Deactivation processes occur simultaneously with the main reactions. 

This heterogeneous process may constitute an interesting alternative to the classical synthetic route. Catalyst deactivation can be slowed down by deliberately poisoning the acidic surface site with added pyridine in the reactant feed. A feasible operation mode for a continuous heterogeneous process consists of reaction and subsequent reoxidation cycles of the catalyst. 

The homogeneous catalytic olefinic substitution, like the carboalkoxylation, does not generally proceed in high yield with aromatic chlorides under the usual conditions. A heterogeneous catalyst, palladium on charcoal, has been reported to cause chlorobenzene and other aromatic chlorides to react with styrene and styrene derivatives, with sodium carbonate as a base at 100° 30, 3]). In our laboratory, we have found the reactions occur as described, but the catalyst is apparently rapidly deactivated. 

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