Deactivation of Heterogenous Catalysts

In heterogenous catalytic reactions, a decrease in catalyst activity is often observed with increasing operation time. There are many reasons for this the most important factors can be classified into three groups  

Poisoning of the catalyst surface by irreversible adsorption and/or reaction of a chemical species, thus makingthe active centers required for the catalyzed reaction inactive. Example is CO adsorption on iron catalysts used for the ammonia 

Coverage of the surface with substances that leads to a mechanical blockage of the catalytically active surface. Example is deposition of coke in various hydrocarbon reactions such as isomerization, cyclization, and cracking. 

For the course of a catalytic reaction whose kinetics can be described as  

the activity factor corresponds to the ratio between the rate constant after a certain time of operation referred and the initial value. It is important to under-hne that this way of including the deactivation is possible only if the deactivation kinetics is separable from the transformation kinetics, viz. the kinetic model for the transformation is not altered by the deactivation process. 

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As in the case of homogeneous catalysis, poisons can also lead to deactivation of heterogeneous catalysts. Soluble or volatile metal or nitrogen compounds can destroy acid sites, while carbon monoxide and sulphur compounds almost invariably poison nickel and noble metal hydrogenation catalysts by bonding strongly with surface metal atoms. These considerations often lead to the selection of less active, but more poison-resistant, catalysts for industrial use. 

In most cases, the deactivation of heterogeneous catalysts can be attributed to one of the four processes displayed in Figure 2.3.6. 

During the operational lifetime of most catalysts, their activity decreases by deactivation. The time period of economic operation can be very different even for commercial catalysts and ranges from a couple of seconds to many years. Catalyst deactivation of heterogeneous catalysts can be attributed four processes ... 

Measurement of heat of adsorption by means of microcalorimetry has been used extensively in heterogeneous catalysis to gain more insight into the strength of gas-surface interactions and the catalytic properties of solid surfaces [61-65]. Microcalorimetry coupled with volumetry is undoubtedly the most reliable method, for two main reasons (i) the expected physical quantities (the heat evolved and the amount of adsorbed substance) are directly measured (ii) no hypotheses on the actual equilibrium of the system are needed. Moreover, besides the provided heat effects, adsorption microcalorimetry can contribute in the study of all phenomena, which can be involved in one catalyzed process (activation/deactivation of the catalyst, coke production, pore blocking, sintering, and adsorption of poisons in the feed gases) [66]. 

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. 

Holzwarth, A. and Maier, W.F. (2000) Catalytic phenomena in combinatorial libraries of heterogeneous catalysts detection of activation and deactivation by emissivity-corrected IR thermography. Platinum Met. Rev., 44, 16. 

Experimental Methods for the Determination of Coke Formation and Deactivation Kinetics of Heterogeneous Catalysts... 

Deactivation of heterogeneous Wacker oxidation catalysts is mainly caused by sintering of the vanadium oxide redox layer, resulting in the accumulation of (inactive) Pd(0), and hence in lower catalytic activity in the oxidation of 1-butene. The sintering process is... 

Most of the heterogeneous catalyst which are in practical use consist of one or more catalytically active compounds which are impregnated on supporting carrier materials. This method can be chosen to immobilise acids and bases as well as salts, oxides or complexes. The major drawback is leaching of one or more component which leads to irreversible deactivation of the catalyst. Physisorption can be enhanced by choosing the appropriate porous, chemical and electronical properties. This leads to catalysts with sufficient long term stability due to e.g. ionic linkages. 

In the second step, the triple bond in 63 is selectively reduced to the cz -alkene using the Lindlar catalyst to form 64. In this case, the Lindlar catalyst is a poisoned heterogeneous palladium catalyst on barium sulfate. The deactivation of the catalyst with quinoline is responsible for the selective hydrogenation to the alkene and not through to the alkane. The reason for the highly stereoselective reduction with syn-addition to the cw-alkene is that one face of the triple bond is shielded by the catalyst surface. 

In the chemical and the petrochemical industries, heterogeneous catalysts are typically operated in a narrow range of reaction conditions. These reaction conditions are chosen so as to achieve an optimal feedstock conversion at minimal catalyst deactivation and are typically either constant over time or are slightly modified to compensate for feedstock conversion loss due to deactivation of the catalyst. 

In the application of heterogeneous catalysts to the chemical and petrochemical industries, either all precautions are taken to minimize deactivation of the catalyst, or the process is designed for regular regenera-tion of the catalyst. In contrast, au-... 

The kinetics are much simpler in homogeneous metallocene-based catalyst systems, especially in base-free cationic catalyzed polymerization systems, than those in heterogeneous systems. The polymerizations with homogeneous metallocene catalysts are no doubt the best systems for kinetic study of Ziegler-Natta polymerization. The a-olefin polymerization with these catalysts also offers a good opportunity to study the durability and deactivation of the catalysts, since the polymerization systems remain homogeneous over a considerable long reaction period [50]. 

One problem in catalysis is gradual loss of activity of the catalyst. There are many reasons underlying the deactivation of heterogeneous metathesis catalysts [42]. The most important causes of catalyst deactivation are (i) intrinsic deactivation reactions, such as the reductive elimination of metallacyclobutane intermediates,... 

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