Heterogenization of catalysts

The Heterogeneity of Catalyst Surfaces for Chemisorption Hugh S. Taylor Alkylation of Isoparaffins V. N. Ipatieff and Louis Schmerling Surface Area Measurements. A New Tool for Studying Contact Catalysts P. H. Emmett... 

The first reaction yields a mixture of all deuterobenzenes, the second one, the deuteration reaction, a mixture of deuterocyclohexanes, mainly C6H6D6 up to C6Di2. The rates of these reactions can be measured with fair accuracy, and additional information can be derived from the isotopic distributions of the reaction products. Schrage and Burwell 36) already demonstrated the usefulness of exchange reactions for studying the heterogeneity of catalyst surfaces. 

The Heterogeneity of Catalyst Surfaces for Chemisorption Hugh S. Taylor... 

The heterogenization of catalysts in membrane is particularly suitable for catalyst design at the atomic and molecular level. One of the main advantages of the membrane reactors, compared to traditional reactors, is the possibility to recycle easily the catalyst. Moreover, the selective transport properties of the membranes can be used to shift the equilibrium conversion (e.g., esterihcation reaction), to remove selectively products and by-products from the reaction mixture, to supply selectively the reagents (e.g., oxygen for partial oxidation reactions). 

Of the numerous poisoning experiments that indicate heterogeneity of catalyst surfaces, reference may be made first to a study of selective poisoning made by Russell et al. (83,84). Russell and Ghering found that the rate of the hydrogenation of ethylene over reduced copper was not markedly affected by nitrous oxide but that the rate of ethane formation decreased rapidly as soon as the adsorbed nitrous oxide was decomposed. 

The non-ideality of catalyst surfaces has ever been one the major difficulties in understanding the detailed mechanisms of contact catalysis. The Advances in Catalysis were opened in 1948 by an article of Taylor on the heterogeneity of catalyst surfaces for chemisorption [4] that the matter was not easy to model is understood by observing that 41 years later the role of particle size on the catalytic activity of supported metals was the subject of of another review in the same series [5] moreover, the family of solids of catalytic interest has since Taylor s review been increased by the availability of new techniques for the preparation of highly dispersed solids, like crystalline zeolites and amorphous aerogels. 

Catalytic membrane reactors (CMRs) are an interesting example of integrated system in which molecular separation and chemical conversions are combined in one step [8]. The heterogenization of catalysts in membrane is particularly suitable for catalyst design at the atomic and molecular... 

These examples show that the area of heterogenization of catalysts represents an enormous potential for research. Some of these catalysts show high activities imder mild conditions with interesting and sometimes unexpected selectivities. The processes for the production of these catalysts, the investigation of their precise structures, and the elucidation of their reaction mechanisms are still at an early stage. 

Figure P6.3 Typical adsorption rate curve for the adsorption of hydrogen on zinc oxide at 1 atm. [Adapted with permission from H. S. Taylor and S. C. Liang, The Heterogeneity of Catalyst Surfaces for Chemisorption, J. Am. Chem. Soc., 69, 1306 (1947). Copyright 1947 by the American Chemical Society.]...

In addition, immobilization of active metal components on the support materials with the aid of ILs provides the following features (i) stabilization of the metal catalysts on the surface of the supports, (ii) use of small amounts of IL, and (iii) accumulation of metals on support materials to facilitate catalytic reactions. As a result, the heterogenization of catalysts and reagents can offer important advantages in separation and reuse. 

The heterogenicity of catalyst surfaces makes problems complicated. If the kinetic data were obtained in a narrow range of experimental conditions, only Langmuir kinetics could be applied. If using the wide range of experimental conditions, micro-kinetic model may involve non-Langmuir effect of surface coverage state. 

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