Normal Supported Catalysts

In support of that explanation, X-ray analysis of the catalyst after use indicated the presence of MgO. Hence, the catalytically active phase was finely divided copper in intimate contact with magnesia, quasi as carrier. The same phenomenon was observed with the Zintl-phase alloys of silver and magnesium. Such catalysts were then deliberately prepared by coprecipitation of copper and silver oxides with magnesium hydroxide, followed by dehydration and reduction. Table I shows that these supported catalysts had the same activation energies as those formed by in situ decomposition of copper and silver alloys with magnesium. 

Since insulating magnesium oxide can be doped to form n-type or p-type semiconductors at moderate temperatures (72), we have here a first example of the electronic effect of a semiconducting support. However, it is unknown what type of conduction was present during the catalytic reaction in the originally undoped specimens used for the preceding experiments. 

Activiation Energies of Model Catalysis and Alloys for Formic Acid Dehydrogenation 

The results with magnesia led us to a planned series of experiments with doped aluminas. Nickel was evaporated in vacuo onto the surface of grains of undoped or doped alumina or, alternatively, onto compact nickel. These preparations were then used as catalysts for the donor model reaction of formic acid dehydrogenation as above. Table II shows the results. 

It is seen again that the activation energy typical of the pure metal is lowered by contact of the metal with an oxidic support. Even more important, p-type doping with bivalent oxides lowers, n-type doping with tetravalent oxides increases the activation energy relative to that observed with undoped alumina. This is consistent with the concept 

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In the initial part of this paper, we mentioned already that the effects described in the preceding section on "normal supported catalysts are understandable only if the amount of semiconductor in the mixed catalysts is very large compared to the amount of metal only then will the number of electrons a semiconductor is able to extract from the metal be sufficient to affect the electron concentration and the Fermi level of a metal. This condition is fulfilled in all of the preceding examples. 

As in the case of normal supported catalysts, we tried with this inverse supported catalyst system to switch over from the thin-layer catalyst structure to the more conventional powder mixture with a grain size smaller than the boundary layer thickness. The reactant in these studies (27) was methanol and the reaction its decomposition or oxidation the catalyst was zinc oxide and the support silver. The particle size of the catalyst was 3 x 10-3 cm hence, not the entire particle in contact with silver can be considered as part of the boundary layer. However, a part of the catalyst particle surface will be close to the zone of contact with the metal. Table VI gives the activation energies and the start temperatures for both methanol reactions, irrespective of the exact composition of the products. 

In the preceding sections the use of catalysts in which vanadium oxides are supported on a more or less inert carrier has been mentioned quite often. Because of the importance of this type of catalyst they are discussed more extensively in this section. Often a distinction is made between the normal supported catalysts and so called monolayer catalysts. In the latter the vanadium oxide is supposed to be dispersed in a monomolecular layer on the support, which may be covered completely or only partly. The normal supported catalysts are usually made by impregnation, either wet or dry, of the porous carrier with an aqueous solution, often of NH4V03, sometimes with oxalate added.12 14,75,95,139,140... 

The ubiquity and inherent stabiUty of C-H bonds have rendered the use of heterogeneous systems rather difficult. Normally, supported catalysts require more drastic reaction conditions than homogeneous counterparts. By using higher temperatures, the somewhat lower activities can be compensated to some extent. To obtain high efficiency, other approaches include the apphcation of non-traditional activation methods, such as microwave irradiation and photochemistry. Especially, heterogeneous photocatalysis thus established offers numerous perspectives for the heterocycie synthesis via C-H activation processes. 

Figure 2.12 shows the rate, the coverages, the reaction orders, and the normalized apparent activation energy, all as a function of temperature. Note the strong variations of all these parameters with temperature, in particular that of the rate, which initially increases, then maximizes and decreases again at high temperatures. This characteristic behavior is expected for all catalytic reactions, but is in practice difficult to observe with supported catalysts because diffusion phenomena come into play. 

The process has also been adapted using resin supported catalysts [e.g. 23-28]. Generally, the reactivity of the alkyl halides follows the normal pattern of I>Br>Cl, but secondary alkyl halides are less reactive and require high reaction temperatures and tertiary alkyl halides fail to react. 

Examples of the Michael-type addition of carbanions, derived from activated methylene compounds, with electron-deficient alkenes under phase-transfer catalytic conditions have been reported [e.g. 1-17] (Table 6.16). Although the basic conditions are normally provided by sodium hydroxide or potassium carbonate, fluoride and cyanide salts have also been used [e.g. 1, 12-14]. Soliddiquid two-phase systems, with or without added organic solvent [e.g. 15-18] and polymer-supported catalysts [11] have been employed, as well as normal liquiddiquid conditions. The micellar ammonium catalysts have also been used, e.g. for the condensation of p-dicarbonyl compounds with but-3-en-2-one [19], and they are reported to be superior to tetra-n-butylammonium bromide at low base concentrations. 

Anchoring the catalyst to polymeric materials has some advantages in easy product separation and catalyst recovery for recycling. The first example of a polymer-supported rhodium catalyst for hydroformylation was reported in 1975. Since then, many reports have been published on polymer-supported catalysts here, we focus on examples of normal-sc QcxiY or enantioselective hydroformylation. 

The term electronics normally refers to the theory and function of the electronic devices and circuits used so universally for measurement, control, and computation. Conventional electronics is an important tool also in the study of supported catalysts. 

The inverse case, a semiconducting catalyst supported by a metal, termed inverse supported catalyst, has been studied systematically only in the last few years. Here, even more drastic effects can be expected because normally the number of free electrons in a metal is several orders of magnitude higher than in semiconductors. The effects are indeed considerably larger as will be shown below. However, the principles and the theory involved are more complex (6-8). 

The experimental results described in this review support the concept that, in certain reactions of the redox type, the interaction between catalysts and supports and its effect on catalytic activity are determined by the electronic properties of metals and semiconductors, taking into account the electronic effects in the boundary layer. In particular, it has been shown that electronic effects on the activity of the catalysts, as expressed by changes of activation energies, are much larger for inverse mixed catalysts (semiconductors supported and/or promoted by metals) than for the more conventional and widely used normal mixed catalysts (metals promoted by semiconductors). The effects are in the order of a few electron volts with inverse systems as opposed to a few tenths of an electron volt with normal systems. This difference is readily understandable in terms of the different magnitude of, and impacts on electron concentrations in metals versus semiconductors. 

This potential, adjusted as a function of the pH of the solution and of the hydrogen pressure, is easily fixed between +0 1 V and — 0.9V/NHE (Normal Hydrogen Electrode). The H2/H+ couple was used to prepare supported catalysts with platinum, palladium, ruthenium, and rhodium modified with deposits of tin, lead, iron, germanium, and bismuth [50-54]. These catalysts were proposed for their good selectivities for different reactions in specialize organic chemistry. 

Fig. 26. Activity versus surface area normalized time, for alumina-supported catalysts at 525 K, 1 atm (10 ppm H2S, H2/CO = 99) (Ref. 194).

Recently, FT synthesis reactions were shown to be independent of metal dispersion on Si02-supported catalysts with 6-22% cobalt dispersion (103). Turnover rates remained nearly constant (1.8-2.7 x 10 s ) over the entire dispersion range. Dispersion effects on reaction kinetics and product distributions were not reported. These tests were performed at very low reactant pressures (3 kPa CO, 9 kPa H2), conditions that prevent the formation of higher hydrocarbons and lead to methane with high selectivity and to CO hydrogenation turnover rates 10 times smaller than those obtained at normal FT synthesis conditions and reported here. These low reactant pressures also lead to kinetics that become positive order in CO pressure. Thus, the reported structure insensitivity (103) may agree only coincidentally with the similar conclusions that we reach here on the basis of our results for the synthesis of higher hydrocarbons on Co. 

As a result, the supported catalyst has a long life under normal operating conditions. The (bulk) 7-phase alumina ensures reasonably high surface area, while the (surface) a- phase alumina minimises undesired acidity at the interface. 

The commercial Co-Mo catalysts used for hydrocracking oil sands bitumen and petroleum residua are normally supported on alumina. Several years ago hydrodesLilphuriEation (HDS) studies with thiophene (ref. 1) indicated that carbon supported catalysts were superior to alumina supported ones. However HDS of benzothiophene (ref. 2) was found to be greater on alumina supported catalysts than on carbon supported ones. Studies of these two supports were performed in our laboratory with Athabasca bitumen (ref, 3). Care was required in choosing the particular carbon support since carbons which have very large surface areas also have small pore sizes which would inhibit or perhaps exclude some of the larger molecular species in resid feedstocks, such as Athabasca bitumen The wide pore carbon that vas available, had a surface area and a rredian pore diameter that were respectively half and more than double those of the alumina support used for comparison. 

Fig. 6.6. Normalized surface area of MCM-supported catalysts Mo-, Ni-, and Co(Ni)-Mo catalysts in oxide form.

Liquid-phase selective oxidations are normally catalysed homogeneously. A small but significant interest has recently arisen in the use of solid catalysts for liquid-phase oxidation, particularly of alkyl aromatics. Shalya et al. have compared the activity of copper, silver, and gold metals as catalysts for cumene oxidation (Table 2). Silver was found to combine good selectivity for the desired product, cumene hydroperoxide, with an activity similar to that of copper. With supported catalysts, silver is considerably more active than copper, while gold is totally inactive. 

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