Catalyst multicomponent

Louis Schmerling anh V. N. Ipatieff Early Studies of Multicomponent Catalysts... 

A general problem existing with all multicomponent catalysts is the fact that their catalytic activity depends not on the component ratio in the bulk of the electrode but on that in the surface layer, which owing to the preferential dissolution of certain components, may vary in time or as a result of certain electrode pretreatments. The same holds for the phase composition of the surface layer, which may well be different from that in the bulk alloy. It is for this reason that numerous attempts at correlating the catalytic activities of alloys and other binary systems with their bulk properties proved futile. 

This review focuses on the cross-metathesis reactions of functionalised alkenes catalysed by well-defined metal carbene complexes. The cross- and self-metath-esis reactions of unfunctionalised alkenes are of limited use to the synthetic organic chemist and therefore outside the scope of this review. Similarly, ill-defined multicomponent catalyst systems, which generally have very limited functional group tolerance, will only be included as a brief introduction to the subject area. 

Although the bulk of this review is concerned with well-defined metal carbene catalysts, it is important to note the contributions made to cross-metathesis chemistry by ill-defined or multicomponent catalysts. A brief discussion of the cross-metathesis reactions of functionalised alkenes using catalysts of this type will therefore be included here [1]. 

Previously acrylonitrile had proved to be inert towards transition metal catalysed cross- and self-metathesis using ill-defined multicomponent catalysts [lib]. Using the molybdenum catalyst, however, acrylonitrile was successfully cross-metathesised with a range of alkyl-substituted alkenes in yields of40-90% (with the exception of 4-bromobut-l-ene, which gave a yield of 17.5%). A dinitrile product formed from self-metathesis of the acrylonitrile was not observed in any of the reactions and significant formation (>10%) of self-metathesis products of the second alkene was only observed in a couple of reactions. 

Muffin-tin scatterers, 34 212 periodic muffin-tin, 34 218 Muller and Schaich formalism, 34 217 Muller-Gault mechanism, 30 17 Multicomponent catalysts, early studies of, 2 81... 

HI. Transition from the Nitride Studies to the Use of Multicomponent Catalysts for the Ammonia Synthesis. 86... 

V. Multicomponent Catalysts for Reactions Other than the Ammonia Synthesis. 96... 

The study of catalysis has expanded with increasing speed since about 1900, both in its technical and in its scientific aspects. In the course of this development, use has been made to a large extent, and with considerable success, of multicomponent catalysts (Mehrstoff-catalysts), that is, of catalysts which contain mixtures of various chemical constituents rather than one single chemical element or one single chemical compound. 

Written by the author mainly from his private notes and with but little literature available, this paper represents the early development of multicomponent catalysts, a field in which he played a decisive role. A few insertions on theoretical points and several references have been added by the translator where it seemed indicated. The theoretical investigations and practical applications of multicomponent catalysts after about 1920 are not included in this paper. 

I. The Use of Multicomponent Catalysts before the Development of the Ammonia Catalyst... 

Although a truly systematic study of multicomponent catalysts did not start prior to 1900, the older literature shows that catalysts of this type were used occasionally in earlier times. 

His observations during the earlier work with nitrides, induced the author to start a systematic search for multicomponent catalysts for the... 

The main emphasis was laid, in this initial work, on Haber s catalysts, e.g., osmium and uranium compounds, as well as on a series of iron catalysts. Some other metals and their compounds which we tested are, as we know today, less accessibble to an activation by added substances than iron. Therefore, they showed no improvement or only small positive effects if used in the form of multicomponent catalysts. Finally, the substances which we added to the metal catalysts in this early stage of our work were mostly of the same type as those which had proved to favor the nitride formation, e.g., the flux promoting chlorides, sulfates, and fluorides of the alkali and alkaline earth metals. Again, we know today that just these compounds do not promote, but rather impair the activity of ammonia catalysts. 

An important property of the multicomponent catalysts was that with highly purified gases, they remained active over long periods, a fact which greatly helped their practical usage for the technical ammonia synthesis. 

Again and again it was confirmed that, as R. WilRtatter expressed it, multicomponent catalysts can be regarded, in their activities as equivalent to new specific substances. ... 

As is well known, starting in 1923, Franz Fischer, Tropsch et al. developed the synthesis of hydrocarbons by carbon monoxide reduction to a technical process. They developed highly efficient multicomponent catalysts of the cobalt, nickel and iron type for this synthesis (43). 

It may be mentioned briefly that also for other catalytic oxidations, e.g., for those of ethylene and of anthracene, multicomponent catalysts proved to be very effective. For the anthracene oxidation, the following... 

Rather unique multicomponent catalysts are those in which one of the components, being volatile, has to be replenished continuously. The oxidation of dilute hydrogen sulfide to elementary sulfur by air with active carbon as a catalyst is such an example. Small amounts of ammonia, if added to the gases entering the reactor, favor and complete this catalytic reaction the ammonia leaves the reactor unchanged (45). 

We shall omit the numerous other examples of successful uses of multicomponent catalysts. Since about 1920, they have been employed to an ever increasing extent in chemical industries all over the world, and it may be considered rather an exception than the rule if a single component compound serves as a catalyst. Nevertheless, it appears that we have merely entered this vast field, particularly in view of further catalytic possibilities in organic chemistry and in biochemistry. 

To a certain extent the expression multicomponent catalysts is an arbitrary one. There is no doubt that the pure chemical elements and pure chemical compounds have to be called single component catalysts. It is, however, questionable whether a material such as steel should be classified as a single component system or as a multicomponent system. Some of the multicomponent catalysts, for instance, the iron-alumina catalyst consist of two separate solid phases but it would be misleading to accept the presence of more than one phase as the decisive criterion for multicomponent catalysts. The more than additive catalytic action of Cu-ions and Fe-ions in an homogeneous aqueous medium represents obviously a case of multicomponent catalysis, although it occurs in a single-phase system. As to solid multicomponent catalysts, they usually consist of more than one single phase, but there are exceptions to this rule, such as in cases in which mixed crystals or solid solutions are formed from the components. 

A purely practical decision whether two substances form, if combined, a multicomponent catalyst, can be reached by investigating if mixtures of both components, made in various proportions, show a change of catalytic activities as a function of their relative amounts (as shown in Fig. 1). 

Further, a stabilization of the total surface of the main catalyst by added substances may explain some promoter effects, but this explanation holds only for a few multicomponent catalysts. For the iron-alumina catalyst, a beneficial stabilizing effect of the promoter alumina on the fine structure of the iron has to be accepted as a partial explanation. The fact that highly dispersed pure iron sinters at temperatures above 300°C. to a considerable extent, and that sintering practically does not occur with iron of the same high dispersion which contains 1 to 2% of alumina, is a strong qualitative support for this concept. In a quantitative way, the work of P. H. Emmett (47) and his associates has proved this point beyond any doubt it gives similarly valuable... 

Whereas some knowledge has been obtained about the working mechanism of ammonia catalysts (51), this does not apply to the same extent to catalysts used for many other processes. However, a few typical cases of multicomponent catalysts have been investigated both in the author s laboratory and by others. The main conclusion to be drawn from these studies is that it would be wrong to seek one universal explanation for the promoter effects in solid catalysts. As outlined above, structural as well as chemical effects may cause the improvements which are observed after certain substances have been added to a given catalyst. 

Since the time when the author retired some twenty years ago from practical work in catalysis, the use and the study of catalysts and particularly of multicomponent catalysts has extended beyond expectations. A great many theoretical (52) and practically important catalytic reactions have been discovered and investigated in all their aspects. This domain of catalytic chemistry will extend over an even wider range when we shall have learned more about the deeper reasons of catalytic action and if we... 

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