Dehydrogenation multiplet theory

Dehydrogenation of the cyclohexanes over platinum, palladium, and nickel is selective in the sense that it affects only those compounds which can form aromatics by this reaction. Six-membered rings with substituent groups in positions such that elimination of hydrogen cannot form a benzene ring, for instance, 1,1-dimethylcyclohexane, undergo dehydrogenation at 300° with difficulty (442). The multiplet theory explains... 

In the discussion of the subject Balandin mentions (15) that Fischer previously postulated that methylene radicals may be produced as an intermediate in the formation of hydrocarbons by his method (116). This mechanism of carbon deposition on platinum supported on oxides of nickel and chromium (oxidized nichrome) through the intermediate formation of methylenes was thought by Balandin to be similar to the mechanism of dehydrogenation over this type of catalyst in that both occur on the boundaries of platinum-nickel and of platinum-chromia and were brought in agreement by him with his multiplet theory (26). 

In the author s paper 68) it was shown that changing the nature of the metal catalyst (with the lattice A1 and A3) greatly affects the rate of dehydrogenation and the energy of activation e. A relationship between e and the interatomic distances or atomic radii is observed, as one should expect from the multiplet theory. This relationship proves to be linear (see Fig. 7). 

Experience shows, in conformity with the multiplet theory, that on oxide catalysts the alcohols are also oriented with their reacting group >CH—OH toward the dehydrogenating catalyst (1.13). This is evident from the equality of the true activation energies of dehydrogenation e, as well as from the equality of the relative adsorption coefficient a for... 

The multiplet theory first gave (6) the sextet model of the dehydrogenation of cyclohexane and its derivatives, a reaction discovered by Zelinskii (197). The reaction takes place in the neighborhood of 300° on metal catalysts. 

Ehrenstein and Bunge (265) found that the derivatives of cis-decahydroquinoline and cis-decalin are more easily dehydrogenated than the corresponding trans forms according to these authors, the facts found by them are in conformity with the multiplet theory. 

The experiments performed have completely confirmed the theoretical calculations cyclohexane, cyclohexene, and piperidine are actually dehydrogenated on cadmium oxide at 400-500° (with partial reduction of CdO to Cd as a side reaction). This is the case of predicting a new catalyst on the basis of the multiplet theory. Cyclohexene and piperidine but not cyclohexane (see above) are, in fact, dehydrogenated on zinc oxide. None of the hydrocarbon studied could be dehydrogenated on beryllium and magnesium oxides. 

Third, the doublet and, especially, sextet models require very precise superimposing of the molecule on the catalyst lattice. We have found that the cyclohexane derivatives, in accordance with the sextet model, smoothly dehydrogenate only on the following metals nickel, cobalt, iridium, palladium, platinum, ruthenium, osmium, and rhenium, all of which crystallize in Al, A3 lattices with certain interatomic distances. These results extend to the alloys of these metals. The catalytic activity of rhenium for this reaction was predicted by the multiplet theory as this metal maintains the square of activity this prediction was realized experimentally in the laboratory of the author. Similar correlations take place in the exchange of cyclanes with deuterium. 

During the early years of development of the multiplet theory, attention was paid chiefly to the correspondence of the structure of reacting molecules and catalyst, especially in relation to the sextet model of dehydrogenation of six-membered cycles on metal catalysts. This work permitted the determination of the group of metals that can act as catalysts for the dehydrogenation of cyclohexane (the so-called Blandin s square of activity ) and the prediction of catalytic activity, e.g., for Re which was unknown as a catalyst for this reaction. 

As the molecular bond lengths became known, correlations were sought between the geometry of the surface and catalytic activity. There developed the multiplet theory of Balandin which was applied successfully to dehydrogenation catalysts. It also provided an adequate explanation of the work of Maxted and others on catalytic poisons and of the behavior of the different plane faces of crystals. There is no inherent conflict between the interpretations based on geometry and those based on the electronic potential of the surface. The two effects are probably complementary. More knowledge is, however, required about the influence of the electronic potential on the decomposition of complex molecules, before a decision can be made on their relative significance. 

The exceptions to this generalization are the dehydrogenation of saturated six-membered rings, and reactions of similar type. Here, surface and reactant geometry have been very closely correlated with catalytic activity and the multiplet theory has achieved notable distinction. 

A theory of catalytic action developed in such detail has naturally been susceptible to experimental test. The following work, most of which has been discussed by Taylor (9), and some of which preceded the multiplet theory and caused its formulation bears on the theory of the mechanism of the dehydrogenation of six-membered rings. 

However, from the multiplet theory, adsorption requires local and congruent active sites for the decomposition of ethanol. Thus, for the dehydrogenation of ethanol, there are adsorption bonding of C-O, C-H and O-H at the surface atoms ( ), according to the following mechanism ... 

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