Rate of dehydrogenation

Some information about structure effects on the rate of dehydrogenation of alcohols to aldehydes and ketones on metals is found in the older literature 129-132) from which it follows that secondary alcohols react more easily than the primary alcohols 129) and that the reactivity decreases with the length of the carbon chain 131). Some series can be correlated by the Taft equation using a constants (Ref. 131, series 103, Cu-Cr203 catalyst, 350°C, four points, slope 18 Ref 132, series 104, Cu catalyst, four points, slope 22). Linear relationships have been used in a systematic way by... 

The effect of orientation is illustrated in an interesting manner by some experiments of Palmer and Constable on the rate of dehydrogenation of alcohols in presence of metallic copper. Primary alcohols appear to be adsorbed with the -CH2OH group attached to the catalyst. The hydrogen is lost from this group in the chemical change, so that it appears reasonable to suppose that this is the portion of the molecule which must be activated. The hydrocarbon chain, therefore, would not be expected to have much influence on the process, and it was indeed found by experiment that the rates of reaction of five primary alcohols are equal. Moreover, the temperature coefficients of the reaction velocity are also equal. 

A major problem in noble metal catalyzed liquid phase alcohol oxidations -which is principally an oxidative dehydrogenation- is poisoning of the catalyst by oxygen. The catalytic oxidation requires a proper mutual tuning of oxidation of the substrate, oxygen chemisorption and water formation and desorption. When the overall rate of dehydrogenation of the substrate is lower than the rate of oxidation of adsorbed hydrogen, noble metal surface oxidation and catalyst deactivation occurs. 

Pshezhetskii et al. (17) have recently expressed the view that in the equation for the rate of dehydrogenation, containing the function p in the form of the Langmuir isotherm, z is the ratio of the rate constants of some partial reactions. 

Rubinshtein and associates (67) found that as Pt is dispersed more and more on carbon, the rate of dehydrogenation of cyclohexane parallels the intensity of X-ray reflections from the (111) faces and not from the others. 

The decrease in the rate of dehydrogenation caused by coke deposition is decreased monotonically with coke content. The deactivation function 4, expressed as the ratio of reaction rates in the presence and absence of coke was tested... 

Fig. 10. Relationship between the rate of dehydrogenation of formic acid wdth Rh-PVA catalyst contains 10 mg. of Rh and 200 mg. PVA. [From Hernandez and Nord, J. Colloid Sci. 3, 377 (1948), Figure 1, p. 379.]...

The kinetics of the ODH of n-butane has been investigated for unpromoted and cesium promoted a-NiMoOa catalysts. The reaction rates of dehydrogenation products as functions of the butane and oxygen partial pressures are described by a kinetic model based on the Mars and van Krevelen mechanism. The effects of Cs on the kinetic parameters can be interpreted on the basis of recently published results concerning the properties of those catalysts. 

Figure 3.5 The rates of dehydrogenation and hydrogenolysis of cyclohexane on ruthenium-copper aggregates as a function of composition (3). (Reprinted with permission from Academic Press, Inc.)...

The selectivity [e.g., the rate of hydrogenolysis relative to the rate of dehydrogenation (both of which are important reactions in the reforming of petroleum distillates)] may also depend on crystallite size ... 

The work of Constable and Palmer 17 regarding the mechanism of catalyst action when alcohol is decomposed in the presence of copper catalysts, has contributed valuable information from a physical-chemical standpoint on the manner in which catalysts act. In the presence of their catalysts, the rate of dehydrogenation of ethanol, propanol, and butanol was the same notwithstanding that the length of the hydrocarbon chain had doubled in the series of alcohols. With isopropanol the velocity was five times that of the others. All primary alcohols, however, contain tile — CHsOH group at the end of the hydrocarbon chain and to explain the action of these alcohols these workers have undertaken to show that the primary alcohols are adsorbed by the catalyst surface with the — CH.. OH group in contact with the surface and the hydrocarbon chains perpendicular to that surface. Such an orientation would then make all of the alcohols alike as far as the catalyst surface was concerned and, hence, make their... 

The influence of the catalyst is related to the value of Kg, kg and ky expressing the affinity for the steam adsorption, the rate of dehydrogenation of CHx species and of the surface reaction respectively. 

Combining both the dehydrogenation and hydrogenation rate expressions Lyubarskii (IS) obtained the following power function model to describe the net rate of dehydrogenation of n-butane. 

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). 

Here Wi is the rate of doublet dehydrogenation of cyclohexane into cyclohexene W[, that of the reverse reaction Ws, the rate of dehydrogenation of cyclohexene into benzene and W2, the rate of the direct dehydrogenation of cyclohexane into benzene. 

Eucken (79, 80), studying alcohol decomposition, was the first to make an attempt at relating the properties of the oxide to the selectivity. He succeeded in establishing a correlation between the selectivity (i.e., the ratio of the rate of dehydrogenation to that of total decomposition) and the quantity... 

Also the presence of foreign metallic ions can enhance the dehydrogenation activity. Freidlin and Levit (86) observed that addition of 5 to 30% K2C03 to silica promotes this reaction. Dalmai et al. (6) showed that not only potassium, but also lithium and magnesium ions act as promoters. Also on ZnO the addition of alkali (Li or Na) increases the rate of dehydrogenation [Shabrowa (94)]. 

As expected from the above discussion, the amounts of cracked products and methane increase by a factor of 2 to 2.5. However, the DHC products (naphthenes and toluene) increase by a factor of 12. Under the experimental conditions, DHC can be both mono- or bi-([metal + alumina]-) functional. The rate of dehydrogenation of MCH to toluene (not shown) actually decreases for the bimetallic catalyst, from 45 to 12 mol/h/kg. 

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