Group VIII metals hydrogenolysis activities

In the work of the author and his associates on bimetallic catalysts comprising various combinations of Group VIII and Group IB metals, it was discovered that the activity of the Group VIII metal for hydrogenolysis reactions of hydrocarbons was decreased markedly by the presence of the Group IB metal (11-15). It was shown that the inhibition of hydrogenolysis leads to improved selectivity for alkane isomerization reactions (11) and for reactions in which saturated hydrocarbons are converted to aromatic hydrocarbons (12,14,15). Interest in bimetallic catalysts increased markedly with the discovery of this selectivity phenomenon. 

In general, inclusion of a Group IB metal with a Group VIII metal markedly decreases the hydrogenolysis activity of the latter but has a much smaller effect on the activity for reactions such as the dehydrogenation and isomerization of hydrocarbons (1,2,5-8). These observations have been supported by the work of other investigators (9-11). 

Figure 2.1 compares the catalytic activities of all of the metals of Group VIII and of rhenium in Group VIIA for ethane hydrogenolysis. Three separate fields represent the metals of the first, second, and third transition series (22,23). The Group IB metals (copper, silver, gold), for which data are not shown, are much less active than the least active of the Group VIII metals (22,23). 

Direct experimental verification of very highly dispersed bimetallic clusters is complicated by limitations in the ability of physical methods to obtain structural information on such systems. In such a system, however, a catalytic reaction can serve as a sensitive probe to obtain evidence of interaction between the atoms of the two metallic components. For supported bimetallic combinations of a Group VIII and a Group IB metal, the hydrogenolysis of ethane to methane is a useful reaction for this purpose. In the case of unsupported bimetallic systems of this type, as discussed previously, the interaction between the Group VIII metal and the Group IB metal results in a marked suppression of the hydrogenolysis activity of the former. 

It is noteworthy that electropositive promoters, e.g. iron and zinc, exhibit significant effects modifying both the activity and the selectivity of ethanol formation in the hydrogenolysis of ethyl acetate on supported Group VIII metal catalysts, such as Pd-Zn/AbOs and Co-Rh-Fe/Ti02. ... 

These considerations are strikingly demonstrated by the volcano-shaped pattern of variation of catalytic activity as shown schematically in Figure 7.3. While the heat of adsorption is steadily decreasing from left to right, the catalytic reaction rates peak at the group VIII metals in the periodic table. Figure 7.3 shows the pattern of variation of catalytic reaction rates across the series of transition metals Re, Os, Ir, Pt, and Au for the hydrogenolysis of the C—C bond in ethane, the C —N bond in methylamine, and the C —Cl bond in methyl chloride. 

Table 11 Activities of Group VIII metal catalysts for the hydrogenolysis of ethane...

The simplest hydrogenolysis is that of ethane. It is catalyzed by almost all Group VIII metals. As explained before, steric hindrance is the reason that the C-C bond, although weaker than a C-H bond, cannot be activated unless the stronger C-H bond is activated first. Thus the following mechanism, valid for ethane... 

Interestingly, tin addition not only suppresses the multiple hydrogenolysis of C-C and C-0 bonds, but also enhances the rate of alcohol formation. This improvement in selectivity can be interpreted by the superficial dilution of group VIII metal by inactive metal atoms of tin (10). Otherwise, to account for the increase in activity, the modification of the nature of the active site has been considered, but not precisely described (11). Furthermore, if the overall composition of the metallic particles is mastered and known, the misunderstanding of the superficial structure and of the nature of the active phase hinders to etablish accurate relations with the catalytic properties. 

A comparison of various metals as catalysts for the hydrogenolysis of hydrocarbons reveals a wide variation in catalytic activity, even among such closely related metals as the noble metals of group VIII of the periodic table. Striking differences in the distribution of hydrogenolysis products have also been revealed in studies on selected hydrocarbon reactants. These features are emphasized in the following discussion of activity patterns and product distributions in hydrogenolysis. 

Fig. 2. Catalytic activities of group VIII noble metals for re-heptane hydrogenolysis. The activities are compared at a temperature of 205° C at 1 atm pressure and a H2/reC7 mole ratio of 5/1 (23).

Fig. 4. Comparison of activity patterns of the group VIII noble metals for cyclopropane hydrogenation and ethane hydrogenolysis. The activities were all determined at hydrogen and hydrocarbon partial pressures of 0.20 and 0.030 atm, respectively (63).

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