Neopentane hydrogenolysis

TABLE 7.10. Kinetic Parameters for Neopentane Hydrogenolysis Over Metal Catalysts... 

Figure 14.1. Compensation plot of Arrhenius parameters for the reactions of (i) neopentane and (ii) n-heptane with hydrogen on various supported metals. They are compared with selected values for ethane hydrogenolysis, the lines being those used to classify the activities of metals for that reaction in Chapter 13 (see Figures 13.3-13.8). Ethane hydrogenolysis Q n-heptane hydrogenolysis v neopentane hydrogenolysis 3 neopentane isomerisation [1.

Engstrom J R, Goodman D W and Weinberg W H 1988 Hydrogenolysis of ethane, propane, n-butane and neopentane... 

This mechanism is general for all alkanes, and for example the hydrogenolysis of isobutane gives methane and propane as the primary products, and overall a 2 1 ratio of methane and ethane at 100% conversion of isobutane. Similarly, neopentane is transformed into a 3 1 methane/ethane mixture (Table 3 and Scheme 22). 

Comparing the product selectivity at low conversion in the hydrogenolysis of 2,2-dimethylbutane for the two catalysts is noteworthy. Zirconium hydride supported on siUca does not produce neopentane, but only isopentane (10%) as a Cs product in agreement with a /1-alkyl transfer as a key step for the carbon-carbon cleavage (no neopentane can be formed through this mechanism, Scheme 25). 

Other types of non-micro-channel, non-micro-flow micro reactors were used for catalyst development and testing [51, 52]. A computer-based micro-reactor system was described for investigating heterogeneously catalyzed gas-phase reactions [52]. The micro reactor is a Pyrex glass tube of 8 mm inner diameter and can be operated up to 500 °C and 1 bar. The reactor inner volume is 5-10 ml, the loop cycle is 0.9 ml, and the pump volume adds a further 9 ml. The reactor was used for isomerization of neopentane and n-pentane and the hydrogenolysis of isobutane, n-butane, propane, ethane, and methane at Pt with a catalyst. 

Above 323 K, the surface hydride catalyzes the hydrogenolysis of neopentane, isobutane, and propane, whereas ethane does not undergo any significant hydrogenolysis. The first step of the reaction is the activation of the C—H bond, whereas the next step is the activation of the C—C bond of the alkyl groups via (l-methyl migration steps. 

These surface hydrides are active catalysts for the hydrogenolysis of alkanes at moderate temperatures. Zirconium hydride can catalyze the hydrogenolysis of neopentane, isobutane, butane, and propane at 323 K but cannot catalyze the hydrogenolysis of ethane.259... 

The temperature dependence of the selectivity for isomerization versus hydrogenolysis depends on the type of catalyst. Thus, over thick platinum film catalysts this selectivity was temperature independent for the reaction of the butanes and neopentane (24). However, in Boudart and Ptak s (122) reaction of neopentane over platinum/carbon the selectivity to isomerization decreased slightly with increasing temperature while Kikuchi et al. (128) found an increased trend for isomerization in the reaction of n-pentane over platinum/silica and platinum/carbon catalysts. 

Neopentane does not undergo isomerization 185) on chromia/alumina (non-acidic) at 537°C, the only significant reaction been hydrogenolysis to methane and iso-C4. However, the reality of isomerization is made clear from, for instance, the formation of xylenes from 2,3,4-trimethylpentane. For o- and p-xylene, the reactions are (24) and (25) 182, 93). These processes are formally quite analogous to those we have described in previous... 

The hydrogenolysis of neopentane occurs primarily by breaking of a single C—C bond to produce methane and isobutane as the major reaction products Small amounts of ethane and propane were also produced. The reaction kinetics were similar on the two surfaces, indicating a similar mechanism for both surfaces. 

Having set out the properties of tantalum and zirconium hydride toward C-H bond activation of alkanes we now describe the catalytic hydrogenolysis of C-C bonds. It was previously shown in the laboratory that supported-hydrides of group 4 metals, and particularly of zirconium, catalyze the hydrogenolysis of alkanes [21] and even polyethylene [5] into an ultimate composition of methane and ethane. However, to our initial surprise, these zirconium hydrides did not cleave ethane. (=SiO)2Ta-H also catalyzes the hydrogenolysis of acyclic alkanes such as propane, butane, isobutane and neopentane. But, unlike the group 4 metals, it can also cleave ethane [10], Figure 3.7 illustrates this difference of behavior between (=SiO)2Ta(H) and [(=SiO)(4.j,)Zr(H) ], x= or 2). With Ta, propane is completely transformed into methane by successive reactions, while with Zr only equimolar amounts of methane and ethane are obtained. 

This phenomenon is also observed during the hydrogenolysis of branched hydrocarbons such as isobutane carried out under the same conditions an excess of 8% of methane versus propane is observed. With neopentane the excess of methane versus isobutane reaches 15%. 

The study of archetype molecules. This method has been proposed and widely used by Rooney, Burwell, Anderson, and others (see, for review, 155,156,160). In this method a molecule is used which can form an archetype of chemisorbed complex ( caged molecules as derivatives of ada-mantane or ethane in its hydrogenolysis, neopentane in exchange with D2 or in reforming reactions, etc.) or which can form several complexes, but the contribution of these complexes to the overall mechanism is easily derived from the product spectrum [as is the case, for example, with neohexane (167, 168). ... 

Fig. 4. 3C complexes, the existence of which can be seen in experimental evidence from exchange [neopentane Rh (Pt)] and from hydrogenolysis and isomerization (neopentane, neohexane). As in Fig. 3, the known ( ) and the speculative aspects ( ) of the 3C complex formation are indicated. 

As mentioned before, nickel usually catalyzes demethylation. For example, besides methane, mainly neopentane (as well as much less ethane and isobutane) is formed when neohexane undergoes hydrogenolysis on nickel.251 264 In contrast,... 

Forge and Anderson studied so-called archetypal molecules to establish the cleavage mode of individual metals.251,265-267 Ethane is an archetype of C2-unit hydro-genolysis (i.e., the cleavage of primary-secondary and secondary-secondary bonds). Neopentane, in turn, is an archetypal molecule for iso-unit hydrogenolysis (i.e., the hydrogenolysis of bonds with at least one tertiary or quaternary carbon atom). 

---