Isobutane hydrogenolysis

Fig. 6. Isobutane hydrogenolysis % products as a function of Sn/Pt atomic ratio, prepared by coimpregnation SI A Pt/Al203 blank X SI without H2.

TABLE 7.6. Kinetic Parameters for Isobutane Hydrogenolysis Over Metal Catalysts... 

Scheme 18.15 Mechanism of isobutane hydrogenolysis and isomerisation on an ensemble" of at least two Pt atoms.

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). 

In some catalytic processes, it is necessary to avoid carbon-carbon bond cleavage. For example, isobutane is mainly transformed into its lower alkane homologues (hydrogenolysis products) on metal surfaces, while it can be converted more and more selectively into isobutene when the Pt catalysts contain an increasing amount of Sn (selective dehydrogenation process) [131]. 

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 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 concept of site isolation is important in catalysis. On metal particles one usually assumes that ensembles of metal atoms are necessary to activate bonds and to accommodate the fragments of molecules that tend to dissociate or to recombine. We present here three examples of such effects the dehydrogenation of decane into 1-decene, the dehydrogenation of isobutane into isobutene and the hydrogenolysis of acids or esters into aldehydes and alcohols. In most cases the effect of tin, present as a surface alloy, wiU be to dilute the active sites, reducing thereby the yield of competitive reactions. 

The presence of tin atoms regularly distributed on the platinum surface isolates the platinum atoms by increasing the distance between two adjacent platinum atoms, as does the copper atoms on a nickel surface [108] or the tin atoms on a rhodium, platinum or nickel surface [106, 109-111]. The presence of tin would thus avoid the hydrogenolysis reaction, leading to a more selective catalyst (Figure 3.37). Indeed, the formation of isobutene from isobutane involves only one platinum atom, with the reaction passing through a simple mechanism of P-H elimination after the first step of C-H bond activation (Scheme 3.26). 

The catalytic behavior of small metal particles in heterogeneous catalysts varies with metallic particle size and shape a phenomenon referred as a structure-sensitivity. Simple alkanes such as ethane, propane, n-butane and isobutane can be used as archetype molecules for studying hydrogenolysis reactions as they... 

Hydrogenolysis of butane was performed using Pt and PtRh nanowires and particles/FSM-16 [23, 32] Table 15.6 summarizes the results. In this reaction, methane, ethane and propane were formed from hydrogenolysis of butane, and isobutane from isomerization (15.1) ... 

From this comparison, the existence of the 4Ca3 complexes does not seem probable at low temperatures. However, it is not completely excluded either. It has been observed with Pt that molecules of 2,2,3,3-tetramethylbutane undergo hydrogenolysis mainly into two molecules of isobutane. This could be evidence of the formation of 4Ca3 complexes (190) on Pt. 

Hydrocarbons higher than C3 can also undergo isomerization on Pt, running in parallel to hydrogenolysis. With butane or isobutane the selectivity for isomerization is rather low, also on Pt, but the higher hydrocarbons show more of isomerization reactions. With higher hydrocarbons some other metals (Ir, Pd) also show some isomerization selectivity. 

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,... 

Since in C2-unit hydrogenolysis both carbon atoms of the C—C bond to be broken must be primary or secondary (isobutane cannot cleave in C2-unit mode through adsorption of its tertiary carbon atom), Anderson formulated the cleavage of neohexane according to Eq. (11.80) involving carbon-metal double bonds (1,2-dicarbene mechanism) 267... 

It was also shown that bond breaking does not necessarily require direct bonding of the carbon atoms of the bond to be broken to surface metal atoms. For example, neither 1,2- nor 1,3-diadsorbed species can explain the favored cleavage of 2,2,3,3-tetramethylbutane to yield isobutane. A 1,4-diadsorbed intermediate, however, could account for the hydrogenolysis of the quaternary-quaternary bond.255... 

The high selectivity to isobutane observed with Pt-Cs2.5 is brought about by the unique roles of protons, which greatly suppress hydrogenolysis (382). When... 

Anderson and his co-workers examined the reactions of small alkanes mainly on platinum and palladium 45-48). Isobutane was isomerized to n-butane on platinum and on palladium, neopentane isomerized to isopentane on platinum, whereas other metals (including palladium) caused hydrogenolysis predominantly or exclusively. It was proposed that the slow step in the isomerization was the formation of a bridged intermediate (C) from an aory-triadsorbed species (A, B) (Fig. 11).1 Hiickel MO calculations based on this proposal suggested,... 

FIG. 12. Possible mechanism of skeletal isomerization and hydrogenolysis of isobutane on Pd (SO). 

To provide adequate background for the work to be described next, some further findings by Anderson and Avery may be mentioned. The selectivity for isomerization versus hydrogenolysis (St = 77/77 ) of isobutane on evaporated films of platinum claimed to expose (111) faces predominantly was found to be enhanced by a factor of 5 relative to unoriented films this enhancement was not observed for n-butane (Table VI). Anderson and Avery (47) proposed that a symmetrical triadsorbed species (Diagram 1) is the preferred reaction intermediate for isobutane, such an intermediate not being possible for n-butane. This intermediate fits the triplets of metal atoms on the (111) plane of platinum, suggesting, they believed, a basis for the enhanced efficiency of the (111) plane for the isomerization of isobutane. We note that inspection of rates of isomerization given in the paper of Anderson and Avery shows a factor of only... 

Anderson and Avery proposed that the same intermediate for isomerization was also responsible for hydrogenolysis of isobutane (47), but very recently Hagen and Somorjai have studied reactions of isobutane and propane on Pt and Ir catalysts in which Au was incorporated in increasing amounts. They concluded from the results that the sites responsible for isomerization are distinct from those causing hydrogenolysis (53a). 

Hydrogenolysis of n-butane and ethane proceeded with higher conversions on the Rh-rich cluster catalysts and only modestly on [Ir j-NaY at temperatures of 373 - 523 K. A negligible yield of isobutane (< 3% of the total butane conversion) by isomerization was obtained on the series of Rh, Ir, and Rhir heterometallic catalysts. 

Indeed, the reaction was first observed in the synthesis of the hydrides. As mentioned above, when 5 is heated under dry hydrogen to 150 °C for three hours, (=SiO)3ZrH (14) is formed together with nine equivalents of methane and three equivalents of ethane. The formation of methane and ethane rather than neopentane was clear evidence of hydrogenolysis under the synthesis conditions [5, 15, 16]. It was observed that the reaction of neopentane occurred by stepwise formation of firstly isobutane and methane, then conversion of the former to a second equivalent of methane and propane which is further converted to ethane and a third equivalent of methane. The C-C bond of ethane cannot be cleaved by P-methyl elimination because a surface metal-ethyl fragment has no methyl group in the S-position. 

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