Alkanes, lower, hydrogenolysis

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]. 

Earlier transition metals, as zirconium and hafnium, are still more active in hydrogenolysis, which allows zirconium hydrides to be used in depolymerization reactions (hydrogenolysis of polyethylene and polypropylene) [89], In this case, the zirconium hydride was supported on silica-alumina. Aluminum hydrides close to [(=SiO)3ZrH] sites would increase their electrophilicity and, thus, their catalytic activity. A catalyst prepared in this way was able to convert low-density polyethylene (MW 125000) into saturated oligomers (after 5h) or lower alkanes at 150°C (100% conversion). It was also able to cleave commercial isotactic polypropylene (MW 250000) under hydrogen at about 190 °C (40% of the starting polypropylene was converted into lower alkanes after 15 h of reaction). 

Whereas the hydrogenolysis of hexane and other alkanes (60, 62, 100, 101) can be observed only at temperatures slightly above the temperatures closing the miscibility gap in Ni-Cu alloys, reaction (II) occurs at substantially lower temperature, where separated phases (if present) are thermodynamically stable. 

The chain length of n-alkanes has a marked influence on reactivities for hydroisomerization, and especially for hydrocracking. To a very small extent a methane and ethane abstracting mechanism, probably hydrogenolysis as predicted in a basic work on bifunctional catalysis (14), is found to be superimposed when lower carbon number feeds (C, Cg, Cg) are used. n-Hexane is excluded from ideal hydrocracking. On the Pt/Ca-Y-zeolite catalyst it is cracked via a mechanism that is mainly hydrogeno-lytic. 

With linear alkanes having five or more carbon atoms, cyclization becomes possible as well as isomerization and hydrogenolysis. With n-pentane, cyclization is minimal and with n-hexane it does not exceed 25% in the range 470-570 K [6] with the latter molecule, isomerization predominates above 520 K. Product selectivities are particle-size sensitive, and Pt/SiC>2 catalysts having lower dispersion give more hydrogenolysis and cyclization. 

Silica-supported Ta hydride (=SiO)2Ta-H (93a) presents unusual properties in the activation of alkanes. It catalyzes the metathesis reaction of alkanes to give higher and lower molecular weight alkanes, and the hydrogenolysis of alkanes such as ethane to methane. This hydride also activates the C H bonds of cycloalkanes to form the corresponding surface metal-cycloaUcyl complexes, and catalyses the H/D exchange reaction between CH4 and CD4, prodncing the statistical distribution of methane isotopomers. ... 

V. Dufaud and J. M. Basset, Catalytic Hydrogenolysis at Low Temperature and Pressure of Polyethylene and Polypropylene to Diesels or Lower Alkanes by a Zirconium Hydride Supported on Silica-Alumina a Step Toward Polyolefin Degradation by the Microscopic Reverse of Ziegler-Natta Polymerization, Angew. Chem. Int. Ed., 37, 806-810 (1998). 

V. Dufaud and J.-M. Basset, Catalytic hydrogenolysis at low temperature and pressure of polyethylene and polypropylene to diesels or lower alkanes by silica-alumina a step toward polyolefin degradation by the microscopic reverse of Ziegler-Natta polymerization, Angew. Chem. Int.Ed., (1998) 37(6) 806-810. I. Nakamura and K. Fujimoto, Development of new disposable catalyst for waste plastics treatment for high quality transportation fuel. Catalysis Today, 27,175-179 (1996)... 

The ensemble control plays a role also in catalytic reforming on platinum or bimetallic catalysv. A good catalyst should have low activity for hydrogenolysis resulting in production of lower alkanes, and it should have a slow build-up of carbon overlayers to maintain stable activity for isomerization, dehydrogenation and dehydrocyciiration ... 

Figure 2.3 The effect of the degree of ruthenium dispersion on its catalytic activity for hydrogenolysis of cyclohexane to lower carbon number alkanes (28). (Solid points represent rates obtained after 10-20 minutes of exposure of the catalyst to reactants open points represent data obtained after 2 hours of exposure.) (Reprinted with permission from Academic Press, Inc.)... 

In addition to hydrogen chemisorption and ethane hydrogenolysis, the reactions of cyclohexane provide a useful chemical probe for investigating ruthenium-copper aggregates. On pure ruthenium, two reactions of cyclohexane are readily observed dehydrogenation to benzene and hydrogenolysis to lower carbon number alkanes. The product of the latter reaction is predominantly methane, even at very low conversions. 

The catalytic properties of this silica-supported tantalum hydrides are noteworthy. First, H/D exchange in D2/CH4 mixture is fast (0.2 mol/mol/s at 150 °C), which shows that these systems readily cleave and reform the C-H bonds of alkanes (Scheme 36(a)). Second, it also converts alkanes into its lower homologs and ultimately methane in the presence of H2 (hydrogeno lysis) at relatively low temperatures (150 °C). " The key step of carbon-carbon bond cleavage probably corresponds to an a-alkyl transfer on a Ta(m) intermediate followed by successive hydrogenolysis steps (Schemes 36(b) and 37). In the case of cycloalkanes, hydrogenolysis yields smaller cycloalkanes, but deactivation is very fast. This phenomenon has been associated with the rapid formation of cyclopentane and, thereby, with the formation of cyclopentadienyl derivatives videsupra Scheme 35), which are inactive for the hydrogenolysis of alkanes. 

Third, besides hydrogenolysis properties, the silica-supported tantalum hydride catalyzes the metathesis of alkanes, which transforms a given alkane into its higher and lower homolog (Scheme 36(c)). This reaction... 

In the context of alkanes, hydrogenolysis is the breaking of C—C bonds by the action of hydrogen, leading to alkanes of lower molar mass. It is not a reaction that is deliberately practised on a large scale, but it is a parasitic reaction that occurs in parallel with other useful reactions of alkanes to be considered in the next chapter. In order to learn how to avoid or minimise it, it becomes necessary to find out as much as possible about it, and this is most easily done with molecules containing only two to four carbon atoms. Alkanes can also be cracked by an acid-catalysed reaction on solid acids or acidic supports, but in this chapter we are solely concerned with reactions that proceed on purely metallic sites the cooperation of metallic and acidic sites in petroleum reforming will be briefly considered in Chapter 14. 

HYDROGENOLYSIS OF THE LOWER ALKANES ON SINGLE METAL CATALYSTS RATES, KINETICS, AND MECHANISMS... 

TABLE 13.1. Apparent Activation Energies (kJ mol ) for Hydrogenolysis (Eh) and for Isomerisation Ei) of the Lower Alkanes... 

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