Hydrogenolysis hydrocarbons

The simplest hydrocarbon hydrogenolysis reaction is that with ethane which can yield methane as the sole reaction product. On the other hand, with larger molecules a range of reaction products is possible since the adsorbed reactant may fragment at more than one C—C bond and, furthermore, even if only one such bond is broken in the reaction of a given molecule, the reactant will often contain more than one stereochemically distinguishable type of C—C bond. 

Various catalytic reactions are known to be structure sensitive as proposed by Boudart and studied by many authors. Examples are the selective hydrogenation of polyunsaturated hydrocarbons, hydrogenolysis of paraffins, and ammonia or Fischer-Tropsch synthesis. Controlled surface reactions such as oxidation-reduction reactions ° or surface organometallic chemistry (SOMC) " are two suitable methods for the synthesis of mono- or bimetallic particles. However, for these techniques. 

Pt-Re Coimpregnation on A1203, Si02, or Y-zeolite, reduced in H2./ - / - Hydrocarbon hydrogenolysis... 

Pt-Re chlorided with CC14, A1203 support Hydrocarbon hydrogenolysis... 

Besides the sextet-doublet model, a slightly modified form of the doublet model was also presented by the Soviet school of catalysis to explain the main features of hydrocarbon hydrogenolysis on ruthenium, osmium, iridium. 

The CO disproportionation reaction (37) has been used as the carbon deposition pathway in the majority of studies 48, 54, 56, 58-60). Carbon has also been deposited via hydrocarbon hydrogenolysis 54, 61-63) and, notably, via the FT synthesis reactions 35-37, 45). Particularly the initial studies involved nickel substrates ruthenium, cobalt, and iron were also studied. 

In Chapter 7 we discuss the unique seven-atom surface-ensemble cluster on the Fe(lll) surface (shown in Fig. 2. IOC) that is optimum for N2 activation. Early suggestions that surface ensembles with a particular number of atoms are necessary for a particular reaction to occur are deduced from alloying studies of reactive transition-metal surfaces, with catalytically inert metals such as Au, Ag, Cu or Sn . For example, the infrared spectrum of CO adsorbed on Pd shows the characteristic signature of CO adsorbed one-fold, twofold or three-fold to surface Pd atoms . Alloying Pd with Ag, to which CO only weakly coordinates, dilutes the surface ensembles. One observes a decrease of the three-fold and the two-fold coordinated CO and the one-fold coordinated CO becomes the dominant species. The effect of alloying a reactive metal with a more inert metal is especially dramatic when one compares hydrocarbon hydrogenation reactions with hydrocarbon hydrogenolysis reactionst . 

M-C interaction in the product as well as reactant state results in weaker interaction between reacting hydrocarbon fragments, so that barriers for the reaction in both directions increase. This in line with the observed lower rate of hydrocarbon hydrogenolysis observed for Pt as compared with Ni. On Ni the rate of C-H activation is lowered more than that for the CH -CHy bond cleavage reaction. Because of the stronger metal-carbon bond of Pt than Ni, the rate of methane formation by recombination of adsorbed hydrogen with adsorbed CH3 will be lower on the former. 

Hydrogenolysis Process. Patty alcohols are produced by hydrogenolysis of methyl esters or fatty acids ia the presence of a heterogeneous catalyst at 20,700—31,000 kPa (3000—4500 psi) and 250—300°C ia conversions of 90—98%. A higher conversion can be achieved using more rigorous reaction conditions, but it is accompanied by a significant amount of hydrocarbon production. 

The catalysts with the simplest compositions are pure metals, and the metals that have the simplest and most uniform surface stmctures are single crystals. Researchers have done many experiments with metal single crystals in ultrahigh vacuum chambers so that unimpeded beams of particles and radiation can be used to probe them. These surface science experiments have led to fundamental understanding of the stmctures of simple adsorbed species, such as CO, H, and small hydrocarbons, and the mechanisms of their reactions (42) they indicate that catalytic activity is often sensitive to small changes in surface stmcture. For example, paraffin hydrogenolysis reactions take place rapidly on steps and kinks of platinum surfaces but only very slowly on flat planes however, hydrogenation of olefins takes place at approximately the same rate on each kind of surface site. 

Reductive removal of fluorme from alk I fluorides requires a potent reducing agent and so is not noimally encountered However, hydrogenolysis of an unacuvated carbon-fluorine bond in, for example, 3 (3-fluorocholestane has been efficiently accomplished in 88% yield with a solution of potassium and dicyclohexyl 18 crown-6 in toluene at 25 C [/] Similarly, sodium naphthaiene in tetrahydrofuran converts 6 tluorohexene-1 and 1-fluorohexane to hydrocarbons in 50% yield at 25 C over a 7-h penod [2]... 

Hydrogen cyanide reactions catalysts, 6,296 Hydrogen ligands, 2, 689-711 Hydrogenolysis platinum hydride complexes synthesis, 5, 359 Hydrogen peroxide catalytic oxidation, 6, 332, 334 hydrocarbon oxidation iron catalysts, 6, 379 reduction... 

Hydrogenolysis of esters to aldehydes or alcohols needs high temperatures and high pressures. Moreover, it leads to the formation of acids, alcohols, and hydrocarbons. In contrast, bimetallic M-Sn alloys (M = Rh, Ru, Ni) supported on sihca are very selective for the hydrogenolysis of ethyl acetate into ethanol [181]. For example while the selectivity to ethanol is 12% with Ru/Si02, it increases up to 90% for a Ru-Sn/Si02 catalyst with a Sn/Ru ratio of 2.5 [182]. In addition, the reaction proceeds at lower temperatures than with the classical catalysts (550 K instead of temperatures higher than 700 K). The first step is the coordination of the ester to the alloy (Scheme 46), and most probably onto the tin atoms. After insertion into the M - H bond, the acetal intermediate decomposes into acetaldehyde and an ethoxide intermediate, which are both transformed into ethanol under H2. 

In discussing the way in which hydrogenolysis occurs, it needs to be recognized at the outset that more than one reaction pathway is possible, and their relative importance depends both on hydrocarbon structure and on the nature of the catalyst. 

In discussing the reaction pathways, we believe that the general evidence leads to the conclusion that hydrogenolysis proceeds via adsorbed hydrocarbon species formed by the loss of more than one hydrogen atom from from the parent molecule, and that in these adsorbed species more than one carbon atom is, in some way, involved in bonding to the catalyst surface. In the case of ethane, this adsorption criterion is met via a 1-2 mode or a v-olefin mode. Mechanistically it is difficult to see how the latter could be involved in C—C bond rupture in ethane. With molecules larger than ethane, other reaction paths are possible One is via adsorption into the 1-3 mode, and another involves adsorption as a ir-allylic species. 

With catalysts such as nickel and rhodium for which it has been shown that 1-2 hydrogenolysis is seriously competitive with 1-3 hydrogenolysis, there is no need to assume that ir-olefin/allyl hydrogenolysis occurs (but neither can it with certainty be excluded). This conclusion is likely to be true for other catalysts such as cobalt and iron which also favor complete hydrocarbon fragmentation to methane. 

Hydrogenolysis reactions of hydrocarbons on metal catalysts have been investigated in some detail. Extensive studies have been conducted on both alkanes and cycloalkanes. While a number of questions still remain with regard to mechanistic and kinetic details of the reactions, the general features seem reasonably clear. 

A further consideration in the mechanism of hydrogenolysis of hydrocarbons is the structure of the chemisorbed species that undergoes scission of the carbon-carbon bond. In the case of ethane hydrogenolysis, one readily visualizes bonding of the two carbon atoms in the species C2H to adjacent metal surface atoms. It is convenient to refer to the adsorbed C2H as a... 

The first reported work on the kinetics of hydrogenolysis reactions of simple hydrocarbons appears to be that of Taylor and associates at Princeton (2-4, 14, 15), primarily on the hydrogenolysis of ethane to methane. The studies were conducted on nickel, cobalt, and iron catalysts. More recently, extensive studies on ethane hydrogenolysis kinetics have been conducted on all the group VIII metals and on certain other metals as well (16,28-83). 

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