Hydrogenolysis reaction mechanism

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. 

The combined information gathered from kinetic studies,184 in situ high-pressure NMR experiments,184,185,195 and the isolation of intermediates related to catalysis, leads to a common mechanism for all the hydrogenolysis reactions of (102)-(104) and other thiophenes catalyzed by triphos- or SULPHOS-rhodium complexes in conjuction with strong Bronsted bases. This mechanism (Scheme 41) involves the usual steps of C—S insertion, hydrogenation of the C—S inserted thiophene to the corresponding thiolate, and base-assisted reductive elimination of the thiol to complete the cycle.184 185 195-198... 

In the absence of radical traps, the radical R is converted immediately into the carbanion R by an ECE or a DISP mechanism, according to the distance from the electrode where it has been formed. B is a strong base (or nucleophile) that will react with any acid (or electrophile) present. Scheme 2.21 illustrates the case where a proton donor, BH, is present. The overall reduction process then amounts to a hydrogenolysis reaction with concomitant formation of a base. This is a typical example of how singleelectron-transfer electrochemistry may trigger an ionic chemistry rather than a radical chemistry. This is not always the case, and the conditions that drive the reaction in one direction or the other will be the object of a summarizing discussion at the end of this chapter (Section 2.7). 

Catalytic hydrogenolysis (continued) M0O3-AI2O3 catalyst, 29 258-269 relative reactivity, 29 255-257 schematic model, 29 254 diphenylmethane kinetics, 29 241-243 reaction mechanism, 29 267 Catalytic oxidation,... 

The reaction mechanism for the catalytic hydrogenolysis of asym DAMs is shown in Fig. 9 for DPM. From the mechanism proposed in Fig. 9, the... 

Finally, the active sites for the hydrogenolysis of asym DAM are Mo(IV) species that originated from the reduction of the octahedral Mo(VI) species. The adsorption of the aryl group occurs on the coordinatively unsaturated molybdenum sites, which have acidic properties this fact, in turn, leads to the reaction mechanism of the interaction between the active species and the substrates. 

At 170°C, under less than 12bar of H2, the reaction of AsPhs with Ni/Al203 can be broken down into two steps. The first is fast and leads to the ill-crystalhzed NiAso.45 bimetaUic nanoparticles its rate is limited by the diffusion of arsenic atom into the metallic particles. The second, much slower, leads to well-crystalhzed NiAs nanoparticles of alloy. Its rate is hmited hydrogenolysis of the As-Ph bond. Scheme 2.45 shows the reaction mechanism. 

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). 

This represents one pathway to the formation of methane, a knovm by-product in iridium catalysed methanol carbonylation. The hydrogenolysis reaction was severely retarded by the presence of excess CO, indicating a mechanism involving initial dissociation of CO from [MeIr(CO)2l3] , prior to activation of H2. The mechanism therefore resembles that for hydrogenolysis of Rh acetyl complexes in hydroformylation. 

A close look at the hydrogenolysis reaction of the Cp2 SmCH(TMS)2 complex with H2Si(SiMe3)2 shows that the two compounds do not react directly. The kinetic profile shows an S-shape form with an induction time. This shape is consistent with a second-order autocatalytic mechanism in which the reaction is catalyzed by a product or an intermediate. Indeed, the induction period was completely eliminated by addition of catalytic amounts of H2, [Cp2 SmH]2, or of the complex [Cp2 Sm (p-H) (p-C CsMe SmCp ], which is a decomposition product of the [Cp2 SmH]2. Thus, two possible mechanistic pathways (Schemes 3 and 4) have been proposed to explain the hydrogenolysis reaction. 

In order to apply ZVI-based dechlorination for site remediation, we have to be sure that no toxic reaction products are produced in the end, and toxic intermediates, if there are any, are degradable. Product distribution is controlled by reaction mechanisms. For the reduction of chlorinated methanes such as carbon tetrachloride (CT), hydrogenolysis, where chlorines are sequentially replaced by hydrogen, takes place in the presence ofZVI (Vogel etal., 1987 Matheson andTratnyek, 1994 Glod etal., 1997) ... 

Bonn6 [1] studied reaction kinetics of tetraphenylporphyrin model compounds over several catalysts and concluded that hydrogenation and hydrogenolysis occur on different sites. Therefore the reaction mechanism of VO-TPP hydrodemetallisation is described by a two-site model. It was also found that a Langmuir-Hinshelwood type of kinetics applies with small inhibition by hydrogen. 

Amada Y, Shinmi Y, Koso S, Kubota T, Nakagawa Y, Tomishige K. Reaction mechanism of the glycerol hydrogenolysis to 1,3-propanediol over Ir-ReOx/SiC>2 catalyst. Appl Catal B Environ. 2011 105 117-27. 

Under pressure of CO and H2, the cobalt catalyst precursor is transformed into cobalt carbonyl hydride, HCo(CO)4. The main steps of the reaction mechanism, first elucidated by D. S. Breslow and R. F. Heck, involve (a) /3-hydrogen transfer to the coordinated olefin, (b) the insertion of CO to form an acyl intermediate, and (c) the hydrogenolysis of the acyl, with formation of the aldehyde product ... 

In this chapter, recent advances in our understanding of catalytic fluorination under heterogeneous conditions are surveyed from the standpoint of catalyst properties, including developments based on the use of mixed metal fluorides having different structural types, and reaction mechanisms. Much of the newer work has been the result of the need to replace chlorofluorocarbons (CFCs) by alternatives, hydrofluorocarbons (HFCs) or, more controversially, hydrochlorofluorocarbons (HCFCs), following adoption of the Montreal and successor Protocols [2,3]. Where relevant, aspects of catalytic hydrogenolysis, where fluorides have been used as replacement supports in the conventional palladium/carbon catalysts, and isomerization reactions are included. 

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