“…In addition to finding that multiple-exchange was more likely at high temperatures (consistent with Kemball’s work above), the authors also found that lower D 2 pressures and larger crystallites promoted multiple exchange; this was ascribed to having more open sites to activate additional C–H bonds . Allylic and pi-bonded intermediates have been proposed as intermediates, similar to studies above with the chiral alkanes, and the mechanism by which multiple exchange occurs has been studied in depth. − More detailed studies on the exchange between cyclopentane and D 2 on Pd/alumina exhibited five product distributions, which the authors suggested indicate five distinguishable processes. , They assigned this distribution to different sets of surface sites; these results show that multideuteration, and thus the propensity to have a second C–H activation occur, is dependent not only on temperature, metal, and crystallite size but also on the facet and crystal termination (e.g., terrace, face, edge, step, corner) at which exchange occurs. This is not surprising because many catalysis studies show dependency of crystallite size and facet …”
Section: H/d Exchange On Metal Catalystssupporting
The catalytic exchange of hydrogen and deuterium (H/D exchange) in light alkanes has been studied for almost a century. While alkanes and their C−H bonds are relatively inert, H/D exchange studies have shown that a large number of materials can catalytically activate these bonds. These studies helped elucidate the mechanisms by which alkane C−H bonds interact with catalytic materials. This Review serves to highlight this area of research, focusing on two main classes of heterogeneous materials, metals and metal oxides, and their trends in reactivities and selectivities are described in detail. Furthermore, the ability of these materials to carry out C−H bond activation and H/D exchange catalysis is compared with that of molecular organometallic complexes, and the mechanistic relationships and similarities in these processes are proposed.
“…In addition to finding that multiple-exchange was more likely at high temperatures (consistent with Kemball’s work above), the authors also found that lower D 2 pressures and larger crystallites promoted multiple exchange; this was ascribed to having more open sites to activate additional C–H bonds . Allylic and pi-bonded intermediates have been proposed as intermediates, similar to studies above with the chiral alkanes, and the mechanism by which multiple exchange occurs has been studied in depth. − More detailed studies on the exchange between cyclopentane and D 2 on Pd/alumina exhibited five product distributions, which the authors suggested indicate five distinguishable processes. , They assigned this distribution to different sets of surface sites; these results show that multideuteration, and thus the propensity to have a second C–H activation occur, is dependent not only on temperature, metal, and crystallite size but also on the facet and crystal termination (e.g., terrace, face, edge, step, corner) at which exchange occurs. This is not surprising because many catalysis studies show dependency of crystallite size and facet …”
Section: H/d Exchange On Metal Catalystssupporting
The catalytic exchange of hydrogen and deuterium (H/D exchange) in light alkanes has been studied for almost a century. While alkanes and their C−H bonds are relatively inert, H/D exchange studies have shown that a large number of materials can catalytically activate these bonds. These studies helped elucidate the mechanisms by which alkane C−H bonds interact with catalytic materials. This Review serves to highlight this area of research, focusing on two main classes of heterogeneous materials, metals and metal oxides, and their trends in reactivities and selectivities are described in detail. Furthermore, the ability of these materials to carry out C−H bond activation and H/D exchange catalysis is compared with that of molecular organometallic complexes, and the mechanistic relationships and similarities in these processes are proposed.
“…The catalytic conversion of olefins under reducing conditions is one of the simplest and most studied catalyzed reactions. − When promoted by solid transition-metal catalysts, the ensuing hydrogenation, dehydrogenation, H−D exchange, and double bond isomerization reactions are accounted for by the so-called Horiuti−Polanyi mechanism (Scheme ). , This deceivingly simple scheme, which is based on a series of stepwise hydrogenation−dehydrogenation steps, can be used to explain many experimental observations, but also hides a number of complications associated with the regio- and stereospecificity of the β-hydride elimination step from the intermediate surface alkyl species. For instance, it has recently been pointed out by us that the exchange of the hydrogen atoms around a CC double bond by deuterium is accompanied by a cis−trans interconversion .…”
Section: Introductionmentioning
confidence: 99%
“…Some trends have been identified by past surface-science studies in connection with this issue, − namely: (1) β-hydride elimination takes place preferentially from the more substituted β-carbons; (2) the desorption temperature of the resulting alkenes shifts to lower temperature with either decreasing chain length or increasing number of substitutions; and (3) the alkene yield is higher (relative to alkane production by the competing reductive elimination reaction) for branched alkyls compared to linear alkyls. Finally, there is the potential of a competing mechanism being operative in olefin conversion involving an initial dehydrogenation at an allylic position. − , Indeed, some allyl intermediates are particularly stable, especially those based on cyclic species, − and are therefore likely to be involved in either hydrogenation or selective oxidation catalytic processes. 1 Horiuti−Polanyi Mechanism…”
The thermal chemistry of 1-methyl-1-cyclopentene (1MCp) and methylene cyclopentane (MeCp) was
investigated on clean and hydrogen- and deuterium-predosed Pt(111) single-crystal surfaces by temperature-programmed desorption and reflection−absorption infrared spectroscopy. It was found that MeCp isomerizes
easily to 1MCp but that the reverse reaction does not occur, at least under our experimental conditions. The
MeCp to 1MCp isomerization is aided by the presence of coadsorbed hydrogen, and occurs through the
formation of a common 1-methyl-1-cyclopentyl (1MCp−Pt) surface intermediate; that intermediate then
undergoes β-hydride elimination selectively at the ring position to form the 1MCp product. In addition to
this isomerization, both 1MCp and MeCp also dehydrogenate on the Pt(111) surface to form a
methylcyclopentadiene species, at 325 and 350 K, respectively. A small amount of benzene desorption is
detected above 500 K with both reactants, indicative of a ring enlargement reaction. No evidence for the
activation of any of the allylic hydrogens was obtained in either molecule.
“…Other workers have proposed concerted " hydrogen switch " mechanisms in which dissociative adsorption of olefin and hydrogen atom addition occurred simultaneously.~9 9 In 1962, it was reported that the dissociation of adsorbed olefin to give adsorbed n-allylic intermediates occurred readily as one of a series of consecutive dissociative steps in the exchange of substituted cyclopentanes with deuterium catalyzed by palladium films. 10 It was suggested that analogous processes might occur in other reactions, and butene isomerization was quoted as an example. Mechanism (2) shows how isomerization may proceed by an " abstraction-addition " mechanism, the abstraction step being the dissociative adsorption of olefin to give adsorbed hydrogen and an adsorbed .n-allylic species.19 11…”
Cobalt powder and alumina-supported cobalt catalyze the reaction :Molecular hydrogen is not required as a reactant. Isomerization takes place at temperatures above 75"C, no other isomers of pentadiene have been detected, and self-hydrogenation is negligible. The isomerization of isotopically labelled cis-penta-l,3-diene at 160°C is described in detail. The results are consistent with a mechanism in which the first step is the dissociation of the dioleh by loss of hydrogen from the methyl group, and the overall process is the 1 : 5-transfer of hydrogen.
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