Catalysis over Supported Metals. III. Comparison of Metals of Known Surface Area for Ethane Hydrogenolysis

Sinfelt, J. H.; Taylor, William F.; Yates, David J.

doi:10.1021/j100885a016

Cited by 120 publications

(44 citation statements)

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“…Specifically, rates increased with increasing H 2 pressure for H 2 /C n H 2n ratios below 40, reached a maximum at 60−65 H 2 /C n H 2n , and then decreased at higher H 2 pressures with a functional dependence described by r ≈ (H 2 ) −λ (where λ > 0). These trends are similar to those previously reported for the hydrogenolysis of ethane, 17,21,23,24,29,30 n-alkanes, 16,29−31 and isoalkanes. 18,30 These λ values differ among cyclohexane reactants (1.5 < λ < 3.0, Table 2; uncertainties of ±0.2−0.5) but become nearly constant at the highest H 2 /C n H 2n ratios (250−275), indicating that λ depends on the number and structure of the alkyl substituents in the cyclohexane ring (methyl, isopropyl, and npropyl).…”

Section: Introductionsupporting

confidence: 91%

Catalytic Ring Opening of Cycloalkanes on Ir Clusters: Alkyl Substitution Effects on the Structure and Stability of C–C Bond Cleavage Transition States

2015

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Rates and locations of C−C cleavage during the hydrogenolysis of alkyl-cyclohexanes determine the isomeric products of ring opening and the yield losses from dealkylation. Kinetically relevant transition states for C−C rupture form by sequential quasi-equilibrated dehydrogenation steps that break C−H bonds, form C−metal bonds, and desorb chemisorbed H atoms (H*) from H*-covered surfaces. Activation enthalpies (ΔH ⧧ ), entropies (ΔS ⧧ ), and the number of H 2 (g) formed with transition states are larger for 3 C− x C rupture than for 2 C− 2 C or 2 C− 1 C cleavage for all cycloalkane reactants and Ir cluster sizes. 3 C− x C rupture transition states bind to surfaces through three or more C atoms, whereas those for less-substituted 2 C− 2 C bonds cleave via α,β species bound by two C atoms. 3 C− x C rupture involves larger ΔH ⧧ than 2 C− 2 C and 2 C− 1 C because the former requires that more C−H bonds cleave and H* desorb than for the latter two. These endothermic steps are partially compensated by C−metal bond formation, whereas the formation of additional H 2 (g) gives larger ΔS ⧧ . C−C rupture transition states for cycloalkanes have less entropy than those for C−C bonds in acyclic alkanes of similar size because C 6 rings decrease the rotational and conformational freedom. ΔH ⧧ values for all C−C bonds in a given reactant decrease with increasing Ir cluster size because the coordination of exposed metal atoms influences the stabilities of the H* atoms that desorb more than those of the transition states. ΔH ⧧ for 3 C− x C cleavage is more sensitive to cluster size because their transition states displace more H* than those for 2 C− 2 C or 2 C− 1 C bonds. These data and their mechanistic interpretation provide guidance for how surface coordination, reaction temperatures, and H 2 pressures can be used to control ring-opening selectivities toward desirable products while minimizing yield losses. These findings are consistent with trends for the hydrogenolysis of acyclic isoalkanes and seem likely to extend to C−X bond cleavage (where X = O, S, and N atoms) reactions during hydrotreating processes.

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Section: Introductionsupporting

confidence: 91%

Catalytic Ring Opening of Cycloalkanes on Ir Clusters: Alkyl Substitution Effects on the Structure and Stability of C–C Bond Cleavage Transition States

2015

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“…The simplest example is the reaction of ethane with hydrogen to yield methane, the kinetics of which were investigated originally on nickel (Morikawa et al, 1936), and subsequently on cobalt (Taylor and Taylor, 1939) and iron (Cimino et al, 1954). More recently, studies of the kinetics of ethane hydrogenolysis have been extended to platinum (Sinfelt, 1964b;Sinfelt et al, 1965), other Group VIII noble metals (Sinfelt and Yates, 1967;, and rhenium (Yates and Sinfelt, 1969). A comparison of metals as hydrogenolysis catalysts reveals a striking variation in activity, even among such similar metals as the noble metals of Group VIII.…”

Section: Hydrogenolysismentioning

confidence: 99%

Catalytic specificity

Sinfelt

1973

AIChE Journal

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SCOPECatalytic specificity refers to the particular ability of a substance or closely related group of substances to catalyze a given type of chemical transformation. Identifying the type of catalyst which is good for a particular reaction is a matter of great practical importance. In the area of heterogeneous catalysis, to which this review is limited, there are few, if any, examples where the factors determining catalytic specificity are understood in detail. However, much progress has been made in establishing patterns of variation in catalytic behavior from one substance to another for various types of reactions. In particular, broad relationships are often evident between the catalytic activity of an element or its compounds and the position of the element in the periodic table. These relationships introduce a measure of order into heterogeneous catalysis and will be useful in the development of a more fundamental understanding of the subject.The present review considers a variety of classes of reactions from the standpoint of assessing patterns of variation in behavior among catalysts. The types of reactions considered include hydrogenation, hydrogenolysis, isomerization, ammonia synthesis and decomposition, oxidation, and the decomposition of alcohols and organic acids. Relationships between catalytic activities and the chemisorption properties of reactants are discussed repeatedly. Both metal and oxide type catalysts are considered. However, most of the emphasis is on metals, for which more extensive data comparing specific catalytic activities are available.

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“…Among these disorders are Parkinson's disease (1), Huntington's and hyperthyroid chorea (2), Gilles de la Tourette's syndrome (3), and the schizophrenias (4). It has also been suggested that catecholamines may play a role in affective disorders (5).…”

Section: Discussionmentioning

confidence: 99%

“…ods for determining the degree of dispersion of supported metals on a routine its basis represents a major advance in the characterization of catalysts (5,6). Earlier studies by Emmett (7).…”

mentioning

confidence: 99%

“…This quantity increases contin-Sts uously with decreasing Rh concentration, which corresponds to increasing I catalyst, the dispersion of the Rh. With a stoichiomethe ratio of the try of one H atom adsorbed per surface the surface to metal atom (5,9), the quantity H/M is a atoms present direct measure of the degree of disvary widely, persion of the metal. The use of selective al form of the chemisorption is especially important for i-Cu alloys are catalysts in which the degree of metal wders with sur-dispersion is extremely high (approach-1 square meter ing unity), since it is very difficult or onding to dis-impossible to obtain quantitative infor-).001.…”

mentioning

confidence: 99%

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Heterogeneous Catalysis: Some Recent Developments

Sinfelt

1977

Science

103

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In recent years major progress has been made in the area of heterogeneous catalysis by metals. Much has been learned about the nature of metal catalysts and of catalytic phenomena on metals. Characteristic patterns of catalytic behavior among the metallic elements have been established for certain classes of reactions, and these patterns provide a first step toward a more comprehensive understanding of catalytic specificity. Studies on metal alloys and related bimetallic catalysts have revived interest in a geometric factor in surface catalysis to complement the traditional electronic factor. Closely related to this geometric factor is the discovery that selectivity, rather than activity alone, is a major factor in reactions on bimetallic catalysts. Concurrent with progress in understanding how catalysts work, advances are also being made in the development of new catalyst systems, examples of which are the bimetallic (or polymetallic) cluster catalysts. Research in this area provides an example of how advances in catalyst technology can be realized within a framework of fundamental research on catalytic phenomena (38).

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Catalysis over Supported Metals. III. Comparison of Metals of Known Surface Area for Ethane Hydrogenolysis

Cited by 120 publications

References 0 publications

Catalytic Ring Opening of Cycloalkanes on Ir Clusters: Alkyl Substitution Effects on the Structure and Stability of C–C Bond Cleavage Transition States

Catalytic Ring Opening of Cycloalkanes on Ir Clusters: Alkyl Substitution Effects on the Structure and Stability of C–C Bond Cleavage Transition States

Catalytic specificity

Heterogeneous Catalysis: Some Recent Developments

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