287111 Rates and Selectivities for Carbon-Carbon Bond Cleavage in Cyclic and Acyclic Hydrocarbons Catalyzed by Metal Clusters

Monday, October 29, 2012: 4:55 PM
319 (Convention Center )
David W. Flaherty, Department of Chemical Engineering, University of California at Berkeley, Berkeley, CA and Enrique Iglesia, Department of Chemical Engineering, University of California, Berkeley, Berkeley, CA

Rates and Selectivities for Carbon-Carbon Bond Cleavage in

Cyclic and Acyclic Hydrocarbons Catalyzed by Metal Clusters

David W. Flaherty and Enrique Iglesia,

University of California at Berkeley, Berkeley, CA

Selective ring opening of arenes requires the cleavage of specific endocyclic C-C bonds without significant losses via dealkylation or multiple hydrogenolysis reactions.   This study shows that ring opening selectivities depend largely on the location of the first C-C bond rupture, whose rates are proportional to surface coverages of active intermediates derived from (cyclo)alkane reactants via equilibrated molecular adsorption and dehydrogenation steps that form a thermodynamic distribution of unsaturated surface species.  The kinetically-relevant rate constants reflect large enthalpic barriers (>200 kJ mol-1), which are sensitive to the degree of substitution at the C-C bond, and large entropic gains (200 – 400 J mol-1 K-1) that reflect the net desorption of H2 molecules upon formation of the relevant transition states. 

On metal clusters covered by chemisorbed hydrogen during catalysis, enthalpy differences between transition states that mediate the cleavage of specifically substituted C-C bonds (e.g., 3C-2C or 2C-2C, where the superscript describes the number of connected C-atoms) and gas-phase alkanes do not depend on the ancillary structure of the specific alkane reactant or on the size of metal clusters and their concomitant changes in surface atom coordination. Enthalpies of activation increase with the degree of substitution of C-C bonds because of the strain induced by the required coordination of hindered C-atoms to surfaces at the transition states.  Enthalpic barriers for cleaving the less hindered C-C bonds, such as 2C-2C and 2C-1C bonds within n-alkanes (C4-C10), are similar.  Consequently, activation entropies, described by statistical mechanics treatments of the prevalent early transition states (reactant-like, α,β-bound hydrocarbons), dictate the turnover rates and selectivities for C-C bond cleavage. 

For isoalkanes and cycloalkanes, less substituted C-C bonds preferentially cleave because of enthalpic differences among the relevant transition states. Overcoming activation barriers for more substituted C-C bonds requires greater entropy gains than for less hindered C-C bonds, which necessitates the release of a larger number of hydrogens before  C-C bond rupture, and therefore occurs via more dehydrogenated transition states.  Accordingly, higher H2 pressures favor cleavage of less substituted C-C bonds within isoalkanes and cycloalkanes by increasing the degree of saturation within the pool of reactant-derived adsorbed species.  For these  reasons, H2 pressure does not influence the position of C-C bond cleavage in n-alkanes, because all C-C bonds have equal activation enthalpies and therefore similarly dehydrogenated transition states.  Activation barriers for cleaving a given type of C-C bond within substituted cyclohexanes are identical to those for C-C bonds of similar substitution in acyclic alkanes.  C5 rings, however, show lower activation barriers for endocyclic C-C bonds (by 50 kJ mol-1) than similarly substituted C-C bonds in acylic alkanes or C6 rings suggesting that ring strain destabilizes C-C bonds in C5 rings relative to the transition state for endocyclic C-C bond cleavage, leading to higher ring opening rates and selectivities.  Activation enthalpies for C-C bond cleavage are independent of metal cluster size, but higher activation entropies cause larger clusters (~10 nm) to give much higher hydrogenolysis turnover rates  than smaller clusters (< 1 nm).  These entropic differences reflect lower metal-carbon bond energies on surfaces with atoms of higher coordination, which prevail on  larger clusters, and which lead, in turn, to transition states with greater translational and vibrational freedom.  Large clusters also give higher selectivities for terminal C-C bond hydrogenolysis, because the flatter nature of low-index planes preferentially exposed on large clusters sterically hinders rotations at transition states.  As a result, large clusters preferentially decrease the rotational entropy of transition states for non-terminal C-C bond cleavage with respect to those for cleavage of  terminal C-C bonds.

Hydrogenolysis rate constants for similarly substituted C-C bonds in alkanes (C2-C10) differ significantly (by a factor of 107).  Among n-alkanes, differences in rate constants solely reflect  differences in activation entropies and measured values agree with entropy estimates from statistical mechanics for early transition states.  Enthalpy barriers for hydrogenolysis increase with C-C bond substitution and reach barriers up to 280 kJ mol-1 for highly substituted 3C-3C bonds.  Catalytic surfaces overcome these large activation barriers for C-C bond cleavage by entropy gains associated with the desorption of hydrogen upon formation of dehydrogenated intermediates involved in the relevant transitions states.

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See more of this Session: Alpha Chi Sigma Award for Enrique Iglesia III
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