(125f) Rates and Selectivities for Carbon-Carbon Bond Cleavage in Cyclic and Acyclic Hydrocarbons Catalyzed by Metal Clusters

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.