(279b) Exocyclic and Endocyclic C-C Bond Cleavage – Mechanism and Site Requirements

The success of metal-catalyzed hydrocarbon ring
opening for fuel upgrading depends on the suppression of dealkylation and multiple
hydrogenolysis, both of which form lower value products. Recent reports state that
turnover rates and selectivities for ring opening may be improved by using bimetallic
clusters [1] or bifunctional catalysts that promote ring contraction pathways via
solid acids [2, 3], without rigorous mechanistic interpretations or clear
implications for the design of more active and selective catalyst sites. Here, we
show how the level of saturation of chemisorbed intermediates and the coverage
of chemisorbed hydrogen atoms, [H*], determine the probability of breaking either
exocyclic or endocyclic C-C bonds, and hence, turnover rates and selectivities.
We also demonstrate how ring opening turnover rates can be increased on
monofunctional Ir catalysts, with little loss of selectivity, by concurrent
increases in temperature and H₂ pressure to maintain the degree of hydrogenation
of the intermediates.

The
hydrogen content of gas-phase cycloalkanes and acyclic alkanes is equilibrated at
all conditions demonstrating that the surface is covered with an equilibrated
mixture of H* and cyclic and acyclic hydrocarbons whose degree of hydrogenation
is determined by chemical equilibrium with the prevalent H₂ pressure.
Cyclic surface intermediates undergo initial C-C bond cleavage at endocyclic and
exocyclic positions, resulting in the formation of ring opening and
dealkylation products, respectively, with individual rate constants for surface
intermediates at every level of hydrogenation. Multiple hydrogenolysis products
form primarily by subsequent C-C bond cleavage of cyclic, dealkylation products,
because acyclic species formed by ring opening are present at very low surface concentrations
due to low dehydrogenation and adsorption equilibrium constants. Figure 1a
shows that turnover rates for 1,3-dimethylcyclohexane (1,3-DMCH) conversion initially
increase with H₂ pressure because of a concomitant increase in the hydrogenation
of adsorbed species leading to a greater fraction of intermediates with
significant rate constants.  Higher H₂ pressures lead to near-saturated
intermediates which bind to the surface weakly such that H* ultimately
displaces all reactive hydrocarbon species and decreases turnover rates for all
reaction pathways.  As shown in Figure 1a, the selectivity for exocyclic C-C
bond rupture, described here as the ratio (β_Exo-Endo =(r_Exo.)/(r_Endo);
 r_Xis the rate of reaction at position X),
decreases monotonically with increasing H₂ pressure because the
aromatic character of highly unsaturated intermediates preferentially stabilizes
endocyclic bonds, thus decreasing  ring opening turnover rates.

Figure 1.  Turnover rates (●) and β_Exo-Endo
(♦), the ratio of the rates of exocyclic
and endocyclic C-C bond rupture.  (a) shows the effect of H₂
pressure for small Ir particles, <d_TEM>  = 0.6 nm, at 20
kPa 1,3-DMCH and 593 K. (b) displays the dependence on Ir dispersion at 3.4 MPa
H₂, 110 kPa 1,3-DMCH and 593 K.

            Turnover rates
and selectivity also depend on the structure of the cycloalkane reactants and
the metal dispersion.  The degree of saturation depends on the C-H bond
strength in reactants. With increasing substitution (cyclohexane →
1,3,5-trimethylcyclohexane), the weaker C-H bonds at tertiary carbon atoms favor
dehydrogenation and lead to more unsaturated surface species. Consequently, alkyl
substitution of the ring decreases turnover rates and ring opening
selectivities.  Figure 1b shows that turnover rates for 1,3-DMCH conversion
decrease with increasing Ir dispersion, however, the selectivity for endocyclic
C-C bond cleavage is significantly higher on smaller clusters.  This
observation correlates with the known tendency of low-index planes, prevalent
on large clusters, to cleave terminal C-C bonds which favors exocyclic C-C bond
scission.  The dependence of selectivity on particle size suggests that low-coordinate
sites lead to a disproportionate increase the adsorption constant of saturated
species in comparison to highly unsaturated species.

The
authors acknowledge financial support from the ExxonMobil Research and
Engineering Co. and technical discussions with Drs. Stuart L. Soled and Guang
Cao.

References

1.     
P. Samoila, M.
Boutzeloit, C. Especel, F. Epron, P. Marecot, Appl. Cat. A 369, 104
(2009)

2.     
G.B. McVicker et al., J. Catal. 210,
137 (2002)

3.     
J.-W. Park, K.
Thomas, J. van Gestel, J.-P. Gilson, C. Collet, J.-P. Dath, M. Houalla, App.
Cat. A: General 388, 37 (2010)