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_X is 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)