The demand for diesel fuel in the US has been increasing because diesel engines have higher fuel economy and produce less greenhouse gas than equivalent gasoline engines.  An important requirement in diesel production is to reduce the amount of aromatic compounds in petroleum, which have poor ignition quality (very low cetane numbers, or CN) and tend to form soot during burning.  Traditional hydrocracking produces a large fraction of low-molecular weight products that cannot be used in diesel fuel.  A better approach is to convert aromatics into saturated cyclic compounds, followed by selective ring opening (SRO).  Iridium possesses high ring-opening activity, but it favors branched isoparaffin products with low CN [1].  Our recent experiments show that the product CN can be influenced by the support and metal dispersion of the Ir catalysts, the degree of substitution of the reactants, as well as the reaction conditions [2].

Existing literature postulates that the dicarbene mechanism, which cleaves secondary-secondary C-C bonds, is dominant over those that open rings at substituted carbon positions (e.g., the metallocyclobutane mechanism), but neither the details of these mechanisms nor the reason why the dicarbene mechanism is favored on Ir is understood.  To address these questions from a different angle, we have performed density functional theory (DFT) calculations to study the reactivity of several cyclohexane species, including cyclohexane, methylcyclohexane, and dimethylcyclohexane, on different Ir surfaces.  The focus of this talk will be on the effects that substitution and surface morphology have on the outcome of the ring-opening reaction.  Our findings will provide clues for enhancing the ability of Ir catalysts or devising new catalysts to more selectively open substituted C-C bonds, thereby increasing the CN of the resultant hydrocarbons.

This research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Division of Scientific User Facilities, U.S. Department of Energy.

[1] G. B. McVicker et al., Selective ring opening of naphthenic molecules. J. Catal. 2002, 210, 137.

[2] P. T. Do, W. E. Alvarez, D. E. Resasco, Ring opening of 1,2- and 1,3-dimethylcyclohexane on iridium catalysts. J. Catal. 2006, 238, 477.