(469c) Theoretical Investigation of the Ring Opening Mechanism of Cyclohexanes on Ir Surfaces

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, which have poor ignition quality (low cetane numbers, or CN) and 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 would be to convert aromatics to saturated cyclic compounds, followed by selective ring opening (SRO). Although catalyst support, metal dispersion, and reaction conditions all make a difference, iridium compared to other metals possesses much higher activity for ring-opening vs. cracking, but it tends to favor branched isoparaffins, which have lower CN than linear paraffins [1-4]. The dicarbene mechanism, which cleaves secondary-secondary C-C bonds, is postulated to dominate 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 by Ir is fully understood.

The focus of this talk will be on the effects that surface morphologyand substitution have on the ring opening reaction on Ir. We are theoretically investigating the ring opening mechanism of cyclohexane and substituted cyclohexanes on the close-packed Ir(111) and stepped Ir(211) surfaces as a basis for understanding the fundamental ability of Ir to promote SRO. We perform detailed density functional theory calculations to identify the more facile reaction channels for each reaction intermediate in order to construct the minimum-energy reaction pathway for the ring opening of a given cycloalkane species. We find that in the low coverage limit, repeated dehydrogenation of cyclohexane at positions closer to the surface is kinetically highly facile, so that neither cyclohexene nor benzene is a major product. Instead, C-C cleavage ensues after 4~5 sequential dehydrogenation steps depending on the surface, producing a metallocycle. This stands in contrast to the decomposition of cyclohexane on Pt, where benzene is a major product [5,6]. The reaction intermediates generally adsorb more strongly on Ir(211) than on Ir(111), but the reaction activation barriers are not always lower on the step edge. Our findings provide clues for enhancing the ability of Ir catalysts to more selectively cleave substituted C-C bonds, thereby increasing the CN of the resultant hydrocarbons.

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