Ring opening of 1,2- and 1,3-dimethylcyclohexane on iridium catalysts

The ring opening reactions of 1,2- and 1,3-dimethylcyclohexane (1,2- and 1,3-DMCH) have been investigated over Ir catalysts supported on Al2O3, SiO2, and TiO2. These two naphthenic molecules are used as probe reactants to investigate the different reaction paths that may occur in the treatment of diesel fuels and gasolines for the improvement of cetane numbers (CNs) and octane numbers. The CN can be improved by ring opening of naphthenics, but only when the CC bond rupture occurs at a substituted position. Otherwise, the resulting product ends up with a high degree of branching, causing the CN to decrease. In this case, the octane number increases, which would be desirable in the production of gasolines. The present contribution shows that the preference of CC bond opening at substituted or unsubstituted positions depends on the type of catalyst used, as well as on the nature of the reactant. Several ring-opening mechanisms can be operative under different conditions depending on the support used. For instance, the dicarbene reaction path results in CC bond opening at unsubstituted positions and typically occurs on Ir/SiO2 catalyst. In consequence, this catalyst may be desirable for increasing octane number while producing low-CN products. Two other mechanisms, those involving adsorbed olefin intermediates or metallocyclobutane intermediates, result in the CC bond opening at substituted positions, which would be more desirable in the production of diesel fuel. Iridium on alumina has shown to be the most selective toward this type of ring opening. This preferred path on alumina-supported catalysts appears to be related to a support effect rather than to an effect of metal dispersion, because catalysts of different metal dispersions on alumina display a comparable selectivity much higher than that obtained on any of the silica-supported catalysts. To quantify these differences, a kinetic study was conducted by following the evolution of the cis- and trans-isomers of 1,2- and 1,3-DMCH as a function of space–time. The kinetic parameters obtained from this study indicate important disparities in the reactivity of the different isomers toward ring opening, and they depend strongly on the support used. To explain these disparities, it is proposed that dicarbene intermediates can be formed from all four isomers. However, only the two conformers that have diequatorial configurations (trans-1,3-DMCH and cis-1,2-DMCH) favor the formation of the intermediates leading to the SRO of substituted CC bonds and hence to CN improvement.