The abundance in supply of shale gas and shale oil hydrocarbon feed stocks has increased interest in valorization of light C2-C5 alkanes. Dehydrogenation to olefins yields valuable chemical and polymer intermediates. Heterogeneous nanoparticle catalysts operate at high temperature and contend with coke formation, while homogeneous organometallic catalysts operate at very low temperature due to poor thermal stability, suffering from thermodynamic limitations. Organometallic complexes including pincer-ligated olefin complexes offer many advantages including high regioselectivity and single metal atom active sites, but the limited prior work on supporting pincer-Ir has shown product inhibition due to high contact times in batch experiments. We developed a continuous-flow gas-phase system for alkane dehydrogenation by pincer-iridium complexes. We synthesized immobilized pincer complexes via condensation of para-substituent leaving groups on the pincers with surface hydroxyl groups on high surface area metal oxide supports. The covalently anchored pincer-Ir catalysts showed long-term catalytic stability under continuous alkane dehydrogenation conditions at temperatures up to 340 °C, 100 °C higher than previously established. We have established through spectroscopic studies that the pincer ligand and organometallic framework remain intact at this high temperature. We have observed that the ligand occupying the open coordination center in the resting state is carbon monoxide regardless of the starting material. Both Ir(H)(Cl) and Ir(C2H4) complexes are converted to Ir(CO) complexes under reaction conditions and remain catalytically active. The role of CO is not that of a catalyst poison, but actually serves as a protecting group to prevent further decomposition of the catalyst. Loss of the CO ligand upon further heating is associated with decomposition of the pincer ligand and decline in activity.

Heterogeneous catalysis by homogeneous complexes: Silica-supported iridium-pincer complexes catalyze alkane dehydrogenation at elevated temperatures

2019 Spring National Meeting

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