The recent boom in unconventional oil recovery has led to an increase in hydrocarbon feed stocks, among which are included the lighter C2-C5 compounds which currently find only limited commercial use. Pincer-ligated iridium complexes in homogeneous batch experiments are known to be effective in synthesizing olefins from the parent alkanes, including high regioselectivity for terminal olefins. Limited work on supporting pincer-Ir for immobilized batch experiments is also known, but is still limited to high contact times and susceptible to product inhibition. Progress towards a practical application for catalytic pincer complexes requires precise synthesis of immobilized species, gas-solid reaction interfaces, and continuous processing conditions. In this work, 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 in a fixed-bed microreactor when supported on silica, but activity decayed on alumina, presumably due to coking catalyzed by the Lewis acidic surface. High temperature experiments demonstrated thermal stability greater than previously reported for homogeneous catalysts. Catalyst speciation was explored pre- and post- reaction with MAS Solid-State NMR to determine reaction and deactivation pathways, indicating that catalysts of the form [Si]-O-(tBu4POCOP)Ir(C2H4) were stable as ligated species above previously explored temperatures, up to 350 °C with hundreds of turnovers per hour.

Activity and speciation of supported pincer-ligated iridium catalysts in continuous-flow gas phase alkane dehydrogenation

255th National Meeting (2018)

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