Silica Sol-Gel Materials as a Catalyst Support for Use in Microwave-Assisted Continuous-Flow Organic Synthesis
Schruder, Christopher William
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The Organ group used microwave irradiation as a means of heating flowed chemical reactions in the development of a microwave-assisted continuous-flow organic synthesis (MACOS) system, to combine the benefits of flow reactions and the effective heating of microwave radiation. MACOS was well suited to the use of heterogeneous catalysts contained within the reactor capillaries, and metal films proved to be effective to not only catalyze reactions, but also to act as a heating source. Unfortunately, the heating of bulk metal films under microwave irradiation was hard to control, leading to intense over-heating of reactions and destruction of the metal films. Transition metal-loaded heterogeneous catalysts immobilized on microwave-transparent silica supports were prepared. The supported catalysts also act as a mixer within the flow channels to prevent laminar flow and allow for high catalyst surface area. The in situ palladium nanoparticle-loaded silica macrospheres were used to catalyze Heck cross-coupling reactions in MACOS, providing good product yield, and can be recycled several times without appreciable decrease in product formation. Dithiocarbamate-functionalized ligands were prepared and used to form amphiphilic transition metal complexes to attempt to selectively load metals at the surface of silica macrospheres. Some selectivity for incorporation of the metal complexes towards the outer surface of the macrospheres was obtained, however, there was a penetration depth of metals into the interior of the macrospheres, and the complexes were not fully characterized. Inorganic-organic hybrid macrospheres were prepared containing functional groups that could coordinate desired transition metals selectively on their surface. These inorganic-organic hybrid catalyst supports were also used to perform Heck cross-coupling reactions in MACOS. Calcination of these metal-loaded macrospheres yielded silica macrospheres with palladium nanoparticles loaded selectively on their surface. The inorganic-organic hybrid macrospheres were able to catalyze the Heck cross-coupling of aryl iodides in MACOS. They were re-usable without significant decrease in product conversion. The calcined macrospheres with palladium nanoparticles loaded selectively on their surface were able to catalyze the Heck cross-coupling of aryl iodides in MACOS, but significant leaching of the catalyst from the support was observed.