Metal Nanoparticle Impregnated Controlled-size Silica Macrospheres as a Microwave-transparent Catalyst System for MACOS

Background: Metal films in microwave-assisted, continuous-flow organic synthesis (MACOS) have shown to act as heterogeneous catalysts for a variety of reactions, but have difficulty due to difficult to control heating and occurrence of laminar flow which limits the contact of the reagents with the catalyst surface. The aim of this paper is to describe a microwave-transparent supported metal catalyst with high surface area and its use in MACOS. Methods: Millimeter sized, monodisperse, macroscopic spherical silica beads loaded with Ni, Cu, and Pd nanoparticles were prepared through use of a single-step emulsion procedure via a sol-gel process and used to perform Heck cross-coupling reactions in MACOS. Results: The size of the spheres was readily controlled to a maximum diameter of 1300 μm by varying the stirring rate of the emulsion mixture. Pd loadings of up to 4.3 wt.% were obtained, and confirmed to be present as nanoparticles through PXRD spectroscopy and TEM imaging. The metal-loaded spheres were found to be essentially microwave-transparent, allowing for use as catalytic beads in microwave flow reactors. In addition, no mechanical dislodgement of the nanoparticles or degradation of their catalytic activity was observed over repeated usage. Conclusion: Metal-nanoparticle-impregnated silica macrospheres were found to be an effective catalyst for use in MACOS by providing access to the use of heterogeneous metal catalysts with controllable heating. Further testing of various metals and reactions can be performed to increase the scope of possible reactions to be catalysed in MACOS using metal-impregnated macrospheres as catalysts.