Multilayer-stacked paper-structured catalysts for microflow Suzuki–Miyaura cross-coupling reaction

Paper-like porous composites of ceramic fibers and ZnO whiskers were prepared using a papermaking technique, followed by the in situ synthesis of a Pd catalyst on the ZnO whiskers using a facile impregnation method. The flexible Pd@ZnO papers had micrometer-sized pores of average diameter ca. 25 μm, which promoted the effective diffusion of reactants passing through an assembly of vertically stacked papers in a flow reactor. The catalytic efficiency of the stacked Pd@ZnO papers in a flow Suzuki–Miyaura cross-coupling (SMC) reaction to synthesize 4-methylbiphenyl from phenylboronic acid and 4-iodotoluene was higher than that of a bead-type Pd particulate catalyst in a reactor. Microchannels originating from the porous fiber-network microstructures in the stacked papers contributed to effective heterogeneous catalysis, possibly by enabling smooth diffusion of substrates to the surfaces of the Pd catalysts, as in a microreactor system. K₂CO₃, which was used as the base in the SMC reaction, was also immobilized in the paper-structured fibrous composites. Stacks of two types of paper, i.e., containing either Pd catalysts or K₂CO₃, significantly affected the SMC catalytic activity in a continuous microflow reaction. A combination of K₂CO₃ papers upstream and Pd@ZnO papers downstream in the flow system provided higher catalytic efficiency via on-site K₂CO₃-mediated borate formation of phenylboronic acid in the initial stage in the reactor. Tailoring of the stacking patterns of the paper-structured composites is expected to be effective for sequential SMC reaction and to improve catalytic process engineering.