Palladium Nanoparticles Supported on a Three-dimensional Polyimide Sponge as a Reusable and Durable Heterogeneous Catalyst for Suzuki Reactions and Reduction of Nitrophenol to Aminophenol

Macroscopic heterogeneous catalysts (“dip catalysts”), which can be easily separated from reaction products and reused, show promise for green and economical use in organic synthesis processes. Although the catalytic performance of sponge-type dip catalysts has been extensively studied, their mechanical and thermal stability upon repeated use has not been sufficiently investigated. This study aims to address this limitation by undertaking a focused and challenging attempt to evaluate their mechanical and structural retention. In this work, using an industrial high-performance aromatic polyimide (PI), we prepared a palladium nanoparticle-loaded PI sponge (Pd@PIS) in the form of a dip catalyst with high catalytic efficiency, reusability, and durability. Pd@PIS was prepared through a Pickering high-internal-phase emulsion and had a porosity of 81% and a surface area of 51 m2/g. In the presence of Pd@PIS, catalytic reduction of 4-nitrophenol (4-NP) to 4-aminophenol proceeded with a high rate constant. In addition, Suzuki–Miyaura cross-coupling reactions between a variety of aryl halides and arylboronic acids with the aid of Pd@PIS produced biphenyl products, with outstanding yields (95–99%) and turnover frequencies of up to 1003 h–1. The Pd@PIS catalyst could be reused for 15 cycles in 4-NP reduction and 5 cycles in Suzuki reactions without significant loss in activity (conversions >95%). The chemical and pore structures, shape, dimensions, and mechanical properties of Pd@PIS were well preserved in the reuse experiments. We conducted additional Suzuki reactions using chlorobenzene as a representative low-reactivity substrate under more challenging conditions (150 °C). Hot filtration and inductively coupled plasma optical emission spectrometry analysis confirmed that Pd leaching was minimal (∼2.0%), and the catalyst retained its porous structure and catalytic performance, demonstrating good thermal robustness. These results are attributed to the inherent chemical, thermal, and mechanical stability of the PI network. Overall, Pd@PIS presents a promising reusable and durable dip catalyst platform for sustainable organic transformations.