Palladium nanoparticle deposition on spherical carbon supports for heterogeneous catalysis in continuous flow

Abstract

Heterogeneous catalysis is widely exploited by the chemical industry, both in batch reactors and in continuous flow, the latter via the use of packed bed reactors. Unfortunately, the transfer of commercially available heterogeneous catalysts to high pressure flow systems is often difficult, with challenges such as catalyst deactivation through metal leaching, and the crushing of pelleted supports. Thus, the limited availability of suitable catalysts for heterogeneous flow processes, which can satisfy all the requirements for its application, is a major bottle neck in the commercial implementation of these systems. Polymer-based spherical activated carbon beads (diameter = 474 ± 96 μm) offer a promising solution: these small, spherical and monodisperse beads have high mechanical strengths and large surface areas (1583 ± 8 m² g⁻¹), offering desirable properties for this task, such as reproducible packing and low pressure drops across packed catalyst beds. Two series of Pd/C spherical bead catalysts were synthesised and compared to a commercial catalyst from Johnson Matthey (1 wt% Pd/C pellets), in small scale screenings (20 mg) via a recirculating batch platform, for their activity in a model nitro reduction reaction. It was observed that small, robust, highly active palladium nanoparticles (PdNPs) supported on spherical carbon beads with a narrow size distribution (e.g. 1e – Pd – d_NP = 5.0 ± 1.4 nm) can be synthesised via solution phase deposition. In contrast, the NP catalysts made via gas phase deposition were much larger (e.g. 2e – Pd – d_NP = 22.8 ± 13.1 nm), less active and unstable due to metal leaching. The applicability of these NP catalysts for use in continuous flow was subsequently demonstrated on a larger scale (0.5–1 g), with a high activity and stability achieved over a two day operating period. This work demonstrates the production of an active, stable heterogeneous catalyst suitable to be employed in a pilot scale continuous flow packed bed reactor, for the production of APIs.