(337bl) Development of Electrochemical Separation Processes for Value-added Molecules

Homogeneous catalysts are known for their high turnover and selectivity. However, catalyst homogeneity can be a double-edged sword; although mononuclear and homogeneous pathways of catalysts show superior kinetics, but it can be difficult to separate them from the product mixtures, posing substantial challenges for economical recycling. Furthermore, homogeneous catalysts often consist of noble metals which of the limited supply cannot keep up with the rising demand. Therefore, energy and resource efficient catalyst recycling technologies are in need for achieving sustainable chemical industry processes heavily relying on homogeneous catalysts. Recently, redox-active materials have risen as promising materials for chemical energy storage and chemical separations due to their selective molecular interactions and electrochemical stability. Here, we introduce a redox-mediated electrochemical recycling platform using redox-metallopolymer-functionalized electrodes for homogeneous catalysts. The recycling platform was examined with key industrially relevant catalysts used in organic synthesis and chemical manufacturing. In detail, Speier’s catalyst and Karstedt’s catalyst were successfully recycled from hydrosilylation and silane etherification, and palladium chloride and bis(triphenylphosphine) palladium dichloride from Wacker oxidation and Suzuki reactions. The redox-metallopolymer electrode demonstrated high uptake (Q_max up to 200 milligrams of platinum per gram of adsorbent) from product mixtures, high selectivity (selectivity factor up to 8 for platinum catalyst and 84 for palladium catalyst against competing anion), and cyclability (over 5000 charging/discharging cycles). More importantly, adsorbed catalysts were released into reaction mixture without deactivation with up to 99.5% recovery and 100% retention of turnover frequency over multiple recycling rounds. The combination of mechanistic studies and electronic structure calculations showed that selective interactions with anionic catalyst intermediates during the catalytic cycle played a key role in the separations. In addition, the versatility of the redox platform was proved with a broad scope of reaction substrates, solvents, and electrolytes as well as other PGM chloroanions. Going forward, we envision the concept of electrochemical separation with redox-metallopolymers to be applied to broader classes of value-added ionic species by molecular engineering of the redox-electrode interface.