Electrochemical recycling of homogeneous catalysts

Abstract

Homogeneous catalysts have rapid kinetics and keen reaction selectivity. However, their widespread use for industrial catalysis has remained limited because of challenges in reusability. Here, we propose a redox-mediated electrochemical approach for catalyst recycling using metallopolymer-functionalized electrodes for binding and release. The redox platform was investigated for the separation of key platinum and palladium homogeneous catalysts used in organic synthesis and industrial chemical manufacturing. Noble metal catalysts for hydrosilylation, silane etherification, Suzuki cross-coupling, and Wacker oxidation were recycled electrochemically. The redox electrodes demonstrated high sorption uptake for platinum-based catalysts ( Q max up to 200 milligrams of platinum per gram of adsorbent) from product mixtures, with up to 99.5% recovery, while retaining full catalytic activity over multiple cycles. The combination of mechanistic studies and electronic structure calculations indicate that selective interactions with anionic intermediates during the catalytic cycle played a key role in the separations. Last, continuous flow cell studies support the scalability and favorable technoeconomics of electrochemical recycling.

Electrochemically Mediated Recovery and Purification of Gold for Sustainable Mining and Electronic Waste Recycling

“…To summarize, cyano-gold capture and release were unaffected by the addition of competing cyano-metals, and the results indicate that gold is selectively bound to oxidized ferrocenium sites. Previous studies suggest the mechanism of selective adsorption with PVF originates from favorable charge-transfer interaction between target heavy metal anion and ferrocenium binding site, , and our results support this mechanism, showing preferential binding of the dicyanoaurate anion to oxidized ferrocene over other cyano-metals. A more detailed investigation of the underlying molecular mechanism for cyano-metal binding, including associated electronic structure calculations, will be carried out in the future.…”

“…Only Au­(I) speciation was observed on the PVF-CNT working electrode after electrosorption of 1 mM [Au­(CN) 2 ] − , and no gold was detected with XPS on the counter electrode from the same experiment. Therefore, this combined spectroscopic evidence suggested that [Au­(CN) 2 ] − anions from solution were electrochemically captured to electrode-bound ferrocene sites without alteration of the oxidation state of the Au­(I) molecule and without any electrodeposition onto the working or counter electrodes, proving the redox-mediated electrosorption mechanism for the process driven by previous observations of ferrocene (Fc)-mediated binding in noble metal catalyst recovery. ,, …”

“…23,33−36 Additionally, redoxmediated electrosorption can offer rapid kinetics for target capture and remarkable efficiency for ion recovery. 35,37 Recently, polyvinyl ferrocene (PVF) was demonstrated as a viable selective separations platform for homogeneous catalyst recovery with industrially pertinent hydrosilylation, cross- coupling, and Wacker oxidation reactions, 38 through selective interactions with anionic platinum complexes.…”

“…Electrochemical separations offer a promising avenue for ion-selective separations from dilute solutions, with capture and release modulated solely by electrochemical potential. − Electrification is a growing movement in the industry due to the advantages of accessing a unified energy source for process units and the possibility of integration with an renewable energy infrastructure and workflow. ,− Further, recent developments with redox-active materials offer a novel electrochemical separations platform, demonstrating a high degree of tunable target selectivity through molecular design and modulation of operational parameters compared to other electrochemical separation methods such as capacitive deionization and electrodeposition. ,− Additionally, redox-mediated electrosorption can offer rapid kinetics for target capture and remarkable efficiency for ion recovery. , Recently, polyvinyl ferrocene (PVF) was demonstrated as a viable selective separations platform for homogeneous catalyst recovery with industrially pertinent hydrosilylation, cross-coupling, and Wacker oxidation reactions, through selective interactions with anionic platinum complexes.…”

“…Therefore, this combined spectroscopic evidence suggested that [Au(CN) 2 ] − anions from solution were electrochemically captured to electrode-bound ferrocene sites without alteration of the oxidation state of the Au(I) molecule and without any electrodeposition onto the working or counter electrodes, proving the redox-mediated electrosorption mechanism for the process driven by previous observations of ferrocene (Fc)mediated binding in noble metal catalyst recovery. 34,37,38 For mining of low-grade gold ores and electronic waste recycling, soluble cyano-gold is typically ultradilute in concentration (1−10 ppm); therefore, to investigate industrial applicability, KAu(CN) 2 adsorption at ultradilute concen-trations with PVF-CNT electrodes was executed over a range of cyano-gold concentrations (0.004−5 mM Au(CN) 2 ). The resulting gold electrosorption isotherm (Figure 3b) was wellcaptured with a Langmuir isotherm model (R 2 of 0.997), and the following Langmuir parameters were estimated: q max = 258 g/g and K = 1.425 mM −1 .…”