Palladium Separation by Pd-Catalyzed Gel Formation via Alkyne Coupling

Selective entrapment of precious metals from industrial wastes containing various metals is a most challenging branch of environmental science. Developed methods like solvent or solid-phase extraction, ion exchange, co-precipitation, membrane filtration, and adsorption rely on chelation, electrostatic attraction, or ion exchange, and these methods present limited selectivity and require post-treatment for further application. Herein, an original concept is reported involving the utilization of the superior catalytic properties of Pd selectively for efficient entrapment of Pd from other metals. Specifically, side chains of poly(vinyl alcohol) (PVA) functionalized with alkynyl groups are catalytically dimerized using Pd(II), which forms gels. The Pd(II) ions are coordinatively encapsulated into the gel networks, while the other metal ions are excluded from the networks, thus allowing the separation and immobilization of Pd. The entrapped Pd(II) is also reduced to Pd(0) nanoparticles (PdNPs) forming PdNPs@alkyne-PVA aerogels that exhibit a high catalytic activity for the Suzuki-Miyaura cross-coupling reaction. PdNPs@alkyne-PVA aerogels are further carbonized to PdNPs@C networks for the efficient electrochemical hydrogen evolution reaction. This method provides an effective way not only to selectively separate Pd but also to utilize the entrapped Pd resources for multiple catalyses without further post-treatment of the entrapped Pd source.