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Precious metals such as palladium (Pd) have many applications, ranging from automotive catalysts to fine chemistry. Platinum group metals are, thus, in massive demand for industrial applications, even though they are relatively rare and belong to the list of critical materials for many countries. The result is an explosion of their price. The recovery of Pd from spent catalysts and, more generally, the development of a circular economy process around Pd, becomes essential for both economic and environmental reasons. To this aim, we propose a sustainable process based on the use of supercritical CO2 (i.e., a green solvent) operated in mild conditions of pressure and temperature (p = 25 MPa, T = 313 K). Note that the range of CO2 pressures commonly used for extraction is going from 15 to 100 MPa, while temperatures typically vary from 308 to 423 K. A pressure of 25 MPa and a temperature of 313 K can, therefore, be viewed as mild conditions. CO2-soluble copolymers bearing complexing groups, such as pyridine, triphenylphosphine, or acetylacetate, were added to the supercritical fluid to extract the Pd from the catalyst. Two supported catalysts were tested: a pristine aluminosilicate-supported catalyst (Cat D) and a spent alumina supported-catalyst (Cat A). An extraction conversion of up to more than 70% was achieved in the presence of the pyridine-containing copolymer. The recovery of the Pd from the polymer was possible after extraction, and the technological and economical assessment of the process was considered. [ABSTRACT FROM AUTHOR]

The synthesis and characterization of a platform of novel functional fluorinated gradient copolymers soluble in liquid and supercritical CO2 is reported. These functional copolymers are bearing different types of complexing units (pyridine, triphenylphosphine, acetylacetate, thioacetate, and thiol) which are well-known ligands for various metals. They have been prepared by reversible addition–fragmentation chain-transfer (RAFT) polymerization in order to obtain well-defined gradient copolymers. The copolymers have been characterized by proton nuclear magnetic resonance (1H-NMR) spectroscopy, matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry, thermal gravimetric analysis (TGA), dynamical scanning calorimetry (DSC) and cloud point measurements in dense CO2. All the investigated metal-complexing copolymers are soluble in dense CO2 under mild conditions (pressure lower than 30 MPa up to 65 °C), confirming their potential applications in processes such as metal-catalyzed reactions in dense CO2, metal impregnation, (e.g., preparation of supported catalysts) or metal extraction from various substrates (solid or liquid effluents). Particularly, it opens the door to greener and less energy-demanding processes for the recovery of metals from spent catalysts compared to more conventional pyro- and hydro-metallurgical methods. [ABSTRACT FROM AUTHOR]

Anionic ring-opening polymerization (AROP) of 2-ethyl-2-oxo-1,3,2-dioxaphospholane (EP) has been utilized to create alternative hydrophilic moieties for utilization in surfactants. The current "go-to" hydrophilic unit is poly(ethylene glycol) (PEG), but this is non-biodegradable and there is some evidence that its use can lead to accumulation of toxic by-products. We have created new approaches leading to water-dispersible, fully degradable amphiphilic block copolymers. Our approach utilized poly(ε-caprolactone)-based macroinitiators in a solvent free process with organocatalyst DBU. We have prepared a comprehensive set of novel amphiphilic PCL-b-PEP block copolymers including linear diblock, triblock and star diblock copolymers characterized by SEC, 1H and 31P NMR and MALDI-TOF analyses. Supercritical carbon dioxide (scCO2) was exploited to efficiently extract the residual EP monomer leading to significantly increased gravimetric yields of purified product compared to conventional techniques such as dialysis. The self-assembly of the amphiphilic copolymers in water was investigated by DLS and cryo-TEM and their surface-activity as a function of concentration was investigated by tensiometry showing behavior that matches or exceeds commercially available surfactants. [ABSTRACT FROM AUTHOR]

Reverse iodine transfer polymerization (RITP) of 1,1,2,2-tetrahydroperfluorodecyl acrylate (FDA) is successfully performed in supercritical carbon dioxide (scCO2) at 70 °C under a CO2 pressure of 300 bar. PolyFDA (PFDA) of increasing molecular weights (from 10 000 to 100 000 g mol−1) is synthesized with good agreement between theoretical, 1H NMR spectroscopy and and size exclusion chromatography/refractive index/right-angle laser-light scattering/differential viscometer (SEC/RI/RALLS/DV)-estimated molecular weights ( [ABSTRACT FROM AUTHOR]

Abstract: Dispersion polymerization of 2-hydroxyethyl methacrylate (HEMA) has been successfully performed in supercritical carbon dioxide at P=370bar and T=65°C with azobis(isobutyronitrile) as initiator and a hydrophilic/CO2-philic poly(ethylene oxide)-b-poly(1,1,2,2-tetrahydroperfluorodecyl acrylate) (PEO-b-PFDA) block copolymer as steric stabilizer. The PEO-b-PFDA (2K/21K) block copolymer was synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization. Spherical particles of poly(HEMA) were obtained in the range of 200–400nm diameter size with a narrow particle size distribution (Dw/Dn<1.1). The effect of the stabilizer concentration on the dispersion polymerization was investigated from 20w/w% down to 3.5w/w% versus HEMA. Precipitation polymerization in the absence of stabilizer lead to the formation of large aggregates of partially coalesced particles whereas discrete spherical particles of poly(HEMA) were obtained by dispersion polymerization even at low concentration of PEO-b-PFDA (3.5w/w% versus HEMA). [Copyright &y& Elsevier]

• CO 2 -soluble Pd-complexing polymers bearing pyridine units have been synthesized. • Cloud point curves of the polymers have been determined in dense CO 2. • Extraction of Pd from supported catalysts was performed in CO 2 at 40 °C and 25 MPa. • Strong effect of the Pd oxidation state on extraction in CO 2 was reported. • 73 % of Pd was extracted from the catalytic support by the complexing polymers. Precious metals, in particular palladium (Pd), have a wide range of daily applications, from automotive catalysts to fine chemistry production. Nevertheless, these metals are relatively rare and highly expensive, considering their massive industrial utilization. In the last decades, different recycling methods have been explored. Nowadays, the most applied methods, namely pyro- and/or hydrometallurgy, involve energy-intensive processes and/or the generation of large amounts of effluents to be treated. Thus, the development of a more sustainable recycling process of precious metals is highly desirable. In the present work, we introduce a sustainable process based on the use of a green solvent, supercritical CO 2 , operated under mild conditions (P = 25 MPa and T = 40 °C). The extraction process is possible thanks to the addition of CO 2 -soluble complexing polymers bearing pyridine units. The proposed method leads to the extraction of more than 70 % of Pd from an aluminosilica-supported catalyst. [ABSTRACT FROM AUTHOR]

[Display omitted] • Green recycling process of cathode materials from LIBs using supercritical CO 2. • scCO 2 as a green solvent for critical metal extraction at 60 °C and 250 bar. • Poly(VBPDA- co -FDA) copolymer as leaching agent, macro-ligand, and extracting agent. • H 2 O 2 and ethanol as reducing agent for Co2+ complexation in scCO 2 from LiCo(III)O 2. • Recovery yield up to 75 % of Co and Li from LiCoO 2. Supercritical CO 2 (scCO 2) extraction assisted by complexing copolymers is a promising process to recover valuable metals from lithium-ion batteries (LIBs). CO 2 , in addition to being non-toxic, abundant and non-flammable, allows an easy separation of metal-complexes from the extraction medium by depressurization, limiting the wastewater production. In this study, CO 2 -philic gradient copolymers bearing phosphonic diacid complexing groups (poly(vinylbenzylphosphonic diacid- co -1,1,2,2-tetrahydroperfluorodecylacrylate), p(VBPDA- co -FDA)) were synthesized for the extraction of lithium and cobalt from LiCoO 2 cathode material. Notably, the copolymer was able to play the triple role of leaching agent, complexing agent and surfactant. The proof of concept for leaching, complexation and extraction was achieved, using two different extraction systems. A first extraction system used aqueous hydrogen peroxide as reducing agent while it was replaced by ethanol in the second extraction system. The scCO 2 extraction conditions such as extraction time, temperature, functional copolymer concentration, and the presence of additives were optimized to improve the metals extraction from LiCoO 2 cathode material, leading to an extraction efficiency of Li and Co up to ca. 75 % at 60 °C and 250 bar. [ABSTRACT FROM AUTHOR]

The use of functional CO 2 -philic polymers in supercritical carbon dioxide (scCO 2 ) for a greener method of extracting Pd from spent catalysts has been investigated. Three fluorinated polymers bearing complexing units (a thiol-terminated poly(1,1,2,2-tetrahydroperfluorodecyl acrylate) homopolymer PFDA-SH, a poly(1,1,2,2-tetrahydroperfluorodecyl acrylate- co -diphenylphosphinostyrene) gradient copolymer P(FDA- co -DPPS), and a thiol-terminated gradient copolymer P(FDA- co -DPPS)-SH) were synthesized and successfully used to extract palladium from commercial Pd/Al 2 O 3 supported catalysts (beads and pellets with 0.5 wt.% and 5 wt.% Pd loading, respectively). These non-destructive extractions were carried out in scCO 2 under mild conditions (40 °C and 25 MPa). Inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis on the Pd/Al 2 O 3 catalysts before and after extraction confirmed the removal of palladium from the catalytic supports. Up to 40% of palladium was extracted from the commercial supported catalysts with full recovery of the original alumina support. [ABSTRACT FROM AUTHOR]

Amphiphilic functional copolymers have been successfully synthesized and used for the supercritical CO 2 -mediated design of Pd supported catalysts. First, a new palladium complex soluble in supercritical CO 2 has been prepared by ligand exchange, in Freon 113 or supercritical CO 2 , between an amphiphilic functional copolymer and a metal precursor (palladium acetate Pd(OAc) 2 ). Then, the solubility behavior of this copolymer/Pd II supramolecular complex has been studied in supercritical CO 2 . The amphiphilic functional gradient copolymer used in this work, poly(1,1,2,2-tetrahydroperfluorodecylacrylate- co -acetoacetoxyethylmethacrylate) copolymer (poly(FDA- co -AAEMA)), was purposely synthesized by controlled radical polymerization in trifluorotoluene or in supercritical CO 2 . The fluorinated monomer units (FDA) impart CO 2 -philic properties to the copolymer while the acetoacetoxy monomer units (AAEMA) bring the complexing character to the copolymer. After reduction of the complex under hydrogen atmosphere, well-dispersed small nanoparticles of Pd° (2–6 nm diameter size range) were obtained within the copolymer matrix. The copolymer/Pd II supramolecular complex was also used as an impregnation agent for the preparation of supported catalysts. Thus, a mesoporous silica material was impregnated by the complex in supercritical CO 2 . After reduction, a composite material composed of nanoparticles of Pd° (2–9 nm size) evenly distributed in the pores of the silica matrix was obtained. Finally, the catalytic activity of the prepared Pd°@SiO 2 supported catalyst was evaluated in the Heck reaction. [ABSTRACT FROM AUTHOR]