Precious-metal recovery from industrial wastewater has received considerable attention because of rapidly increasing amounts of electronic waste. Existing technologies have yet to be widely applied due to their high cost and low selectivity toward precious-metal ions. Herein, we report a direct Z-scheme tannin–TiO2 heterostructure for selective gold adsorption from electronic waste under solar irradiation. The tannin-coated TiO2 nanoparticles were prepared by a simple dipping method, and under light illumination, both tannin and TiO2 can serve as photosensitive components for the reduction of metal ions, with metal-to-ligand charge transfer from TiO2 to tannin extending the lifetime of the excited electrons. Moreover, no additional electron donors are required because the tannin layer scavenges the reactive oxygen species generated by the holes from the light-activated TiO2 surface. The heterostructure allows for the highly efficient photocatalytic recovery of gold ions, with 11 times higher adsorption capacity in the light compared to the dark. High selectivity toward gold ions was also demonstrated using a metal ion mixture including nine different metal ions that are commonly found in electronic waste. Our findings suggest that the Z-scheme heterostructure of polyphenol and semiconductor provides a promising photochemical pathway for efficient and selective metal ion recovery from electronic waste.
The redox and non-redox CO2 utilization: Dry reforming of methane and catalytic cyclic carbonate formation
CO₂ emissions are too large to tackle with a single process, but a combination of avoidance with chemical utilization may be able to slow global warming. In this Focus Review, we identify two large-scale CO₂ conversion processes based on their viability and opposite energy requirements. In the high-energy, stationary path, CO₂ reforming of methane could provide gigatons of CO₂ utilization through synthesis gas. The main problem is the lack of a durable, effective, low-cost dry reforming catalyst. The exothermic cyclic carbonate formation from CO₂ and organic epoxides offers a low-energy, mobile, nonredox route. The catalysts, however, must be metal-free and robust, have a high surface area, and be low-cost while being easily scalable. These two processes could potentially address at least a quarter of all current CO₂ emissions.
Triazatruxene Based Ordered Porous Polymer: High Capacity CO2, CH4, H2 Capture, Heterogeneous Suzuki-Miyaura Catalytic Coupling and Thermoelectric Properties
A. E. Sadak, E. Karakuş, Y. Chumakov, N. A. Dogan, C. T. Yavuz
A hypercrosslinked ultramicroporous and ordered organic polymer network was synthesized from a planar trimer indole building block called triazatruxene (TAT) through anhydrous FeCl3 catalyzed Friedel–Crafts alkylation using methylal as a crosslinker. The polymer network is stable in a variety of chemicals and thermally durable. The hypercrosslinked network TATHCP shows a high BET (Brunauer–Emmet–Teller) specific surface area of 997 m2 g–1 with CO₂ uptake capacity of 12.55 wt % at 273 K, 1.1 bar. Gas selectivities of 38.4 for CO₂/N₂, 7.8 for CO₂/CH₄, 40.6 for CO₂/O₂, and 32.1 for CO₂/CO were achieved through IAST calculation. The PXRD analysis has revealed that TATHCP has a fully eclipsed structure in full agreement with Pawley refinement. The ordered 2D layers provide anisotropy that could be used in catalysis and thermoelectric measurements. After loading with Pd(II), TATHCP-Pd showed high catalytic activity in Suzuki–Miyaura cross coupling reaction with a wide range of reagents and excellent reaction yields of 90–98% with good recyclability. The structure of TATHCP-Pd was found to have two independent molecules of Pd(OAc)2 in the asymmetric unit cell which are arranged between two TATHCP layers. Thermoelectric properties of TATHCP showed a high Seebeck coefficient and ZT, a first and promising example in HCPs with applications in all-organic thermal energy recovery devices.
Thiourea-based extraction and deposition of gold for electroless nickel immersion gold process
Gold electroless plating for surface finishing of electronic circuits, named electroless nickel immersion gold (ENIG), is widely practiced in the electronics packaging industry. Noncyanide substitutions of the current cyanide bath for immersion gold are being sought for environmental and safety reasons. Herein, as a promising option, a bath using a noncyanide gold complex, Au(I)–thiourea, was developed. The kinetics of gold deposition were estimated with respect to gold concentration, thiourea concentration, pH, and temperature; the transfer coefficient of gold concentration and activation energy were found to be 0.697 and 36.69 kJ·mol–1, respectively. In addition, the quality of gold coating in terms of corrosion resistance was verified by electrochemical analysis. The relationship between particle size and corrosion resistance of the coating was confirmed by morphology observation through scanning electron microscopy and Tafel plots. The corrosion potential of the gold layer with thiourea was found to be −62 mV, close to that of the layer using a thiosulfate–sulfite bath, with an advantage of faster deposition rate. The results suggest Au(I)–thiourea can serve as an eco-friendly and field-implementable option for the ENIG process, helping to realize a closed-loop process of gold: recovering the precious metal from electronic wastes and reusing it in new products.
Quaternary Ammonium Salt Grafted Nanoporous Covalent Organic Polymer for Atmospheric CO2 Fixation and Cyclic Carbonate Formation
D. Kim, S. Subramanian, D. Thirion, Y. Song, M. Otaibi, A. Jamal, C. T. Yavuz
Non-redox carbon dioxide utilization through cycloaddition of CO₂ to epoxides offers great promise but suffers from lack of heterogeneous catalysts that don’t need additives or pressure. Here we report a systematic post-synthetic modification procedure on a highly porous hydrocarbon framework for efficient grafting of quaternary ammonium salts. The active sites were tuned and characterized while maintaining the porous structure. The metal free and cost-effective catalysts showed quantitative selectivity and very high conversion yields in atmospheric pressure catalysis for cyclic carbonate formation from CO₂ and epoxides. The reaction proceeded without additives or co-catalysts, and tolerant for a wide substrate scope. We found that the steric hindrance of the alkyl units on ammonium salts affect microporosity, CO₂ binding and the kinetics of cycloaddition reactions. The catalysts were also recyclable, an attractive prerequisite for industrial implementation.
Quantifying nitrogen effect on CO2 capture using isoporous network polymers
The impact of nitrogen atoms on CO₂ binding was evaluated for two isostructural porous bisimidazole-linked polymers (BILPs), which serendipitously had identical surface areas and pore size distributions, a very rare observation. The two structures differ only in the core of the trialdehyde component, the nitrogen atom (BILP-19) versus benzene ring (BILP-5). Such a slight difference, however, has brought about a stronger CO₂ capture capacity of BILP-19 and hence increased CO₂/N₂ separation capability.
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