Visible Light-Driven Cascade Carbon-Carbon Bond Scission for Organic Transformations and Plastics Recycling.

Significant efforts are devoted to developing artificial photosynthetic systems to produce fuels and chemicals in order to cope with the exacerbating energy and environmental crises in the world now. Nonetheless, the large-scale reactions that are the focus of the artificial photosynthesis community, such as water splitting, are thus far not economically viable, owing to the existing, cheaper alternatives to the gaseous hydrogen and oxygen products. As a potential substitute for water oxidation, here, a unique, visible light-driven oxygenation of carbon-carbon bonds for the selective transformation of 32 unactivated alcohols, mediated by a vanadium photocatalyst under ambient, atmospheric conditions is presented. Furthermore, since the initial alcohol products remain as substrates, an unprecedented photodriven cascade carbon-carbon bond cleavage of macromolecules can be performed. Accordingly, hydroxyl-terminated polymers such as polyethylene glycol, its block co-polymer with polycaprolactone, and even the non-biodegradable polyethylene can be repurposed into fuels and chemical feedstocks, such as formic acid and methyl formate. Thus, a distinctive approach is presented to integrate the benefits of photoredox catalysis into environmental remediation and artificial photosynthesis.

New insights into the photocatalytic activity of 3-D core-shell P25@silica nanocomposites: impact of mesoporous coating.

In this report, a three-dimensional (3-D) network of core-shell TiO2 (P25)-mesoporous SiO2 (P25@mSiO2) nanocomposites was prepared via a controllable surfactant-assisted sol-gel method. The nanocomposites were investigated for photocatalytic reactions of organic dye degradation, water splitting, and CO2 reduction to understand the roles of the mSiO2 shell in these photocatalytic reactions. It was found that the mSiO2 shell accelerates the photodegradation of the organic dye, but dramatically reduces the photocatalytic activity of P25 in water splitting and CO2 reduction. The roles played by the mSiO2 shell in the photocatalytic reactions are summarized as: (1) effective prevention of agglomeration of P25 nanoparticles, (2) facilitating the transfer of uncharged photo-generated ˙OH radicals via the abundant -OH groups on the mesoporous surface, (3) provision of increased reaction sites between ˙OH radicals and dye molecules by its mesoporous nanostructure and large surface area, and (4) prevention of diffusion of the photo-generated charge carriers (photoelectrons and photoholes) because of its insulating nature.

Selective photocatalytic C-C bond cleavage under ambient conditions with earth abundant vanadium complexes.

Selective C-C bond cleavage under ambient conditions is a challenging chemical transformation that can be a valuable tool for organic syntheses and macromolecular disassembly. Herein, we show that base metal vanadium photocatalysts can harvest visible light to effect the chemoselective C-C bond cleavage of lignin model compounds under ambient conditions. Lignin, a major aromatic constituent of non-food biomass, is an inexpensive, accessible source of fine chemical feedstocks such as phenols and aryl ethers. However, existing lignin degradation technologies are harsh and indiscriminately degrade valuable functional groups to produce intractable mixtures. The selective, photocatalytic depolymerization of lignin remains underexplored. In the course of our studies on lignin model compounds, we have uncovered a new C-C activation reaction that takes place under exceptionally mild conditions with high conversions. We present our fundamental studies on representative lignin model compounds, with the aim of expanding and generalizing the substrate scope in the future. Visible light is employed in the presence of earth-abundant vanadium oxo catalysts under ambient conditions. Selective C-C bond cleavage leads to valuable and functionally rich fine chemicals such as substituted aryl aldehydes and formates. Isotope labeling experiments, product analyses, and intermediate radical trapping, together with density functional theory studies, suggest a unique pathway that involves a photogenerated T1 state during the C-C bond cleavage reactions. Our study demonstrates a sustainable approach to harvest sunlight for an unusual, selective bond activation, which can potentially be applied in organic transformations and biomass valorization.

Photodegradation of organic dyes in the presence of [Fe(III)-salen]Cl complex and H2O2 under visible light irradiation.

Photodegradation of persistent organic dyes (Rhodamine B (RhB), Malachite Green Oxalate (MG) and Crystal Violet 10B (CV)) is studied with Fe(III)-salen complex (λ(max) 494 nm), and hydrogen peroxide under visible light irradiation (λ≥400 nm). The complete decolourization of the dyes (60 mg/L each) was achieved in the aqueous medium. The pseudo-first-order degradation rate constants of RhB, MG, CV were found to be 2.83×10(-3) s(-1), 1.57×10(-3) s(-1) and 1.34×10(-3) s(-1), respectively. The effect of various parameters like concentration of H(2)O(2), pH of the medium, and influence of electrolytes are investigated on the degradation of RhB. A modified benzoic acid hydroxylation method has been used to detect the active oxygen species (OH radicals) in this study. The hydroxyl radical production is increased with the increase in irradiation time. Interestingly, even an excess amount of scavenger could not arrest the degradation of the dyes. This may be due to the formation of some secondary oxidants. Here, active ferryl ion was identified as the secondary oxidant. Degradation products of the dye (RhB) were determined by GC-MS, and phthalic acid was identified as the major one. From the results, a possible photodegradation mechanism has been proposed.