Degradation of plastic wastes to commercial chemicals and monomers under visible light

Recent advances in oxidative degradation of plastics

Oxidative degradation is a powerful method to degrade plastics into oligomers and small oxidized products. While thermal energy has been conventionally employed as an external stimulus, recent advances in photochemistry have enabled photocatalytic oxidative degradation of polymers under mild conditions. This tutorial review presents an overview of oxidative degradation, from its earliest examples to emerging strategies. This review briefly discusses the motivation and the development of thermal oxidative degradation of polymers with a focus on underlying mechanisms. Then, we will examine modern studies primarily relevant to catalytic thermal oxidative degradation and photocatalytic oxidative degradation. Lastly, we highlight some unique studies using unconventional approaches for oxidative polymer degradation, such as electrochemistry.

Recent Progresses and Challenges in Upcycling of Plastics through Selective Catalytic Oxidation

Chemical upcycling of plastics provides an important direction for solving the challenging issues of plastic pollution and mitigating the wastage of carbon resources. Among them, catalytic oxidative cracking of plastics to produce high-value chemicals, such as catalytic oxidation of polyethylene (PE) to produce fatty dicarboxylic acids, catalytic oxidation of polystyrene (PS) to produce benzoic acid, and catalytic oxidation of polyethylene terephthalate (PET) to produce terephthalic acid under mild conditions has attracted increasing attention, and some exciting progress has been made recently. In this article, we will review recent progresses on the catalytic oxidation upcycling of plastics and provide our understanding on the current challenges in catalytic oxidation upcycling of plastics.

The effect of ancillary ligands on hydrocarbon C-H bond functionalization by uranyl photocatalysts

The aqueous uranyl dication has long been known to facilitate the UV light-induced decomposition of aqueous VOCs (volatile organic compounds), via the long-lived highly efficient, uranyl excited state. The lower-energy visible light excited uranyl ion is also able to cleave unactivated hydrocarbon C-H bonds, yet the development of this reactivity into controlled and catalytic C-H bond functionalization is still in its infancy, with almost all studies still focused on uranyl nitrate as the precatalyst. Here, hydrocarbon-soluble uranyl nitrate and chloride complexes supported by substituted phenanthroline (Ph2phen) ligands are compared to each other, and to the parent salts, as photocatalysts for the functionalization of cyclooctane by H atom abstraction. Analysis of the absorption and emission spectra, and emission lifetimes of Ph2phen-coordinated uranyl complexes demonstrate the utility of the ligand in light absorption in the photocatalysis, which is related to the energy and kinetic decay profile of the uranyl photoexcited state. Density functional theory computational analysis of the C-H activation steps in the reaction show how a set of dispersion forces between the hydrocarbon substrate and the Ph2phen ligand provide control over the H atom abstraction, and provide predictions of selectivity of H atom abstraction by the uranyl oxo of the ring C-H over the ethyl C-H in an ethylcyclohexane substrate.

Photocatalytic Upcycling and Depolymerization of Vinyl Polymers

Photocatalytic upcycling and depolymerization of vinyl polymers have emerged as promising strategies to combat plastic pollution and promote a circular economy. This mini review critically summarizes current developments in the upcycling and degradation of vinyl polymers including polystyrene and poly(meth)acrylates. Of these material classes, polymethacrylates possess the unique possibility to undergo a photocatalytic depolymerization back to monomer under thermodynamically favourable conditions, thus presenting significant advantages over traditional thermal strategies. Our perspective on current formidable challenges and potential future directions are also discussed.

Photouranium-Catalyzed C-F Activation Hydroxylation via Water Splitting

The C-F bond is the strongest covalent single bond (126 kcal/mol) in carbon-centered bonds, in which the highest electronegativity of fluorine (χ = 4) gives rise to the shortest bond length (1.38 Å) and the smallest van der Waals radius (rw = 1.47 Å), resulting in enormous challenges for activation and transformation. Herein, C-F conversion was realized via photouranium-catalyzed hydroxylation of unactivated aryl fluorides using water as a hydroxyl source to deliver multifunctional phenols under ambient conditions. The activation featured cascade sequences of single electron transfer (SET)/hydrogen atom transfer (HAT)/oxygen atom transfer (OAT), highly integrated from the excited uranyl cation. The *UO22+ prompted water splitting under mild photoexcitation, caging the active oxygen in a peroxo-bridged manner for the critical OAT process and releasing hydrogen via the HAT process.

Light-driven polymer recycling to monomers and small molecules

Only a small proportion of global plastic waste is recycled, of which most is mechanically recycled into lower quality materials. The alternative, chemical recycling, enables renewed production of pristine materials, but generally comes at a high energy cost, particularly for processes like pyrolysis. This review focuses on light-driven approaches for chemically recycling and upcycling plastic waste, with emphasis on reduced energy consumption and selective transformations not achievable with heat-driven methods. We focus on challenging to recycle backbone structures composed of mainly C‒C bonds, which lack functional groups i.e., esters or amides, that facilitate chemical recycling e.g., by solvolysis. We discuss the use of light, either in conjunction with heat to drive depolymerization to monomers or via photocatalysis to transform polymers into valuable small molecules. The structural prerequisites for these approaches are outlined, highlighting their advantages as well as limitations. We conclude with an outlook, addressing key challenges, opportunities, and provide guidelines for future photocatalyst (PC) development.

Catalytic conversion of mixed polyolefins under mild atmospheric pressure

The chemical recycling of polyolefin presents a considerable challenge, especially as upcycling methods struggle with the reality that plastic wastes typically consist of mixtures of polyethylene (PE), polystyrene (PS), and polypropylene (PP). We report a catalytic aerobic oxidative approach for polyolefins upcycling with the corresponding carboxylic acids as the product. This method encompasses three key innovations. First, it operates under atmospheric pressure and mild conditions, using O2 or air as the oxidant. Second, it is compatible with high-density polyethylene, low-density polyethylene, PS, PP, and their blends. Third, it uses an economical and recoverable metal catalyst. It has been demonstrated that this approach can efficiently degrade mixed wastes of plastic bags, bottles, masks, and foam boxes.

Recent advancement on photocatalytic plastic upcycling

More than 8 billion tons of plastics have been generated since 1950. About 80% of these plastics have been dumped in landfills or went into natural environments, resulting in ever-worsening contamination. Among various strategies for waste plastics processing (e.g., incineration, mechanical recycling, thermochemical conversion and electrocatalytic/photocatalytic techniques), photocatalysis stands out as a cost-effective, environmentally benign and clean technique to upcycle plastic waste at ambient temperature and pressure using solar light. The mild reaction conditions for photocatalysis enable the highly selective conversion of plastic waste into targeted value-added chemicals/fuels. Here, we for the first time summarize the recent development of photocatalytic plastic upcycling based on the chemical composition of photocatalysts (e.g., metal oxides, metal sulfides, non-metals and composites). The pros and cons of various photocatalysts have been critically discussed and summarized. At last, the future challenges and opportunities in this area are presented in this review.