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Nature highlights the presentation! A beam of light saves polystyrene from landfills

[Introduction]

A "Research Highlights" article in the March 31, 2022 issue of Nature is about a paper published in JACS on March 23, 2022 The paper, titled "A Beam of Light Rescues Polystyrene from Landfills. A new treatment method was used to convert waste polystyrene, a common plastic, into valuable small molecules. The authors of this JACS article are Sewon Oh and Erin E. Stache, from the Department of Chemistry and Chemical Biology at Cornell University.

Nature highlights the presentation! A beam of light saves polystyrene from landfills

[Note] Current issue covers of Nature and JACS journals

Nature highlights the presentation! A beam of light saves polystyrene from landfillsNature highlights the presentation! A beam of light saves polystyrene from landfills

Products made from polystyrene range from foam egg cartons to CD cases. It also accounts for about one-third of the world's landfill waste, prompting scientists to search for a scalable, energy-efficient process to convert it into useful compounds. sewon Oh and Erin Stache discovered this process by dissolving 20 mg of commercial polystyrene and compound ferric chloride in the liquid solvent acetone. They irradiated the resulting mixture with white light in an oxygen-rich environment.

After 20 hours, the polystyrene has broken down into small molecules, mainly benzoic acid, a crystalline solid commonly used as a food preservative. The researchers applied their method to several commercial polystyrenes and found that the molecular breakdown was very effective. They also demonstrated how the technique converts grams of polystyrene into benzoic acid in just a few hours.

[JACS Article]

Chemical upgrading of catalytic photo-oxidation of industrial polystyrene

Nature highlights the presentation! A beam of light saves polystyrene from landfills

Upcycling polystyrene into targeted small molecules by chemical means is an ideal way to reduce plastic pollution (editor's note: upcycling, or upcycling, is a hot term in recent years, using innovative ways to transform something to make it work in new ways). This JACS article reports on the upcycling of polystyrene to benzoyl products, mainly benzoic acid, using a catalyst-controlled photo-oxidative degradation method. Under white light irradiation, FeCl3 undergoes homolytic cleavage to generate a chlorine radical that detaches an electron-rich hydrogen atom from the polymer backbone. In an oxygen-rich environment, high MW (molecular weight) polystyrene (> 90 kg/mol) is reduced to < 1 kg/mol and produces up to 23 mol% (molar ratio) of benzoyl products. A series of mechanistic studies have shown that chlorine radicals promote degradation by dissociating hydrogen atoms. The high degradation efficiency of commercial polystyrene in our approach shows the compatibility of our system with polymeric fillers. Finally, we demonstrate the potential to scale up our method in a photocurrent process to convert gram quantities of PS to benzoic acid.

Synthetic chemists and engineers have optimized and simplified the synthesis of polystyrene (PS) products, an essential commodity in everyday life. While these rigid materials inspire consumer confidence, current recycling rates are less than 1%, and PS products, particularly expanded foam, account for approximately one-third of landfills worldwide. With so much effort invested in the development of PS alternatives and post-functionalization, production costs cannot overcome the financial incentive to continue producing virgin PS. Recycling of polystyrene, mainly by pyrolysis and microbial chemical degradation, is an active area of research. However, a fundamental requirement for PS waste degradation is to meet scalable, energy-efficient processes that can convert PS into individual products that are compatible with several different forms of commercial PS, including solids, expanded foam, copolymers, and polymer fillers.

Inspired by recent advances in photocatalysis and the opportunities of cheap visible light in scalable processes, we envisioned a photon-driven, catalyst-controlled oxidative degradation pathway to upgrade commercial PS to benzoic acid and benzoyl derivatives. UV light (λ = 254 nm) irradiation of PS generates radicals on the polymer backbone and is directly oxidized by surrounding atmospheric oxygen to generate peroxyl radicals, which eventually lead to the breakage of polymer chains. To increase the degradation rate, photoinitiators such as quinones, peroxides, benzophenones, and metal catalysts have been added to accelerate the oxidation of the backbone, possibly by hydrogen atom transfer (HAT) (Scheme 1A). Although these advances show promise, the conversion of polystyrene to a single product, such as benzoic acid, remains a challenge.

Nature highlights the presentation! A beam of light saves polystyrene from landfills

Scheme 1 Upgraded polystyrene schematic

Since HAT is a key step in photo-oxidative degradation, we hypothesized that HAT reagents generated by visible light would allow us to avoid the use of high-energy UV light and allow for more specific control of the abstraction step. Chlorine radicals are widely used as hydrogen atom abstraction agents due to their selectivity for strong, electron-rich hydrocarbon bonds (the enthalpy of dissociation of HCl is 103 kcal/mol). Considering these properties, we hypothesized that the introduction of chlorine radicals could lead to the dissociation of a phenyl C - H bond from the PS backbone and the generation of peroxyl radicals capable of cleaving the C - C bond upon oxidation (Scheme 1B). In addition, the use of a solvent containing an electrophile C - H bond avoids unwanted side reactions caused by the HAT promoted by the chlorine radical.

Nature highlights the presentation! A beam of light saves polystyrene from landfills

[Note] Emission spectrum of the white LED light source used

While there are many ways to generate chlorine radicals, PS waste degradation applications require an inexpensive, scalable option, for example, using visible light. Under visible light irradiation, FeCl3 and related compounds are catalysts for the generation of chlorine radicals. Numerous reports have demonstrated that HAT of hydrocarbons can effectively promote C - H oxidation or functionalization. We hypothesized that irradiated PS in the presence of sub stoichiometric amounts of FeCl3, in the presence of oxygen, would generate chlorine radicals and, through HAT, promote oxidative degradation of PS under mild conditions while compatible with polymer waste streams containing fillers, composites and other polymers (Scheme 1C).

Nature highlights the presentation! A beam of light saves polystyrene from landfills

[Note] Experimental setup for PS degradation under air flow environment

Our study began with irradiation of PS under ambient airflow using a broad-spectrum white LED lamp in acetone (containing 10 wt % FeCl3). 20 h later, the PS polymer was converted to PS oligomers (Mn = 0.8 kg/mol), 11 mol % (per monomer basis) of small molecules were produced, consisting mainly of benzoyl products: benzoic acid, benzaldehyde, benzoyl chloride, and acetophenone (Table 1). Trace amounts of benzene and chlorobenzene were detected, and the remaining mass balance consisted of styrene oxide oligomers. We also investigated other common light sources where FeCl3 produces chlorine radicals and found that 390 and 427 nm light sources had similar effects on polymer degradation, but yielded fewer benzoyl products. The control experiments confirmed that light, FeCl3 and air are necessary for effective degradation. Furthermore, when the reaction was carried out under ambient conditions but in a sealed bottle, only 1 mol % of benzoyl product was produced and the mass balance contained mildly degraded polystyrene.

Nature highlights the presentation! A beam of light saves polystyrene from landfills

[Figure Note] Distribution of PS degradation products (under different light conditions)

Table 1 Optimization of benzoyl product formation

Nature highlights the presentation! A beam of light saves polystyrene from landfills

To elucidate the importance of chlorine radicals, we compared our approach with benzophenone, a known catalyst for hydrogen extraction, used in previous PS degradation studies. When some degradation was observed, the production of benzoyl products was significantly reduced, indicating the specific properties of the chlorine radical. Experiments testing FeCl3 incorporation showed that the use of 5% wt % FeCl3 produced less than 9 mol % of product, while greater than 10 wt % FeCl3 produced no improvement. The degradation rate was slightly slowed when FeCl3-6H2O was used as a substitute. Although chloride-based additives have previously been shown to promote chlorine radical formation and C-H dissociation, the addition of LiCl, NaCl or NH4Cl did not improve the formation of benzoyl products under our scheme. Since oxygen is necessary for effective degradation, we saturated the system with oxygen and, other things being equal, we observed a more efficient degradation reaction with 23 mol % product formation in only 20 hours.

Nature highlights the presentation! A beam of light saves polystyrene from landfills

[Note] Distribution of products after degradation under different conditions (airflow, catalyst, benzophenone)

Nature highlights the presentation! A beam of light saves polystyrene from landfills

[Note] Distribution of PS degradation products under different iron trichloride content

Nature highlights the presentation! A beam of light saves polystyrene from landfills

[Figure Note] Distribution of PS degradation products under different chloride additive conditions

Nature highlights the presentation! A beam of light saves polystyrene from landfills

[ILLUSTRATION NOTE] Experimental setup for PS degradation under oxygen conditions

Under our optimized conditions, we set out to investigate the kinetics of polymer degradation and benzoyl product formation. Since high molecular weight PS cannot be completely dissolved in acetone, we simultaneously ran identical samples, each removed from the light source at a given time point and analyzed. By the 12 h time point, the total amount of product exceeded 20 mol %. From 20 h to 48 h, the formation of benzoic acid plateaued, indicating that the oxidation of PS oligomers slowed down significantly (Figure 1a). It is noteworthy that the amount of benzaldehyde tended to increase and then decrease during 12 h, while the amount of benzoic acid increased with time and eventually became the main product. To confirm the conversion of benzaldehyde to benzoic acid, we performed experiments in the presence of 10 wt % 4-tert-butylbenzaldehyde dye (Figure 1c).1H NMR NMR data confirmed the quantitative conversion of 4-tert-butylbenzaldehyde to 4-tert-butylbenzoic acid (75%), 4-tert-butylbenzoyl chloride (25%) or 4-tert-butylbenzene (trace), confirming that: under our reaction conditions , benzaldehyde was oxidized to benzoic acid or benzoyl chloride over time. after 12 h, benzoyl chloride also started to decrease, probably by hydrolysis to benzoic acid and regeneration of chlorine.

Nature highlights the presentation! A beam of light saves polystyrene from landfills

Fig. 1 (a) Scatter plot of the decomposition of benzoic acid, benzaldehyde and benzoyl chloride at each time point in the presence of O2. (b) Degradation of oxidized PS in the absence of FeCl3. (c) Control experiments to understand the mechanism of small molecule production. (d) Proposed mechanism of chain breakage and major pathways of benzoyl products.

We conducted further mechanistic studies to elucidate the main pathways of main chain oxidation and benzoyl product generation. First, we sought to determine whether the oxidized PS backbone could be degraded by the Norrish type I dissociation pathway under white light and in the absence of FeCl3. We irradiated the oxidized polystyrene with white light and observed no further degradation in the absence of FeCl3 (Figure 1b). The results suggest that white light is unlikely to promote Norrish type I dissociation of PS oxidation. Therefore, these experiments support our hypothesis that HAT promotes the primary degradation pathway via chlorine or oxygen-centered radicals.

Based on these data, we propose the mechanism as shown in Figure 1d. The photogeneration of chlorine radicals promotes the dissociation of C-H from the PS backbone to generate benzyl radicals, which react with oxygen to give peroxy radicals. The dissociation of the hydrogen atom gives a hydrogen peroxide molecule, and the reduction reaction with Fe(II) likely gives a peroxy radical on the backbone and regenerates Fe(III). β-Cutting off process promotes the splitting of the carbon chain, leaving a phenyl ketone chain end and a Bur group. There are several pathways above this main radical and eventually a benzyl group is formed at the end of the chain. The oxygen-centered radicals generated by the oxidation and β-cutting processes give benzaldehyde and regenerate a primary radical. Oxidation of benzaldehyde to benzoic acid is the end product of the oxidative cleavage process. At the end of the reaction, evaluation experiments on the total product and oligomers showed that there was some mass loss, possibly in the form of formaldehyde or formic acid, indicating that some oxidation of the main radical may have occurred.

Based on our optimized conditions and mechanistic basis, we envision that mild reaction conditions should enable the degradation of commercial PS samples that may contain additives such as UV absorbers, free radical scavengers and composites to improve the durability of PS products. Therefore, we selected several types of PS products, ranging from packaging materials to coffee cup lids, to test the feasibility of our approach. We first measured Mn and dispersion by SEC analysis (Scheme 2) and obtained 1H NMR spectra. As expected, high molecular weights (> 60 kg/mol) and wide dispersion rates (> 2.00) were observed in all PS samples corresponding to the experiments. The lids of the coffee cups also contained additional signals in the 1H NMR spectra, which could be some form of cross-linked polymers or composites. All samples were used without purification.

Nature highlights the presentation! A beam of light saves polystyrene from landfills

Program 2 Degradation of commercial PS products

We observed that the degradation of PS oligomers by black coffee cup lids was minimal under our reaction conditions. Despite the low degradation efficiency, 2.2 mol % of benzoyl product was obtained. We also noted a decrease in dispersibility and observed the appearance of insoluble high molecular weight materials after the reaction. Based on these data, we hypothesized that the black dye inhibited light penetration, but the smaller PS chains in the sample were still effectively degraded. Satisfactorily, white coffee cup lids, polystyrene foam and clear cup lids degraded to PS oligomers. These results indicate that our system can effectively degrade commercial PS samples, even with the addition of composites and insoluble materials. Furthermore, in the case of polystyrene foam and clear lids, we obtained similar levels of benzoyl products with benzoic acid production rates of 13.8 and 16.0 mol %, respectively.

So far, we have demonstrated the feasibility of commercial PS degradation under mild conditions to generate benzoyl products, and we have attempted to demonstrate the scalability of our approach. Photochemistry has evolved to a larger scale, most notably focusing on the transition to photofluidic processes because of the lack of efficiency of intermittent reactions in light-driven processes. With the increase in photon efficiency and biphasic gas/liquid flow systems, we envisioned a degree of degradation at the gram unit level in a few hours for commercial PS (Scheme 3). Our simple device consisted of two syringe pumps and two LED lights placed inside a 3D printed photoreactor. After some optimization, we found that in just a few hours, 1 g of polystyrene bubbles (Mn = 74 kg/mol) were degraded into styrene oxide oligomers (Mn = 0.6 kg/mol). Satisfactorily, we also obtained 10.8 mol % of benzoic acid for a total of 20.4 mol % of benzoyl product, comparable to our experimental results in small batches.

Nature highlights the presentation! A beam of light saves polystyrene from landfills

[Figure Note] Distribution of degradation products of different PS commodities

Nature highlights the presentation! A beam of light saves polystyrene from landfills

Scheme 3 Degradation of commercial PS in a flow-through device

Nature highlights the presentation! A beam of light saves polystyrene from landfills

[Fig. Note] Optical flow experimental setup

We have revealed a gentle and inherently scalable experimental process to chemically upcycle commercial PS into valuable small molecule products. Under mild conditions, high molecular weight PS can be degraded to oligomers and form specific small molecules such as benzoic acid. In addition, we applied the method to several classes of commercial PS products and observed efficient levels of degradation. Finally, we demonstrated the scalability of our degradation method on the gram unit scale by flow chemistry experiments.

Nature highlights the presentation! A beam of light saves polystyrene from landfills

[Note] The possible PS degradation mechanism proposed in this paper

Nature highlights the presentation! A beam of light saves polystyrene from landfills

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