Efficient catalytic upcycling of polyester and polycarbonate plastics using NNN-based iron catalyst

Efficient catalytic upcycling of polyester and polycarbonate plastics using NNN-based iron catalyst | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Efficient catalytic upcycling of polyester and polycarbonate plastics using NNN-based iron catalyst Maofu Pang, Xiaoxiao Chu, Guoren Zhou, Chongyan Ren, Xiaoshi Zhang, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5750869/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted You are reading this latest preprint version Abstract The development of efficient, eco-friendly recycling methods for mitigating the environmental impact of polyester waste remains a significant challenge. Herein, we establish an efficient catalytic system based on an NNN-based iron pincer catalyst, which can facilitate the hydrogenative depolymerization of polyester plastics using two methods. The first method is to depolymerize the polyester into ester monomers via methanolysis and facilitate subsequent transfer hydrogenation using ammonia borane as a hydrogen source to obtain diol products under mild conditions. The second method is to use molecular hydrogen as a hydrogen source for the direct catalytic hydrogenolysis of the plastic to obtain diol products. The catalyst [Fe(NNHN)Cl 2 ] 2 demonstrates high catalytic efficiency in the degradation of polyester and polycarbonate plastics, including when using plastic waste from daily life as raw materials. Physical sciences/Chemistry/Green chemistry/Sustainability Physical sciences/Chemistry/Environmental chemistry/Pollution remediation Physical sciences/Chemistry/Catalysis/Homogeneous catalysis Figures Figure 1 Figure 2 Figure 3 Figure 4 Introduction Polyesters are important polymeric materials extensively used in packaging, textiles, and engineering plastics 1 . Polyester plastic production has significantly growth and changed in recent years. In 2022, the global production of plastics reached 400.3 million metric tons, highlighting the scale of the plastic pollution problem 2 . However, plastic pollution is the world’s second largest environmental issue after climate change. Polyester plastics, due to their wide application and difficulty degradation, are one of the primary pollutants in soil and water 3 , 4 . Microplastics, especially polyester microplastics, contaminate soil, freshwater, and marine ecosystems, affecting wildlife and plant life. They can absorb and carry toxic substances, further harming the environment 5 . Marine organisms’ ingestion of microplastics can lead to bioaccumulation in the food chain, potentially affecting human health. Microplastic pollution is linked to respiratory issues and other health concerns 6 . Addressing polyester plastic pollution requires coordinated efforts from industries, governments, and consumers to adopt sustainable practices and innovative solutions. Therefore, efficient, environmentally friendly plastic recycling methods should be developed 7 . Traditional methods, such as landfilling, and mechanical methods for the recycling of plastic wastes have various drawbacks, such as considerable equipment requirements, high energy consumption, and potential secondary pollution 8 . These limitations highlight the need for eco-friendlier approaches. The chemical recycling and upcycling of plastic waste are vital components of sustainable development 9 . These processes convert plastic waste into valuable chemicals. Compared with traditional chemical recycling methods 10 , 11 , such as hydrolysis 12 , 13 , methanolysis 14 , 15 , and glycolysis 16 , 17 , upcycling is more advantageous 18 . The recently developed transition-metals-catalyzed hydrogenative depolymerization of polyester plastic is gaining increasing attention from both industry and academia due to its high atom economy and potential to enhance sustainability and resource efficiency 19 . Several ruthenium pincer complexes have been utilized as catalysts for the hydrogenation depolymerization of polyester plastics with molecular hydrogen (Fig. 1 a) 20 – 22 . Xie et al. developed a highly efficient strategy for the hydrogenation of polyester waste that involves the initial transesterification of polyester into more degradable oligomeric fragments in the presence of CH 3 OH, followed by hydrogenation using a quinaldine-based Ru complex under mild conditions: 80°C, 1 bar H 2 23 . In comparison with precious metal catalysts, Liu et al. reported the catalytic hydrogenolysis of polyester and polycarbonate polymers using a molecular NHC-based manganese pincer complex, in which a 150°C temperature and a 50 bar H 2 pressure were essential 24 . The transfer hydrogenation of polyesters and polycarbonates is less reported than hydrogenation, although it has milder reaction conditions and does not require special reaction equipment. Here, two catalytic degradation methods are presented (Fig. 1 b). The first is transfer hydrogenation with ammonia borane as the hydrogen source. The degradation reaction is divided into two steps; the first step is the methanolysis of polyester which produces plastic monomers 25 , 26 , and the second is the transfer hydrogenation of the monomers (which produces diols). The second catalytic degradation method is direct hydrogenolysis with hydrogen as the hydrogen source, where the polyester plastic is directly degraded into diols. Both methods use the synthesized phosphine-free NNN-based iron pincer catalyst [Fe(NNHN)Cl 2 ], which has high reactivity in methanolysis, transfer hydrogenation, or hydrogenolysis. Results and discussion The Fe dipyridyl amine pincer complex [Fe(NNHN)Cl 2 ] 2 ( Fe1 )was synthesized as previously described 27 . Bis(pyridin-2-ylmethyl)amine and FeCl 2 were added to THF, and the mixture was heated to 50°C and stirred for 2 h (Eq. 1). The suspension was recrystallized at − 30°C, and yellow microcrystals were obtained. Analysis of the crystal structure reveals that this is a dimer complex, with two chlorine atoms linking the iron center (Fig. 2 ). Catalytic transfer hydrogenation of esters In degrading plastics, such as polyethylene terephthalate (PET), we initially assessed the capability of complex Fe1 to facilitate the transfer hydrogenation of ester compounds. In initial experiments, the transfer hydrogenation of methyl benzoate was examined using 2 mol% Fe1 in conjunction with a base and H 3 N·BH 3 as the hydrogen source (Table 1 ). Utilizing NaOH as the base at 50°C resulted in low conversion, but the use of employing tetrahydrofuran (THF) as the solvent slightly improved this, achieving 37% conversion (Table 1 , entries 1–3). The conversion improved with NaOMe as the base (Table 1 , entry 4) and increased significantly to 83% with KO t Bu as the base (Table 1 , entry 5). Elevating the reaction temperature to 60°C further increased conversion to 96% (Table 1 , entry 6). Control experiments were conducted in the absence of Fe1 (Table 1 , entry 7) or the base (Table 1 , entry 8), and no conversion of methyl benzoate occurred. After the optimal conditions were established, we examined the substrate scope of the reaction using the catalyst [Fe(NNHN)Cl 2 ] 2 ( Fe1 ). As shown in Table 2 , Fe1 was an efficient precatalyst for the reduction of various (hetero)aromatic and aliphatic esters. The transfer hydrogenation of 1mmol methyl benzoate with H 3 N·BH 3 at 60°C in THF resulted in a 90% isolated yield of phenylmethanol (Table 2 , entry 1). Ethyl benzoate, isopropyl benzoate, and butyl benzoate were also subjected to hydrogenation, affording phenylmethanol with high yields ranging from 85–92% under the same conditions (Table 2 , entries 2–4). Then, the substituents were changed at the para position of the benzene ring; methyl 4-methoxybenzoate was smoothly hydrogenated, leading to an 88% yield of (4-methoxyphenyl)methanol (Table 2 , entry 5). The transfer hydrogenation of methyl 4-acetylbenzoate resulted in an 83% yield of 1-(4-(hydroxymethyl)phenyl)ethan-1-ol, confirming that carbonyl groups are reducible within this catalytic system (Table 2 , entry 6). For halogens, including fluorine and chlorine, the reaction system exhibited good compatibility, with good yields of the corresponding alcohols (Table 2 , entries 7 and 8). Esters with electron-donating or electron-withdrawing groups underwent transfer hydrogenation, forming the corresponding alcohols with yields ranging from good to very good (Table 2 , entries 4, 5, 7, 8). Methyl 4-cyanobenzoate was hydrogenated to yield 75% of 4-(hydroxymethyl)benzonitrile; this relatively low yield was ascribed to competing nitrile coordination, with the cyano group remaining intact, demonstrating the catalytic reaction's good functional group tolerance (Table 2 , entry 9). Upon the relocation of the substituent to the 2-position of the benzene ring, the catalytic reaction remained unimpaired. The hydrogenation of methyl 2-fluorobenzoate yielded 89% of (2-fluorophenyl)methanol (Table 2 , entry 10). Subsequently, altering the functional group on the C-O single-bond side of the ester had no discernible impact on the reaction. Phenyl benzoate, 4-fluorophenyl acetate, and 2-chlorophenyl acetate (Table 2 , entries 11–13) were successfully hydrogenated, yielding the desired products with good to excellent yields of 81–88%. The introduction of fused-ring functional groups, such as naphthyl, into the catalytic system revealed that both methyl 1-naphthoate and methyl 2-naphthoate exhibited good reactivity, producing the corresponding alcohols (Table 2 , entries 14 and 15). This also showed that a large steric hindrance had little effect on the reaction. The hydrogenation of [1,1′-biphenyl]-4-yl benzoate resulted in [1,1′-biphenyl]-4-ol with an 83% yield (Table 2 , entry 16). The catalyst demonstrated robust catalytic activity for aliphatic esters; for instance, methyl cyclohexanecarboxylate was efficiently converted into cyclohexylmethanol with an 86% yield (Table 2 , entry 17). Methyl pentanoate was reduced to pentan-1-ol with a 74% yield (Table 2 , entry 18). Similarly, methyl isobutyrate was converted into isobutanol with a 77% yield (Table 2 , entry 19). These low isolated yields were attributed to the volatility of the products. Methyl nonanoate was converted into nonan-1-ol with an 86% yield (Table 2 , entry 20). The hydrogenation of vinyl hexanoate yielded hexanol with a 75% yield (Table 2 , entry 21). For lactone compounds, this system also facilitated efficient hydrogenation. Chroman-2-one and 5-butyldihydrofuran-2(3H)-one were successfully hydrogenated to produce 2-(3-hydroxypropyl)phenol and octane-1,4-diol with yields of 90% and 88%, respectively (Table 2 , entries 22 and 23, respectively). Additionally, carbonate compounds were tested; they were efficiently converted into the corresponding alcohols with excellent isolated yields of 89–93%, indicating the broad applicability of the reaction system to carbonate substrates (Table 2 , entries 24–26). Catalytic methanolysis of PET The hydrogenation depolymerization of PET typically involves two sequential steps. The first step is cleaving the ester bonds within the PET via alcoholysis to produce ester monomers. The second step is hydrogenating these ester monomers to yield the corresponding diols. Given that esters have been hydrogenated and our catalytic system effectively facilitated the alcoholysis of PET, the subsequent hydrogenolysis would be feasible, resulting in the formation of diol products. We investigated the reaction conditions for PET alcoholysis and discovered that in the existing catalytic system, methanol can serve as a solvent for the methanolysis of PET (Table 3 ), yielding dimethyl terephthalate (DMT) and ethylene glycol (EG). At reaction heating temperatures not exceeding 60°C, the separation yield of DMT was not high, with the highest isolated yield being only 71% (Table 3 , entries 1–3). When the temperature increased to 80°C, the isolated yield reached 90% (Table 3 , entry 4). The use of alternative solvents, including tetrahydrofuran and toluene, resulted in no detectable formation of DMT (Table 3 , entries 5 and 6). Reports in the literature indicate that potassium tert-butoxide can catalyze the alcoholysis of PET independently ( 20 ); thus, we conducted a control experiment: Under identical reaction conditions, omitting the catalyst and ammonia borane resulted in an isolated yield of DMT of only 10% (Table 3 , entry 7). Conversely, in the absence of potassium tert-butoxide, the reaction yield was negligible, regardless of whether the catalyst or ammonia borane was present (Table 3 , entry 8). Therefore, the iron-hydride complex generated in situ by the reaction of Fe1 with KO t Bu and H 3 N·BH 3 is the catalyst for methanolysis. Its presence greatly increased the reaction rate. Based on the above results, our catalytic system effectively facilitated the methanolysis of PET and obtained DMT and EG products with high isolated yields. The Fe1 precatalyst was an effective ester transfer hydrogenation catalyst and PET methanolysis catalyst. The combination of its two catalytic capabilities efficiently hydrogenated and depolymerized PET. Polyester degradation through methanolysis/hydrogenative depolymerization The first degradation reaction was conducted in the presence of the catalyst [Fe(NNHN)Cl 2 ] 2 and KO t Bu with aromatic polyester plastics, such as PET and polybutylene terephthalate (PBT). Our catalytic system depolymerized PET into DMT and EG, or PBT into DMT and 1,4-butandiol, at 80°C through methanolysis and then facilitated the transfer hydrogenation of DMT to 1,4-benzene dimethanol and methanol at 60°C with excellent isolated yields (Fig. 3 a, methanol is not drawn for clarity, same as below). Through this methanolysis/transfer hydrogenation reaction, PET and PBT can be efficiently hydrogenated and depolymerized to obtain 1,4-benzene dimethanol and methanol. This method effectively addresses plastic pollution, as PET is a wildly produced and most consumed polyester plastic. Most of the water and other beverage bottles used in daily life are made of PET, which is also the most common type of waste plastic discarded by humans. The aliphatic polyester polycaprolactone (PCL) could also be methanolized to produce methyl 6-hydroxyhexanoate with a 90% isolated yield, which then underwent transfer hydrogenation, producing 1,6-hexanediol with an 87% isolated yield (Fig. 3 b). Chain lipids were less reactive in our catalytic system and required more catalyst (5 mol%) for higher yields. Polylactic acid (PLA) could be converted into methyl 2-hydroxypropanoate and then into 1,2-propanediol with an 89% isolated yield under the same catalytic conditions (Fig. 3 c). Although PCL and PLA are biodegradable plastics that are widely used in daily life and industrial production, their natural degradation conditions are harsh and require prolonged periods, and the degradation products are mainly carbon dioxide, a greenhouse gas. Therefore, hydrogenative degradation is appropriate for these types of plastics. Polycarbonate plastics, typically those based on bisphenol A (PC), can be hydrogenated directly to bisphenol A and methanol using [Fe(NNHN)Cl 2 ] 2 and KO t Bu as the catalytic system (Fig. 3 d). Polyester degradation through hydrogenolysis In the degradation of polyester plastics, methanolysis/hydrogenative depolymerization leads to milder reaction conditions, easier operation, and more convenient mechanism exploration. However, the reaction must be divided into two steps with different solvents, making it unsuitable for practical applications, especially industrial production. Thus, we also attempted direct hydrogenation for plastic degradation and found that the use of hydrogen as the hydrogen source achieved the hydrogenolysis of polyester plastics at high temperature and pressure with [Fe(NNHN)Cl 2 ] 2 and KO t Bu as the catalyst system. In this hydrogenolysis experiment, common 0.5-liter PET water bottle was unsealed, cut into pieces, and placed in a 250-mL autoclave. At 120°C and a 20 bar H 2 pressure, diol products with an 84% isolated yield were obtained at a catalyst loading of 2 mol% Fe1 and 5 mol% KO t Bu (Fig. 4 , top row; Table 4 , entry 1). PCL, a common biodegradable plastic widely used in the medical field, was selected for this experiment, with medical 3D printing supplies serving as the reaction substrates. Under the appropriate reaction conditions, the 3D printing supplies, which were cut into small pieces, were converted into 1,6-hexanediol as a white crystalline solid with a 76% separation yield at a high catalyst loading of 3 mol% Fe1 and 6 mol% KO t Bu (Fig. 4 , second row; Table 4 , entry 2). The blue coating on 3D printing supplies was presumably nonreactive and could be easily separated via filtration. A PLA beverage cup was cut into small pieces and placed in the autoclave under the established hydrogenolysis conditions, and 1,2-propanediol was obtained as a light yellow oil with an 87% isolated yield at a rather low catalyst loading of 1.5 mol% Fe1 and 3 mol% KO t Bu (Fig. 4 , third row; Table 4 , entry 3). In the subsequent reaction, three CDs were cut into small pieces and added into the reaction system to form bisphenol A and methanol smoothly using 3 mol% Fe1 and 6 mol% KO t Bu (Fig. 4 , bottom row; Table 4 , entry 4). Due to the thick coating, the separation yield in this reaction was comparatively low at 69%, and the unreacted coating could also be directly removed via filtration. In summary, an NNN-based iron pincer catalyst was developed for the efficient upcycling of polyester and polycarbonate plastics. The catalyst’s ability to facilitate (1) methanolysis and transfer hydrogenation and (2) direct catalytic hydrogenolysis under mild conditions is a breakthrough in plastic waste management. The high yields and broad applicability of the catalyst across various types of plastics (PET, PBT, PCL, PLA, and polycarbonates) underscore its potential for industrial-scale recycling. This research contributes to the academic community by deepening the understanding of catalytic upcycling and offers practical solutions to address the environmental challenges of plastic pollution, aligning with the goals of a circular economy. By converting waste plastics into value-added diols, this study aligns with the circular economy model, promoting a more sustainable future. Methods Materials All reagents were purchased from Sigma-Aldrich, TCI or Acros and used without further purification. Methanol, toluene, and tetrahydrofuran were purified using a Glass Contour solvent purification system consisting of a neutral alumina, copper catalyst, and activated molecular sieves, then passed through an in-line, 2 µm filter immediately before being dispensed. Physical methods NMR spectra were recorded on Bruker Avance 500 spectrometer in NMR tubes at room temperature. Chemical shifts (δ) were reported in parts per million (ppm) with referenced to the proton signal of the deuterated solvent. CDCl 3 were dried over CaH 2 and purified by vacuum transfer. MS (HRMS) measured with ThermoFisher Q-Exactive Mass Spectrometer. Synthetic methods General procedure for transfer hydrogenation of esters In a glovebox under a nitrogen atmosphere, a scintillation vial equipped with a magnetic stir bar was charged with esters (1.0 mmol) and H 3 N·BH 3 (1.0 mmol, 31 mg). The catalyst Fe1 (0.02 mmol, 7 mg), KO t Bu (0.05 mmol, 6 mg), and THF (2 mL) were added. The mixture was stirred at 60°C. After the indicated time, the reaction mixture was isolated through chromatography on silica gel to obtain the products. General procedure for catalytic methanolysis/hydrogenative depolymerization of polyester and polycarbonate plastics For all polymers, the molar amount used was calculated based on the corresponding repetition units. In a glovebox under an N 2 atmosphere, a scintillation vial (with a magnetic stir bar) was charged with Fe1 (0.02 mmol, 7 mg), KO t Bu (0.05 mmol, 6 mg), H 3 N·BH 3 (0.02 mmol, 1 mg), and MeOH (2 mL). Then, the polymer (1.0 mmol) was added. The mixture was stirred at 80°C. After the indicated time, the reaction mixture was isolated via chromatography on silica gel to produce the ester products. The obtained esters were then processed according to Section S3 to obtain the diols. General procedure for autoclave reactions The Processed polymer was placed in the glass insert of the autoclave, a stir bar was added, and the insert was placed in the 250 mL steel autoclave. The autoclave was evacuated and backfilled with N 2 three times. Fe1 and KO t Bu were weighed in a 10 mL Schlenk tube and dissolved in 5 mL THF. The solution was transferred into the autoclave in a N 2 counter-stream using a syringe equipped with a cannula. The autoclave was pressurized with 20 bar H 2 . The reaction was stirred at 120°C for 24 h. After completion of the reaction time, the autoclave was cooled down to room temperature in an ice bath and carefully vented to the atmosphere. The reaction mixture was isolated via chromatography on silica gel to obtain the products. Declarations Data Availability All data supporting the findings of this study are available within the article and its Supplementary Information or from the corresponding author upon reasonable request. CCDC Number: 2433516. Acknowledgements This work was supported by National Science Foundation of Shandong Province (No. ZR2022QB036), Shandong Laboratory of Advanced Materials and Green Manufacturing at Yantai (No. AMGM2023F10, AMGM2023F11). Ethics declarations Competing interests The authors have no competing interests as defined by Nature Portfolio, or other interests that might be perceived to influence the results and/or discussion reported in this paper. References Zheng, K. et al. Progress and perspective for conversion of plastic wastes into valuable chemicals. Chem. Soc. Rev. 52, 8–29 (2023). Schwab, S. T., Baur, M., Nelson, T. F. & Mecking, S. Synthesis and deconstruction of polyethylene-type materials. Chem. Rev. 124, 2327–2351 (2024). Sullivan, K. P. et al. Mixed plastics waste valorization through tandem chemical oxidation and biological funneling. Science 378, 207–211 (2022). Rahimi, A. & García, J. M. Chemical recycling of waste plastics for new materials production. 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Supplementary Files checkcif.pdf Checkcif Cif.cif Cif SupplementaryInformation.pdf Supplementary Information Tables.docx Cite Share Download PDF Status: Under Review Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-5750869","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":437864308,"identity":"ec2d3e2f-daee-4443-ac87-020ae0eb2bf8","order_by":0,"name":"Maofu 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University","correspondingAuthor":false,"prefix":"","firstName":"Nuoyan","middleName":"","lastName":"Zhao","suffix":""}],"badges":[],"createdAt":"2025-01-02 10:10:30","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5750869/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5750869/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":81282289,"identity":"ca261d50-e490-4451-9464-a524e8ed7e30","added_by":"auto","created_at":"2025-04-24 10:22:17","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":63197,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eHydrogenative depolymerization of polyester and polycarbonate plastics. a \u003c/strong\u003ePrevious work on polyesters hydrogenolysis by homogeneous catalysts. \u003cstrong\u003eb \u003c/strong\u003eThe present work on catalytic methanolysis/transfer hydrogenation or hydrogenolysis of polyesters and polycarbonate using an iron pincer catalyst\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-5750869/v1/11f156844b7d32abf1d6599a.png"},{"id":81283581,"identity":"7af15e9f-6a17-4152-8c76-4a8bb061dd8a","added_by":"auto","created_at":"2025-04-24 10:30:17","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":39417,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eThe synthesis of [Fe(NNHN)Cl\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e]\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e.\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-5750869/v1/da2ca6f99995566465e2e785.png"},{"id":81282293,"identity":"e680f3f8-b8f0-4d3c-aefb-a52e19c8b416","added_by":"auto","created_at":"2025-04-24 10:22:17","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":55544,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eCatalytic methanolysis/hydrogenative depolymerization of polyester and polycarbonate plastics. a \u003c/strong\u003eThe catalytic methanolysis and hydrogenation of PET and PBT. \u003cstrong\u003eb\u003c/strong\u003e The catalytic methanolysis and hydrogenation of PCL. \u003cstrong\u003ec\u003c/strong\u003e The catalytic methanolysis and hydrogenation of PLA. \u003cstrong\u003ed\u003c/strong\u003e The catalytic transfer hydrogenation of PLA.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-5750869/v1/e2cda9aac35d9e51361f1625.png"},{"id":81282294,"identity":"aa5b4e04-2cd3-4449-9605-07157e62797e","added_by":"auto","created_at":"2025-04-24 10:22:17","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":436123,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eHydrogenolysis of selected polyesters and polycarbonate consumer products.\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-5750869/v1/d3acfdcf7019272b0ab4882a.png"},{"id":81284358,"identity":"36817024-1125-495c-8ebb-ca8043c9c86d","added_by":"auto","created_at":"2025-04-24 10:38:34","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1141007,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5750869/v1/7818b5bc-b757-414e-a595-df016f6bbf90.pdf"},{"id":81282291,"identity":"ae2a11eb-2009-4217-8531-e3cfaea976a1","added_by":"auto","created_at":"2025-04-24 10:22:17","extension":"pdf","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":129859,"visible":true,"origin":"","legend":"Checkcif","description":"","filename":"checkcif.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5750869/v1/fc04f324c0517e9715afd935.pdf"},{"id":81282295,"identity":"3f7bb261-5fae-4c64-9bd2-9ebeb8292e09","added_by":"auto","created_at":"2025-04-24 10:22:17","extension":"cif","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":1122334,"visible":true,"origin":"","legend":"Cif","description":"","filename":"Cif.cif","url":"https://assets-eu.researchsquare.com/files/rs-5750869/v1/2614405cc6fac5c42bdf4ffc.cif"},{"id":81282312,"identity":"d12068d1-12df-47b8-a13e-bc91db5bc250","added_by":"auto","created_at":"2025-04-24 10:22:18","extension":"pdf","order_by":3,"title":"","display":"","copyAsset":false,"role":"supplement","size":2677068,"visible":true,"origin":"","legend":"Supplementary Information","description":"","filename":"SupplementaryInformation.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5750869/v1/f94b10cc16b50b0d19851318.pdf"},{"id":81282297,"identity":"21a42679-fda4-4fd4-b926-f666008d0449","added_by":"auto","created_at":"2025-04-24 10:22:17","extension":"docx","order_by":4,"title":"","display":"","copyAsset":false,"role":"supplement","size":142434,"visible":true,"origin":"","legend":"","description":"","filename":"Tables.docx","url":"https://assets-eu.researchsquare.com/files/rs-5750869/v1/e77380e3bf2ce8fd9aabe049.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Efficient catalytic upcycling of polyester and polycarbonate plastics using NNN-based iron catalyst","fulltext":[{"header":"Introduction","content":"\u003cp\u003ePolyesters are important polymeric materials extensively used in packaging, textiles, and engineering plastics\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e. Polyester plastic production has significantly growth and changed in recent years. In 2022, the global production of plastics reached 400.3\u0026nbsp;million metric tons, highlighting the scale of the plastic pollution problem\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e. However, plastic pollution is the world\u0026rsquo;s second largest environmental issue after climate change. Polyester plastics, due to their wide application and difficulty degradation, are one of the primary pollutants in soil and water\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e,\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e. Microplastics, especially polyester microplastics, contaminate soil, freshwater, and marine ecosystems, affecting wildlife and plant life. They can absorb and carry toxic substances, further harming the environment\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e. Marine organisms\u0026rsquo; ingestion of microplastics can lead to bioaccumulation in the food chain, potentially affecting human health. Microplastic pollution is linked to respiratory issues and other health concerns\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e. Addressing polyester plastic pollution requires coordinated efforts from industries, governments, and consumers to adopt sustainable practices and innovative solutions. Therefore, efficient, environmentally friendly plastic recycling methods should be developed\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e. Traditional methods, such as landfilling, and mechanical methods for the recycling of plastic wastes have various drawbacks, such as considerable equipment requirements, high energy consumption, and potential secondary pollution\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e. These limitations highlight the need for eco-friendlier approaches.\u003c/p\u003e \u003cp\u003eThe chemical recycling and upcycling of plastic waste are vital components of sustainable development\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e. These processes convert plastic waste into valuable chemicals. Compared with traditional chemical recycling methods\u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e,\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u003c/sup\u003e, such as hydrolysis\u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e,\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e, methanolysis\u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e,\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e, and glycolysis\u003csup\u003e\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e,\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e, upcycling is more advantageous\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e. The recently developed transition-metals-catalyzed hydrogenative depolymerization of polyester plastic is gaining increasing attention from both industry and academia due to its high atom economy and potential to enhance sustainability and resource efficiency\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e. Several ruthenium pincer complexes have been utilized as catalysts for the hydrogenation depolymerization of polyester plastics with molecular hydrogen (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea)\u003csup\u003e\u003cspan additionalcitationids=\"CR21\" citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e. Xie et al. developed a highly efficient strategy for the hydrogenation of polyester waste that involves the initial transesterification of polyester into more degradable oligomeric fragments in the presence of CH\u003csub\u003e3\u003c/sub\u003eOH, followed by hydrogenation using a quinaldine-based Ru complex under mild conditions: 80\u0026deg;C, 1 bar H\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e23\u003c/sup\u003e. In comparison with precious metal catalysts, Liu et al. reported the catalytic hydrogenolysis of polyester and polycarbonate polymers using a molecular NHC-based manganese pincer complex, in which a 150\u0026deg;C temperature and a 50 bar H\u003csub\u003e2\u003c/sub\u003e pressure were essential\u003csup\u003e\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe transfer hydrogenation of polyesters and polycarbonates is less reported than hydrogenation, although it has milder reaction conditions and does not require special reaction equipment. Here, two catalytic degradation methods are presented (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb). The first is transfer hydrogenation with ammonia borane as the hydrogen source. The degradation reaction is divided into two steps; the first step is the methanolysis of polyester which produces plastic monomers\u003csup\u003e\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e,\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e, and the second is the transfer hydrogenation of the monomers (which produces diols). The second catalytic degradation method is direct hydrogenolysis with hydrogen as the hydrogen source, where the polyester plastic is directly degraded into diols. Both methods use the synthesized phosphine-free NNN-based iron pincer catalyst [Fe(NNHN)Cl\u003csub\u003e2\u003c/sub\u003e], which has high reactivity in methanolysis, transfer hydrogenation, or hydrogenolysis.\u003c/p\u003e"},{"header":"Results and discussion","content":"\u003cp\u003eThe Fe dipyridyl amine pincer complex [Fe(NNHN)Cl\u003csub\u003e2\u003c/sub\u003e]\u003csub\u003e2\u003c/sub\u003e (\u003cstrong\u003eFe1\u003c/strong\u003e)was synthesized as previously described\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e. Bis(pyridin-2-ylmethyl)amine and FeCl\u003csub\u003e2\u003c/sub\u003e were added to THF, and the mixture was heated to 50\u0026deg;C and stirred for 2 h (Eq. 1). The suspension was recrystallized at \u0026minus;\u0026thinsp;30\u0026deg;C, and yellow microcrystals were obtained. Analysis of the crystal structure reveals that this is a dimer complex, with two chlorine atoms linking the iron center (Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e).\u003c/p\u003e\n\u003cp\u003eCatalytic transfer hydrogenation of esters\u003c/p\u003e\n\u003cp\u003eIn degrading plastics, such as polyethylene terephthalate (PET), we initially assessed the capability of complex \u003cstrong\u003eFe1\u003c/strong\u003e to facilitate the transfer hydrogenation of ester compounds. In initial experiments, the transfer hydrogenation of methyl benzoate was examined using 2 mol% \u003cstrong\u003eFe1\u003c/strong\u003e in conjunction with a base and H\u003csub\u003e3\u003c/sub\u003eN\u0026middot;BH\u003csub\u003e3\u003c/sub\u003e as the hydrogen source (Table \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e). Utilizing NaOH as the base at 50\u0026deg;C resulted in low conversion, but the use of employing tetrahydrofuran (THF) as the solvent slightly improved this, achieving 37% conversion (Table \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e, entries 1\u0026ndash;3). The conversion improved with NaOMe as the base (Table \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e, entry 4) and increased significantly to 83% with KO\u003csup\u003e\u003cem\u003et\u003c/em\u003e\u003c/sup\u003eBu as the base (Table \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e, entry 5). Elevating the reaction temperature to 60\u0026deg;C further increased conversion to 96% (Table \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e, entry 6). Control experiments were conducted in the absence of \u003cstrong\u003eFe1\u003c/strong\u003e (Table \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e, entry 7) or the base (Table \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e, entry 8), and no conversion of methyl benzoate occurred.\u003c/p\u003e\n\u003cp\u003eAfter the optimal conditions were established, we examined the substrate scope of the reaction using the catalyst [Fe(NNHN)Cl\u003csub\u003e2\u003c/sub\u003e]\u003csub\u003e2\u003c/sub\u003e (\u003cstrong\u003eFe1\u003c/strong\u003e). As shown in Table \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e, \u003cstrong\u003eFe1\u003c/strong\u003e was an efficient precatalyst for the reduction of various (hetero)aromatic and aliphatic esters. The transfer hydrogenation of 1mmol methyl benzoate with H\u003csub\u003e3\u003c/sub\u003eN\u0026middot;BH\u003csub\u003e3\u003c/sub\u003e at 60\u0026deg;C in THF resulted in a 90% isolated yield of phenylmethanol (Table \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e, entry 1). Ethyl benzoate, isopropyl benzoate, and butyl benzoate were also subjected to hydrogenation, affording phenylmethanol with high yields ranging from 85\u0026ndash;92% under the same conditions (Table \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e, entries 2\u0026ndash;4). Then, the substituents were changed at the para position of the benzene ring; methyl 4-methoxybenzoate was smoothly hydrogenated, leading to an 88% yield of (4-methoxyphenyl)methanol (Table \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e, entry 5). The transfer hydrogenation of methyl 4-acetylbenzoate resulted in an 83% yield of 1-(4-(hydroxymethyl)phenyl)ethan-1-ol, confirming that carbonyl groups are reducible within this catalytic system (Table \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e, entry 6). For halogens, including fluorine and chlorine, the reaction system exhibited good compatibility, with good yields of the corresponding alcohols (Table \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e, entries 7 and 8). Esters with electron-donating or electron-withdrawing groups underwent transfer hydrogenation, forming the corresponding alcohols with yields ranging from good to very good (Table \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e, entries 4, 5, 7, 8). Methyl 4-cyanobenzoate was hydrogenated to yield 75% of 4-(hydroxymethyl)benzonitrile; this relatively low yield was ascribed to competing nitrile coordination, with the cyano group remaining intact, demonstrating the catalytic reaction\u0026apos;s good functional group tolerance (Table \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e, entry 9). Upon the relocation of the substituent to the 2-position of the benzene ring, the catalytic reaction remained unimpaired. The hydrogenation of methyl 2-fluorobenzoate yielded 89% of (2-fluorophenyl)methanol (Table \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e, entry 10). Subsequently, altering the functional group on the C-O single-bond side of the ester had no discernible impact on the reaction. Phenyl benzoate, 4-fluorophenyl acetate, and 2-chlorophenyl acetate (Table \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e, entries 11\u0026ndash;13) were successfully hydrogenated, yielding the desired products with good to excellent yields of 81\u0026ndash;88%. The introduction of fused-ring functional groups, such as naphthyl, into the catalytic system revealed that both methyl 1-naphthoate and methyl 2-naphthoate exhibited good reactivity, producing the corresponding alcohols (Table \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e, entries 14 and 15). This also showed that a large steric hindrance had little effect on the reaction. The hydrogenation of [1,1\u0026prime;-biphenyl]-4-yl benzoate resulted in [1,1\u0026prime;-biphenyl]-4-ol with an 83% yield (Table \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e, entry 16). The catalyst demonstrated robust catalytic activity for aliphatic esters; for instance, methyl cyclohexanecarboxylate was efficiently converted into cyclohexylmethanol with an 86% yield (Table \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e, entry 17). Methyl pentanoate was reduced to pentan-1-ol with a 74% yield (Table \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e, entry 18). Similarly, methyl isobutyrate was converted into isobutanol with a 77% yield (Table \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e, entry 19). These low isolated yields were attributed to the volatility of the products. Methyl nonanoate was converted into nonan-1-ol with an 86% yield (Table \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e, entry 20). The hydrogenation of vinyl hexanoate yielded hexanol with a 75% yield (Table \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e, entry 21). For lactone compounds, this system also facilitated efficient hydrogenation. Chroman-2-one and 5-butyldihydrofuran-2(3H)-one were successfully hydrogenated to produce 2-(3-hydroxypropyl)phenol and octane-1,4-diol with yields of 90% and 88%, respectively (Table \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e, entries 22 and 23, respectively). Additionally, carbonate compounds were tested; they were efficiently converted into the corresponding alcohols with excellent isolated yields of 89\u0026ndash;93%, indicating the broad applicability of the reaction system to carbonate substrates (Table \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e, entries 24\u0026ndash;26).\u003c/p\u003e\n\u003cp\u003eCatalytic methanolysis of PET\u003c/p\u003e\n\u003cp\u003eThe hydrogenation depolymerization of PET typically involves two sequential steps. The first step is cleaving the ester bonds within the PET via alcoholysis to produce ester monomers. The second step is hydrogenating these ester monomers to yield the corresponding diols. Given that esters have been hydrogenated and our catalytic system effectively facilitated the alcoholysis of PET, the subsequent hydrogenolysis would be feasible, resulting in the formation of diol products.\u003c/p\u003e\n\u003cp\u003eWe investigated the reaction conditions for PET alcoholysis and discovered that in the existing catalytic system, methanol can serve as a solvent for the methanolysis of PET (Table \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e), yielding dimethyl terephthalate (DMT) and ethylene glycol (EG). At reaction heating temperatures not exceeding 60\u0026deg;C, the separation yield of DMT was not high, with the highest isolated yield being only 71% (Table \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e, entries 1\u0026ndash;3). When the temperature increased to 80\u0026deg;C, the isolated yield reached 90% (Table \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e, entry 4). The use of alternative solvents, including tetrahydrofuran and toluene, resulted in no detectable formation of DMT (Table \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e, entries 5 and 6). Reports in the literature indicate that potassium tert-butoxide can catalyze the alcoholysis of PET independently (\u003cem\u003e20\u003c/em\u003e); thus, we conducted a control experiment: Under identical reaction conditions, omitting the catalyst and ammonia borane resulted in an isolated yield of DMT of only 10% (Table \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e, entry 7). Conversely, in the absence of potassium tert-butoxide, the reaction yield was negligible, regardless of whether the catalyst or ammonia borane was present (Table \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e, entry 8). Therefore, the iron-hydride complex generated in situ by the reaction of \u003cstrong\u003eFe1\u003c/strong\u003e with KO\u003csup\u003e\u003cem\u003et\u003c/em\u003e\u003c/sup\u003eBu and H\u003csub\u003e3\u003c/sub\u003eN\u0026middot;BH\u003csub\u003e3\u003c/sub\u003e is the catalyst for methanolysis. Its presence greatly increased the reaction rate.\u003c/p\u003e\n\u003cp\u003eBased on the above results, our catalytic system effectively facilitated the methanolysis of PET and obtained DMT and EG products with high isolated yields. The \u003cstrong\u003eFe1\u003c/strong\u003e precatalyst was an effective ester transfer hydrogenation catalyst and PET methanolysis catalyst. The combination of its two catalytic capabilities efficiently hydrogenated and depolymerized PET.\u003c/p\u003e\n\u003cp\u003ePolyester degradation through methanolysis/hydrogenative depolymerization\u003c/p\u003e\n\u003cp\u003eThe first degradation reaction was conducted in the presence of the catalyst [Fe(NNHN)Cl\u003csub\u003e2\u003c/sub\u003e]\u003csub\u003e2\u003c/sub\u003e and KO\u003csup\u003e\u003cem\u003et\u003c/em\u003e\u003c/sup\u003eBu with aromatic polyester plastics, such as PET and polybutylene terephthalate (PBT). Our catalytic system depolymerized PET into DMT and EG, or PBT into DMT and 1,4-butandiol, at 80\u0026deg;C through methanolysis and then facilitated the transfer hydrogenation of DMT to 1,4-benzene dimethanol and methanol at 60\u0026deg;C with excellent isolated yields (Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ea, methanol is not drawn for clarity, same as below). Through this methanolysis/transfer hydrogenation reaction, PET and PBT can be efficiently hydrogenated and depolymerized to obtain 1,4-benzene dimethanol and methanol. This method effectively addresses plastic pollution, as PET is a wildly produced and most consumed polyester plastic. Most of the water and other beverage bottles used in daily life are made of PET, which is also the most common type of waste plastic discarded by humans. The aliphatic polyester polycaprolactone (PCL) could also be methanolized to produce methyl 6-hydroxyhexanoate with a 90% isolated yield, which then underwent transfer hydrogenation, producing 1,6-hexanediol with an 87% isolated yield (Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eb). Chain lipids were less reactive in our catalytic system and required more catalyst (5 mol%) for higher yields. Polylactic acid (PLA) could be converted into methyl 2-hydroxypropanoate and then into 1,2-propanediol with an 89% isolated yield under the same catalytic conditions (Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ec). Although PCL and PLA are biodegradable plastics that are widely used in daily life and industrial production, their natural degradation conditions are harsh and require prolonged periods, and the degradation products are mainly carbon dioxide, a greenhouse gas. Therefore, hydrogenative degradation is appropriate for these types of plastics. Polycarbonate plastics, typically those based on bisphenol A (PC), can be hydrogenated directly to bisphenol A and methanol using [Fe(NNHN)Cl\u003csub\u003e2\u003c/sub\u003e]\u003csub\u003e2\u003c/sub\u003e and KO\u003csup\u003e\u003cem\u003et\u003c/em\u003e\u003c/sup\u003eBu as the catalytic system (Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ed).\u003c/p\u003e\n\u003cp\u003ePolyester degradation through hydrogenolysis\u003c/p\u003e\n\u003cp\u003eIn the degradation of polyester plastics, methanolysis/hydrogenative depolymerization leads to milder reaction conditions, easier operation, and more convenient mechanism exploration. However, the reaction must be divided into two steps with different solvents, making it unsuitable for practical applications, especially industrial production. Thus, we also attempted direct hydrogenation for plastic degradation and found that the use of hydrogen as the hydrogen source achieved the hydrogenolysis of polyester plastics at high temperature and pressure with [Fe(NNHN)Cl\u003csub\u003e2\u003c/sub\u003e]\u003csub\u003e2\u003c/sub\u003e and KO\u003csup\u003e\u003cem\u003et\u003c/em\u003e\u003c/sup\u003eBu as the catalyst system.\u003c/p\u003e\n\u003cp\u003eIn this hydrogenolysis experiment, common 0.5-liter PET water bottle was unsealed, cut into pieces, and placed in a 250-mL autoclave. At 120\u0026deg;C and a 20 bar H\u003csub\u003e2\u003c/sub\u003e pressure, diol products with an 84% isolated yield were obtained at a catalyst loading of 2 mol% \u003cstrong\u003eFe1\u003c/strong\u003e and 5 mol% KO\u003csup\u003e\u003cem\u003et\u003c/em\u003e\u003c/sup\u003eBu (Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e, top row; Table \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e, entry 1). PCL, a common biodegradable plastic widely used in the medical field, was selected for this experiment, with medical 3D printing supplies serving as the reaction substrates. Under the appropriate reaction conditions, the 3D printing supplies, which were cut into small pieces, were converted into 1,6-hexanediol as a white crystalline solid with a 76% separation yield at a high catalyst loading of 3 mol% \u003cstrong\u003eFe1\u003c/strong\u003e and 6 mol% KO\u003csup\u003e\u003cem\u003et\u003c/em\u003e\u003c/sup\u003eBu (Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e, second row; Table \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e, entry 2). The blue coating on 3D printing supplies was presumably nonreactive and could be easily separated via filtration. A PLA beverage cup was cut into small pieces and placed in the autoclave under the established hydrogenolysis conditions, and 1,2-propanediol was obtained as a light yellow oil with an 87% isolated yield at a rather low catalyst loading of 1.5 mol% \u003cstrong\u003eFe1\u003c/strong\u003e and 3 mol% KO\u003csup\u003e\u003cem\u003et\u003c/em\u003e\u003c/sup\u003eBu (Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e, third row; Table \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e, entry 3). In the subsequent reaction, three CDs were cut into small pieces and added into the reaction system to form bisphenol A and methanol smoothly using 3 mol% \u003cstrong\u003eFe1\u003c/strong\u003e and 6 mol% KO\u003csup\u003e\u003cem\u003et\u003c/em\u003e\u003c/sup\u003eBu (Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e, bottom row; Table \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e, entry 4). Due to the thick coating, the separation yield in this reaction was comparatively low at 69%, and the unreacted coating could also be directly removed via filtration.\u003c/p\u003e\n\u003cp\u003eIn summary, an NNN-based iron pincer catalyst was developed for the efficient upcycling of polyester and polycarbonate plastics. The catalyst\u0026rsquo;s ability to facilitate (1) methanolysis and transfer hydrogenation and (2) direct catalytic hydrogenolysis under mild conditions is a breakthrough in plastic waste management. The high yields and broad applicability of the catalyst across various types of plastics (PET, PBT, PCL, PLA, and polycarbonates) underscore its potential for industrial-scale recycling. This research contributes to the academic community by deepening the understanding of catalytic upcycling and offers practical solutions to address the environmental challenges of plastic pollution, aligning with the goals of a circular economy. By converting waste plastics into value-added diols, this study aligns with the circular economy model, promoting a more sustainable future.\u003c/p\u003e"},{"header":"Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003cp\u003eMaterials\u003c/p\u003e \u003cp\u003eAll reagents were purchased from Sigma-Aldrich, TCI or Acros and used without further purification. Methanol, toluene, and tetrahydrofuran were purified using a Glass Contour solvent purification system consisting of a neutral alumina, copper catalyst, and activated molecular sieves, then passed through an in-line, 2 \u0026micro;m filter immediately before being dispensed.\u003c/p\u003e \u003cp\u003ePhysical methods\u003c/p\u003e \u003cp\u003eNMR spectra were recorded on Bruker Avance 500 spectrometer in NMR tubes at room temperature. Chemical shifts (δ) were reported in parts per million (ppm) with referenced to the proton signal of the deuterated solvent. CDCl\u003csub\u003e3\u003c/sub\u003e were dried over CaH\u003csub\u003e2\u003c/sub\u003e and purified by vacuum transfer. MS (HRMS) measured with ThermoFisher Q-Exactive Mass Spectrometer.\u003c/p\u003e \u003cp\u003eSynthetic methods\u003c/p\u003e \u003cp\u003eGeneral procedure for transfer hydrogenation of esters\u003c/p\u003e \u003cp\u003eIn a glovebox under a nitrogen atmosphere, a scintillation vial equipped with a magnetic stir bar was charged with esters (1.0 mmol) and H\u003csub\u003e3\u003c/sub\u003eN\u0026middot;BH\u003csub\u003e3\u003c/sub\u003e (1.0 mmol, 31 mg). The catalyst \u003cb\u003eFe1\u003c/b\u003e (0.02 mmol, 7 mg), KO\u003cem\u003et\u003c/em\u003eBu (0.05 mmol, 6 mg), and THF (2 mL) were added. The mixture was stirred at 60\u0026deg;C. After the indicated time, the reaction mixture was isolated through chromatography on silica gel to obtain the products.\u003c/p\u003e \u003cp\u003eGeneral procedure for catalytic methanolysis/hydrogenative depolymerization of polyester and polycarbonate plastics\u003c/p\u003e \u003cp\u003eFor all polymers, the molar amount used was calculated based on the corresponding repetition units. In a glovebox under an N\u003csub\u003e2\u003c/sub\u003e atmosphere, a scintillation vial (with a magnetic stir bar) was charged with \u003cb\u003eFe1\u003c/b\u003e (0.02 mmol, 7 mg), KO\u003cem\u003et\u003c/em\u003eBu (0.05 mmol, 6 mg), H\u003csub\u003e3\u003c/sub\u003eN\u0026middot;BH\u003csub\u003e3\u003c/sub\u003e (0.02 mmol, 1 mg), and MeOH (2 mL). Then, the polymer (1.0 mmol) was added. The mixture was stirred at 80\u0026deg;C. After the indicated time, the reaction mixture was isolated via chromatography on silica gel to produce the ester products. The obtained esters were then processed according to Section S3 to obtain the diols.\u003c/p\u003e \u003cp\u003eGeneral procedure for autoclave reactions\u003c/p\u003e \u003cp\u003eThe Processed polymer was placed in the glass insert of the autoclave, a stir bar was added, and the insert was placed in the 250 mL steel autoclave. The autoclave was evacuated and backfilled with N\u003csub\u003e2\u003c/sub\u003e three times. \u003cb\u003eFe1\u003c/b\u003e and KO\u003cem\u003et\u003c/em\u003eBu were weighed in a 10 mL Schlenk tube and dissolved in 5 mL THF. The solution was transferred into the autoclave in a N\u003csub\u003e2\u003c/sub\u003e counter-stream using a syringe equipped with a cannula. The autoclave was pressurized with 20 bar H\u003csub\u003e2\u003c/sub\u003e. The reaction was stirred at 120\u0026deg;C for 24 h. After completion of the reaction time, the autoclave was cooled down to room temperature in an ice bath and carefully vented to the atmosphere. The reaction mixture was isolated via chromatography on silica gel to obtain the products.\u003c/p\u003e \u003c/div\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eData Availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll data supporting the findings of this study are available within the article and its Supplementary Information or from the corresponding author upon reasonable request. CCDC Number: 2433516.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was supported by National Science Foundation of Shandong Province (No. ZR2022QB036), Shandong Laboratory of Advanced Materials and Green Manufacturing at Yantai (No. AMGM2023F10, AMGM2023F11).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthics declarations\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eCompeting\u0026nbsp;interests\u003c/p\u003e\n\u003cp\u003eThe authors have no competing interests as defined by Nature Portfolio, or other interests that might be perceived to influence the results and/or discussion reported in this paper.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eZheng, K. et al. Progress and perspective for conversion of plastic wastes into valuable chemicals. Chem. Soc. Rev. 52, 8\u0026ndash;29 (2023).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSchwab, S. T., Baur, M., Nelson, T. F. \u0026amp; Mecking, S. Synthesis and deconstruction of polyethylene-type materials. Chem. Rev. 124, 2327\u0026ndash;2351 (2024).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSullivan, K. P. et al. Mixed plastics waste valorization through tandem chemical oxidation and biological funneling. Science 378, 207\u0026ndash;211 (2022).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eRahimi, A. \u0026amp; Garc\u0026iacute;a, J. M. Chemical recycling of waste plastics for new materials production. Nat. Rev. Chem. 1, 0046\u0026ndash;0056 (2017).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eAhmed, T. et al. Biodegradation of plastics: current scenario and future prospects for environmental safety. Environ. Sci. Pollut. Res. 25, 7287\u0026ndash;7298 (2018).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eMacleod, M., Arp, H. H., Tekman, M. B. \u0026amp; Jahnke, A. The global threat from plastic pollution. Science 373, 61\u0026ndash;65 (2021).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eGibb, B. C. Plastics are forever. Nat. Chem. 11, 394\u0026ndash;395 (2019).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eKwon, D. Three ways to solve the plastics pollution crisis. Nature 616, 234\u0026ndash;237 (2023).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLi, H. et al. Expanding plastics recycling technologies: chemical aspects, technology status and challenges. Green Chem. 24, 8899\u0026ndash;9002 (2022).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSimon, N. et al. A binding global agreement to address the life cycle of plastics. Science 373, 43\u0026ndash;47 (2021).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eGeyer, R., Jambeck, J. R. \u0026amp; Law, K. L. Production, use, and fate of all plastics ever made. Sci. Adv. 3, e1700782 (2017).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZhang, S. et al. Depolymerization of polyesters by a binuclear catalyst for plastic recycling. Nat. Sustain. 6, 965\u0026ndash;973 (2023).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eGao, Z. G., Ma, B., Chen, S., Tian, J. \u0026amp; Zhao, C. Converting waste PET plastics into automobile fuels and antifreeze components. Nat. Commun. 13, 3343 (2022).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eTanaka, S. et al. Depolymerization of polyester fibers with dimethyl carbonate-aided methanolysis. ACS Mater. Au 4, 335\u0026ndash;345 (2024).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eEllis, L. D. et al. Chemical and biological catalysis for plastics recycling and upcycling. Nat. Catal. 4, 539\u0026ndash;556 (2021).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBartolome, L. et al. Superparamagnetic γ-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e nanoparticles as an easily recoverable catalyst for the chemical recycling of PET. Green Chem. 16, 279\u0026ndash;286 (2014).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eJehanno, C. et al. Selective chemical upcycling of mixed plastics guided by a thermally stable organocatalyst. Angew. Chem. Int. Ed. 60, 6710\u0026ndash;6717 (2021).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eKorley, L. T. J., Epps III, T. H., Helms, B. A. \u0026amp; Ryan, A. J. Toward polymer upcycling-adding value and tackling circularity. Science. 373, 66\u0026ndash;69 (2021).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBalaraman, E., Gnanaprakasam, B., Gunanathan, C., Milstein, D. \u0026amp; Zhang, J. Novel ruthenium complexes and their uses in processes for formation and/or hydrogenation of esters, amides and derivatives thereof, U.S. Patent WO2012052996A2 (2013).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWesthues, S., Idel, J. \u0026amp; Klankermayer, J. Molecular catalyst systems as key enablers for tailored polyesters and polycarbonate recycling concepts. Sci. Adv. 4, eaat9669 (2018).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eKrall, E. M. et al. Controlled hydrogenative depolymerization of polyesters and polycarbonates catalyzed by ruthenium(ii) PNN pincer complexes. Chem. Commun. 50, 4884\u0026ndash;4887 (2014).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eFuentes, J. A. et al. On the functional group tolerance of ester hydrogenation and polyester depolymerisation catalysed by ruthenium complexes of tridentate aminophosphine ligands. Chem. Eur. J. 21, 10851\u0026ndash;10860 (2015).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHu, Y. et al. Highly efficient depolymerization of waste polyesters enabled by transesterification/hydrogenation relay under mild conditions. Angew. Chem. Int. Ed. 62, e202312564 (2023).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWei, Z., Li, H., Wang, Y. \u0026amp; Liu, Q. A tailored versatile and efficient NHC-based NNC-pincer manganese catalyst for hydrogenation of polar unsaturated compounds. Angew. Chem. Int. Ed. 62, e202301042 (2023).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eTanaka, S., Sato, J. \u0026amp; Nakajima, Y. Capturing ethylene glycol with dimethyl carbonate towards depolymerisation of polyethylene terephthalate at ambient temperature. Green Chem. 23, 9412\u0026ndash;9416 (2021).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eAbe, R., Komine, N., Nomura, K. \u0026amp; Hirano, M. La(III)-Catalysed degradation of polyesters to monomers via transesterifications. Chem. Commun. 58, 8141\u0026ndash;8144 (2022).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003ePerez, M., Elangovan, S., Spannenberg, A., Junge, K. \u0026amp; Beller, M. Molecularly defined manganese pincer complexes for selective transfer hydrogenation of ketones. ChemSusChem 10, 83\u0026ndash;86 (2017).\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"},{"header":"Tables","content":"\u003cp\u003eTables 1 to 4 are available in the Supplementary Files section.\u003c/p\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-5750869/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5750869/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe development of efficient, eco-friendly recycling methods for mitigating the environmental impact of polyester waste remains a significant challenge. Herein, we establish an efficient catalytic system based on an NNN-based iron pincer catalyst, which can facilitate the hydrogenative depolymerization of polyester plastics using two methods. The first method is to depolymerize the polyester into ester monomers via methanolysis and facilitate subsequent transfer hydrogenation using ammonia borane as a hydrogen source to obtain diol products under mild conditions. The second method is to use molecular hydrogen as a hydrogen source for the direct catalytic hydrogenolysis of the plastic to obtain diol products. The catalyst [Fe(NNHN)Cl\u003csub\u003e2\u003c/sub\u003e]\u003csub\u003e2\u003c/sub\u003e demonstrates high catalytic efficiency in the degradation of polyester and polycarbonate plastics, including when using plastic waste from daily life as raw materials.\u003c/p\u003e","manuscriptTitle":"Efficient catalytic upcycling of polyester and polycarbonate plastics using NNN-based iron catalyst","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-04-24 10:22:12","doi":"10.21203/rs.3.rs-5750869/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"communications-chemistry","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"commschem","sideBox":"Learn more about [Communications Chemistry](http://www.nature.com/commschem/)","snPcode":"","submissionUrl":"","title":"Communications Chemistry","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Communications Series","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"bcabd7ca-bba6-4cbb-83e8-593dda911982","owner":[],"postedDate":"April 24th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":46625147,"name":"Physical sciences/Chemistry/Green chemistry/Sustainability"},{"id":46625148,"name":"Physical sciences/Chemistry/Environmental chemistry/Pollution remediation"},{"id":46625149,"name":"Physical sciences/Chemistry/Catalysis/Homogeneous catalysis"}],"tags":[],"updatedAt":"2025-04-24T13:35:24+00:00","versionOfRecord":[],"versionCreatedAt":"2025-04-24 10:22:12","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-5750869","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5750869","identity":"rs-5750869","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}