Advances in the Heterogeneous Catalytic Hydrogenative Upcycling of Waste Polyesters

Kaizhi Wang; Qiang Lin

doi:10.7536/PC20250714

Progress in Chemistry ›› 2026, Vol. 38 ›› Issue (4) : 672-688 DOI: 10.7536/PC20250714

research-article

Scan to read & Share

Advances in the Heterogeneous Catalytic Hydrogenative Upcycling of Waste Polyesters

Kaizhi Wang ¹^,^*
, Qiang Lin ²^,^*

Author information +

History +

PDF

Abstract

Since the advent of plastics in the 20th century， human life has been greatly facilitated by their excellent properties. However， their non-biodegradable nature has also led to environmental pollution and resource waste. Regarding waste PET and PLA as valuable carbon reservoirs， their upcycling through heterogeneous catalytic hydrogenation has become a central theme in contemporary chemical recycling research. This process can efficiently produce high-value chemicals and new polymer materials under mild conditions， demonstrating excellent atom economy and rich product diversity. This review systematically summarizes the latest progress in the heterogeneous catalytic hydrogenation upcycling of waste polyesters， mainly PET and PLA， deeply analyzes the structure-activity relationship of catalyst active sites， and the synergistic mechanism of hydrogen sources （H₂， alcohols， and endogenous hydrogen）. It also explores in detail the two main pathways of depolymerization， hydrogenation and non-depolymerization hydrogenation， as well as their coupling strategies， revealing the significant impact of key factors such as hydrogen sources， reaction temperature， pressure， and solvents on product selectivity and conversion efficiency. Meanwhile， the review comprehensively summarizes the main bottlenecks of current technologies in terms of catalyst stability， impurity compatibility， economy， and environmental performance， and points out that the development of non-precious metal catalysts， efficient utilization of hydrogen sources， and the scaling up and life cycle assessment of processes are key issues that urgently need to be addressed. It aims to provide a comprehensive reference for the in-depth study of polyester degradation mechanisms and the continuous optimization of efficient upcycling technologies， and to contribute to the synergistic realization of plastic circular utilization and the “dual carbon” goals.

Contents

1 Introduction

2 Controlled reduction and targeted conversion pathways for polyesters

3 Advances in polyester upcycling

3.1 Reductive upcycling of PET

3.2 Reductive upcycling of PLA

4 Techno-economic analysis and life cycle assessment

5 Conclusion and outlook

Graphical abstract

Keywords

waste polyesters / heterogeneous catalytic hydrogenation / hydrogen sources / upcycling / hydrogenation pathways

Cite this article

Download citation ▾

Kaizhi Wang, Qiang Lin. Advances in the Heterogeneous Catalytic Hydrogenative Upcycling of Waste Polyesters. Progress in Chemistry, 2026, 38(4): 672-688 DOI:10.7536/PC20250714

登录浏览全文

4963

注册一个新账户忘记密码

1 Introduction

Under the dual challenges of resource scarcity and environmental pollution, the efficient recycling of waste polyesters has become a core issue in promoting the circular economy^[1-2]. According to data from the United Nations Environment Programme (UNEP), among the approximately 380 million tons of plastic waste generated globally in 2025, polyester materials such as polyethylene terephthalate (PET) and polylactic acid (PLA) account for over 32%^[3]. These materials are widely used in packaging, textiles, and biomedicine due to their excellent physical and chemical properties. However, their stable chemical structure results in natural degradation cycles lasting hundreds of years; millions of tons of waste polyester are landfilled or incinerated annually, which not only releases microplastics that pollute the environment^[4], but also causes the waste of approximately 120 million tons of carbon resources^[5]. Currently, the global plastic recycling rate is less than 10%^[6]; due to issues such as complex composition, high molecular weight, and impurity contamination, the effective recycling rate of waste polyester is only 9%^[7], while the disposal of the remaining portion exacerbates ecological risks and leads to the permanent loss of significant carbon resources.

Regarding polyester recycling, traditional chemical recycling methods (such as hydrolysis, pyrolysis, and alcoholysis) have been partially applied in industry, but their technical limitations severely restrict large-scale promotion^[8]. Hydrolysis requires strong acid/alkali conditions, consumes high energy, involves complex product purification, and struggles to process high-molecular-weight polyesters^[9]; pyrolysis processes, due to high-temperature conditions (above approximately 1000 °C), lead to surging energy consumption, yield complex mixtures requiring costly separation and purification, and suffer from insufficient selectivity^[10]. For example, traditional hydrolysis achieves a recovery rate of less than 40% for PET textiles containing impurities^[11], while the monomer purity of pyrolysis products often falls below industrial standards^[12], limiting their circular utilization value. Previously, alcoholysis suffered from poor compatibility with composite materials and low product separation efficiency due to reversible reaction kinetics and the cost of alcohol solvents^[13]; in the past five years, alcoholysis technology has achieved low-energy, low-emission closed-loop demonstrations and become the mainstream solution for commercial recycling. Among these, methanolysis and glycolysis can quantitatively convert PET or PLA into bis(2-hydroxyethyl) terephthalate (BHET)/dimethyl terephthalate (DMT) or methyl lactate under mild conditions, with monomer yields exceeding 95%, and have been applied to regenerate bottle-grade polyesters (r-PET or r-PLA) and high-value aromatic derivatives^[14-15]. However, although researchers have improved traditional processes by developing novel acidic catalysts (such as MgCl₂-based, potassium carbonate/guaiacol, and other Lewis acid catalysts^[16-18]) or microwave-assisted technologies^[19], alcoholysis remains sensitive to mixed plastics (PVC, dyes, metal residues), requiring high-purity raw materials to ensure product quality; furthermore, products are limited to monomers/oligomers, making it difficult to directly upgrade to high-performance fuels, polymer monomers, or oxygenated fine chemicals in a single step^[19-21].

In recent years, catalytic oxidation and catalytic reduction, especially catalytic reduction, have leveraged their advantages in directional bond cleavage selectivity and precise controllability of molecular structure (Figure 1), becoming a core pathway for achieving high-value conversion in the resource regeneration of polyester waste^[22-23]. Through carefully designed catalyst systems, combined with green hydrogen and bio-based alcohols as small hydrogen-containing molecules^[23-26]and other diverse hydrogen sources, waste polyesters can undergo hydrogenolytic depolymerization under relatively mild conditions^{[23-25,27-40]}or hydrodeoaromatization^[41-45]and other reduction pathways to be directionally converted into high-value-added chemicals or novel biodegradable polyesters. This process achieves an efficient transformation from "waste" to "resource," breaking through the limitations of traditional catalysts in stability and selectivity. It demonstrates significant advantages in handling complex matrices, regulating product selectivity, and process scale-up, thereby enabling the efficient conversion of waste polyesters from laboratory research to industrial application.

Homogeneous and heterogeneous catalysis each possess unique advantages, driving the upcycling of polyester waste. Homogeneous catalysis offers high activity and selectivity at the molecular level but suffers from poor stability and stringent synthesis and storage conditions^[46-50]; in contrast, heterogeneous catalysis has garnered significant attention due to its high stability and ease of recovery and reuse^[51-53]. This review focuses on the heterogeneous catalytic reductive upcycling of PET and PLA, deeply exploring depolymerization and non-depolymerization hydrogenation pathways along with coupling strategies. It analyzes the structure-activity relationships of catalytic systems and the impact of surface and interface microstructure design on efficiency. By integrating techno-economic assessment and life cycle analysis, it examines the application potential of existing technologies, providing a scientific basis and practical reference for the polyester recycling industry.

2 Controllable Reduction and Directed Conversion Pathways of Polyesters

The core of heterogeneous catalytic reduction upcycling technology for waste polyester lies in the synergistic design of hydrogen source types and conversion pathways: based on external hydrogen (H₂) or hydrogen-containing organic molecules (methanol, ethylene glycol, etc.) as hydrogen donation mechanisms, through two pathways—depolymerization reduction (hydrogenolytic depolymerization, hydrodeoxygenative depolymerization) and non-depolymerization hydrogenation (direct activation of C=C bonds)—achieving selective bond cleavage of polyester functional groups (—COOR) and precise control over product distribution (Figure 2).

Based on the above hydrogen source classification, external hydrogen and hydrogen-containing organic molecules drive polyester conversion through differentiated hydrogen supply mechanisms. Among them, external high-purity hydrogen (H₂) can directionally activate target chemical bonds with the synergistic effect of metal active sites and supports under low pressure (<5 MPa) and temperature (<300 ℃). For example, Ru/TiO₂ catalyst under 0.3 MPa H₂ conditions preferentially and selectively cleaves the ester bonds of PET while preserving the aromatic ring structure, generating p-xylene (PX)^[31]; Ir-Re alloy catalysts efficiently hydrogenolyze the cyclic units of PET under 3 MPa H₂ to prepare high-value-added cycloalkanes^[44]. In contrast, methanol^[25-26], ethylene glycol^[24,27] and other hydrogen-containing organic molecules can release active hydrogen through in situ cracking or reforming, avoiding the limitations of high-pressure hydrogen and reducing energy consumption. For instance, Cu/SiO₂ catalysts catalyze the methanolysis depolymerization of PET to generate DMT intermediates, which are subsequently converted to PX via in situ hydrodeoxygenation, with the entire reaction process requiring no high-pressure hydrogen source^[26].

Focusing on the selectivity differences at bond cleavage sites, depolymerization and non-depolymerization pathways achieve polyester conversion through differentiated chemical bond activation mechanisms. In the depolymerization-reduction pathway, acidic or basic supports preferentially cleave ester bonds to release monomers or oligomers, which are subsequently converted into target products via metal-site hydrogenolysis or hydrodeoxygenation. For example, C/MoO₂catalysts can hydrogenolytically depolymerize PET into terephthalic acid (TPA) and small molecules such as ethylene under solvent-free conditions, with yields exceeding 90%^[28]. Non-depolymerization hydrogenation directly activates unsaturated C=C bonds in the polyester chain, utilizing Ru/C or Pd/C catalysts to achieve selective hydrogenation, thereby partially or completely dearomatizing the benzene rings to generate biodegradable materials such as poly(ethylene 1,4-cyclohexanedicarboxylate) (PECHD)^[54]. The main difference between the two pathways lies in the catalyst's selectivity towards functional groups and the hydrogen source synergy mechanism; however, both require optimization of active site distribution and hydrogen transport efficiency to balance catalytic activity and product selectivity.

To break through the bottlenecks of traditional catalytic systems, synergistic optimization is required in three aspects: hydrogen sources, reaction pathways, and catalysts. External hydrogen (such as H₂) offers high hydrogenation efficiency but requires optimized high-pressure storage and transportation to reduce costs; internal hydrogen (such as methanol reforming) is suitable for complex matrices due to its safety and economic viability. In terms of pathway design, depolymerization with hydrogenation is suitable for preparing high-purity monomers (such as TPA), while non-depolymerization with hydrogenation is more efficient for preparing polymer intermediates such as PECHD. For catalyst optimization, single-atom catalysts (such as RuMo/TiO₂and diatomic catalysts^[36]) can maximize the utilization efficiency of active sites, while bimetallic catalysts (such as Pd-CuCo₂O₄) significantly enhance selectivity through interfacial synergistic effects. In the future, it is necessary to develop catalysts with stronger toxicity resistance and higher stability, or multi-catalyst relay catalysis systems involving the combined use of multiple catalysts^[55-57]to address interference from metals, pigments, or blended polymers in complex matrices. For example, the team of Cao Yong at Fudan University^[53]developed a heterogeneous relay catalysis strategy. Through the cascade reaction of Pd/r-GO and og-CuZn catalysts, PET was efficiently converted into 1,4-cyclohexanedimethanol (CHDM) via the PECHD intermediate, achieving an isolated yield of 87%, providing an engineerable technical pathway for the closed-loop recycling of polyesters.

3 Research Progress on Polyester Reduction Upgrading

3.1 PET reduction and upgrading conversion

In the field of catalytic reductive upgrading of PET, precise control over high-value products has become a core direction for technological research and development. Aromatics, as key basic raw materials for polyester fibers, engineering plastics, and high-performance materials, are projected to exceed a global market size of $120 billion by 2030, with a compound annual growth rate (CAGR) of 5.8%. Among these, PX, as the core monomer in the polyester industry chain, is expected to account for over 65% of global production capacity by 2030.^[58]; low-carbon alkanes such as ethylene and propylene, serving as the cornerstone of the petrochemical industry chain, are experiencing an annual production capacity demand growth rate exceeding 4.3%.^[28]; naphthenic-based aviation fuels, due to their high thermal stability characteristics, are seeing an annual demand growth rate exceeding 6% in the specialty fuel sector, with the market volume expected to surpass $32 billion by 2030.^[43]. Furthermore, oxygen-containing high-value chemicals such as CHDM, owing to their irreplaceability in fields like high-end copolyesters, bio-based polyesters, and weather-resistant coatings, are becoming core target products for the chemical upgrading of recycled polyester. Driven by global circular economy policies, their regeneration preparation technologies are rapidly iterating, with industrial conversion rates increasing to over 38%.^[53]. Meanwhile, PET-based biodegradable materials, through molecular structure design and functional modification, are demonstrating a compound annual growth rate of 12% in green industry sectors such as packaging, agricultural films, and medical consumables.^[59]. Based on the market demand for the aforementioned diversified high-value products, recent research has focused on achieving directional reductive conversion of PET waste through strategies such as hydrodeoxygenation, hydrogenolysis, and catalytic hydrogenation. Key scientific issues under investigation include the design of catalyst active sites (e.g., construction of metal-acid bifunctional sites), hydrogen source synergy mechanisms (adaptability of green hydrogen/biomass hydrogen donors), and reaction pathway regulation (correlation between bond cleavage sequence and product selectivity) (Figure 3).

3.1.1 Preparation of aromatic hydrocarbons and low-carbon alkanes

In the field of PET catalytic conversion for preparing aromatic hydrocarbons and low-carbon alkanes, multiple research teams have achieved significant progress through innovative catalyst design and optimization of technical pathways. The Marks team at Northwestern University in the United States^[28]developed a carbon-supported single-atom molybdenum dioxide catalyst (C/MoO₂) that, under solvent-free conditions at 260 °C and 1 atm H₂, achieves efficient PET depolymerization via a synergistic catalytic mechanism of Mo⁴⁺ dual-oxygen sites, selectively generating TPA and ethylene with an 87% yield (Figure 4a). Its core mechanism lies in: the Mo-O-C interface preferentially initiates β-C—O bond cleavage of the PET chain to generate vinyl benzoate intermediates, followed by precise hydrogenolysis of C—O bonds by Mo active hydrogen species. To further improve product selectivity, the team designed a Hf(OTf)₄-Pd/C tandem catalytic system, which, under conditions of 265 °C, 24 h, and 1 atm H₂, degrades PET into TPA and ethane, achieving a TPA yield as high as 98%^[29]. In this catalytic system, homogeneous Hf(OTf)₄ is responsible for precisely depolymerizing the polyester backbone, while heterogeneous Pd/C eliminates intermediate accumulation through thermodynamic driving (hydrogenolysis of olefins with ΔG being negative), forming a self-sustaining reaction cycle and breaking through the limitations of traditional single catalysts regarding stepwise control of depolymerization and hydrogenation.

Yan Ning et al. from the National University of Singapore^[30]on 0.5% Co/TiO₂catalyst, combining depolymerization and hydrodeoxygenation (HDO) in a one-pot reaction to convert PET into aromatics (Figure 4b). Under conditions of 340 °C and an initial hydrogen pressure of 30 bar, the catalyst induced depolymerization and HDO reactions via C—O bond cleavage. After 4 hours of reaction using PET as the substrate, an aromatic yield of 78.9 mol% (toluene and xylene) was obtained. Furthermore, Duan Xuezhi et al. from East China University of Science and Technology^[31]innovatively designed a Ru/TiO₂catalyst system that, under conditions of 230 °C and 0.3 MPa H₂, directionally converted the hydrogenolysis products of PET-containing mixed plastics into the single compound methane with a yield exceeding 91%. By decoupling the geometric and support effects of ruthenium-based catalysts, they clarified the influence of the ruthenium coordination environment on the adsorption configuration of PET hydrogenolysis intermediates, pointing out that low-coordination Ru sites induce "upright adsorption" of intermediates to suppress side reactions. Based on this, precise regulation of the coordination environment of ruthenium-based catalysts achieved directional hydrogenolysis of PET (Figure 4c). Building on this, research focused on tackling the directional conversion of product systems from mixed aromatics to single components. Based on precise regulation of the metal-support interface and alloy electronic structure, the Co-Fe-Al catalyst developed by Hu Xun's team at the University of Jinan, under conditions of 210 °C and 4.0 MPa H₂, increased PX selectivity to over 99% by constructing a CoFe alloy to regulate the coordination environment of active sites^[32]. The core mechanism lies in: the electronic effect of the CoFe alloy weakens H₂adsorption and activation capability, while preferentially adsorbing the upright configuration of oxygen-containing intermediates, suppressing deep hydrogenation side reactions caused by parallel adsorption of the benzene ring. By decoupling the geometric effects (alloy particle size/crystal phase) of Co-based catalysts and the synergistic action of acidic sites on the support, the dual regulation law of CoFe alloys on selective C—O bond hydrogenolysis and aromatic ring stabilization was revealed: low-coordination CoFe sites promote directional deoxygenation of carboxyl/hydroxyl groups, while electron-rich Co species reduce the adsorption strength of aromatic ring π bonds, blocking the ring hydrogenation pathway. Based on the concept of adsorption configuration regulation, Li Zhenxing et al. from China University of Petroleum^[33]developed a RuFe bimetallic catalyst that, under conditions of 350 °C and 5 MPa H₂, achieved a vertical adsorption configuration of DMT intermediates by constructing a Ru-N₃coordination structure and the synergistic confinement effect of ruthenium clusters (~2 nm). In situ EXAFS and DFT calculations indicated that this configuration reduced the C—O bond dissociation energy to 1.91 eV (a 13% decrease compared to the parallel adsorption configuration) while increasing the activation energy for aromatic ring hydrogenation to 2.20 eV, ultimately generating PX with 88.9% selectivity.

Addressing the bottleneck of dependence on external hydrogen sources, the team led by Wang Yanqin at East China University of Science and Technology^[24]innovatively achieved the efficient conversion of PET into BTX (benzene, toluene, and PX) for the first time under conditions without added hydrogen sources via an aqueous phase reforming pathway of ethylene glycol (Figure 5a). This study used Ru/Nb₂O₅as a catalyst to fully utilize the structural hydrogen in the ethylene glycol units of PET molecules. Through a three-stage tandem reaction mechanism of "hydrolysis-reforming-hydrogenolysis/decarboxylation," waste plastics were successfully converted into BTX products dominated by alkyl aromatics (with toluene and PX accounting for 74.5%). This endogenous hydrogen strategy breaks the dependence of traditional PET conversion on external hydrogen sources, has been successfully applied to the conversion of actual waste plastics such as cola bottles and polyester films, and provides a new paradigm for the closed-loop recycling of PET. The team at the Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences^[25]took a different approach, using methanol solvent as a hydrogen carrier. Through the synergistic effect between Pt nanoparticles (coordination number 3.2) on the surface of the Pt/TiO₂catalyst and Lewis acid sites (acid amount 1.2 mmol∙g₂carrier (acid amount 1.2 mmol∙g^-1)) of the TiO2 carrier, a coupled channel for methanol reforming to produce hydrogen and in-situ hydrogenolysis reactions was constructed. At 400 °C, this system precisely regulated the C—O bond cleavage pathway through an "upright adsorption" configuration of intermediates, increasing the selectivity of directional PET conversion to BTX to 77.96%. Furthermore, the team led by Professor Zhao Chen at East China Normal University^[26]developed a Cu/SiO₂cascade catalytic system (Figure 5b): NaCl-induced hydrothermal synthesis was used to create a dense copper silicate precursor (with a specific surface area reaching 327 m²∙g^-1). In methanol solvent at 210 °C, by precisely regulating the ratio of Cu⁺/Cu⁰active sites, methanol dehydrogenation and selective hydrodeoxygenation of DMT intermediates were simultaneously achieved, with both PX and ethylene glycol yields reaching 100%. Its core breakthrough lies in the fact that the dense structure of the copper silicate precursor suppresses the over-reduction of Cu species through spatial confinement effects, maintaining a hydrogen supply network for methanol decomposition dominated by highly active Cu⁺sites, realizing an upright adsorption configuration of DMT intermediates, and making the C—O bond hydrogenolysis rate four times that of ester group hydrogenation. Both studies decoupled the geometric effects of metal sites (regulation of Cu⁺/Cu⁰ratio, coordination state of Pt nanoparticles) and carrier effects (SiO₂precursor structure engineering, TiO₂acidity regulation), revealing the decisive role of the catalyst coordination environment on the adsorption configuration of intermediates, and providing a theoretical basis for the precise control of plastic hydrogenolysis reaction pathways.

In summary, the selectivity of PET hydrogenation to aromatics/alkanes is jointly determined by the dual factors of "upright adsorption and electronic regulation." While >90% directional conversion has been achieved on model substrates, bottlenecks such as insufficient C—O bond activation under low temperature and pressure, the activity-stability trade-off of non-noble metals, and impurity-induced flipping of adsorption configurations still coexist with narrowed product spectra and a sharp decline in selectivity. Future research must incorporate impurity tolerance and broad-spectrum tunability into the core of catalyst design to break through the limitations of "high pressure, noble metals, and pure substrates," thereby maintaining simultaneous high selectivity for aromatic ring and alkane products under mild conditions.

3.1.2 Preparation of cyclic hydrocarbons and fuels

Addressing the technical demand for catalytic conversion of PET waste into high-value cycloalkane fuels, the team of Liu Zhimin at the Institute of Chemistry, Chinese Academy of Sciences^[43]developed a Ru/TiO₂ catalyst for upgrading PET into alkanes in the presence of H₂ and water (Figure 6a). Under optimal conditions (200 °C, 60 bar H₂, and a small amount of water), PET can be completely converted into alkanes, primarily cyclohexane and methane. Studies indicate that the strong interaction between TiO₂ and Ru nanoparticles enables electron transfer from TiO₂ to Ru nanoparticles, thereby endowing Ru/TiO₂ with the capability to simultaneously catalyze PET hydrolysis and hydrogenation. In contrast, the team of Liu Guozhu at Tianjin University^[44]developed a novel synergistic catalytic system of Ir-ReO_x/SiO₂ and HZSM-5 zeolite (Figure 6b), achieving directional conversion of PET to 1,4-dimethylcyclohexane (DMCH) with a yield exceeding 95% under mild conditions of 190 °C and 3 MPa H₂. In Ir-ReO_x, ReO_x modulates the hydrogenation activity of Ir through an electron transfer effect (Ir→Re charge amount of 0.12 e), while the Brønsted acid sites of HZSM-5 accelerate intermediate dehydration, reducing the activation energy for decarboxylation from 98 kJ/mol to 72 kJ/mol. Moreover, the aqueous solvent promotes parallel adsorption of aromatic rings on the catalyst surface via an "on-water" effect, suppressing side reactions caused by premature C—O bond cleavage. Through cascade regulation of hydrogenation-depolymerization-deoxygenation, this system achieves precise product orientation: PET molecules first undergo selective hydrogenation of the aromatic ring, followed by hydrogenolytic ring-opening of the acyl C—O bond to generate saturated esters, and finally complete decarboxylation and dehydroxylation via multi-stage hydrodeoxygenation (HDO) to convert into single-component DMCH. Based on this, we know that cycloalkane selectivity follows the sequence of "aromatic ring hydrogenation first, then deoxygenation." The commonality lies in the precise matching of electron transfer and C—O bond cleavage energy barriers by metal-acid dual sites to achieve yields of single-ring products like DMCH greater than 90%. However, dependence on high hydrogen pressure (≥3 MPa) and noble metals (Ru, Ir) remains prevalent. Furthermore, the "on-water" parallel adsorption model is only applicable to clean substrates; impurities such as chlorine or dyes can reverse the aromatic ring adsorption configuration, leading to a sharp decline in ring selectivity accompanied by methanation side reactions. Therefore, the seesaw effect between narrowing the cyclic hydrocarbon product spectrum and excessive aromatic ring hydrogenation remains a core obstacle for controllable conversion under mild conditions.

3.1.3 Preparation of high-value oxygenated chemicals

In the field of PET reductive preparation of oxygen-containing high-value chemicals, innovations in different catalytic strategies have provided multidimensional solutions for the resource utilization of waste plastics. Hou Zhaoyin et al. from Zhejiang University^[45]Addressing the challenge of directional conversion of PET degradation intermediates, they designed a mesoporous HZSM-5 supported ruthenium catalyst (Ru/m-HZSM-5) with a main channel of 5.32 nm. By regulating the pore structure via sequential desilication and dealumination, the efficiency of benzene ring hydrogenation was significantly improved. This catalyst, at 120 °C and 3 MPa H₂, efficiently hydrogenated the PET glycolysis product BHET into BHCD with a yield of 95.6%. The mesoporous structure effectively promoted the diffusion of large molecular substrates (1.82 nm), while the synergistic effect between highly dispersed Ru nanoparticles (3.5 nm) and acidic sites enhanced hydrogen activation capability. Experiments confirmed its universality for various PET degradation products such as DMT, providing an efficient catalytic method for the high-value conversion of plastic waste. Ma Ding et al. from Peking University^[27]reported an innovative strategy for the efficient one-pot conversion of PET into the high-value chemical 1,4-cyclohexanedicarboxylic acid (CHDA) without external hydrogen supply. By designing a tandem reaction system integrating PET depolymerization, aqueous phase reforming of ethylene glycol for hydrogen production, and TPA hydrogenation, and utilizing the synergistic effect of Ru/C and Pd/C catalysts under alkaline conditions, the CHDA yield exceeded 90% (Figure 7a).

Regarding the asymmetric hydrogenation of PET, Li Zelong from Lanzhou University^[34]et al. developed a Cu/m-ZrO₂bifunctional catalyst, which selectively hydrogenates the PET methanolysis intermediate DMT to methyl p-toluate (MMB, 86% selectivity) via a two-step method. The key lies in the weak Lewis acidity of the monoclinic ZrO₂support, which moderately regulates the adsorption strength of intermediates, successfully suppressing the side reaction pathway of deep benzene ring hydrogenation to generate PX (Figure 7b). Meanwhile, Mei Qingqing et al. from Zhejiang University^[35]utilized a PtW/MCM-48 catalyst, leveraging the synergy between the moderate acid sites of WO_xspecies and hydrogen spillover from Pt nanoparticles to achieve one-pot conversion of PET into p-toluic acid (p-TA, 53.4% yield) with co-production of PX (36.4% yield) in an acidic medium. By precisely regulating the moderate acid sites of the catalyst, selective hydrogenation of waste PET to prepare p-toluic acid was realized in a one-pot process (Figure 7c). Among these, the gradient acid site design increased the p-TA desorption rate by 2.7 times compared to traditional catalysts, effectively avoiding over-hydrogenation of the carboxyl group. These two works, approaching from crystal phase confinement effects and acid-base relay catalysis respectively, provide complementary solutions for the selective retention of ester/carboxyl groups in PET molecules, jointly expanding the technical boundaries for the directional conversion of plastic waste into high-value oxygenated chemicals.

Regarding research on the high-value production of CHDM, multiple innovative pathways have emerged. Early studies were dominated by noble metal catalytic systems: Cao Yong et al. from Fudan University^[53]constructed a dual-catalyst relay catalytic system (Figure 8), which directly and efficiently converts PET into high-value-added trans-isomer-enriched CHDM through a hydrogenation/hydrogenolysis tandem reaction (Figure 9a). This system employs a Pd/r-GO catalyst to first hydrogenate PET into the intermediate PECHD, followed by an og-CuZn catalyst prepared via an oxalate gel method to achieve hydrogenolysis of PECHD, with a total yield of 95% for the two-stage reaction. Wang Yanqin et al.^[54]employed a Pd-modified Ni/CeO₂catalyst to realize a tandem reaction of PET methanolysis and benzene ring hydrogenation, efficiently preparing the intermediate DMCD via a one-pot method with a yield of 86.5%, followed by hydrogenolysis using a CuMgAl catalyst to obtain CHDM with an 83.8% yield. Yao Siyu et al. from Zhejiang University^[55]utilized Ru/MnO₂to catalyze the coupling of low-temperature PET methanolysis and benzene ring hydrogenation, combined with CuZnZr mixed oxides to achieve a 78% yield from DMCD to CHDM (Figure 9b). Furthermore, the Wang Yong team at Zhejiang University^[56]reported the use of a physically mixed Pd/C and CuMn-xrelay catalytic system, achieving an excellent CHDM yield (92.6%) in the continuous hydrogenation of DMT; moreover, through a three-step tandem reaction process comprising methanolysis, phenyl hydrogenation, and ester group hydrogenation, they ultimately achieved a direct CHDM yield of 85.7% from PET, wherein phenyl and ester group hydrogenation occurred on segmented loads of Pd and Cu catalysts. Meanwhile, the Hou Zhaoyin team^[57]reported a defect-type sulfonated UiO-66_def-SO₃H-supported Ru catalyst, which can efficiently convert waste PET plastic directly into DMCD with a yield as high as 97.7% under conditions of 170 °C, 3 MPa H₂, and a reaction time of 6 h.

To reduce reliance on precious metals, subsequent research has shifted to non-precious metal systems. Li Xiaohong et al.^[58]employed a NiLa catalyst to achieve PET methanolysis into DMT followed by hydrogenation to DMCD, combined with a CuFeAl catalyst to complete ester group hydrogenolysis, achieving a total yield of 90.2%, with products efficiently separable via boiling point differences (Figure 9c). Meanwhile, progress has been made in exploring green solvent systems: Liu Yue et al.^[59]utilized Pd/C to catalyze PET hydrolysis into TPA with simultaneous hydrogenation to CHDA, then employed a Ru-Sn/C bimetallic catalyst to achieve carboxyl group hydrogenolysis, ultimately obtaining 80.1% CHDM; this system innovatively leverages the autocatalytic effect of the intermediate CHDA to accelerate PET hydrolysis (Figure 9d). Hou Zhaoyin et al.^[60]employed a Cu₁/Mg₃Sc₂O₆catalyst under mild conditions (250 °C, 2 MPa, WHSV_DMCD = 0.49 h⁻¹) to efficiently hydrogenate DMCD derived from PET into CHDM, achieving a yield as high as 96.3%, with catalyst performance remaining stable for 300 hours. Further investigation revealed that replacing Sc with Al in the catalyst, Cu/MgAl₂O₄exhibited excellent performance for the hydrogenation of BHCD derived from PET into CHDM (Figure 9e), at 240 °C, 4 MPa, and 0.525 g·g_cat⁻¹·h⁻¹, BHCD was completely converted, achieving a CHDM yield of 98%^[61].

Thus, key technological breakthroughs ranging from noble to non-noble metal catalysis, from organic solvents to aqueous systems, and from multi-step separation to tandem reactions have provided diverse solutions for PET resource utilization. Currently, noble metals (Ru, Pd) or single-atom Cu, via synergistic "metal-acid" dual sites, can achieve precise stepwise control of ester group hydrogenation and benzene ring saturation under conditions of 2–4 MPa H₂ and ≤250 °C. By reinforcing macromolecular diffusion through mesoporous/defective supports and combining gradient acid sites to suppress excessive carboxyl hydrogenation, CHDM yields generally exceed 90%. However, all high-yield systems rely on high hydrogen pressure to drive thermodynamics, pure PET substrates to avoid Cl⁻/dye poisoning, and suffer intermediate separation losses due to multi-step relay processes. When hydrogen pressure drops to ≤1 MPa or real impurities are introduced, CHDM selectivity plummets by 20%–30%. Furthermore, non-noble Cu-based catalysts undergo grain sintering (>5 nm) above 240 °C, leading to irreversible decay in ester hydrogenolysis activity. Currently, the lack of universal descriptors capable of simultaneously maintaining benzene ring saturation and ester group retention within a "low-pressure, non-noble metal, complex matrix" framework constitutes the core bottleneck for the directed synthesis of high-value oxygenated chemicals from PET under mild conditions.

3.1.4 Preparation of new materials

For the hydrogenation upcycling of PET waste to prepare biodegradable polyesters, the team of Zhu Weipu at Zhejiang University^[62]Using bio-based hydrogenated dimer acid (HDA) as a raw material, under molten reaction conditions utilizing only the residual antimony catalyst in PET waste without any added external catalysts, partial TPA units in the macromolecular chains of PET waste were replaced via a novel carboxyl transesterification reaction, yielding biodegradable poly(hydrogenated dimer acid-co-ethylene terephthalate) (PEHT) (Figure 10a). The number-average molecular weight of this material reaches 48.5 kDa (when HDA/TPA=0.5), with a tensile strength of 28~35 MPa (comparable to PBAT), and a degradation rate of 24.4% after 28 weeks of soil composting. This material exhibits excellent mechanical properties, tunable flexibility, good biodegradability, and favorable film-forming performance, showing potential for large-scale use as high-value-added biodegradable mulch films. The team of Martin^[63]developed a new pathway of "catalytic hydrogenation modification," employing a Ru/C catalyst to prepare PET-PECHD copolyesters through selective hydrogenation of aromatic rings (Figure 10b). By optimizing reaction conditions, polyesters with an aromatic/alicyclic ring ratio ofx/yranging from 100/0 to 0/100 can be obtained. Whenx/y>87/13, the molecular weight remains at 43 kDa (original PET 53 kDa), tensile strength is maintained at 35 MPa, while elongation at break increases to 400% (PET only 4%). This material shows an 84% degradation rate in 1 M NaOH within 84 h (PET only 11%). It is evident that whenx/y>87/13, PET-PECHD is comparable to PET in terms of thermal stability and mechanical strength, while its elongation at break, toughness, and barrier properties are significantly improved, demonstrating excellent degradability in acidic/alkaline solutions and moist soil.

3.2 PLA restoration, upgrade, and utilization

In the reductive upgrading of PLA, targeted conversion into high-value-added products has become a research hotspot. As a key raw material for cosmetics, pharmaceuticals, and polyester synthesis, the global market size of propylene glycol is expected to exceed $11.7 billion by 2030^[64]; amino acids such as alanine are core components of food additives and pharmaceutical intermediates, with an annual demand growth rate of 8.5%^[38]; methyl methacrylate (MMA), as a monomer for organic glass and coatings, will reach a market value of $9.8 billion by 2027^[65]. These market demands have driven innovative breakthroughs in PLA catalytic hydrogenation technology. Researchers, through strategies such as catalytic hydrogenation, hydrolysis-coupled reduction, and hydrogenolytic decarboxylation, combined with multi-active site catalyst design, regulation of support interface properties, and optimization of hydrogen species transport pathways, are gradually overcoming the technical bottlenecks in PLA reductive upgrading (Figure 11).

3.2.1 Preparation of propylene glycol

For the high-value production of 1,2-propanediol, the Liu Zhimin team^[36]developed a Ru₄Mo₁/TiO₂diatomic catalyst (3.76 wt% Ru loading) that achieves 100% selective conversion of PLA to 1,2-propanediol via aqueous-phase reaction at 180 °C and 4 MPa H₂. The core lies in the Mo-induced Ru-O-Mo diatomic sites: Ru single atoms activate H₂to generate active hydrogen, while the Ru-O-Mo interface elongates the C=O bond in lactic acid molecules (generated from PLA hydrolysis) to 1.32 Å (originally 1.23 Å) through electron transfer, promoting hydrogenation to intermediates and suppressing deoxygenation side reactions (Figure 12a). The catalyst shows no activity decay after five consecutive cycles, attributed to the strong metal-support interaction (SMSI effect) between the TiO₂support and Mo. In contrast, Martin et al.^[37]designed a CuZnSi-400 catalyst that produces propanediol under solvent-free conditions (180 °C, 4 MPa H₂) with 99.5% selectivity and a space velocity of 0.65 g·g_cat^-1·h^-1. Its mesoporous structure formed by the Zn-Si oxide support (pore size 8~12 nm) significantly enhances mass transfer efficiency for molten PLA (softened at 180 °C), while the Cu-Zn alloy phase reduces the H₂dissociation energy to 0.78 eV via d-band center regulation, a 35% reduction compared to pure Cu catalysts (1.2 eV) (Figure 12b). This process eliminates solvent separation steps, reducing energy consumption by 22%; however, Cu grains grow from 5 nm to 12 nm during reaction, requiring SiO₂coating to suppress sintering. Based on this, PLA hydrogenation to 1,2-propanediol at diatomic or alloy sites (Ru-O-Mo, Cu-Zn) can achieve kinetic separation of preferential C=O hydrogenation and deoxygenation side reactions by lowering H₂dissociation energy and elongating the lactic acid C=O bond. However, all highly efficient systems still rely on high hydrogen pressure to compensate for insufficient hydrogen concentration in aqueous or solvent-free environments. Furthermore, these systems use pure lactic acid oligomer substrates; once PLA pigments or trace metal auxiliaries are introduced, the Ru-O-Mo interface becomes occupied or Cu grains rapidly grow. Moreover, under low pressure (≤1 MPa), the C=O hydrogenation rate decays exponentially, causing deoxygenation pathways to regain dominance. Currently, there is a lack of universal descriptors within the "low pressure-non-noble metal-complex matrix" framework to simultaneously suppress grain sintering and deoxygenation side reactions, constituting the core bottleneck for the directed synthesis of propanediol from PLA under mild conditions.

3.2.2 Preparation of amino acids and monomers

For alanine synthesis, the Martin team^[38]utilized Ru/TiO₂ (2 wt% Ru) at 140 °C, without external H₂, to achieve a 70% yield via a cascade reaction of PLA dehydrogenation, amination, and re-hydrogenolysis. In situ characterization indicates that PLA first interacts with NH₃ at TiO₂ Lewis acid sites (surface Ti³⁺) to form lactamide, which is subsequently hydrolyzed to ammonium lactate; active hydrogen generated on the surface of Ru nanoparticles (~3 nm) drives its decarboxylation to produce alanine (Figure 13a). The research team led by Researcher Chen Yong from the Technical Institute of Physics and Chemistry, Chinese Academy of Sciences^[39]proposed a "polylactic acid depolymerization-oxidation-hydrogenation" cascade reaction strategy. By designing a CoP/CdS composite photocatalyst, waste PLA plastic can be directly and highly selectively converted into alanine small molecules under mild conditions. This catalyst exhibits excellent catalytic activity (2.4 mmol∙g_cat^-1∙h^-1) and stability. A series of in situ and time-resolved experimental results indicate that CoP grown in situ on CdS can rapidly extract photogenerated electrons from CdS for proton reduction to generate active hydrogen species, while simultaneously accelerating the oxidation of PLA by photogenerated holes to form imine intermediates; finally, active hydrogen reacts with the imine intermediates to produce alanine (Figure 13b). For MMA production, Martin et al.^[40]further designed a two-step strategy: first, under α-MoC catalysis, PLA undergoes hydrogenolysis-esterification in methanol at 200 °C to generate methyl propionate (98% selectivity); subsequently, aldol condensation with formaldehyde is achieved at 380 °C using a Cs-La/SiO₂ bifunctional catalyst (15 wt% Cs-5 wt% La), reaching a total MMA yield of 79% (Figure 13c). However, plasticizers in commercial PLA tableware (such as phthalates) can cover the active sites of α-MoC, reducing the yield to 74%~85%, necessitating the development of a core-shell structured MoC@C (with a 2 nm thick carbon layer) to shield toxic substances. Furthermore, the team led by Qiu Bocheng from Nanjing Agricultural University^[66]developed a "thermochemical-plasma-electrochemical" system, achieving for the first time the direct synthesis of alanine from waste PLA plastic, air, and water. Through in situ characterization combined with density functional theory (DFT) calculations, the team confirmed that the reaction proceeds through three key steps: first, nitrate is electrochemically reduced to generate hydroxylamine (NH₂OH); subsequently, hydroxylamine spontaneously couples with pyruvate to form an oxime; finally, the oxime is further hydrogenated to generate the target product, alanine. The study found that the strain effect present in the CuBi alloy significantly accelerates the desorption process of hydroxylamine and the hydrogenation step of the oxime, which is the key mechanism for achieving efficient synthesis. This technology not only successfully converts waste plastic into high-value chemicals but also innovatively uses air as a nitrogen source (replacing traditional ammonia sources with high carbon footprints). The entire process is characterized by being green and efficient, providing a completely new technical pathway for the synthesis of environmentally friendly chemicals. Thus, it is evident that both the preparation of amino acids and MMA from PLA first construct C—N or C=C bonds via a "dehydrogenation-amination/esterification" cascade, and then utilize active hydrogen provided by Ru/TiO₂, α-MoC, or CoP/CdS to complete C—C cleavage/hydrogenation, achieving directional conversion with C-utilization >75%; however, there is currently a lack of universal descriptors within the "in situ nitrogen fixation-impurity tolerance" framework to synergistically regulate the energy barriers of C—N/C=C coupling and C—C cleavage, which has become the core bottleneck for the precise synthesis of high-value nitrogen-containing/ester-containing monomers from PLA under mild conditions.

3.2.3 Preparation of propionic acid and biofuels

Regarding the preparation of propionic acid, the Liu Zhimin team^[41]reported a strategy for depolymerizing PLA into propionic acid using halide anion ionic liquids (Figure 14a). Using a composite system of 1-butyl-2,3-dimethylimidazolium bromide and Pd/C under hydrogen conditions, PLA can be degraded into corresponding carboxylic acids and alkanes. Furthermore, in the reductive upgrading utilization of PLA, biofuels, as key products replacing fossil energy, have attracted significant attention due to their high heating value and low carbon characteristics. Kang Shimin et al. from Dongguan University of Technology^[42]developed a Pd/C catalyst (1 wt% Pd loading) that converts PLA into high-heating-value liquid fuels (heating value 29.9 MJ∙kg^-1) via hydrogenolysis at 270 ℃ and 3.0 MPa H₂, comprising ethanol (32%), ethyl propionate (28%), ethyl lactate (25%), and diethyl acetal (15%), with a carbon utilization rate of 99% and no solid residue. The reaction pathway involves heterolytic activation of H₂by Pd nanoparticles (~5 nm) to generate H⁺and H^-species: the PLA ester bonds are attacked and cleaved by H⁺in the molten state (>170 ℃), generating lactic acid intermediates, which subsequently undergo decarboxylation (producing CO) and hydrogenation on the Pd surface, ultimately forming C₂~C₃oxygenated compounds. The byproduct CO (accounting for 85% of product gases) can recover H₂through the water-gas shift reaction (WGS), reducing overall hydrogen consumption by 18% (Figure 14b). For pigmented PLA waste (such as red straws), pretreatment using supercritical CO₂(60 ℃, 15 MPa) extracts and removes 95% of phthalocyanine dyes, restoring biofuel yield from 82% to 93%, and increasing the heating value to 30.3 MJ∙kg^-1, approaching that of diesel (35~45 MJ∙kg^-1). However, industrial scale-up requires addressing the coking issue of Pd catalysts in molten PLA (activity decreases by 40% every 50 h); encapsulating Pd particles within a mesoporous structure (pore size 6 nm) via an Al₂O₃coating can extend stability to 200 h.

3.2.4 Preparation of new materials

Currently, there are no literature reports on the hydrogenative upcycling of PLA waste into biodegradable polyesters. Although the molecular structure of PLA is relatively simple, research exploring its chemical upcycling into other new materials has been conducted. Among these, upgrading PLA to PLA-based polyurethanes (PLA-PU) represents a typical pathway. For instance, the team led by Academician Wang Yuzhong at Sichuan University^[67]proposed a solvent-free, one-pot recycling strategy with low catalyst loading. This method can efficiently convert waste PLA and its disposable plastic products into PLA-PU polymers with superior performance. The core route involves two key steps: first, degrading PLA into oligomers with hydroxyl groups at both chain ends via alcoholysis; subsequently, without purifying or separating the intermediate product, a chain extender is directly added to the reaction system for chain extension, ultimately yielding the target product, PLA-PU. This efficient and concise recycling and upcycling strategy provides important insights for the high-value utilization of PLA waste. In addition to the aforementioned polyurethane pathway, other directions exist for the chemical upcycling of PLA, such as Qin Yusheng et al. from Yantai University^[68]who utilized guanidine-zinc complexes to catalyze the alcoholysis/aminolysis of PLA for preparing novel diol monomers, which were subsequently used to synthesize polyesteramides. However, in the specific technical route of preparing new materials via hydrogenative modification, research remains a blank.

4 Techno-economic analysis and evaluation

In the hydrogenation upcycling of PET, techno-economic analysis (TEA) and life cycle assessment (LCA) reveal the dual impact of catalyst design and reaction conditions on economic viability and environmental benefits (Table 1). Taking catalytic hydrogenolysis of PET as an example (Figure 15), using Ru-based catalysts at 200–250 °C and 3–5 MPa hydrogen pressure can efficiently convert PET into TPA and ethylene glycol, but separation costs account for over 40% of the total investment cost. TEA studies indicate that by optimizing catalyst lifetime (from 200 h to 1000 h), separation energy consumption can be reduced by 30%, lowering the total production cost from $1.5/kg to $1.1/kg, approaching the production cost of virgin PET ($0.8–1.0/kg). However, LCA data show that the global warming potential (GWP) of the hydrogenolysis process is 0.8 kg CO_2e·kg^⁻¹ of product, primarily from hydrogen production (accounting for 65% of emissions); if green hydrogen (produced via water electrolysis using renewable energy) is adopted, the GWP can be reduced to 0.3 kg CO_2e·kg^⁻¹, but costs increase by 20%. In contrast, PET hydrocracking generates alkane fuels at higher temperatures (350–400 °C) and pressures (10–15 MPa); although monomer separation is unnecessary, the rapid catalyst deactivation rate (requiring regeneration every 50 h) leads to a 25% increase in operating costs, and the GWP reaches as high as 1.2 kg CO_2e·kg^⁻¹, highlighting the advantages of hydrogenolysis in closed-loop recycling^[69].

Furthermore, among the various routes for PET hydrogenation upgrading and recycling, the conversion of PET to CHDM offers significant economic benefits and application prospects, necessitating a TEA and LCA. Taking Wang Yanqin et al.^[54]who reported on the NiPd/CeO₂and CuMgAl systems as examples, a process model for an annual production of 10,000 tons of CHDM was established. The production cost of CHDM is approximately 22,500 CNY/ton, which is lower than the market price of 31,000 CNY/ton. Meanwhile, Wang Yong et al.^[56]conducted an economic feasibility assessment using Pd/C and CuMn-xcatalytic systems. The main equipment of this process includes two reactors for sequential alcoholysis and hydrogenation reactions, two flash evaporators for separating low-boiling components (methanol and hydrogen), one static mixer, and three distillation columns for solvent recovery (methanol and 1,4-dioxane) and product purification. Process simulation indicates that this process can produce 10,000 tons of high-purity CHDM (99.2 wt%) annually, with a calculated production cost of 22,450 CNY/ton, representing a 27.4% reduction compared to the current market price (31,000 CNY/ton).

Furthermore, while gram-to-kilogram-scale laboratory results are emerging rapidly, the industrial scale-up of CHDM still faces the rigid bottleneck that "high yield ≠ high throughput." Existing reports almost exclusively operate with ≤10 g PET·L⁻¹"dilute solutions" or ≤0.5 h⁻¹WHSV, resulting in space-time yields (STY) generally <30 kg_CHDM·kg_cat⁻¹·d⁻¹; whereas industrial polyester depolymerization units have single-line feed rates ≥5 t PET·d⁻¹, requiring STY ≥200 kg_CHDM·kg_cat⁻¹·d⁻¹to control catalyst loading within the economically viable range of "hundreds of kilograms per ton." Precious metal systems, due to their high active site density, can approach this metric at 2~4 MPa; however, Ru and Pd prices are ≥300,000 RMB·kg⁻¹, meaning the metal cost for producing each ton of CHDM is ≥1,500 RMB, accounting for 15%~20% of the product selling price, making them unable to compete with petroleum-based CHDM. Although non-precious Cu-based catalysts reduce metal costs by 90%, they are limited by grain sintering and low density of ester hydrogenolysis active sites. Under industrial space velocities (WHSV ≥1 h⁻¹), conversion declines by 30%~40%, necessitating a 2~3-fold increase in catalyst volume for compensation. This scales the reactor from "tubular" to "shell-and-tube with recycle," increasing fixed investment by ≥50%. Furthermore, support mechanical strength and pressure drop become "scale-up killers": mesoporous materials such as UiO-66_def-SO₃H and MCM-48 tend to powder under bed pressure differences of 1~3 MPa, generating 1‰~5‰ fine powder, which can cause erosion of circulating hydrogen compressor impellers, forcing a reduction in the annual maintenance cycle of the unit. Therefore, further research on catalysts for CHDM industrialization is still needed.

Regarding the hydrogenation upgrading recycling of PLA, current research is still in the exploratory stage, but its unique bio-based characteristics offer potential for low-carbon pathways. The hydrodeoxygenation reaction of PLA can be carried out at 200~250 ℃ and 2~4 MPa hydrogen pressure using Ru₄Mo₁/TiO₂ catalysts to convert PLA into propylene glycol and lactic acid derivatives; however, the high water content of PLA (>5%) leads to catalyst coking, reducing the conversion rate from 95% to 70%^[38]. TEA simulations indicate that developing hydrophobic MoS₂ catalysts could lower the reaction temperature to 180 ℃ and hydrogen pressure to 1.5 MPa, potentially reducing operating costs from $2.2/kg to $1.6/kg, though this remains higher than the production cost of virgin PLA ($1.3~1.5/kg)^[70]. In terms of LCA, the GWP of PLA hydrogenation is 1.0 kgCO_2e∙kg^-1 of product, with 60% originating from PLA raw material production (if industrial compostable PLA waste is used, GWP can be reduced by 30%), whereas the GWP of traditional mechanical recycling of PLA is 0.7 kgCO_2e∙kg^-1, indicating a need for further optimization of reaction selectivity to reduce carbon emissions from by-products. Achieving synergistic hydrogenation of mixed PLA and PET waste (e.g., using bimetallic Cu-Pd/TiO₂ catalysts) would allow processing both types of plastics in the same reactor, significantly lowering sorting costs, but requires addressing the mismatch between the PLA degradation temperature (160~180 ℃) and the PET hydrogenolysis temperature (200~250 ℃). Compared to PLA oxidation reactions, the potential of the hydrogenation upgrading recycling strategy remains to be explored^[71].

The industrialization of future PET and PLA hydrogenation technologies requires overcoming multiple bottlenecks. For PET, developing high-stability non-noble metal catalysts (such as Co-Mo) is key; their cost is 40% lower than Ru-based catalysts, and they can tolerate chlorine impurities (from PVC contamination), reducing raw material pretreatment costs by 15%. PLA hydrogenation needs to address product separation challenges; for instance, extracting propylene glycol (purity >99%) from the reaction mixture via membrane separation technology can significantly reduce separation energy consumption. Furthermore, integrated plant designs (such as sharing a hydrogen circulation system between PET hydrogenolysis and PLA hydrogenation) can lower hydrogen consumption costs, but require precise control of reactor series parameters. From an LCA perspective, if PET-PLA mixed hydrogenation systems adopt bio-based hydrogen and renewable energy power supply, the comprehensive GWP can be further reduced by 10%~35% compared to separate treatment, although initial investment will increase, necessitating economic balance through carbon tax policies or green subsidies. These technological breakthroughs will drive hydro-upcycling to achieve targets of PET processing costs < $1/kg and PLA processing costs < $1.5/kg before 2030, providing a dual-track solution for the circular plastic economy.

5 Conclusion and Outlook

In summary, the heterogeneous catalytic reductive upcycling of waste polyesters has advanced from laboratory exploration to the forefront of process scale-up. Targeting the two mainstream polyester materials, PET and PLA, researchers have achieved precise control ranging from ester bond cleavage to carbon-carbon bond activation through the design of metal coordination environments and supports. This has successfully yielded a series of high-value chemicals and novel biodegradable polymers, including PX, CHDM, aromatic fuels, propylene glycol, alanine, and MMA. The synergistic application of various hydrogen source pathways—such as direct hydrogen, alcohol reforming, and aqueous-phase ethylene glycol reforming—not only reduces reliance on high-pressure hydrogen but also enables efficient conversion at atmospheric or low pressure via endogenous hydrogen strategies, significantly enhancing energy efficiency and safety. The two main pathways, depolymerization-hydrogenation and non-depolymerization hydrogenation, each have distinct advantages: the former relies on acid-base supports to cleave ester bonds followed by stepwise hydrogenolysis, while the latter bypasses intermediate generation to directly target polymer segments, achieving one-step directional hydrogenation of intrachain C—C/C—O bonds.

From the perspective of techno-economic and environmental life cycle assessment, the catalytic hydrogenolysis process faces pressure regarding separation costs and hydrogen sources. By extending catalyst lifetime (200→1000 h) and introducing green hydrogen, production costs can be reduced from approximately $1.5/kg to $1.1/kg, while GWP is reduced from 0.8 kgCO_2e∙kg^-1to 0.3 kgCO_2e∙kg^-1; whereas hydrocracking and non-separation fuel preparation, despite having the advantage of simplified reactions, do not show significant advantages in GWP and operating costs due to catalyst deactivation and high-temperature, high-pressure conditions. Although PLA research started later, breakthroughs have been achieved in the highly selective production of products such as propylene glycol, alanine, and MMA by leveraging aqueous-phase reactions and dual-atom/dual-functional catalytic systems; integrated RFC (reaction-separation-recycling) design is expected to further improve economic viability. Mixed waste co-conversion and integrated plant designs provide a pathway for future large-scale operations—accommodating optimal conditions for both PET (200~250 ℃) and PLA (160~180 ℃) within the same system could reduce sorting and separation costs by at least 50%, utilizing carbon tax incentives or green subsidies to balance initial investments.

Nevertheless, to truly achieve a closed-loop cycle and scaled application of polyester recycling, multiple challenges remain to be overcome: first, the efficient and stable design of non-noble metal catalysts to reduce raw material pretreatment costs and enhance tolerance to impurities (such as PVC and metal residues); second, the deep coupling of multi-hydrogen source mechanisms, including the supply balance between endogenous and exogenous hydrogen, and the combined use of enzymatic and chemical catalysis; third, the synergy between integrated reaction and separation technologies (such as membrane separation, microreactors, and online methanol reforming) and digital factory platforms (high-throughput screening plus machine learning models) to create a closed-loop R&D process of "design-verification-optimization"; fourth, minimizing carbon emissions and hydrogen source costs under green electrolytic hydrogen and large-scale renewable energy power supply systems.

In the next decade, with the deep integration of catalytic materials science, reaction engineering, green chemistry, and artificial intelligence, it is expected to drive waste polyester from a "resource consumption-waste emission" model toward a full industrial chain closed loop of "high-value chemicals-repolymerization cycling." Through interdisciplinary collaboration and policy incentives, achieving the organic unity of "zero plastic waste," carbon neutrality, and a circular economy by 2050 will no longer be a distant vision, but an inevitable destination driven by both technology and the market.

References

Publishing order | Descend order by publishing year | Descend order by cited within

[1]	Zhang M Q, Wang M, Sun B, Hu C Q, Xiao D Q, Ma D. Chem, 2022, 8(11): 2912.

[2]	Weng Y J, Hong C B, Zhang Y L, Liu H C. Green Chem., 2024, 26(2): 571.

[3]	Wang K Z, Sun Z H, Yang T, Guo W D, Gao F F, Chen M G, Liu Y M, He H Y, Cao Y. ChemCatChem, 2025, 17(11): e202500018.

[4]	Zhao H Y, Ye Y D, Zhang Y B, Yang L, Du W C, Wang S L, Hou Z Y. Chem. Commun., 2024, 60(94): 13832.

[5]	Van Roijen E, Miller S A, Davis S J. Science, 2025, 387(6730): 176.

[6]	Korley L T J, Epps T H III, Helms B A, Ryan A J. Science, 2021, 373(6550): 66.

[7]	Houssini K, Li J H, Tan Q Y. Commun. Earth Environ., 2025, 6: 257.

[8]	Qin J B, Wu F Y, Dou Y B, Zhao D, Hélix-Nielsen C, Zhang W J. Adv. Mater., 2025, 37(23): 2418138.

[9]	Chu M Y, Liu Y, Lou X X, Zhang Q, Chen J X. ACS Catal., 2022, 12(8): 4659.

[10]	Chu M Y, Tu W L, Zhang Q, Huang W Y, Chen J X, Ma D. Nat. Rev. Clean Technol., 2025, 1(5): 320.

[11]	Gritsch S M, Mihalyi S, Bartl A, Ipsmiller W, Jenull-Halver U, Putz R F, Quartinello F, Guebitz G M. Resour. Conserv. Recycl., 2023, 188: 106701.

[12]	Li Y Y, Yi H L, Li M J, Ge M Q, Yao D G. J. Clean. Prod., 2022, 366: 132985.

[13]	Zhang S, Zhao X, Liu X H, Chen L, Bai L, Xu S M, Wang Y Z. Prog. Polym. Sci., 2025, 164: 101958.

[14]	Ye B Y, Zhou R R, Zhong Z X, Wang S L, Wang H, Hou Z Y. Green Chem., 2023, 25(18): 7243.

[15]	Ye B Y, Zhou R R, Wang C L, Wang Z X, Zhong Z X, Hou Z Y. Appl. Catal. A Gen., 2023, 649: 118936.

[16]	Yu Y K, Zhang Y F, Zhu S M, Mei Q Q. Angew. Chem. Int. Ed., 2025, 64(28): e202503469.

[17]	Ye Y D, Wang H, Zhang Y B, Zhao H Y, Du W C, Chen P, Hou Z Y. Appl. Catal. A Gen., 2024, 686: 119917.

[18]	Zhao W C, Guo Z P, He J H, Zhang Y T. Angew. Chem. Int. Ed., 2025, 64(9): e202420688.

[19]	Najmi S, Vance B C, Selvam E, Huang D, Vlachos D G. Chem. Eng. J., 2023, 471: 144712.

[20]	Kang M J, Yu H J, Jegal J, Kim H S, Cha H G. Chem. Eng. J., 2020, 398: 125655.

[21]	Wang Y X, Biddle T, Jiang C L, Luong T, Chen R J, Brown S, Jie X Y, Hu J L. Chem. Eng. J., 2023, 465: 142918.

[22]	Kumar S, Sajwan D, Sharma D, Krishnan V. Adv. Sustain. Syst., 2025, 9(4): 2500003.

[23]	Bian W G, Xu X C, Bai P Z, Xie Y J. Chin. Sci. Bull., 2025, 70(18): 2838.

[24]	(边文格, 徐向超, 白培志, 谢银君. 科学通报, 2025, 70(18): 2838.)

[25]	Lu S L, Jing Y X, Feng B, Guo Y, Liu X H, Wang Y Q. ChemSusChem, 2021, 14(19): 4242.

[26]	Liu H Y, Zhang J, Gu J, Shan R, Yuan H R, Chen Y. J. Clean. Prod., 2023, 425: 138924.

[27]	Gao Z W, Ma B, Chen S, Tian J Q, Zhao C. Nat. Commun., 2022, 13: 3343.

[28]	Chen H Y, Guo Z B, Liang X, Zhang M Q, Wang M, Ma D. ACS Catal., 2025, 15(8): 6287.

[29]	Kratish Y, Li J Q, Liu S F, Gao Y S, Marks T J. Angew. Chem. Int. Ed., 2020, 59(45): 19857.

[30]	Kratish Y, Marks T J. Angew. Chem. Int. Ed., 2022, 61(9): e202112576.

[31]	Hongkailers S, Jing Y X, Wang Y Q, Hinchiranan N, Yan N. ChemSusChem, 2021, 14(19): 4330.

[32]	Ye M X, Li Y R, Yang Z R, Yao C, Sun W X, Zhang X X, Chen W Y, Qian G, Duan X Z, Cao Y Q, Li L N, Zhou X G, Zhang J. Angew. Chem. Int. Ed., 2023, 62(19): e202301024.

[33]	Shao Y W, Fan M J, Sun K, Gao G M, Li C, Li D Q, Jiang Y C, Zhang L J, Zhang S, Hu X. Green Chem., 2023, 25(24): 10513.

[34]	Li J, An Z D, Kong Y, Zhang L, Yang J Q, Wang X J, Wang J T, Duan D L, Zhang Q, Long R, Vlachos D G, Li Z X. Appl. Catal. B Environ. Energy, 2024, 357: 124307.

[35]	Cheng J N, Xie J, Xi Y J, Wu X J, Zhang R H, Mao Z H, Yang H F, Li Z L, Li C. Angew. Chem. Int. Ed., 2024, 63(11): e202319896.

[36]	Zhu Y, Mao Z Y, Wu W X, Han B X, Mei Q Q. J. Am. Chem. Soc., 2025, 147(12): 10662.

[37]	Tang M H, Shen J, Wang Y D, Zhao Y F, Gan T, Zheng X S, Wang D S, Han B X, Liu Z M. Nat. Commun., 2024, 15: 5630.

[38]	Mi R L, Zeng L Z, Wang M L, Tian S H, Yan J, Yu S X, Wang M, Ma D. Angew. Chem. Int. Ed., 2023, 62(28): e202304219.

[39]	Tian S H, Jiao Y C, Gao Z R, Xu Y, Fu L K, Fu H, Zhou W, Hu C Q, Liu G S, Wang M, Ma D. J. Am. Chem. Soc., 2021, 143(40): 16358.

[40]	Liu C X, Liu K S, Xu Y J, Wang Z, Weng Y X, Liu F L, Chen Y. Angew. Chem. Int. Ed., 2024, 63(16): e202401255.

[41]	Sun B, Zhang J, Wang M L, Yu S X, Xu Y, Tian S H, Gao Z R, Xiao D Q, Liu G S, Zhou W, Wang M, Ma D. Nat. Sustain., 2023, 6(6): 712.

[42]	Zeng W, Zhao Y F, Zhang F T, Li R X, Tang M H, Chang X Q, Wang Y, Wu F T, Han B X, Liu Z M. Nat. Commun., 2024, 15: 160.

[43]	Xu J L, Zhou K, Fu J X, Tan Z M, Qin L L, Duan P G, Xu Y J, Kang S M. Fuel, 2023, 334: 126609.

[44]	Li R X, Zeng W, Zhao R Y, Zhao Y F, Wang Y P, Zhang F T, Tang M H, Wang Y, Chang X Q, Wu F T, Liu Z M. Nano Res., 2023, 16(10): 12223.

[45]	Wang N, Liu J Y, Wei J D, Liu S B, Liu G Z. Sci. China Chem., 2025, 68(4): 1563.

[46]	Ou W T, Wang H, Ye Y D, Zhao H Y, Zhang Y B, Hou Z Y. J. Hazard. Mater., 2024, 476: 134964.

[47]	Hu Y, Gu Y W, Dong Y J, Wang Y Q, Xu J F, Han Y Y, Zhang C J, Xie Y J. Angew. Chem. Int. Ed., 2025, 64(14): e202502923.

[48]	Hu Y, Zhang S Y, Xu J F, Liu Y, Yu A A, Qian J, Xie Y J. Angew. Chem. Int. Ed., 2023, 62(45): e202312564.

[49]	Spicer A J, Brandolese A, Dove A P. ACS Macro Lett., 2024: 189.

[50]	Fernandes A C. ChemSusChem, 2021, 14(19): 4228.

[51]	Westhues S, Idel J, Klankermayer J. Sci. Adv., 2018, 4(8): eaat9669.

[52]	Li M, Zhang S B. ACS Catal., 2024, 14(9): 6717.

[53]	Tan T, Wang W, Zhang K, Zhan Z X, Deng W P, Zhang Q H, Wang Y. ChemSusChem, 2022, 15(14): e202200522.

[54]	Sun Z H, Wang K Z, Lin Q, Guo W D, Chen M G, Chen C, Zhang C, Fei J C, Zhu Y F, Li J B, Liu Y M, He H Y, Cao Y. Angew. Chem. Int. Ed., 2024, 63(37): e202408561.

[55]	Mo S L, Kou J, Zeng J R, Song K P, Zhang Y Y, He S M, Hu Y F, Guo Y, Liu X H, Chen X, Wang Y Q. Chem. Eng. J., 2024, 500: 157249.

[56]	Huang Y C, Si Y X, Guo X S, Qin C, Huang Y K, Wang L, Gao X F, Yao S Y, Cheng Y W. ACS Catal., 2025, 15(6): 4570.

[57]	Liu W C, Wang Z Y, Ban H, Tang C Y, Zhang L W, Lu B, Zhang K C, Mao S J, Wang Y. Appl. Catal. B Environ. Energy, 2025, 377: 125466.

[58]	Ou W T, Ye Y D, Zhang Y B, Zhao H Y, Du W C, Hou Z Y. Chin. J. Catal., 2025, 71: 363.

[59]	Ren W Z, Gang C, Zhao C, Wu P, Li X H. Appl. Catal. A Gen., 2025, 698: 120233.

[60]	Fang S H, Gao Z W, Shi X R, Zhang M, Zhang W, Liu Y. Sci. China Chem., 2025, 68(5): 2101.

[61]	Chang Z L, Ye B Y, Zhong Z X, Wang S L, Wang H, Du W C, Hou Z Y. J. Mater. Chem. A, 2024, 12(2): 1003.

[62]	Jiang X L, Chang Z L, Yang L, Du W C, Hou Z Y. Fuel, 2024, 363: 130944.

[63]	Fang T X, Jiang W P, Zheng T F, Yao X X, Zhu W P. Adv. Mater., 2024, 36(46): 2403728.

[64]	Guo Z B, Zhang H R, Chen H Y, Zhang M Q, Tang X Y, Wang M, Ma D. Angew. Chem. Int. Ed., 2025, 64(6): e202418157.

[65]	Główka M, Krawczyk T. ACS Sustainable Chem. Eng., 2023, 11(19): 7274.

[66]	Darabi Mahboub M J, Dubois J L, Cavani F, Rostamizadeh M, Patience G S. Chem. Soc. Rev., 2018, 47(20): 7703.

[67]	Ma Y X, Guo X Y, Han M X, Ma J Z, Han M Z, Cai L J, Nicolosi V, Wang W L, Dong W L, Jiang M, Qiu B C. Angew. Chem. Int. Ed., 2025, 64(39): e202511466.

[68]	Luo Z X, Tian G Q, Chen S C, Wu G, Wang Y Z. Macromolecules, 2024, 57(14): 6828.

[69]	Liu S, Hu L, Liu J Y, Zhang Z S, Suo H Y, Qin Y S. Macromolecules, 2024, 57(10): 4662.

[70]	Zhang Z D, Wang J, Ge X H, Wang S L, Li A, Li R Z, Shen J, Liang X, Gan T, Han X D, Zheng X S, Duan X Z, Wang D S, Jiang J C, Li Y D. J. Am. Chem. Soc., 2023, 145(41): 22836.