Solvent‐ and Hydrogen‐Free Upcycling of Polystyrene by Layered Double Oxides

ABSTRACT

Chemical upcycling of plastic waste into value‐added feedstocks holds great promise for advancing the circular economy, while confronting formidable challenges. Herein, we report a cost‐effective catalyst MgAl‐layered double oxide (MgAl‐LDO) for upcycling polystyrene (PS) into liquid fuel substitute components with rich styrene under solvent‐ and H₂‐free conditions. The operando characterizations and theoretical simulations demonstrate that the MgAl‐LDO, featuring rich basic sites (Mg─O), is favorable for efficient activation of C─H bonds and selective cleavage of C(sp³)‐C(sp³) bonds. In particular, MgO in MgAl‐LDO affords a low barrier for the rate‐determining step for the formation of styrene while simultaneously suppressing the side reaction of further hydrogenation. As a result, the MgAl‐LDO achieves a liquid fuel yield of 59.5% with a styrene selectivity of 70.9% at 280°C. Further, the obtained styrene monomer was copolymerized with methyl methacrylate (MMA) to prepare high value‐added MMA‐styrene copolymer, which offers pivotal insights into advanced chemical upcycling strategies, facilitating the transition to a sustainable plastics economy and the reduction of environmental impacts.

We present MgAl‐layered double oxide as a cost‐effective catalyst enabling the upcycling of polystyrene into styrene‐rich liquid fuel substitute components, with the entire process proceeding without solvents or H₂. By offering critical insights to optimize chemical upcycling efficiency, this research plays a vital role in facilitating the transition to a sustainable plastics economy while reducing adverse environmental effects.

1. Introduction

Plastics are low‐density, cost‐effective, and long‐lasting while having strong corrosion resistance and insulation properties, being the primary material of choice for various applications [1, 2, 3]. Each year, over 360 million tons of plastics are produced globally, representing an estimated 3–4% of all carbon processed, with ∼80% of them ending up as waste in landfills or entering water systems [4, 5]. Despite its status as the fifth most produced plastic globally, polystyrene (PS) constituted approximately 20% of landfill waste and was recycled at a rate of merely 1% [6, 7, 8]. While ongoing research explores PS alternatives and post‐functionalization methods, the economic viability of virgin PS production continues to dominate the market, constituting a significant constraint on sustainable development [9]. Conventional mechanical recycling, which involves melting and re‐extruding PS fails to restore the material to its original quality due to unavoidable chemical alterations and the current impracticality of flawless waste sorting [10]. In contrast, chemical recycling overcomes the fundamental limitations of mechanical methods by converting plastics into value‐added feedstocks such as aromatic nitrogen‐containing compounds [11], alkylbenzenes [12, 13], aromatic carbonyl compounds [14, 15]. Moreover, chemical recycling enables altering the heating mode for PS upcycling, for example, microwave‐assisted or photothermal processes, thereby offering the versatile and efficient route for upcycling of plastics [16, 17, 18]. Despite these promising progress in the chemical upcycling of plastics, the development of scalable and energy‐efficient processes remains a pivotal challenge [19, 20, 21, 22]. An optimal solution must not only enable the conversion of PS into a single high‐value product but also align with economic viability to facilitate its widespread implementation.

Currently, catalytic upcycling of PS has garnered intensive research attention. The presence of catalysts not only lowers the reaction temperature but also plays a role in regulating the conversion and increasing the selectivity for target products [23]. To date, various catalytic systems have been explored for PS‐to‐fuel transformations (Scheme 1). In particular, several catalytic systems with metal active sites such as Ru/Nb₂O₅, Co‐N‐Ni dual‐atom catalyst, and Fe₂N@C are developed for facilitating the hydrogenolysis of C(sp³)‐C(sp³) bonds with the assistance of H₂ [24, 25, 26]. Moreover, acid catalysts (e.g., AlCl₃, ZSM‐5) enable control over the reaction pathways of PS conversion, tuning product distributions and enhancing selectivity toward desirable compounds [27, 28]. Notable advances have also been achieved in the catalytic solvothermal processing of polystyrene [29, 30, 31]. Despite great efforts, the key challenge in the chemical upcycling of polystyrene lies in the selective activation and cleavage of the inert C(sp³)‐C(sp³) bonds in its backbone [32]. Effective catalytic strategies must overcome the high bond dissociation energy of these linkages to depolymerize PS, which necessitates harsh reaction conditions such as temperatures exceeding 300°C, solvent‐assisted environments, or H₂ atmospheres for high conversion [33, 34]. Therefore, it is essential to develop catalytic systems that enable the selective upcycling of PS waste into valuable monomers or fuels under energy‐efficient conditions, such as solvent‐ and H₂‐free conditions.

SCHEME 1.

The comparison of previous works and our work on thermal chemical upcycling of PS.

Herein, a series of layered double oxides (LDO) obtained by topotactic transformation of XAl‐layered double hydroxides (LDHs) precursors (X═Co, Ni, Zn, Ca, Mg) were used for upcycling PS under solvent‐ and H₂‐free conditions. The strong basic sites Mg─O on MgAl‐LDO not only attack C─H bonds and trigger β‐cleavage of the C─C bond, but also lower the energy barrier of C(sp³)‐C(sp³) cleavage to facilitate selective styrene formation, while imposing a high energy barrier for further hydrogenation to suppress side product formation. As a result, the MgAl‐LDO exhibits high performance for upcycling PS into styrene, and styrene‐rich liquid products was copolymerized with methyl methacrylate (MMA) to synthesize MMA‐styrene copolymer as a proto resin ready for further development. Additionally, diverse real‐world PS products are amenable to upcycling by our system, demonstrating viability and broad applicability. This study establishes the paradigm for the chemical upcycling of PS waste under solvent—and H_{2 –} free conditions, thereby fostering the circular utilization of plastic materials with profound implications.

2. Results and Discussion

2.1. The Fabrication and Structural Characterization of MgAl‐LDO

The MgAl‐LDO was synthesized by a topotactic transformation strategy, which included hydrothermal synthesis of MgAl‐LDH and subsequent calcination treatment (Figure 1a). The powder X‐ray diffraction (XRD) pattern diffraction peaks at 2θ= 11.62°, 23.42°, 34.2°, 60.5°, and 61.9° indexed to the (003), (006), (009), (010), and (113) planes of the LDHs structure (Figure 1b) [35]. After calcination at 450°C, the disappearance of characteristic LDHs peaks indicated the removal of interlayer anions, and H₂O leads to the collapse of the layered architecture (Figures S1–S5). Concurrently, broadened diffraction peaks emerged at 2θ = 43.2° and 62.5°, corresponding to the (200) and (220) planes for MgAl‐LDO. The obtained MgAl‐LDO retained the hexagonal platelet morphology of LDHs precursor while exhibiting a porous surface texture (Figure 1c and Figure S6). This structural evolution arises from the volatilization of interlayer species during calcination, generating interconnected pores that facilitate mass diffusion in catalysis [36].

FIGURE 1.

(a) Schematic illustration for the fabrication of LDOs (SNAS: Separate nucleation and aging steps). (b) XRD patterns of MgAl‐LDH and MgAl‐LDO. (c) SEM, (d) TEM, (e) High‐resolution TEM image, (f) lattice fringe image, and (g) EDS elemental mapping images of MgAl‐LDO with Mg: Al = 3:1.

In addition, a well‐defined lattice fringes with a spacing of 0.15 nm ascribed to the (220) plane of crystalline MgO is observed (Figure 1d–f), being consistent with the XRD result (Figure 1b). Furthermore, the energy‐dispersive X‐ray spectroscopy (EDS) elemental mapping (Figure 1g) confirmed the homogeneous distribution of Mg, Al, and O on the MgAl‐LDO nanoplate (Figure S7).

2.2. The Evaluation of Upcycling PS

The performance of synthesized MgAl‐LDO for upcycling PS was then evaluated under solvent‐ and H₂‐free conditions. To investigate the upcycling of PS plastic waste, we first performed a catalyst‑free experiment, which produced no liquid products at 280°C (Figure S8). Then, we examined the reaction temperatures under 260°C, 280°C, and 300°C (Figure 2a). The results show that the conversion of PS increases from 16% to 86.5% as the temperature increases from 260°C to 280°C, while further increasing the temperature to 300°C, the yield of liquid products is not significantly increased. Moreover, the yield of gas products is gradually enhanced with the increase of reaction temperature. Then, the optimal temperature is determined to be 280°C. When the mass ratio exceeds 2.0:0.6, the insufficient provision of surface‐active sites for PS activation results in sluggish reaction kinetics and low conversion. When the mass ratio is lower than 2.0:0.6, LDO agglomeration would prolong the diffusion pathways of the reactants and hinder the accessibility of PS molecules to active sites [37]. Based on the comparison, the mass ratio of PS to the catalyst is optimized to be 2.0:0.6 (Figure S9). Based on the above investigation, the optimized conditions that reaction time of 280°C and PS‐to‐catalyst ratio of 2.0:0.6 were applied to subsequent studies on catalytic upcycling of PS.

FIGURE 2.

Effect of reaction conditions or catalysts on product distribution in the upcycling PS process (a) reaction temperature from 260°C, 280°C to 300°C over MgAl‐LDO. (b) particle size from 5250, 145 to 55 nm. (c) Mg/Al molar ratio from 2:1, 3:1 to 4:1. (d) XAl‐LDO, X═Co. Ni, Zn, Ca, Mg. (e) acid catalysts (ZSM‐5, MMT, γ‐Al₂O₃). (f) basic catalysts (MgO, CaO, ZnO).

As shown in Figure 2b, the conversion of PS exhibits an increase from 60.5% to 86.5% corresponding to decreased average particle size from ∼5250 to ∼55 nm, indicating the catalytic activity is affected by the particle size (Figures S10–S12). This is attributed to the smaller particle size of MgAl‑LDO, which has a higher surface area, being conducive to the adsorption of PS on the catalyst surface and exposing more active sites. Further regulating the catalytic performance of LDO by adjusting the Mg/Al ratio, the MgAl‐LDO with a Mg/Al ratio of 3:1 exhibited optimal catalytic performance with 59.5% yield of liquid products (Figure 2c). The catalyst with a low Mg/Al molar ratio (Mg/Al < 3) containss a relatively high amount of Al³⁺, which filled sites that would have otherwise been occupied by Mg²⁺ ions, resulting in low catalytic activity [38]. However, in samples with low Al content (Mg/Al > 3), the formation of a surface amorphous AlO_y structure partially covers the Mg‑O pairs and decreases the concentration of surface active sites, resulting in a remarkable decrease in the yield of liquid products [39].

In addition to adjusting the Mg/Al ratio of MgAl‐LDO, the structure of catalyst can be modified by incorporating other transition metal elements into layered struxture, thereby adjusting the active sites of the catalyst. Figure 2d shows that the CoAl‐LDO, NiAl‐LDO, ZnAl‐LDO, and CaAl‐LDO have low conversion and the yield of liquid products compared to the MgAl‐LDO. Especially, CoAl‐LDO and NiAl‐LDO display high solid residue yields of 64.5% and 63.0%, respectively. Due to coverage of transition metal sites (Co^{2 +}, Ni^{2 +}) by oligomers under solvent‐ and H₂‐free conditions, CoAl‐LDO and NiAl‐LDO suffered deactivation during the initial reaction stage.

Compared with the acid (ZSM‐5, Montmorillonite (MMT), γ‐Al₂O₃) and basic catalysts (MgO, CaO, ZnO) for PS conversion (Figure 2e,f), the MgAl‐LDO exhibited exceptional performance with 86.5% conversion, and the yield of liquid products reached 59.5%. It is found that the PS conversion on the commercial acid and basic catalysts is less than 60%, and the yield of liquid products is less than 40% under the same reaction conditions. Of note, acid catalysts can activate C─H bonds in PS chains through protonation, initiating the formation of intermediates that subsequently trigger the cleavage of C(sp³)‐C(sp³) bonds [40]. However, without external hydrogen donors such as H₂ or hydrogen‐donating solvents, the protonation process suffers from kinetic limitations. In contrast, typical solid bases such as MgO, CaO, ZnO possess strongly basic sites to efficiently attack the polymer backbone composed of C(sp³)‐C(sp³) single bonds through a β‐cleavage mechanism, with the catalytic process involving three key steps: proton abstraction, β‐cleavage, and termination [41, 42]. Although typical solid bases exhibit high selectivity in PS conversion compared with acid catalysts, base catalysts have a low specific surface area of less than 50 m²·g⁻¹ (Figures S13 and S14), and the types of monotonous basic sites may lead to their weak activation ability toward the C─H bond in PS [37, 41]. In contrast, the 2D LDH‐derived MgAl‐LDO maintains the morphology of the precursor after calcination, while also exhibiting a well‐developed porous structure, high specific surface area, and abundant Lewis basic sites. The Lewis basic sites are crucial for activating PS through a deprotonation mechanism, enabling effective MgAl‐LDO mediated PS upcycling proceeds effectively without hydrogen donors and achieving high performance with a conversion of 86.5% and liquid yield of 59.5% [43].

The chemical functional group composition of the liquid products was characterized by the Fourier Transform infrared spectroscopy (FTIR spectrum) and compared with that of pristine PS (Figure 3a and Figure S15). A band at 3030 cm⁻¹ corresponded to aromatic C─H stretching vibrations (νC─H), the absorption bands at 1490 cm⁻¹ and 1450 cm⁻¹ were indicative of aromatic skeletal vibrations, and the peaks at 750 and 700 cm⁻¹ were characteristic of the out‐of‐plane bending vibrations of aromatic C─H stretching vibrations (νC─H), all of which indicated the presence of benzene rings in the liquid products. Moreover, a peak at 3070 cm⁻¹ was attributed to the C─H stretching vibrations (νC─H) in vinyl groups. The peak at 2930 cm⁻¹ was ascribed to the C‐H stretching vibrations (νC─H) bond of the alkyl bond, which is likely related to methylene groups in styrene derivatives. Additionally, a distinct peak at 1625 cm⁻¹ is attributed to the conjugated C═C stretching vibrations (νC═C) [44, 45]. Most importantly, the analysis of gas chromatography‐mass spectrometry (GC‐MS) validates a relatively simple composition dominated by compounds with carbon numbers of C₇, C₈, C₉, C₁₀, C₁₃, C₁₄, C₁₅, and C₁₆ (Figure 3b and Figure S16 and S17). Further analysis of liquid products by MS identified that the C₈ component with the highest proportion was styrene monomer (Figure 3c and Figure S18) [46].

FIGURE 3.

Characterization of product distribution from catalytic conversion of polystyrene over MgAl‐LDO with a Mg: Al molar ratio of 3:1. (a) FTIR spectra of liquid products. (b) Carbon number distribution of liquid products. (c) Mass spectrometry signal peak matching of liquid products. (d) NMR spectra of liquid products. (e) Selectivity of liquid products. (f) Copolymerization of styrene and MMA (Azobisisobutyronitrile (AIBN), tetrahydrofuran (THF)).

To gain a clear understanding of the structural composition of the liquid products, the nuclear magnetic resonance (NMR) spectroscopy validated the conclusions. The peaks at 7.44 and 7.33 ppm were assigned to the ortho‐protons (2H) and meta‐/para‐protons (3H) of the benzene ring of styrene. The remaining peaks between 7.00‐7.50 ppm corresponded to protons on the benzene ring in other monocyclic aromatics (e.g., ethylbenzene, toluene). Signals between 4.70–6.80 ppm corresponded to olefinic protons, among them, 6.72, 5.76, and 5.25 ppm corresponded to the ‐CH = and = CH₂ protons of styrene, respectively (Figure 3d and Figures S19, S20). The ¹³C NMR analysis identified peaks at 127–128 ppm as aromatic carbons of styrene, with the critical vinyl carbon peak appearing at 136–137 ppm, attributed to substituent effects that maximize the chemical shift of the vinyl carbon (Figure S21). Additionally, terminal olefinic carbons at 114–116 ppm exhibited a downfield shift due to conjugation with the phenyl ring. Furthermore, subsequent quantitative integration determined 70.9% styrene selectivity in the liquid products, illustrating that the MgAl‐LDO efficiently generates styrene by selectively breaking C(sp³)‐C(sp³) bonds in PS (Figure 3e and Figures S22–S25) [12, 47, 48]. Additionally, the analysis of the main composition for gas products showed that the main components are H₂ and C₁ over the MgAl‐LDO catalyst (Figure S26).

The liquid product rich in styrene monomer from PS upcycling was copolymerized with MMA in a flask, and a white powder was obtained upon the completion of the reaction (Figure 3f). FTIR spectroscopy verified the successful copolymerization of MMA and waste polystyrene‐derived styrene, with characteristic peaks confirming the presence of styrene segments (aromatic C‐H stretching, aromatic skeletal vibrations, monosubstituted benzene ring bending, and MMA units (ester carbonyl stretching, methyl/methylene C‐H stretching), and the absence of vinyl C═C peaks ruling out residual monomers (Figure S27) [49]. Gel permeation chromatography (GPC) analysis showed the resultant MMA‐styrene copolymer exhibited a number‐average molecular weight (Mn) of 7152 g·mol⁻¹ and a narrow molecular weight distribution (PDI = 1.38), which validated the successful preparation of pure MMA‐St copolymer (Figure S28) [50]. Furthermore, XRD characterization revealed the copolymer had a low‐crystallinity amorphous structure, which is conducive to the transparency of the MMA‐St copolymer (Figure S29) [51].

2.3. The Reaction Mechanism of PS Upcycling

In the MgAl‐LDO catalyst, both Mg and Al exist in the form of oxides. To further elucidate whether there is a synergistic effect between them, we evaluated the physically mixed MgO + γ‐Al₂O₃ catalyst under the same reaction conditions. The results demonstated that the PS conversion of this physical mixture was lower than that of the MgAl‐LDO catalyst, elucidating that the layered structure of MgAl‐LDO provides a catalytic advantage for PS conversion (Figure S30). To investigate the mechanism of PS upcycling, we compared the chemical states of Mg, Al, and O between fresh and used MgAl‐LDO by X‐ray photoelectron spectroscopy (XPS). The shift in binding energy is observed for Mg 2p and O 1s (Figure 4a,b), indicating that Mg and O are primary active sites. The Al 2p peak exhibited no significant shift, indicating that the Al³⁺ species is not the site of upcycling PS, the primary role of Al is to stabilize the host structure in the MgAl‐LDO catalyst (Figure S31). The peak at binding energy of 49.69 eV is ascribed to Mg 2p for fresh MgAl‐LDO. After catalysis, the binding energy shifted to 50.29 eV (Figure 4a). This phenomenon can be attributed to the adsorption of PS reaction intermediates by surface Mg sites, which induces variation of the coordination environment and modification electron density. As shown in Figure 4b, the peaks located at low binding energy of 530.86 eV and high binding energy of 532.38 eV were attributed to lattice oxygen (O_α) and surface adsorbed oxygen (O_β) for fresh MgAl‐LDO, respectively [52, 53]. Remarkably, the O_α binding energy decreased from 530.86 to 530.73 eV after reaction. This phenomenon may be attributed to hydrogen bonding (Oα─H─C) facilitated by the polarized C─H bonds in PS. During adsorption, electronegative Oα interacts with H atoms of PS, leading to partial electron transfer from O atoms of MgAl‑LDO to the hydrogen‑bonding region, which causes a slight increase in the electron cloud density around Oα [54, 55]. Moreover, as the catalytic activity strongly depends on the basicity of the catalysts, CO₂‐TPD analyzes the distribution of basic sites across various LDOs. CO₂ desorption peaks with temperature (Td) below 200°C are classified as weak alkaline sites, 300°C–450°C as moderately alkaline sites, and above 500°C as strongly alkaline sites (Figure 4c) [56]. In the comparison, MgAl‐LDO uniquely combines moderate‐strength (Al‐O‐Mg) and strong (Mg‐O) basic sites, which is crucial for efficient PS conversion and promotes the growth nucleophilic attack on C‐H bonds, triggering β‐cleavage (Figure 4d) [57]. In contrast, LDOs derived from Ni, Zn, or Co exhibit weaker basicity, resulting in inferior PS conversion and low yields of liquid products (Figure 2d). These results highlight the critical role of strong basic Mg‐O in the MgAl‐LDO for both the reaction activity and selectivity.

FIGURE 4.

(a) Mg 2p and (b) O1s of XPS spectra for MgAl‐LDO. (c) CO₂‐TPD spectra of XAl‐LDO (X= Co. Ni, Zn, Ca, Mg). (d) The illustration of basic sites over MgAl‐LDO. (e) Operando FT‐IR spectra for upcycling PS into styrene over MgAl‐LDO with reaction time. The molar ratio of Mg: Al is 3:1. (f) The Gibbs free energy diagram, and (g) schematic of MgAl‐LDO upcycling of PS into styrene under solution‐ and H₂ ‐ free condition.

As previously discussed, the strong basic Mg‐O plays a crucial role in upcycling PS over the catalysts. To elucidate the rate‐determining step and gain deeper insight into the reaction mechanism, we employed a radical trapping experiment to confirm the presence of key intermediates during the reaction [58]. The radical intermediate trapping experiment confirmed that the transformation was suppressed when the 2, 2, 6, 6‐tetramethylpiperidinyl‐1‐oxide (TEMPO) was added, indicating that a radical pathway is likely involved (Figure S32). Moreover, high‐resolution mass spectrometry (HRMS) analysis of the reaction mixture revealed the formation of a TEMPO‐benzyl adduct, consistent with trapping of a benzylic radical intermediate (Figure S33). Based on the analysis of the above results, we proposed that benzylic carbon radicals were likely involved in the reaction pathway.

To shed light on the rate‐determining step and gain insight into the reaction mechanism, the operando FT‐IR is used to study the adsorbed species and reaction intermediates. As the reaction time increased, an absorption peak attributed to the O─H stretching vibrations ν(O─H) stretching emerged at 3700 cm⁻¹ (Figure 4e). The mobility of the chain segments in the molten PS polymer may be enhanced, ensuring that PS can more easily form hydrogen bonds with the strong basic sites (Mg─O) on the MgAl‐LDO during the catalytic process [37]. Meanwhile, the characteristic bands were observed in the C─H stretching vibrations (νC─H) of alkanes at 2950–2850 cm⁻¹. However, these signals gradually weakened during the process. This illustrates the strong basic sites (Mg─O) exposed on the surface of MgAl‐LDO attack the C─H bond in the main chain of PS through a nucleophilic mechanism, triggering the β‐cleavage reaction. Most importantly, the C═C stretching vibration at around 1600 cm^{− 1}, the in‐plane bending vibration of aromatic C─H at 1150 cm⁻¹ 1, and the characteristic bands of terminal vinyl groups at approximately 910 and 990 cm⁻¹ 1 gradually increased (Figure S34). This indicates the main chain of PS undergoes β‐cleavage mechanism, and the C(sp³)‐C(sp³) skeleton of PS is broken and converted into styrene monomer under the activation of strong basic sites (Mg‐O) over MgAl‐LDO.

NMR spectroscopy revealed the structural evolution of PS during the reaction (Figure S35). Strong resonance signals at 7–8 and 1–5 ppm corresponded to the aromatic and alkyl hydrogens of PS, respectively [59]. These changes indicate that the main chain of PS undergoes continuous scission and is gradually converted into styrene monomer and other low‐molecular‐weight aromatic products. Simultaneously, the Gibbs free energy calculations on the catalytic reaction were conducted, in combination with operando experimental characterizations to give an understanding of reaction pathway (Figure 4f) [22, 59, 60, 61]. Initially, PS is adsorbed onto the basic Mg‐O sites of the MgAl‐LDO catalyst surface, where Mg‐O basic sites polarize adjacent C─H bonds, leading to a reduction in the electron density of the C─H bonds. (Step I.). This system exhibits a free energy reduction of ∼0.40 eV, indicating a thermodynamically spontaneous trend. Afterward, the basic Mg─O sites attack the C─H bonds in the PS backbone via a nucleophilic mechanism, forming H‐Mg‐O intermediates and generating unstable carbon radicals (Step II.), with the free energy further decreasing to −4.28 eV, demonstrating enhanced reaction spontaneity. Subsequently, radical‐induced β‐cleavage at the β‐position of the radical site ruptures C(sp³)‐C(sp³) bonds, producing styrene monomers and shorter‐chain radical fragments. This chain propagation cycle dominantly produces styrene (Step III.). Notably, step III exhibits the highest energy barrier (∼5.70 eV), which is identified as the rate‐determining step. This observation indicates that the β‐cleavage of PS proceeds on Mg‐O of MgAl‐LDO, dominating the overall reaction kinetics. Then, styrene monomer is more easily desorbed from the surface of MgAl‐LDO (Step IV.) relative to remaining intermediates, which undergo side reactions with hydrogen atoms to generate ethylbenzene (Step V.).

To validate whether MgAl‐LDO suppresses hydrogenation, we performed an additional control experiment. Specifically, the hydrogenation of styrene was conducted over MgAl‐LDO under an H₂ atmosphere, applying the identical reaction conditions (temperature and time) as in the upcycling process of PS. Analysis of the liquid products collected after the reaction revealed the absence of ethylbenzene formation, confirming that MgAl‐LDO does not catalyze the hydrogenation of styrene under the employed conditions (Figure S36). In comparison, the carbanion intermediate hydrogenation process has a higher energy barrier, which indicates that strong basic sites, Mg─O, effectively suppress secondary reactions that could form byproducts. The operando experiment combined with theoretical calculation thus demonstrates the high selectivity of MgAl‐LDO for upcycling PS to styrene under solvent‐ and H₂ – free conditions (Figure 4g). In summary, the proposed reaction mechanism demonstrates that MgAl‐LDO effectively catalyzes the upcycling of polystyrene to styrene monomer. This pathway proceeds via selective cleavage of C(sp³)‐C(sp³) bonds, leading to high efficiency in styrene production. Notably, the transformation occurs under solvent‐free and H₂‐free conditions at moderate reaction temperatures.

2.4. Evaluation of Catalytic Upcycling of Real‐World Plastic and Economic Viability Analysis

After validating the feasibility of plastic upcycling over the MgAl‐LDO catalyst, we further demonstrate the scalability of upcycling real‐world complex plastic waste. The evaluation of various PS‐based plastic wastes encountered in real‐world includes representative discarded items such as cup lids, foam, cups, weighing tray, and fill material (Figure 5a). Due to the influence of additives, the MgAl‐LDO exhibited a reduced average conversion down to 70% alongside a diminished average liquid product yield dropping to 41.6%. Following the upcycling of mixed real‐world plastics, MgAl‐LDO had a selectivity of 53.3% for styrene, and the gas products were mainly H₂ and C₁ (Figure S37–S40). The slight decay of conversion was observed in five consecutive recycling experiments, but analyses revealed that the MgAl‐LDO catalyst maintained its structural and morphological integrity after five consecutive reaction cycles, demonstrating exceptional thermal stability (Figure 5b and Figures S41–S43). An integrated process for the chemical upcycling of PS waste was established, including feedstock pretreatment, feeding systems, catalytic conversion, and product separation, yielding styrene and other valuable chemicals (Figure 5c). Based on the existing economic assessment, the process flow for our catalytic upcycling of PS was simulated to evaluate its economic viability [62]. The analysis demonstrates that this technology generates an annual revenue of USD 2.531 million from 3000 t·year⁻¹ of PS. After deducting operational expenditures (OPEX) of USD 0.795 million, it achieving an annual net profit of USD 1.735 million, representing a profit margin of 69.0% (Figure 5d). The initial capital expenditure (CAPEX) of USD 3.300 million can be recovered within 1.90 years. Furthermore, the economic viability of this upcycling route can be further enhanced by converting recovered styrene into methyl methacrylate‐styrene copolymer. The market price of the copolymer is approximately 2–3 times that of pure styrene monomer, significantly expanding the high‐value application prospects of recycled polystyrene.

FIGURE 5.

(a) Products yield from catalytic conversion of real‐world plastic wastes by MgAl‐LDO. (b) Catalytic stability evaluation of MgAl‐LDO for PS upcycling via multiple reaction cycle. (c) Process flow diagram for producing styrene from PS.(d) Economic viability analysis of producing styrene from PS in plant.

3. Conclusion

In summary, a series of LDOs derived from XAl‐LDH (X═Co, Ni, Zn, Ca, Mg) via topotactic transformation were employed for upcycling of PS under solvent‐ and H₂‐ free conditions. The strong basic sites Mg‐O on the MgAl‐LDO not only mediate C─H bond activation and trigger β‐cleavage of the C─C bond, but also reduce the energy barrier for C(sp³)‐C(sp³) bond cleavage. Simultaneously, MgAl‐LDO facilitates the desorption of styrene while raising a high energy barrier for hydrogenation to suppress by‐product formation, which endows a favorable path for converting PS to styrene. Without H₂ or a solvent, this catalytic system enables upcycling of real‐world PS waste, demonstrating the successful preparation of MMA‐styrene copolymer using the styrene‐rich liquid product derived from PS upcycling, and highlighting the economic feasibility and practical potential. By establishing a new paradigm for solvent‐ and H₂‐free chemical upcycling of PS waste, this study provides critical support for the circular utilization of plastic materials, with implications that extend deeply into related research and potential industrial practices.

Conflicts of Interest

The authors declare no conflict of interest.

Supporting information

Supporting File: smll73855‐sup‐0001‐SuppMat.docx.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.