Carbonylolysis of waste polyesters into high-value organic acids

Domain: biology
Published: 2026-03-10
Editorial outcome: major_revision

Abstract
Polyesters such as PET contribute substantially to global plastic waste, yet current recycling approaches are hindered by high energy demands, inefficient product separation, and limited valorization pathways. We report a one-pot “carbonylolysis” strategy that couples polyester depolymerization with in situ carbon-chain reconstruction, producing high-value C3+ carboxylic acids under relatively mild conditions (170 °C, 2 MPa CO). Using a Rh–iodide catalyst, PET is quantitatively converted to terephthalic acid (99%) and propionic acid (96%). Mechanistic studies show that ethylene glycol released from PET hydrolysis undergoes iodide-assisted elimination followed by Rh-catalyzed carbonylation. The method applies broadly to diverse polyester wastes, including textiles and bio-based plastics. Life-cycle assessment and techno-economic analysis reveal substantial gains in energy efficiency, carbon footprint reduction, and wastewater minimization over conventional recycling routes. By integrating molecular-level reconstruction into polyester recycling, carbonylolysis establishes a sustainable blueprint for converting waste polyesters into high-value carboxylic acid.

Introduction
Plastics are essential to contemporary society and industrial processes, with widespread use across packaging, electronics, and construction due to their durability and malleability1,2. However, the majority of plastic waste is either incinerated (19%) or landfilled (49%), raising pressing concerns regarding resource loss, environmental pollution, and greenhouse gas emissions3,4. Polyethylene terephthalate (PET) is the most widely used polyester and a major contributor to plastic waste due to its extensive use and slow degradation5,6. Mechanical recycling remains the dominant strategy for managing PET waste7,8, but suffers from material downgrading and limited feedstock tolerance. Chemical recycling methods such as pyrolysis, solvolysis and hydrogenolysis offer alternatives for recovering monomers or converting waste into value-added chemicals9–11. Among them, hydrolysis stands out as the most direct and atom-efficient route to break PET into its constituent monomers, terephthalic acid (TPA) and ethylene glycol (EG)12,13 (Fig. 1a-Ⅰ). However, its industrial adoption is constrained by harsh reaction conditions, high energy consumption, and laborious downstream separation14,15.

These limitations have prompted increasing interest in one-step catalytic processes that couple depolymerization with the simultaneous conversion of hydrolysis intermediates into high-value products. Especially, EG derived from PET hydrolysis presents a key opportunity for value enhancement within a circular economy framework. Direct EG capture and conversion afford a wide range of chemicals containing the –OCH2CH2O– motif9,16, but they are usually constrained by the reaction equilibria. More extensively studied approaches circumvent this limitation by exploiting the intrinsic reductive nature of EG, which enables dehydrogenation with concomitant H2 release or transfer hydrogenation17,18, typically co-producing oxygenates or carbon. Selective C–C bond cleavage of EG affords C1 oxygenates such as formic acid19,20, while backbone-preserving pathways yield C2 products including glycolic and acetic acids21–23. However, these PET-derived EG conversion routes generally rely on excess strong base, as EG dehydration is typically promoted under strongly basic conditions across thermal, electrochemical, and photochemical methods. Moreover, nonthermal strategies usually require prior PET depolymerization into EG, which also involves large base inputs19,24. Notably, excess base typically converts TPA and other acidic products into salts that require re-acidification, consuming large amounts of acid and base while generating substantial saline wastewater, complicating integration into practical recycling systems (Fig. 1a-Ⅱ). Therefore, overcoming this reliance on excess base remains a core challenge for sustainable PET upcycling.

To date, the product spectrum from PET-derived EG has been largely confined to low-carbon (C1–C2) molecules11,19–23. Crucially, C–C chain-extended C3+ products are rarely pursued11,25,26, despite their markedly higher industrial value. C3–C6 carboxylic acids, such as propionic, glutaric, and adipic acids, remain predominantly fossil-derived via energy- and emissions-intensive oxidation processes27–29, representing especially appealing targets for sustainable production (Fig. 1a-Ⅲ). This gap underscores the urgent need for carbon-reconstructive catalytic pathways that can upgrade waste-derived diols into longer-chain carboxylic acids. Such pathways must combine selective bond scission, controlled C–C coupling, and oxygenate functionalization, ideally under low- or no-base conditions. Achieving this would unlock higher-value products and significantly broaden the chemical space accessible from plastic waste within sustainable recycling systems.

Herein, we report a previously unexplored carbonylation-assisted depolymerization strategy, termed “carbonylolysis”, that couples PET backbone cleavage with in situ carbon chain reconstruction of EG (Fig. 1b). Carbonylation offers a highly atom-economy, regioselective, and thermodynamically favorable route for upgrading alcohols into longer-chain acids30, with its highly exothermic nature supplying thermal energy to drive self-sustaining processes. Crucially, as a fundamental C1 building block, carbon monoxide (CO) can be sustainably sourced through biomass conversion, waste valorization, or CO2 reduction31,32. By integrating carbonylation with polyester depolymerization, we establish a single-step process that converts PET into both TPA and high-value C3+ acids under mild, scalable conditions.

Results
Catalytic system development for PET carbonylolysis
The proposed carbonylolysis process integrates simultaneous PET depolymerization with carbonyl insertion into the hydrolysis-derived diol fragments. To identify an effective catalytic system, we systematically screened a range of metal catalysts (RhCl3, RuCl3, IrCl3, PdCl2, CoCl2, NiCl2), iodide additives (LiI, NaI, ZnI2, CH3I, and CuI), and solvents commonly employed in carbonylation chemistry, including toluene (TL), water, acetonitrile (ACN), hexafluoroisopropanol (HFIP), cyclohexane (CYH), acetic acid (HAc)33, as well as the commonly used bio-based solvents dimethyl isosorbide (DMI) and γ-valerolactone (GVL). Among the tested metals, Rh3+ exhibited the highest catalytic activity for PET carbonylolysis (Supplementary Fig. S2), while the effect of the chloride counter-anion was negligible (Supplementary Table S1). Both iodide source and solvent were identified as critical parameters governing reaction efficiency34,35. CH3I and HI is the most effective iodide additives, owing to their ability to activate the Rh catalyst and sustain a high free-iodide concentration under reaction conditions (Supplementary Fig. S3). HFIP was identified as the optimal solvent (Supplementary Fig. S4), benefiting from its excellent PET-dissolving capability and high compatibility with the carbonylolysis system36,37.

In this transformation, terephthalic acid (TPA) is the primary product from PET depolymerization, while propionic acid (PA) is formed via carbonyl insertion into EG, the diol released from PET. Using commercially sourced, low-shear–pulverized PET powder as the feedstock (without additional ball-milling pretreatment that could alter the polymer structure38), we systematically optimized key reaction parameters to maximize PA yield, including the amounts of CH3I, H2O, and Rh3+, as well as the CO pressure (Fig. 2a–c, Supplementary Fig. S5).

The presence of an iodide source was essential for carbonylation activity, as a sufficiently high iodide concentration is required to stabilize the catalytically active Rh–iodide carbonyl species39–42. At low CH3I loadings, iodide activity was insufficient to sustain the active Rh speciation, resulting in incomplete carbonylation and predominant formation of EG (Fig. 2a). As CH3I is increased into an intermediate regime, the steady-state population of catalytically competent anionic Rh–iodide–carbonyl species is established, leading to a sharp rise in PA formation (Supplementary Table S2). Further increases in CH3I result in a plateau in activity and selectivity, indicative of saturation of the active Rh species. At much higher CH3I loadings, excess iodide over-competes with CO for coordination, driving Rh speciation toward iodide-saturated CO-deficient environments and limiting CO insertion, which reduces PA yield (Fig. 2a)43. This behavior is fully consistent with established Rh–iodide carbonylation chemistry34,44. Water addition was also found to be crucial (Fig. 2b). Small amounts of H2O significantly promoted PET depolymerization and subsequent carbonylation. Increasing H2O content enhanced EG and PA formation initially (EG increased from 0.3% to 18%, PA from 1.5% to 28%). Further addition of water led to near-complete consumption of EG and a substantial increase in PA yield reaching 96%. However, excessive water compromised the reaction by hydrolyzing anionic Rh(I) carbonyl species into less stable species (e.g., [Rh(CO)2I2]– to [Rh(CO)(H2O)I2]–45), and promoting the water-gas shift reaction, reducing CO availability and decreasing the PA yield to 81%. CO pressure was another key parameter. Increasing the CO pressure from 0.5 MPa to 2.0 MPa progressively improved the PA yield from 69% to 96% (Fig. 2c), though little improvement was observed beyond 2.0 MPa. Therefore, 2.0 MPa was chosen as the optimal pressure of CO. Likewise, PA yield increased with Rh3+ content up to 0.05 mmol, and further increases provided minimal additional benefit (Supplementary Fig. S5). Reaction temperature and time were then optimized (Fig. 2d, e). Increasing the temperature from 150 °C to 170 °C significantly improved PA yield from 80% to 96% (Fig. 2d). However, due to the highly exothermic nature of the carbonylation process, further temperature increases reduced the PA yield to 76%. Kinetic profiling under optimal conditions (170 °C, 2.0 MPa CO, 0.05 mmol Rh3+, 0.5 mmol CH3I, 500 µL H2O) showed a progressive increase in TPA yield, which stabilized above 99% over time (Fig. 2e). EG concentration initially increased and then declined, while PA yield gradually rose from 2.2% to 96% confirming effective coupling of EG with CO. These results demonstrate the viability of a Rh3+/I–/HFIP system in mediating tandem PET depolymerization and EG carbonylation, achieving near-quantitative yields of both TPA and PA.

Substrate scope and real-world waste plastic compatibility
The robustness of the carbonylolysis strategy was demonstrated using a range of commercial PET samples containing common additives and contaminants. Six types of real-world PET waste (including beverage bottles, non-woven fabrics, colored trays, transparent films, garments and woven materials) were successfully upcycled, affording PA yields of 90–98% and TPA yields of 92–98% (Fig. 3a). The recycling of blended textiles presents significant challenges due to the frequent incorporation of cotton, spandex, nylon, and synthetic dyes. Remarkably, even PET fabrics blended with cotton (15–35%), spandex (5%), chinlon (35%), or rayon (43%) achieved high product yields (PA: 89–93%; TPA: 90–94%) under the optimized conditions (Fig. 3b). Notably, all PET samples were manually cut into small pieces without further powdering or micronization, minimizing potential structural modification associated with intensive mechanical pretreatments such as ball milling38. Further size-activity experiments demonstrate that PET powdering is unnecessary for efficient conversion under the present HFIP-mediated conditions (Supplementary Table S3 and Supplementary Fig. S7), owing to the strong solubility of HFIP toward PET, which enables the direct processing of real waste streams without energy-intensive size reduction. The scope of this strategy was further extended to various polyester substrates, including polyethylene 2,5-furandicarboxylate (PEF), polyethylene glycol succinate (PES), polyethylene glycol (PEG), polyethylene adipate (PEA), polytrimethylene terephthalate (PTT), polybutylene terephthalate (PBT) and polybutylene adipate-co-terephthalate (PBAT). The process proved highly efficient across this diverse range of glycol- and diol-derived polyesters. For glycol-based polyesters such as PEF, PES, PEG, and PEA, both the carbonylation product (PA) and the hydrolysis-derived monomer were obtained in yields exceeding 90%. Polyesters based on longer-chain diols (PTT, PBT, PBAT) also underwent efficient transformation. In these cases, where the hydroxyl-bearing carbon atoms are non-adjacent, the reaction yields dicarboxylic or hydroxy acids as products41, alongside high TPA recovery, with both product classes exceeding 90% yield (Fig. 3c).

Kinetic dissection of the hydrolysis–carbonylation cascade
Understanding the mechanism of PET carbonylolysis is essential to rationalize its high selectivity and efficiency. Conceptually, the carbonylation reaction may proceed via activation of either the ester functionalities in the PET backbone or the hydroxyl groups of the depolymerized intermediate, ethylene glycol (EG). To differentiate these possibilities, we systematically investigated the carbonylation reactivity of PET, bis(2-hydroxyethyl) terephthalate (BHET), and EG under identical reaction conditions. All three substrates yielded comparable amounts of PA (Fig. 4a–c and Supplementary Table S4), strongly suggesting that PET is first hydrolyzed to release EG, which is subsequently carbonylated. The absence of byproducts such as 3-hydroxypropionic acid (3-HP) or succinic acid (SA), which would otherwise result from direct ester carbonylation, further supports this sequential pathway. Based on these findings, the overall carbonylolysis process can be divided into two distinct yet coupled stages: (i) hydrolytic depolymerization of PET into EG and TPA, and (ii) catalytic carbonylation of EG into C3+ organic acids. To gain further insights, we performed time-resolved kinetic studies under optimized conditions (Fig. 4), which revealed important distinctions in the dynamics and rate-limiting features of the two stages.

Due to its inertness and sluggish hydrolysis kinetics, PET remained largely undecomposed in water at 170 °C, even in the presence of Rh3+ alone (Fig. 4d-Ⅰ). To enhance depolymerization, we employed HFIP, a hydrogen-bonding solvent known to disrupt polymer crystallinity and improve the solubility of esters and polyamides46,47. HFIP significantly promoted PET dissolution and hydrolysis, achieving a 43% yield of TPA after 4 hours at 170 °C (Fig. 4d-Ⅰ). The addition of a small amount (50 µL) of PA, a downstream product of EG carbonylation, further elevated the TPA yield to 48%, likely due to acid-promoted ester hydrolysis48. Under optimized carbonylolysis conditions with CO, the TPA yield increased to 86%, and kinetic analysis showed a significantly accelerated hydrolysis rate compared to conventional systems (Fig. 4d-Ⅰ). This improvement arises from the coupling of EG carbonylation, which not only upgrades a hydrolysis product but also thermodynamically pulls the depolymerization equilibrium forward (Supplementary Table S7). These results underscore the dual importance of HFIP-mediated solubilization and product-driven equilibrium shifting in Stage 1.

Turning to Stage 2, kinetic profiles revealed that the concentrations of both EG and TPA increased rapidly at the early stage of reaction, with TPA quickly plateauing, indicative of fast and near-complete hydrolysis (Fig. 4d-Ⅱ). In contrast, EG concentration exhibited a transient peak followed by gradual decline, while PA concentration steadily increased throughout the reaction. These profiles strongly suggest that EG carbonylation is the rate-limiting step in the overall process. Interestingly, the measured TPA yield temporarily exceeded the combined yields of EG and PA, implying that a fraction of EG followed an alternative transformation route beyond direct carbonylation. Supporting this, NMR analysis detected the formation of 1,2-diiodoethane within the first hour of the reaction (Supplementary Fig. S9), and GC analysis identified ethylene as a transient intermediate that appeared shortly after EG and was nearly completely converted to PA within 12 hours (Fig. 4d-Ⅱ). These observations align with literature reports indicating that vicinal diols can undergo iodide-mediated substitution and β-elimination to generate alkenes41,49. The role of 1,2-diiodoethane as a key intermediate was further confirmed by independent reactions using it as a substrate, which also produced PA in high yield (Supplementary Fig. S10). Mechanistically, iodide species serves a dual function in carbonylation reaction: activating EG for substitution and stabilizing the Rh catalytic species39. When the iodine dosage was insufficient (e.g., 0.1 mmol, Fig. 2a), PA formation was significantly suppressed, highlighting its essential role in facilitating this reaction. Moreover, control experiments without Rh3+ led to trace levels of ethylene and negligible PA formation (Fig. 4d-Ⅲ). This confirms that both iodide and Rh catalysis play indispensable catalytic roles in the carbonylative transformation50. In addition, coupling with the carbonylation reaction facilitates ethylene generation to some extent. Collectively, these findings support a tandem cascade reaction mechanism in which EG is first converted to 1,2-diiodoethane, followed by elimination to ethylene, and subsequently Rh-catalyzed carbonylation to yield PA. The reaction kinetics, particularly the delayed consumption of EG and gradual PA formation, further suggest that the iodide-mediated substitution/elimination sequence constitutes the rate-determining step in the overall process.

Selectivity: β-elimination vs oxidative carbonylation
Notably, this mechanistic route presents a surprising ambiguity: one might anticipate that carbonylation of EG would yield 3-hydroxypropionic acid (3-HP) or succinic acid (SA) rather than PA. To clarify this, we performed density functional theory (DFT) calculations to map the competing reaction pathways and their associated Gibbs free energy barriers (Fig. 4e). The analysis focused on the rate-determining iodide-mediated substitution and elimination steps, starting from 1,2-diiodoethane (IM1), formed via substitution of EG’s hydroxyl groups by iodide49. From IM1, two distinct reaction channels diverge: indirect carbonylation via ethylene formation through β-elimination41 (Pathway A)51, as suggested by kinetic analysis, and direct carbonylation via oxidative addition of the organic iodide (Pathway B)40, in line with established halide carbonylation chemistry. The catalytically active [HRh(CO)2I2]– and [Rh(CO)2I2]– species coexist and are well documented to promote olefin and organic halide carbonylation, respectively39,40. In Pathway A, IM1 undergoes iodide-assisted β-elimination of I– via TS1 to generate ethylene (IM2). This ethylene then coordinates to the Rh center of [HRh(CO)2I]–40 to form IM2′, followed by protonation via TS2 to yield the alkyl–Rh intermediate (IM3). Subsequent CO insertion through TS3 produces the acyl species (IM4), which, upon hydrolysis, releases PA. In this pathway, β-elimination (IM1→TS1) is the rate-determining step, with a calculated free-energy barrier of 29.9 kcal/mol. By contrast, Pathway B involves direct oxidative addition of IM1 to [Rh(CO)2I2]–39. IM1 first coordinates to the Rh center, after which oxidative insertion into the C–I bond occurs via TS2′ to form the six-coordinate intermediate IM3′. Subsequent CO insertion through TS3′ affords the acyl intermediate (IM4′), which, upon hydrolysis, generates 3-HP. This product can undergo further analogous transformations to yield SA. The rate-determining step in this pathway is oxidative addition (IM1→TS2′), associated with a substantially higher free-energy barrier of 40.8 kcal/mol. The markedly lower barrier of the β-elimination pathway (29.9 vs. 40.8 kcal/mol) explains the selective formation of PA while suppressing C3 hydroxy acids and succinic acid. For polyesters lacking a β-diol motif, hydroxy acids or diacids dominate, consistent with the absence of an accessible β-elimination pathway (Fig. 3).

Integrating kinetic, experimental, and computational evidence, we propose the following overall mechanism for the carbonylolysis process. PET is initially hydrolyzed to TPA and EG. In the presence of iodide, EG is converted to 1,2-diiodoethane, which undergoes β-elimination to ethylene rather than direct carbonylation. This ethylene is subsequently carbonylated to form PA. This cascade reconstructs the carbon backbone via carbonyl insertion, achieving high product selectivity and excellent atom economy (Fig. 4).

Economic and environmental assessment of carbonylolysis
Process design is crucial for evaluating reaction feasibility, separation strategies, and system scalability52,53. Given that the CH3I–CO system generates acetic acid, which increases separation energy demands and leads to carbon loss44, we evaluated HI as a more sustainable alternative. Remarkably, the addition of 0.4 mmol of HI afforded >99% yield of TPA and 93% yield of PA, confirming its high catalytic efficacy (Supplementary Table S8). In homogeneous carbonylation reactions, the stability of Rh catalysts is strongly dependent on CO partial pressure. At low CO pressure, the active Rh species tend to form RhI3 precipitates, resulting in catalyst deactivation and metal loss from the system54. Based on this mechanistic understanding, we designed a separation protocol (Fig. 5a) in which solid TPA is first recovered by pressure filtration under CO, followed by atmospheric filtration of the remaining solution to recover the Rh catalyst with removal of CO. PA is subsequently isolated by vacuum distillation of the mother liquor. CO is stoichiometrically consumed during carbonylation, and excess CO can be readily recovered and recycled within the process. We conducted laboratory-scale cycling experiments to simulate and validate the PET carbonylolysis process (Supplementary Fig. S11), which served as the basis for further developing a preliminary process scheme (Supplementary Fig. S15). To assess the broader viability of this strategy, we performed a comprehensive system-level evaluation, including mass and energy balances (Supplementary Fig. S15 and Supplementary Tables S9–S12), life cycle assessment (LCA), and techno-economic assessment (TEA).

Full LCA results are presented in Supplementary Tables S13–S15, with the system boundary illustrated in Fig. 5b. Specifically, CO was treated as a purchased industrial feedstock, and its upstream environmental impacts were fully incorporated in the LCA model55,56. Compared to conventional routes, the carbonylolysis process not only generates high-value products and circumvents the need for EG separation, but also substantially reduces non-renewable energy use (NREU) and greenhouse gas emissions (GWP). As shown in Fig. 5c, converting PET waste into TPA and PA dramatically lowers environmental impact relative to incineration or landfill (baseline: 10.51 kg CO2-eq/kg PET)57. In particular, the process achieved NREU values of 22.9 MJ/kg PET in China and 20.9 MJ/kg PET in Europe, with corresponding GWP values of 1.42 and 1.17 kg CO2-eq/kg PET, respectively (Supplementary Tables S14 and S15). By comparison, conventional hydrolysis exhibits NREU and GWP values of 51 MJ/kg PET and 4.47 kg CO2-eq/kg PET, respectively. Thus, the carbonylolysis route achieves reductions of 28.1 MJ/kg in NREU and 3.05 kg CO2-eq/kg in GWP. These environmental advantages are primarily attributed to the exothermic nature of the carbonylation reaction, which thermodynamically drives PET depolymerization while minimizing external energy input. In addition, the integrated valorization of EG into PA eliminates the need for EG purification, thereby simplifying downstream processing and reducing overall process complexity.

Economic analysis from Aspen Process Economic Analyzer in Aspen Plus V11 (Fig. 5d and Supplementary Table S16) illustrated the distribution of costs including plant cost (installed cost, equipment cost, and capital cost), working costs (utilities cost and operating cost), raw materials costs (market for waste PET chips and CO), and product sales (market for TPA and PA). The total plant cost was USD 15.03 million, and the annual working cost was USD 3.1 million/year at an annual treatment of 100,000 tons waste PET chips. While raw materials represented the major cost component (USD 55.41 million/year), substantial product sales (USD 109.46 million/year) offset these expenses. The process demonstrates favorable economic viability with an annual profit of USD 35.92 million27 and represents a promising return on investment for industrial implementation.

Together, the LCA and TEA highlight the carbonylolysis strategy as a promising and scalable platform for industrial PET recycling. This approach achieves high selectivity and atom economy, while minimizing environmental impacts and operational costs. Importantly, the strategy aligns with global sustainability objectives by integrating carbon efficiency, resource recovery, and energy minimization. These attributes make carbonylolysis particularly well-suited for regional industrial symbiosis, waste-to-chemicals initiatives, and petrochemical decarbonization efforts. These features position it as a compelling solution for large-scale, circular plastic valorization.

Methods
Catalytic tests
In a typical experiment, a mixture consisting of 0.192 g (1 mmol structural unit) PET powder, 500 μL RhCl3 aqueous solution (0.05 mmol of Rh3+), 0.071 g (0.5 mmol) CH3I and 2 mL of HFIP were introduced into the reactor. The autoclave and container were then sealed and purged three times with CO to remove air, and then charged with 2.0 MPa CO. The reactor was then heated to 170 °C, controlled by a temperature control device, and stirred at a speed of 700 rpm. After reaching the desired reaction time, the reactor was immediately quenched in cold water. Gas products were collected by an airbag and analyzed using an Agilent 6890N gas chromatography equipped with an Agilent GS-Q GC Column. The liquid phase was diluted with 8 mL reaction solvent. Quantitative analysis of PA and EG in the liquid phase is performed using Shimadzu GC-2014 gas chromatography equipped with Shimadzu SH-WAX Polyethylene glycol GC Column, and TPA is performed using Bruker Avance neo (1H at 500 MHz) Nuclear magnetic resonance (NMR). Chloroethane was selected as the gas internal standard. A mixture of 100 μL chloroethane and 400 μL reaction gas was identified by GC to quantify the yield of ethylene. To the reaction suspension, 1.0 mmol mesitylene was added as an internal standard. The suspensions were centrifuged at 2555 × g to separate the solid and liquid phases, and the supernatant was taken for product identification by GC to quantify the conversion rate of PA. The remaining liquid was vaporized to remove the solvent HFIP, 1 mmol of fumaric acid was added as an internal standard, and the suspension was centrifuged at 2555 × g to separate the solid and liquid phases. 50 μL of the supernatant was mixed with 550 μL D2O for NMR analysis to quantify the conversion rate of TPA.

Computational methods
All calculations were carried out using the Gaussian 16 package58. Both geometry optimizations and single-point energy calculations were optimized using the hybrid M06-2X functionals in combination with the def2-TZVP basis set for C, H, O, and the SDD basis set for I, Rh. Frequency analyses were conducted at the same level of theory to obtain thermal correction and confirm the stationary points to be minimal or transition states. The solvation effect of HFIP46 and H2O was introduced using the SMD model59. All the species involved were treated in standard conditions.

Lifecycle assessment and techno-economic analysis
LCA was conducted using Open LCA following the ISO 14040 series standard. The non-renewable energy use (NREU) and global warming potential (GWP) of our carbonylolysis versus currently reported PET recycling routes60–62 were assessed. The functional unit is 1 kg of amorphous PET resin in chips form. The system boundary is “cradle-to-gate”, which includes: the collection and transportation of waste PET, the production of PET flakes, and the carbonylolysis process to obtain TPA and PA (Fig. 5a, b). The background processes were modeled using generic datasets from the Ecoinvent v3.9.1 database, primarily based on globally averaged values. The CML-IA method was employed for NREU (expressed in megajoules, MJ), while the IPCC 2013 methodology was used for GWP (expressed in kilograms of CO2-equivalents, kg CO2-eq), both evaluated over a 100-year time horizon.

The experimental procedure under scrutiny was diligently formulated and modeled via the chemical process simulation software Aspen Plus V11, with a comprehensive schematic illustrated in the Supplementary Information. A cost-revenue analysis of 100,000-ton waste PET per year was performed using the established techno-economic assessment model. The summary for the estimated parameters and total investment cost included raw materials, plant working, plant construction, and product sales. The cost of raw materials and product sales was calculated from the elemental balance and unit price. The cost of plant working (including operating costs and utilities) was directly derived from Aspen Plus V11. Detailed data and assumptions can be found in the Supplementary Information. In this process simulation, the ASPEN simulation used the NRTL physical property method. The process commenced by introducing PET waste (WAS-PET, 12500 kg/h) mixed with water, HFIP, HI and RhCl3 catalyst, and subsequently introduced into the reactor with CO. The depolymerization reaction was conducted at 170 °C under precise control.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Liu et al. present a very interesting study on the valorization of PET using a rhodium-based carbonylation catalyst. Compared to existing approaches for PET valorization, they do not rely on hydrolysis of the polymer backbone but instead use CO to transform the C2 building block ethylene glycol into the C3 molecule propionic acid. The study is well designed and executed, encompassing not only thorough chemical analysis but also an environmental assessment and a preliminary outline for technological implementation. Overall, I am supportive of the publication of this article in Nature Communications after the authors conduct several additional experiments and provide clarification on specific aspects of the manuscript.\n\nMajor questions\n- Role of CH3I loading. Why is such a large excess of CH3I required relative to the catalyst? In the classical Cativa process, iodine functions as a co-catalyst. Here, a distinct change in reactivity is observed between 0.1 and 0.3 equivalents of CH3I, yet no explanation is provided for this nearly quantitative shift from EG to PA.\n- Stability of HFIP under reaction conditions. How stable is HFIP in the presence of the Rh catalyst and CO? Are any fluorine species formed? A control experiment with Rh3+, HFIP, and CO in the absence of PET would be useful. 19F NMR may reveal small signals compared to the dominant HFIP resonance; in this case, an adapted solvent suppression sequence should be employed.\n- Counter ions in metal salts. The Supporting Information lists the metal salts as chlorides. This information should be included in the main text. In this context, I am curious about the role of chloride: how does the reaction perform if RhI3 is used directly, without an additional iodine additive?\n- Figure 4D (b & c). The time axis presentation is confusing. Either equal step/tick distances or a logarithmic scale should be used. In panel (c), arrows are needed to link curves to the correct y-axes. Furthermore, the secondary axis showing “120% yield” is misleading — yields should be limited to 100%.\n- Preparation of PET powder. How was the PET powder obtained? Was it ball-milled? Pretreatment methods such as ball milling can significantly affect polymer structure; see, for example, Vollmer et al. (10.1039/D4MR00098F; 10.26434/chemrxiv-2025-nkf6j). The method should be clearly described.\n- Supporting Information. The SI should include at least one set of representative GC chromatograms from a catalytic test, to allow readers to assess the raw data quality and the data-processing workflow. It would be even better if the entire dataset is made available together with the analysis script.\n- Reaction equipment. The autoclave manufacturer, type, and total volume must be specified.\n\nMinor comments and clarifications\n- Mechanistic statement. The manuscript states: \"The presence of iodide was essential, as CH3I reacts with Rh to form the catalytically active intermediate [Rh(CO)2I2]⁻.\" However, Reference 36 shows that CH3I reacts with [Rh(CO)2I2]⁻ to form [Rh(CO)2I3(CH3)]⁻. The statement should be revised to reflect the actual species involved.\n- Environmental analysis. The sentence \"Compared with conventional hydrolysis, the carbonylolysis route reduced 28.1 MJ/kg in NREU and 3.05 kg CO₂-eq/kg in GWP, respectively\" is unclear. Are the numbers linked to the reference hydrolysis process. Please rephrase for clarity.\n- Figure S5. The broad signals need to be assigned. Presumably, they correspond to water and the carboxylic acid proton?\n- Scheme 1B. What does \"R\" represent?\n- Figure 4E. Pathways A and B need to be explained explicitly.\n- Competing interests. The authors declare no competing interests. Is this accurate? Was no patent application filed for this process?\n\nTypos\n- p. 4, l. 87: “mental catalyst” → “metal catalyst.”", "reviewer_2": "This manuscript reports a one-pot “carbonylolysis” strategy that couples polyester depolymerization with in-situ carbon-chain reconstruction, producing high-value C3+ carboxylic acids under relatively mild conditions. The experimental methodology is systematic. I believe it meets the publication standards of Nature Communication and can be published after the authors addressed the following comments.\n\nMy comments are as follows:\n1.Section 2.1: The authors selected HFIP as the solvent due to its favorable solubility for PET. However, the manuscript does not address the potential use of alternative bio-based solvents, such as dimethyl isosorbide (DMI), which are more environmentally friendly and also exhibit good solubility for polyester materials. It is recommended that the authors clarify whether such green solvents were considered, or explain why they may not be suitable for the present reaction system.\n2.Figure 2: The condition optimization experiments are presented without error bars. It is unclear whether the reported data represent results from a single experiment or the average of multiple replicates. If the latter, the corresponding error bars should be provided.\n3.Section 2.4: The authors propose a catalyst separation and recovery strategy, but no experimental data are provided to validate its effectiveness. It is necessary to clarify whether catalyst recycling experiments were conducted. In particular, evidence is required to demonstrate that Rh can be completely recovered and that the recovered Rh retains comparable activity and selectivity in subsequent reaction cycles.\n4.Section 2.4: CO is employed as a reactant in the carbonylolysis process. However, CO is also a pollutant. It is recommended that the authors clarify why the energy consumption and emissions associated with CO production were not included in the LCA. As this omission may significantly affect the reliability of the LCA results.\n5.Figure 5: The authors mention circulating reaction solution, but no experimental evidence is provided to validate the feasibility of multiple reuse cycles. It is recommended that the authors provide data on solvent recovery efficiency and the purity after repeated use to demonstrate the practicality of solvent recycling.", "reviewer_3": "The authors report a one-pot “carbonylolysis” strategy to couple polyester depolymerization with in-situ carbon-chain reconstruction, producing high-value C3 carboxylic acids under relatively mild conditions (170°C, 2 MPa 15 CO)，which is indeed an innovative method. However, there are some major problems that hasn’t been addressed. First, Rh is a very expensive metal, so that when using it as a catalyst, it needs to have high TON or high stability to be recycled. Here, the author proposed to separate the Rh from the reaction system by converting it to inert RhI3. It seems unpractical for actual application. Besides, more details need to be provided, such as the recovery percentage and the reactivation pathway of the catalyst. Second, iodide reagent is also kind of expensive, the author needs to address how to separate and recycle the iodide additive. Third, the author used hexafluoroisopropanol [HFIP] as solvent for its good properties. However, it is an expensive solvent and have serious environmental impact, so that its recovery needs to be addressed. Fourth, the author claims that the carbonylation mechanism is through the ethylene insertion into [Rh(CO)2I2]– active catalytic species. Actually, the well-recognized active catalytic species is supposed to be [HRh(CO)I3]−. You can follow a recently published paper J. Am. Chem. Soc. 2025, 147, 38, 34589–34600."}, "author_response": "We appreciate the reviewer for the positive comments and suggestions for improving this manuscript.\n\nMajor questions\nComments 1: Role of CH I loading. Why is such a large excess of CH I required 3 3 relative to the catalyst? In the classical Cativa process, iodine functions as a co-catalyst. Here, a distinct change in reactivity is observed between 0.1 and 0.3 equivalents of CH I, yet no explanation is provided for this nearly quantitative shift from EG to PA. 3\nResponse: We thank the reviewers for their insightful comments regarding the role and loading of CH I in the catalytic system. In Rh-catalyzed carbonylation chemistry, 3 an excess iodide source is widely recognized as essential for sustaining catalytic activity through control of Rh speciation. In the present system, CH I functions as both 3 the iodide source and a co-catalyst, governing the formation and stability of the catalytically competent Rh–iodide carbonyl species.\nAs established in classical Monsanto/Cativa-type carbonylation chemistry, oxidative addition of CH I to Rh constitutes a key elementary step, and the steady- 3 state population of active Rh–iodide species is highly sensitive to iodide activity. Consequently, a sufficient excess of CH I is required to maintain an iodide-rich 3 environment that stabilizes the active Rh intermediates necessary for efficient carbonylation (Sci. Adv., 2018, 4, eaaq0266; Green Chem., 2019, 21, 4434–4442; Angew. Chem. Int. Ed., 2017, 56, 14868–14872; ACS Catal., 2021, 11, 7249–7256; Green Chem., 2024, 26, 7302–7311).\nThe abrupt change in reactivity observed between 0.1 and 0.3 equivalents of CH I 3 reflects a speciation-controlled threshold, rather than a linear concentration effect. At low CH I loadings (e.g., 0.1 equiv), iodide activity is insufficient to sustain the 3 catalytically active Rh–iodide species, leading to incomplete carbonylation and predominant formation of EG. Once the iodide concentration exceeds a critical level, the catalytically competent Rh-iodide cycle becomes fully established, resulting in near-quantitative conversion to PA.\nTo further support this interpretation, we examined an intermediate CH I loading 3 (0.2 equiv), which afforded a TPA yield of 99% and a PA yield of 83%, with no detectable EG formation. These results are comparable to those obtained at 0.3 equivalents of CH I, indicating that once the critical iodide threshold is exceeded, 3 further increases in CH I loading do not lead to proportional enhancements in activity. 3 This behavior is consistent with a speciation-controlled catalytic regime rather than a stoichiometric dependence on CH I. 3\nRevisions made: The manuscript has been revised to clarify the role of CH I loading: 3 “The presence of iodide was essential for carbonylation activity, as a sufficiently high iodide concentration is required to stabilize the catalytically active Rh–iodide carbonyl species. At low CH I loadings, iodide activity was insufficient to sustain the 3 active Rh speciation, resulting in incomplete carbonylation and predominant formation of EG. A sharp increase in activity was observed upon increasing CH I 3 loading from sub-threshold to iodide-rich conditions, followed by a plateau upon moderate further increases, indicating that a minimum iodide level is required to establish the active Rh catalytic cycle rather than a linear dependence on CH I loading. 3 At higher CH I loadings, excessive iodide led to reduced PA yields, likely due to over- 3 stabilization of Rh–iodide complexes and diminished catalytic turnover (Fig. 2A).”\nComments 2: Stability of HFIP under reaction conditions. How stable is HFIP in the presence of the Rh catalyst and CO? Are any fluorine species formed? A control experiment with Rh3+, HFIP, and CO in the absence of PET would be useful. 19F NMR may reveal small signals compared to the dominant HFIP resonance; in this case, an adapted solvent suppression sequence should be employed.\nResponse: We thank the reviewer for the helpful comments regarding the stability of HFIP, which is indeed critical for assessing the robustness of the reaction system. Following these suggestions, we conducted a series of control experiments to evaluate HFIP stability. First, reactions were carried out in the absence of PET but under otherwise identical conditions (Rh, HFIP, CH I, and CO at the same temperature 3 and pressure). Second, both PET and iodide additives were omitted, leaving only Rh, HFIP, and CO under identical conditions. In both cases, 19F NMR spectra analysis showed no detectable changes in the dominant HFIP resonance and no additional fluorine-containing signals, indicating that HFIP remains chemically stable in the presence of Rh and CO. These spectra were acquired with appropriate signal suppression to ensure that minor fluorinated species, if present, would be observable. Furthermore, solvent recycling experiments demonstrated that HFIP can be reused multiple times without loss of catalytic performance, further confirming its stability under the reaction conditions.\nRevisions made: In the revised Supplementary Information, the corresponding 19F NMR data (Figure S7) and recycling experiments have been added to explicitly demonstrate the chemical integrity and reusability of HFIP under Rh/CO conditions (see also our response to Reviewer 2, Comment 5).\nFigure R1. 19F NMR spectra (D O) of the reaction solutions: (A) without PET; (B) without 2 PET and CH I; (C) HFIP standard. Corresponding data have been added as Figure S7 in the 3 revised Supplementary Information.\nComments 3: Counter ions in metal salts. The Supplementary Information lists the metal salts as chlorides. This information should be included in the main text. In this context, I am curious about the role of chloride: how does the reaction perform if RhI 3 is used directly, without an additional iodine additive?\nResponse: We thank the reviewer for the insightful comments. Following your suggestion, we have now explicitly stated in the main text that all metal salt catalysts used in this study were chloride salts.\nUnder the iodide-rich carbonylation conditions employed here, chloride is not mechanistically essential and is rapidly displaced to form the catalytically active Rh/iodide species, consistent with established Monsanto/Cativa-type systems (Organometallics, 1994, 13, 3215–3226; Coord. Chem. Rev., 2003, 243, 125–142; J. Am. Chem. Soc., 2022, 144, 2034–2050). To examine whether chloride influences catalytic performance, we conducted a series of control experiments (Table R1). When RhI was used in place of RhCl in combination with the HI promoter, full catalytic 3 3 activity comparable to the standard system was observed, attributable to rapid in situ anion exchange generating the active Rh–iodide carbonyl species. However, when RhI was used as the sole rhodium precursor without any additional iodide additive, 3 only low activity (13%) was observed. In contrast, when RhCl was used as the sole 3 rhodium precursor, only trace formation of PA was detected. These results confirm that chloride is not required for catalytic activity and does not provide any promotional effect. Instead, they indicate that the iodide supplied stoichiometrically by RhI is 3 insufficient to establish the high free-iodide activity required to promote oxidative addition, stabilize key acyl-forming intermediates, and sustain the Cativa/Monsanto- type catalytic cycle.\nCollectively, these findings demonstrate that catalytic efficiency is governed by the concentration of free iodide rather than the identity of the Cl⁻ counter-anion, and that maintaining a sufficiently high iodide activity is essential for efficient catalytic turnover.\nRevisions made: In the revised manuscript, the catalysts (Rh3+, Ru3+, Ir3+, Pd2+, Co2+, Ni2+) have been updated to their corresponding chloride salts (RhCl , RuCl , 3 3 IrCl , PdCl , CoCl , NiCl ), and the statement “the effect of the chloride counter-anion 3 2 2 2 was negligible (Table S1)” has been added. In the revised Supplementary Information, Table S1 has been included.\nTable R1. Catalytic performance of different anionic rhodium salts. (Reproduced as Table S1 in the revised Supplementary Information)\nNo. Rh HI TPA Yield (%) PA Yield (%) EG Yield (%)\nStandard reaction conditions: PET (0.192g, 1 mmol structural unit), HFIP (2 mL), H O (0.5 mL), HI 2 (0.4 mmol), CO (2 MPa) and catalyst (RhCl or RhI , 5 mol%) at 170°C for 12 h. 3 3\nComments 4: Figure 4D (b & c). The time axis presentation is confusing. Either equal step/tick distances or a logarithmic scale should be used. In panel (c), arrows are needed to link curves to the correct y-axes. Furthermore, the secondary axis showing “120% yield” is misleading — yields should be limited to 100%.\nResponse: We thank the reviewer for the constructive suggestions regarding the presentation of Figure 4D (b & c). We agree that the previous time-axis formatting could be confusing, and the panels have now been revised to use uniform tick spacing, ensuring consistent visual interpretation of the kinetic profiles. In panel (c), arrows have been added to clearly associate each curve with its corresponding y-axis, and the dual-axis labeling has been redrawn for improved clarity. In addition, the secondary axis previously extending beyond 100% yield has been corrected, as yields should not exceed 100%. The revised figure now limits the yield scale to 0–100% and presents the kinetic and selectivity data in a clearer and more accurate manner.\nRevisions made: In the revised manuscript, Figure 4D (b & c) has been updated.\nFigure R2. Revised Figure 4D (b & c) of the manuscript. Kinetic studies: (a) time profiles of different reaction systems: PET (0.192 g, 1 mmol structural unit), HFIP (2 mL), H O (0.5 mL), 2 RhCl (5 mol%), PA (50 μL). (b) kinetic profile of PET carbonylolysis; (c) time-dependent ethylene 3 and PA evolution during EG carbonylation with and without Rh3+.\nComments 5: Preparation of PET powder. How was the PET powder obtained? Was it ball-milled? Pretreatment methods such as ball milling can significantly affect polymer structure; see, for example, Vollmer et al. (10.1039/D4MR00098F; 10.26434/chemrxiv-2025-nkf6j). The method should be clearly described.\nResponse: We thank the reviewer for highlighting this important point.\nThe PET powder used in this study (Mn ≈ 150,000) was commercially supplied by Tesulang Chemical Materials Co., Ltd. According to the supplier specifications, the material is produced via conventional industrial size-reduction processes (low-shear– pulverization) designed for polymer handling, rather than mechanochemical milling. It was used as received and not subjected to any high-energy mechanochemical pretreatment (e.g., ball milling), and is therefore not expected to induce significant chain scission or structural modification.\nWe fully agree that high-energy ball milling and related mechanochemical pretreatments can substantially alter polymer structure, as demonstrated in the studies cited by the reviewer. However, PET powder preparation is not a prerequisite for this transformation. As shown in Figure 3A, real-world waste PET samples in various practical physical forms were manually cut into small pieces without further micronization or powdering. This simple treatment minimizes potential structural modification induced by mechanical pretreatment. The transformation performance observed across these PET forms is comparable, demonstrating that the process is tolerant to realistic PET morphologies and enabling direct application to real plastic waste streams.\nRevisions made: In the revised manuscript, the sentence “We systematically optimized key parameters to enhance PA yield” has been revised to “Using commercially sourced, low-shear–pulverized PET powder as the feedstock (without additional ball-milling pretreatment that could alter the polymer structure), we systematically optimized key parameters to maximize PA yield.” We also added: “Notably, all PET samples were manually cut into small pieces without further powdering or micronization, thereby minimizing potential structural modification associated with intensive mechanical pretreatments such as ball milling. These results demonstrate that PET powder preparation is not a prerequisite for the transformation, enabling direct application to real waste streams without energy-intensive size reduction.” (p. 4, line 12) In the revised Supplementary Information: additional information on the PET source and handling has been added.\nComments 6: Supplementary Information. The SI should include at least one set of representative GC chromatograms from a catalytic test, to allow readers to assess the raw data quality and the data-processing workflow. It would be even better if the entire dataset is made available together with the analysis script.\nResponse: We thank the reviewer for the constructive suggestion regarding data transparency and evaluation of the GC analysis. In response, representative GC chromatograms from typical catalytic experiments have been added to the Supplementary Information, displaying the raw detector signals together with the corresponding peak assignments used for quantification. In addition, the peak integration and yield calculation has been provided, enabling readers to assess both the chromatographic data quality and the analysis workflow. Additional raw GC files are available from the authors upon reasonable request.\nRevisions made: Representative GC chromatograms and the associated data- processing procedures have been added to the revised Supplementary Information (Section 2.6, Quantification of Product Yields, Fig. S6-I–III).\nFigure R3. (A) representative GC chromatograms from a typical catalytic run; (B) Calibration curve of PA using mesitylene as the internal standard; (C) Calibration curve of EG using mesitylene as the internal standard.\nComments 7: Reaction equipment. The autoclave manufacturer, type, and total volume must be specified.\nResponse: We thank the reviewer for the suggestion to enhance the experiment details. In response, the Experimental section has been revised to explicitly describe the reaction apparatus. All carbonylolysis reactions were conducted in a stainless-steel stirred autoclave manufactured by Ruisheng Laboratory Equipment Factory (Lüshunkou District, Dalian). The reactor is a high-pressure-resistant steel vessel equipped with a PTFE liner. The total internal volume of the autoclave was determined to be 24 mL, and the usable volume of the PTFE liner was 19.4 mL. The reactor was purged and pressurized with CO following the procedure described in the manuscript.\nRevisions made: In the revised Supplementary Information, the manufacturer, reactor type, and total and usable volumes of the autoclave have been added to Section 1.2, Reactor Equipment, with corresponding photos shown in Fig. S1.\nFigure R4. Details of the reaction equipment. Photograph of the high-pressure reactor (A); Cutaway view of the high-pressure reactor (B); Cross-sectional view of the reactor liner (C); Side view of the reactor liner (D).\nMinor comments and clarifications\nComments 8: Mechanistic statement. The manuscript states: \" The presence of iodide was essential, as CH I reacts with Rh to form the catalytically active intermediate 3 [Rh(CO) I ]⁻.\" However, Reference 36 shows that CH I reacts with [Rh(CO) I ]⁻ to 2 2 3 2 2 form [Rh(CO) I (CH )]⁻. The statement should be revised to reflect the actual species 2 3 3 involved.\nResponse: We thank the reviewer for pointing out the inaccurate mechanistic wording regarding the role of CH I. We agree that CH I does not directly react with Rh to form 3 3 the anionic Rh(I) species [Rh(CO) 2 I 2 ]–. Instead, as established in Rh-catalyzed carbonylation chemistry, CH I primarily serves as an iodide promoter, while oxidative 3 addition of CH 3 I to [Rh(CO) 2 I 2 ]– can generate a transient methyl–Rh intermediate [(CH 3 )Rh(CO) 2 I 3 ]–, as described in Reference 36. This intermediate is short-lived and does not represent the catalytically dominant species.\nAccordingly, the manuscript has been revised to remove the incorrect causal statement and to clarify that the essential role of CH I is to maintain a sufficiently high 3 iodide activity required to stabilize the anionic Rh(I) carbonyl species responsible for carbonylation catalysis. The corrected text no longer implies that CH I directly 3 generates [Rh(CO) 2 I 2 ]–. As multiple Rh-iodide species may be involved (discussed in the Mechanism section), individual Rh-iodide intermediates are not explicitly specified in this paragraph.\nRevisions made: In the revised manuscript, The mechanistic wording in the optimization discussion has been corrected to accurately describe the role of iodide and CH I. (p. 2, line 18) 3\nComments 9: Environmental analysis. The sentence \"Compared with conventional hydrolysis, the carbonylolysis route reduced 28.1 MJ/kg in NREU and 3.05 kg CO₂- eq/kg in GWP, respectively\" is unclear. Are the numbers linked to the reference hydrolysis process. Please rephrase for clarity.\nResponse: We thank the reviewer for pointing out the ambiguity. The sentence has been revised to explicitly state that the reported reductions in NREU and GWP are referenced to the conventional hydrolysis process. The revised text now clearly reports both the absolute values for the hydrolysis benchmark and the corresponding values for the carbonylolysis route, eliminating any ambiguity in the comparison. (p. 7, line 14)\nComments 10: Figure S5. The broad signals need to be assigned. Presumably, they correspond to water and the carboxylic acid proton?\nResponse: We thank the reviewer for this helpful observation regarding the broad signals in Figure S5. Accordingly, the broad resonance at δ ≈ 3.5 ppm has been assigned to residual water in HFIP, which typically appears as a broadened peak due to strong hydrogen bonding. The broader signal at δ ≈ 8.0 ppm is attributed to proton exchange involving trace water and HFIP rather than carboxylic acid proton.\nRevisions made: In the revised Supplementary Information, the original Figure S5 has been renumbered as Figure S8, and the broad resonances have been assigned and annotated.\nComments 11: Scheme 1B. What does \"R\" represent?\nResponse: We thank the reviewer for pointing out the ambiguity.\nAccordingly, Scheme 1B has been revised to include clear definitions of both R and m. In the updated scheme, R denotes the divalent organic residue of the diol monomer, which is the portion of the monomer between the two hydroxyl groups, and m represents the number of carbon atoms in this residue. These clarifications have been incorporated directly into the scheme.\nRevisions made: In the revised manuscript, Scheme 1B has been updated to explicitly define the symbols R and m for improved clarity.\nComments 12: Figure 4E. Pathways A and B need to be explained explicitly.\nResponse: We thank the reviewer for the insightful comment. In the revised manuscript, we have clarified Pathways A and B. As illustrated in Fig. 4E, both pathways originate from the common intermediate IM1 and diverge via β-elimination (Pathway A) or direct oxidative addition (Pathway B). In Pathway A, IM1 undergoes β-elimination to generate ethylene, which is subsequently carbonylated to form PA. In Pathway B, IM1 directly coordinates to the catalyst and undergoes oxidative addition followed by carbonylation, potentially producing 3- iodopropionic acid and succinic acid. DFT calculations indicate that β-elimination has a substantially lower free-energy barrier (29.9 vs. 40.8 kcal/mol), consistent with the selective formation of PA observed experimentally. These clarifications are now explicitly included in the main text to remove any ambiguity regarding Fig. 4E.\nRevisions made: In the revised manuscript, the mechanistic descriptions of Pathways A and B in Fig. 4E have been expanded and clarified in the main text to clearly convey their distinct sequences and outcomes (p. 6, line 9).\nComments 13: Competing interests. The authors declare no competing interests. Is this accurate? Was no patent application filed for this process?\nResponse: We thank the reviewer for raising this important point regarding the declaration of competing interests. We confirm that, at the time of the original manuscript submission, no patent application had been filed and the statement “The authors declare no competing interests” was accurate.\nFollowing your comment and a further evaluation of the potential applicability of this work, we have now decided to pursue a patent application related to the reported process. To ensure full transparency, the Competing Interests statement has been updated accordingly.\nRevisions made: In the revised manuscript, the Competing Interests section has been revised to reflect the intention to file a patent application.\nTypos\nComments 14: p. 4, l. 87: “mental catalyst” → “metal catalyst.”\nResponse: We thank the reviewer for pointing out this typographical error. “Mental catalyst” has been corrected to “metal catalyst” in the revised manuscript.\n\nReviewer #2 (Remarks to the Author):\nThis manuscript reports a one-pot “carbonylolysis” strategy that couples polyester depolymerization with in-situ carbon-chain reconstruction, producing high-value C3+ carboxylic acids under relatively mild conditions. The experimental methodology is systematic. I believe it meets the publication standards of Nature Communication and can be published after the authors addressed the following comments.\nResponse: We appreciate the reviewer for the positive comments and suggestions for improving this manuscript.\n\nMy comments are as follows:\nComments 1: Section 2.1: The authors selected HFIP as the solvent due to its favorable solubility for PET. However, the manuscript does not address the potential use of alternative bio-based solvents, such as dimethyl isosorbide (DMI), which are more environmentally friendly and also exhibit good solubility for polyester materials. It is recommended that the authors clarify whether such green solvents were considered, or explain why they may not be suitable for the present reaction system.\nResponse: We thank the reviewer for the valuable suggestion regarding the potential use of bio-based solvents in place of HFIP. We fully agree that the development of greener solvent systems is important and merits careful consideration.\nAccordingly, we evaluated representative bio-based solvents, including dimethyl isosorbide (DMI) and γ-valerolactone (GVL), for PET carbonylolysis under otherwise identical conditions. As shown in Table R2, PET depolymerization was incomplete using DMI as the solvent, affording only 30.7% TPA, while GVL enabled efficient PET depolymerization (95.4% TPA). However, neither solvent led to detectable formation of PA, indicating that the carbonylolysis pathway was not operative in these media. These results suggest that, despite adequate PET solubility benefit for depolymerization, DMI and GVL do not support the key carbonylolysis steps required for PA formation.\nThis incompatibility can be attributed to the unique solution properties of HFIP. HFIP provides a strongly hydrogen-bond-donating, highly ionizing, and low- nucleophilicity environment that promotes HI dissociation, stabilizes iodide and protonated intermediates, and supports the formation and persistence of anionic Rh/I catalytic species. These combined effects generate a superacid-like medium that is critical for efficient C–O activation and subsequent carbonyl insertion steps (Synth., 2007, 2007, 2925–2943; J. Am. Chem. Soc., 2006, 128, 8421–8426). In contrast, DMI and GVL lack sufficient hydrogen-bond donation and ion-stabilization capacity to sustain the iodide activity and acidic conditions necessary for the iodine-mediated carbonylolysis sequence (Synth., 2007, 2007, 2925–2943). Therefore, despite their greener origin and ability to dissolve polyesters to enhance depolymerization, these solvents are intrinsically incompatible with the mechanistic requirements of the present integrated carbonylolysis process.\nWe fully agree that identifying greener solvent systems capable of meeting these mechanistic requirements is an important direction and is currently under exploration in our ongoing work.\nRevisions made: In the revised manuscript, bio-based solvents (DMI and GVL) have been included in the solvent evaluation. (p. 2, line 22) In the revised Supplementary Information, the corresponding reaction results for these bio-based solvents are provided in Figure S4.\nTable R2. Reaction performance of bio-based solvents. (These data have been added to Fig. S4)\nNo. Sol. TPA Yield (%) EG Yield (%) PA Yield (%)\n2 γ-Valeroactone 95.4 18.0 -\nStandard reaction conditions: PET (0.192g, 1 mmol structural unit), solvent (2 mL), H O (0.5 mL), 2 CH I (0.5 mmol), CO (2 MPa) and catalyst (RhCl , 5 mol%) at 170°C for 12 h. 3 3\nComments 2: Figure 2: The condition optimization experiments are presented without error bars. It is unclear whether the reported data represent results from a single experiment or the average of multiple replicates. If the latter, the corresponding error bars should be provided.\nResponse: We thank the reviewer for pointing out the need for greater clarity regarding the data presented in Figure 2. The optimization data were obtained as averages from at least three parallel experimental replicates and showed good reproducibility with small standard deviations. Error bars were not included in the original submission to maintain visual clarity and facilitate comparison of overall trends across a broad parameter space. We fully agree that inclusion of error bars improves scientific rigor. Following your suggestion, we have now added the corresponding error bars (standard deviations) to Figure 2, and the figure caption has been revised accordingly.\nRevision made: In the revised manuscript, Figure 2 and the caption has been updated.\nFigure R6. Updated Figure 2 of the manuscript. Construction of the catalytic system and optimization of conditions. Reaction conditions: PET (0.192 g, 1 mmol structural unit), HFIP (2 mL), H O (0.5 mL), CH I (0.5 mmol), CO (2 MPa) and catalyst (RhCl , 5 mol%) at 170°C 2 3 3 for 12 h. (A) The amount of CH I; (B) Water volume; (C) CO pressure; (D) Temperature and 3 (E) Reaction time.\nComments 3: Section 2.4: The authors propose a catalyst separation and recovery strategy, but no experimental data are provided to validate its effectiveness. It is necessary to clarify whether catalyst recycling experiments were conducted. In particular, evidence is required to demonstrate that Rh can be completely recovered and that the recovered Rh retains comparable activity and selectivity in subsequent reaction cycles.\nResponse: We thank the reviewer for the insightful comment regarding catalyst recovery and recyclability. We have conducted dedicated experiments to experimentally validate Rh catalyst recovery and reuse via an RhI -based route. 3\nUsing an RhI -based laboratory recovery protocol, a closed Rh mass balance was 3 achieved within experimental uncertainty. The experimental details: After completion of the reaction, the crude mixture was centrifuged to separate the solid fraction (Solid a) from the liquid phase (liquid a). Solid a was washed with aqueous HI to dissolve any trace Rh precipitate that might be present, and the wash solution was collected as liquid b. Liquid a was distilled to recover the HI/HFIP mixture, which was directly reused in the repeated catalytic cycle. The remaining liquid was evaporated to recover PA. The final residue, containing Rh complexes, was combined with Liquid b (denoted as Liquid c) and thermally aged to promote precipitation of RhI . The mixture was 3 subsequently centrifuged to isolate solid RhI . 3\nUsing this procedure, more than 95% of the total Rh was recovered, with the remaining loss attributed to unavoidable handling and analytical uncertainties inherent to small-scale experiments. Under standard reaction conditions, the recycled Rh catalyst (Rh-cyclic) exhibited activity and selectivity comparable to fresh RhCl and 3 simulated recycled RhI (prepared by thermal aging of RhCl in excess aqueous HI) 3 3 when used with fresh HFIP and HI, confirming efficient reactivation and preservation of catalytic integrity (Table R3).\nRevision made: Details of the Rh recovery procedure and recycling performance have been added to the revised Supplementary Information (Fig. S10 and Table S4).\nFigure R7. Rhodium -catalyzed cyclic processes within the laboratory setting.\nTable R3. Reaction performance of different anionic rhodium salts.\nNo. Rh I − TPA Yield (%) PA Yield (%) EG Yield (%)\nStandard reaction conditions: PET (0.192g, 1 mmol structural unit), HFIP (2 mL), H O (0.5 mL), 2 CH I (0.5 mmol) or HI (0.4 mmol), CO (2 MPa) and catalyst (RhCl , 5 mol%) at 170°C for 12 h. 3 3\nComments 4: Section 2.4: CO is employed as a reactant in the carbonylolysis process. However, CO is also a pollutant. It is recommended that the authors clarify why the energy consumption and emissions associated with CO production were not included in the LCA. As this omission may significantly affect the reliability of the LCA results.\nResponse: We thank the reviewer for raising this important point regarding the treatment of CO in the LCA. We agree that the handling of CO-related energy consumption and emissions must be clearly defined to ensure the reliability and transparency of the results.\nIn our analysis, CO is treated as a reactant (raw material) rather than an emission source. The environmental burdens associated with CO production were fully included through upstream inventory datasets incorporated into the LCA model (Figure 5D and Tables S12–S13).\nSpecifically, the CO consumed in this process was modeled using standard industrial carbon monoxide production datasets, which incorporates the energy input, resource consumption, and emissions associated with CO production (typically via syngas generation or partial oxidation routes). As a result, the environmental impacts of CO production are already embedded within the system boundary as part of the raw- material inputs, in accordance with ISO 14040/14044 guidelines (Chem. Eng. J., 2024, 489, 151326; Green Chem., 2017, 19, 2244–2259).\nWe acknowledge that CO is a regulated pollutant. However, in industrial practice, CO is highly reactive process gas and is routinely recovered and recycled in large- scale carbonylation and hydroformylation processes, such as Cativa/Monsanto-type systems, to minimize emissions and improve efficiency. A similar CO recycling strategy would be applicable to the present process, thereby preventing atmospheric release of unreacted CO.\nRevision made: In the revised manuscript, we have revised that CO is treated as a purchased industrial feedstock (p. 7, line 1) and that its upstream environmental impacts are included in the LCA model (Figure 5D). In the revised Supplementary Information, the specific values are provided in Tables S12–S13.\nComments 5: Figure 5: The authors mention circulating reaction solution, but no experimental evidence is provided to validate the feasibility of multiple reuse cycles. It is recommended that the authors provide data on solvent recovery efficiency and the purity after repeated use to demonstrate the practicality of solvent recycling.\nResponse: We thank the reviewer for this constructive suggestion regarding the feasibility of circulating the reaction solution. Following your suggestion, additional experiments were conducted to evaluate the recovery efficiency, purity, and reusability of HFIP under the reaction conditions.\nHFIP recycling procedure: After completion of the reaction, the crude mixture was centrifuged to separate the solid fraction from the liquid phase. The liquid phase was then subjected to low-temperature distillation to recover an HI/HFIP mixed solution, which was directly reused in subsequent catalytic cycles without further purification. Because HFIP and HI function cooperatively as the reaction medium, separation of HI from HFIP is neither required nor desirable for effective solvent circulation. Moreover, the co-recycling of solvent and iodide promoter is consistent with established industrial practice in Monsanto/Cativa-type carbonylation processes.\nHFIP was recovered after each run with near-quantitative efficiency (>99%) and reused for at least three consecutive reaction cycles. 1H, 13C and 19F NMR analyses of the recycled solvent showed no detectable impurity accumulation relative to fresh HFIP. Consistently, PET conversion and product selectivity remained essentially unchanged over multiple cycles with fresh RhCl , indicating that solvent purity and 3 catalytic performance were preserved upon reuse (Figures R8–R10, Table R4).\nThese results experimentally validate the feasibility of circulating the reaction solution and demonstrate that HFIP can be efficiently recovered and reused without loss of solvent purity or catalytic performance, validating the feasibility of circulating the reaction medium.\nRevision made: In the revised Supplementary Information, the 1H, 13C and 19F NMR spectra of the fresh HFIP standard and the recycled HFIP solution are provided in Fig. S11–S13, and the corresponding catalytic results are summarized in Tables S5.\nFigure R8. 1H-NMR (DMSO-d ) of standard sample: (A) Fresh HFIP; (B) HFIP 6 recycled three times.\nFigure R9. 13C-NMR (DMSO-d ) of standard sample: (A) HFIP; (B) HFIP recycled 6 three times.\nFigure R10. 19F-NMR (D O) of standard sample: (A) HFIP; (B) HFIP recycled three 2 times.\nTable R4. Recyclability Test of HFIP\nCycle Number Temp. Sol. TPA Yield (%) PA Yield (%)\n1 170°C HFIP 99 96.3\n2 170°C HFIP 99 96.7\n3 170°C HFIP 99 96.4\nStandard reaction conditions: PET (0.192g, 1 mmol structural unit), HFIP (2 mL), H O (0.5 mL), CH I 2 3 (0.5 mmol), CO (2 MPa) and catalyst (RhCl , 5 mol%) at 170°C for 12 h. 3\n\nReviewer #3 (Remarks to the Author):\nThe authors report a one-pot “carbonylolysis” strategy to couple polyester depolymerization with in-situ carbon-chain reconstruction, producing high-value C3 carboxylic acids under relatively mild conditions (170°C, 2 MPa CO), which is indeed an innovative method. However, there are some major problems that hasn’t been addressed.\nResponse: We appreciate the reviewer for the positive comments and suggestions for improving this manuscript.\n\nComments 1: First, Rh is a very expensive metal, so that when using it as a catalyst, it needs to have high TON or high stability to be recycled. Here, the author proposed to separate the Rh from the reaction system by converting it to inert RhI . It seems 3 unpractical for actual application. Besides, more details need to be provided, such as the recovery percentage and the reactivation pathway of the catalyst.\nResponse: We thank the reviewer for the insightful comments regarding Rh recovery and catalyst reuse. We fully agree that, given the high cost of Rh, efficient recovery and reusability of the catalyst are critical considerations.\nIt is important to note that conversion of Rh species to RhI is not required for 3 catalyst reuse under the studied reaction conditions. During catalysis, Rh remains fully dissolved in the homogeneous reaction mixture and retains full catalytic activity. Formation of RhI occurs only after removal of CO and volatile solvent components 3 and therefore does not represent the operative catalytic species. After reaction completion, solvent recovery, and product isolation, the majority of Rh remains in the non-volatile residue, which could, in principle, be directly reused. Although industrial- scale operation was not evaluated, this behavior is conceptually consistent with established principles of Rh retention in liquid-phase carbonylation processes (RU Fuanin et al., JPS581974B).\nLaboratory RhI -based recycling procedure: After completion of the reaction, the 3 crude mixture was centrifuged to separate the solid fraction (Solid a) from the liquid phase (liquid a). Solid a was washed with aqueous HI to dissolve any trace Rh precipitate that might be present, and the wash solution was collected as liquid b. Liquid a was distilled to recover the HI/HFIP mixture, which was directly reused in the repeated catalytic cycle. The remaining liquid was evaporated to recover PA. The final residue, containing Rh complexes, was combined with Liquid b (denoted as Liquid c) and thermally aged to promote precipitation of RhI . The mixture was 3 subsequently centrifuged to isolate solid RhI . Using this method, more than 95% of 3 the total Rh was recovered, with the remaining loss attributed to unavoidable handling and analytical uncertainties inherent to small-scale experiments. Under standard reaction conditions, the recycled Rh catalyst (Rh-cyclic) exhibited activity and selectivity comparable to fresh RhCl and simulated recycled RhI (prepared by 3 3 thermal aging of RhCl in excess aqueous HI) when used with fresh HFIP and HI, 3 confirming efficient reactivation and preservation of catalytic integrity (Table R3).\nReactivation pathway: Upon re-exposure to CO and HI under the reaction conditions, isolated RhI is converted in situ into catalytically active Rh–iodide 3 carbonyl species (H Rh(CO) I ) (RU Fuanin et al., JPS581974B; Coord. Chem. Rev., z x y 2003, 243, 125–142; J. Mol. Catal., 1987, 39, 115–136). This conversion occurs via coordination of CO and HI to Rh, which restores the Rh center to its homogeneous, catalytically competent state. The resulting Rh–iodide carbonyl complexes are well-known active species in liquid-phase carbonylation reactions, consistent with established Rh carbonyl chemistry. This reactivation ensures that the recycled Rh catalyst retains comparable activity and selectivity to fresh RhCl .\nRevision made: Details of the Rh recovery procedure, recovery efficiency, catalyst reactivation pathway, and recycling performance have been added to the revised Supplementary Information (Section 5.1 Fig. S10 and Table S4).\nFigure R7. Rhodium -catalyzed cyclic processes within the laboratory setting.\nTable R3. Reaction performance of different anionic rhodium salts.\nNo. Rh I − TPA Yield (%) PA Yield (%) EG Yield (%)\nStandard reaction conditions: PET (0.192g, 1 mmol structural unit), HFIP (2 mL), H O (0.5 mL), 2 CH I (0.5 mmol) or HI (0.4 mmol), CO (2 MPa) and catalyst (RhCl , 5 mol%) at 170°C for 12 h. 3 3\nComments 2: Second, iodide reagent is also kind of expensive, the author needs to address how to separate and recycle the iodide additive.\nResponse: We thank the reviewer for raising this important point regarding the separation and recycling of the iodide additive. In this system, iodide is distributed between two fractions after the reaction. The major fraction is present as HI, which is dissolves in HFIP and remains volatile, while the remainder is retained within the non- volatile Rh-containing catalytic complex. Consequently, iodide recycling is intrinsically integrated with the recycling of HFIP and the Rh catalyst. In line with Comment 1, for laboratory demonstration, the RhI -based recovery procedure was 3 employed to illustrate iodide recycling. The experimental procedure is as follows:\nAfter completion of the reaction, the crude mixture was centrifuged to separate the solid fraction (Solid a) from the liquid phase (liquid a). Solid a was washed with aqueous HI to dissolve any trace Rh precipitate that might be present, and the resulting suspension was centrifuged. The recovered solid was then recrystallized to afford purified TPA, and the wash solution was collected as Liquid b. Liquid a was distilled to recover the HI/HFIP mixture, which was directly reused in the repeated catalytic cycle. The remaining liquid was evaporated to recover PA. The final residue, containing Rh complexes, was combined with Liquid b (denoted as Liquid c) and thermally aged to promote precipitation of RhI . The recovered aqueous HI solution 3 could be reused in the subsequent TPA washing step. It is worth noting that this aqueous HI is specifically reserved for the TPA purification cycle and does not enter the catalytic reaction. The resulting non-volatile Rh-containing residue including RhI , 3 together with the recovered HI/HFIP mixture, was directly reused in subsequent repeated catalytic cycle without the addition of extra iodide. Using this approach, the system delivered 98% TPA yield and 90.9% PA yield. Although minor iodide loss is unavoidable under laboratory-scale, non-closed recycling conditions, these results demonstrate that the iodide additive is largely retained and can be efficiently recycled within the present system. This laboratory recovery protocol provides a conservative mass-balance demonstration and aligns with the well-established retention and reactivation behavior of Rh/iodide species in liquid-phase carbonylation systems.\nRevision made: The iodide recycling procedure has been added to the revised Supplementary Information (Section 5.3, Fig. S10).\nFigure R11. The cyclic process of the PET carbonylolysis reaction within the laboratory setting.\nComments 3: Third, the author used hexafluoroisopropanol [HFIP] as solvent for its good properties. However, it is an expensive solvent and have serious environmental impact, so that its recovery needs to be addressed.\nResponse: We thank the reviewer for highlighting the cost and environmental considerations associated with HFIP. We agree that demonstrating efficient solvent recovery and reuse is essential for assessing the practical viability of this process.\nAccordingly, we conducted solvent-recycling experiments to evaluate the recovery efficiency and chemical stability of HFIP under the reaction conditions.\nHFIP recycling procedure: After completion of the reaction, the crude mixture was centrifuged to separate the solid fraction from the liquid phase. The liquid phase was then subjected to low-temperature distillation to recover an HI/HFIP mixed solution, which was directly reused in subsequent catalytic cycle. Because HFIP and HI function cooperatively as the reaction medium, separation of HI from HFIP is neither required nor desirable for effective solvent circulation. Moreover, the co- recycling of solvent and iodide promoter is consistent with established industrial practice in Monsanto/Cativa-type carbonylation processes.\nHFIP was recovered after each run with near-quantitative efficiency (>99%), and 1H, 13C and 19F NMR analyses (see also our response to Reviewer 2, Comment 5) confirmed that the recovered solvent retained high chemical purity without detectable degradation. Reuse of the recycled HFIP with fresh RhCl resulted in no observable 3 loss of catalytic activity or product selectivity, demonstrating that HFIP can be efficiently recycled and reused without compromising performance (Table R4). These results establish the practical viability of HFIP recycling, and exploration of alternative greener solvent systems remains an important direction for future work.\nRevision made: In the revised Supplementary Information, the NMR spectra of fresh HFIP and recycled HFIP are provided in Figures S11–S13, and the corresponding catalytic performance data are summarized in Table S5.\nTable R4. Recyclability Test of HFIP\nCycle Number Temp. Sol. TPA Yield (%) PA Yield (%)\n1 170°C HFIP 99 96.3\n2 170°C HFIP 99 96.7\n3 170°C HFIP 99 96.4\nStandard reaction conditions: PET (0.192g, 1 mmol structural unit), HFIP (2 mL), H O (0.5 mL), 2 CH I (0.5 mmol), CO (2 MPa) and catalyst (RhCl , 5 mol%) at 170°C for 12 h. 3 3\nComments 4: Fourth, the author claims that the carbonylation mechanism is through the ethylene insertion into [Rh(CO) I ]– active catalytic species. Actually, the well- 2 2 recognized active catalytic species is supposed to be [HRh(CO)I ]−. You can follow a 3 recently published paper J. Am. Chem. Soc. 2025, 147, 38, 34589–34600.\nResponse: We thank the reviewer for the important mechanistic clarification. We fully agree that, according to well-established carbonylation chemistry, the hydride complex [HRh(CO)I 3 ]– is the widely recognized active catalytic species, as comprehensively documented in the recent report by J. Am. Chem. Soc. 2025, 147, 34589–34600 and related literature (M Beller, Catalytic carbonylation reactions, 2006, 18, Springer Science & Business Media; J. Mol. Catal. A: Chem., 1995, 104, 17–85; J. Catal., 2014, 319, 211–219; ACS Catal., 2022, 4203–4215).\nIn light of this, we have revised the mechanistic description throughout the manuscript to identify [HRh(CO)I 3 ]– as the primary active species governing ethylene insertion and subsequent carbonylation steps. Consistent with this revision, we have reconducted the DFT calculations using [HRh(CO)I 3 ]– as the catalytic center, and the updated results are now presented in the revised manuscript and Supplementary Information. The new DFT results confirm that ethylene coordination and insertion into the Rh–H bond of [HRh(CO)I 3 ]– proceed with accessible activation barriers under the reaction conditions, followed by CO migration and hydrolysis to afford the observed products, in full agreement with the established hydride-based catalytic cycle.\nRevisions made: The mechanistic section and catalytic cycle have been updated to reflect the hydride-mediated pathway via [HRh(CO)I 3 ]–, and the DFT analysis has been revised accordingly (Figure 4E in the revised manuscript and Section 4.1 in the revised Supplementary Information).\nFigure R12. Revised Figure 4E of the manuscript. Gibbs free-energy profile for the carbonylation of 1,2-diiodoethane.\nD G (k c a l/m IM 1 0 .0 o l) T2 S 1 9 .9 IM1 9 2.0 R IM3 0 e a 2 ' .0 c tio n TS 2 ' 4 0 .8 TS 2 3 9 .8 c o o r d in a te fo r th e c IM 3 2 2 .0 IM 3 ' 7 .1 a r b o n y lo ly s is o f E G T S 3 3 1 .8 T S 3 ' 2 3 .1 pp a th y w a th y w IM 4 1 4 .7 IM 4 ' -4 .4 aa yy A B"}, {"round": 2, "reviewer_comments": {"reviewer_1": "After reviewing the revisions made by Liu et al., I am fully supportive of the publication of the manuscript in Nature Communications. The authors have fully addressed the three reviewers' concerns by not only adding additional clarifications where needed, but by also providing additional experimental data which support their claims made throughout the manuscript.", "reviewer_2": "This manuscript can be accepted as it is.", "reviewer_3": "The author has answered most of the questions. But I think the answers to the reviewer’s comments are not sufficient, the following issues are still need to be addressed.\n1. The author claimed in the response that “A sharp increase in activity was observed upon increasing CH3I loading from sub-threshold to iodide-rich conditions, followed by a plateau upon moderate further increases, indicating that a minimum iodide level is required to establish the active Rh catalytic cycle rather than a linear dependence on CH3I loading.” However, only three data provided in not enough to draw this conclusion. I think more data should be added to draw a relationship line. Besides, the author should use this data to correlate to the catalytically active Rh–iodide species and give a reasonable explanation.\n2. The author mentioned in the response that “Notably, all PET samples were manually cut into small pieces without further powdering or micronization, thereby minimizing potential structural modification associated with intensive mechanical pretreatments such as ball milling.” Is there any study on the effect of the PET sample size? It seems unreasonable that the result is irrelevant to the substrate size.\n3. The author described a complex procedure for the catalyst recycling. First, only 95% of Rh can be recycled, which would lead to significant high cost. A reasonable production cost needs to be provided. Second, the recycling efficiency of iodide species should be calculated, which also contribute to the high cost of the procedure."}, "author_response": "We sincerely thank you for your careful review and thoughtful evaluation. Your insightful comments and suggestions have been invaluable in improving both the clarity and quality of our work. We are truly honored by your positive recommendation and greatly appreciate your recognition and support.\n\nThank you very much for your careful review and encouraging assessment of our work. Your feedback has significantly contributed to improving the clarity and presentation of our manuscript. We are deeply honored by your positive evaluation and sincerely appreciate your constructive guidance.\n\nWe sincerely thank you for the insightful and constructive feedback. We greatly appreciate your recognition of our efforts to address the initial concerns and your emphasis on the remaining key aspects. We fully acknowledge that these are critical factors that require careful consideration, and each has been thoroughly addressed in the revised manuscript. Below, we provide detailed responses to the specific points you raised.\n\nComments 1: The author claimed in the response that “A sharp increase in activity was observed upon increasing CH I loading from sub-threshold to iodide-rich 3 conditions, followed by a plateau upon moderate further increases, indicating that a minimum iodide level is required to establish the active Rh catalytic cycle rather than a linear dependence on CH I loading.” However, only three data provided in not 3 enough to draw this conclusion. I think more data should be added to draw a relationship line. Besides, the author should use this data to correlate to the catalytically active Rh–iodide species and give a reasonable explanation.\nResponse: We thank the reviewer for this insightful comment and fully agree that the initial presentation of the CH I dependence, based on a limited number of data points 3 within the critical concentration window, was insufficient to rigorously substantiate the proposed non-linear relationship. In response to the reviewer’s suggestion, we have now inserted additional data points specifically within the key concentration region (0.10–0.30 mmol per 1 mmol PET), where the sharp change in catalytic behavior occurs, thereby enabling a clearer construction of the activity–iodide relationship (Table R1).\nThe refined dataset within this critical window reveals a pronounced non-linear dependence of catalytic performance on CH I loading. At low CH I levels (<0.10 3 3 mmol per 1 mmol PET), PA formation is strongly suppressed and EG remains the dominant product. Upon increasing CH I into an intermediate regime, PA yield 3 increases sharply over a narrow concentration range (0.10–0.20 mmol per 1 mmol PET). Further increases in CH I result in only marginal changes in activity and 3 selectivity, indicative of saturation-type behavior (0.20–0.50 mmol per 1 mmol PET), while at higher CH I loadings (~0.80 mmol per 1 mmol PET) a decrease in PA yield 3 is observed.\nThis non-linear trend can be directly correlated with changes in steady-state Rh– iodide speciation arising from competitive coordination between iodide and CO at the Rh center. At sub-critical iodide activities, iodide is unable to effectively compete with CO for coordination, and Rh is predominantly present in iodide-deficient or neutral carbonyl environments that are ineffective for oxidative addition and subsequent CO insertion, resulting in low PA formation. Increasing CH I raises iodide activity in 3 solution, enabling iodide to successfully compete with CO and stabilize mixed anionic Rh–iodide–carbonyl species (commonly invoked in Rh-catalyzed carbonylation cycles, such as [RhI (CO) ]⁻ or [HRh(CO)I ]⁻-type intermediates), which are catalytically 2 2 3 competent for oxidative addition and CO insertion.\nOnce these anionic Rh–iodide–carbonyl species become the dominant steady- state form, the carbonylation cycle is fully established and PA formation proceeds efficiently. Within this CH I concentration window, catalytic performance remains 3 essentially constant, giving rise to the observed activity plateau. At higher CH I 3 loadings, however, excess iodide overcompetes with CO for coordination and drives Rh speciation toward iodide-saturated, CO-deficient environments, limiting the availability of Rh–CO species required for CO insertion. As a result, further increases in CH I do not enhance and may even suppress catalytic activity. Thus, the observed 3 CH I dependence reflects modulation of the steady-state population of catalytically 3 active Rh–iodide species through iodide–CO coordination balance, rather than a direct kinetic dependence on CH I concentration itself. While this interpretation is based on 3 catalytic trends rather than direct in situ speciation measurements, it is fully consistent with established Rh–iodide carbonylation chemistry. (Catal. Sci. Technol., 2022, 12, 664-673; ACS Catal., 2021, 11, 7249-7256; J. Catal., 2014, 319, 211-219; Sci. China, Ser. B: Chem, 2004, 47, 41-49; J. Catal., 2015, 325, 1-8; The Canadian Journal of Chemical Engineering, 2021, 99, 1137-1145.)\nRevisions made: In the revised manuscript, the discussion has been rephrased to explicitly correlate the CH I dependence with the formation of catalytically active Rh– 3 iodide species. Specifically, we now state that “As CH I is increased into an 3 intermediate regime, the steady-state population of catalytically competent anionic Rh–iodide–carbonyl species is established, leading to a sharp rise in PA formation (Table S2). Further increases in CH I result in a plateau in activity and selectivity, 3 indicative of saturation of the active Rh species. At much higher CH I loadings, excess 3 iodide overcompetes with CO for coordination, driving Rh speciation toward iodide- saturated CO-deficient environments and limiting CO insertion, which reduces PA yield (Fig. 2A). This behavior is fully consistent with established Rh–iodide carbonylation chemistry”\nIn addition, Table S2 in the revised Supplementary Information Section 2.6 includes the newly added data points within the critical CH I concentration window, 3 providing explicit experimental support for the non-linear dependence and the requirement of a minimum iodide level to establish the active Rh catalytic cycle.\nTable R1. Reaction performance of different CH I content. 3\nNo. CH I loading TPA Yield (%) PA Yield (%) 3\n1 0.10 95.1 0.33\n2 0.12 96.2 5.0\n3 0.15 96.8 6.8\n4 0.18 97.3 17.3\n5 0.20 99.0 83.0\n6 0.30 97.5 85.4\nStandard reaction conditions: PET (0.192g, 1 mmol structural unit), solvent (2 mL), H O (0.5 mL), CH I 2 3 (0.5 mmol), CO (2 MPa) and catalyst (RhCl , 5 mol%) at 170°C for 12 h. 3\nComments 2: The author mentioned in the response that “Notably, all PET samples were manually cut into small pieces without further powdering or micronization, thereby minimizing potential structural modification associated with intensive mechanical pretreatments such as ball milling.” Is there any study on the effect of the PET sample size? It seems unreasonable that the result is irrelevant to the substrate size.\nResponse: We thank the reviewer for this insightful comment and fully agree that, in general, the particle size of polymeric substrates can significantly influence reaction kinetics by affecting surface area and mass-transfer processes. To directly address this concern, we have performed additional experiments using PET bottle flakes with different dimensions (0.3 × 0.3, 0.5 × 0.5, 1.0 × 1.0, 1.5 × 1.5, 2.0 × 2.0 cm2 and randomly irregular mixture) under otherwise identical carbonylation conditions (Fig. R1 and Fig. R2). The results show that the yields of terephthalic acid (TPA) and propionic acid (PA) remain essentially unchanged across this particle-size range, indicating that PET particle size exhibits negligible dependence within the reaction regime investigated in this work.\nThis behavior can be rationalized by the nature of the reaction medium and mechanism. HFIP is a strong solvent for PET and can readily swell and dissolve the polymer under the reaction conditions. This rapid solvent penetration substantially alleviates mass-transfer limitations associated with solid–liquid interfaces, such that the transformation is not strictly governed by heterogeneous interfacial reactions, particularly after the initial solvation and depolymerization stages. Once PET is solubilized and depolymerized to molecular intermediates (e.g., EG), the subsequent carbonylation step proceeds predominantly via homogeneous molecular pathways. Consequently, the overall reaction rate and selectivity are governed primarily by the homogeneous carbonylation chemistry rather than by the specific surface area of the solid polymer.\nIt is noteworthy that this observation is specific to the present HFIP-mediated system and does not imply that PET particle size is universally unimportant across all depolymerization processes. Under the conditions employed here, efficient conversion can be achieved without energy-intensive mechanical pretreatments such as ball milling or micronization.\nRevisions made: Additional particle-size–dependent experiments have been included in the revised Supplementary Information Section 2.8 (Fig. S7 and Table S3). The revised manuscript text has been revised to clarify that PET powdering is unnecessary for efficient conversion specifically under the HFIP-mediated reaction conditions studied here, due to the strong solubility of HFIP toward PET, thereby avoiding any implication of universal size-independence (p. 4, line 16).\nFig. R1 Different sizes of PET bottle flakes. (A-B) 0.3 × 0.3 cm2; (C-D) 0.5 × 0.5 cm2; (E-F) 1.0 × 1.0 cm2; (G-H) 1.5 × 1.5 cm2; (I-J) 2.0 × 2.0 cm2 (These data have been added to Fig. S7).\nTable R2. Reaction performance of different PET samples sizes. (These data have been added to Table. S3)\nNo. PET samples size TPA Yield (%) PA Yield (%)\n1 2.0 × 2.0 94.6 93.6\n2 1.5 × 1.5 91.2 90.6\n3 1.0 × 1.0 95.1 92.5\n4 0.5 × 0.5 92.7 91.6\n5 0.3 × 0.3 96.0 92.2\n6 randomly irregular mixture 94.1 92.1\nStandard reaction conditions: PET (0.192g, 1 mmol structural unit), solvent (2 mL), H O (0.5 mL), CH I 2 3 (0.5 mmol), CO (2 MPa) and catalyst (RhCl , 5 mol%) at 170°C for 12 h. 3\nComments 3: The author described a complex procedure for the catalyst recycling. First, only 95% of Rh can be recycled, which would lead to significant high cost. A reasonable production cost needs to be provided. Second, the recycling efficiency of iodide species should be calculated, which also contribute to the high cost of the procedure.\nResponse: We thank the reviewer for raising these important concerns, and we fully agree that efficient resource retention and economic viability are critical for practical applications.\nRh recovery and cost implications: The recovery of “~95% Rh as RhI ” should 3 not be directly equated with an intrinsic ~5% catalyst loss, as it does not represent the exact behavior of the catalytic system under reaction conditions. After TPA separation, distillation of volatile components (HFIP/HI and PA) of the reaction mixture leave a non-volatile Rh-containing residue that constitutes the actual recycled catalytic phase. ICP–MS analysis of this residue indicates near-quantitative Rh retention (~99%, within analytical error), and direct reuse of this Rh-containing residue in subsequent cycles, without deliberate precipitation of RhI , delivers 97% PA yield, 3 consistent with fresh runs, demonstrating effective recyclability and catalytic stability under closed-system operation. In a realistic process design, Rh would therefore be continuously retained in this non-volatile catalytic residue and recycled following distillation of volatile solvents and products, rather than being isolated or converted to RhI between 3 cycles. This behavior aligns with established industrial practice in liquid-phase Rh-catalyzed carbonylation, where Rh is maintained in a high-boiling catalytic phase and recovered primarily via product distillation. The ~95% Rh recovery was determined gravimetrically after intentional RhI isolation, which was employed solely as a 3 laboratory-scale method for rigorous mass-balance accounting under non-closed handling conditions. This procedure involves introducing additional transfer, washing, and phase-separation steps that are not required in a practical process and introduces experimental errors at small scale. Importantly, isolation of Rh as RhI is an extreme 3 laboratory representation of the complex mixture of Rh species present in the catalytic residue, rather than the actual form in a practical process.\nBased on the near-quantitative Rh retention measured by ICP–MS and the successful direct reuse of the Rh-containing residue, the effective Rh cost contribution is expected to be only a small fraction of overall process costs in a closed-system operation. The primary determinant of catalyst-related cost is therefore not intrinsic chemical consumption, but the process to retain Rh in a non-volatile catalytic residue and recycle it directly after product distillation, as confirmed experimentally. For TEA purposes, RhI is adopted as a representative Rh species to simplify accounting and 3 modeling, while intrinsic Rh loss is neglected in accordance with the ICP–MS results. This provides a conservative, order-of-magnitude estimate rather than a fully detailed process model. Consistent with extensive literature, Rh–iodide carbonylation catalysts sustain high-turnover operation through continuous retention, such that the Rh loading should be regarded as a retained catalytic inventory rather than a consumable input (Coord. Chem. Rev., 2003, 243, 125–142; J. Mol. Catal., 1987, 39, 115–136; Adv. Catal., 2010, 53, 1–45). In parallel, we are pursuing further cost optimization via two complementary strategies: development of readily separable heterogeneous catalytic systems, and exploration of low-cost, non-precious metal alternatives to Rh.\nIodide recycling efficiency: In this system, iodide recovery is intrinsically coupled with the recovery of both the solvent and the catalyst. After reaction, most of the iodide remains dissolved in HFIP as HI and can be recovered by distillation and directly reused, while the remaining iodide is retained in the non-volatile Rh- containing catalytic residue. In the laboratory RhI -based recovery protocol, the iodide 3 associated with Rh is quantitatively preserved and returned to the system upon catalyst reactivation. Iodometric titration of the distilled HI/HFIP solution (revised Supporting Information Section 4.4), together with the independently determined 99% Rh retention, indicates an overall iodide recovery of >93% (defined as total iodide recovered relative to iodide initially charged) under non-closed laboratory operating conditions. The small losses observed are primarily due to open distillation and manual handling rather than intrinsic chemistry and represent conservative laboratory artifacts. Multiple catalytic cycles were carried out without adding any external iodide, demonstrating excellent practical recyclability. Because iodide functions catalytically and is effectively recovered with both the solvent and the catalyst, these minor lab- scale losses have a negligible impact on total operating costs. In industrial-scale closed or semi-closed operations, such handling-related losses can be further minimized, further reducing the effective cost contribution of iodide.\nAbout the cost: From a practical perspective, the present recycling experiments are intended to demonstrate catalyst and promoter retention, reactivation capability, and mechanistic robustness, rather than to provide a full industrial process design. In our system, Rh is recovered in a non-volatile catalytic residue, while iodide is largely retained in the volatile HFIP/HI phase with well-established methods available to minimize volatile losses. Based on our experiments and established industrial practice in liquid-phase Rh carbonylation, material losses of both Rh and iodide are expected to be minimal under closed or semi-closed operation. Accordingly, Rh can be reasonably regarded as a retained catalytic inventory rather than a consumable reagent. Consequently, the dominant economic factors are expected to be process operation and energy consumption, which have been considered qualitatively in our current techno- economic assessment using RhI as a conservative reference. Exact quantitative cost 3 estimation under industrial conditions would require scaled-up experiments and detailed engineering studies, which are beyond the scope of the current work. Future work will focus on process intensification, simplified catalyst recovery strategies, and alternative catalyst designs to further enhance operational efficiency and economic competitiveness.\nRevisions made: The discussion section in the revised manuscript now includes a sentence highlighting limitations and future directions: “large-scale deployment requires advances in cost-efficient non-noble catalysts and robust performance with highly mixed or contaminated feedstocks, with catalyst recovery and operational efficiency being particularly critical.” Revised Supporting Information Sections 4.1 and 4.3 report ICP–MS–determined Rh retention (~99%, within analytical error) and iodide recovery (>93%), and Section 4.4 details the iodide quantification method. The following key points are also clarified in the Supporting Information: in practical recycling, Rh is retained in the non-volatile catalytic residue and directly reused, enabling near-quantitative catalyst retention. Under closed or semi-closed operation, where Rh is continuously retained, the effective Rh cost is expected to be dominated by catalyst inventory amortization rather than consumption. RhI is used as a 3 representative Rh species for accounting and modeling purposes, and any minor laboratory-scale loss does not impact overall production costs, as Rh retention is effectively quantitative."}, {"round": 3, "reviewer_comments": {"reviewer_3": "I think all the questions have been answered,it is ready to be accepted"}, "author_response": ""}]

Decision letter