Defect-engineered MIL-101(Cr) via facile low-temperature calcination for efficient CO₂ cycloaddition under mild conditions

Abstract

Defective metal organic frameworks have emerged as promising catalysts for CO₂ conversion owing to their tunable active sites and exceptional catalytic activities. Despite their potential, achieving high defect densities through simple and scalable methods remains a major challenge. To address this, we developed an innovative low-temperature calcination strategy (300 °C, 10 min) to introduce abundant ligand defects into MIL-101(Cr) while maintaining crystallinity, thereby exposing additional Cr³⁺ Lewis acid sites to participate in epoxide activation. The resulting defective MIL-101 demonstrates remarkable catalytic efficiency in CO₂ cycloaddition, achieving a >99% yield under mild conditions (80 °C, 0.1 MPa) within 6 h, significantly surpassing that of pristine MIL-101 (52%). Furthermore, the catalyst exhibited outstanding recyclability, retaining full activity over multiple reaction cycles. In addition to synthetic simplicity, this work provides fundamental insights into the critical role of defects in promoting epoxide activation and ring opening, establishing a sustainable pathway for CO₂ utilization under mild conditions and the energy-efficient production of cyclic carbonates.

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1. Introduction

The catalytic cycloaddition of CO₂ to epoxides represents a crucial pathway for converting greenhouse gases into value-added cyclic carbonates, which find extensive applications such as electrolytes in lithium-ion batteries, and versatile building blocks for polymers and fine chemicals.^1–4 While diverse catalysts, including heteroatom-doped carbon,⁵ ionic liquids,⁶ metal complexes and zeolites,^7,8 have been explored, metal organic frameworks (MOFs) offer superior advantages because of their tunable porosity, high surface areas, and designable active sites, positioning them as ideal catalysts for this reaction.^9–12 Among MOF catalysts, Materials of Institute Lavoisier (MIL) catalysts, particularly chromium-based MIL-101(Cr), stand out for CO₂ fixation owing to their remarkable stability, strong Lewis acidity, and large cages.¹³ Zalomaeva et al. first reported the use of MIL-101(Cr) for catalyzing the cycloaddition of CO₂ with epoxides, achieving a 95% yield of styrene carbonate after 48 h under reaction conditions of 8 atm CO₂ and 25 °C.¹⁴ Subsequent research focused on enhancing MIL-101's performance under milder conditions, aiming to achieve high yields of cyclic carbonates with reduced energy input.

Current strategies to increase the CO₂ cycloaddition efficiency of MIL-101 under mild conditions include ligand functionalization (e.g., introducing –OH/–NH₂ groups),¹⁵ ionic liquid encapsulation,¹⁶ heteroatom doping and defect engineering.^17,18 As established by prior research, the Cr³⁺ metal centers in MIL-101 function as efficient Lewis acid sites for epoxide activation.^19–21 Consequently, partial ligand removal from the MIL framework should theoretically expose additional Cr³⁺ centers, thereby increasing their Lewis acidity to boost their catalytic performance.²² In this context, defect engineering, especially partial ligand removal, holds particular promise for accelerating the CO₂ cycloaddition efficiency by exposing additional uncoordinated Cr³⁺ ions as Lewis acid sites. Current approaches for generating ligand defects in MOF materials primarily fall into two categories: direct synthesis and post-synthetic treatment.^23,24 Direct synthesis methods typically involve controlling synthesis conditions, adding template agents or modulators, and adjusting ligand types. Post-synthetic approaches include acid/base treatment, thermal treatment, solvent-assisted defect engineering, and high-energy irradiation, which have been reviewed in the literature.^25,26 However, conventional defect engineering methods are usually complex and time-consuming, presenting major barriers to their practical deployment and scalable manufacturing.²⁷

To address this limitation, we developed a rapid low-temperature calcination protocol (300 °C, 10 min) that selectively generates ligand defects in MIL-101(Cr) while maintaining crystallinity. In contrast to previously reported defect engineering strategies, our approach not only avoids complex processing and the use of solvents or additional chemicals but also requires only low-temperature calcination at 300 °C for a short duration of only 10 min. This method is characterized by mild conditions, short processing time, and operational simplicity while efficiently creating a high density of defect sites. Such a straightforward and economical strategy demonstrates great potential for large-scale production of defective MIL materials. More importantly, the resulting defect-engineered MIL-101 exhibited enhanced Lewis acidity, achieving >99% epichlorohydrin carbonate yield within 6 h at 80 °C and 0.1 MPa CO₂ and maintaining >99% yield over five cycles. This facile strategy not only significantly enhances the catalytic efficiency of MIL-101 but also mechanistically clarifies the important role of ligand defects in promoting CO₂ cycloaddition efficiency.

2. Experimental

2.1 Synthesis of pristine MIL-101 and defective MILs

MIL-101(Cr) was synthesized via a modified hydrothermal procedure adapted from the literature.²⁸ Briefly, 3.28 g (19.7 mmol) of terephthalic acid (H₂BDC, Aladdin, >99%) and 8.0 g (20.0 mmol) of Cr(NO₃)₃·9H₂O (Sinopharm Chemical Reagent) were dissolved in 100 mL of a CH₃COONa (Sinopharm Chemical Reagent) solution (0.05 mol L⁻¹) under stirring for 30 min to form a homogeneous solution. The mixture was then transferred to a 200 mL Teflon-lined autoclave and reacted at 200 °C for 12 h. The resulting precipitate was isolated via centrifugation, alternately washed with dimethylformamide (DMF, ≥99.5%, Sinopharm Chemical Reagent) (60 °C) and ethanol (99.7%, Sinopharm Chemical Reagent) (60 °C) three times, and then vacuum-dried at 80 °C to obtain pristine MIL-101.

Defective MILs were prepared via controlled pyrolysis of pristine MIL-101 (0.2 g of sample was evenly distributed in a porcelain boat) under an air atmosphere (muffle furnace; the heating rate was set to 10 °C min⁻¹) at specified temperatures and durations. The resulting samples are denoted as PM-x/y, where x and y represent the calcination temperature (°C) and duration (min), respectively. For example, PM-300/10 denotes MIL-101 calcined in air at 300 °C for 10 min. A control sample, labeled PM-300/10 (N₂), was also prepared under a N₂ atmosphere at 300 °C for 10 min.

2.2 Characterizations

Detailed material characterization methods, including powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), N₂ sorption tests, thermogravimetric analysis (TGA), NH₃-temperature programmed desorption (NH₃-TPD) and transmission electron microscopy (TEM), are provided in the SI. Catalytic evaluations for CO₂ cycloaddition were performed under solvent-free conditions at atmospheric pressure. The typical reaction mixture contained 50 mg of catalyst, 65 mg (0.2 mmol) of tetrabutylammonium bromide (TBAB, Sinopharm, 99.0%), and 780 µL (10 mmol) of epoxide in a CO₂-purged flask, which was reacted at 80 °C. The reaction progress was monitored by gas chromatography (HF-901A, Shandong Huifen). Catalyst recovery involved centrifugation, ethanol washing, and vacuum drying (80 °C) for the recycling test. The substrate scope was evaluated under identical conditions and included epichlorohydrin (Aladdin Industrial, 99.5%), epibromohydrin (99.5%, Aladdin Industrial), cyclohexene oxide (99.0%, Energy Chemical), styrene oxide (98%, Aladdin Industrial), benzyl(R)–(–)-glycidyl ether, and 1,2-epoxybutane (99%, Aladdin Industrial).

3. Results and discussion

3.1 Material characterization

Fig. 1 illustrates the low-temperature rapid calcination strategy employed in this work to engineer ligand-deficient MIL-101, enhancing its Lewis acidity for efficient CO₂ cycloaddition under mild conditions. As epoxide ring opening on Lewis acidic sites usually constitutes the rate-determining step in this reaction,²⁹ increasing the Lewis acidity of catalysts can theoretically increase the catalytic CO₂ cycloaddition efficiency. Here, we found that short-duration calcination in air (e.g., 300 °C, 10 min) can enable the selective removal of partial ligands while preserving the MIL-101 framework, thereby exposing additional Cr³⁺ active centers as Lewis acidic sites. These exposed Cr³⁺ Lewis acid sites can effectively adsorb and activate epoxides to form ring-opened intermediates, which subsequently react with adsorbed CO₂, followed by CO₂ insertion and ring-closure steps to yield cyclic carbonates as the sole product.

Fig. 1 Schematic illustration of (a) the synthesis of defective MIL-101 via facial calcination treatment for (b) CO₂ fixation to produce cyclic carbonates.

Fig. 2a displays the XRD patterns of pristine MIL-101 and its pyrolyzed derivatives. MIL-101 exhibits characteristic diffraction peaks at 5.2°, 5.8°, 8.2°, 9.1°, and 10.3°, corresponding to the (333), (531), (606), (753), and (666) crystal planes, respectively.³⁰ The XRD pattern of PM-200/10 is identical to that of pristine MIL-101, whereas PM-300/10 preserves a similar XRD pattern but with a reduced peak intensity. In contrast, a distinct phase transition occurred for PM-400/10, where new peaks emerge at 33.6°, 36.2°, 41.5°, and 54.9°, corresponding to the characteristic peaks of Cr₂O₃ (PDF#038-1479),^31,32 with some typical peaks of MIL-101 disappearing. When the pyrolysis temperature was fixed at 300 °C, extending the duration from 10 to 30 or 60 min reduces the peak intensity and broadens the peak width, indicating partial degradation of the MIL framework and the formation of an amorphous crystal structure. On the basis of the XRD analysis, it can be concluded that the crystal structure of MIL-101 can be maintained at ≤300 °C for short durations (<30 min). Beyond these calcination conditions, the MIL-101 structure began to degrade, which was accompanied by the formation of Cr₂O₃. In addition, the sample calcined in N₂ (300 °C, 10 min) has a crystal structure identical to that of its air-calcined counterpart (PM-300/10), suggesting that the pyrolysis atmosphere did not affect the crystal structure of MIL under the same pyrolysis temperature and duration.

Fig. 2 (a) XRD patterns, (b) FTIR spectra, and (c and d) N₂ sorption isotherms of MIL-101 and PM-x/y.

Fig. 2b presents the FTIR spectra of MIL-101 and its derivatives under different calcination conditions. All the samples retain the characteristic functional groups of MIL-101, including peaks at 1300–1700 cm⁻¹ (H₂BDC linker), 590 cm⁻¹ (Cr–O vibration), and a broad band centered at 3400 cm⁻¹ (edge –OH groups),^33,34 confirming the preservation of the MIL framework. However, when the pyrolysis time was increased from 10 to 60 min at 300 °C or when the pyrolysis temperature was increased to 400 °C, the intensity of the peaks corresponding to the BDC linkers decreased, suggesting the occurrence of partial decomposition of organic linkers, which is in agreement with the XRD analysis. Fig. 2c and d show the N₂ adsorption–desorption isotherms. Pristine MIL-101 has the highest specific surface area of 1840 m² g⁻¹, while subsequent calcination universally reduces the surface area (Table S1). Specifically, increasing the calcination temperature progressively decreased the surface area to 1829 m² g⁻¹ (PM-200/10), 1279 m² g⁻¹ (PM-300/10), and 62 m² g⁻¹ (PM-400/10). Notably, the surface area of PM-300/10 (N₂) (1175 m² g⁻¹) was slightly lower than that of PM-300/10, suggesting that the presence of oxygen (in air) during calcination might facilitate more effective ligand removal and enhances the surface area compared to an inert atmosphere. Similarly, an extended calcination duration at 300 °C reduced the surface area from 1279 m² g⁻¹ (PM-300/10) to 1147 m² g⁻¹ (PM-300/30) and 233 m² g⁻¹ (PM-300/60). Consistent with the XRD and FTIR analysis results, this degradation stems from partial ligand decomposition causing localized framework collapse under mild pyrolysis conditions (<400 °C) and Cr₂O₃ formation under harsh pyrolysis conditions (e.g., 400 °C).

Fig. 3 displays the morphological evolution of MIL-101 under various calcination conditions. Pristine MIL-101 displays a distinct octahedral rhombic structure with an average particle size of ∼200 nm and a smooth surface (Fig. 3a), whereas PM-300/10 shows minimal morphological changes (Fig. 3b). In contrast, PM-400/10 displays fragmented particles forming agglomerates (Fig. 3c), indicating that severe structural degradation occurred at 400 °C (as confirmed by XRD, as shown in Fig. 2a). This degradation can be further verified by TEM analysis. TEM images reveal that PM-300/10 (Fig. S1) and pristine MIL-101 (Fig. 3g) display similar intact architectures. However, PM-400/10 exhibits significant fragmentation of the original large particles into smaller fragments (Fig. 3h), accompanied by the generation of numerous large pores (evident as bright spots in Fig. 3i). Similarly, extending the calcination duration from 10 min to 30–60 min at 300 °C progressively compromises particle integrity to form fragmented particles, as displayed in Fig. 3d and e. The N₂-calcined sample (PM-300/10 (N₂)) retains particle dimensions similar to those of its air-calcined counterpart (PM-300/10), and it tends toward a rounded morphology with less distinct edges, as displayed in Fig. 3f.

Fig. 3 SEM images of (a) pristine MIL-101, (b) PM-300/10, (c) PM-400/10, (d) PM-300/30, (e) PM-300/60, and (f) PM-300/10 (N₂) and TEM images of (g) MIL-101 and (h and i) PM-400/10.

Fig. 4 compares the CO₂ adsorption properties and calculated adsorption enthalpies among different samples under different calcination conditions at 0 °C and 25 °C. The CO₂ uptake capacity at 0 °C follows the order of PM-200/10 (40.5 cm³ g⁻¹) ≈ MIL-101 (40.1 cm³ g⁻¹) > PM-300/10 (36.3 cm³ g⁻¹) > PM-300/60 (33.0 cm³ g⁻¹) > PM-300/30 (24.7 cm³ g⁻¹), whereas MIL-101 demonstrates the highest CO₂ uptake at 25 °C (34.8 cm³ g⁻¹), followed by PM-200/10 (25.6 cm³ g⁻¹), PM-300/10 (21.5 cm³ g⁻¹), PM-300/60 (19.4 cm³ g⁻¹), and PM-300/30 (15.1 cm³ g⁻¹). This reduction in CO₂ adsorption stems from ligand loss during calcination, which diminishes both the surface area and the number of adsorption-active sites on the linkers. Notably, an increased calcination temperature/duration accelerated ligand depletion, further reducing CO₂ uptake (Fig. 4c). Analysis of the adsorption data reveals enhanced CO₂ adsorption enthalpies in the calcined samples (Fig. 4d). CO₂ adsorption enthalpies were estimated via the virial equation method on the basis of the literature.³⁵ The samples calcined at 300 °C had significantly improved enthalpies of 20.7, 28.3 and 24.4 kJ mol⁻¹ for PM-300/10, PM-300/30 and PM-300/60, respectively, versus 18.4 kJ mol⁻¹ for pristine MIL-101 and 17.2 kJ mol⁻¹ for PM-200/10. Considering that the CO₂ adsorption enthalpies of all samples were less than 40 kJ mol⁻¹, the CO₂ adsorption on MIL-101 and PM-x/y samples was predominantly physisorption.

Fig. 4 CO₂ adsorption isotherms at (a) 0 °C and (b) 25 °C, (c) CO₂ uptake capacity and (d) adsorption enthalpy of CO₂ adsorption for MIL-101 and PM-x/y.

3.2 Catalytic performance

Epichlorohydrin was selected as the model substrate to evaluate its catalytic activity in the thermally driven CO₂ cycloaddition reaction (80 °C, 0.1 MPa). No cyclic carbonate was detected in the absence of catalysts, confirming the indispensable role of catalysts in catalyzing CO₂ fixation into epoxides. Furthermore, PM-300/10 exhibited a low yield of 36.8% at 6 h in the absence of TBAB, confirming the essential role of TBAB as the cocatalyst in facilitating the cycloaddition process under the described reaction conditions. Comparative analysis of MIL-101 and its calcined derivatives under identical conditions revealed that PM-300/10 was the optimal catalyst, achieving a high yield of >99% (selectivity >99%) within 6 h, significantly surpassing that of pristine MIL-101 (52%), as displayed in Fig. 5a. The corresponding turnover number (TON) and turnover frequency (TOF) were calculated based on chromium active sites (see SI for details), and MIL-101 exhibited a TON of 24.9 and a TOF of 4.15 h⁻¹, while PM-300/10 showed significantly higher values of 47.8 and 7.97 h⁻¹, respectively. In contrast, PM-200/10 and PM-400/10 yielded only 68% and 48% (selectivity >99%) at 6 h, respectively. Similarly, extending the calcination time from 300 °C to 30/60 min decreased the yield to 91% (PM-300/30) and 70% (PM-300/60), as shown in Fig. 5b. The N₂-calcined sample, namely, PM-300/10 (N₂), delivered a yield of 89% at 6 h, which slightly underperformed that of PM-300/10. In general, the catalytic activity decreased in the order of PM-300/10 > PM-300/10 (N₂) > PM-300/30 > PM-200/10 > PM-300/60 > PM-400/10 ≈ pristine MIL-101, establishing 300 °C/10 min in air as the optimal calcination protocol to increase the catalytic activity of MIL-101.

Fig. 5 Yields of target cyclic carbonate at different reaction times for (a) MIL-101 and PM-x/10 and (b) PM-300/x; (c) cycling stability test and (d) substrate tolerance of PM-300/10.

In addition, PM-300/10 also demonstrated exceptional recyclability (Fig. 5c), maintaining >99% yield over five consecutive cycles (80 °C, 0.1 MPa, 6 h). XRD analysis of the recycled sample (Fig. S2) confirmed its structural integrity. Substrate scope evaluation (Fig. 5d) revealed >99% yields at 6 h for benzyl (R)-(–)-glycidyl ether, 1,2-epoxybutane, and epibromohydrin, highlighting good substrate tolerance for aliphatic/halogenated epoxides. In contrast, steric hindrance limited yields for bulky substrates such as styrene oxide (55%) and cyclohexene oxide (9%).

3.3 Proposed mechanism

Thermogravimetric (TG) analysis revealed distinct thermal degradation patterns in pristine MIL-101 and its derivatives (Fig. 6a). The thermal decomposition of pristine MIL-101 exhibited a two-stage process with an initial gradual mass loss below 200 °C, corresponding to the removal of physisorbed solvent molecules, followed by a sharp 63% mass loss between 200–800 °C attributed to the decomposition of H₂BDC linkers within the organic framework. Only 30.4% of the residual mass remained at 800 °C, corresponding to Cr₂O₃ as the final residual product.³⁶ In contrast, the mass loss values between 200–800 °C for PM-300/10, PM-200/10 and PM-400/10 were less than those of pristine MIL-101, which were 54%, 51% and 10%, respectively, while the residual mass ratio of Cr₂O₃ remained relatively high, suggesting a defective nature in the PM-x/y samples. Theoretically, the defect-free MIL-101 contains an average of three BDC ligands per secondary building unit (SBU). Ligand defect density calculations based on TG data (detailed in the SI) revealed that PM-200/10 and PM-300/10 have similar ligand defect densities, with missing linker numbers per SBU of approximately 1.25 and 1.11, respectively, whereas pristine MIL-101 is almost defect free (the missing linker number per SBU is 0.1).

Fig. 6 (a) TG curves of MIL-101 and PM-x/y, (b) XPS spectra of Cr 2p, (c) C 1s, (d) O 1s, (e) NH₃-TPD, and (f) calculation of activation energies for PM-300/10 and pristine MIL-101.

Subsequent XPS analysis further reveals the ligand deficiency nature of PM-300/10 versus pristine MIL-101 through peak shifting of the Cr 2p peaks (Fig. 6b). Specifically, the Cr 2p spectrum of MIL-101 exhibited characteristic peaks at 577.6 eV (Cr 2p_3/2) and 587.6 eV (Cr 2p_1/2),³⁷ and these peaks shifted to higher binding energies at 578.8 eV and 588.4 eV for PM-300/10, representing positive shifts of 1.2 eV and 0.8 eV, respectively. This shift reflects altered Cr coordination environments in PM-300/10, where ligand loss reduces the electron density around Cr through coordinative unsaturation. Deconvolution of the C 1s spectra (Fig. 6c) of both MIL-101 and PM-300/10 showed three sub-peaks corresponding to C–C, C–O, and O=C–O, whereas the O 1s spectra (Fig. 6d) displayed peaks indicating Cr–O, C=O and C–O.³⁸ Specifically, elemental quantification further confirms ligand deficiency because the Cr content increases from 4.7 wt% in MIL-101 to 7.5 wt% in PM-300/10, whereas the C content decreases from 66.6 wt% to 64.1 wt% because of the removal of partial ligands from the MIL structure.

To evaluate the Lewis acid sites in PM-300/10, NH₃-TPD analysis was conducted (Fig. 6e). The desorption temperature range was set at 100–300 °C, as calcination above 300 °C induces particle fragmentation and Cr₂O₃ formation according to the XRD and SEM analyses. PM-300/10 exhibited a broad peak (150–200 °C), indicating weak acid sites, alongside an intense peak at 275 °C, indicating abundant medium-strength acid sites.³⁹ Both the weak and medium acid site densities in PM-300/10 substantially exceed those in pristine MIL-101. In the CO₂ cycloaddition reaction, epoxide activation primarily occurs at Lewis acid sites.⁴⁰ According to literature, the acidic sites in MOFs, particularly in MIL-type materials, mainly originate from their metal ion centers.²⁷ These metal centers activate the oxygen atom of the epoxide, facilitating ring-opening and subsequent CO₂ insertion. Therefore, partial ligand removal is expected to not only increase the specific surface area of the material but also expose more metal ion active sites, thereby enhancing the Lewis acidity of the MIL material. In this study, controlled calcination (300 °C/10 min, in air) removes ligands, resulting in the exposure of additional Cr³⁺ centers that increase the Lewis acidity to facilitate epoxide activation.

To probe the contribution of ligand deficiency to catalytic enhancement, the activation energy (E_a) of CO₂ with epoxide over MIL-101 and PM-300/10 was calculated via Arrhenius plots using yield data at various temperatures (Fig. S3).⁴¹ The experimental results show that E_a decreases from 17.6 kJ mol⁻¹ for pristine MIL-101 to 17.4 kJ mol⁻¹ for PM-300/10 (Fig. 6f). The CO₂ cycloaddition process involves epoxide activation, epoxide ring opening, CO₂ insertion, and ring-closure processes to eventually form cyclic carbonates, among which the ring-opening process is usually the rate-determining step. Obviously, the removal of partial organic ligands results in the exposure of Cr³⁺ active sites serving as highly active Lewis acids to accelerate the ring-opening process, as displayed in Fig. 7. Our proposed facile calcination strategy enables the construction of highly defective MIL-101 while preserving its crystalline structure, endowing the material with abundant Lewis acid sites that significantly increase its efficiency in CO₂ cycloaddition. Compared with pristine MIL-101, the resulting defect-rich MIL-101 (especially PM-300/10) exhibits dramatically improved catalytic performance and outperforms most reported MIL-based materials as well as other representative catalysts, including UiO-66, zeolitic imidazolate frameworks (ZIFs), HKUST-1, covalent organic frameworks (COFs), porous carbon, SBA-15, g-C₃N₄, and metal compounds, as quantitatively compared in Table S2, advancing the practical application of MILs in CO₂ utilization and green cyclic carbonate production.

Fig. 7 Schematic illustration of the CO₂ cycloaddition reaction catalyzed by defective MIL-101.

4. Conclusion

This work demonstrated that targeted ligand-defect engineering of MIL-101(Cr) via low-temperature calcination (300 °C, 10 min, in air) unlocks exceptional catalytic efficiency for CO₂ cycloaddition under ambient conditions. By selectively removing organic linkers while preserving crystallinity, this method exposes coordinatively unsaturated Cr³⁺ Lewis acid sites, which drive epoxide activation through oxygen polarization and reduce the kinetic barrier to 17.4 kJ mol⁻¹. As a result, the resultant defective MIL-101 enabled a >99% cyclic carbonate yield within 6 h at 80 °C and 0.1 MPa CO₂. Crucially, characterizations such as XPS, NH₃-TPD and TG establish a structure–activity relationship, revealing how ligand deficiency enhances Lewis acidity to favor host–guest interactions, offering a sustainable paradigm for energy-efficient CO₂ utilization and green cyclic carbonate production.

Conflicts of interest

There is no declaration of competing interest.

Data availability

Data will be available on request.

Supplementary information (SI): detailed information for material characterizations, calculation of linker missing number, calculation of conversion, selectivity, yield and activation energy; TEM image of PM-300/10; XRD patterns, N₂ sorption isotherms of pristine and recycled PM-300/10 and SEM image of recycled PM-300/10; yields at different reaction temperatures for MIL-101 and PM-300/10; table of pore structure information for MILs, and performance comparison of catalysts reported in the literature and as-synthesized PM-300/10. See DOI: https://doi.org/10.1039/d5ta07230a.

Acknowledgements

The authors are grateful for the financial support of the National Natural Science Foundation of China (21808112). We also thank Advanced Analysis & Testing Center, Nanjing Forestry University for sample tests.

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