Si-doped γ-Al₂O₃ from solid-phase grinding for highly efficient one-step production of L,L-lactide

Abstract

L,L-Lactide, the important precursor of biodegradable polylactic acid, has attracted significant attention. The one-step synthesis based on nanoconfinement primarily utilizes a porous catalyst to induce the directional cyclization of L-lactic acid dimers into L,L-lactide; however, the reported porous catalysts suffer from complex fabrication methods and high costs. We have developed a new Si-doped γ-Al₂O₃ catalyst via solid-phase grinding (SPG-Si-Al₂O₃), where the incorporation of trace Si precisely modulated the pore size to create an optimal spatial confinement environment for directing L-lactic acid conversion to L,L-lactide. After a 3 h 170 °C reaction of 0.5 g of 90 wt% L-lactic acid and 0.25 g of catalyst in 10 mL of o-xylene, L,L-lactide was obtained with a yield of 77.9% (determined using an HPLC external standard method), purity of 79.4% (determined by ¹H NMR), and 100% optical selectivity (analyzed by chiral GC). By increasing the amount of catalyst to 0.30 g, 82% yield and purity can be achieved; by prolonging the reaction time to 5 h, 92% yield and 83.3% purity were achieved. Furthermore, after mild leaching regeneration, the catalyst maintains an L,L-lactide yield of around 80% over 5 cycles. The proposed catalyst is simply synthesized and low-cost, and this work provides a simple, low-energy, and low-cost approach for L,L-lactide production.

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Green foundation

1. The liquid-phase one-step production of lactide is both energy- and cost-saving, and the key lies in the use of porous confinement catalysts. We report the greenest catalyst in views of safe composition and preparation method.

2. The catalyst is Si-doped γ-Al₂O₃, made from a simple and easily scaled-up strategy of solid-phase grinding.

3. The proposed catalyst not only can achieve the one-step conversion of L-lactic acid into L,L-lactide with a high yield of 93.4% and high purity of 83.3% (100% optical purity), which is at the top level among those reported in the literature, but also has the shortest fabrication period and the lowest cost.

Introduction

Plastic pollution has turned out to be one of the most pressing environmental threats.^1–3 To address that, biodegradable plastics are being developed and have made impressive advances.^4–6 As one of the most well-known commercial biomass-based, biodegradable and biocompatible polymers, polylactic acid (PLA) has proved to be an effective alternative to petroleum-based plastics.^7–10 To date, two general methods have been developed to synthesize PLA: one is the self-polycondensation of lactic acid (LA) and another is the ring-opening polymerization (ROP) of lactide (LD).^11–15 Owing to the advantages in achieving much higher molecular weight polymers and precisely controlling the polydispersity index of resultant polymers, the ROP of LD is currently the preferred route in industrial production.¹⁶

Commercial LD is typically manufactured via a two-step process, wherein the LA oligomers are first formed through the thermal dehydration of LA and then the depolymerization of the oligomers and cyclization over non-recyclable soluble metal salt catalysts are conducted.^17–19 High temperature and vacuum conditions are inevitable in the two-step process.²⁰ Meanwhile, the soluble metal salt catalysts make the post-treatment process cumbersome and generate more toxic waste, and racemization at high temperatures (usually 200–250 °C) is apparently harmful for the quality of LD and the resultant PLA.^21–23 In order to overcome these shortcomings of the two-step method, scientists have developed one-step methods, including gas-phase and liquid-phase processes.²⁴ The former generally converts aqueous LA into steam (or methyl lactate) and then catalytically transforms it into LD via a catalyst-fixed-bed reactor, in which high temperature and pressure-tightness are required.^25–28 The latter mainly uses porous catalysts, including shape-selective zeolites,^29–32 covalent organic frameworks (COFs),³³ metal–organic frameworks (MOFs),³⁴etc., to spatially confine the direct dehydration and in situ cyclization of two LA molecules to LD. However, the requirements for large doses of catalyst (usually 50% of LA) and the prohibitive synthetic cost and long synthesis times for conventional confinement catalysts (e.g., zeolites, COFs, Scheme 1a and b)^30,31,33 drive up the manufacturing costs of LD, hindering PLA mass production and limiting its competitiveness with petroleum-based plastics. Therefore, it remains an urgent need to develop catalysts with low costs and short synthesis times to prepare L,L-LD of high purity and high yield.

Scheme 1 The simplified fabrication and synthesis times of the typical catalysts reported: (a) zeolites and (b) COFs; and (c) SPG-Si-Al₂O₃ used in this work for one-step production of L,L-LD.

γ-Al₂O₃, a kind of cost-effective catalyst or catalyst support, might have great potential for the synthesis of L,L-LD.³⁵ There have been reports that the commercial γ-Al₂O₃ can improve the optical purity of LD, but it promotes further polycondensation of LA oligomers, leading to a relatively low yield of LD.³⁶ Inspired by the confinement catalysts mentioned above, we suppose that porous γ-Al₂O₃ with a suitable pore size might help to obtain L,L-LD from L-LA with high purity and high yield, but low cost.

Porous γ-Al₂O₃ can be prepared through numerous methods; the most simple and energy-efficient way is the solid-phase grinding (SPG) of NH₄HCO₃ and Al salts, wherein the surface area and pore volume of the resultant γ-Al₂O₃ can be easily adjusted by changing the feeding ratios of reactants and calcination temperature.³⁷ Besides, this SPG strategy allows the addition of dopants, which is very convenient to achieve the synthesis of doped γ-Al₂O₃, and Si-doping has the potential to alter the pore channels, pore volume, surface area as well as the surface acidic sites of γ-Al₂O₃.^38–40

In this work, we prepare Si-doped γ-Al₂O₃via a simple SPG method (named SPG-Si-Al₂O₃) to construct an appropriate confining microenvironment for promoting the one-step selective conversion of L-LA into L,L-LD (Scheme 1c). The main reactants include NH₄HCO₃, Al(NO₃)₃·9H₂O, tetraethoxysilane (TEOS, for Si dopant only) and polyethylene glycol 400 (PEG-400). The synthesis and characterization of Al₂O₃-based catalysts with different methods, different precursors and various feeding ratios, the one-step catalysis of L-LA to L,L-LD, and the mechanism are systematically discussed. The results demonstrate that the proposed SPG-Si-Al₂O₃ leads to superfine confinement for restricting the formation of LA oligomers, and both the L,L-LD yield and purity can reach up to 82% when 0.5 g of 90 wt% L-lactic acid and 0.30 g of catalyst in 10 mL of o-xylene were reacted at 170 °C for 3 h. To the best of our knowledge, this is the simplest and green confinement catalyst for producing L,L-LD with high purity and yield, and would find wide application in the field of biocompatible polymers.

Results and discussion

Synthesis and characterization of SPG-Si-Al₂O₃

It is well-known that the surface feature and catalytic effect of γ-Al₂O₃ are related to the precursors and synthesis methods, so we designed several synthetic strategies (detailed operation is given in the SI). Among them, the ammonium aluminum carbonate hydroxide (AACH) precursor made from the SPG method was mainly used. A different Al salt of Al₂(SO₄)₃·18H₂O (the resultant product named SPG-Al₂(SO₄)₃-Si-Al₂O₃) and different feeding ratios of NH₄HCO₃/Al(NO₃)₃·9H₂O/TEOS were tested. The same feeding ratio of reactants but a different synthesis method using a hydrothermal process (the resultant product marked as HT-Si-Al₂O₃) and the same hydrothermal process but different precursors (final products named HT-AACH-Al₂O₃ and HT-AlOOH-Al₂O₃, respectively) were also tested for comparison.

We first discuss the γ-Al₂O₃ synthesized from different methods. As shown in Fig. S1, except for the amorphous SPG-Al₂(SO₄)₃-Si-Al₂O₃, others show the X-ray diffraction (XRD) patterns of the crystalline γ-Al₂O₃ phase (JCPDS #01-075-0921)³⁷ as those acquired via the SPG method, as shown in Fig. 1a. Compared with the undoped SPG-Al₂O₃, the Si-doped SPG-Si-Al₂O₃ with different doping amounts of Si (0.18%, 0.36% and 0.54% of Si/Al) and feeding ratios of NH₄HCO₃/Al (3 : 1, 4 : 1, and 5 : 1) have slightly lower-shifted peaks at 37.1, 45.9 and 66.9 degrees in XRD patterns (Fig. S2), which indicates that Si has been incorporated into the lattice of Al₂O₃. Besides, the different Al salt, various feeding ratios of NH₄HCO₃/Al/Si and precursors exhibit significant influence on the surface area and the pore size (Fig. S3, Fig. 1b, c and Table S1). The AACH precursor results in a larger surface area and pore volume than the AlOOH precursor, and the SPG process also leads to a larger surface area and pore volume than the hydrothermal process. Replacing Al(NO₃)₃·9H₂O with Al₂(SO₄)₃·18H₂O results in amorphous Al₂O₃ with a minimum surface area of 49.4 m² g⁻¹ and a pore volume of 0.07 mL g⁻¹.

Fig. 1 (a) XRD patterns; (b) N₂ adsorption–desorption isotherms; and (c) pore size distribution profiles of SPG-Al₂O₃ and SPG-Si-Al₂O₃ with different feeding ratios of NH₄HCO₃/Al/Si as indicated; (d) elemental distribution mapping of SPG-Si-Al₂O₃ with NH₄HCO₃/Al/Si at 400/100/0.36.

The feeding ratios of NH₄HCO₃/Al noticeably alter the surface features, and the 4 : 1 ratio results in the highest surface area among the ratios investigated, with the most abundant smaller pores of 3.8 nm. Compared with the pure SPG-Al₂O₃ without Si doping (NH₄HCO₃/Al = 4 : 1), the doping of Si at a Si/Al ratio of 0.18% significantly decreases the larger pores of ∼16 nm, but increases the distribution of smaller pores at 7.7 and 3.8 nm, which leads to obvious enlargement of the surface area from 340.8 m² g⁻¹ to 387.8 m² g⁻¹. When further increasing the Si/Al ratio to 0.36%, the distribution of 16 nm and 7.7 nm pores is continuously reduced, and that of 3.8 nm becomes dominant, while the surface area is maintained at 387.2 m² g⁻¹. When the Si/Al ratio is set at 0.54%, the distribution of smaller pores at 3.8 nm is less, while that of the pores at 8 and 16 nm becomes more, and the surface area is 362.6 m² g⁻¹.

From the above results, we can see that SPG of the reactants with NH₄HCO₃/Al/Si at a 400/100/0.36 ratio produces SPG-Si-Al₂O₃ with a high surface area of 387.2 m² g⁻¹ and the most abundant pores of 3.8 nm, which might have the best confinement to restrict the formation of LA oligomers (will be proved in the subsequent discussion). Thus, in the following text, SPG-Si-Al₂O₃ prepared from NH₄HCO₃/Al/Si at 400/100/0.36 is the focus of the follow-up discussion.

As shown in Fig. 1d, SPG-Si-Al₂O₃ has the morphology of nanoparticles, and the Al, O and Si elements are homogeneously distributed within the nanoparticles. Since the catalytic capability of the γ-Al₂O₃ catalysts is usually related to the acidic sites, the pyridine infrared spectra of SPG-Al₂O₃ (purple line) and SPG-Si-Al₂O₃ with 0.36% of Si (red line) are recorded to disclose the effect of Si-doping on the acidic sites. As shown in Fig. 2, the Lewis acidic sites (denoted as L) at 1444 cm⁻¹ can be clearly seen, while the Brønsted acidic sites (denoted as B) at 1540 cm⁻¹ show negligible signal in all spectra at 40, 200 and 350 °C.⁴¹ The Lewis acidic sites mainly originate from Al(III) on the γ-Al₂O₃ surface, and the doping of a small quantity of Si will cover a few Al(III) sites and result in a slight decrease in the amount of Lewis acidic sites (Table S2).

Fig. 2 The pyridine infrared spectra of SPG-Al₂O₃ (NH₄HCO₃/Al = 400/100, purple lines) and SPG-Si-Al₂O₃ (NH₄HCO₃/Al/Si = 400/100/0.36, red lines).

One-step catalytic synthesis of L,L-LD

Azeotropic distillation was set up for the catalytic reactions (Fig. S4). The SPG-Si-Al₂O₃ made from NH₄HCO₃/Al/Si at a 400/100/0.36 ratio and SPG-Al₂O₃ without Si were used as catalysts. A catalyst-free reaction was also performed for demonstrating the confinement effect of the catalysts. 90 wt% LA was used as the reactant and o-xylene was used as the solvent (detailed operation in the SI). The supernatant centrifuged from the reaction mixture after catalysis (marked as the reaction liquid) is dried by nitrogen blowing, re-dissolved and analysed by high-performance liquid chromatography (HPLC) with a diode array detector and/or mass spectrometry (MS) detector, ¹H-NMR and gas chromatography (GC)-MS, respectively. The solid catalysts after use were harvested and washed ultrasonically three times, and the resultant supernatants assigned Extracts 1, 2 and 3 were analysed by HPLC.

The composition of the 90 wt% L-LA reactant and its adsorption on SPG-Si-Al₂O₃ and SPG-Al₂O₃ were first analyzed (Fig. S5). There are several peaks in the HPLC chromatogram of 90 wt% LA, and the biggest peak lies at the retention time (RT) of 3.3 min, which corresponds to L-LA, and the free L-LA quantified using an HPLC external standard method is about 67% (the same as the reported value).⁴² The room temperature adsorption of L-LA and oligomers of 90 wt% L-LA onto SPG-Si-Al₂O₃ and SPG-Al₂O₃ is clearly observed as revealed by the decreased peak areas in HPLC chromatograms after adsorption. SPG-Si-Al₂O₃ exhibits much higher adsorption selectivity for L-LA than for the oligomers as the decreased peak area percentages of L-LA, L₂A, L₃A, L₄A and L₅A are 39.6%, 2.0%, 1.2%, 2.8% and 2.1%, respectively. In contrast, SPG-Al₂O₃ shows lower selectivity, since the decreased peak area percentages of L-LA, L₂A, L₃A, L₄A and L₅A are 59.8%, 29.8%, 30.4%, 35.3% and 31.3%, respectively. These data demonstrate the substantial adsorption of oligomers on SPG-Al₂O₃ with large pores, but very slight adsorption of oligomers on SPG-Si-Al₂O₃ with small pores.

For illustrating the catalytic outcomes, the reactions performed for 3 h are examined as the example (Fig. 3). The reaction liquid in the absence of a catalyst contains LD at a RT of 8.2 min, but with an apparent L-LA peak at 3.3 min and more than 9 peaks of LA oligomers. In contrast, the HPLC chromatogram of the reaction liquid catalysed by SPG-Al₂O₃ shows no LA peak and much fewer oligomers, and that by SPG-Si-Al₂O₃ shows the least oligomers and no LA peak. Meanwhile, the content of L-LA and other compositions in Extracts 1–3 of SPG-Si-Al₂O₃ is less than that of SPG-Al₂O₃, but L,L-LD produced by SPG-Si-Al₂O₃ is more than that produced by SPG-Al₂O₃. The catalysis with SPG-Si-Al₂O₃ gives the yield and purity of L,L-LD in the reaction liquid of 77.9% and 79.4%, which are much higher than the corresponding values of 68.0% and 60.5% for SPG-Al₂O₃. These data give strong evidence to support the fine confinement of the proposed SPG-Al₂O₃ and SPG-Si-Al₂O₃, and the small amount of doped Si significantly promotes both L,L-LD yield and purity.

Fig. 3 HPLC chromatograms of (a) the reaction liquids of 90 wt% L-LA without a catalyst and catalysed by SPG-Al₂O₃ (NH₄HCO₃/Al = 400/100) and SPG-Si-Al₂O₃ (NH₄HCO₃/Al/Si = 400/100/0.36), and the standard solutions of L-LA, L,L-LD and o-xylene; (b and c) the Extracts 1–3 of (b) SPG-Al₂O₃ and (c) SPG-Si-Al₂O₃ (0.5 g of 90 wt% L-LA, 0.25 g of catalyst, 10 mL of o-xylene, 170 °C oil bath, 3 h).

Further elaborative analysis of the catalytic data of SPG-Si-Al₂O₃ at different reaction times reveals the composition changes during the reaction (Fig. 4). The most noteworthy feature is that L-LA and L-LA dimers in the reaction liquid of 1 h are very tiny, and they both disappear at 2 h. L,L-LD, the target product, increases gradually in 1–5 h. Other L_nA oligomers (mainly L₃A, L₄A and L₅A, as revealed by LC-MS analysis in Fig. S6) reach the maximum within 2 h, and tend to decline at 3–5 h. Consequently, the L,L-LD yield measured by HPLC and the purity measured by ¹H-NMR in reaction liquids show a gradual increase from 1 h to 5 h. For the 5 h reaction, the L,L-LD purity reaches 83.3%, and the L,L-LD yield reaches 92.0%, demonstrating the strong competition of SPG-Si-Al₂O₃ with other porous catalysts (Table S3).^{26,30–34,43–45}

Fig. 4 The catalytic reactions catalysed by SPG-Si-Al₂O₃ (NH₄HCO₃/Al/Si = 400/100/0.36): (a) HPLC chromatograms; (b) ¹H-NMR and (c) the L,L-LD yield measured by HPLC and purity determined from ¹H-NMR of the reaction liquids at 1–5 h (0.5 g of 90 wt% L-LA, 0.25 g of catalyst, 10 mL of o-xylene, 170 °C oil bath).

Besides, the LD catalysed by the proposed SPG-Si-Al₂O₃ has 100% optical purity as ascertained by the chiral GC-MS (Fig. 5) and ¹H-NMR analyses (Fig. 4b). In chiral GC-MS analysis, the two peaks with RTs of 11.082 and 11.507 min in the total ion chromatogram (TIC) of the racemic-LD show the same mass spectra, while only one peak at 11.082 min exists in the TIC of standard L,L-LD and the reaction liquid, proving that only L,L-LD has been produced when using L-LA as the reactant. Meanwhile, the total absence of a signal at 5.35–5.40 ppm for meso-LD further supports the 100% optical purity of the catalytic products.⁴⁶

Fig. 5 GC-MS analysis of racemic LD, the standard L,L-LD and the reaction liquid: (a) TIC, (b and c) mass spectra of the composition at RT of (b) 11.082 min and (c) 11.507 min (reaction conditions as in Fig. 3).

The higher reaction temperature when using a solvent with higher boiling point is beneficial for acquiring higher yield and purity of L,L-LD (Fig. S7a and b), thus L-LA conversion to L,L-LD catalysed by the proposed SPG-Si-Al₂O₃ also runs with the ring closing of L-LA dimers as a determining step of the reaction rate. o-Xylene shows the highest yield and purity, and its volume exhibits great influence on the conversion (Fig. S7c and d). Low volume (5 mL) is insufficient for the dispersion of the catalyst and L-LA, and results in low L,L-LD yield; however, the purity of L,L-LD in the reaction liquid is very similar to that for volumes of 10, 15 and 20 mL. This is most possibly ascribed to the adsorption of L-LA and LA oligomers on SPG-Si-Al₂O₃ during the reaction. When 15 mL of o-xylene is used, a yield of 93.4% and purity of 80.6% can be achieved.

No obvious deactivation is observed for SPG-Si-Al₂O₃ recovered by simply washing with a 50/50 (v/v) acetonitrile/water mixture and drying in an air oven at 80 °C. As shown in Fig. S8, the proposed catalyst maintains an L,L-LD yield of nearly 80% in all 5 cycles (for the detailed ¹H-NMR and HPLC chromatograms, see Fig. S9). A gradual decrease in L,L-LD purity is observed upon recycling, which is ascribed to the decreased surface area and pore volume as shown in Fig. S10. Fortunately, the proposed catalyst is very cost-effective and composed of the Earth-abundant Al and Si elements; thus, even catalyst disposal will lead to negligible pollution.

Mechanism of one-step catalysis by SPG-Si-Al₂O₃

To understand why the low-cost and simply-synthesized SPG-Si-Al₂O₃ can produce highly pure L,L-LD with high yield, we compared the catalytic outcomes of different SPG-Si-Al₂O₃ dosages, SPG-Si-Al₂O₃ synthesized with different feeding ratios of NH₄HCO₃/Al/Si, and other Al₂O₃ catalysts prepared from other precursors and synthetic methods.

Fig. S11 shows the data of the reaction liquid catalysed with various dosages of SPG-Si-Al₂O₃ (NH₄HCO₃/Al/Si = 400/100/0.36). In the range of 0.15–0.30 g, both the yield and purity of L,L-LD keep increasing with the increase of dosage, while when the dosage is further increased to 0.35 g, the purity slightly reduces to 79.0% and the yield decreases to 55.9%. This asynchronous change is mainly caused by the simultaneous decrease of L,L-LD and LA oligomers at 0.35 g dosage. For confinement catalysis, the relative amount of catalysts and the reactants is crucial. With the same L-LA feeding, if the catalyst amount is low (0.15 and 0.20 g), many L-LA molecules cannot be dispersed into the pores of the catalyst, and they dehydrate themselves and the restriction of LA oligomers is decreased accordingly. In contrast, if the catalyst is superfluous, the adsorbed L-LA becomes more and fewer L-LA molecules can take part in the reactions. In our case, 0.3 g of catalyst and 0.50 g of 90 wt% L-LA suit very well, and both the yield and purity reach 82.0%.

The catalytic data for SPG-Si-Al₂O₃ produced with different feeding ratios of NH₄HCO₃/Al/Si (Fig. 6) and other Al₂O₃ catalysts prepared from other precursors and synthetic methods (Fig. 7 and Fig. S12) further support the confinement effect. Fig. 6 shows that the NH₄HCO₃/Al ratio of 4 : 1 results in the same L,L-LD purity as that for 3 : 1, but leads to the highest L,L-LD yield, which corresponds to the larger surface area and pore volume, and more abundant small pores of 3.8 nm for the 4 : 1 ratio (Table S1 and Fig. 1c). The Si-doping obviously promotes the abundance of the 3.8 nm pores, thus preventing the formation of LA oligomers and enhancing the L,L-LD purity of the reaction liquids. Meanwhile, the abundance of the 3.8 nm pores is also in line with the L,L-LD yield, the more 3.8 nm pores, the higher LD yields are observed.

Fig. 6 L,L-LD purity and yield of the reaction liquids catalyzed by SPG-Si-Al₂O₃ at different feeding ratios of (a and b) NH₄HCO₃/Al and (c and d) Al/Si: (a and c) ¹H-NMR spectra and (b and d) HPLC chromatograms.

Fig. 7 L,L-LD purity and yield of the reaction liquids catalyzed by other Al₂O₃ catalysts prepared from other precursors and synthetic methods.

As for other precursors and synthetic methods (Fig. 7), the AACH precursor leads to a larger surface area and pore volume (which benefit the adsorption and catalysis) than AlOOH, and some pores of 3.4 nm of HT-AACH-Al₂O₃ can partly restrict the formation of LA oligomers; thus HT-AACH-Al₂O₃ results in higher L,L-LD purity than HT-AlOOH-Al₂O₃. It is worth noting that SPG-Si-Al₂O₃ and HT-Si-Al₂O₃ are prepared with the same reactants, but with SPG and hydrothermal methods, respectively. The L,L-LD yields produced by them are similar, but the L,L-LD purity obtained from SPG-Si-Al₂O₃ is much higher than that from HT-Si-Al₂O₃, which not only demonstrates the merit of the SPG strategy over the hydrothermal method, but also supports the main role of high surface area and distribution of 3.8 nm pores. The much lower L,L-LD yield and purity produced by SPG-Al₂(SO₄)₃-Si-Al₂O₃ than by SPG-Si-Al₂O₃ are in accordance with the much lower surface area and pore volume of the former than those of the latter. The amorphous structure of SPG-Al₂(SO₄)₃-Si-Al₂O₃, in contrast to γ-Al₂O₃, is another reason for its low yield.

Conclusions

We have demonstrated a simple and energy-saving SPG strategy for preparing the eco-friendly and cost-effective catalyst named SPG-Si-Al₂O₃ for one-step production of L,L-LD from L-LA. The surface feature of SPG-Si-Al₂O₃ can be easily tuned by adjusting the feeding ratios of NH₄HCO₃, Al(NO₃)₃·9H₂O, and TEOS, and the catalytic outcomes (purity and yield of L,L-LD) show close relationship with the surface feature. The high surface area and more abundant pores of about 3.8 nm have a main effect on the purity and yield of L,L-LD. The suitable Lewis acidic sites afford appropriate adsorption of L-LA onto the catalysts, and the right-sized pores restrict the formation of LA oligomers, and thereby the purity and yield of L,L-LD are elevated. Moreover, the SPG-Si-Al₂O₃ catalyst suppresses racemization, ensuring exceptional 100% optical purity of the products. Under the mild conditions of low temperature and normal pressure, the SPG-Si-Al₂O₃ catalyst can achieve L,L-LD with 83.3% purity and 93.4% yield. Besides, the yield of L,L-LD remains around 80% for 5 cycles just by the simple regeneration of the catalyst through a washing procedure. This work presents a simple, low-cost and facile catalyst for confined conversion of L-LA into L,L-LD with high yield and high purity, and the fabrication of the proposed catalyst is facile to scale-up, and thus would find promising application in the fields of LD and ecofriendly plastics.

Author contributions

Min-Min Wang and Hong Guo: methodology, investigation, data curation, formal analysis, writing – original draft preparation, and visualization. Zheng-Wu Wang and Yu-Quan Yan: data curation and formal analysis. He-Fang Wang and Yi-Zhou Zhu: conceptualization, methodology, supervision, writing – reviewing and editing, visualization, resources, and funding acquisition.

Conflicts of interest

The authors declare no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d5gc04167h.

Acknowledgements

The support by the National Natural Science Foundation of China (No. 22376105 and 21974072) and the Undergraduate Teaching Reform Research Project of Tianjin (B231005504) is greatly appreciated.

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