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Communication

Novel and Highly Efficient Carboxylative Cyclization of CO2 to 2-Oxazolidinones Using Nano-SiO2-Supported Ionic Liquid Sustainable Catalysts

by
Yulin Hu
1,
Zongyan Tang
1 and
Xiaobing Liu
2,*
1
College of Chemistry and Chemical Engineering, Anshun University, Anshun 561000, China
2
College of Chemistry and Chemical Engineering, Jinggangshan University, Ji’an 343009, China
*
Author to whom correspondence should be addressed.
Molecules 2025, 30(3), 633; https://doi.org/10.3390/molecules30030633
Submission received: 26 December 2024 / Revised: 25 January 2025 / Accepted: 30 January 2025 / Published: 31 January 2025
(This article belongs to the Special Issue Efficient Catalytic CO2 Chemical Fixation)
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Abstract

:
The conversion of CO2 into 2-oxazolidinones through carboxylative cyclization with propargylic amines is considered an effective method for utilizing waste gas as a sustainable C1 resource and mitigating the greenhouse effect. In this context, a series of nano-SiO2-supported ionic liquids have been prepared and developed as multifunctional heterogeneous catalysts in the carboxylative cyclization of propargylic amines with CO2. The catalyst IL-SbF6@nano-SiO2 demonstrated a high compatibility with various propargylic amines, achieving excellent yields (90~98%) for the desired 2-oxazolidinones under mild conditions. Additionally, IL-SbF6@nano-SiO2 can be easily separated from the reaction mixture and reused for up to six cycles without any significant activity loss. This is important for sustainable chemistry, as it reduces waste and potentially lowers costs. This study offers novel insights into the development and design of green and efficient catalysts for the synthesis of 2-oxazolidinones from carbon dioxide.

1. Introduction

The use of carbon dioxide (CO2) as an inexpensive, non-toxic, renewable, and non-flammable C1 source for the chemical construction of valuable chemicals has excellent potential from the perspective of source employment and environmental conservancy [1,2,3,4,5,6]. Among various CO2 chemical fixation approaches, the carboxylative cyclization reaction between propargylic amines and CO2, resulting in the formation of 2-oxazolidinones, has garnered significant research interest [7]. This is primarily due to the 100% atom economy of the carboxylative cyclization reaction and the extensive applications of target products 2-oxazolidinones in chemical intermediates, pharmaceuticals, and auxiliaries [8]. In this regard, numerous catalytic systems, such as AgNO3/DBU [9], ZnCl2(TBD)2 [10], CuI/DBU [11], MOFs [12,13,14,15], Ag@TpPa-1/DBU [16], COFs [17,18,19], Pd(II)@bpy-CTF [20], MCM-41 [21], and dendritic NHC–gold(I) complex [22], have been explored for the carboxylative cyclization of propargylic amines and CO2. Nevertheless, most catalytic systems use bases and toxic solvents while requiring high gas pressure, which inevitably impedes their practical application to a certain extent. Therefore, the development of novel and efficient catalysts for the chemical fixation of CO₂ into 2-oxazolidinones under mild conditions remains a highly sought-after goal.
Ionic liquids (Ils), which are low-melting salts with customizable cations and active nucleophilic anions, have been described as green solvents with significant advantages over many other catalysts, primarily due to their extraordinary activity and selectivity, good thermostability, low vapor pressure, and non-flammability [23,24,25,26,27]. By modifying their cations and/or anions, Ils can serve as efficient catalysts for the capture and conversion of CO2 into 2-oxazolidinones [28,29,30]. However, Ils encounter challenges such as catalyst separation and high viscosity, which inevitably impede their practical application to a certain extent. In this regard, there is increasing interest in the heterogenization of various Ils. Ils can be immobilized onto various carriers such as zeolite, mesoporous silica, and porous carbon material, etc., which possess the benefits of sufficient contact and easy separation of heterogeneous catalysis for CO2 transformation [31,32,33,34]. Compared with inorganic and organic carriers nano-SiO2 nanoparticles possess unique structural features such as their large surface area, well-defined surface, low price, and appropriate thermal and mechanical properties, and are an ideal type of solid carrier for the immobilization of Ils [35,36,37,38]. The literature clearly indicates that silica-supported, imidazolium-based Ils could serve as effective heterogeneous catalysts in CO2 conversions. However, compared to homogeneous Ils, most heterogeneous counterparts generally show inferior reaction conditions and conversions. With this in mind, there remains a significant demand for developing highly efficient heterogeneous catalysts that can match the catalytic performance of homogeneous imidazolium-based Ils. Herein, a series of nano-SiO2-supported ionic liquids were synthesized via a covalent bond (https://www.sciencedirect.com/topics/pharmacology-toxicology-and-pharmaceutical-science/covalent-bond, URL (accessed on 29 January 2025) linkage method. The as-developed nanocomposite catalysts were explored as the heterogeneous catalysts for the carboxylative cyclization reaction of propargylic amines and CO2, providing excellent activity to produce 2-oxazolidinones under mild conditions (Scheme 1).

2. Results and Discussion

The supported ionic liquid nanocomposites were prepared via the reaction of 7-(1H-imidazol-1-yl)heptan-1-ol and (3-chloropropyl) triethoxysilane, followed by the anion functionalization of ionic liquid with sodium salts, and the further immobilization of functionalized ionic liquid on nano-SiO2 via a grafting method, and the procedure for the preparation of supported ionic liquids was shown in Scheme S1. The synthesized supported ionic liquid nanocomposites were characterized using FT-IR spectra (Figure 1). The samples exhibited absorption peaks at about 3540–3280 cm−1 and 972–967 cm−1, attributed to the stretching vibration of the –OH. The absorption peaks at about 1062–1045 cm−1 indicated the presence of a Si–O–Si bond. The stretching vibration peaks corresponding to the CH3 and CH2 groups were observed at around 2962–2847 cm−1. The absorption peaks at about 1637–1633 cm−1 and 1257–1251 cm−1 correspond to the C=C and C–N stretching vibrations of the imidazole ring, respectively [23,26,27]. The peaks at about 726–724 cm−1 were attributed to the bending vibrations of CH2 units. Additionally, the absorption peaks at around 841 cm−1, 678 cm−1, 692 cm−1, and 746 cm−1 were attributed to the Mo-O, B-F, Sb-F, and Al-O stretching vibrations of MoO4, BF4, SbF6, and AlO2 anions, respectively [39,40,41,42]. The above results confirmed the successful immobilization modification of ionic liquid on the silica support.
The morphologies of the supported nanocomposites were characterized by scanning electron microscopy (SEM) (Figure 2). Compared with the surface of the original nano-SiO2 (Figure 2i), that of all the functionalized supported ionic liquid nanocomposites (Figure 2a–h) appeared rougher because of surface swelling during the modification. These structures resulted from the tight aggregation of irregular particles, with sizes ranging from several hundred nanometers to a few micrometers. In addition, the incorporation of functional ionic liquid groups has not caused significant structural damage to the silica support, indicating their remarkable mechanical stability. Additionally, the element mapping image (EDS) (Figure S1) demonstrated the uniform dispersion of C, N, O, Si, Mo, B, Sb, Al, and F throughout the structure of these supported ionic liquid nanocomposites, indicating the even distribution of ionic liquid units within the framework. Furthermore, the XRD spectra of the supported ionic liquid nanocomposites are presented in Figure S2. Diffraction peaks corresponding to the (02-2) plane of amorphous silica were observed at 22–25° for all samples (JCPDS Card No. 39-1425). The distinct diffraction peaks emerged at 2θ = 9.2° and 28.5°, corresponding to the (100) and (210) crystal planes of the Mo-O, respectively, (JCPDS Card No. 21-0569). Additionally, the characteristic diffraction peaks emerged at 2θ = 20.3°, 28.2°, 32.3°, 37.8°, and 47.1°, corresponding to the crystal planes of Al-O (JCPDS Card No. 10-0425). Conversely, for all the supported organic ionic liquid nanocomposites, we did not observe signals of organic ionic liquids, suggesting the uniform and low-amount ratio dispersion of these functionalized ionic liquids in silica frameworks.
The as-prepared supported ionic liquid catalysts were applied in the carboxylative cyclization of N-benzylprop-2-yn-1-amine with CO2 for the production of 3-benzyl-5-methyleneoxazolidin-2-one (Table 1). Different catalysts were tested to demonstrate the effectiveness of supported ionic liquids in this carboxylative cyclization (Table 1, entries 1–8), and the results indicated that the catalyst IL-SbF6@nano-SiO2 has superior catalytic activity, with a 97% product yield (Table 1, entry 5) compared to other supported catalysts. Additionally, the catalytic performance of bulk ionic liquids or nano-SiO2 support and the catalysis efficiency of the carboxylative cyclization were also evaluated. Under the same catalytic conditions, the bulk ionic liquids (IL-MoO4, IL-SbF6, IL-AlO2) exhibited significantly lower activity (Table 1, entries 9–11), and the reaction did not proceed in the presence of nano-SiO2 support (Table 1, entry 12). The results also demonstrated that there were no 2-oxazolidinone products in the absence of any catalysts (Table 1, entry 13). Notably, IL-SbF6@nano-SiO2 was the best catalyst for the carboxylative cyclization reaction. In addition, the amount of IL-SbF6@nano-SiO2 catalyst was optimized for the reactivity. As the catalyst amount decreased from 0.4 g to 0.1 g, 0.2 g, and 0.3 g, a notable reduction in product yield was observed (Table 1, entries 14–16). The yield reached 97% with 0.4 g of catalyst, and no significant improvement was observed with further increases in the catalyst amount (Table 1, entry 17).
Due to the exceptional activity of IL-SbF6@nano-SiO2, it was selected as the model catalyst to optimize the reaction parameters. Figure 3a illustrates the effect of different solvents on the reaction. The influence of seven solvents, namely toluene, EtOH, MeOH, DMF, H2O, acetone, and acetonitrile, on the reaction, was assessed. It was revealed that the highest product yield was achieved when H2O was employed as the solvent. Subsequently, the influence of reaction temperature was assessed. The results, presented in Figure 3b, demonstrated significant variations in yield as the reaction temperature increased to 50 °C. According to the figure, employing 50 °C is enough to complete the reaction, and a high product yield of 97% was achieved, while lowering the temperature to 30 °C resulted in a decrease in the reaction yield. After that, with a further increase in the temperature to 70 °C, the product yield was slightly decreased due to the formation of side products. The influence of CO₂ pressure on catalytic activity was investigated. As shown in Figure 3c, increasing the reaction pressure from 0.1 to 0.3 MPa significantly enhanced the catalytic activity. Further increasing the pressure to 0.5 MPa led to a slower decrease in activity. Consequently, 0.3 MPa was identified as the optimal reaction pressure. The effect of reaction time was also examined (Figure 3d). As shown, by increasing the reaction time from 1 h to 3 h, the catalytic activity increased and reached the maximum value at 3 h, and no significant improvement was observed with further increases in reaction time.
The thermal stability of the nano-SiO2@IL-SbF6 catalyst was determined using thermogravimetric analysis (TGA) (Figure S3). The initial weight loss of 2.77% observed below 100 °C was ascribed to the desorption of adsorbed water and solvent molecules. The major weight loss of 27.46% occurring above 150 °C was due to the thermal decomposition of the chemically grafted ionic liquid segment. TGA confirmed that the superior IL-SbF6@nano-SiO2 catalyst exhibit sufficient thermal stability for subsequent application in CO2 carboxylative cyclization reactions. Reusability is an important factor in the evaluation of heterogeneous catalysts. The recycling performance of IL-SbF6@nano-SiO2 was investigated using N-benzylprop-2-yn-1-amine as the model substrate under optimized conditions. In each cycle, the catalyst was recovered through filtration. After rinsing with ethanol and vacuum drying, it was reused in the next cycle. The results depicted in Figure 4 show that the yield was still as high as 90% even after six runs, indicating its commendable reusability. In addition, a hot filtration experiment was performed (Figure S4). The reaction was halted after 1.5 h, and the catalyst was promptly removed via hot filtration. The reaction mixture was then allowed to proceed under the same conditions for an additional five hours. It can be seen that there are no obvious conversions in the reaction, suggesting the heterogeneous features and good stability of the IL-SbF6@nano-SiO2 catalyst. To perform a thorough evaluation of the reusability and stability of the catalyst, the reused catalyst nano-SiO2@IL-SbF6 was subjected to characterization using FT-IR (Figure S5) and XPS spectroscopy (Figure S6) after six runs, compared with those of the fresh IL-SbF6@nano-SiO2. The recovered sample exhibited all the original characteristic peaks, which indicated that the chemical structure and composition of the reused IL-SbF6@nano-SiO2 did not change. SEM images (Figure S7) also revealed no significant changes in morphology. Therefore, the novel supported catalyst has excellent stability and recyclability for CO2 carboxylative cyclization.
To explore the applicability scope of IL-SbF6@nano-SiO2, various propargylic amines were employed under optimized conditions (Table 2). The results showcase that a series of propargylic amines with electron-donating and electron-withdrawing groups could be converted into the respective products in excellent yields (94–98%) (Table 2, entries 2–7). Interestingly, other propargylic amines of N-(thiophen-2-ylmethyl)prop-2- yn-1-amine, N-(prop-2-yn-1-yl)butan-1-amine, and N-(prop-2-yn-1-yl)cyclohexanamine were also efficiently converted to their corresponding products in excellent yields (90–97%) (Table 2, entries 8–10). These findings provide additional evidence of the effective catalytic performance of the heterogeneous supported catalyst.
In light of the experimental data and literature references [11,12,13,14,15,16], a possible reaction mechanism for the carboxylative cyclization of CO2 with propargylic amines, catalyzed by IL-SbF6@nano-SiO2, was proposed and depicted in Scheme 2. Initially, the OH groups and SbF6 of the nanocatalyst work together to activate the propargylic amines through hydrogen bonding, followed by the deprotonation of the N-H bond to form the intermediate I. The outstanding catalytic performance of IL-SbF6@nano-SiO2 may be attributed to the synergistic effect of the two active sites. Next, the intermediate I carries out an electrophilic attack on propargylic amines by CO2, forming a carbamate intermediate II. Subsequently, intramolecular cyclization ensues, wherein the negatively charged oxygen in the carbamate triggers an attack on the triple bond to form the intermediate III. This sequence culminates in the generation of the final product via the protonation process, thereby regenerating the catalyst and completing the catalytic cycle.

3. Materials and Methods

The materials and methods section of materials, synthesis of catalysts, and catalytic carboxylative cyclization is shown in the Supplementary Materials.

4. Conclusions

In summary, a type of nano-SiO2-supported ionic liquid heterogeneous catalyst was developed to catalyze the carboxylative cyclization of propargylic amines with CO2 to produce 2-oxazolidinones. Among the synthesized supported ionic liquids, IL-SbF6@nano-SiO2 showed the better catalytic property, and the reactions could proceed smoothly under mild conditions to obtain 2-oxazolidinones products in excellent yields. As a highly active and recyclable catalyst, IL-SbF6@nano-SiO2 effectively facilitated the CO2 carboxylative cyclization reaction under mild conditions and demonstrated excellent cycling performance, maintaining its catalytic activity over multiple cycles, with a negligible loss of activity. The green protocol is attractive in terms of the simplicity of the procedure, easy separation of the catalyst, environmental compatibility, high yields, recycle exploitation, and excellent isolated yields. This study offers a promising avenue for converting CO2 into high-value-added chemicals, representing a sustainable alternative to conventional methods.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/molecules30030633/s1. Scheme S1: Preparation of supported ionic liquids; Figures S1 and S2: EDS images, XRD diffractograms of supported ionic liquids, respectively; Figure S3: TGA curve of IL-SbF6@nano-SiO2 catalyst; Figure S4: Hot filtration test during the catalytic process; Figure S5: FT-IR spectra of IL-SbF6@nano-SiO2 catalyst before and after reaction; Figure S6: XPS spectra of fresh and reused IL-SbF6@nano-SiO2 catalyst; and Figure S7: SEM images of fresh and six times recycled IL-SbF6@nano-SiO2 catalyst.

Author Contributions

Conceptualization, Y.H. and X.L.; methodology, Y.H.; software, Z.T.; validation, Y.H. and X.L.; formal analysis, Z.T.; investigation, X.L.; resources, X.L.; data curation, Z.T.; writing—original draft preparation, Z.T.; writing—review and editing, X.L.; visualization, X.L.; supervision, X.L.; project administration, X.L.; funding acquisition, Y.H. and X.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China, grant number 22268023.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The authors confirm that all the data needed to support this study are presented within the article.

Acknowledgments

The authors thank Jinggangshan University for their financial support.

Conflicts of Interest

The authors declare no conflicts of interest.

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Molecules 30 00633 sch001
Scheme 1. Catalytic synthesis of 2-oxazolidinones.
Scheme 1. Catalytic synthesis of 2-oxazolidinones.
Molecules 30 00633 sch001
Molecules 30 00633 g001
Figure 1. FT-IR spectra of (a) IL−MoO4@nano−SiO2, (b) IL−OH@nano−SiO2, (c) IL−HOCH2COO@nano−SiO2, (d) IL−BF4@nano−SiO2, (e) IL−SbF6@nano−SiO2, (f) IL−CH3COO@nano−SiO2, (g) IL−AlO2@nano−SiO2, and (h) IL−HCO3@nano−SiO2.
Figure 1. FT-IR spectra of (a) IL−MoO4@nano−SiO2, (b) IL−OH@nano−SiO2, (c) IL−HOCH2COO@nano−SiO2, (d) IL−BF4@nano−SiO2, (e) IL−SbF6@nano−SiO2, (f) IL−CH3COO@nano−SiO2, (g) IL−AlO2@nano−SiO2, and (h) IL−HCO3@nano−SiO2.
Molecules 30 00633 g001
Molecules 30 00633 g002
Figure 2. SEM images of (a) IL−MoO4@nano−SiO2, (b) IL−OH@nano−SiO2, (c) IL−HOCH2COO@nano−SiO2, (d) IL−BF4@nano−SiO2, (e) IL−SbF6@nano−SiO2, (f) IL−CH3COO@nano−SiO2, (g) IL−AlO2@nano−SiO2, (h) IL−HCO3@nano−SiO2, and (i) nano−SiO2.
Figure 2. SEM images of (a) IL−MoO4@nano−SiO2, (b) IL−OH@nano−SiO2, (c) IL−HOCH2COO@nano−SiO2, (d) IL−BF4@nano−SiO2, (e) IL−SbF6@nano−SiO2, (f) IL−CH3COO@nano−SiO2, (g) IL−AlO2@nano−SiO2, (h) IL−HCO3@nano−SiO2, and (i) nano−SiO2.
Molecules 30 00633 g002
Molecules 30 00633 g003aMolecules 30 00633 g003b
Figure 3. (a) Effect of solvent (10 mmol N-benzylprop-2-yn-1-amine, 0.4 g IL-SbF6@nano-SiO2, 10 mL solvent, 0.3 MPa CO2, 50 °C, and 3 h), (b) effect of temperature (10 mmol N-benzylprop-2-yn-1-amine, 0.4 g IL-SbF6@nano-SiO2, 10 mL H2O, 0.3 MPa CO2, and 3 h), (c) effect of pressure (10 mmol N-benzylprop-2-yn-1-amine, 0.4 g IL-SbF6@nano-SiO2, 10 mL H2O, CO2, 50 °C, and 3 h), and (d) effect of reaction time on the reaction (10 mmol N-benzylprop-2-yn-1-amine, 0.4 g IL-SbF6@nano-SiO2, 10 mL H2O, 0.3 MPa CO2, and 50 °C).
Figure 3. (a) Effect of solvent (10 mmol N-benzylprop-2-yn-1-amine, 0.4 g IL-SbF6@nano-SiO2, 10 mL solvent, 0.3 MPa CO2, 50 °C, and 3 h), (b) effect of temperature (10 mmol N-benzylprop-2-yn-1-amine, 0.4 g IL-SbF6@nano-SiO2, 10 mL H2O, 0.3 MPa CO2, and 3 h), (c) effect of pressure (10 mmol N-benzylprop-2-yn-1-amine, 0.4 g IL-SbF6@nano-SiO2, 10 mL H2O, CO2, 50 °C, and 3 h), and (d) effect of reaction time on the reaction (10 mmol N-benzylprop-2-yn-1-amine, 0.4 g IL-SbF6@nano-SiO2, 10 mL H2O, 0.3 MPa CO2, and 50 °C).
Molecules 30 00633 g003aMolecules 30 00633 g003b
Molecules 30 00633 g004
Figure 4. Catalytic activity of the recycled IL-SbF6@nano-SiO2 catalyst.
Figure 4. Catalytic activity of the recycled IL-SbF6@nano-SiO2 catalyst.
Molecules 30 00633 g004
Molecules 30 00633 sch002
Scheme 2. Possible mechanism of carboxylative cyclization.
Scheme 2. Possible mechanism of carboxylative cyclization.
Molecules 30 00633 sch002
Table 1. Catalyst screening for carboxylative cyclization a.
Table 1. Catalyst screening for carboxylative cyclization a.
EntryCatalystCatalyst (g)Time (h)Yield (%) b
1IL−MoO4@nano−SiO20.4384
2IL−OH@nano−SiO20.4565
3IL−HOCH2COO@nano−SiO20.4560
4IL−BF4@nano−SiO20.4570
5IL−SbF6@nano−SiO20.4397
6IL−CH3COO@nano−SiO20.4656
7IL−AlO2@nano−SiO20.4380
8IL−HCO3@nano−SiO20.4351
9IL−MoO40.4382
10IL−SbF60.4387
11IL−AlO20.4379
12nano−SiO20.524trace
13none0240
14IL−SbF6@nano−SiO20.1549
15IL−SbF6@nano−SiO20.2567
16IL−SbF6@nano−SiO20.3388
17IL−SbF6@nano−SiO20.5397
a Reaction conditions: N-benzylprop-2-yn-1-amine (10 mmol), H2O (10 mL), CO2 (0.3 MPa), and 50 °C. b Isolated yield.
Table 2. Catalytic synthesis of various 2-oxazolidinones with IL-SbF6@nano-SiO2 a.
Table 2. Catalytic synthesis of various 2-oxazolidinones with IL-SbF6@nano-SiO2 a.
EntrySubstrateProductTime (h)Yield (%) b
1Molecules 30 00633 i001Molecules 30 00633 i002397
2Molecules 30 00633 i003Molecules 30 00633 i004396
3Molecules 30 00633 i005Molecules 30 00633 i006396
4Molecules 30 00633 i007Molecules 30 00633 i008398
5Molecules 30 00633 i009Molecules 30 00633 i010398
6Molecules 30 00633 i011Molecules 30 00633 i012396
7Molecules 30 00633 i013Molecules 30 00633 i014394
8Molecules 30 00633 i015Molecules 30 00633 i016397
9Molecules 30 00633 i017Molecules 30 00633 i018492
10Molecules 30 00633 i019Molecules 30 00633 i020490
a Reaction conditions: propargylic amine (10 mmol), IL-SbF6@nano-SiO2 (0.4 g), H2O (10 mL), CO2 (0.3 MPa), and 50 °C. b Isolated yield.
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MDPI and ACS Style

Hu, Y.; Tang, Z.; Liu, X. Novel and Highly Efficient Carboxylative Cyclization of CO2 to 2-Oxazolidinones Using Nano-SiO2-Supported Ionic Liquid Sustainable Catalysts. Molecules 2025, 30, 633. https://doi.org/10.3390/molecules30030633

AMA Style

Hu Y, Tang Z, Liu X. Novel and Highly Efficient Carboxylative Cyclization of CO2 to 2-Oxazolidinones Using Nano-SiO2-Supported Ionic Liquid Sustainable Catalysts. Molecules. 2025; 30(3):633. https://doi.org/10.3390/molecules30030633

Chicago/Turabian Style

Hu, Yulin, Zongyan Tang, and Xiaobing Liu. 2025. "Novel and Highly Efficient Carboxylative Cyclization of CO2 to 2-Oxazolidinones Using Nano-SiO2-Supported Ionic Liquid Sustainable Catalysts" Molecules 30, no. 3: 633. https://doi.org/10.3390/molecules30030633

APA Style

Hu, Y., Tang, Z., & Liu, X. (2025). Novel and Highly Efficient Carboxylative Cyclization of CO2 to 2-Oxazolidinones Using Nano-SiO2-Supported Ionic Liquid Sustainable Catalysts. Molecules, 30(3), 633. https://doi.org/10.3390/molecules30030633

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