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Communication

Zinc Iodide-Metal Chloride-Organic Base: An Efficient Catalytic System for Synthesis of Cyclic Carbonates from Carbon Dioxide and Epoxides under Ambient Conditions

by
Masatoshi Mihara
*,
Shuichi Nakao
,
Takeo Nakai
and
Takumi Mizuno
Osaka Research Institute of Industrial Science and Technology, 1-6-50 Morinomiya, Joto-ku, Osaka 536-8553, Japan
*
Author to whom correspondence should be addressed.
Catalysts 2023, 13(8), 1214; https://doi.org/10.3390/catal13081214
Submission received: 21 June 2023 / Revised: 9 August 2023 / Accepted: 10 August 2023 / Published: 16 August 2023
(This article belongs to the Special Issue Recent Progress of Catalysis in “Dual Carbon Targets”)
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Abstract

:
Development of an effective catalytic system for the cycloaddition of carbon dioxide to epoxides for the preparation of cyclic carbonates under mild conditions is of great importance. Herein, a mixture of zinc iodide, metal chlorides, and strong organic bases is demonstrated to be a useful catalytic system that works at room temperature under atmospheric pressure. The most efficient combination, zinc iodide-niobium chloride-7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (1.2-0.3-3.0 mol%), gave styrene carbonate (95%) from styrene oxide and CO2 (balloon) at 25 °C for 24 h. Another combination, zinc iodide-zinc chloride-1,8-diazabicyclo[5.4.0]undec-7-ene (1.2-0.8-4.0 mol%), kept the catalytic activity for the preparation of propylene carbonate until the fourth run. Therefore, the reaction system was operationally simple, highly efficient, and proceeded under ambient conditions. The catalyst is composed of readily available reagents and is reusable. Thus, the method presented is a powerful tool for utilizing CO2 as the starting material for the production of valuable chemicals.

Graphical Abstract

1. Introduction

CO2 is regarded as an ideal carbon source for organic synthesis because it is inexpensive, abundant, nontoxic, and renewable. Therefore, efficient transformation of CO2 into valuable chemicals is important for creating greener and more sustainable industries. The cycloaddition of CO2 and epoxides to afford five-membered cyclic carbonates is one of the most promising methods for utilizing CO2, owing to the 100% atom economy of the reaction. Carbonates are widely used as polar aprotic solvents, electrolytes in lithium-ion batteries, and intermediates in the production of pharmaceuticals and fine chemicals. Accordingly, a wide range of methods involving metal-based catalysts and organocatalysts have been reported for the synthesis of cyclic carbonates using CO2 [1,2,3]. For example, the catalytic components employed in this study—zinc halides [4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19], NbCl5 [20,21,22,23,24], and 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD) [25]—have been previously used as catalysts for the cycloaddition of CO2, often in combination with onium salts. In addition, zinc-based heterogeneous catalysts have been continuously reported [4,5,6,8,9,11,12,13,14,17,18,19,26,27,28,29,30]. Despite the extensive research in this area, pressurizing or/and heating are usually required to obtain cyclic carbonates from CO2 and epoxides in satisfactory yields.
Owing to the growing environmental and energy concerns, the development of synthetic methodologies for the preparation of cyclic carbonates by harnessing CO2 under mild conditions is gaining interest. Recently, metal complexes [31,32,33,34,35,36,37,38,39,40,41,42], metal-organic frameworks [43,44,45], and solid-supported reagents [46,47,48,49,50,51,52] have been reported for facile conversion of CO2 and epoxides into cyclic carbonates, even under ambient conditions (around 25 °C under 0.1 MPa). However, these materials typically have complex chemical structures with relatively high molecular weights. Organocatalysts have been utilized under ambient conditions; however, they usually require loadings greater than 5 mol% [53,54,55,56,57,58,59]. These limitations have spurred catalytic research toward the development of more effective and practical methods for the synthesis of cyclic carbonates under milder conditions.
We recently reported the selective guanidine-accelerated synthesis of carbonates from CO2, glycerol, and alkyl halides [60]. In this study, we observed that a zinc complex formed between ZnI2 and guanidine displayed catalytic activity for the cycloaddition of CO2 and epoxides. Continuing our investigations in this area of research, we report the development of an efficient catalytic system for the conversion of CO2 to cyclic carbonates using CO2 at room temperature and atmospheric pressure. The catalyst was developed using simple metal halides and strong organic bases, and was catalytically active under ambient conditions.

2. Results

Synthesis of styrene carbonate, 2a, from styrene oxide, 1a, and CO2 was investigated as a model reaction in the presence of metal halides and guanidine (Table 1). The combination of ZnI2 and pentaalkylguanidine led to a 26% conversion to 2a (entry 1). In contrast, the addition of ZnCl2 as a cocatalyst substantially improved the conversion (entry 2). Catalytic activity almost completely disappeared in the absence of ZnI2 and/or guanidine (entries 3 and 4). Thus, the combination of ZnI2, ZnCl2, and guanidine enhances the catalytic activity, indicating the presence of synergistic effects that contribute to the acceleration of the reaction.
The catalyst combination was further optimized by evaluating several metal chlorides, organic bases, and metal iodides to replace ZnCl2, N,N′,N′,N′′,N′′-tert-butyltetramethylguanidine, and ZnI2, respectively (Table 2). The effect of the metal chlorides was first examined under the conditions described in entry 2 of Table 1 (combination A). Although all metal chlorides enhanced the catalytic reaction, their effects were significantly different. NbCl5 afforded the best conversion (83%), which might be due to the suitable Lewis acidity of NbCl5 for the cycloaddition in the presence of guanidines and ZnI2. The effect of organic bases was then examined using NbCl5 as a metal halide (combination B). The results indicated that a strong basicity was necessary to achieve good conversion, which was further evidenced by the low conversion observed for imidazole. Furthermore, the presence of sterically demanding substituents on the guanidines did not affect the cycloaddition conversion, and MTBD demonstrated the best conversion (86%). The effect of the metal iodide was further examined using NbCl5 and MTBD (combination C). ZnI2 was found to be the best iodide anion source despite its low number of iodine atoms per metal center. The catalytic activity of ZnI2 was higher than that of ZnBr2, which can be attributed to the facile generation of the iodide anion from ZnI2 compared with the generation of the bromide anion from ZnBr2. Additionally, although NbCl5-Bu4NBr (0.5–1 mol%) was reported to be an effective catalyst under mild conditions (45 °C) [20], this catalytic system did not deliver good results (conversion: 11%) under the present conditions (NbCl5-Bu4NBr: 0.5–2 mol%, 25 °C). Our optimization results indicate that ZnI2-NbCl5-MTBD is the best combination, which probably leads to effective cooperative catalysis of acidic sites and iodide anions.
ZnI2 and MTBD complexes were prepared prior to NbCl5 addition, and the catalytic system prepared by this process was used for the cycloaddition reaction. However, the simultaneous mixing of ZnI2, MTBD, and NbCl5 did not affect product conversion (Table 3). Therefore, NbCl5 was found to be effective upon the direct addition of free guanidine. We further observed that the conversion depended significantly on the ZnI2, MTBD, and NbCl5 mole ratio. The use of a suitable ratio led to almost complete consumption of the starting material with a prolonged reaction time [ZnI2:MTBD:NbCl5 = 1.2:3.0:0.3, reaction time (24 h), conversion (99%), and isolated yield (95%)]. Overall, the optimized method is operationally simple, does not require the preformation of the catalyst, and allows the efficient cycloaddition of CO2 and epoxides under ambient conditions.
The optimized conditions were used for the conversion of several epoxides in the presence of CO2, and the corresponding five-membered cyclic carbonates were isolated in moderate to excellent yields (Table 4). Various terminal epoxides bearing aryl, alkyl, ether, chloro, long alkyl chains, and unsaturated groups reacted smoothly with CO2 (atmospheric pressure) at room temperature to furnish the corresponding carbonates in high yields (entries 1–6). The bis-epoxide was also converted to the corresponding bis-cyclic carbonate in excellent yield (entry 7). However, the use of N-methylpyrrolidone (NMP) as a solvent was required to homogenize the reaction mixture in cases where the product precipitation lowered the yield by hindering continuous stirring of the reaction mixture. Accordingly, the ZnI2-NbCl5-MTBD-catalyst system allowed the selective cycloaddition of CO2 with terminal epoxides under solvent-free conditions and proceeded without by-product formation. However, internal epoxides did not afford the desired carbonates under ambient conditions. Nevertheless, the desired cycloaddition was achieved by increasing the reaction temperature and CO2 pressure (entries 8 and 9). Remarkably, even a sterically hindered internal epoxide was converted to the corresponding carbonate product in moderate yield.
After studying the scope of the catalytic reaction, we evaluated the reusability of the catalyst system. The reusability of ZnI2-NbCl5-MTBD was assessed using the reaction of propylene oxide (1b) with CO2 (Table 5). After the first experiment, the product was isolated by the distillation of the crude mixture because of the homogeneous system, and the residual catalyst in the reaction vessel was used for two subsequent runs under the same reaction conditions without any pretreatment. Unfortunately, a slight decrease in yield was observed in the second run, which was maintained in the third run (catalyst A). The reduction in catalytic efficiency can be attributed to the partial decomposition of NbCl5 and MTBD during distillation. Therefore, we further examined the cycloaddition using a metal chloride and an organic base, which are less moisture- and heat-sensitive under similar conditions. The use of ZnCl2 and DBU instead of NbCl5 and MTBD led to similar conversions during the recycling experiments, although larger quantities of the reagents were required (catalyst B). Thus, the ZnI2-ZnCl2-DBU system is a highly stable cycloaddition catalyst that does not display any significant decrease in catalytic activity until the fourth run.

3. Discussion

The catalytic system was examined by 1H-NMR in DMSO-d6 using tetramethylsilane as an internal standard. The addition of ZnI2 or NbCl5 to the MTBD led to a downfield shift in the peaks corresponding to the MTBD (Figure 1A). In addition, the addition of NbCl5 to the MTBD and ZnI2 resulted in a further downfield shift in the MTBD peaks. These changes are indicative of MTBD coordination to ZnI2 and NbCl5. Moreover, the addition of styrene oxide (1a) to the complexes led to a slight downfield shift of the 1a signals, which suggests coordination of the formed complexes with 1a (Figure 1B). However, because bubbling CO2 into the mixture did not cause changes in the chemical shifts, the MTBD complex coordinated with ZnI2 and NbCl5 might be less able to interact with CO2. Furthermore, the coordination of ZnI2 and NbCl5 to MTBD was also confirmed by the changes in the IR spectra (Figure S1). Accordingly, considering the key contribution of iodide to the catalyst activity (almost no reaction occurred in the absence of ZnI2, Table 1, entries 2 vs. 3), the main catalytic effect should operate via the synergistic activation of epoxides by the acidity of the niobium and zinc complexes, followed by the nucleophilic attack of the iodide anion. The proposed catalytic cycle for the synthesis of cyclic carbonates is shown in Scheme 1 [61,62,63]. The epoxide ring was activated by coordination with the Nb and Zn acidic sites. The iodide anion generated from the reaction between ZnI2 and guanidines simultaneously attacks the epoxide ring to form an intermediate [I] (path A). The other path B, including the attack of MTBD followed by exchanging the MTBD moiety by the iodide, can also be proposed. Nucleophilic addition of the alkoxide to CO2 forms a hemicarbonate intermediate [II]. Because the iodide anion is a good leaving group, the ring closure of the metal carbonate occurs smoothly, affording the desired cyclic carbonate.
Previous reports on styrene carbonate synthesis revealed that the preceding combination of metal halides and organic bases afforded good yields under severe conditions. For example, SnCl4 [64] or ZnI2 [5,13,15] were employed in combination with bases under 0.3 MPa-75 °C or 1–3 MPa-60–150 °C, respectively. Additionally, ZnI2 [6,7,8] or NbCl5 [24] in combination with base-HX also were used under heating and pressurized CO2 conditions (30–130 °C, 1–2 MPa). Therefore, the higher efficiency reported herein supports the occurrence of cooperative catalysis by ZnI2, NbCl5, and MTBD. In this catalytic system, the coordination of ZnI2 and NbCl5 to MTBD may hold I− derived from ZnI2 closer to the Lewis acids and increase the solubility of their metal halides in the reaction mixture, thus enhancing the activity of this reaction. Furthermore, although several effective catalysts for the preparation of styrene carbonate, even under ambient conditions, have already been reported [31,32,33,34,35,36,37,38,39,40,41,42,44,45,46,48,49,50,51,52,53,54,55,58,59], they usually require a long time for multistep preparation (overnight to 30 days, 1–3 steps), simultaneous use of a relatively high-loading tetrabutylammonium halide (1–50 mol%), and/or the use of significant quantities despite low catalytic loadings (73–1737 mg for 10 mmol of styrene oxide). However, our reaction allows convenient coupling with a relatively low-loading catalyst composed of readily available reagents (ZnI2-NbCl5-MTBD: 92 mg for 10 mmol). In addition, while TON (mol of product/mol of metal) and TOF [TON/time (h)] of previous catalysts cited here are 16–2760 and 0.7–58 for the production of styrene carbonate, respectively, those of our one are 63 and 2.6, respectively. Thus, our catalytic system shows near average values among them except the highest TON and TOF [46] attained in the presence of tetrabutylammonium bromide (7.2 mol%) as a cocatalyst.

4. Materials and Methods

4.1. Materials

ZnI2, ZnCl2, NbCl5, DBU, MTBD, epoxides, deuterated solvents, and CO2 (99.5%) were used without further purification. NMP (super dehydrated) and EtOH (super dehydrated), purchased from Wako Pure Chemical Industries Ltd., were used without further treatment.

4.2. Methods

A general procedure for ZnI2/organic base + metal chloride-catalyzed synthesis of styrene carbonate (2a) from styrene oxide (1a) and CO2
ZnI2 (0.1 mmol), organic bases (0.22 mmol), EtOH (0.4 mL), and a magnetic stirring bar were placed in a glass vessel connected to an injection port with a 3-way cock. The mixture was stirred at 60 °C for 3 h and was evaporated around 25 °C under reduced pressure to afford the ZnI2/organic base complex. To the complex, metal chlorides (0.05 mmol) and 1a (10 mmol) were added, and the vessel was charged with CO2 from a balloon. The reaction mixture was stirred at 25 °C for 24 h under a 0.1 MPa pressure of CO2. The conversions to the desired carbonates were determined based on the 1H NMR area ratios (2a/1a+2a).
A general procedure for ZnI2-NbCl5-MTBD-catalyzed synthesis of cyclic carbonates 2 from epoxides 1 and CO2
ZnI2 (0.12 mmol), MTBD (0.3 mmol), NbCl5 (0.03 mmol), and 1 (10 mmol) were placed with a magnetic stirring bar in a glass vessel connected to an injection port with a 3-way cock. NMP was added as the solvent in the experiments for glycidyl phenyl ether (1c) and 1,2-epoxydecane (1e). The vessel was charged with CO2 from a balloon, and the reaction mixture was stirred at 25 °C for 24 h under a 0.1 MPa pressure of CO2. The obtained crude product was purified by silica gel column chromatography (silica gel 60) to isolate 2. The products 2b and 4-chloromethyl-1,3-dioxolan-2-one (2d) were purified by distillation instead of purification on silica gel column chromatography.
A procedure for reuse of ZnI2-NbCl5-MTBD and ZnI2-ZnCl2-DBU in the synthesis of propylene carbonate (2b) from propylene oxide (1b) and CO2
The carbonate 2b was synthesized from 1b and CO2 according to the procedure described above, using ZnI2-NbCl5-MTBD or ZnI2-ZnCl2-DBU. The crude 2b was purified by distillation under reduced pressure to obtain pure 2b. To the residual catalyst in the reaction vessel, fresh 1b (10 mmol) was added, CO2 was charged from a balloon, and the reaction mixture was stirred at 25 °C for 24 h in next recycle experiment. The resulting liquid was purified by distillation to afford 2b, and the residual catalyst was further used for the third run.
A procedure for 1H NMR measurement to examine the reaction mechanism.
ZnI2 (0.04 mmol), MTBD (0.1 mmol), NbCl5 (0.01 mmol), and DMSO-d6 + 0.03% TMS (0.75 mL) were placed in an NMR tube, for studying the coordination of metal halides to MTBD. The 1H NMR spectrum was acquired. After that, 1a (0.1 mmol) was added to the complex, and 1H NMR was acquired again. CO2 was bubbled into the mixture, and the 1H NMR was acquired again.

5. Conclusions

The ZnI2-NbCl5-MTBD-catalyzed synthesis of carbonates is a powerful tool for utilizing CO2 under extremely mild conditions. The catalytic system allowed the conversion of various terminal epoxides into the corresponding cyclic carbonates in high yields (85–95%) with a relatively low-loading catalyst (92 mg for 10 mmol of epoxides) under ambient conditions. The catalyst was also prepared in situ by simply mixing readily available reagents and could be used under solvent-free conditions; thus, it is a practical and environmentally benign reagent. To the best of our knowledge, no catalysts composed of two different metal halides or organic bases have been reported for the synthesis of carbonates. As has already been observed for some metal-organic framework catalysts [65,66], the use of a dual Lewis acid catalytic system in combination with nucleophiles is believed to be a promising strategy for the development of synergistic catalysts for the cycloaddition of CO2.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal13081214/s1, Characterization of Compounds 2a–i [34,67,68,69], Figure S1: IR spectra of MTBD, ZnI2+MTBD, NbCl5+MTBD, and ZnI2+NbCl5+MTBD.

Author Contributions

Conceptualization, M.M. and T.M.; methodology, T.N., S.N. and M.M.; formal analysis, T.N., S.N. and M.M.; investigation, M.M.; data curation, M.M.; writing—original draft preparation, M.M.; writing—review and editing, M.M.; supervision, T.M.; project administration, M.M.; funding acquisition, M.M., T.N. and T.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by JSPS KAKENHI Grant Number 17K05959.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 13 01214 g001 550
Figure 1. 1H-NMR of a mixture of catalysts, styrene oxide (1a), and CO2; (A) ZnI2 (0.04 mmol), NbCl5 (0.01 mmol), and MTBD (0.1 mmol); (B) ZnI2-NbCl5-MTBD, 1a (0.1 mmol), and CO2.
Figure 1. 1H-NMR of a mixture of catalysts, styrene oxide (1a), and CO2; (A) ZnI2 (0.04 mmol), NbCl5 (0.01 mmol), and MTBD (0.1 mmol); (B) ZnI2-NbCl5-MTBD, 1a (0.1 mmol), and CO2.
Catalysts 13 01214 g001
Catalysts 13 01214 sch001 550
Scheme 1. Plausible mechanism for the cycloaddition of styrene oxide and CO2.
Scheme 1. Plausible mechanism for the cycloaddition of styrene oxide and CO2.
Catalysts 13 01214 sch001
Table
Table 1. Cycloaddition of styrene oxide (1a) and CO2 in the presence of zinc halides and guanidine a.
Table 1. Cycloaddition of styrene oxide (1a) and CO2 in the presence of zinc halides and guanidine a.
Catalysts 13 01214 i001
EntryCatalystCocatalystConversion (%) b
1ZnI2/tBuN=C(NMe2)2none26
2ZnI2/tBuN=C(NMe2)2ZnCl281
3tBuN=C(NMe2)2ZnCl21
4noneZnCl20
a Reaction conditions: 1a (10 mmol), CO2 (0.1 MPa), ZnI2/tBuN=C(NMe2)2 (1.0/2.2 mol%), ZnCl2 (0.5 mol%), 25 °C, 18 h; ZnI2/tBuN=C(NMe2)2 was prepared by mixing ZnI2 and tBuN=C(NMe2)2 in ethanol followed by solvent removal. b Determined by 1H-NMR area ratio.
Table
Table 2. Metal iodide/base and metal chlorides-catalyzed synthesis of styrene carbonate (2a) from styrene oxide (1a) and CO2 a.
Table 2. Metal iodide/base and metal chlorides-catalyzed synthesis of styrene carbonate (2a) from styrene oxide (1a) and CO2 a.
Catalysts 13 01214 i002
Combination A: ZnI2/tBuN=C(NMe2)2 + metal chloride
Metal chlorideNbCl5ZnCl2ZrCl4BiCl3FeCl3MgCl2CuCl2
Conversion b (%)83817575737141
Combination B: ZnI2/base + NbCl5
Base cMTBDDBUtBuGnBuGTBDPhG1,2-dimethylimidazole
Conversion b (%)86848383806522
Combination C: Metal iodide (bromide)/MTBD + NbCl5
Metal iodide (bromide)ZnI2SnI4TiI4BiI3ZnBr2
Conversion b (%)8680555044
a Reaction conditions: 1a (10 mmol), CO2 (0.1 MPa), metal iodide/base (1.0/2.2 mol%), metal chloride (0.5 mol%), 25 °C, 18 h. b Determined by 1H-NMR area ratio. c DBU: 1,8-diazabicyclo[5.4.0]undec-7-ene, G: N=C(NMe2)2, TBD: 1,5,7-triazabicyclo[4.4.0]dec-5-ene.
Table
Table 3. Comparison of conversions to styrene carbonate (2a) with and without the preformed ZnI2/MTBD a.
Table 3. Comparison of conversions to styrene carbonate (2a) with and without the preformed ZnI2/MTBD a.
Catalysts 13 01214 i003
Catalyst bConversion c (%)
ZnI2/MTBD + NbCl5 d86
ZnI2-MTBD-NbCl5 e86
a Reaction conditions: 1a (10 mmol), CO2 (0.1 MPa), 25 °C, 18 h. b ZnI2 (1.0 mol%), MTBD (2.2 mol%), NbCl5 (0.5 mol%). c Determined by 1H-NMR area ratio. d After the preformation of ZnI2/MTBD, NbCl5 was added. e ZnI2, MTBD, and NbCl5 were added simultaneously.
Table
Table 4. Efficient synthesis of a cyclic carbonate (2) under mild conditions a.
Table 4. Efficient synthesis of a cyclic carbonate (2) under mild conditions a.
Catalysts 13 01214 i004
EntryR1R22Yield (%) b
1PhH2a95
2MeH2b90
3 cCH2OPhH2c92
4CH2ClH2d85
5 cC8H15H2e89
6CH2OCH2CH=CH2H2f89
7 dCatalysts 13 01214 i005H2g95
8 e(CH2) 42h38 f
9 ePh (trans)Ph2i53 g
a Reaction conditions: epoxides (10 mmol), CO2 (0.1 MPa), ZnI2 (1.2 mol%), NbCl5 (0.3 mol%), MTBD (3.0 mol%), 25 °C, 24 h. b Isolated yields. c NMP (1 mL) was added. d ZnI2 (2.4 mol%), NbCl5 (0.6 mol%), MTBD (6.0 mol%). e CO2 (1.0 MPa), ZnI2 (4.0 mol%), NbCl5 (1.0 mol%), MTBD (10.0 mol%), 120 °C. f Mixture of cis/trans (84/16). g Only trans.
Table
Table 5. Reuse of catalyst in the synthesis of propylene carbonate (2b) from propylene oxide (1b) and CO2 a.
Table 5. Reuse of catalyst in the synthesis of propylene carbonate (2b) from propylene oxide (1b) and CO2 a.
Catalysts 13 01214 i006
Catalyst A: ZnI2-NbCl5-MTBD b
Cycle123
Yield (%)846768
Catalyst B: ZnI2-ZnCl2-DBU c
Cycle1234
Yield (%)78818583
a Reaction conditions: 1b (10 mmol), CO2 (0.1 MPa), 25 °C, 24 h. b ZnI2 (1.2 mol%), NbCl5 (0.3 mol%), MTBD (3.0 mol%). c ZnI2 (1.2 mol%), ZnCl2 (0.8 mol%), DBU (4.0 mol%).
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Mihara, M.; Nakao, S.; Nakai, T.; Mizuno, T. Zinc Iodide-Metal Chloride-Organic Base: An Efficient Catalytic System for Synthesis of Cyclic Carbonates from Carbon Dioxide and Epoxides under Ambient Conditions. Catalysts 2023, 13, 1214. https://doi.org/10.3390/catal13081214

AMA Style

Mihara M, Nakao S, Nakai T, Mizuno T. Zinc Iodide-Metal Chloride-Organic Base: An Efficient Catalytic System for Synthesis of Cyclic Carbonates from Carbon Dioxide and Epoxides under Ambient Conditions. Catalysts. 2023; 13(8):1214. https://doi.org/10.3390/catal13081214

Chicago/Turabian Style

Mihara, Masatoshi, Shuichi Nakao, Takeo Nakai, and Takumi Mizuno. 2023. "Zinc Iodide-Metal Chloride-Organic Base: An Efficient Catalytic System for Synthesis of Cyclic Carbonates from Carbon Dioxide and Epoxides under Ambient Conditions" Catalysts 13, no. 8: 1214. https://doi.org/10.3390/catal13081214

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