2022 Volume 5 Issue 1

Open Access

INEOS OPEN, 2022, 5 (1), 7–9  Journal of Nesmeyanov Institute of Organoelement Compounds of the Russian Academy of Sciences Download PDF DOI: 10.32931/io2202a

A Mixture of Triethanolamine and Iodine as an Accessible and Effective Catalyst
for the Formation of Organic Carbonates from Epoxides and CO₂

Abstract

An effective catalyst formed by mixing molecular iodine with triethanolamine is suggested for the addition of CO₂ to epoxides.

Key words: triethanolamine, iodine, epoxides, CO₂ fixation.

 

Introduction

The high available volumes of СО₂, in particular, largely emitted as an industrial by-product, and the ease of its condensation predetermine the low cost of this gas. Furthermore, carbon dioxide is nonflammable and less toxic than many organic solvents and reagents [1]. However, wide application of СО₂ is hampered by its low solubilizing ability and inertness associated with the zero dipole moment and dielectric permittivity ranging from 1.1 to 1.5 depending on the temperature and pressure [2]. From the synthetic point of view, one of the most efficient and convenient directions of the carbon dioxide utilization is its insertion into organic molecules, resulting in the formation of organic cyclic carbonates [3–6]. This process does not require the use of a solvent since СО₂ serves simultaneously as a reaction medium and reagent. It should be noted that organic carbonates are extensively used as fuel additives, electrolytes for lithium-ion batteries, polar solvents, and monomers for the production of polycarbonates and non-isocyanate polyurethanes [5]. In order to further optimize the process, of particular interest is the development of effective catalysts that would be able to promote the coupling of СО₂ with epoxides and feature maximum affordability. Recently, we have shown that the molecular complexes of [R₃NI]⁺I^– composition, which result from simple mixing of monoamines or amino acids with molecular iodine, can serve as effective catalysts for this reaction, providing the quantitative conversion [7, 8]. Taking into account the co-catalytic effect of hydroxy groups during the addition of СО₂ to epoxides, which has been previously demonstrated with ammonium salts [9, 10], we decided to obtain the molecular complexes of iodine with di- and triethanolamines and evaluate their activity in this reaction. Of note are the low cost of ethanolamines, which are produced on a large scale, and low toxicity, which stipulates their use in cosmetic products.

Herein, we report on the preparation of the catalysts based on iodine and di- and triethanolamine and their application in the addition of СО₂ to epoxides.

Results and discussion

The catalysts obtained in situ from equimolar mixtures of di- (1) and triethanolamine (2) with iodine (1 mol %) were studied first in the addition of СО₂ to propylene oxide (4a, R = H, Scheme 1).

Scheme 1. Addition of СО₂ to epoxides.

It was found that the catalytic system based on triethanolamine, which has a higher content of the hydroxy groups than diethanolamine, provides the higher conversion (Table 1, entries 1, 2). The application of preformed molecular complex 3 led to additional conversion enhancement (entry 3). An increase in the СО₂ pressure almost did not affect the reaction efficiency (entry 4). To optimize the results, we turned to the promoting effect of a silica gel substrate, which was observed earlier with the ammonium salts [4, 11]. For this purpose, two mass equivalents of silica gel were added during the complex formation (see the Experimental section). This simple manipulation allowed us not only to increase the conversion (entries 4, 5) but also to make the catalyst a free-flowing powder and, therefore, more convenient to handle compared to the oily complex lacking the substrate. The promoting effect of silica gel was also confirmed in the reaction at a higher temperature (130 °С) but for a shorter period of time (entries 6, 7). An increase in the reaction duration to 2 h afforded the quantitative conversion (entry 8); under these conditions, the immobilized catalyst can be reused while keeping the complete conversion; however, the third run afforded a reduction in the conversion to 96%. This is likely to be associated with the partial leaching of the catalyst from the substrate. It should be noted that neither molecular iodine, nor silica gel, nor their mixture does not catalyze the process under consideration.

Table 1. СО₂ addition to propylene oxide

Entry	Amine	Additive	Time, h	T, °C	P, atm	Conversion, %
1	1	–	3	105	10	43
2	2	–	3	105	10	52
3	2a	–	3	105	10	61
4	2a	–	3	105	56	63
5	2a	SiO2	3	105	10	76
6	2a	–	1	130	10	77
7	2a	SiO2	1	130	10	85
8	2a	SiO2	2	130	10	100b

a preformed catalysts; b 97% yield.

The catalytic system based on triethanolamine and iodine immobilized on silica gel was further tested in the coupling of СО₂ with epoxides 4b–f (Scheme 1, Table 2).

Table 2. СО₂ addition to epoxides 4b–f (105 °С, СО₂ 10 atm)

Entry	Epoxide	Time, h	Conversion, %	Yield, %
1	4b	3	77	–
2	4b	4	100	95
3	4c	4	100	97
4	4d	4	100	96
5	4e	4	100	97
6	4f	4	100	97

In the case of epoxide 4b bearing a bulky phenyl substituent, the conversion achieved 77% in 3 h. An increase in the reaction time to 4 h afforded the complete conversion. Under these conditions, substrates 4b–f were also quantitatively converted to the corresponding carbonates.

Molecular complex 3 was studied by NMR spectroscopy. The expected shifts of the signals of the CH₂ units relative to those of initial triethanolamine 2 were observed both in the ¹H and ¹³C NMR spectra (Table 3), which confirms the formation of the bond between the halogen atom and the amine.

Table 3. NMR spectroscopic data for compound 2 and its molecular complex with iodine

	1H NMR ((CD3)2SO, δ)			13C NMR ((CD3)2SO, δ)
	CH2N	CH2O		CH2N	CH2O
2	2.54 (t, J = 4.0 Hz)	3.40 (t, J = 4.0 Hz)		57.60	58.72
3	3.30 (t, J = 4.0 Hz)	3.75 (t, J = 4.0 Hz)		55.51	55.55

Adduct 3 was also studied by Raman spectroscopy (Fig. 1). At the laser power of 0.01 mW, the resulting spectrum displayed the characteristic lines of I₃⁻ and I₅⁻ ions [12, 13]. As the laser power was enhanced to 1 mW, the intensity of the line at 165 cm^–1 corresponding to polyiodide I₅⁻ reduced and a new line at 227 cm^–1 appeared that can be attributed to released molecular iodine.

Figure 1. Raman spectra of complex 3 at the laser power of 0.01 mW (a) and 1 mW (b).

Based on the results presented, it can be concluded that molecular complex 3 is analogous to the well-known complexes of iodine with amines, among which an iodine–pyridine complex is the most popular one (Scheme 2) [14–23].

Scheme 2. Iodine complex with pyridine.

Experimental

The NMR spectra were registered on a Bruker Avance 400 spectrometer. The Raman spectra were recorded on a JobinYvon LabRAM 300 spectrometer (He-Ne laser 632.8 nm, 1 mW) in the range of 100–4000 cm^–1. Epoxides 4a–f and silica gel 0.06–200 mm, 60 Ǻ were purchased from commercial sources (Sigma-Aldrich and Acros).

Iodine adduct of triethanolamine

Triethanolamine (149 mg, 1 mmol) and molecular iodine (254 mg, 1 mmol) were dissolved in methanol (5 mL) under stirring for 5 min. Then, the solvent was removed under vacuum (30 Torr). The complex immobilized on silica gel was obtained analogously by the addition of the substrate (806 mg) to a methanol solution of the complex followed by the solvent removal.

Synthesis of carbonates 5а–f from the epoxides

A 10 mL autoclave was charged with the corresponding amine (0.06 mmol), iodine (15 mg, 0.06 mmol), and the preformed complex or complex immobilized on silica gel (0.06 mmol). Then, epoxide (6 mmol) was added. The autoclave was filled with СО₂ and heated at the required temperature in a thermostatically controlled chamber. After the reaction completion, the autoclave was cooled to 5 °С, СО₂ was depressurized, and, after addition of CDCl₃ (1 mL), the reaction mixture was filtered through a thin layer of silica gel to remove the residual catalyst. The resulting solution was analyzed by NMR spectroscopy. The spectral characteristics of the carbonates were in good agreement with the earlier published data [24–26].

Conclusions

Hence, we developed an accessible and effective catalyst for the CO₂ addition to epoxides, which can be formed by simple mixing of triethanolamine with iodine. The efficiency of this catalytic system increases with the immobilization on silica gel. The latter also imparts heterogeneity and recyclability to the suggested catalyst.

Acknowledgements

This work was supported by the Ministry of Science and Higher Education of the Russian Federation (agreement no. 075-00697-22-00) and was performed using the equipment of the Center for Molecular Composition Studies of INEOS RAS.

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