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. 2021 Oct 5;6(41):27527–27535. doi: 10.1021/acsomega.1c04516

CeO2-Catalyzed Synthesis of 2-Imidazolidinone from Ethylenediamine Carbamate

Jie Peng †, Masazumi Tamura ‡,*, Mizuho Yabushita †, Ryotaro Fujii †,§, Yoshinao Nakagawa †, Keiichi Tomishige †,*
PMCID: PMC8529688  PMID: 34693173

Abstract

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CeO2 acted as an effective and reusable heterogeneous catalyst for the direct synthesis of 2-imidazolidinone from ethylenediamine carbamate (EDA-CA) without further addition of CO2 in the reaction system. 2-Propanol was the best solvent among the solvents tested from the viewpoint of selectivity to 2-imidazolidinone, and the use of an adequate amount of 2-propanol provided high conversion and selectivity for the reaction. This positive effect of 2-propanol on the catalytic reaction can be explained by the solubility of EDA-CA in 2-propanol under the reaction conditions and no formation of solvent-derived byproducts. This catalytic system using the combination of the CeO2 catalyst and the 2-propanol solvent provided 2-imidazolidinone in up to 83% yield on the EDA-CA basis at 413 K under Ar. The reaction conducted under Ar showed a higher reaction rate than that with pressured CO2, which clearly demonstrated the advantage of the catalytic system operated at low CO2 pressure or even without CO2.

Introduction

Cyclic ureas are important intermediates for pharmaceuticals, agricultural chemicals, natural products, chiral auxiliaries, and so on.1 2-Imidazolidinone is the simplest cyclic urea, and the synthesis from ethylenediamine (EDA) is often used as a model reaction for cyclic urea syntheses. The methods for the synthesis of 2-imidazolidinone are shown in Scheme 1. 2-Imidazolidinone is conventionally synthesized from EDA and hazardous carbonylation agents such as phosgene2 and CO,3 and the stoichiometric amount of salts is produced by neutralization. Acyclic ureas4 and organic carbonates,5 both of which are typically synthesized with phosgene or CO as the carbonyl source, are also used as alternative carbonyl agents. The direct synthesis of 2-imidazolidinone from CO2 and EDA is one of the promising and environmentally benign methods because the byproduct is only water, and various methods have been reported such as non-catalytic systems6−8 and catalytic systems including homogeneous catalyst systems,9−11 modified ionic liquids,12−14 and heterogeneous catalyst systems.15−19 However, in these reaction systems, pure and high-pressure CO2 is required for obtaining the high yield of the target product based on EDA, which means that additional processes, equipment, and energy costs such as CO2 desorption, purification, and compression are necessary.20,21 Therefore, the direct transformation of captured CO2 is a simple and desirable process.

Scheme 1. Various Methods for the Synthesis of 2-Imidazolidinone from EDA and This Work.

Scheme 1

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CO2 chemical absorption with amines is a useful and commercialized method,22−25 and the direct synthesis of ureas from CO2 captured by amines (namely, amine carbamate) without desorption, purification, and compression is promising. Some researchers reported on the transformation of amine carbamates into organic ureas.26−29 Barzagli and co-workers reported that the CuCl2 catalyst was effective for the synthesis of urea from ammonium carbamate with a yield of 54% (413 K, 3 days)26 and 1,3-dialkyl ureas from the corresponding carbamates with a yield of 37–44% (423 K, 5 h).27 Recently, Manaka and co-workers found that 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), an organic strong base, worked as an effective catalyst for the synthesis of urea from ammonium salts including ammonium carbamate, ammonium bicarbonate, and ammonium carbonate, and 35% yield of urea was obtained from the ammonium salts in dimethyl sulfoxide at 373 K for 3 days.28 However, these catalyst systems suffered from the low yields (<60%). Very recently, Choi and co-workers reported an effective homogeneous catalyst of Cp2Ti(OTf)2 for the synthesis of ureas including cyclic ureas from amine carbamates which were prepared from CO2 and amines, and high yields of the target ureas (70–99%) were obtained.29 However, these catalyst systems used homogeneous catalysts, and heterogeneous catalysts are preferable to homogeneous ones from the viewpoints of the durability, reusability, and ease of separation of catalysts from reaction mixtures. Hence, the development of effective heterogeneous catalysts for the synthesis of organic ureas from the corresponding carbamates is highly desirable. In the previous reports, CeO2 is one of the effective heterogeneous catalysts for organic urea synthesis from amines with pure and pressurized CO215,17,30 as well as for the synthesis of organic carbonates31 from alcohols and CO2 and organic carbamates32,33 from amines, alcohols, and CO2. Therefore, CeO2 is a promising candidate of the heterogeneous catalysts for the conversion of amine carbamates to the corresponding ureas.

Herein, we aimed to develop effective systems using heterogeneous catalysts for the synthesis of 2-imidazolidinone from ethylenediamine carbamate (EDA-CA) (Scheme 1). We found that CeO2 was an effective heterogeneous catalyst for the reaction under Ar with an appropriate amount of the 2-propanol solvent, providing a high 2-imidazolidinone yield of 83% and high utilization of captured CO2 in EDA-CA.

Results and Discussion

At first, the catalytic performance of metal oxides in the synthesis of 2-imidazolidinone (1) from EDA-CA at 413 K under Ar was investigated (Table 1). Without any catalysts (entries 19 and 20), the conversion of EDA-CA was 51%; however, no formation of 1 was observed even at 463 K (entry 20), which suggests that about half of EDA-CA was decomposed to EDA and CO2 by heating in the absence of catalysts (Scheme 2). La2O3, Y2O3, ZnO, Gd2O3, Pr6O11, Dy2O3, Sm2O3, Eu2O3, SiO2, γ-Al2O3, SiO2–Al2O3, MgO, K2CO3, and Cs2CO3 (entries 5–18) showed almost no formation of 1 with about 50% conversion of EDA-CA, and EDA was formed as the main product by the decomposition of EDA-CA. On the other hand, CeO2, ZrO2, and TiO2 showed activity in the reaction (entries 1–4). ZrO2 and TiO2 provided low 1 yields of 2 and 1%, respectively (entries 3 and 4); in contrast, CeO2 gave high 1 yields of 8% at 1 h and 44% at 24 h (entries 1 and 2). Therefore, CeO2 has high activity for the formation of 1 from EDA-CA compared with the decomposition of EDA-CA, leading to higher selectivity to 1 than the other metal oxides. CeO2 is the most effective catalyst for the formation of 1.

Table 1. Synthesis of 2-Imidazolidinone (1) from EDA-CA with Various Metal Oxides under Ara.

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          selectivityc/%
  
entry metal oxide Sb/m2·g–1 t/h conv.c/% 1 EDA yieldc/%
1 CeO2 84 1 30 27 73 8
2 CeO2 84 24 46 96 4 44
3 ZrO2 44 24 52 4 96 2
4 TiO2 44 24 51 1 99 1
5 La2O3 107 24 45 <1 >99 <1
6 Y2O3 40 24 50 <1 >99 <1
7 ZnO 12 24 46 <1 >99 <1
8 Gd2O3 60 24 49 <1 >99 <1
9 Pr6O11 105 24 47 <1 >99 <1
10 Dy2O3 58 24 45 <1 >99 <1
11 Sm2O3 79 24 50 <1 >99 <1
12 Eu2O3 69 24 49 <1 >99 <1
13 SiO2 453 24 45 <1 >99 <1
14 γ-Al2O3 86 24 52 <1 >99 <1
15 SiO2–Al2O3 360 24 47 <1 >99 <1
16 MgO 31 24 50 <1 >99 <1
17 K2CO3   24 53 <1 >99 <1
18 Cs2CO3   24 52 <1 >99 <1
19 none   24 51 <1 >99 <1
20 noned   24 51 <1 >99 <1
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a

Reaction conditions: metal oxide 0.5 mmol (based on the metal), EDA-CA 1.04 g (9.8 mmol), 2-propanol 10 mL, 413 K.

b

Specific surface area.

c

Based on EDA-CA.

d

463 K, Ar 1 MPa (r.t.).

Scheme 2. Decomposition of EDA-CA to EDA and CO2.

Scheme 2

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CeO2 samples calcined at different temperatures were also applied to the same reaction (Figure S3 and Table S1). The conversion (24 h) showed a volcano-type tendency with respect to the calcination temperature, and CeO2 calcined at 873 K provided the highest yield of 1. This conversion behavior toward the calcination temperature is similar to that of our previous reports on CeO2-catalyzed non-reductive transformation of CO2 into organic carbonates34−42 and carbamates.32,43 This volcano-type relationship can be explained by the crystallinity and surface area of the catalysts. At a low calcination temperature of 673 K, the surface area of CeO2 is high (∼140 m2 g–1), but the crystallinity of CeO2 is relatively low, that is, an amorphous phase is included, while the crystallinity of CeO2 catalysts calcinated at higher temperatures than 873 K is high (see Figure S4). The amorphous phase of CeO2 calcinated at 673 K will provide a negative effect on catalytic performance, leading to the low activity. On the other hand, a higher calcination temperature decreased the surface area, resulting in a decrease in active site amount and activity. Therefore, the intermediate calcination temperature of 873 K provided the most active CeO2 catalyst. In the following studies, CeO2 calcined at 873 K was used as the standard catalyst.

The effect of solvents was studied with methanol, ethanol, 1-propanol, 2-propanol, tert-butanol, CH3CN, N-methylpyrrolidone (NMP), tetrahydrofuran (THF), and H2O (Table 2). The yields of 1 in NMP (58%, entry 7) and THF (35%, entry 8) were much lower than those in the other solvents such as alcohols and CH3CN (over 67%, entries 1–6). CH3CN provided about 1 mmol solvent-derived products of N-(2-aminoethyl)acetamide and N,N′-diacetylethylenediamine. These byproducts were formed via the reaction between EDA and CH3CN (Scheme 3) or EDA and acetamide, which was formed by hydration of CH3CN (Scheme 4) because the hydration of CH3CN can be catalyzed by CeO2.37,44,45 In the cases of primary alcohols such as methanol, ethanol, and 1-propanol (entries 1–3), a small amount of byproducts were produced by N-alkylation of EDA with the alcohols. In contrast, the secondary alcohol of 2-propanol (entry 4) and the tertiary alcohol of tert-butanol (entry 5) provided no byproduct with 1 yields of 83% and 67%, respectively. The bulkiness around the OH group suppressed the N-alkylation of EDA, leading to no production of byproducts. With the H2O solvent (entry 9), the yield of 1 was only 4%. Even though EDA-CA can be dissolved in H2O easily, the formation of 1 was suppressed by the presence of H2O due to the equilibrium. Therefore, in terms of yield and selectivity, 2-propanol was selected as a suitable solvent for the reaction.

Table 2. Effect of Solvents on the Synthesis of 2-Imidazolidinone (1) from EDA-CAa.

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a

Reaction conditions: CeO2 0.34 g (2.0 mmol), EDA-CA 2.08 g (19.6 mmol), solvent 15 mL, 413 K, 24 h, Ar 1 MPa (r.t.).

b

Based on EDA-CA.

Scheme 3. Formation of N-(2-Aminoethyl)acetamide via the Reaction of EDA and CH3CN.

Scheme 3

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Scheme 4. Formation of N-(2-Aminoethyl)acetamide via the Reaction of EDA and Acetamide.

Scheme 4

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The effect of the 2-propanol solvent amount was then studied in the range between 1 and 30 mL (standard volume: 15 mL), and the conversion and yield at 24 h are shown in Figure 1 (details are shown in Table S2). The conversion and yield increased with the increase of the 2-propanol amount up to 15 mL, and they were constant with a higher amount of 2-propanol (about 80% conversion and yield). Focusing on the byproduct, the formation of N,N′-bis(2-aminoethyl)urea, a linear urea byproduct, was observed with less than 10 mL of 2-propanol. The conversion with 1 and 5 mL of 2-propanol is larger than the yield, and the difference between conversion and yield increased with the decrease of the 2-propanol amount due to the decomposition of a part of EDA-CA to EDA and CO2 (Table S2 and Scheme 2), which suggests that the 2-propanol solvent can play a role in the suppression of the decomposition of EDA-CA. These results indicate that with a small amount of the 2-propanol solvent (0–10 mL) (Figure 1), EDA produced by decomposition of EDA-CA will react with 1, providing N,N′-bis(2-aminoethyl)urea (Scheme 5), and the low reactivity will be related to the presence of EDA or CO2 produced by decomposition of EDA-CA. To examine the reactivity of 1 with EDA, the reaction of 1 and EDA was conducted in the presence or absence of the CeO2 catalyst (Table S3). The formation of N,N′-bis(2-aminoethyl)urea was confirmed in both cases, and the reaction can proceed without CeO2. The formation of N,N′-bis(2-aminoethyl)urea was also supported in terms of the formation energy, which was estimated by DFT calculations (Table S4). The formation of N,N′-bis(2-aminoethyl)urea from 1 and EDA can be exothermic (−31 kJ mol–1, entry 5, Table S4), whose energy change is larger than the case of the formation of N,N′-bis(2-aminoethyl)urea from CO2 and EDA (−19 kJ mol–1, entry 4, Table S4). Therefore, the formation of N,N′-bis(2-aminoethyl)urea from 1 and EDA is easier than that from CO2 and EDA, and the EDA formed by the decomposition of EDA-CA in the case of a small amount of the 2-propanol solvent can react with 1 to provide the linear urea of N,N′-bis(2-aminoethyl)urea. As above, the proper amount of the 2-propanol solvent (≥15 mL) is required for the high conversion and selectivity.

Figure 1.

Figure 1

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Effect of 2-propanol solvent amount on the synthesis of 2-imidazolidinone (1) from EDA-CA over CeO2 [diamonds: conversion, white bars: yield of 1, black bars: yield of N,N′-bis(2-aminoethyl)urea]. Reaction conditions: CeO2 0.34 g (2.0 mmol), EDA-CA 2.08 g (19.6 mmol), 2-propanol, 413 K, 24 h, Ar 1 MPa (r.t.).

Scheme 5. Formation of N,N′-bis(2-Aminoethyl)urea from 2-Imidazolidinone (1) and EDA.

Scheme 5

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The reaction temperature dependence was examined in the range between 373 and 453 K. The time courses at each temperature are shown in Figure 2 (the detailed data are in Table S5). The yield of 1 increased smoothly with the reaction time at any reaction temperatures and reached the equilibrium in 72 h at higher reaction temperatures than 403 K. Focusing on the equilibrium yields (Figure 2d–f), the yield of 1 decreased from 83 to 70% with the increase of the reaction temperature. Based on the DFT calculations (Table S4), the formation of 1 from EDA-CA is an endothermic reaction (+27 kJ mol–1, entry 3, Table S4). Therefore, the higher equilibrium yield at a lower reaction temperature is probably due to the entropic effect of the reaction because the 2 moles of molecules, 1 and water, are formed from the 1 mole of EDA-CA molecules. The highest yield of 83% was obtained at 413 K with a proper amount of the 2-propanol solvent (15 mL). The formation rate was calculated from the slope of the time course at a low conversion level (<40%, Figure S5 and Table S6) to be 0.60 h–1 at 413 K. The value is a little higher than that with the reported homogeneous catalyst system of Cp2Ti(OTf)2 (0.47 h–1) at 433 K29 despite the 20 K lower reaction temperature. Koizumi and co-workers reported that CeO2 (<25 nm) was active for the synthesis of 1 from EDA-CA, but the activity was low (yield 29%, 443 K).29 In contrast, in our case, by using nano-CeO2 (84 m2/g, 9 nm), the yield reached 83% at a low temperature of 413 K, and such a lower reaction temperature is beneficial for the suppression of EDA-CA decomposition. These results mean that our CeO2 catalyst is quite active for the reaction. In terms of the activity and equilibrium yield, 413 K was used in the following studies.

Figure 2.

Figure 2

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Time courses of the synthesis of 2-imidazolidinone (1) from EDA-CA over CeO2 at (a) 373 K, (b) 393 K, (c) 403 K, (d) 413 K, (e) 433 K, and (f) 453 K (diamonds: conversion, circles: yield). Reaction conditions: CeO2 0.34 g (2.0 mmol), EDA-CA 2.08 g (19.6 mmol), 2-propanol 15 mL, Ar 1 MPa (r.t.).

The effect of CO2 addition on the synthesis of 1 from EDA-CA was investigated (Figure 3 and the detailed data are in Table S7). The reaction rate clearly decreased with the increase of the CO2 pressure. Therefore, CO2 gives a negative effect on the reaction rate, demonstrating that the catalytic system for converting EDA-CA to 1 without the further CO2 addition is advantageous over the reactions operated with CO2. This result supports that the lower reaction rate in the case of a smaller amount of the 2-propanol solvent (Figure 1) is caused by CO2 produced via the decomposition of EDA-CA. The negative effect of the presence of external CO2 on the formation rate of 2-imidazolidinone from EDA-CA can be explained by the conversion of the reactive amino group of EDA-CA adspecies with CO2 to the corresponding carbamic acid with lower reactivity.

Figure 3.

Figure 3

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Effect of external CO2 addition on the synthesis of 2-imidazolidinone (1) from EDA-CA over CeO2 (diamonds: conversion, circles: yield). Reaction conditions: CeO2 0.34 g (2.0 mmol), EDA-CA 2.08 g (19.6 mmol), 2-propanol 15 mL, (a) 1 MPa Ar (r.t.), (b) 0.5 MPa CO2 (r.t.) (ca. 40 mmol), (c) 1 MPa CO2 (r.t.) (ca. 80 mmol), and (d) 2 MPa CO2 (r.t.) (ca. 175 mmol).

The reusability of the CeO2 catalyst in the synthesis of 1 from EDA-CA was elucidated (Figure 4, the detailed data are summarized in Table S8). After each reaction, the spent CeO2 catalyst was dried at 383 K for 3 h for the next run. In the reuse test, the yield was not changed at least three times. The X-ray diffraction (XRD) patterns of CeO2 catalysts before and after the reaction were not changed, and no decrease of the specific surface area was observed (Figure 5). These results indicate that CeO2 is a reusable heterogeneous catalyst and robust against the reaction and regeneration treatment.

Figure 4.

Figure 4

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Reusability test of CeO2 in the synthesis of 2-imidazolidinone (1) from EDA-CA (diamonds: conversion, black bars: yield of 1). Reaction conditions: CeO2 0.34 g (2.0 mmol), EDA-CA 2.08 g (19.6 mmol), 2-propanol 15 mL, 413 K, 8 h, Ar 1 MPa (r.t.).

Figure 5.

Figure 5

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XRD patterns, specific surface area, and particle size of CeO2 catalysts in the reusability test. (a) Fresh CeO2, (b) CeO2 after the first run, (c) CeO2 after the second run, (d) CeO2 after the third run, (e) and CeO2 after the fourth run.

The pressure and temperature of the autoclave reactor during the heating and reaction were monitored with the following three cases (Figure 6): (i) 2-propanol + Ar, (ii) 2-propanol + EDA-CA + Ar, and (iii) 2-propanol + EDA-CA + CeO2 + Ar. The pressure increased with the increase of the reaction temperature in all the cases. In case (i), the pressure reached about 1.8 MPa. Considering that the values of (vapor) pressure of 1 MPa Ar and 2-propanol are about 1.37 and 0.67 MPa at 413 K based on the ideal gas, respectively, the ideal total pressure of case (i) is about 2.0 MPa, which is similar to the observed one (1.8 MPa). On the other hand, in the cases with EDA-CA [cases (ii) and (iii) in Figure 6], the pressure reached about 2.0 MPa at 413 K, which is higher than that of case (i) (1.8 MPa). The difference in the pressures is about 0.2 MPa. The conversion (or decomposition) of EDA-CA was also determined by GC in the cases of (ii) and (iii) (Table S9). The decomposition in the case of (ii) was about 38%, and 7.5 mmol of CO2 and 7.5 mmol of EDA were formed (entry 1 in Table S9). Considering the estimated (vapor) pressure of EDA at 413 K (∼0.2 MPa)46 and 7.5 mmol CO2 (∼0.14 MPa), the observed pressure difference (0.2 MPa) is lower than the total estimated (vapor) pressure of EDA and CO2 (0.34 MPa) at 413 K, which will be due to the suppression of EDA-CA decomposition or the dissolution of EDA and CO2 in the solvent. The pressure of actual reaction [case (iii) in Figure 6] was slightly lower than that without the CeO2 catalyst [case (ii) in Figure 6], which may be due to the formation of 1 by CeO2 in case (iii).

Figure 6.

Figure 6

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Behavior of pressure and temperature in the autoclave reactor. (i) 2-Propanol + Ar (pressure ▲, temperature Δ), (ii) 2-propanol + EDA-CA + Ar (pressure ■, temperature □), and (iii) 2-propanol + EDA-CA + CeO2 + Ar (pressure ●, temperature ○). Conditions of (i) 2-propanol 15 mL, Ar 1 MPa (r.t.); (ii) EDA-CA 2.08 g (19.6 mmol), 2-propanol 15 mL, Ar 1 MPa (r.t.); (iii) CeO2 0.34 g (2.0 mmol), EDA-CA 2.08 g (19.6 mmol), 2-propanol 15 mL, Ar 1 MPa (r.t.).

Moreover, to investigate the state of the reaction mixture during heating more intuitively, direct monitoring with a transparent glass tube was conducted with the same three cases above. The reactor volume was 10 mL, and the scale was reduced to one-fifth of that of the reaction conditions for the autoclave reactor employed above. The behavior of the pressure and temperature is shown in Figure S7, and the pictures of reaction media during heating are shown in Figure S8. In the direct monitoring, similar pressure changes and a similar pressure gap between cases (i) and (ii) to the cases of the autoclave reactor (Figure 6) were observed (0.1–0.2 MPa) (Figure S7). When the temperature reached 413 K (21 min), the pressures were almost the same in all the cases. Afterward, the pressure of cases (ii) and (iii) increased up to about 81 min and decreased gradually at a longer reaction time, while the pressure of case (i) was not changed. The pressure increases after 21 min in the cases of (ii) and (iii) can be attributed to the decomposition of EDA-CA to CO2 and EDA. Focusing on the pictures of the reaction media (Figure S8), case (i) showed that the liquid was always transparent (Figure S8). In contrast, in the cases of (ii) and (iii) (Figure S8), EDA-CA was in the solid state up to 413 K (0–21 min) and subsequently started to be dissolved and completely dissolved at 81 min. The starting time of solid dissolution was the same as the time when the pressure gap between case (i) and cases (ii) and (iii) appeared, which supports that the pressure gap is due to the decomposition of EDA-CA to CO2 and EDA. These results also suggest that the 2-propanol solvent suppressed the decomposition of EDA-CA, which is related to the results of the solvent amount effect (Figure 1). Some white solids were formed on the wall of the top of the tube after 81 min in the cases of (ii) and (iii) (Figure S8). This is because CO2 and EDA, which were produced by the decomposition of EDA-CA, reacted at the top of the reactor to form EDA-CA again on the wall of the test tube, leading to the decrease of the total pressure.

Conclusions

We found that CeO2 was the most effective catalyst for the synthesis of 2-imidazolidinone from EDA-CA in the absence of external CO2 among the various metal oxides tested. 2-Propanol was the best solvent by the suppression of the formation of solvent-derived products. The combination of the CeO2 catalyst and an adequate amount of the 2-propanol solvent provided 2-imidazolidinone in high yield of 83% at 413 K, and CeO2 was a reusable heterogeneous catalyst and robust against the reaction and recovery treatment. The investigation of the solvent amount effect of 2-propanol on the reaction of EDA-CA suggests that the formation of N,N′-bis(2-aminoethyl)urea is suppressed with the sufficient amount of the 2-propanol solvent, which can be interpreted by the suppression of decomposition of EDA-CA in the presence of the 2-propanol solvent. According to the direct monitoring of the reaction media and the measurement of the total pressure of the autoclave during the reaction, most EDA-CA can be solved in the 2-propanol solvent, suggesting that EDA-CA is directly converted to 2-imidazolidinone catalyzed by CeO2. The negative effect of the external CO2 on the reaction also demonstrates the superiority of the catalytic conversion of EDA-CA to 2-imidazolidinone over CeO2 in the absence of external CO2.

Experimental Section

Catalysts and Reagents

Metal oxides were commercially available and used as received or after calcination: CeO2 (Daiichi Kigenso Kagaku Kogyo Co., Ltd., CeO2–HS, calcined at 873 K in air for 3 h), ZrO2 (Daiichi Kigenso Kagaku Kogyo Co., Ltd., RC-100 P, calcined at 873 K in air for 3 h), TiO2 (Nippon Aerosil P25, calcined at 873 K in air for 3 h), La2O3 (Kanto Chemical Co., Inc., used as received), Y2O3 (Kanto Chemical Co., Inc., used as received), ZnO (Kanto Chemical Co., Inc., used as received), Gd2O3 (Kanto Chemical Co., Inc., used as received), Pr6O11 (Kanto Chemical Co., Inc., used as received), Dy2O3 (Kanto Chemical Co., Inc., used as received), Sm2O3 (Kanto Chemical Co., Inc., used as received), Eu2O3 (Kanto Chemical Co., Inc., used as received), SiO2 (Fuji Silysia Chemical Ltd., G6, calcined at 773 K in air for 3 h), γ-Al2O3 (Nippon Aerosil, calcined at 873 K in air for 3 h), SiO2–Al2O3 (Reference Catalyst Division of the Catalysis Society of Japan, JRC-SAL-2, a SiO2/Al2O3 ratio of 5.5, used as received), and MgO (Ube Industries, Ltd., MgO 500A, calcined at 873 K in air for 3 h).

It was already reported that EDA-CA typically in an EDA/CO2 = 1:1 complex can be obtained from EDA and low-pressure CO2 of 0.1 or 0.01 MPa and even CO2 (∼0.04%) in air.29,47−50 The substrate of EDA-CA used in this work was synthesized in our laboratory. 20 g of EDA and 20 g of ethanol (solvent) were put into a 190 mL autoclave. The autoclave was purged with 1 MPa CO2 twice and then was pressurized to 5 MPa by CO2. The mixture in the autoclave was stirred at room temperature and 300 rpm for 20 h. Then, it was filtered, washed with ethanol, and dried at 333 K overnight. The obtained white solid (yield 41%) was characterized by GC, thermogravimetric analysis–differential thermal analysis (TG-DTA) (Figure S1), and CHNS elemental analysis [C: 34.71, H: 7.85, N: 27.16; ideal composition (C: 34.61, H: 7.74, N: 26.91, O: 30.74)]. The purity of EDA-CA in the obtained solid was 98%, and the impurity of EDA in the obtained solid was estimated to be 2% based on GC and CHNS elemental analysis results. Other chemicals were purchased from chemical companies and used without further purification.

Characterization of Catalysts

The specific surface areas of metal oxides were measured by the BET method (N2 adsorption) with a Micromeritics Gemini VII 2360. The XRD patterns were obtained using a Rigaku MiniFlex600 diffractometer [Cu Kα (λ = 0.154 nm), 40 kV, 20 mA] under air. The TG and DTA data were recorded using a Rigaku Thermo Plus EVOII instrument. The CHNS elemental analysis was carried out with an Elementar vario EL cube.

Typical Procedure for the Synthesis of 2-Imidazolidinone (1) from EDA-CA

The reactions were carried out in a 190 mL autoclave reactor. The typical procedure for the reaction was as follows: 0.34 g (2.0 mmol) of CeO2, 15 mL of 2-propanol, and 2.08 g (19.6 mmol) of EDA-CA were put into the autoclave with a spinner. The autoclave was sealed, purged twice with 1 MPa Ar, and pressurized to 1 MPa by Ar at room temperature. The autoclave was heated to 413 K, and the time when the temperature just reached 413 K was defined as 0 h of reaction time. After the reaction, the autoclave was cooled down to room temperature in a water bath. EDA-CA decomposes into EDA and CO2 in the injection chamber of the GC system due to high temperature, and hence, the direct analysis of EDA-CA is impossible. EDA-CA easily dissolves in H2O but cannot dissolve in ethanol. In contrast, EDA can easily dissolve in ethanol. Based on the different dissolution properties of EDA and EDA-CA, the following collecting method was adopted, and the procedure is illustrated in Figure S2. The reaction mixture including CeO2 and the white solid was collected with ethanol (25 g). The liquid phase (i.e., ethanol solution containing 2-imidazolidinone and EDA) and the solid phase containing CeO2 and unreacted EDA-CA were separated by filtration. The filtrate was analyzed by FID-GC (Shimadzu) with a capillary column (InertCap for Amine column, 0.32 mm, 30 m) by using 1-hexanol as an internal standard. The collected solid phase containing CeO2 and unreacted EDA-CA were washed with H2O (30 g), and CeO2 was separated by filtration. The filtrate was analyzed using the same GC system. The qualitative analysis of products was performed on GC–MS (EI and CI, InertCap for Amine capillary column).

The conversion of EDA-CA, yield of products based on EDA-CA, and balance based on EDA were calculated as follows:

graphic file with name ao1c04516_m001.jpg
graphic file with name ao1c04516_m002.jpg
graphic file with name ao1c04516_m003.jpg

where nEDA-CAintroduced is the amount of EDA-CA (in mmol) that was introduced into the reactor, nEDA-CA is the amount of EDA-CA (in mmol) that was detected, nproducts is the amount of products (in mmol), and nEDA impintroduced is the amount of EDA impurity (in mmol) that was introduced into the reactor.

For the reactions performed in a CO2 atmosphere, the yield of products was based on EDA and calculated as follows:

graphic file with name ao1c04516_m004.jpg

Reusability Test

The reusability test of CeO2 was conducted as follows: after the reaction, the used catalyst was dried at 383 K for 3 h. During the recovery procedure, about 10 wt % CeO2 was lost; therefore, multiple batches were conducted at the same time under the same reaction conditions to collect an enough amount of the used catalyst for the next run (first run: 4 batches, second run: 3 batches, third run: 2 batches, and fourth run: 1 batch).

Acknowledgments

A part of this work was based on the results obtained from a project, JPNP18016, subsidized by the New Energy and Industrial Technology Development Organization (NEDO), and it was also supported by JSPS KAKENHI 18H05247. Parts of the direct monitoring test were performed on the equipment in Watanabe Lab, Research Center of Supercritical Fluid Technology, Department of Chemical Engineering, Graduate School of Engineering, Tohoku University, Japan. We acknowledge the support from Prof. Masaru Watanabe and Dr. Qingxin Zheng in Tohoku University.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.1c04516.

  • Detailed data for the graph in the main text and characterization results including XRD, BET, TG-DTA, and NMR (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao1c04516_si_001.pdf (1.3MB, pdf)

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Associated Data

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Supplementary Materials

ao1c04516_si_001.pdf (1.3MB, pdf)

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