Reusable Polystyrene-Functionalized Basic Ionic Liquids as Catalysts

Research Article pubs.acs.org/journal/ascecg

Reusable Polystyrene-Functionalized Basic Ionic Liquids as Catalysts for Carboxylation of Amines to Disubstituted Ureas Duy Son Nguyen,† Jin Ku Cho,†,‡ Seung-Han Shin,†,‡ Dinesh Kumar Mishra,*,‡,§ and Yong Jin Kim*,†,‡,§ †

Department of Green Process and System Engineering, University of Science and Technology (UST), 176 Gajeong-dong, 305-350 Yuseong-gu, Daejeon, South Korea ‡ Department of Green Process and System Engineering, Korea Institute of Industrial Technology, 89, Yangdaegiro-gil, Ipjang-myeon, Cheonan-si 331-822, South Korea § Green Process Material Research Group, Korea Institute of Industrial Technology, 89, Yangdaegiro-gil, Ipjang-myeon, Cheonan-si 331-822, South Korea S Supporting Information *

ABSTRACT: A series of polystyrene (PS)-functionalized basic ionic liquids (BILs) were prepared and used as catalysts for synthesis of disubstituted ureas (DSUs) from amines and carbon dioxide (CO2). The PS-BILs as prepared were characterized by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FT-IR) spectroscopy. For investigating the catalytic activities, all PS-BILs were tested in a model reaction of cyclohexylamine (CHA) and CO2 to synthesize dicyclohexylurea (DCU). Poly-2 having branched [bis-imidazolium]/ [bis-bicarbonate] was found to show the highest activity for the DCU formation among all PS-BILs catalysts. For a comparison point of view, ceria (CeO2) as a typical catalyst and original Merrifield’s resin (MR) was also applied for the reaction. Reaction conditions were optimized by varying reaction temperature, pressure, reaction time, and amount of catalyst used. Under optimized conditions, reactions of various amines with CO2 to synthesize the corresponding diureas were carried out in the presence of the most active catalyst (poly-2). Furthermore, poly-2 could be easily recovered and reused up to seven consecutive cycles with no significant loss of the catalytic activity. KEYWORDS: Amines, Disubstituted urea, CO2, Carboxylation, Polystyrene, Ionic liquids

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INTRODUCTION The synthesis of carbonyl compounds using carbon dioxide (CO2) has gained growing concern regarding carbon capture and utilization (CCU). More recently, the utilization of CO2 as a carboxylating reagent attracted great significance for the replacement of conventional phosgene and carbon monoxide.1,2 Even though few commercial processes have been established until now, several meaningful CO2 fixation processes have been made on the catalytic conversion of CO2 to form a variety of organic carbonyl compounds such as oxazolidinones, carbamates, and disubstituted ureas (DSUs).3−19 The DSUs can be easily converted to carbamates, which can be used as precursors for preparing isocyanates by thermal cracking.20−24 Meanwhile, the share of polyurethanes within all polymers on the EU market was 7.0%. Taking into account the increased importance of polyurethanes for the global market, alternative synthetic routes avoiding the use of tremendously toxic phosgene for producing isocyanates is obligatory from the viewpoint of sustainability.25 In recent years, the synthesis of DSUs from direct carboxylation between amines and CO2 has been extensively © XXXX American Chemical Society

studied. Deng et al. reported an effective strategy for generating symmetric ureas in high yields using [Bmim]Cl/CsOH as reaction media and catalyst.26 Undoubtedly, several catalytic systems have also been well developed such as Ph3SbO/P4S10,27 DBU/PBu3, Et3N/CCl4, and DMAN/CCl4.28−30 The desired products could be obtained in good yields (ca. 45−95%). However, the use of stoichiometric qualities of bases such as DBU and Et3N or the use of expensive ionic liquid as solvent made these processes disadvantageous. More effective homogeneous systems such as KOH/PEG1000,31 TBA2[WO4],32 and Cs 2CO3 /NMP 33 displayed from moderate to excellent activities; however, they are plagued by prolonged reaction time (ca. 10−24 h) and/or difficulties in catalyst recovery. As far as the authors concerned, Tamura et al. reported the exploitation of CeO2 as the solely heterogeneous catalyst that Special Issue: Ionic Liquids at the Interface of Chemistry and Engineering Received: October 26, 2015 Revised: December 15, 2015

A

DOI: 10.1021/acssuschemeng.5b01369 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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mL), and dried under vacuum at 80 °C. Finally, the solid (1′) and sodium bicarbonate (2.0 g) were mixed together in anhydrous methanol (35.0 mL); the suspension was stirred for 24 h at ambient temperature. The final product (named poly-2) was separated, washed with deionized water until neutralized, and dried under vacuum. Besides bicarbonate [HCO3]−, different anions such as [Cl]−, [OAc]−, [OH]−, [OMs]−, and [OTs]− were also introduced into poly-2 for activity comparisons. Poly-3 and Poly-4 were also prepared using a similar procedure with pyridine and tributylamine, respectively. Procedure for Preparation of Linear Catalysts (Poly-5 and Poly-6). Preparations of poly-5 and poly-6 are shown in Scheme 3. First, a mixture of Merrifield’s resin (2.0 g) and imidazole (1.1 g, 16.0 mmol) in anhydrous acetonitrile (35.0 mL) was stirred at 100 °C for 24 h under an Ar atmosphere. The product (2′) was separated by filtration and then was added again to the solution of dichloromethane (30.0 mL) containing 1,4-dibromobutane (1.45 mL, 12.0 mmol). After refluxing for another 24 h, the solid (3′) was separated and washed with methanol (4 mL × 20.0 mL) and acetone (2 × 20.0 mL). To immobilize the second ionic liquid moiety, solid (3′) (2.0 g) and pyridine (1.29 mL, 16.0 mmol) in anhydrous dimethylformamide (35.0 mL) were vigorously stirred at 80 °C for 24 h under Ar. The obtained solid was collected, washed with acetone (5 mL × 20.0 mL), and dried under vacuum to give a product containing two bromide anions. Lastly, the bicarbonate-bearing poly-5 was achieved by mixing with NaHCO3 (2.0 g) in methanol (35.0 mL) for 24 h. Poly-6 was prepared with a similar procedure by replacing pyridine with tributylamine, respectively. Procedure for Carboxylation of Amines to Disubstituted Urea. All the carboxylating reactions were conducted in a 100.0 mL high-pressurized stainless steel reactor equipped with a magnetic stirrer and an electrical heater. The reactor was charged with amine (40.0 mmol), 10.0 mL of N-methylpyrolidone (NMP), and PS-BILs as catalysts (0.25 g). After that, 10 bar of CO2 pressure (P) was purged into the reactor to deoxygenate the reaction mixture three times. The reactor was then pressurized to 50 bar of CO2 and heated to the desired temperature. The reactor was further pressurized to 90 bar when the temperature reached 170 °C. During the reaction, CO2 pressure (90 bar) inside the reactor was maintained by using a gas reservoir equipped with a back-pressure regulator and a pressure transducer. After the desired reaction time, the product mixture was allowed to cool and was filtered to separate out the solid containing the DSU product, PS-BILs, and carbamate salt. The obtained solid was washed with water and THF several times to remove the carbamate salt from the solid mixture. The remaining solid was dried in a vacuum oven overnight. After drying, the solid mixture containing the DSU product and catalyst PS-BILs was weighed, and the amount of catalyst PS-BILs was excluded to get the isolated yield of DSU product. For recycling, the DSU product was separated from the solid catalyst PSBILs by dissolving it in hot methanol. The catalyst was dried at 80 °C under vacuum for 24 h and then used for the next cycle without any treatment. To the filtrate, an external standard (isooctane) was added, and the solution was analyzed by gas chromatography (GC).

converted diamines into their corresponding five- and sixmembered ring ureas.34 Even though CeO2 was demonstrated to be a quite active and highly reusable catalyst, it could not achieve a satisfactory yield in a short span of reaction time, except for 12 h. Ionic liquids (ILs) have been developed for task-specific purposes, especially for their role as active catalysts in organic synthesis.35−37 The notable one among those examples has been the supported IL system that combines the advantages of homogeneous and heterogeneous catalysts, which enhances both catalytic activity and reusability.38 However, there has been no report on the supported IL as an efficient heterogeneous catalyst for the synthesis of DSUs from amines and CO2. In this paper, we report the synthesis of new polystyrene (PS)-functionalized basic ionic liquids (BILs) (designated as PS-BILs), which are highly recyclable catalysts for producing DSUs from direct carboxylation of amines and CO2. Furthermore, the DSU products can be obtained in reasonable yields after only a 4 h reaction time without requiring any additive or dehydrating agent (Scheme 1). Scheme 1. Synthesis of DSU from Amines and CO2 Using PS-BILs as Catalysts

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EXPERIMENTAL SECTION

Materials. All chemicals and reagents were purchased from SigmaAldrich Chemical Co., U.S.A., and used immediately without any further purification. All solvents were of analytic grades and were distilled with appropriate drying agents under a nitrogen atmosphere prior to use. An over 99% purity carbon dioxide cylinder was purchased from Gong-Dan Industrial Gas Co, Korea. The liquid CO2 was transported into a gas reservoir through a HSK-600 liquid pump and was gasified at room temperature (RT). Procedure for Preparation of Branched Catalysts (Poly-2, Poly-3, and Poly-4). Preparation of catalysts (poly-2, poly-3, and poly-4) is shown in Scheme 2. First, Merrifield’s resin (1% crosslinked, 3.5−4.5 mmol Cl/g, 2.0 g) and bis(2-chloroethyl)amine (12.0 mmol) were taken into a two-necked flask containing anhydrous acetonitrile (35.0 mL) and refluxed for 12 h under an Ar atmosphere. The product (1) was separated by filtration and then was suspended again in a solution of anhydrous dimethylformamide (DMF) (35.0 mL) containing 1-methylimidazole (32.0 mmol). The mixture was stirred at 80 °C for another 24 h under Ar atmosphere. The solid (1′) product was collected by filtration, washed with acetone (6 mL × 20.0

Scheme 2. Preparation of Poly-2, Poly-3, and Poly-4 Corresponding to 1-Methylimidazolium, Pyridinium, and Tributylammonium, Respectively

B


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Scheme 3. Preparation of Poly-5 and Poly-6 Corresponding to Pyridinium and Tributylammonium, Respectively

Scheme 4. Formation of DCU from CHA through Carbamate Salt Intermediate

Table 1. Effect of Anionic Species on Formation of DCUa

Quantitative analyses were made on an Agilent 6890N gas chromatograph equipped with a flame ionization detector (FID) and qualitative analyses on an Agilent 6890N-5975 mass spectrometer-gas chromatograph (MSD-GC) equipped with an HP-5 column (30 m × 0.32 m × 0.25 μm) Catalyst Characterization. Scanning electron microscopy (SEM) images were recorded on a Jeol JSM 6701F electron microscopy. The PS-BILs as prepared were characterized by X-ray photoelectron spectroscopy (XPS, Escalab MK II spectrometer) using an Al X-ray excitation source (Kα = 1487.6 eV). The FT-IR spectra were recorded in the range from 500 to 2500 cm−1 on a Nicolet 6700 spectrometer.

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RESULTS AND DISCUSSION Activities of Catalysts Bearing Different Anionic Species. When aliphatic amine such as cyclohexylamine (CHA) is placed under CO2 pressure, carbamate salt is formed spontaneously (Scheme 4). In order to produce dicylohexylurea (DCU) from a carbamate salt intermediate, basic catalysts are usually required for generating DCU through the dehydration process.39 In order to investigate the activities of basic counteranions, the carboxylation reactions of CHA to DCU were carried out using poly-2 bearing [HCO3]− and its analogues bearing other anionic species such as chloride [Cl]−, mesylate [OMs]−, tosylate [OTs]−, acetate [OAc]−, and hydroxyl [OH]−. The obtained results are summarized in Table 1. It is noteworthy to mention that carbamate salt was the only byproduct found after the reaction (Scheme 4). Carbamate salt from a product mixture was removed by adding water as described in the Experimental Section. Since DCU could not be dissolved in water, a solid product mixture consisting of DCU and the catalyst was easily separated by simple filtration. When the reaction was carried out in the absence of the catalyst, only 19.5% DCU yield at 38.8% CHA conversion could be produced. For comparison, ceria (CeO2) as a typical solid catalyst was tested in a separate reaction. It was observed that CeO2 remained almost inactive during the course of the reaction, and the obtained DCU yield was low, i.e., 22.5% (entry 2), with no significant improvement compared to that of noncatalyzed reaction. Interestingly, Merrifield’s resin (MR) somehow displayed activity toward the formation of DCU (29.5%, entry 3), and the result was almost similar to that of the

entry

anion

CHA conversionb [%]

DCU yieldc [%]

1 2 3 4 5 6 7 8 9

no catalyst CeO2 MR [Cl]− [OMs]− [OTs]− [OAc]− [OH]− [HCO3]−

38.8 40.2 42.8 50.2 52.1 69.1 77.2 78.8 97.5

19.5 22.5 29.5 30.3 32.5 43.2 44.5 48.5 78.3

a

Reaction conditions: CHA = 40.0 mmol, Poly-2 = 0.25 g, NMP = 10.0 mL, CO2 pressure = 90 bar, Temperature = 170 °C, Time = 4 h. b Determined by GC-FID from the remaining product mixture with isooctane as an external standard. cIsolated yield.

catalyst bearing [Cl]− (30.3%, entry 4). By replacing the anion with [OMs]−, [OTs]−, [OAc]−, and [HCO3]−, the yields of DCU increased. It is clear that poly-2 bearing all other anions was found to exhibit much lower activity than poly-2 bearing [HCO3]−, generating DCU in a range of 30.3−44.5% yields (entries 5−7). Poly-2 bearing [HCO3]− gave the highest yield of DCU (78.3%) at almost complete CHA conversion (97.5%) (entry 9). Interestingly, the activities of the catalysts were found to somehow increase in order of increasing basicity of the anions: [HCO3]− (pKa = 6.35, entry 9) > [OAc]− (pKa = 4.76, entry 7) > [OTs]− (pKa = −0.6, entry 6) > [OMs]− (pKa = −2.8, entry 5) > [Cl]− (pKa = −8.0, entry 4), which were in good agreement with the literature.40,41 Choi et al. reported that it was somewhat reasonable to say that the more basic anion would interact more strongly to amino groups and therefore facilitated the insertion of CO2 into the N−H bond of amine molecules, which in turn led to a higher yield of DCU.40 However, in the case of [OH]− (entry 8, pKa = 16), which is considered to be a stronger base than [HCO3]−, the obtained DCU yield was still moderate. It is suggested that the basicity of anions is not the only determined factor for the reaction and that the anion’s structure itself could be a more important contributor. The [HCO3]− anion with a planar structure displayed higher activity than that of [OH]−, possibly through C


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ACS Sustainable Chemistry & Engineering stabilization of the transition state. Rather than [OH]−, all three oxygen atoms of [HCO3]− are arranged in the same plane to interact with the amine, CO2 molecule, and C(2)-acidic hydrogen of the imidazolium rings at the same time.40 Catalytic Activities of PS-BILs on Carboxylation of CHA to DCU. Since it had been demonstrated that poly-2 having a bis-bicarbonate anion showed the highest activity for the synthesis of DCU from CHA and CO2, the catalytic activities of PS-BILs having different types of cationic parts such as branched bis-imidazolium (poly-2), bis-pyridinium (poly-3), bis-ammonium (poly-4), and linear monoimidazolium (poly-5, poly-6) with the same bicarbonate counteranion were evaluated. The results are presented in Figure 1. The actual

Besides polystyrene-based catalysts, two more different types of silica-based catalysts having the same bis-imidazolium and [HCO3]− subunits (represented as Si-7 and Si-8) were prepared from their respective MCM-41 and SBA-15 support materials (Supporting Information, Figure S1), and their catalytic activities were examined. However, the silica-based catalysts showed overall lower yields than those of the PS-BILs results, 35.8% and 32.0%, respectively, probably due to the unreacted silanol groups (Si−OH) that were playing a poisoning role by lowering the basicity of the bis-bicarbonate anion during the reaction. Optimization of Reaction Conditions for Carboxylation of CHA to DCU. For optimization, carboxylation of CHA to DCU was carried out using poly-2 as the catalyst. Figure 2

Figure 2. Effect of reaction time on the carboxylation reaction. Reaction conditions: CHA = 40.0 mmol, Poly-2 = 0.25 g, NMP = 10.0 mL, CO2 pressure = 90 bar, Temperature = 170 °C.

Figure 1. Catalytic activity comparison results in the carboxylation reaction using different types of support materials and their alkyl moieties. Reaction conditions: CHA = 40.0 mmol, NMP = 10.0 mL, CO2 pressure = 90 bar, Temperature = 170 °C, Time = 4 h, Molar ratio of (substrate/catalyst) = 64 (Table S1).

shows the effect of reaction time on the carboxylation reaction performed with poly-2 at 170 °C and 90 bar of CO2 pressure. We found that the yield and the conversion increased sharply with an increase in reaction time from 1 to 4 h, indicating that there was a strong dependency on reaction time for the formation of DCU. Beyond the cited range of time, it was not too pronounced and increased slightly and remained almost constant until 6 h. The low CHA conversion with low DCU yield at a short reaction period of less than 2 h was due to the formation of carbamate salt (CyNH3+CyNHCO2−), which was readily converted back to CHA via a backward reaction upon exposure to the atmosphere. To study the influence of reaction temperature on catalytic activity, a series of carboxylation reactions were carried out at a varying temperature range from 150 to 190 °C for 4 h at a constant pressure of 90 bar. The results illustrated in Figure 3 show that the DCU yield increased rapidly up to 78.9% yield at almost complete conversion when the temperature reached 170 °C but then slightly decreased at 190 °C. At higher temperatures than 170 °C, the effect of temperature appeared to be negative because the reaction was exothermic and the high temperature facilitated a reversible reaction.31,40,43 The influence of pressure on the DCU formation was also evaluated in the range of 20−110 bar, and the results are presented in Figure 4. The DCU yield was insignificant at a pressure lower than 50 bar due to poor solubility of CO2 in the reaction phase. The effect of pressure from 70 to 90 bar showed a positive effect on DCU yields. It was found that DCU yield increased when the pressure increased up to 90 bar, and thereafter, the yield of DCU became constant.

weights of PS-BILs used for these reactions were determined based on the elemental analysis results, and the calculated molar ratios (substrate/catalyst) were in the range from 59 to 64 (Supporting Information, Table S1). On comparing the activities of PS-BILs, as shown in Figure 1, all branched PS-BILs (poly-2 to poly-4) showed higher activities toward the formation of DCU than those of the linear PS-BILs (poly-5, poly-6). It has been reported that the overall activity of the catalyst is due to hydrogen bonding interactions among the imidazolium cation, bicarbonate anion, and intermediate species (CO2-amine adduct) in transition states.42 The higher activities of branched PS-BILs can be cautiously attributed to the large freedom of access for the formation of the intermediate species through the sp3hybridized branched linker. On the other hand, the linear PSBILs catalysts (poly-5, poly-6) provide less accessibility due to the more restricted sp2-hybridized linkers of imidazolium moieties, which in turn are responsible for the lower activity of the linear PS-BILs. Another possible explanation for the poor activity is that the rigid “linker” imidazolium groups are located in the vicinity of the PS surface; hence, it can be expected to be sterically crowded to form the intermediate species. As expected, the poly-2 bearing bis-imidazolium subunits and bicarbonate anions afforded the highest catalytic activity, producing DCU in 78.3% yield with 97.5% CHA conversion. D


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Table 3 shows the effect of solvents on the carboxylation of CHA using poly-2 as a catalyst (molar ratio of CHA/poly-2 = Table 3. Effect of Solvent on Carboxylation Reactiona

Figure 3. Effect of reaction temperature on carboxylation reaction. Reaction conditions: CHA = 40.0 mmol, Poly-2 = 0.25 g, NMP = 10.0 mL, CO2 pressure = 90 bar, Time = 4 h.

CHA conversionb [%]

DCU yieldc [%]

1 2 3 4 5

0.08 0.15 0.25 0.38 0.80

200 100 64 40 20

52.1 75.0 97.5 98.1 99.2

38.6 50.1 78.3 84.9 88.0

DCU yieldc [%]

1 2 3 4 5 6 7 8 9

benzene toluene DMPU NMP PEGDM DMF DMSO morpholine 1,4-dioxane

22.7 30.1 90.5 97.5 53.3 70.2 87.0 15.5 23.8

3.8 8.0 55.5 78.3 21.1 0.0 47.7 8.9 11.6

64) at 170 °C and 90 bar of CO2 pressure for 4 h. In nonpolar solvents like benzene and toluene, CHA was not carboxylated at all, while in polar solvents such as DMSO, DMPU (1,3dimethyl-3,4,5,6-tetrahydro-2(H)-pyrimidione), and NMP, CHA was carboxylated from moderate to good yields, 47.7%, 55.5%, and 78.3%, respectively. Among those aprotic polar solvents, NMP was considered to be the best solvent because it was capable not only of absorbing CO233,44 but also of assisting the reaction by forming the hydrogen bonding interactions between carbonyl group in its structure with various reactants such as Cy-NH2, Cy-NHCO(OH) (carbamic acid intermediate), and (Cy-NH)2C(OH)2, thus facilitating nucleophilic attack of a second CHA molecule and rate of the dehydration process.43 However, the other polar solvents such as morpholine, 1,4-dioxane, and DMF did not show any good performance. Moreover, PEGDM (polyethylene glycol dimethyl ether), a well-known polar solvent for pyrolysis process,45 gave only 21.1% DCU yield and 53.3% CHA conversion. These results indicated that the carboxylation reaction was largely dependent on the solvent used. Interpretation of Characterization Data. SEM analysis was performed to characterize the morphology of the original Merrifield’s resin and poly-2, as shown in Figure 5. Figure 5a showed that the particle sizes were found in the range of 50− 100 μm. On comparing Figure 5a and b, Figure 5b showed that the surface of poly-2 was much plainer, indicating that Merrifield’s resins were successfully functionalized with organo-moieties, and the entire poly-2 was still robust after the synthetic procedures.46

Table 2. Effect of Molar Ratio on Formation of DCUa molar ratio

CHA conversionb [%]

Reaction conditions: CHA = 40.0 mmol, Poly-2 = 0.25 g, NMP = 10.0 mL, CO2 pressure = 90 bar, Temperature = 170 °C, Time = 4 h. b Determined by GC-FID from the remaining product mixture with isooctane as an external standard. cIsolated yield

Therefore, for obtaining reasonable yield of DCU using poly2 as a catalyst, the reaction parameters should meet the following requirements: at least 4 h of reaction time, 170 °C of temperature, not lower than 50 bar of CO2 pressure. The carboxylation of CHA was carried out by varying the molar ratio of CHA/poly-2 from 20 to 200, and the results are summarized in Table 2. The product yield increased from

poly-2 (g)

solvent

a

Figure 4. Effect of CO2 pressure on the carboxylation reaction. Reaction conditions: CHA = 40.0 mmol, Poly-2 = 0.25 g, NMP = 10.0 mL, Temperature = 170 °C, Time = 4 h.

entry

entry

a

Reaction conditions: CHA = 40.0 mmol, NMP = 10.0 mL, CO2 pressure = 90 bar, Temperature = 170 °C, Time = 4 h. bDetermined by GC-FID from the remaining product mixture with isooctane as an external standard. cIsolated yield.

38.6% to 78.3% when the molar ratio decreased from 200 to 64. Beyond the cited range, the effect of molar ratio was no longer significant. The DCU yield at molar ratio 20 was 88.0% and was slightly higher than that of molar ratio 64. Therefore, the suitable molar ratio of CHA/poly-2, i.e., 64, was chosen for further investigations.

Figure 5. SEM images of original Merrifield’s resin (a) and poly-2 (b). E


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demonstrated a successful replacement of Cl group with Ncontaining species. If we have a closer look at the spectrum of N 1s, which is described in Figure 6c, it is clear that set of peaks at both 401.38 and 402.28 eV were responsible for the N 1s of quaternary ammonium atoms of imidazolium rings, and the peak at 399.6 eV was attributed to the N 1s of aliphatic amine atoms47 of the linkers. These results suggested that the polystyrene resins were well functionalized. To confirm the presence of a bicarbonate anion, FT-IR spectra of poly-2 and their precursors, compounds 1, 1′, and Merrifield’s resin, were recorded accordingly and are shown in Figure 7. The disappearance of two major peaks at 662 and

Figure 6a and b represents the XPS spectra of Merrifield’s resin and poly-2, respectively.

Figure 7. FT-IR spectra of poly-2 and precursors.

1236 cm−1 (Figure 7c and d), which were responsible for CH2−Cl bending frequencies, and the appearance of a new peak at 1170 cm−1 (Figure 7a and b), which was attributed to C−N stretching frequency, clearly suggested that the C−Cl bond had been replaced by a C−N bond. Moreover, the spectral changes from Figure 7b to a showed a new set of characteristic stretching frequencies of bicarbonate (HCO3−) at 1652, 1632, and 1409 cm−148. It was concluded that the chloride anions (Cl)− had been undoubtedly exchanged with HCO3−. Poly-3, 4, 5, and 6 were also determined by FT-IR, and they showed the same characteristic peaks for the bicarbonate anion (Supporting information, Figures S2−S5). These results demonstrated that all the ionic liquid moieties were properly anchored to PS and the anions were successfully exchanged with (HCO3−) as well. In addition, the complete anionic exchange of chloride to bicarbonate in poly-2 was further confirmed by a silver nitrate (AgNO3) test (Supporting Information, Figure S6). There was no sign of silver chloride (AgCl) precipitate, while the phenomena were apparent with the intermediate 1′. Recyclability of Catalysts. The recyclability of a catalyst is one of the most important merits for the heterogeneous catalysis in terms of process economy. In our work, the recycling studies of poly-2, poly-3, and poly-4 were investigated under the optimized conditions. As shown in Figure 8, poly-2 showed a higher activity than those of poly-3 and poly-4 (1st cycle). The catalytic strength of poly-2 decreased very slightly from the first cycle to the seventh cycle;

Figure 6. XPS spectra of (a) Merrifield’s resin, (b) poly-2, and (c) magnifying N 1s of poly-2.

Original Merrifield’s resin did not show any peak related to nitrogen in Figure 6a, whereas a characteristic peak of the nitrogen atom appeared clearly at 400.55 eV in poly-2 as shown in Figure 6b. Moreover, the dramatic decrease in the peak intensity at 199.13 eV responsible for the Cl 2p atom in poly-2 (Figure 6b) compared to that of the original Merrifield’s resin F


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disubstituted urea in the range from 32.8% to 83.3% yields (entries 3−6). It was observed that the DSU yields were greatly influenced by changing the reactants having different alkyl chain length. Typically, all aliphatic amines possessed almost similar basicity, and they were expected to have similar reactivity toward CO2. However, the experiments showed that the amine with the longer alkyl chain length gave the higher DSU yield. It might be suggested that a long alkyl chain probably enhanced the dehydration process of the corresponding carbamate salts as a result of a microhydrophobic environment created by these alkyl groups.43 Cyclic amines such as CHA (entry 7), cycloheptylamine (entry 8), and cyclododecylamine (entry 9) were also transformed to the corresponding ureas in 78.3%, 61.4%, and 80.1% yields, respectively. However, for the cases of diamines, most of the products were found to be oligomers instead of disubstituted ureas (entries 10−13). All oligomer structures were identified using FT-IR spectroscopy by matching the frequency bands with the presynthesized polymers obtained from a separate reaction of diamines and corresponding diisocyanates49 (Supporting Information, Figures S7 and S8). GPC analysis also indicated that these oligomers have molecular weights (Mw) ranging from 540 to 830. It is noteworthy that the longer reaction time (8 h) in the reaction of HMDA with CO2 (entry 11) produced an almost polymeric level (Mw = 3000). For chemoselectivity, equal amounts of butylamine and cyclododecylamine were used together as substrates. The mixture was carboxylated for 4 h with all other variables constant. After the reaction, it was found that the dibutylurea yield was only half of the yield (24.5%) as when used separately (53.0%; Table 4, entry 4), while dicyclododecylurea, which was obtained at 57.3% yield, decreased approximately by a third compared to the original yield (i.e., 80.1%; Table 4, entry 9). Interestingly, the collective yield of both products was almost identical to that of cyclododecylamine if used as a single substrate. This result indicated that even though the overall activity of the catalyst remained unchanged the catalyst poly-2 facilitated the carboxylation step of the more active substrate (cyclododecylamine) to produce DSU faster and hence became comparative to the weaker amine (butylamine), therefore decreasing the overall reactivity of the weaker one. From these results, poly-2 was proven to be an ideal heterogeneous catalyst applicable to a variety of amines, producing the corresponding disubstituted ureas in moderate to good yields, except with the formation of oligomers in some cases as tabulated.

Figure 8. Recycling tests of poly-2, poly-3, and poly-4 in carboxylation of CHA to DCU. Reaction conditions: CHA = 40.0 mmol, Catalysts = 0.25 g, NMP = 10.0 mL, CO2 pressure = 90 bar, Temperature = 170 °C, Time = 4 h.

however, activities of poly-3 and poly-4 dropped quickly with a faster rate than poly-2. Poly-2 displayed an excellent recycling performance with no significant loss of its original activity. As indicated by the SEM image in Figure 9, the size and surface of

Figure 9. SEM image of poly-2 after 7 cycles.

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poly-2 were changed slightly. The small decrement in the catalytic activity of poly-2 might be due to this reason. It is important to mention that all catalysts, poly-2, poly-3, and poly-4, can be reused by simply washing and drying after each cycle. All of these results revealed that poly-2, compared to poly-3 and poly-4, was robust, efficient, and highly reusable. To broaden the scope of this carboxylation reaction, various mono- and diamines were examined as substrates for the carboxylation reaction. Poly-2 was chosen as an active catalyst with a fixed molar ratio (substrate/catalyst) of 64, and the reaction conditions were 170 °C, 90 bar of CO2, and 4 h. The results summarized in Table 4 showed that aniline (entry 1) was not carboxylated at all because of its extremely low basicity (pKa = 4.25), whereas benzylamine was readily converted to the corresponding dibenzylurea in an acceptable yield (entry 2), probably due to the relatively higher basicity of benzylamine (pKa = 9.33) than that of aniline (pKa = 4.25). Aliphatic monoamines like propylamine, butylamine, hexylamine, and octylamine were all accordingly converted to the corresponding

CONCLUSIONS A series of polystyrene-functionalized basic ionic liquids (PSBILs) were prepared, and their catalytic activities were investigated in the carboxylation reaction of amines and CO2 to produce corresponding disubstituted ureas. Poly-2 comprising bis-imidazolium cations and bicarbonate anions was found to exhibit the highest activity as it could produce 78.3% yield of DCU at 97.5% conversion of CHA within only 4 h. Poly-2 was also found to be an active catalyst toward various of mono- and diamines to produce good yields of the corresponding diureas (ca. 58.7−83.3%) and/or oligomers. The catalytic behavior of poly-2 is believed to be influenced by not only good interactions of bicarbonate anion to both substrates and CO2 but also by the flexibility of the aliphatic “linker” of the cationic part, which affords large freedom of access. Reaction variables such as temperature, time, and pressure also affected the overall catalytic activity. In addition, poly-2 could be easily recovered G


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ACS Sustainable Chemistry & Engineering Table 4. Synthesis of Ureas from Various Substratesa,*

Reaction conditions: Substrate = 40.0 mmol, Poly-2 = 0.25 g, NMP = 10.0 mL, CO2 pressure = 90 bar, Temperatur e= 170 °C, Time = 4 h. Isolated yield (1H NMR spectra of all products are given in SI, Figure S10). cDetermined by GC-FID from final product mixture with isooctane as an external standard. dDetermined by GPC with m-cresol as solvent. eHMDA = Hexamethylenediamine, fReaction time: 8 h. g1,4-DACH = 1,4diaminocyclohexane hMethyleneBis = 4 4′-methylenebis(cyclohexylamine). *GC-MS (SI, Figure S9) and elemental analysis data (SI, Table S2) for all DSU products. a b

by simply washing−drying after each cycle and reused at least seven times with no significant loss of catalytic activity. Further investigation on the mechanistic pathway will be necessary to clarify the details of this fascinating and important catalytic reaction using this novel and robust heterogeneous catalyst system.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the Ministry of Science, ICT & Future Planning as “Fusion Research Program for Green Technologies (2012M3C1A1054497)” and as “C1 Gas Refinery Program (2015M3D3A1A01064895) through the National Research Foundation of Korea.

ASSOCIATED CONTENT

S Supporting Information *

This material is available free of charge via the Internet. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b01369. Experimental procedures, spectroscopic characterization of all the catalysts. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +82 41 5898469. Fax: +82 41 5898580 (Dinesh Kumar Mishra). *E-mail: [email protected]. Phone: +82 41 5898469. Fax: +82 41 5898580 (Yong Jin Kim). Notes

The authors declare no competing financial interest. H


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Reusable Polystyrene-Functionalized Basic Ionic Liquids as Catalysts

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