Hydroxyl-functionalized pyrazolium ionic liquids to catalyze chemical fixation of CO2: Further benign reaction condition for the single-component catalyst

Journal of Molecular Liquids 293 (2019) 111479

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Hydroxyl-functionalized pyrazolium ionic liquids to catalyze chemical fixation of CO2: Further benign reaction condition for the singlecomponent catalyst Tengfei Wang a,b,c,1, Yuan Ma a,b,c,1, Jiamin Jiang a,b,c, Xinrui Zhu a,b,c, Baowan Fan a,b,c, Guanyao Yu d, Ningning Li d, Shasha Wang d, Tiegang Ren a,b,c,⁎, Li Wang a,b,c,⁎, Jinglai Zhang a,b,c,⁎ a

Institute of Upconversion Nanoscale Materials, PR China Henan Provincial Engineering Research Center of Green Anticorrosion Technology for Magnesium Alloy, PR China c College of Chemistry and Chemical Engineering, Henan University, Kaifeng, Henan 475004, PR China d Minsheng College, Henan University, Kaifeng, Henan 475004, PR China b

a r t i c l e

i n f o

Article history: Received 24 June 2019 Received in revised form 28 July 2019 Accepted 30 July 2019 Available online 31 July 2019 Keywords: Hydroxyl-functionalized pyrazolium ionic liquids Epoxides CO2 Catalysis Density functional theory

a b s t r a c t Lots of ionic liquids have been utilized as catalyst for the coupling reaction of carbon dioxide with epoxides, however, catalyzed conditions could not be regarded as benign, especially when no co-catalyst and/or organic solvent is involved. A series of hydroxyl-functionalized pyrazolium ionic liquids are firstly synthesized. They would efficiently catalyze the cycloaddition of carbon dioxide and propylene oxide under 110 °C and 1.0 MPa carbon dioxide initial pressure with 1 mol% catalyst during 4 h resulting in the product yield of 91.2%. The catalytic condition is greatly refined as compared with other single-component ionic liquids with simple anion. Simultaneously, the effect of reaction temperature, amount of catalyst, carbon dioxide initial pressure, and reaction time is explored along with the reusability of catalyst. For most of epoxides with terminal substituted group, HEEPzBr presents acceptable catalytic activity. The difference of HEMPzBr, HEEPzBr, and HPEPzBr is also explored by the density functional theory calculations. © 2019 Elsevier B.V. All rights reserved.

1. Introduction The utilization of carbon dioxide (CO2) has attracted continuous attentions from academic and industrial communities [1], since it is an efficient pathway to control the CO2 concentration in the air. The utilization of CO2 is not limited to alleviate the environmental problems but afford the inexpensive, nontoxic, and abundant C1 resources to moderate the depletion of fossil fuels [2,3]. The coupling reaction of CO2 with epoxides to produce five-membered cyclic carbonates [4,5] is one of the most promising routes. It is an atomic efficiency route resulting in valuable products with wide applications, such as electrolytes [6], polar aprotic solvents [7], and chemical intermediates [8]. It is a challenge to promote reaction under mild conditions due to the high thermostability and kinetic inertness of CO2. The involvement of high-efficiency catalyst is a powerful and simple pathway to refine the reaction conditions.

⁎ Corresponding authors at: Institute of Upconversion Nanoscale Materials, PR China. E-mail addresses: [email protected] (T. Ren), [email protected] (L. Wang), [email protected] (J. Zhang). 1 These authors contributed equally to this work.

https://doi.org/10.1016/j.molliq.2019.111479 0167-7322/© 2019 Elsevier B.V. All rights reserved.

Ionic liquids (ILs) [9,10] are one of the most prominent catalysts in various catalytic systems with distinct advantages including high thermostability, negligible vapor pressure, and excellent chemical stability [11]. The reaction condition catalyzed by single-component ILs is still not regarded as benign. Direct inclusion of co-catalyst, normally Lewis acid, in the catalytic system is a facile and simple method to improve the catalytic activity. However, the reaction condition is harsh due to the water sensitivity of Lewis acids [12]. Later, an alternative method is proposed to introduce the solvent containing hydroxyl-group into the IL-based catalytic system [13]. Inspired by it, the hydroxylfuntionalized imidazolium ILs have been synthesized by Sun et al. [14] Besides the nucleophilic activation, the hydroxyl group would activate the epoxide leading to the energy-rich intermediate [15]. The carboxylic acid group is the stronger Brønsted acid along with better hydrogen bond donor as compared with hydroxyl group. Consequently, carboxyl-functionalized imidazolium ILs are developed with the better catalytic activity [16]. Later, carboxyl-functionalized quaternary ammonium salts [17], carboxyl-functionalized pyridinium ILs [16], and others are synthesized in succession with the critical aim to develop the highefficiency single-component ILs. Although some progresses have been made, there are still some problems required to be resolved. The reaction temperature and required CO2 pressure are still too high, which is

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T. Wang et al. / Journal of Molecular Liquids 293 (2019) 111479

difficult to be controlled [18]. Or the reaction time is very long resulting in the lower turnover frequency (TOF) [19]. To equilibrate several reaction conditions is the more difficult in design of catalysts along with other benefits such as, facile synthesis, economic available, easy purification, and others. It is found that the task specified imidazolium ILs is one of the most popular ILs with better catalytic activity as compared with other task specified ILs, including task-functionalized quaternary ammonium salts or pyridinium ILs. The special five-member ring of imidazole would play key role in improving the catalytic activity. Pyrazole has the similar five-member ring with imidazole, which would perhaps present the similar or better catalytic activity. It has been testified in our previous work. Some pyrazolium ILs [20,21] have been employed as the catalyst for the title reaction presenting the comparable catalytic activity with imidazolium ILs. However, reaction conditions including reaction temperature and pressure are not greatly refined. To our best knowledge, task specified pyrazolium ILs are rarely reported till now. Or they are rarely applied as catalyst for the coupling reaction of CO2 and propylene oxide (PO). In this work, five hydroxyl-functionalized pyrazolium ILs including 1-methyl-2-hydroxyethyl pyrazolium bromide (HEMPzBr), 1-ethyl-2hydroxyethyl pyrazolium bromide (HEEPzBr), 1-ethyl-2-hydroxypropyl pyrazolium bromide (HPEPzBr), 1-ethyl-2-hydroxyethyl-3-methyl pyrazolium bromide (HEEMPzBr), and 1-ethyl-2-hydroxyethyl-3,5dimethyl pyrazolium bromide (HEEDMPzBr) are synthesized to be catalysts for the title reaction. On the basis of HEEPzBr, the optimal reaction conditions are further refined. Finally, the reaction mechanism is elucidated by density functional theory (DFT). Our central goal is to find a high efficient catalyst, which would greatly decrease the reaction temperature but not increase other conditions. 2. Experimental and theoretical details 2.1. Instruments and materials High resolution mass spectra (HR-MS) was measured in Agilent 1290 Infinity LC with 6224 TOF MSD. 1H NMR (400 MHz) and 13C NMR (101 MHz) spectra were obtained via Bruker Avance III HD spectrometer with tetramethylsilane (TMS) as the internal standard. The thermal decomposition temperature was analyzed with a thermal gravimetric analyzer (Mettler Toledo TGA/SDTA851e). GC analyses were performed on Agilent GC-7890B with a flame ionization detector. The 1-methylpyrazole, 1-ethylpyrazole, 3-methylpyrazole, 3,5dimethylpyrazole, bromoethanol, and 3-bromo-1-propanol were purchased from Shanghai Macklin Biochemical Co. Ltd.. PO, epoxy chloropropane, styrene oxide, and other epoxides were purchased from Aladdin Industrial Co. Other ordinary used organic chemical regents were produced from Sinopharm Chemical Reagent Co. Ltd. All reactants were used directly as received without any further purification. The CO2 (99.9%) was purchased from Kaifeng Xinri Gas Co. 2.2. Preparation of hydroxyl pyrazolium ionic liquids These hydroxyl-functionalized pyrazolium ionic liquids were synthesized according to the literature [14] with some revision. For example, the synthesis procedure of 1-methyl-2-hydroxyethyl pyrazolium ILs is shown in Scheme 1. 1-methyl-2-hydroxyethyl pyrazolium bromide (HEMPzBr). In a three-necked bottle, 0.82 g (10 mmol)1-methylpyrazole and 1.25 g (10 mmol) bromoethanol were added into 10 mL CH3CN. Then, the mixtures were stirred at reflux temperature for 48 h in a nitrogen atmosphere. After the reaction, the volatiles were removed under reduced pressure. The residual was washed several times by ethyl acetate to give a white solid. After filtration and drying in vacuum, the pure HEMPzBr was obtained: 1.532 g, yield 74%, white solid. 1H NMR (400 MHz, DMSO‑d6) δ 8.58 (dd, J = 21.9, 2.8 Hz, 2H, Pz-H), 6.86 (t, J

= 3.1 Hz, 1H, Pz-H), 5.26 (s, 1H, –OH), 4.61 (t, J = 5.0 Hz, 2H, –CH2-), 4.20 (s, 3H, –CH3), 3.76 (t, J = 5.2 Hz, 2H, –CH2-).13C NMR (101 MHz, D2O) δ 138.38, 137.98, 107.43, 59.34, 52.34, 37.61. HR-MS (QTOF) calcd. for [C6H11N2O+] (m/z): 127.0866, found: 127.0868. Other hydroxyl-functionalized pyrazolium ionic liquids were synthesized by alkylpyrazole and bromhydrin with the identical procedure for HEMPzBr. 1-ethyl-2-hydroxyethyl pyrazolium bromide (HEEPzBr). White solid, 1.348 g, yield 61%; m.p: 43–44 °C·1H NMR (400 MHz, DMSO‑d6) δ 8.67 (m, 1H, Pz-H), 8.58 (m, 1H, Pz-H), 6.93 (t, J = 3.0 Hz, 1H, Pz-H), 5.27 (t, J = 5.4 Hz, 1H, –OH), 4.63 (t, J = 5.0 Hz, 2H, –CH2-), 4.58 (q, J = 7.2 Hz, 2H, –CH2-), 3.78 (q, J = 5.2 Hz, 2H, –CH2-), 1.45 (t, J = 7.2 Hz, 3H, –CH3). 13C NMR (101 MHz, DMSO‑d6) δ 138.33, 137.00, 107.77, 59.43, 52.40, 45.60, 14.62. HR-MS (QTOF) calcd. for [C7H13N2O+] (m/z): 141.1022, found: 141.1025. 1-ethyl-2-hydroxypropyl pyrazolium bromide (HPEPzBr). White solid, 1.363 g, yield 58%; m.p: 54–55 °C·1H NMR (400 MHz, D2O) δ 8.14 (dt, J = 2.6, 1.3 Hz, 2H, Pz-H), 6.70 (td, J = 3.0, 0.9 Hz, 1H, Pz-H), 4.48 (t, J = 7.2 Hz, 2H, –CH2-), 4.40 (q, J = 7.3 Hz, 2H, –CH2-), 3.59–3.54 (m, 2H, –CH2-), 2.11–2.01 (m, 2H, –CH2-), 1.48 (td, J = 7.3, 0.9 Hz, 3H, –CH3). 13C NMR (101 MHz, DMSO‑d6) δ 137.77, 136.88, 107.84, 57.43, 47.37, 45.35, 31.84, 14.50. HR-MS (QTOF) calcd. for [C8H15N2O+] (m/z): 155.1179, found: 155.1178. 1-ethyl-2-hydroxyethyl-3-methyl pyrazolium bromide (HEEMPzBr). White solid, 1.504 g, yield 64%; m.p: 41–42 °C·1H NMR (400 MHz, D2O) δ 8.03 (m, 1H, Pz-H), 6.53 (m, 1H, Pz-H), 4.48 (q, J = 4.9 Hz, 2H, –CH2-), 4.38 (tt, J = 11.9, 6.3 Hz, 2H, –CH2-), 3.85 (dt, J = 10.9, 5.1 Hz, 2H, –CH2-), 2.41 (m, 3H, –CH3), 1.33 (m, 3H, –CH3). 13C NMR (101 MHz, D2O) δ 147.72, 136.96, 107.95, 59.26, 51.91, 42.26, 13.40, 11.34. HR-MS (QTOF) calcd. for [C8H15N2O+] m/z: 155.1179, found: 155.1177. 1-ethyl-2-hydroxyethyl-3,5-dimethyl pyrazolium bromide (HEEDMPzBr). White solid, 1.606 g, yield 65%; m.p: 118–119 °C. 1H NMR (400 MHz, DMSO‑d6) δ 6.61 (s, 1H, Pz-H), 4.54–4.45 (m, 4H, – CH2-CH2-), 3.69 (t, J = 5.0 Hz, 2H, –CH2-), 2.46 (d, J = 4.8 Hz, 6H, – CH3), 1.27 (t, J = 7.2 Hz, 3H, –CH3). 13C NMR (101 MHz, DMSO‑d6) δ 147.57, 146.32, 108.46, 59.58, 49.52, 42.37, 14.49, 12.28, 11.74. HR-MS (QTOF) calcd. for [C9H17N2O+] m/z: 169.1335, found: 169.1338. 1H and 13C NMR spectra of five hydroxyl functionalized pyrazolium ionic liquids are showed in Fig. S1. 2.3. Coupling reaction of CO2 with epoxides Epoxides and ionic liquids were firstly added into the stainless steel autoclave (100 mL) at ambient temperature. Then, CO2 (1.0–3.0 MPa) was introduced to the reactor vessel and heated up to the designed temperature. The reaction was carried out at 90–140 °C for 1–5 h. After that, the reactor was cooled to ambient temperature and the remaining CO2 was slowly released. Finally, the products were isolated and yields were obtained. Some reactions were repeated to ensure the reproducibility of yields was ±3%. The structures of various cyclic carbonates were characterized by 1H NMR (see SI). 2.4. Computational details Geometric optimization and vibration analysis of the reactants, intermediates, and transition states were carried out by the Becke's three parameters exact exchange-functional combined with Perdew and Wang (B3PW91) [22,23] method along with the 6-31G(d,p) basis set [24]. On the basis of the optimized transition states, the minimumenergy path (MEP) was constructed following the intrinsic reaction coordinates (IRC) [25]. The energy was refined at the M06/6-311 + G (2d,2p) level [26] without variation of the optimized geometry. And the solvent effect of ethyl ether (Et2O) is considered by the polarized continuum model (PCM) [27,28]. The non-covalent interactions are considered by non-covalent interactions (NCI) [29,30] and atoms in

T. Wang et al. / Journal of Molecular Liquids 293 (2019) 111479

3

Scheme 1. Synthetic route and structures of the hydroxyl functionalized pyrazolium ILs.

molecule (AIM) [31]. The atomic charge is calculated by natural bond orbital (NBO) analysis [32,33] The aforementioned electronic calculations were performed by the Gaussian 09 program [34]. 3. Results and discussion 3.1. Effect of catalysts The catalytic performance of five hydroxyl-functionalized pyrazolium ILs is tested for the coupling reaction of CO2 and PO. The corresponding data are tabulated in Table 1. For entries 1–3, the product yield catalyzed by HEEPzBr (entry 2) is the larger than items catalyzed

by HEMPzBr (entry 1) and HPEPzBr (entry 3), which is attributed to their different alkyl chain length. The ethyl group on N1 atom along with hydroxyethyl group on the other N atom is the more suitable combination. Except for the alkyl chain length, the catalytic activity would be also affected by the substituted position of alkyl group. The catalytic activity of HEEMPzBr (entry 4) and HEEDMPzBr (entry 5) is comparable, which is even larger than that of HEEPzBr (entry 2) suggesting that the inclusion of methyl group on pyrazole ring would further increase the catalytic activity. The overall performance is not enhanced with the further increasing functional group number, which is similar to the hydroxyl-functionalized quaternary ammonium salt [13]. The NEt(HE)3Br (ethyltrihydroxyethyl ammonium bromide) has the higher

Table 1 Catalytic performance of various catalysts.a Entry

a b

Structure

Catalyst

Yieldb (/%)

1

HEMPzBr

85.3

2

HEEPzBr

91.2

3

HPEPzBr

89.5

4

HEEMPzBr

92.9

5

HEEDMPzBr

92.2

6

DEPzBr [22]

76.1

Reaction conditions: PO 0.1 mol, catalyst loading 1 mol%, temperature 110 °C, initial CO2 pressure 1.0 MPa, time 4 h. Isolated yield.

4

T. Wang et al. / Journal of Molecular Liquids 293 (2019) 111479

catalytic activity than N(HE)4Br (tetrakis(2-hydroxyethyl) ammonium bromide). Under the same reaction parameters, the catalytic activity of DEPzBr (entry 6) [21] is much lower than that of HEEPzBr indicating that the inclusion of hydroxyl group greatly refine the reaction. As compared with HEMImBr (see entry S3 in Table S1) [14], the product yield catalyzed by HEEPzBr is the slightly lower (99.2% for HEMImBr v.s. 91.2% for HEEPzBr). However, the required reaction temperature and CO2 initial pressure are much higher for HEMImBr. When the reaction temperature is decreased by 15 °C reaching 110 °C, the product yield catalyzed by HEMImBr is decreased to be 90% (entry S4 in Table S1) [35]. At the same reaction condition (Cat. 1.6 mol%, 125 °C, 2.0 MPa, 1 h), the catalytic activity of HETBAB (entry S5 in Table S1) is the smaller than that of HEMImBr [14]. If both temperature and CO2 initial pressure are decreased, the catalytic activity of HETBAB would be even worse, which is expected to be lower than that of HEEPzBr. When the temperature and CO2 initial pressure are decreased to be 120 °C and 1.5 MPa, the product yield is only 81% for NEt3(HE)Br (entry S6 in Table S1). When carboxyl- and amino-functionalized imidazolium ILs 3-carboxyethyl-1-methyl imidazolium bromide (CEMImBr) and 3-aminopropyl-1-methyl imidazolium bromide (APMImBr) [36] are employed as catalysts, the reaction conditions are catalyst 1 mol%, temperature 120 °C, CO2 pressure 1.5 MPa, and time 1.5 h leading to the product yields of 84.0% for CEMImBr (entry S7 in Table S1) and 88.6% for APMImBr (entry S8 in Table S1). Although the reaction temperature and CO2 initial pressure are decreased, the catalytic activity is also decreased. Only CMMImBr (entry S9 in Table S1) [37] could be comparable with HEEPzBr. However, not only the catalyst dosage but also CO2 initial pressure is the higher for CMMImBr. In general, HEEPzBr presents the better catalytic activity than aforementioned ionic liquids. More importantly, the reaction parameters, especially for reaction temperature and CO2 initial pressure, are greatly decreased, which is better than most of single component ionic liquid reported previously. 3.2. Effect of reaction parameters The HEEPzBr is chosen as an example to investigate the influence of reaction parameter on the catalytic activity. Reaction temperature has a remarkable effect on the catalytic activity (see Fig. 1). The yield of propylene carbonate (PC) enhanced about 12.4% when the reaction temperature is increased from 90 °C to 110 °C. Afterwards, the yield of PC has a slight decrease with the further enhancement of temperature, which is ascribed to the occurrence of side reactions. The selectivity is almost invariably in the whole process. The HEEPzBr is stable even at 206 °C without obvious weight loss suggesting that the HEEPzBr

would not dissolve in the catalytic reaction. The corresponding thermogravimetric analysis (TGA) curves for HEMPzBr, HEEPzBr, HPEPzBr, HEEMPzBr, and HEEDMPzBr are plotted in Fig. 2. The CO2 initial pressure also has a significant effect on the catalytic activity (Fig. 3). The PC yield is increased by 11.9% when the CO2 pressure varies from 0.5 MPa to 1.0 MPa. After that, the PC yield is kept till that the CO2 pressure is increased to be 2.5 MPa. However, there is slight decrease when the CO2 pressure reaches 3.0 MPa. Initially, the increased CO2 pressure would enhance the concentration of reactant, which is beneficial for promoting the reaction. The concentration of ionic liquids would be decreased with the more CO2 entering the system, the reaction is suppressed to some extent resulting in the less product yield. Next, the effect of catalyst loading is also considered. As shown in Fig. 4, the PC yield increases sharply from 53.8% to 87.9% with catalyst amount increasing from 0.2 mol% to 0.5 mol%. However, PC yield has a slight enhancement even up to 1 mol% of catalyst loading. Finally, the dependence of reaction time on the PC yield is plotted in Fig. 5. The reaction proceeds rapidly at the initial time and the conversion of PO is almost finished within 4 h. The elongation of reaction time would not promote the reaction. In general, the optimal reaction conditions are reaction temperature 110 °C, CO2 initial pressure 1.0 MPa, HEEPzBr 1 mol%, and reaction time 4 h. Both reaction temperature and CO2 initial pressure are greatly decreased, which are two important items to arouse the potential dangerous during the reaction. The reaction temperature is 130 °C for alkyl pyrazolium ILs [38] and protic pyrazolium ILs [20], which is 20 °C higher than the employed temperature in this work. Although the reaction temperature for the amino-functionalized pyrazolium ILs [39] is decreased to be 110 °C, CO2 initial pressure is still as high as 1.5 MPa. Even for the hydroxyl-functionalized imidazolium ILs [14], the reaction temperature is 125 °C along with CO2 initial pressure of 2.0 MPa. The reaction temperature and CO2 initial pressure for the coupling reaction catalyzed by HEEPzBr is much better than most of previous reported single-component ionic liquids with the exception of phenolic hydroxyl-functionalized imidazolium ILs [40]. However, the cost to synthesize phenolic hydroxyl-functionalized imidazolium ILs is too expensive to be used in large scale. It is expected that it would present the better result. Perhaps, the heterogeneous catalysts [41,42] have the higher product yields, however, the reaction temperature is as high as 140 °C, which is even harsh (see Table S2). 3.3. Reusability of catalyst Under the optimized conditions, the HEEPzBr is employed to catalyze the reaction and recovered by distillation and reused for subsequent reactions (see Fig. 6). The PC yield is not significantly decreased

HEMPzBr HEEPzBr HPEPzBr HEEMPzBr HEEDMPzBr

100

90 80

80

70

60

Weight (%)

Yield and Selectivity (%)

100

Yield Sel.

90

100

110

120

130

60 40 20

140

Temperature (oC)

0 100

200

300

400

500

Temperature (°C) Fig. 1. Influence of temperature on PC yield and selectivity (reaction condition: PO 0.1 mol, HEEPzBr 1 mol%, initial CO2 pressure 1.0 MPa, 4 h).

Fig. 2. TGA curves of five hydroxyl pyrazolium ILs.

600

T. Wang et al. / Journal of Molecular Liquids 293 (2019) 111479

100

Yield and Selectivity (%)

Yield and Selectivity (%)

100

90

80

70

60

5

Yield Sel.

90

80

70

Yield Sel.

60

50

0.5

1.0

1.5

2.0

2.5

1

3.0

2

Initial CO2 pressure (MPa) Fig. 3. Influence of CO2 initial pressure on PC yield and selectivity (reaction condition: PO 0.1 mol, HEEPzBr 1 mol%, 110 °C, 4 h).

if the HEEPzBr is reused in the second time. The distillation process is complex, which is not suitable for large scale application. After the second run, the catalyst is not separated. The catalyst is reused with addition of the 0.1 mol PO in the reaction system to ensure the reaction. The PC yield is decreased for every run, which is attributed to the consume of catalyst. At the sixth run, additional 25% HEEPzBr is involved in the reaction system leading to the increase of PC yield, which is even larger than the first run. It is not necessary to separate the catalyst every time. The addition of slight amount of catalyst would further promote the reaction, which is easy to be operated.

3.4. Applicability of catalyst The suitability of HEEPzBr are tested for other epoxides, the corresponding data are tabulated in Table 2. The product yield is slightly affected by the size of substituted group on C atom with the exception of cyclohexene oxide. The large bulk of substituted group would block the activation of C\\O bond of PO. However, the anion of HEEPzBr would activate the other carbon atom with nonsubstitution. Consequently, HEEPzBr has the least catalytic activity towards cyclohexene oxide (7a) because of the largest bulk block for both carbon atoms.

4

5

Fig. 5. Influence of reaction time on PC yield and selectivity (reaction condition: PO 0.1 mol, HEEPzBr 1 mol%, CO2 initial pressure 1.0 MPa, 110 °C).

3.5. Elucidation of different catalytic activity by DFT method To further understand the difference between various ionic liquids, the reaction mechanism is simulated by the DFT method. The threestep reaction mechanism has been thoroughly elucidated in previous literature along with the rate determining step of ring-opening of PO [43,44]. In this work, only the rate determining step is studied following the three-step mechanism. It has been testified that the result obtained by “Double-IL” model is more reliable than “Single-IL” model [45]. Therefore, only the former model is employed in this work. The cation is responsible for the electrophilic activation and the anion is in charge of the nucleophilic activation. HEEPzBr is still taken as the example to explore the mechanism. Possible routes by “Double-IL” model are considered according to the different electrophilic activation. First, both hydroxyl groups are employed to activate the O atom of PO; second, one hydroxyl group is employed to activate the O atom of PO and the other is used to stabilize the Br− anion; Third, one HEEPzBr plays both the electrophile and nucleophile to activate the C\\O bond and the other one stabilize the system by weak interactions. The corresponding structures and barrier heights are plotted in Table S3. If the different hydrogen atoms are used to activate PO or stabilize the whole system, lots of other possible routes would be confirmed. However, the deviation among them would not

100

100

*

90

25% of the initial amount of HEEPzBr were added

80

80

Yield (%)

Yield and Selectivity (%)

3

Time (h)

70

*

*

*

60

40

60 Yield Sel.

50 40

20

0

0.2

0.4

0.6

0.8

1.0

1

2

3

Reuse

4

5

6

Catalyst loading (mol%) Fig. 4. Influence of HEEPzBr loading on PC yield and selectivity (reaction condition: PO 0.1 mol, CO2 initial pressure 1.0 MPa, 110 °C, 4 h).

Fig. 6. Reused performance of the catalyst (Reaction conditions: PO 0.1 mol, HEEPzBr 1 mol%, CO2 initial pressure 1.0 MPa, temperature 110 °C, time 4 h), N99% PC selectivity is maintained. * indicates another 0.1 mol PO was added into the reaction mixture.

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T. Wang et al. / Journal of Molecular Liquids 293 (2019) 111479

Table 2 Cycloaddition reaction of CO2 and various epoxides catalyzed by HEEPzBr.a Entry

Epoxide

Cyclic carbonate

Yieldb (%) 91.2

1 1a 1b 2

89.4 2a 2b 87.4

3 3a 3b

74.3

4 4a 4b 5

94.4 5a 5b 94.8

6

6a 6b 7c

63.6 7a

a b c

7b

Reaction conditions: epoxides 100 mmol, HEEPzBr 1 mol% (mol% of pyrazolium salt moieties to epoxide), CO2 pressure 1.0 MPa, reaction time 4.0 h, temperature 110 °C. Isolated yield. Reaction conditions:temperature 130 °C.

be large since the difference among them is only aroused by the configuration rotation. They are not considered one by one. The route 1 is the most favorable one with the lowest barrier height. Following the similar pathway, the rate-determining step catalyzed by HEMPzBr and HPEPzBr are further confirmed. The potential profiles are plotted in Fig. 7. The barrier heights decrease in the order of D-HEMPzBr (HEMPzBr, 13.85 kcal/mol) N D-HPEPzBr (HPEPzBr, 12.12 kcal/mol) N D-HEEPzBr (HEEPzBr, 10.43 kcal/mol), which is totally consistent with the experimental product yields.

On the basis of the natural bond orbital (NBO) analysis (see Fig. S2), the charge of H atom in hydroxyl group is almost the same in three transition states suggesting the similar interactions of static electricity. Besides it, the weak interactions also play a critical role to activate the ring opening. The difference between them is mainly aroused by the weak interaction. The non-covalent interaction is analyzed by atoms in molecule (AIM) and noncovalent interactions (NCI), which is listed in Table 3 and Fig. 8. There is hydrogen bond between H atom of hydrogen group and O atom of PO with the positive value of ∇2ρ. Two

Fig. 7. Potential energy profiles and sketch structures of transition states for the ring-opening step along routes D-HEMPzBr, D-HEEPzBr, and D-HPEPzBr calculated at the M06/6-311 + G (2d,2p) (PCM)//B3PW91/6-31G(d,p) level.

T. Wang et al. / Journal of Molecular Liquids 293 (2019) 111479

7

Table 3 Selected topological parameters of the bond critical points (BCP) and the charge of some important atoms in the transition states calculated at the B3PW91/6-31G(d,p) level of theory.

D-HEMPzBr

D-HEEPzBr

D-HPEPzBr

X-Y…Z

Sign(λ2)ρ

ρ

∇2 ρ

G

V

H

G/︱V︱

O1-H…O3 O2-H…O3 Br1…C1 O1-H…O3 O2-H…O3 Br1…C1 O1-H…O3 O2-H…O3 Br1…C1

−0.04317 −0.04970 −0.03921 −0.03942 −0.05479 −0.03785 −0.04024 −0.05268 −0.03952

0.04317 0.04970 0.03921 0.03942 0.05479 0.03785 0.04024 0.05268 0.03952

0.12694 0.13310 0.07215 0.11578 0.14527 0.06839 0.12159 0.14158 0.06872

0.03259 0.03614 0.02098 0.02958 0.04025 0.01972 0.03061 0.03877 0.02034

−0.03345 −0.03900 −0.02392 −0.03021 −0.04418 −0.02234 −0.03082 −0.04215 −0.02350

−0.00086 −0.00286 −0.00294 −0.00063 −0.00393 −0.00262 −0.00021 −0.00338 −0.00316

0.97434 0.92657 0.87708 0.97907 0.91098 0.88260 0.99313 0.91992 0.86561

hydrogen bonds O1-H…O3 and O2-H…O3 are not equilibrium with the different ρ value. The hydrogen bond, O1-H…O3, is the weaker than O2-

H…O3 with the smaller ρ value indicating the less important role. In contrast, the other hydrogen bond plays the more important role to be

Fig. 8. NCI plots of D-HEMPzBr, D-HEEPzBr, and D-HPEPzBr. The corresponding 3D plots are displayed on the right with blue regions representing strong electrostatic interactions and green regions representing more dispersion attractive interactions.

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T. Wang et al. / Journal of Molecular Liquids 293 (2019) 111479

electrophile. The strength of O2-H…O3 increases in the order of DHEEPzBr N D-HPEPzBr N D-HEMPzBr, which is consistent with the variation of barrier heights. Between in C1…Br1 interaction for three transition states is too small to differentiate them, which is not the major item to arouse the diversity of catalysts. The hydrogen bond interaction not only plays important role to activate the ring-opening of PO along with the electrostatic interaction but also is the main factor resulting in the difference of various catalysts. 4. Conclusions Five hydroxyl-functionalized pyrazolium ionic liquids with different chain length or substituted group are first synthesized. All of them present high catalytic efficiency for the cycloaddition reaction of CO2 and PO without any solvent and co-catalyst. HEEMPzBr is the best with the product yield of 92.9% under 110 °C and 1.0 MPa CO2 initial pressure with 1 mol% catalyst during 4 h. It is even better than most of previous reported imidazolium ILs. More important, the reaction temperature and pressure are much refined as compare with other singlecomponent ionic liquids. After that the reaction conditions including reaction temperature, amount of catalyst, CO2 initial pressure, and reaction time are optimized one by one to confirm the most optimal reaction condition. Moreover, HEEPzBr has good suitability for most of epoxides with satisfied product yields. Finally, the difference among HEMPzBr, HEEPzBr, and HPEPzBr is elucidated by DFT calculation. The hydrogen bond between catalyst and PO is the main factor to arouse the difference of three catalysts rather than the nucleophilic activity. Acknowledgments We thank the National Supercomputing Center in Shenzhen (Shenzhen Cloud Computing Center) and Changsha (Changsha Cloud Computing Center) for providing computational resources and softwares. This work was supported by the National Natural Science Foundation of China (21476061, 21503069, 21676071), Key Scientific Research Project of Colleges and Universities in Henan Province (18A150024), Innovation and entrepreneurship Support Project for Minsheng College of He’nan University (MSCXCY2018005, MSCXCY2018006). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.molliq.2019.111479. References [1] N.L. Panwar, S.C. Kaushik, S. Kothari, Role of renewableenergy sources in environmental protection: a review, Renew. Sust. Energ. Rev. 15 (2011) 1513–1524. [2] B.A. Vara, T.J. Struble, W.W. Wang, M.C. Dobish, J.N. Johnston, Nantioselective small molecule synthesis by carbon dioxidefixation using a dual brønsted acid/base organocatalyst, J. Am. Chem. Soc. 137 (2015) 7302–7305. [3] D.J. Tao, F.F. Chen, Z.Q. Tian, K. Huang, S.M. Mahurin, D.E. Jiang, S. Dai, Highly efficient carbon monoxide capture by carbanion-functionalized ionic liquids through C-site interactions, Angew. Chem. 129 (2017) 6947–6951. [4] M.S. Liu, M. Liang, X. Li, X.X. Gao, J.M. Sun, Novel urea derivative-based ionic liquids with dual-functions: CO2 capture and conversion under metal- and solvent-free conditions, Green Chem. 18 (2017) 2851–2863. [5] M.S. Liu, J.W. Lan, L. Liang, J.M. Sun, M. Arai, Heterogeneous catalytic conversion of CO2 and epoxides to cyclic carbonates over multifunctional tri-s-triazine terminallinked ionic liquids, J. Catal. 347 (2017) 138–147. [6] T. Sakakura, J.C. Choi, H. Yasuda, Transformation of carbon dioxide, Chem. Rev. 107 (2007) 2365–2387. [7] T. Sakakura, K. Kohno, The synthesis of organic carbonates from carbon dioxide, Chem. Commun. (11) (2009) 1312–1330. [8] V. Laserna, G. Fiorani, C.J. Whiteoak, E. Martin, E. Escudero-Adán, A.W. Kleij, Carbon dioxide as a protecting group: highly efficient and selective catalytic access to cyclic cis-diol scaffolds, Angew. Chem. Int. Ed. 53 (2014) 10416–10419. [9] F.F. Chen, K. Huang, Y. Zhou, Z.Q. Tian, X. Zhu, D.J. Tao, D.E. Jiang, S. Dai, Multi-molar absorption of CO2 by the activation of carboxylate groups in amino acid ionic liquids, Angew. Chem. Int. Ed. 55 (2016) 7166–7170.

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