Insight on asym-pyrazolium ionic liquids for chemical fixation of CO2

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Kinetics, Catalysis, and Reaction Engineering

Insight on asym-pyrazolium ionic liquids for chemical fixation of CO2 and propylene epoxide into propylene carbonate without organic solvent and metal Yuan Ma, Yue Zhang, Ci Chen, Jingshun Zhang, Baowan Fan, Tengfei Wang, Tiegang Ren, Li Wang, and Jinglai Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02318 • Publication Date (Web): 20 Sep 2018 Downloaded from http://pubs.acs.org on September 26, 2018

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Industrial & Engineering Chemistry Research

Graphic abstract

Synopsis: Asym-pyrazolium ionic liquids synergistically promote cycloaddition reaction of CO2 with propylene epoxide in the absence of solvent and metal.

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Insight on asym-pyrazolium ionic liquids for chemical fixation of CO2 and propylene epoxide into propylene carbonate without organic solvent and metal Yuan Maa,b, Yue Zhangb, Ci Chenb, Jingshun Zhanga,b, Baowan Fana,b, Tengfei Wanga,b, Tiegang Ren*a,b, Li Wang*b, Jinglai Zhang*b aEngineering b

Laboratory for Flame Retardant and Functional Materials of Henan Province,

College of Chemistry and Chemical Engineering, Henan University, Kaifeng, Henan 475004, P.R. China

Abstract From the perspective of environmental protection and resource utilization, the fixation of carbon dioxide (CO2) is an interesting and meaningful topic. In this work, a series of asym-dialkylpyrazolium ionic liquids are synthesized by us to explore their catalytic activity for the cycloaddition of CO2 and propylene epoxide (PO) to produce propylene carbonate (PC). They could be easily synthesized by a simple reaction, which is the premise for the large scale application. The effect of alkyl chain length in cation on catalytic performance is investigated. Although asym-pyrazolium ILs presents the less catalytic activity than sym-pyrazolium ILs, the product yield catalyzed by EPPzBr is as high as 85% with PC selectivity of 99%, which is better than most of non-functionalized ionic liquids. Moreover, the catalyst could be reused for at least five times without significant loss of catalytic activity. To elucidate the structure-property relationship, the difference between asym-pyrazolium ILs and sym-pyrazolium ILs is discussed in detail by Double-IL model associated with the Corresponding author E-mail: [email protected] Corresponding author E-mail: [email protected] * Corresponding author E-mail: [email protected] * *

1


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non-covalent interactions

and atoms in molecule analysis. The kind of

single-component catalysts for the fixation of CO2 is further enriched.

Keywords: asym-pyrazolium ILs; CO2 fixation; catalytic mechanism; Double-IL model

1. Introduction Carbon dioxide (CO2) is not limited to be greenhouse gas but also the non-toxic, inexpensive, and abundant C1 building block.1 The conversion and chemical fixation of CO2 to high-value compounds is particularly attractive in both environmental and social issues.2 However, the thermodynamic stability and kinetic inertness of CO2 impede its various conversions. The incorporation of catalyst has been testified to be an efficient route to promote the CO2 conversion. In past decades, remarkable progress has been achieved to utilize CO2 to synthesize methane,3 methanol,4 and formate.5 Among them, cyclic carbonate is one of the most attractive derivatives since it is not only aprotic polar solvent but also key structural motifs for synthesizing fine chemicals and pharmaceuticals.6,7 Various catalysts have been developed for the cycloaddition of CO2 and PO including metal-oxide,8 MOFs,9 and functional polymers,10 however, high catalyst loading, poor recyclability, and inclusion of metal and solvent is still inevitable in these systems. Therefore, the development of robust and single-component catalyst is a great challenge for academic researchers. The emergence of ionic liquids (ILs) offers us a novel choice that it is not only the catalyst but also the solvent.11 Moreover, there are some ubiquitous characteristics for ILs due to their distinct structures. If there is no need for the involvement of co-catalyst, especially for metal, it would be a green and “perfect” catalyst. 2



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In past decades, lots of single-component ILs have been developed to be taken as the homogeneous catalysts for the incorporation of CO2 and propylene epoxide (PO) to produce propylene carbonate (PC) including imidazolium ILs,12,13 quaternary ammonium salt,14,15 quaternary phosphonium salt,16 and guanidinium salt.17 Among them, imidazolium ILs have attracted most of attentions due to their excellent catalytic activity, easier synthetic route, and higher product yield. However, almost no attention has been paid to the pyrazolium ILs. As early as 1951, Wizinger et al. synthesized 1,2-dimethyl-5-styrylpyrazolim iodide and its derivatives.18 Later in 1975, Pande et al. developed a series of pyrazolium bromides and investigated their mass spectrometry by field desorption, field ionization, and electron impact.19 In 2003, Abu-lebdeh

et

al.

synthesized

a

series

of

N,N’-cyclized

pyrazolium

trifluoromethanesulfonimide salts and utilized them as lithium battery electrolyte with the acceptable performance.20 However, they are never employed as the catalyst for the coupling of CO2 and PO. Recently, we utilized a series of sym-dialkylpyrazolium ILs including dimethylpyrazolium iodide (DMPzI), diethylpyrazolium iodide (DEPzI), and dipropylpyrazolium iodide (DPPzI), to catalyze the chemical fixation of CO2.21 Moreover, the product yield of DEPzI is as high as 96%, which could be comparable to that of imidazolium ILs.22 The additional solvent or co-catalyst is not required. Pyrazolium ILs perhaps are taken as a new kind of efficient single-component catalyst for the title reaction. Since the sym-pyrazolium ILs are developed, it is easy to infer that the asym-pyrazolium ILs also have the similar catalytic properties. In this work, five 1-ethyl-2-alkylpyrazolium bromides are synthesized and employed to promote the cycloaddition of CO2 with epoxides with the central goal to explore the influence of alkyl chain length substituted on N2 atom on the catalytic 3


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performance. Moreover, the effect of catalyst loading, initial CO2 pressure, reaction temperature, and reaction time is studied to confirm the optimal reaction condition. The reusability and suitability of diethylpyrazolium bromide (DEPzBr) are also investigated. Finally, the reaction mechanism is elucidated by density functional theory (DFT) to uncover the difference between sym-pyrazolium ILs and asym-pyrazolium ILs from the atomic viewpoints. It is expected that pyrazolium ionic liquids would open new hot research areas in catalytic field after imidazolium ILs. Combination of theoretical and experimental studies would find an express pathway to search for efficient and single-component ionic liquids to promote the chemical transformation of CO2 in a proper condition. 2. Results and discussion 2.1 Synthesis of ILs The asym-alkylpyrazolium bromide is synthesized by N-alkylation reaction of pyrazole according to the literature,23 which is shown in Scheme 1. The reaction proceeds smoothly under mild condition (38-82°C) with yields over 60%. As the second substituted alkyl chain length increases, the corresponding yield is gradually decreased. Mild reaction condition is also beneficial to suppress the appearance of side reaction and by-product. 2.2 Catalytic Evaluation Our central aim is to investigate the influence of different alkyl chain length in pyrazolium cation on the catalytic activity. Five pyrazolium ILs with different alkyl group on N2 atom and ethyl group on N1 atom have been prepared. No product is detected in the absence of catalyst for the cycloaddition of CO2 and PO (entry 1, Table 1) in a stainless steel autoclave equipped with a magnetic stirrer at 2.0 MPa and 130°C for 8 h, which is consistent with reports in previous literature.21 Even if the 4



KBr is incorporated, the product yield of PC is only 3.3% (entry 2) indicating that it is not a good candidate. Then, KBr along with alkaline source is used to catalyze the reaction. When imidazole or pyrazole is taken as the alkaline source, PC yields of 88.0% and 74.3% can be obtained with 4 h reaction time, respectively (entries 3-4). However, there is almost no catalytic activity using KBr and triethylamine as catalysts (entry 5). It would be attributed to the hydrogen bond formed between PO and active hydrogen atoms in imidazole and pyrazole. In contrast, the absence of active hydrogen atom in triethylamine weakens the possibility of activation. Although the product yield is in an acceptable region, it is still difficult to apply them in large scale due to the inclusion of metal and inferior recyclability. All N,N-1-ethyl-2-alkylpyrazolium ILs present good catalytic activity with the product yields over 76.8% (entries 6-10) under the same condition. The substituted group on N1 atom is the same in all synthesized pyrazolium ILs, while the alkyl chain length substituted on N2 atom varies from methyl, ethyl, propyl, butyl to amyl (entries 6-10). When the alkyl chain length varies from methyl (entry 6) to ethyl (entry 7), the yield has a dramatic increase (76.8% to 92.4%). Then there is a distinct decrease for product yield from 92.4% (entry 7) to 84.6% (entry 8). After that there is no distinct variation with further increase of alkyl chain length (entry 8, entry 9, and entry 10). DEPzBr presents the best catalytic activity, which is attributed to its suitable bulk. The longer alkyl chain length would retard the interaction between cation and PO resulting in the lower catalytic activity. Additionally, DEPzBr has the symmetric configuration. The catalytic performance of sym-dialkylpyrazolim ILs has been investigated in our previous work.21 As compared with the asym-dialkylpyrazolium ILs, the corresponding sym-pyrazolium ILs have the better catalytic activity, such as EPPzBr v.s. DPPzBr, EBPzBr v.s. DBPzBr and EAmPzBr v.s. DAmPzBr (entries 8-10 in 5


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Table 1 v.s. entries s1-s3 in Table S1).21 Enhancement of the symmetry of cation is more effective than increasing the length of alkyl chain. In addition, the catalytic performance of DEPzI21 and EPPzI is also studied to confirm the activity of anion (entries 11-12) under the same reaction condition. The catalytic activity of DEPzI/EPPzI is superior to that of DEPzBr/EPPzBr, which is consistent with the stronger nucleophilic ability of I- anion.24 It is reasonable to infer that the catalytic activity of DEPzCl/EPPzCl would be lower than that of DEPzBr/EPPzBr due to the less nucleophilic ability of Cl- anion. They are not synthesized anymore. To confirm the stability of asym-pyrazolium ILs under the reaction temperature, the thermal stability of five 1-ethyl-2-alkylpyrazolium ILs is investigated by thermogravimetric analysis (TGA) (See Fig. S1). Most of them present good thermal stability with the degradation started in a range of 183-216°C. All of them are stable under the experimental temperature. In order to compare the catalytic activity of asym-pyrazolium ILs with other traditional ILs, the EPPzBr is employed as catalyst under two different reaction conditions (See Table S2). The catalytic efficiency of EPPzBr is higher than 1-ethyl-2-methylimidazolium bromide (EMImBr),25 tetraethylammonium bromide (NEt4Br),26 1-propyl-triphenyl phosphonium bromide ([Ph3PC3H7]Br)27 guanidinium ILs of [TMGC2H4CH3]Br and [TMGC2H5]Br.17 Under the totally identical reaction condition, the catalytic activity of EPPzBr is better than that of NEt4Br (63.0%, entry s5). The product yield in the presence of EPPzBr is much higher than that of EMImBr (52.7%, entry s3) during the shorter reaction time with other identical reaction condition. [TMGC2H4CH3]Br and [TMGC2H5]Br have the worst catalytic activity, especially for [TMGC2H5]Br. Even if the temperature is as high as 150°C, the product 6



yield of PC catalyzed by [Ph3PC3H7]Br (entry s6 in Table S2) is only 74.0%. The performance of EPPzBr is even close to that of BMImBr (entry s4),28 which is popularly regarded as the best catalyst in alkylimidazolium bromide ILs for reaction of CO2 and PO. Although the catalytic activity of asym-pyrazolium ILs are slightly worse than that of sym-pyrazolium ILs, they are better than most of existed non-functionalized ILs. The catalysts for the coupling reaction of CO2 and PO are further enriched. Actually, ionic liquids is not only the catalyst but also the solvent, which has been testified in previous literature.29 Therefore, no additional organic solvent is required. 2.3 Effect of Reaction Conditions In the following research, DEPzBr is selected to refine the reaction condition due to its highest catalytic activity for cycloaddition of CO2 with PO. The effects on the catalytic activity by various reaction parameters such as catalyst loading, initial CO2 pressure, reaction temperature, and reaction time is explored in sequence. First, the influence of catalyst loading on PC yields is investigated under 2.0 MPa initial CO2 pressure, 130°C, 4 h (See Fig. 1). Initially, the PC yield is sharply increased from 48.5% to 83.5% when the DEPzBr loading is increased from 0.2 mol% to 0.5 mol%. While the PC yield is only enhanced by 8.9% with further increase of DEPzBr amount from 0.5 mol% to 1.0 mol%. After that there is only slight enhancement for the PC yield less than 0.2% even if the DEPzBr amount is enlarged to 1.25 mol%. Therefore, 1.0 mol% is the optimal catalyst loading for DEPzBr. The PC selectivity is almost impervious (>99%) over the whole process. Next, the influence of initial CO2 pressure on the yield and selectivity of PC is studied (See Fig. 2). The PC yield rises at the beginning and then reduces. The turning point is 2.0 MPa for initial CO2 pressure. Although the optimal reaction condition is 7


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2.0 MPa for initial CO2 pressure, the difference for PC yield between 1.0 MPa and 2.0 MPa is as small as 5.7%. To get a better ratio for cost/product, the 1.0 MPa for initial CO2 pressure is a better choice, especially for industrial application. The higher CO2 initial pressure would increase CO2 concentration in the liquid, which is favorable for promoting the reaction leading to the higher PC yield. However, the high CO2 pressure would reduce the concentration of PO around ILs.30 Consequently, the PC yield is reduced. Moreover, the PC selectivity is not affected by CO2 pressure with above 99% throughout. Third, how reaction temperature affects PC yield and selectivity is considered (See Fig. 3). When the reaction is carried out at 100°C, the PC yield is 84.2%. Although the PC yield increases with the temperature increasing from 100°C to 130°C, the enhanced extent is as small as 8.4%. On the contrary, the PC yield appears to decrease when the temperature is higher than 140°C. The high experimental temperature would increase the pressure in the autoclave, which is not favorable for the reaction.31 Similarly, the selectivity of PC has almost no changes in the whole process. As shown in TGA analysis of DEPzBr (See Fig. 4, the thermogravimetric curves of fresh DEPzBr), its thermal decomposition temperature is higher than 183°C indicating that it is stable in the whole reaction process. Finally, the effects of reaction time on PC yield and selectivity are shown in Fig. 5. The PC yield increases from 79.5% to 92.4% within the first 4 h. Then, there is only a mild enhancement when further prolonged one hour reaction time. The PC selectivity is >99% throughout. In general, the optimal reaction condition is under 130°C, 2.0 MPa initial CO2 pressure and 4 h with 1 mol% catalyst loading. In addition, the influence of concentration of PO on PC yield are conducted under the optimal reaction condition (See Fig. 6). With the increase of the 8



concentration of PO, the yield of PC is enhanced mildly and then has a slight decrease. The suitable concentration of PO is favorable to promoting the reaction. Note that aforementioned product yields have been measured by 1-5 times to avoid the random error and decrease the inherent error. As a result, the more reliable results would be obtained. All corresponding experimental results are listed in Table S3-S7. The results aroused by random error are marked by red color, which is omitted to have the final results. The final results are the average results of all experimental result excluding the random error. The relative error is also listed in the corresponding tables. 2.4 Catalyst Recycling The recyclability of catalyst is a critical item to evaluate its performance especially in industrial production. Moreover, the reusability is vulnerable for homogeneous catalyst.29 The reusability of DEPzBr is examined under the optimal reaction condition (130°C, 2.0 MPa initial CO2 pressure and 4 h with 1 mol% catalyst loading). After the catalytic reaction, the mixture is distilled under reduced pressure to remove large amounts of propylene carbonate, and the residue is washed by ethyl acetate. The ILs are obtained by centrifugalization. Subsequently, the catalyst is dried under vacuum to be utilized in next return. As shown in Fig. 7, DEPzBr maintains high activity after five times without significant loss for catalytic activity. The slightly decreased activity may be due to the decreased amount of catalyst during the regeneration process. The thermogravimetric curve of reused DEPzBr is also shown in Fig. 4. Two curves were almost the same indicating that the structure of the used catalyst is kept. In addition, 1H NMR and FT-IR analysis of recycled DEPzBr compared with fresh DEPzBr is listed in Fig. S4 and Fig. S13, respectively. It is clearly presented that the catalyst has no obvious change after the recycle. Similarly, it 9


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is easy to refer that other 1-ethyl-2-alkylpyrazolium ILs would also represent good reusability, which is a potential practicable catalyst for the cycloaddition of CO2 with PO. 2.5 Catalytic Activity toward Other Epoxide Besides the excellent catalytic activity, an extensive suitability is also an important factor for a catalyst. The catalytic activity for DEPzBr is testified to be applied to the chemical fixation of CO2 with various epoxides. The corresponding results are listed in Table 2. DEPzBr would efficiently catalyze most of terminal epoxides with product yield over 81.1%. In the reaction of styrene oxide with CO2, the catalytic yield of BMImBr would achieve to be 99% under the reaction condition of 10 mol% catalyst, 0.5 MPa CO2, 150°C, 4 h.32 As compare to DEPzBr (Table 2, entry 4), the initial CO2 pressure is decreased, but the reaction temperature is enhanced. Moreover, the catalyst dosage is as large as 10 mol%. When the catalyst dosage is reduced to be 1 mol%, the product yield decreases to be 29%.32 If the DEPzBr is taken as catalyst, the product yield of 85.3% could be obtained under reaction condition of 1 mol% catalyst, 2.0 MPa CO2, 130°C, and 4 h. Although the CO2 initial pressure is higher for the latter, other reaction conditions are more benign than the former. In addition, the large hindrance of epoxides would decrease the possibility of attack from catalyst resulting in the poor product yield. The product yield of 8b is only 65.5% even after 24 h. The substituted hexatomic ring in 8a blocks the nucleophilic attack from the catalyst leading to the lowest product yield. 2.6 Reaction Mechanism On the basis of above experimental result, DEPzBr has the best catalytic activity in all five investigated pyrazolium ILs. From the structural viewpoint, DEPzBr has the 10



symmetric substituted alkyl groups, while other four pyrazolium ILs have the asymmetric substituted alkyl groups. To uncover the difference among them from atomic level, the mechanism of coupling reaction of CO2 and PO catalyzed by five pyrazolium ILs is elucidated by B3PW91 method33,34 with the 6-31G(d,p)35 basis set. At the same level, the vibrational frequency is calculated to confirm the feature of stationary point. The minimum is confirmed to have all positive frequencies, while transition state has only one imaginary frequency. The energies are refined at the M06/6-311+G(2d,2p) level35,36 on the basis of optimized geometries. The solvent effect is taken into consideration by the polarized continuum model (PCM) in ethyl ether (Et2O) solvent.37,38 Aforementioned calculations are performed by the Gaussian 09 program package.39 The reaction would proceed ring-opening of PO, CO2 insertion, and ring-closure to generate PC. The first step is the rate-determining step with the highest barrier height, therefore, only the ring-opening step is studied in the following theoretical study. According to our previous work,21 three possible routes (route S1, route S2, and route S3) catalyzed by EPPzBr are located (See Table S8). The barrier height of ring-opening step for route S2 is lower than those of route S1 and route S3. The slight difference between latter two transition states is attributed to the asymmetric alkyl substitution. According to the most optimum route, transition states catalyzed by other pyrazolium ILs are also located. The order of catalytic activity reflected by the calculated barrier heights (See Table S9) is not consistent with the corresponding order of experimental product yield. To obtain the more reliable barrier heights, Double-IL model is employed to elucidate the reaction mechanism (See Table S10). One EPPzBr is taken as the electrophile to activate the O atom of PO, while the other one plays a role in stabilizing the Br- anion (route D1). The former EPPzBr is 11


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fixed; then, the latter one is rotated to get routes D2, D3, and D4. Because of the asymmetric alkyl substitution of EPPzBr, the hydrogen atoms in the other side could also activate the O atom of PO (route D5). Then, routes D6, D7, and D8 are subsequently located by the same model like routes D2, D3, and D4. Next, the H1 and H5 could also activate the O atom of PO to get routes D9, D10, D11, and D12. The fourth situation is that both EPPzBr activate the O atom of PO. There are three transition states on the basis of the different attacking hydrogen atoms. The most favorable one is route D2. On the basis of the same attacking model, it is easy to infer other transition states catalyzed by other pyrazolium ILs. However, the number of routes catalyzed by DEPzBr is less due to the symmetric configuration of DEPzBr. Only six routes are finally confirmed for DEPzBr, which are tabulated in Table S11. It is interesting that the most favorable route for DEPzBr is different from that for EPPzBr. According to the similar attacking model, the most favorable routes catalyzed by other asymmetric pyrazolium ILs are finally calculated (See Table 3). The corresponding potential energy surface is plotted in Fig. 8. The barrier height of route DB is the lowest, which is in agreement with its highest product yield. While the barrier heights of ring-opening step for route DC, route DD, and route DE are similar with the deviation within 0.19 kcal/mol, which corresponds to their closer product yields. The highest barrier height of route DA is consistent with its much less product yield. In general, the calculated sequence of barrier heights solidly agrees well with the variation of product yield. A scheme with the most probable mechanism is presented (See Scheme 2) following three steps, i.e., ring-opening of PO, CO2 insertion, and ring-closure to generate PC. In general, both the hydrogen atom in pyrazole ring or imidazole ring and that in alkyl chain near to the N atom would activate the PO. There is an activated hydrogen 12



atom in imidazole ring, which would play a role activating the PO. The hydrogen atom in alkyl chain connected to the N atom is difficult to be taken as electrophile because of the steric hindrance, which is judged from our previous study for the same reaction catalyzed by amino imidazolium ILs.12 To our best knowledge, Double-IL model has been applied to study the reaction mechanism of CO2 and PO catalyzed by limited ILs, which would be further reported in our next work to further testify our conjecture.40 In most of previous literature, the reaction mechanism of PO with CO2 catalyzed by ionic liquids has been widely investigated by the Single-IL model.41 Although the reaction mechanism is the same, three-step (ring-opening of epoxide, CO2 insertion, and ring-closure to generate product) and two-step (ring-opening of epoxide as well as CO2 insertion and ring-closure to generate product), the barrier heights are too close to differentiate them.41 If only the reaction mechanism is studied, the Single-IL model is good enough. 2.7 AIM and NBO Analysis To further uncover the essential items to result in the difference among various routes, the non-covalent interactions played in transition states are analyzed by atoms in molecule (AIM)42 (See Table 4) and non-covalent interactions (NCI)43,44 (See Fig. 9) methods. It is well known that the ring-opening of PO is promoted by both the electrophilic attack from the cation and the nucleophilic attack from the anion. Only the non-covalent interactions in the central catalytic region are considered. Not only the C-H1…O and C-H2…O but also C…Br are the non-covalent interactions with the positive Laplacian values of the electron density (  2  ). Moreover, they have the similar electron density (  ) values in TSDC, TSDD, and TSDE indicating that they play the similar role in respective transition states leading to the closer barrier heights. Although the  value of 0.02299 a.u. for the C-H1…O in TSDA is larger than that in 13


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aforementioned three transition states, the other interaction for C-H2 … O is less. Moreover, it has the less C…Br interaction in TSDA resulting in the highest barrier height. The situation for TSDB is totally different from them. It not only has the less C…Br interaction but also the unfavorable C-H…O interaction. However, one more hydrogen atom is involved in the electrophilic attacking, which is a possible reason for its lowest barrier height. In the other side, the atomic charge is calculated by natural bond orbital (NBO) method45,46 (See Fig. S14). There is not great difference among the positive charges for attacking hydrogen atoms and negative charge for O atom of PO suggesting the similar static interaction between them. However, the negative charge for Br- anion in TSDB is much large than that in other transition states. The static interaction between Br- anion and C atom of PO is the strongest for TSDB, which is beneficial for the ring-opening of PO. The symmetric and asymmetric pyrazolium ILs present great difference between various aspects, including the reaction mechanism, the weak interaction, and the catalytic activity, although there is only a slight difference between their structures. To uncover the relationship between structure and property is the promise for the rational design and developing the efficient catalyst. 3. Conclusions Four asym-dialkylpyrazolium ILs are synthesized in this work. Moreover, their catalytic performance for the coupling reaction of CO2 and PO is investigated under the reaction condition of 130°C, 2.0 MPa initial CO2 pressure, and 4 h with 1 mol% catalyst loading. Although the product yield of asym-pyrazolium ILs is slightly poor than that of corresponding sym-pyrazolium ILs, they are better than most of reported non-functionalized ILs. As compared with the several popular reported ILs, the catalytic activity of EPPzBr is only slightly less than that of BMImBr. Under the 14



identical or even more moderate reaction condition, the product yield catalyzed by EPPzBr is better than EMImBr, NEt4Br,[Ph3PC3H7]Br, and [TMGC2H4CH3]Br. The optimum reaction condition is confirmed taken the DEPzBr as an example. The catalyst would be reused by five times with a slight product loss. And DEPzBr presents excellent suitability for most of terminal epoxides. Finally, the mechanism of coupling reaction catalyzed by five pyrazolium ILs are studied at the M06/6-311+ G(2d,2p)//B3PW91/6-31G(d,p) (PCM) level by Double-IL model. The optimum reaction route is different for asym-pyrazolium ILs and sym-pyrazolium ILs. The strongest static interaction and one more hydrogen bond in the central catalytic region finally leads to the lowest barrier height for DEPzBr. The slight structural variation would result in the totally different reaction mechanism. Therefore, elucidation of the structure-property relationship is necessary to develop new catalysts. The suitable bulk is beneficial to strengthen the interaction between catalyst and PO. Consequently, the catalytic activity is enhanced. It is expected that the developed asym-pyrazolium ILs would further enrich the single-component catalyst for the fixation of CO2. Supporting Information The experimental section, detailed 1H NMR, 13C NMR and FT-IR spectrum, the compare of the catalytic performance between our ILs with other ILs, optimized geometries for the intermediates and transition states, NMR, FT-IRspectrum and graphs of ILs (fresh and reused five times). This material is available free of charge via the Internet at http://pubs.acs.org. Acknowledgments We thank the National Supercomputing Center in Shenzhen (Shenzhen Cloud Computing Center) for providing computational resources and softwares. This work was supported by the National Natural Science Foundation of China (21476061, 15


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(9) Gao, W. Y.; Tsai, C. Y.; Wojtas, L.; Thiounn, T.; Lin, C. C.; Ma, S. Q. Interpenetrating metal-metalloporphyrin framework for selective CO2 uptake and chemical transformation of CO2. Inorg. Chem. 2016, 55, 7291-7294. (10) Wu, X.; Wang, M.; Xie, Y.; Chen, C.; Li, K.; Yuan, M.; Zhao, X.; Hou, Z. Carboxymethyl cellulose supported ionic liquid as a heterogeneouscatalyst for the cycloaddition of CO2 to cyclic carbonate. Appl. Catal. A Gen. 2016, 519, 146-154. (11) Chen, F. F.; Huang, K.; Fan, J. P.; Tao, D. J. Chemical solvent in chemical solvent: A class of hybrid materials for effective capture of CO2. Aiche J. 2018, 64, 632-639. (12) Chen, C.; Ma, Y.; Zheng, D.; Wang, L.; Li, J.; Zhang, J.; He, H.; Zhang, S. Insight into the role of weak interaction played in the fixation of CO2 catalyzed by the amino-functionalized imidazolium-based ionic liquids. J.CO2 Util. 2017, 18, 156-163. (13) Liu, M.; Lan, J.; Liang, L.; Sun, J.; Arai, M. Heterogeneous catalytic conversion of CO2 and epoxides to cyclic carbonates over multifunctional tri-s-triazine terminal-linked ionic liquids. J. Catal. 2017, 347, 138-147. (14) Yang, H. Q.; Wang, X.; Ma, Y.; Wang, L.; Zhang, J. L. Quaternary ammonium-based ionic liquids bearing different numbers of hydroxyl groups as highly efficient catalysts for the fixation of CO2: a theoretical study by QM and MD. Catal. Sci. Technol. 2016, 6 (21), 7773-7782. (15) Liu, M.; Li, X.; Liang, L.; Sun, J. Protonated triethanolamine as multi-hydrogen bond donors catalyst for efficient cycloaddition of CO2 to epoxides under mild and cocatalyst-free conditions. J. CO2 Util. 2016, 16, 384-390. (16) Tao, D. J.; Chen, F. F.; Tian, Z. Q.; Huang, K.; Mahurin, S. M.; Jiang, D. E.; Dai, S. Highly efficient carbon monoxide capture by carbanion-functionalized ionic liquids through C-site interactions. Angew Chem. 2017, 129, 6947-6951. 17


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(17) Dai, W. L.; Bi, J.; Luo, S. L.; Luo, X. B.; Tu, X. M.; Au, C. T. Novel functionalized guanidinium ionic liquids: Efficient acid-base bifunctional catalysts for CO2 fixation with epoxides. J. Mol. Catal. A: Chem. 2013, 378, 326-332. (18) Wizinger, V. R.; Grob-Albrecht, V. Über Pyrazoliumcyanine. Z. Naturforsch. B. 1951, 6, 242-246. (19) Pande, U.C.; Egsgaard, H.; Larsen, E.; Begbrup, M. Mass spectrometry of pyrazolium salts. J. Mass. Spectrom. 1981, 16, 377-380. (20) Abu-Lebdeh, Y.; Alarco, P. J.; Armand, M. Conductive organic plastic crystals based on pyrazolium imides. Angew. Chem. Int. Ed. 2003, 42, 4499 -4501. (21) Ma, Y.; Chen, C.; Wang, T. F.; Zhang, J. S.; Wu, J. J.; Liu, X. D.; Ren, T. G.; Wang, L.; Zhang, J. L. Dialkylpyrazolium ionic liquids as novel catalyst for efficient fixation of CO2 with metal-and solvent-free. Appl. Catal. A: Gen. 2017, 547, 265-273. (22) Xiao, L. F.; Lv, D. W.; Su, D.; Wu, W.; Li, H. F. Influence of acidic strength on the catalytic activity of brønsted acidic ionic liquids on synthesizing cyclic carbonate from carbon dioxide and epoxide. J. Clean. Prod. 2014, 67, 285-290. (23) Huynh, H.V.; Han, Y.; Jothibasu, R.; Yang, J. A.

C NMR spectroscopic

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determination of ligand donor strengths using N-heterocyclic carbene complexes of palladium (ii). Organometallics. 2009, 28, 5395-5404. (24) Xu, B. H.; Wang, J. Q.; Sun, J.; Huang Y.; Zhang J. P.; Zhang X. P.; Zhang S. J. Fixation of CO2 into cyclic carbonates catalyzed by ionic liquids: a multi-scale approach. Green Chem. 2014, 17 (1), 108-122. (25) Yue, C.; Su, D.; Zhang, X.; Wu, W.; Xiao, L. Amino-functional imidazolium ionic liquids for CO2 activation and conversion to form cyclic carbonate. Catal. Lett. 2014, 144, 1313-1321. (26) Cheng, W. G.; Xiao, B. N.; Sun, J.; Dong, K.; Zhang, P.; Zhang, S. J.; Ng, F. T. T. 18



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Effect of hydrogen bond of hydroxyl-functionalized ammonium ionic liquids on cycloaddition of CO2. Tetrahedron Lett. 2015, 56 (11), 1416-1419. (27) Liu, Y.; Cheng, W.; Zhang, Y.; Sun, J.; Zhang, S. Controllable preparation of phosphonium-based polymeric ionic liquids as highly selective nanocatalysts for the chemical conversion of CO2 with epoxides. Green Chem. 2017, 19, 2184-2193. (28) Wang, J. Q.; Cheng, W. G.; Sun, J.; Shi, T. Y.; Zhang, X. P.; Zhang, S. J. Efficient

fixation

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into

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(40) Chen, C.; Ma, Y.; Zheng D. N.; Zhang, J. S.; Ren, T. G.; Wang, L.; Zhang, J. L. Influence of different substitution in pyrazolium ionic liquids on catalytic activity for the fixation of CO2, under solvent- and metal-free conditions. Tetrahedron, 2018, 74 (15), 1776-1784. (41) Marmitt S.; Gonçalves P. F. B. A DFT study on the insertion of CO2 into styrene oxide catalyzed by 1-butyl-3-methyl-imidazolium bromide ionic liquid. J. Comput. Chem. 2015, 36 (17), 1322-1333. (42) Bader, R. F. W.; Beddall, P. M. Virial field relationship for molecular charge distributions and the spatial partitioning of molecular properties. J. Chem. Phys. 1972, 56, 3320-3329. (43) Johnson, E. R.; Keinan, S.; Mori-Sánchez, P.; Contreras-García, J.; Cohen, A. J.; Yang, W. T. Revealing noncovalent interactions. J. Am. Chem. Soc. 2010, 132, 6498-6506. (44) Contreras-García, J.; Johnson, E. R.; Keinan, S.; Chaudret, R.; Piquemal, J. P.; Beratan, D. N.; Yang, W. T. NCIPLOT: a program for plotting noncovalent interaction regions. J. Chem. Theory Comput. 2011, 7, 625-632. (45) Reed, A. E.; Weinstock, R. B.; Weinhold, F. Natural population analysis. J. Chem. Phys. 1985, 83, 735-746. (46) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Intermolecular interactions from a natural bond orbital, donor-acceptor viewpoint. Chem. Rev. 1988, 88, 899-926.

21


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Scheme 1 Synthetic route and structures of the 1-ethyl-2-alkylpyrazolium ionic liquids (EAPzILs).

22



Scheme 2 The proposed mechanism.

23


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Fig. 1 Influence of catalyst loading on PC yield and selectivity (reaction condition: PO 0.1 mol, initial CO2 pressure 2.0 MPa, 130°C, 4 h).

24



Fig. 2 Influence of CO2 pressure on PC yield and selectivity (reaction condition: PO 0.1 mol, DEPzBr 1 mol%, 130ºC, 4 h).

25


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Fig. 3 Influence of temperature on PC yield and selectivity (reaction condition: PO 0.1 mol, DEPzBr 1 mol%, initial CO2 pressure 2.0 MPa, 4 h).

26



Fig. 4 TGA curves of fresh DEPzBr and recycled DEPzBr.

27


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Fig. 5 Influence of reaction time on PC yield and selectivity (reaction condition: PO 0.1 mol, DEPzBr 1 mol%, initial CO2 pressure 2.0 MPa, 130°C).

28



Fig. 6 Influence of concentration of PO on PC yield (reaction condition: DEPzBr 1 mol%, initial CO2 pressure 2.0 MPa, 130°C, 4 h).

29


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Fig. 7 Reused performance of the catalyst (Reaction condition: PO 0.1 mol, DEPzBr 1 mol%, initial CO2 pressure 2.0 MPa, 130ºC, 4 h).

30



Page 32 of 40

Fig. 8 Energy profiles for the cycloaddition reaction along route DA, route DB, route DC,

route

DD,

and

route

DE

calculated

2p)//B3PW91/6-31G(d, p) (PCM) level in Et2O solvent.

31


at

the

M06/6-311+G(2d,

Page 33 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46


32


Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

Fig. 9 NCI plots for TSDA, TSDB, TSDC, TSDD, and TSDE. The corresponding 3D plots are displayed below with blue regions representing strong electrostatic interactions and green regions representing more dispersive attractive interactions. 33


Page 34 of 40

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Table 1 Catalytic performance of various catalystsa Entry

Catalysts

Time/h

Yield b/%

1

None

8

-

2

KBr

8

3.3

3

KBr/imidazole c

4

88.0

4

KBr/pyrazole c

4

74.3

5

KBr/triethylaminec

4

1.3

6

EMPzBr

4

76.8

7

DEPzBr

4

92.4 (96.2) d

8

EPPzBr

4

84.6

9

EBPzBr

4

84.9

10

EAmPzBr

4

85.1

11

DEPzI21

4

94.4

12

EPPzI

4

91.1

Reaction condition: PO 0.1 mol, catalyst 1 mol%, 130°C, initial CO2 pressure 2.0

a

MPa. bIsolated yield. cUse 1 mol% KBr and 1 mol% alkaline source as co-catalyst. Yield was determined by GC.

d

34



Page 36 of 40

Table 2 Cycloaddition of CO2 with various epoxides catalyzed by DEPzBra. Entry

Epoxide

Cyclic carbonate

1

Yieldb/%

93.8 1a

1b

2

92.4 2a

2b

3

86.4 3a

3b

4

85.3 4a

4b

5

82.5 5a

5b

6

86.1 6a

6b

35


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7

81.1 7a

7b

8

65.5 c 8a

8b

Reaction condition: epoxide 0.1 mol, DEPzBr 1 mol%, 2.0 MPa CO2, 130°C, 4 h.

a

Isolated yield. cReaction time: 24 h.

b

36



Page 38 of 40

Table 3 The corresponding schematic structures of transition states and barrier heights catalyzed by EMPzBr, DEPzBr, EPPzBr, EBPzBr, and EAmPzBr at the M06/6-311+G(2d,2p) (PCM)//B3PW91/6-31G(d,p) level calculated by double-IL model. routes

schematic structures of

barrier heights

transition states

(kcal/mol)

route DA (EMPzBr)

20.46

route DB (DEPzBr)a

17.88

route DC (EPPzBr)

18.25

37


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route DD (EBPzBr)

18.22

route DE (EAmPzBr)

18.06

route DB is the same as route 1 in Table S11 in the supporting information.

a

38



Page 40 of 40

Table 4 Selected topological parameters of the bond critical point in the most stable transition state calculated at the B3PW91/6-31G(d, p) level of theory. route

route DA (EMPzBr)

route DB (DEPzBr)

route DC (EPPzBr)

route DD (EBPzBr)

route DE (EAmPzBr)

Transition state

TSDA

TSDB

TSDC

TSDD

TSDE

X-Y…Z

Sign(2)



2

C-H1…O

-0.02299

0.02299

0.06228

C-H2…O

-0.03403

0.03403

0.10430

C…Br

-0.05454

0.05454

0.06607

C-H2…O

-0.02861

0.02861

0.07253

C-H2…O

-0.02603

0.02603

0.07199

C-H7…O

-0.01120

0.01120

0.03374

C…Br

-0.03519

0.03519

0.07161

C-H1…O

-0.01267

0.01267

0.04472

C-H2…O

-0.04021

0.04021

0.11014

C…Br

-0.05529

0.05529

0.06617

C-H1…O

-0.01265

0.01265

0.04460

C-H2…O

-0.04051

0.04051

0.11035

C…Br

-0.05559

0.05559

0.06596

C-H1…O

-0.01284

0.01284

0.04492

C-H2…O

-0.03992

0.03992

0.10987

C…Br

-0.05552

0.05552

0.06607

39


Insight on asym-pyrazolium ionic liquids for chemical fixation of CO2

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