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Chemical transformations of captured CO2 into cyclic and polymeric carbonates
Journal of CO₂ Utilization 32 (2019) 196–201
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Chemical transformations of captured CO2 into cyclic and polymeric carbonates
T
Jotheeswari Kothandaramana, Jun Zhangb, Vassiliki-Alexandra Glezakoub, Michael T. Mockb, ⁎ David J. Heldebranta, a b
Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, WA, 99354, United States Physical Sciences Division, Pacific Northwest National Laboratory, Richland, WA, 99352, United States
ARTICLE INFO
ABSTRACT
Keywords: CO2 Polymerization Alkylcarbonate Catalysis
Alkylcarbonate ionic liquids are shown to be chemically active species for trans-carboxylating cyclohexene oxide to produce polycarbonates. Catalytic equivalents of alkylcarbonate ionic liquids can be added to co-polymerizations of cyclohexene oxide/CO2 to increase the yield and selectivity of the polymer. On the other hand, in the case of propylene oxide, polymerization is suppressed, and propylene oxide is converted to propylene carbonate. The suppression of polymerization is due to the high activity of the alkylcarbonate ionic liquids towards carboxylation of epoxides for cyclic carbonate formation. The reactivity of alkylcarbonate ionic liquids with propylene oxide is sufficient enough that propylene carbonate is produced in the absence of catalyst at pressures as low as 1 atm at 60 °C.
1. Introduction There has been a recent interest to utilize vast quantities of anthropogenic CO2 being emitted into the atmosphere as a part of a larger carbon reduction strategy to help mitigate the impacts of climate change [1,2]. CO2 is considered stable in kinetic and thermodynamic standpoint, making it slow to coordinate and be activated by catalysts. While the conversion of CO2 to chemicals is potentially more environmentally responsible when replacing toxic and highly reactive chemicals, the magnitude of anthropogenic CO2 emissions dwarfs the volume of useful products that can be made from CO2. It is known that the magnitude of CO2 far exceeds the volume of products that can be made, but CO2 always remain an attractive chemical feedstock because of its abundance, low cost and low eco-toxicity [2,3]. There are many desired products to be made from CO2, ranging from methanol, methane, formic acid, carbonates etc. [4,5]. Cyclic carbonates and linear polycarbonate polymers stand out because of their simplicity and low cost. The energetics of capture and compression of CO2 is an oftenoverlooked component of chemical transformations of CO2. The energy intensive compression and storage steps after the CO2 capture can be avoided when the captured CO2 is directly used for the chemical transformations. Since Inoue et al’s first report on the CO2/epoxide copolymerization
⁎
using diethylzinc/H2O derived catalyst, there has been many advances made in the heterogenous metal catalyzed synthesis of polymers [6,7]. However, most of these systems require high catalyst loading, providing polymers with broad polydispersity indices (PDI) and low selectivity [8]. Similarly, elegant catalytic systems have been developed for either direct carboxylation or co-polymerization of epoxides/CO2 in presence of homogenous catalysts, but they often are limited to high pressures and in the case of polymerizations, often suffer from long reaction times or chain termination through intramolecular cyclization of the growing carbonate chain [7,9,10]. Cyclic carbonates are useful as intermediates in the synthesis of pharmaceutical/fine chemicals, monomers in polymerization and solvents for lithium-ion rechargeable batteries and in many applications [11–13]. Catalysts are continuously developed to incrementally enhance the reactivity of CO2, the lone variable that has yet to be changed in such systems is the CO2 itself [14–18]. We hypothesized that by employing a transcarboxylating agent, reaction selectivity, kinetics and catalyst design, may be changed, opening new pathways, processes and products that may not be available from CO2 directly. Conversion of captured CO2 to value-added chemicals/fuels such as urea derivatives, [19], formate [20,21], methanol [22] etc… have already been investigated by various groups. We also have demonstrated recently that carbonates (formed from captured CO2) can be hydrogenated directly to formate and methanol, and it is likely that such systems could be
Corresponding author. E-mail address: [email protected] (D.J. Heldebrant).
https://doi.org/10.1016/j.jcou.2019.04.020 Received 19 November 2018; Received in revised form 18 April 2019; Accepted 24 April 2019 Available online 02 May 2019 2212-9820/ © 2019 Elsevier Ltd. All rights reserved.
Journal of CO₂ Utilization 32 (2019) 196–201
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or cyclic carbonate products were detected (entry 1, Table 1). However, upon heating the reaction mixture to 95 °C, PCHC (polycyclohexyl carbonate) was formed with 81% selectivity, which is comparable to Darensbourg et al.’s results [9]. The polyether and cyclic carbonate products were formed with 14% and 5% selectivity respectively based on 1H NMR spectroscopy. To our delight, the addition of SWIL increased both the selectivity and yield for PCHC (entry 2–5, Table 1). Using 2 equivalents of SWIL with respect to the catalyst, PCHC yield as high as 82% was obtained with 99% selectivity for PCHC and PDI of 1.14. We also observed that the presence of DBU by itself increased the formation of PCHC, albeit with lower selectivity and yields for PCHC compared to SWIL (entry 5 vs 6, Table 1). Increasing the temperature from 82 °C to 95 °C improved the yield of PCHC from 82 to 94% with slight drop in selectivity (98%) with a TOF of 206 h−1, which is to the best of our knowledge the highest ever reported for the Cr(Salen)Cl catalyst shown in Scheme 3 (entry 7, Table 1) [32]. Presence of SWIL increased the rate of copolymerization by > 12 fold (entry 2 and 7, Table 1). When the DBU and 1-hexanol mixture was used directly to produce the SWIL in-situ instead of preforming it, PCHC was formed, however with relatively lower selectivity (92%) and yield (41%) for PCHC (entry 8, Table 1). It is important to note that both the yield and selectivity for PCHC did not improve with increasing DBU concentration (entry 6 and 9, Table 1), which is probably because of increased coordination ability of DBU to the catalyst, leading to inhibition of chain initiation step [33]. Lewis basic ionic liquids such as [DBUH+]OAc−, [DBUH+]Cl−, [TBDH+]Cl−, [MImH+]Cl− etc were identified previously by He at al as efficient catalysts for the formation of cyclic carbonates [34]. Under our reaction condition, in the presence of Cr(salen)Cl catalyst, CHO/ CO2 formed primarily PCHC with good selectivity (96%, entry 10, Table 1) in the presence of [DBUH+]OAc− as a co-catalayst, although with low yield, 39%. This suggests that the SWIL is acting both as nucleophile and trans-carboxylating agent but [DBUH+]OAc is only acting as a nucleophilic co-catalyst. Next, we studied the influence of CO2 pressure on the polymer formation. Even at lower pressure (under 10 bar), the polymer, PCHC was formed with good yield, 63% (entry 11, Table 1) in the presence of SWIL. We also observed that the cyclic carbonate yield increased from traces to 6% upon lowering the pressure from 44 bar to 10 bar (entry 7 and 11, Table 1). A reaction mechanism is proposed for the polymerization in Scheme 4, which is similar to Darensbourg’s study on the effect of Lewis basic co-catalysts on the Cr-Salen catalyst [35]. First the chain initiation step occurs where the Cl initiates the ring opening of epoxide followed by the coordination of the hexylcarbonate (from SWIL) to Cr. The nucleophilic attack of the ring opened epoxide (metal alkoxide) on the hexylcarbonate carbon leads to the release of free 1-hexanol and DBU. The free DBU and 1-hexanol reacts with fresh CO2 and trans-carboxylate with the growing polymer chain.
Scheme 1. The DBU hexylcarbonate switchable ionic liquid (SWIL).
Scheme 2. Cyclic carbonate formation from backbiting of the polymer.
developed for carboxylation reactions [23,24]. Alkyl carbonate ionic liquids are suitable CO2 mimics for CO2 catalysis, as they could act as a CO2 carrier, concentrator, and solvent, but also (under anhydrous conditions) could prevent chain termination in polymerization reactions [25]. Alkyl carbonate ionic liquids (Scheme 1) contain a 0.5 mol fraction of “CO2” in solution, offering high concentrations of CO2 at atmospheric pressure, far better than conventional solvents that require high pressures of CO2 (often > 50 atm) to achieve such concentrations [26]. Alkyl carbonate ionic liquids continuously capture and activate CO2 and being polar, they allow stabilization of charged/polar intermediates common in catalytic reactions [25,27,28]. Alkyl carbonates are also negatively charged, favoring complexation to cationic homogeneous catalysts. Furthermore, in co-polymerizations of CO2 and epoxides, growing polycarbonate chains often backbites on itself, forming cyclic carbonates, thus terminating the polymer chain (Scheme 2) [7,29]. Under anhydrous conditions, alkylcarbonates could mitigate the backbiting compared to neutral CO2 by either enhancing the rate of coordination of a CO2 equivalent to the catalyst, or by alternatively by having an excess of alkylcarbonate in the first-solvation sphere in solution to trans-carboxylate to the growing polymer chain. Alkyl carbonate ionic liquids are a system that implores exploration, as they can perform all of the aforementioned roles of CO2 fixation, concentration, and activation towards potential catalytic pathways to cyclic carbonate and polycarbonate products and be catalytic with respect to capture solvent. We present here the reactivity, feasibility and reaction mechanisms of the reaction between alkylcarbonate ionic liquids and cyclohexene/ propylene oxide to make cyclic carbonates and polycarbonates. 2. Results and discussion 2.1. Co-polymerization of PCHC and CO2 using alkylcarbonate We chose Jacobsen’s catalyst, Cr(salen)Cl, to determine if transition metal catalysts were tolerant of alkylcarbonate chemistry and to discern the difference in reactivity and selectivity between neutral CO2 and alkylcarbonates [30]. Jacobsen used Cr(salen)Cl complex for the addition of TMSN3 to meso-epoxides by asymmetric ring opening [30]. Followed by his study, the groups of Nguyen and Darensbourg showed that the Jacobsen’s catalyst can be used for the cyclic carbonate and polycarbonate formation in the presence of Lewis-base co-catalysts, DMAP and N-Methylimdazole, respectively [31,9]. The results for the co-polymerization of cyclohexeneoxide/CO2 using diazabiciclo[5.4.0]undec-7-ene hexylcarbonate switchable ionic liquid (SWIL) are tabulated in Table 1. Scheme 3 shows the polycarbonate, ether and cyclic carbonate linkages observed in the system. Alternating copolymerization of epoxide/CO2 of the growing polymer chain by chain propagation pathway forms the polycarbonate. Backbiting or back-to-back epoxide enchainment of the polymer chain gives the cyclic carbonate and ether linkages respectively. In the absence of any catalysts or additives, no detectable amount of polymer was identified by 1H NMR spectroscopy. Even in the presence of Cr(salen)Cl catalyst at 84 °C under 44 bar CO2 pressure, no polymer
2.2. Co-polymerization/coupling of propylene oxide and alkylcarbonate Polymerizations of CO2 and propylene oxide were performed with SWIL using a heterogenous catalyst, zinc glutarate (Zn-glutarate). The Zn-glutarate catalyst was prepared following the procedure by Darensbourg et al using zinc oxide and glutaric acid [36]. Under our reaction conditions (60 °C and 44 bar CO2 pressure), polypropylene carbonate (PPC) was formed with 76% selectivity from the coupling of propylene oxide and CO2 (Scheme 5 and entry 1, Table 2). Interestingly, addition of SWIL formed propylene carbonate (PC) with excellent selectivity (entry 2, Table 2). A control experiment performed using DBU as an additive, produced relatively less amount of cyclic carbonate (entry 3, Table 2). Even in the absence of Zn-glutarate catalyst, SWIL selectively formed PC at temperature as low as 60 °C under 10 bar CO2 pressure with good yield (entry 4, Table 2). 197
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Table 1 Co-polymerization of CHO and CO2. Entry
T (˚C)
CO2 (bar)
[Cr] : [CHO]: [Co-Cat]
PCHC : CHC + PEC linkagesa
PCHCa
TON
TOF
Mwb (g/mol)
Mn
1 2 3 4 5 6 7 8 9 10 11 12
84 95 84 84 82 82 95 95 92 92 92 92
44 44 44 44 44 44 44 44 44 44 10 10
1 : 3550 : 1 : 3550 : 1: 3550 : 0.5 (SWIL) 1 : 3550 : 1 (SWIL) 1 : 3550 : 2 (SWIL) 1 : 3550 : 1 (DBU) 1 : 3550 : 2 (SWIL) 1 : 3550 : 1 ([DBUH+]C6H13O−) 1 : 3550 : 2 (DBU) 1 : 3550 : 2 ([DBUH+]OAc−) 1 : 2550 : 2 (SWIL) 1 : 2550 : 2 (DBU)
– 81%:19% 93%:7% 96%:4% 99%:1% 93%:7% 98%:2% 92%:8% 95%:5% 96%:4% 92%:8% 90%:10%
– 7% 41% 51% 82% 39% 94% 41% 41% 39% 63% 35%
– 260 1450 1800 2900 1400 3300 1450 1450 1400 1600 900
– 16 91 112 181 87 206 92 91 87 100 56
– 2270 4440 6550 11900 4930 12600 7550 12700 9030 7360 7440
– 3000 4230 6160 10500 4520 10300 6170 9310 6740 6470 6420
b
(g/mol)
PDIb – 2.29 1.05 1.06 1.14 1.09 1.22 1.22 1.36 1.34 1.14 1.16
CHO = Cyclohexene oxide, PCHC = polycyclohexene carbonate, CHC = cyclohexene carbonate, PEC = polyether carbonate, Cr(salen)Cl = 0.02 mmol, neat CHC was used for all the experiments and the reactions were performed in an autoclave for 16 h. a Selectivity and yield of all entries were calculated by 1H NMR spectroscopy using 1,3,5-trimethoxybenzene as an internal standard. b Determined by GPC using polystyrene standards. Turnover number (TON)=(mol of repeating unit)/(mol of Cr). Turnover frequency (TOF)=TON/h.
Scheme 5. Co-polymerization/Coupling of PO with alkylcarbonates.
intermediate A (Scheme 6). The high concentration of this transient peak suggests this to be a stable species that undergoes another transformation to PC (13C chemical shifts: 154.9, 73.9, 71.0 ppm). The broadness at the beginning of the reaction is due to the high viscosity of the ionic liquid. As PC begins to build up in concentration, the viscosity of the solution begins to decrease, improving the resolution of the later spectra. Also, the slight shifting of the SWIL peaks up field over the reaction profile is attributed to a change in the chemical environment, as the ionic liquid is consumed. Evidence for the transient intermediate A was also observed using in-situ REACT-IR (with [DBUH+] [C6H13OCO2−] baseline subtracted). The intermediate A builds up at 1750 cm-1 concurrently with the formation of PC at 1800 cm-1 (Fig. 2). The IR and NMR data described above suggested that the bi-functional nature of [DBUH]+ C6H13OCO2−, i.e. the Bronsted (conjugate) acid and the nucleophilic (anionic) hexylcarbonate were contributing to the observed ring opening of PO to make propylene carbonate, however, it was unclear as to whether the [DBUH]+ or the C6H13OCO2− was the reactive species. To assess the reactivity, we performed DFT calculations for all of the species in the proposed reaction cycle (Scheme 6). The energetics of the cycle show that the formation of [DBUH]+ C6H13OCO2− is favorable by −9.7 kcal/mol, in accord with experimental observations. The complete dissociation of the [DBUH]+ C6H13OCO2- pair is predicted to be endothermic, as was an individual attack on PO by either [DBUH]+ or C6H13OCO2−. The results suggest that protonation of PO by [DBUH]+ to make a protonated epoxide is
Scheme 3. Co-polymerization of CHO and CO2 using alkylcarbonate additives.
Based on DFT-based calculations, we propose a mechanism that proceeds through intermediates including: a carbonate/protonated amine complex, an acid promoted ring opening of propylene oxide by a proton transfer from [DBUH+], and finally a nucleophilic attack of carbonate on the protonated epoxide to form the proposed ester intermediate A. With the addition of a second CO2, intermediate A subsequently cyclizes to form PC. Similar H+-catalyzed mechanisms have been proposed in cyclic carbonate formation in ionic liquids [34]. The cyclization liberates 1-hexanol which can then further react with DBU and CO2 to reform SWIL and the cycle continues (Scheme 6). The metal-free reaction between PO and SWIL was studied using both time resolved 13C NMR and REACT-IR. The 13C time resolved spectra (Fig. 1) at the bottom, shows the amidinium [DBUH+] and hexylcarbonate [C6H13OCO2−] signals around 162 ppm. As the reaction proceeds (Fig. 1) we observe a transient species with a 13C shift of 165 ppm, which we attribute to the protonated form of the ester
Scheme 4. Proposed reaction mechanism. 198
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Table 2 Coupling of PO and CO2. Entry
T (˚C)
CO2 (bar)
[Zn-glutarate] : [PO]: [Co-Cat]
PPC : PC + PEC linkagesa
PPCa
PC
TON
TOF
Mwb(g/mol)
Mn
1 2 3 4
60 60 60 60
44 44 44 10
1 : 180 : 1 : 180 : 1 (SWIL) 1: 180: 1 (DBU) - : 4 : 1 (SWIL)
76%:24% 0%:100% 0%:100% 0%:100%
25% – – –
7% 22% 4% 53%
47 – – –
2 – – –
67100 – – –
41600 – – –
b
(g/mol)
PDIb 1.61 – – –
PC = propylene carbonate, PPC = polypropylene carbonate, PEC = polyethercarbonate, Zn-Glutarate = 0.77 mmol, PO = 0.14 mol, 60 °C, 24 h. a Selectivity and yield of all entries were calculated by 1H NMR spectroscopy using 1,3,5-trimethoxybenzene as an internal standard. b Determined by GPC using polystyrene standards. Turnover number (TON)=(mol of repeating unit)/(mol of Zn). Turnover frequency (TOF)=TON/h.
unfavorable, and also lessens the likelihood of direct nucleophilic attack on PO from C6H13OCO2− as individual steps. The calculations indicate that a concerted H+ transfer and nucleophilic attack from ionic pair [DBUH]+ C6H13OCO2− is at play. The proposed linear ester (A) is energetically favored by −10 kcal/mol. Upon further reaction with CO2, it produces propylene carbonate also favored by −15.8 kcal/mol. The results from these calculations overall suggest that the proposed mechanism, shown in Scheme 6, is favorable energetically and that the ion pair is likely the in-situ catalyst for the cycloaddition reaction. Based on the computational results and optimized structures, it is postulated that the strength and orientation of hydrogen bonding between [DBUH]+ C6H13OCO2− is critical by tuning the charge transfer that facilitates the concerted reaction. The bifunctional nature of the SWIL allows both modes of epoxide opening on the same system. The proposed concerted reaction is similar to other published cycloadditions where a H-bond donor and electrophile are used to jointly stabilize the opened epoxide prior to cyclization [37]. Our previous studies have also shown that the orientation and speciation of hydrogen bonding between the cation and alkylcarbonate anions in SWIL systems is key to controlling the physical and thermodynamic properties of the system [38,39]. It is likely that the strong association between cation and anion in this case provides the needed regio-selectivity and charge transfer that enables this concerted cycloaddition reaction even in the absence of a transition metal catalyst. However, in the presence of a transition metal catalyst (Scheme 4), the strong association between the cation and anion of the SWIL can be broken, as shown previously in the presence of a Ru catalyst [23]. The effect of SWIL on the propylene oxide/CO2 copolymerization was then studied in the presence of Cr catalyst (Table 3). In the absence of any co-catalyst at room temperature, the selectivity for the copolymer formation was high in the presence of Cr(salen)Cl however, the reaction rate was very slow (TOF = 4 h−1). The addition of co-catalyst such as DBU or SWIL slightly favored the formation of cyclic carbonate (entry 2 and 3, Table 3). Increasing the temperature to 50 °C and in the absence of any co-catalyst significantly increased the reaction selectivity to cyclic carbonate formation. The co-catalysts DBU and [DBUH+]OAc- increased the yield for polymer with the TON of 1300 and 1225 respectively (entry 5 and 7, Table 3) likely due to the unavailability of the acetate to undergo SWIL-promoted cycloaddition under this reaction condition (Scheme 6). Unlike the CHO case, the presence of SWIL increased the selectivity for the cyclic carbonate due to its higher direct reactivity with SWIL as described above (entry 6, Table 3). Similarly, when glycidol was used instead of propylene oxide in entry 6, Table 3, the yield for glycerol carbonate (64%) increased in presence of SWIL compared to the experiment without the SWIL (38%). In order to extent the scope of SWIL, we screened various epoxides for the cycloaddition reaction and the results were shown in Fig. 3. It is evident from the results that diverse epoxides can tolerate the reaction condition and form corresponding cyclic carbonate with moderate to good yields. Under the reaction conditions, the low boiling epoxides such as propylene oxide (bp. 34 °C) and 1,2-butylene oxide (bp. 63 °C) probably exist as vapor in the head space of the reactor, which accounts for relatively lower yield for 1a (53%) and 1b (56%). However, various cyclic carbonates can be prepared by this method via one-pot CO2 capture and conversion.
Scheme 6. Proposed mechanism of the reaction between SWIL and PO.
Fig. 1. Time-resolved 13C NMR of neat SWIL + PO (1 atm, 60 °C, 14 h). Array referenced to propylene carbonate (74.0 ppm).
Fig. 2. Time-resolved IR spectra of neat SWIL + PO (1 atm CO2, 60 °C) with SWIL baseline subtracted.
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Table 3 Coupling of PO and CO2 in the presence of Cr catalyst and different co-catalysts. Entry
T (˚C)
CO2 (bar)
[Cr] : [PO]: [Co-Cat]
PPC : PC + PECa
PPCa
TON
TOF h−1
Mwb (g/mol)
Mn
1 2 3 4 5 6 7
25 25 25 50 50 50 50
44 44 44 44 44 44 44
1 1 1 1 1 1 1
90%:10% 83%:17% 86%:14% 27%:73% 83%:17% 43%:57% 81%:19%
8% 6% 11% 2% 35% 23% 33%
290 215 375 81 1300 856 1225
4 3 5 3 43 28 41
8080 155 592 – 8400 6200 14700
611 119 147 – 4500 4020 10100
: : : : : : :
3550 : 3550 : 1 (DBU) 3550 : 1 (SWIL) 3700 : 3700 : 1 (DBU) 3700 : 1 (SWIL) 3700: 1 ([DBUH+]OAc−)
b
(g/mol)
PDIb 13.2 1.3 4.02 – 1.87 1.54 1.46
PC = propylene carbonate, PPC = polypropylene carbonate, PEC = polyethercarbonate. Cr(salen)Cl = 0.08 mmol, PO = 0.30 mol, entry 1–3 = 72 h, entry 4–7 = 30 h. a Selectivity and yield of all entries were calculated by 1H NMR spectroscopy using 1,3,5-trimethoxybenzene as an internal standard. b Determined by GPC using polystyrene standards. Turnover number (TON) = (mol of repeating unit)/(mol of Cr). Turnover frequency (TOF) = TON/h. catalysis and life cycle assessment, Chem. Rev. 118 (2) (2018) 434–504. [3] S.-Y. Pan, P.-C. Chiang, W. Pan, H. Kim, Advances in state-of-art valorization technologies for captured CO2 toward sustainable carbon cycle, Crit. Rev. Environ. Sci. Technol. 48 (5) (2018) 471–534. [4] Z. Jiang, T. Xiao, V.L. Kuznetsov, P.P. Edwards, Turning carbon dioxide into fuel, Philos. Trans. R. Soc. A 368 (1923) (2010) 3343–3364. [5] J. Klankermayer, W. Leitner, Harnessing renewable energy with CO2 for the chemical value chain: challenges and opportunities for catalysis, Philos. Trans. R. Soc. A 374 (2061) (2016). [6] S. Inoue, H. Koinuma, T. Tsuruta, Copolymerization of carbon dioxide and epoxide, J. Polym. Sci. Part B: Polym. Lett. 7 (1969) 287–292. [7] D.J. Darensbourg, Making plastics from carbon dioxide: salen metal complexes as catalysts for the production of polycarbonates from epoxides and CO2, Chem. Rev. 107 (2007) 2388–2410. [8] K. Soga, E. Imai, I. Hattori, Alternating copolymerization of CO2 and propylene oxide with the catalysts prepared from Zn(OH)2 and various dicarboxylic acids, Polym. J. 13 (1981) 407–410. [9] D.J. Darensbourg, J.C. Yarbrough, Mechanistic aspects of the copolymerization reaction of carbon dioxide and epoxides, using a chiral salen chromium chloride catalyst, J. Am. Chem. Soc. 124 (2002) 6335–6342. [10] S.J. Poland, D.J. Darensbourg, A quest for polycarbonates provided via sustainable epoxide/CO2 copolymerization processes, Green Chem. 19 (21) (2017) 4990–5011. [11] A.-A.G. Shaikh, S. Swaminathan, Organic carbonates, Chem. Rev. 96 (1996) 951–976. [12] B. Scrosati, J. Hassoun, Y.-K. Sun, Lithium-ion batteries. A look into the future, Energy Environ. Sci. 4 (2011) 3287–3295. [13] V. Besse, F. Camara, C. Voirin, A. Caillol, R. Sylvain, B. Boutevin, Synthesis and applications of unsaturated cyclocarbonates, Polym. Chem. 4 (2013) 4545–4561. [14] M.R. Kember, A. Buchard, C.K. Williams, Catalysts for CO2/epoxide copolymerisation, Chem. Commun. 47 (1) (2011) 141–163. [15] C. Martin, G. Fiorani, A.W. Kleij, Recent advances in the catalytic preparation of cyclic organic carbonates, ACS Catal. 5 (2) (2015) 1353–1370. [16] R.R. Shaikh, S. Pornpraprom, V. D’Elia, Catalytic strategies for the cycloaddition of pure, diluted, and waste CO2 to epoxides under ambient conditions, ACS Catal. 8 (1) (2018) 419–450. [17] H. Buttner, L. Longwitz, J. Steinbauer, C. Wulf, T. Werner, Recent developments in the synthesis of cyclic carbonates from epoxides and CO2, Top. Curr. Chem. 375 (3) (2017) 50–55. [18] M. Alves, B. Grignard, R. Mereau, C. Jerome, T. Tassaing, C. Detrembleur, Organocatalyzed coupling of carbon dioxide with epoxides for the synthesis of cyclic carbonates: catalyst design and mechanistic studies, Catal. Sci. Technol. 7 (13) (2017) 2651–2684. [19] Z.Z. Yang, L.N. He, Y.N. Zhao, B. Li, B. Yu, CO2 capture and activation by superbase/polyethylene glycol and its subsequent conversion, Energy Environ. Sci. 4 (10) (2011) 3971–3975. [20] J. Kothandaraman, A. Goeppert, M. Czaun, G.A. Olah, G.K.S. Prakash, CO2 capture by amines in aqueous media and its subsequent conversion to formate with reusable ruthenium and iron catalysts, Green Chem. 18 (21) (2016) 5831–5838. [21] Y.N. Li, L.N. He, A.H. Liu, X.D. Lang, Z.Z. Yang, B. Yu, C.R. Luan, In situ hydrogenation of captured CO2 to formate with polyethyleneimine and Rh/monophosphine system, Green Chem. 15 (10) (2013) 2825–2829. [22] J. Kothandaraman, A. Goeppert, M. Czaun, G.A. Olah, G.K. Prakash, Conversion of CO2 from air into methanol using a polyamine and a homogeneous ruthenium catalyst, J. Am. Chem. Soc. 138 (3) (2016) 778–781. [23] D.B. Lao, B.R. Galan, J.C. Linehan, D.J. Heldebrant, The steps of activating a prospective CO2 hydrogenation catalyst with combined CO2 capture and reduction, Green Chem. 18 (18) (2016) 4871–4874. [24] J. Kothandaraman, R.A. Dagle, V.L. Dagle, S.D. Davidson, E.D. Walter, S.D. Burton, D.W. Hoyt, D.J. Heldebrant, Condensed-phase low temperature heterogeneous hydrogenation of CO2 to methanol, Catal. Sci. Technol. 8 (2018) 5098–5103. [25] P.G. Jessop, S.M. Mercer, D.J. Heldebrant, CO2-triggered switchable solvents, surfactants, and other materials, Energy Environ. Sci. 5 (6) (2012) 7240–7253. [26] X. Gui, Z. Tang, W. Fei, Solubility of CO2 in alcohols, glycols, ethers, and ketones at high pressures from (288.15 to 318.15) K, J. Chem. Eng. Data 56 (2011) 2420–2429.
Fig. 3. Cycloaddition of CO2 to various epoxides. Reaction conditions: 0.036 mol SWIL, 0.14 mol epoxide, 10 bar CO2, 60 °C, 24 h, aSelectivity and yield of all entries were calculated by 1H NMR spectroscopy using 1,3,5-trimethoxybenzene as an internal standard.
3. Conclusions CO2 captured as amidinium alkylcarbonates react with CHO analogously to CO2. In polymerization reactions, selectivity of the copolymer was found to increase (> 98%) with catalytic equivalents of the alkyl carbonate. Catalyst equivalents of SWIL also increased the yield (from 7% to 94%) of polymers. Time resolved IR and 13C NMR spectroscopic studies elucidated that SWIL was highly reactive towards PO, as direct carboxylation of PO occurs in the absence of metal catalyst at 60 °C at atmospheric pressure. The metal-free direct carboxylation of PO at atmospheric pressure confirmed the unique reactivity of alkyl carbonates, as direct carboxylation of PO is unavailable with neutral CO2. DFT calculations confirmed the energetic feasibility of the catalytic cycle, revealing the role of ionic pair as the in-situ catalyst for the cycloaddition of CO2 contained in an alkylcarbonate to PO. Funding sources We thank the United States Department of Energy’s Office of Science Basic Energy Sciences Early Career Research ProgramFWP 67038 for funding. Acknowledgment Pacific Northwest National Laboratory (PNNL) is proudly operated by Battelle for the US Department of Energy. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jcou.2019.04.020. References [1] M. Mikkelsen, M. Jørgensen, F.C. Krebs, The teraton challenge. A review of fixation and transformation of carbondioxide, Energy Environ. Sci. 2010 (3) (2009) 43–81. [2] J. Artz, T.E. Muller, K. Thenert, J. Kleinekorte, R. Meys, A. Sternberg, A. Bardow, W. Leitner, Sustainable conversion of carbon dioxide: an integrated review of
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J. Kothandaraman, et al. [27] L. Phan, D. Chiu, D.J. Heldebrant, H. Huttenhower, J. Ejae, X. Li, P. Pollet, R. Wang, C.A. Eckert, C.L. Liotta, P.G. Jessop, Switchable solvents consisting of amidine/ alcohol or guanidine/alcohol mixtures, Ind. Eng. Chem. Res. 47 (3) (2008) 539–545. [28] D.J. Heldebrant, C.R. Yonker, P.G. Jessop, L. Phan, Organic liquid CO2 capture agents with high gravimetric CO2 capacity, Energy Environ. Sci. 1 (4) (2008) 487–493. [29] D.J. Darensbourg, A.D. Yeung, Thermodynamics of the carbon dioxide-epoxide copolymerization and kinetics of the metal-free degradation: a computational study, Macromolecules 46 (1) (2013) 83–95. [30] E.N. Jacobsen, Asymmetric catalysis of epoxide ring-opening reactions, Acc. Chem. Res. 33 (6) (2000) 421–431. [31] R.L. Paddock, S.T. Nguyen, Chemical CO2 fixation: Cr(III) salen complexes as highly efficient catalysts for the coupling of CO2 and epoxides, J. Am. Chem. Soc. 123 (46) (2001) 11498–11499. [32] D.J. Darensbourg, R.M. Mackiewicz, A.L. Phelps, D.R. Billodeaux, Copolymerization of CO2 and epoxides catalyzed by metal salen complexes, Acc. Chem. Res. 37 (11) (2004) 836–844. [33] D.J. Darensbourg, R.M. Mackiewicz, J.L. Rodgers, C.C. Fang, D.R. Billodeaux, J.H. Reibenspies, Cyclohexene oxide/CO2 copolymerization catalyzed by chromium (III) salen complexes and N-methylimidazole: effects of varying salen ligand substituents and relative cocatalyst loading, Inorg. Chem. 43 (19) (2004) 6024–6034.
[34] Z. Yang, L. He, C. Miao, S. Chanfreau, Lewis basic ionic liquids-catalyzed conversion of carbon dioxide to cyclic carbonates, Adv. Synth. Catal. 352 (2010) 2233–2240. [35] D.J. Darensbourg, R.M. Mackiewicz, J.L. Rodgers, Role of the cocatalyst in the copolymerization of CO2 and cyclohexene oxide utilizing chromium salen complexes, J. Am. Chem. Soc. 127 (40) (2005) 14026–14038. [36] D.J. Darensbourg, N.W. Stafford, T. Katsurao, Supercritical carbon dioxide as solvent for the copolymerization of carbon dioxide and propylene oxide using a heterogeneous zinc carboxylate catalyst, J. Mol. Catal. A: Chem. 104 (1995) L1–L4. [37] J. Gao, Q.W. Song, L.N. He, C. Liu, Z.Z. Yang, X. Han, X.D. Li, Q.C. Song, Preparation of polystyrene-supported Lewis acidic Fe(III) ionic liquid and its application in catalytic conversion of carbon dioxide, Tetrahedron 68 (20) (2012) 3835–3842. [38] D. Malhotra, P.K. Koech, D.J. Heldebrant, D.C. Cantu, F. Zheng, V.A. Glezakou, R. Rousseau, Reinventing design principles for developing low-viscosity carbon dioxide binding organic liquids (CO2BOLs) for flue gas clean up, ChemSusChem 10 (2017) 636–642. [39] D.C. Cantu, J. Lee, M.S. Lee, D.J. Heldebrant, P.K. Koech, C.J. Freeman, R. Rousseau, V.A. Glezakou, Dynamic acid/base equilibrium in single component switchable ionic liquids and consequences on viscosity, J. Phys. Chem. Lett. 7 (9) (2016) 1646–1652.
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Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou
Chemical transformations of captured CO2 into cyclic and polymeric carbonates
T
Jotheeswari Kothandaramana, Jun Zhangb, Vassiliki-Alexandra Glezakoub, Michael T. Mockb, ⁎ David J. Heldebranta, a b
Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, WA, 99354, United States Physical Sciences Division, Pacific Northwest National Laboratory, Richland, WA, 99352, United States
ARTICLE INFO
ABSTRACT
Keywords: CO2 Polymerization Alkylcarbonate Catalysis
Alkylcarbonate ionic liquids are shown to be chemically active species for trans-carboxylating cyclohexene oxide to produce polycarbonates. Catalytic equivalents of alkylcarbonate ionic liquids can be added to co-polymerizations of cyclohexene oxide/CO2 to increase the yield and selectivity of the polymer. On the other hand, in the case of propylene oxide, polymerization is suppressed, and propylene oxide is converted to propylene carbonate. The suppression of polymerization is due to the high activity of the alkylcarbonate ionic liquids towards carboxylation of epoxides for cyclic carbonate formation. The reactivity of alkylcarbonate ionic liquids with propylene oxide is sufficient enough that propylene carbonate is produced in the absence of catalyst at pressures as low as 1 atm at 60 °C.
1. Introduction There has been a recent interest to utilize vast quantities of anthropogenic CO2 being emitted into the atmosphere as a part of a larger carbon reduction strategy to help mitigate the impacts of climate change [1,2]. CO2 is considered stable in kinetic and thermodynamic standpoint, making it slow to coordinate and be activated by catalysts. While the conversion of CO2 to chemicals is potentially more environmentally responsible when replacing toxic and highly reactive chemicals, the magnitude of anthropogenic CO2 emissions dwarfs the volume of useful products that can be made from CO2. It is known that the magnitude of CO2 far exceeds the volume of products that can be made, but CO2 always remain an attractive chemical feedstock because of its abundance, low cost and low eco-toxicity [2,3]. There are many desired products to be made from CO2, ranging from methanol, methane, formic acid, carbonates etc. [4,5]. Cyclic carbonates and linear polycarbonate polymers stand out because of their simplicity and low cost. The energetics of capture and compression of CO2 is an oftenoverlooked component of chemical transformations of CO2. The energy intensive compression and storage steps after the CO2 capture can be avoided when the captured CO2 is directly used for the chemical transformations. Since Inoue et al’s first report on the CO2/epoxide copolymerization
⁎
using diethylzinc/H2O derived catalyst, there has been many advances made in the heterogenous metal catalyzed synthesis of polymers [6,7]. However, most of these systems require high catalyst loading, providing polymers with broad polydispersity indices (PDI) and low selectivity [8]. Similarly, elegant catalytic systems have been developed for either direct carboxylation or co-polymerization of epoxides/CO2 in presence of homogenous catalysts, but they often are limited to high pressures and in the case of polymerizations, often suffer from long reaction times or chain termination through intramolecular cyclization of the growing carbonate chain [7,9,10]. Cyclic carbonates are useful as intermediates in the synthesis of pharmaceutical/fine chemicals, monomers in polymerization and solvents for lithium-ion rechargeable batteries and in many applications [11–13]. Catalysts are continuously developed to incrementally enhance the reactivity of CO2, the lone variable that has yet to be changed in such systems is the CO2 itself [14–18]. We hypothesized that by employing a transcarboxylating agent, reaction selectivity, kinetics and catalyst design, may be changed, opening new pathways, processes and products that may not be available from CO2 directly. Conversion of captured CO2 to value-added chemicals/fuels such as urea derivatives, [19], formate [20,21], methanol [22] etc… have already been investigated by various groups. We also have demonstrated recently that carbonates (formed from captured CO2) can be hydrogenated directly to formate and methanol, and it is likely that such systems could be
Corresponding author. E-mail address: [email protected] (D.J. Heldebrant).
https://doi.org/10.1016/j.jcou.2019.04.020 Received 19 November 2018; Received in revised form 18 April 2019; Accepted 24 April 2019 Available online 02 May 2019 2212-9820/ © 2019 Elsevier Ltd. All rights reserved.
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or cyclic carbonate products were detected (entry 1, Table 1). However, upon heating the reaction mixture to 95 °C, PCHC (polycyclohexyl carbonate) was formed with 81% selectivity, which is comparable to Darensbourg et al.’s results [9]. The polyether and cyclic carbonate products were formed with 14% and 5% selectivity respectively based on 1H NMR spectroscopy. To our delight, the addition of SWIL increased both the selectivity and yield for PCHC (entry 2–5, Table 1). Using 2 equivalents of SWIL with respect to the catalyst, PCHC yield as high as 82% was obtained with 99% selectivity for PCHC and PDI of 1.14. We also observed that the presence of DBU by itself increased the formation of PCHC, albeit with lower selectivity and yields for PCHC compared to SWIL (entry 5 vs 6, Table 1). Increasing the temperature from 82 °C to 95 °C improved the yield of PCHC from 82 to 94% with slight drop in selectivity (98%) with a TOF of 206 h−1, which is to the best of our knowledge the highest ever reported for the Cr(Salen)Cl catalyst shown in Scheme 3 (entry 7, Table 1) [32]. Presence of SWIL increased the rate of copolymerization by > 12 fold (entry 2 and 7, Table 1). When the DBU and 1-hexanol mixture was used directly to produce the SWIL in-situ instead of preforming it, PCHC was formed, however with relatively lower selectivity (92%) and yield (41%) for PCHC (entry 8, Table 1). It is important to note that both the yield and selectivity for PCHC did not improve with increasing DBU concentration (entry 6 and 9, Table 1), which is probably because of increased coordination ability of DBU to the catalyst, leading to inhibition of chain initiation step [33]. Lewis basic ionic liquids such as [DBUH+]OAc−, [DBUH+]Cl−, [TBDH+]Cl−, [MImH+]Cl− etc were identified previously by He at al as efficient catalysts for the formation of cyclic carbonates [34]. Under our reaction condition, in the presence of Cr(salen)Cl catalyst, CHO/ CO2 formed primarily PCHC with good selectivity (96%, entry 10, Table 1) in the presence of [DBUH+]OAc− as a co-catalayst, although with low yield, 39%. This suggests that the SWIL is acting both as nucleophile and trans-carboxylating agent but [DBUH+]OAc is only acting as a nucleophilic co-catalyst. Next, we studied the influence of CO2 pressure on the polymer formation. Even at lower pressure (under 10 bar), the polymer, PCHC was formed with good yield, 63% (entry 11, Table 1) in the presence of SWIL. We also observed that the cyclic carbonate yield increased from traces to 6% upon lowering the pressure from 44 bar to 10 bar (entry 7 and 11, Table 1). A reaction mechanism is proposed for the polymerization in Scheme 4, which is similar to Darensbourg’s study on the effect of Lewis basic co-catalysts on the Cr-Salen catalyst [35]. First the chain initiation step occurs where the Cl initiates the ring opening of epoxide followed by the coordination of the hexylcarbonate (from SWIL) to Cr. The nucleophilic attack of the ring opened epoxide (metal alkoxide) on the hexylcarbonate carbon leads to the release of free 1-hexanol and DBU. The free DBU and 1-hexanol reacts with fresh CO2 and trans-carboxylate with the growing polymer chain.
Scheme 1. The DBU hexylcarbonate switchable ionic liquid (SWIL).
Scheme 2. Cyclic carbonate formation from backbiting of the polymer.
developed for carboxylation reactions [23,24]. Alkyl carbonate ionic liquids are suitable CO2 mimics for CO2 catalysis, as they could act as a CO2 carrier, concentrator, and solvent, but also (under anhydrous conditions) could prevent chain termination in polymerization reactions [25]. Alkyl carbonate ionic liquids (Scheme 1) contain a 0.5 mol fraction of “CO2” in solution, offering high concentrations of CO2 at atmospheric pressure, far better than conventional solvents that require high pressures of CO2 (often > 50 atm) to achieve such concentrations [26]. Alkyl carbonate ionic liquids continuously capture and activate CO2 and being polar, they allow stabilization of charged/polar intermediates common in catalytic reactions [25,27,28]. Alkyl carbonates are also negatively charged, favoring complexation to cationic homogeneous catalysts. Furthermore, in co-polymerizations of CO2 and epoxides, growing polycarbonate chains often backbites on itself, forming cyclic carbonates, thus terminating the polymer chain (Scheme 2) [7,29]. Under anhydrous conditions, alkylcarbonates could mitigate the backbiting compared to neutral CO2 by either enhancing the rate of coordination of a CO2 equivalent to the catalyst, or by alternatively by having an excess of alkylcarbonate in the first-solvation sphere in solution to trans-carboxylate to the growing polymer chain. Alkyl carbonate ionic liquids are a system that implores exploration, as they can perform all of the aforementioned roles of CO2 fixation, concentration, and activation towards potential catalytic pathways to cyclic carbonate and polycarbonate products and be catalytic with respect to capture solvent. We present here the reactivity, feasibility and reaction mechanisms of the reaction between alkylcarbonate ionic liquids and cyclohexene/ propylene oxide to make cyclic carbonates and polycarbonates. 2. Results and discussion 2.1. Co-polymerization of PCHC and CO2 using alkylcarbonate We chose Jacobsen’s catalyst, Cr(salen)Cl, to determine if transition metal catalysts were tolerant of alkylcarbonate chemistry and to discern the difference in reactivity and selectivity between neutral CO2 and alkylcarbonates [30]. Jacobsen used Cr(salen)Cl complex for the addition of TMSN3 to meso-epoxides by asymmetric ring opening [30]. Followed by his study, the groups of Nguyen and Darensbourg showed that the Jacobsen’s catalyst can be used for the cyclic carbonate and polycarbonate formation in the presence of Lewis-base co-catalysts, DMAP and N-Methylimdazole, respectively [31,9]. The results for the co-polymerization of cyclohexeneoxide/CO2 using diazabiciclo[5.4.0]undec-7-ene hexylcarbonate switchable ionic liquid (SWIL) are tabulated in Table 1. Scheme 3 shows the polycarbonate, ether and cyclic carbonate linkages observed in the system. Alternating copolymerization of epoxide/CO2 of the growing polymer chain by chain propagation pathway forms the polycarbonate. Backbiting or back-to-back epoxide enchainment of the polymer chain gives the cyclic carbonate and ether linkages respectively. In the absence of any catalysts or additives, no detectable amount of polymer was identified by 1H NMR spectroscopy. Even in the presence of Cr(salen)Cl catalyst at 84 °C under 44 bar CO2 pressure, no polymer
2.2. Co-polymerization/coupling of propylene oxide and alkylcarbonate Polymerizations of CO2 and propylene oxide were performed with SWIL using a heterogenous catalyst, zinc glutarate (Zn-glutarate). The Zn-glutarate catalyst was prepared following the procedure by Darensbourg et al using zinc oxide and glutaric acid [36]. Under our reaction conditions (60 °C and 44 bar CO2 pressure), polypropylene carbonate (PPC) was formed with 76% selectivity from the coupling of propylene oxide and CO2 (Scheme 5 and entry 1, Table 2). Interestingly, addition of SWIL formed propylene carbonate (PC) with excellent selectivity (entry 2, Table 2). A control experiment performed using DBU as an additive, produced relatively less amount of cyclic carbonate (entry 3, Table 2). Even in the absence of Zn-glutarate catalyst, SWIL selectively formed PC at temperature as low as 60 °C under 10 bar CO2 pressure with good yield (entry 4, Table 2). 197
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Table 1 Co-polymerization of CHO and CO2. Entry
T (˚C)
CO2 (bar)
[Cr] : [CHO]: [Co-Cat]
PCHC : CHC + PEC linkagesa
PCHCa
TON
TOF
Mwb (g/mol)
Mn
1 2 3 4 5 6 7 8 9 10 11 12
84 95 84 84 82 82 95 95 92 92 92 92
44 44 44 44 44 44 44 44 44 44 10 10
1 : 3550 : 1 : 3550 : 1: 3550 : 0.5 (SWIL) 1 : 3550 : 1 (SWIL) 1 : 3550 : 2 (SWIL) 1 : 3550 : 1 (DBU) 1 : 3550 : 2 (SWIL) 1 : 3550 : 1 ([DBUH+]C6H13O−) 1 : 3550 : 2 (DBU) 1 : 3550 : 2 ([DBUH+]OAc−) 1 : 2550 : 2 (SWIL) 1 : 2550 : 2 (DBU)
– 81%:19% 93%:7% 96%:4% 99%:1% 93%:7% 98%:2% 92%:8% 95%:5% 96%:4% 92%:8% 90%:10%
– 7% 41% 51% 82% 39% 94% 41% 41% 39% 63% 35%
– 260 1450 1800 2900 1400 3300 1450 1450 1400 1600 900
– 16 91 112 181 87 206 92 91 87 100 56
– 2270 4440 6550 11900 4930 12600 7550 12700 9030 7360 7440
– 3000 4230 6160 10500 4520 10300 6170 9310 6740 6470 6420
b
(g/mol)
PDIb – 2.29 1.05 1.06 1.14 1.09 1.22 1.22 1.36 1.34 1.14 1.16
CHO = Cyclohexene oxide, PCHC = polycyclohexene carbonate, CHC = cyclohexene carbonate, PEC = polyether carbonate, Cr(salen)Cl = 0.02 mmol, neat CHC was used for all the experiments and the reactions were performed in an autoclave for 16 h. a Selectivity and yield of all entries were calculated by 1H NMR spectroscopy using 1,3,5-trimethoxybenzene as an internal standard. b Determined by GPC using polystyrene standards. Turnover number (TON)=(mol of repeating unit)/(mol of Cr). Turnover frequency (TOF)=TON/h.
Scheme 5. Co-polymerization/Coupling of PO with alkylcarbonates.
intermediate A (Scheme 6). The high concentration of this transient peak suggests this to be a stable species that undergoes another transformation to PC (13C chemical shifts: 154.9, 73.9, 71.0 ppm). The broadness at the beginning of the reaction is due to the high viscosity of the ionic liquid. As PC begins to build up in concentration, the viscosity of the solution begins to decrease, improving the resolution of the later spectra. Also, the slight shifting of the SWIL peaks up field over the reaction profile is attributed to a change in the chemical environment, as the ionic liquid is consumed. Evidence for the transient intermediate A was also observed using in-situ REACT-IR (with [DBUH+] [C6H13OCO2−] baseline subtracted). The intermediate A builds up at 1750 cm-1 concurrently with the formation of PC at 1800 cm-1 (Fig. 2). The IR and NMR data described above suggested that the bi-functional nature of [DBUH]+ C6H13OCO2−, i.e. the Bronsted (conjugate) acid and the nucleophilic (anionic) hexylcarbonate were contributing to the observed ring opening of PO to make propylene carbonate, however, it was unclear as to whether the [DBUH]+ or the C6H13OCO2− was the reactive species. To assess the reactivity, we performed DFT calculations for all of the species in the proposed reaction cycle (Scheme 6). The energetics of the cycle show that the formation of [DBUH]+ C6H13OCO2− is favorable by −9.7 kcal/mol, in accord with experimental observations. The complete dissociation of the [DBUH]+ C6H13OCO2- pair is predicted to be endothermic, as was an individual attack on PO by either [DBUH]+ or C6H13OCO2−. The results suggest that protonation of PO by [DBUH]+ to make a protonated epoxide is
Scheme 3. Co-polymerization of CHO and CO2 using alkylcarbonate additives.
Based on DFT-based calculations, we propose a mechanism that proceeds through intermediates including: a carbonate/protonated amine complex, an acid promoted ring opening of propylene oxide by a proton transfer from [DBUH+], and finally a nucleophilic attack of carbonate on the protonated epoxide to form the proposed ester intermediate A. With the addition of a second CO2, intermediate A subsequently cyclizes to form PC. Similar H+-catalyzed mechanisms have been proposed in cyclic carbonate formation in ionic liquids [34]. The cyclization liberates 1-hexanol which can then further react with DBU and CO2 to reform SWIL and the cycle continues (Scheme 6). The metal-free reaction between PO and SWIL was studied using both time resolved 13C NMR and REACT-IR. The 13C time resolved spectra (Fig. 1) at the bottom, shows the amidinium [DBUH+] and hexylcarbonate [C6H13OCO2−] signals around 162 ppm. As the reaction proceeds (Fig. 1) we observe a transient species with a 13C shift of 165 ppm, which we attribute to the protonated form of the ester
Scheme 4. Proposed reaction mechanism. 198
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Table 2 Coupling of PO and CO2. Entry
T (˚C)
CO2 (bar)
[Zn-glutarate] : [PO]: [Co-Cat]
PPC : PC + PEC linkagesa
PPCa
PC
TON
TOF
Mwb(g/mol)
Mn
1 2 3 4
60 60 60 60
44 44 44 10
1 : 180 : 1 : 180 : 1 (SWIL) 1: 180: 1 (DBU) - : 4 : 1 (SWIL)
76%:24% 0%:100% 0%:100% 0%:100%
25% – – –
7% 22% 4% 53%
47 – – –
2 – – –
67100 – – –
41600 – – –
b
(g/mol)
PDIb 1.61 – – –
PC = propylene carbonate, PPC = polypropylene carbonate, PEC = polyethercarbonate, Zn-Glutarate = 0.77 mmol, PO = 0.14 mol, 60 °C, 24 h. a Selectivity and yield of all entries were calculated by 1H NMR spectroscopy using 1,3,5-trimethoxybenzene as an internal standard. b Determined by GPC using polystyrene standards. Turnover number (TON)=(mol of repeating unit)/(mol of Zn). Turnover frequency (TOF)=TON/h.
unfavorable, and also lessens the likelihood of direct nucleophilic attack on PO from C6H13OCO2− as individual steps. The calculations indicate that a concerted H+ transfer and nucleophilic attack from ionic pair [DBUH]+ C6H13OCO2− is at play. The proposed linear ester (A) is energetically favored by −10 kcal/mol. Upon further reaction with CO2, it produces propylene carbonate also favored by −15.8 kcal/mol. The results from these calculations overall suggest that the proposed mechanism, shown in Scheme 6, is favorable energetically and that the ion pair is likely the in-situ catalyst for the cycloaddition reaction. Based on the computational results and optimized structures, it is postulated that the strength and orientation of hydrogen bonding between [DBUH]+ C6H13OCO2− is critical by tuning the charge transfer that facilitates the concerted reaction. The bifunctional nature of the SWIL allows both modes of epoxide opening on the same system. The proposed concerted reaction is similar to other published cycloadditions where a H-bond donor and electrophile are used to jointly stabilize the opened epoxide prior to cyclization [37]. Our previous studies have also shown that the orientation and speciation of hydrogen bonding between the cation and alkylcarbonate anions in SWIL systems is key to controlling the physical and thermodynamic properties of the system [38,39]. It is likely that the strong association between cation and anion in this case provides the needed regio-selectivity and charge transfer that enables this concerted cycloaddition reaction even in the absence of a transition metal catalyst. However, in the presence of a transition metal catalyst (Scheme 4), the strong association between the cation and anion of the SWIL can be broken, as shown previously in the presence of a Ru catalyst [23]. The effect of SWIL on the propylene oxide/CO2 copolymerization was then studied in the presence of Cr catalyst (Table 3). In the absence of any co-catalyst at room temperature, the selectivity for the copolymer formation was high in the presence of Cr(salen)Cl however, the reaction rate was very slow (TOF = 4 h−1). The addition of co-catalyst such as DBU or SWIL slightly favored the formation of cyclic carbonate (entry 2 and 3, Table 3). Increasing the temperature to 50 °C and in the absence of any co-catalyst significantly increased the reaction selectivity to cyclic carbonate formation. The co-catalysts DBU and [DBUH+]OAc- increased the yield for polymer with the TON of 1300 and 1225 respectively (entry 5 and 7, Table 3) likely due to the unavailability of the acetate to undergo SWIL-promoted cycloaddition under this reaction condition (Scheme 6). Unlike the CHO case, the presence of SWIL increased the selectivity for the cyclic carbonate due to its higher direct reactivity with SWIL as described above (entry 6, Table 3). Similarly, when glycidol was used instead of propylene oxide in entry 6, Table 3, the yield for glycerol carbonate (64%) increased in presence of SWIL compared to the experiment without the SWIL (38%). In order to extent the scope of SWIL, we screened various epoxides for the cycloaddition reaction and the results were shown in Fig. 3. It is evident from the results that diverse epoxides can tolerate the reaction condition and form corresponding cyclic carbonate with moderate to good yields. Under the reaction conditions, the low boiling epoxides such as propylene oxide (bp. 34 °C) and 1,2-butylene oxide (bp. 63 °C) probably exist as vapor in the head space of the reactor, which accounts for relatively lower yield for 1a (53%) and 1b (56%). However, various cyclic carbonates can be prepared by this method via one-pot CO2 capture and conversion.
Scheme 6. Proposed mechanism of the reaction between SWIL and PO.
Fig. 1. Time-resolved 13C NMR of neat SWIL + PO (1 atm, 60 °C, 14 h). Array referenced to propylene carbonate (74.0 ppm).
Fig. 2. Time-resolved IR spectra of neat SWIL + PO (1 atm CO2, 60 °C) with SWIL baseline subtracted.
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Table 3 Coupling of PO and CO2 in the presence of Cr catalyst and different co-catalysts. Entry
T (˚C)
CO2 (bar)
[Cr] : [PO]: [Co-Cat]
PPC : PC + PECa
PPCa
TON
TOF h−1
Mwb (g/mol)
Mn
1 2 3 4 5 6 7
25 25 25 50 50 50 50
44 44 44 44 44 44 44
1 1 1 1 1 1 1
90%:10% 83%:17% 86%:14% 27%:73% 83%:17% 43%:57% 81%:19%
8% 6% 11% 2% 35% 23% 33%
290 215 375 81 1300 856 1225
4 3 5 3 43 28 41
8080 155 592 – 8400 6200 14700
611 119 147 – 4500 4020 10100
: : : : : : :
3550 : 3550 : 1 (DBU) 3550 : 1 (SWIL) 3700 : 3700 : 1 (DBU) 3700 : 1 (SWIL) 3700: 1 ([DBUH+]OAc−)
b
(g/mol)
PDIb 13.2 1.3 4.02 – 1.87 1.54 1.46
PC = propylene carbonate, PPC = polypropylene carbonate, PEC = polyethercarbonate. Cr(salen)Cl = 0.08 mmol, PO = 0.30 mol, entry 1–3 = 72 h, entry 4–7 = 30 h. a Selectivity and yield of all entries were calculated by 1H NMR spectroscopy using 1,3,5-trimethoxybenzene as an internal standard. b Determined by GPC using polystyrene standards. Turnover number (TON) = (mol of repeating unit)/(mol of Cr). Turnover frequency (TOF) = TON/h. catalysis and life cycle assessment, Chem. Rev. 118 (2) (2018) 434–504. [3] S.-Y. Pan, P.-C. Chiang, W. Pan, H. Kim, Advances in state-of-art valorization technologies for captured CO2 toward sustainable carbon cycle, Crit. Rev. Environ. Sci. Technol. 48 (5) (2018) 471–534. [4] Z. Jiang, T. Xiao, V.L. Kuznetsov, P.P. Edwards, Turning carbon dioxide into fuel, Philos. Trans. R. Soc. A 368 (1923) (2010) 3343–3364. [5] J. Klankermayer, W. Leitner, Harnessing renewable energy with CO2 for the chemical value chain: challenges and opportunities for catalysis, Philos. Trans. R. Soc. A 374 (2061) (2016). [6] S. Inoue, H. Koinuma, T. Tsuruta, Copolymerization of carbon dioxide and epoxide, J. Polym. Sci. Part B: Polym. Lett. 7 (1969) 287–292. [7] D.J. Darensbourg, Making plastics from carbon dioxide: salen metal complexes as catalysts for the production of polycarbonates from epoxides and CO2, Chem. Rev. 107 (2007) 2388–2410. [8] K. Soga, E. Imai, I. Hattori, Alternating copolymerization of CO2 and propylene oxide with the catalysts prepared from Zn(OH)2 and various dicarboxylic acids, Polym. J. 13 (1981) 407–410. [9] D.J. Darensbourg, J.C. Yarbrough, Mechanistic aspects of the copolymerization reaction of carbon dioxide and epoxides, using a chiral salen chromium chloride catalyst, J. Am. Chem. Soc. 124 (2002) 6335–6342. [10] S.J. Poland, D.J. Darensbourg, A quest for polycarbonates provided via sustainable epoxide/CO2 copolymerization processes, Green Chem. 19 (21) (2017) 4990–5011. [11] A.-A.G. Shaikh, S. Swaminathan, Organic carbonates, Chem. Rev. 96 (1996) 951–976. [12] B. Scrosati, J. Hassoun, Y.-K. Sun, Lithium-ion batteries. A look into the future, Energy Environ. Sci. 4 (2011) 3287–3295. [13] V. Besse, F. Camara, C. Voirin, A. Caillol, R. Sylvain, B. Boutevin, Synthesis and applications of unsaturated cyclocarbonates, Polym. Chem. 4 (2013) 4545–4561. [14] M.R. Kember, A. Buchard, C.K. Williams, Catalysts for CO2/epoxide copolymerisation, Chem. Commun. 47 (1) (2011) 141–163. [15] C. Martin, G. Fiorani, A.W. Kleij, Recent advances in the catalytic preparation of cyclic organic carbonates, ACS Catal. 5 (2) (2015) 1353–1370. [16] R.R. Shaikh, S. Pornpraprom, V. D’Elia, Catalytic strategies for the cycloaddition of pure, diluted, and waste CO2 to epoxides under ambient conditions, ACS Catal. 8 (1) (2018) 419–450. [17] H. Buttner, L. Longwitz, J. Steinbauer, C. Wulf, T. Werner, Recent developments in the synthesis of cyclic carbonates from epoxides and CO2, Top. Curr. Chem. 375 (3) (2017) 50–55. [18] M. Alves, B. Grignard, R. Mereau, C. Jerome, T. Tassaing, C. Detrembleur, Organocatalyzed coupling of carbon dioxide with epoxides for the synthesis of cyclic carbonates: catalyst design and mechanistic studies, Catal. Sci. Technol. 7 (13) (2017) 2651–2684. [19] Z.Z. Yang, L.N. He, Y.N. Zhao, B. Li, B. Yu, CO2 capture and activation by superbase/polyethylene glycol and its subsequent conversion, Energy Environ. Sci. 4 (10) (2011) 3971–3975. [20] J. Kothandaraman, A. Goeppert, M. Czaun, G.A. Olah, G.K.S. Prakash, CO2 capture by amines in aqueous media and its subsequent conversion to formate with reusable ruthenium and iron catalysts, Green Chem. 18 (21) (2016) 5831–5838. [21] Y.N. Li, L.N. He, A.H. Liu, X.D. Lang, Z.Z. Yang, B. Yu, C.R. Luan, In situ hydrogenation of captured CO2 to formate with polyethyleneimine and Rh/monophosphine system, Green Chem. 15 (10) (2013) 2825–2829. [22] J. Kothandaraman, A. Goeppert, M. Czaun, G.A. Olah, G.K. Prakash, Conversion of CO2 from air into methanol using a polyamine and a homogeneous ruthenium catalyst, J. Am. Chem. Soc. 138 (3) (2016) 778–781. [23] D.B. Lao, B.R. Galan, J.C. Linehan, D.J. Heldebrant, The steps of activating a prospective CO2 hydrogenation catalyst with combined CO2 capture and reduction, Green Chem. 18 (18) (2016) 4871–4874. [24] J. Kothandaraman, R.A. Dagle, V.L. Dagle, S.D. Davidson, E.D. Walter, S.D. Burton, D.W. Hoyt, D.J. Heldebrant, Condensed-phase low temperature heterogeneous hydrogenation of CO2 to methanol, Catal. Sci. Technol. 8 (2018) 5098–5103. [25] P.G. Jessop, S.M. Mercer, D.J. Heldebrant, CO2-triggered switchable solvents, surfactants, and other materials, Energy Environ. Sci. 5 (6) (2012) 7240–7253. [26] X. Gui, Z. Tang, W. Fei, Solubility of CO2 in alcohols, glycols, ethers, and ketones at high pressures from (288.15 to 318.15) K, J. Chem. Eng. Data 56 (2011) 2420–2429.
Fig. 3. Cycloaddition of CO2 to various epoxides. Reaction conditions: 0.036 mol SWIL, 0.14 mol epoxide, 10 bar CO2, 60 °C, 24 h, aSelectivity and yield of all entries were calculated by 1H NMR spectroscopy using 1,3,5-trimethoxybenzene as an internal standard.
3. Conclusions CO2 captured as amidinium alkylcarbonates react with CHO analogously to CO2. In polymerization reactions, selectivity of the copolymer was found to increase (> 98%) with catalytic equivalents of the alkyl carbonate. Catalyst equivalents of SWIL also increased the yield (from 7% to 94%) of polymers. Time resolved IR and 13C NMR spectroscopic studies elucidated that SWIL was highly reactive towards PO, as direct carboxylation of PO occurs in the absence of metal catalyst at 60 °C at atmospheric pressure. The metal-free direct carboxylation of PO at atmospheric pressure confirmed the unique reactivity of alkyl carbonates, as direct carboxylation of PO is unavailable with neutral CO2. DFT calculations confirmed the energetic feasibility of the catalytic cycle, revealing the role of ionic pair as the in-situ catalyst for the cycloaddition of CO2 contained in an alkylcarbonate to PO. Funding sources We thank the United States Department of Energy’s Office of Science Basic Energy Sciences Early Career Research ProgramFWP 67038 for funding. Acknowledgment Pacific Northwest National Laboratory (PNNL) is proudly operated by Battelle for the US Department of Energy. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jcou.2019.04.020. References [1] M. Mikkelsen, M. Jørgensen, F.C. Krebs, The teraton challenge. A review of fixation and transformation of carbondioxide, Energy Environ. Sci. 2010 (3) (2009) 43–81. [2] J. Artz, T.E. Muller, K. Thenert, J. Kleinekorte, R. Meys, A. Sternberg, A. Bardow, W. Leitner, Sustainable conversion of carbon dioxide: an integrated review of
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J. Kothandaraman, et al. [27] L. Phan, D. Chiu, D.J. Heldebrant, H. Huttenhower, J. Ejae, X. Li, P. Pollet, R. Wang, C.A. Eckert, C.L. Liotta, P.G. Jessop, Switchable solvents consisting of amidine/ alcohol or guanidine/alcohol mixtures, Ind. Eng. Chem. Res. 47 (3) (2008) 539–545. [28] D.J. Heldebrant, C.R. Yonker, P.G. Jessop, L. Phan, Organic liquid CO2 capture agents with high gravimetric CO2 capacity, Energy Environ. Sci. 1 (4) (2008) 487–493. [29] D.J. Darensbourg, A.D. Yeung, Thermodynamics of the carbon dioxide-epoxide copolymerization and kinetics of the metal-free degradation: a computational study, Macromolecules 46 (1) (2013) 83–95. [30] E.N. Jacobsen, Asymmetric catalysis of epoxide ring-opening reactions, Acc. Chem. Res. 33 (6) (2000) 421–431. [31] R.L. Paddock, S.T. Nguyen, Chemical CO2 fixation: Cr(III) salen complexes as highly efficient catalysts for the coupling of CO2 and epoxides, J. Am. Chem. Soc. 123 (46) (2001) 11498–11499. [32] D.J. Darensbourg, R.M. Mackiewicz, A.L. Phelps, D.R. Billodeaux, Copolymerization of CO2 and epoxides catalyzed by metal salen complexes, Acc. Chem. Res. 37 (11) (2004) 836–844. [33] D.J. Darensbourg, R.M. Mackiewicz, J.L. Rodgers, C.C. Fang, D.R. Billodeaux, J.H. Reibenspies, Cyclohexene oxide/CO2 copolymerization catalyzed by chromium (III) salen complexes and N-methylimidazole: effects of varying salen ligand substituents and relative cocatalyst loading, Inorg. Chem. 43 (19) (2004) 6024–6034.
[34] Z. Yang, L. He, C. Miao, S. Chanfreau, Lewis basic ionic liquids-catalyzed conversion of carbon dioxide to cyclic carbonates, Adv. Synth. Catal. 352 (2010) 2233–2240. [35] D.J. Darensbourg, R.M. Mackiewicz, J.L. Rodgers, Role of the cocatalyst in the copolymerization of CO2 and cyclohexene oxide utilizing chromium salen complexes, J. Am. Chem. Soc. 127 (40) (2005) 14026–14038. [36] D.J. Darensbourg, N.W. Stafford, T. Katsurao, Supercritical carbon dioxide as solvent for the copolymerization of carbon dioxide and propylene oxide using a heterogeneous zinc carboxylate catalyst, J. Mol. Catal. A: Chem. 104 (1995) L1–L4. [37] J. Gao, Q.W. Song, L.N. He, C. Liu, Z.Z. Yang, X. Han, X.D. Li, Q.C. Song, Preparation of polystyrene-supported Lewis acidic Fe(III) ionic liquid and its application in catalytic conversion of carbon dioxide, Tetrahedron 68 (20) (2012) 3835–3842. [38] D. Malhotra, P.K. Koech, D.J. Heldebrant, D.C. Cantu, F. Zheng, V.A. Glezakou, R. Rousseau, Reinventing design principles for developing low-viscosity carbon dioxide binding organic liquids (CO2BOLs) for flue gas clean up, ChemSusChem 10 (2017) 636–642. [39] D.C. Cantu, J. Lee, M.S. Lee, D.J. Heldebrant, P.K. Koech, C.J. Freeman, R. Rousseau, V.A. Glezakou, Dynamic acid/base equilibrium in single component switchable ionic liquids and consequences on viscosity, J. Phys. Chem. Lett. 7 (9) (2016) 1646–1652.
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