Kinetic Study on the Coupling of CO2 and Epoxides Catalyzed by Co

Feb 5, 2013 - The big difference in the energies of activation for cyclic carbonate versus copolymer formation accounts for the excellent selectivity ...
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Kinetic Study on the Coupling of CO2 and Epoxides Catalyzed by Co(III) Complex with an Inter- or Intramolecular Nucleophilic Cocatalyst Jie Liu, Wei-Min Ren,* Ye Liu, and Xiao-Bing Lu* State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, China S Supporting Information *

ABSTRACT: Trivalent cobalt complexes of salicylaldimine in the presence of an inter- or intramolecular nucleophilic cocatalyst have proven to be excellent catalysts for the copolymerization of CO2 and epoxides to selectively afford the corresponding polycarbonates in perfectly alternating nature. Especially, bifunctional cobalt(III)−salen complexes bearing an appended quaternary ammonium salt are more efficient in catalyzing this copolymerization even at high temperatures and extremely low catalyst loading. The present study focuses on comparative kinetics of two different catalyst systems (binary catalyst system of salen(III)X 1/nBu4NX and bifunctional catalyst 2 bearing an appended quaternary ammonium salt, X = 2,4-dinitrophenoxide) for coupling CO2 and epoxides (propylene oxide or cyclohexene oxide) by means of in situ infrared spectroscopy. An induction period was readily found in the binary catalyst system, and its length significantly depends on catalyst loading. Contrarily, no induction period was observed in the bifunctional catalyst 2, in which the overall reaction pathway is consistent with the firstorder dependence on catalyst concentration. A reaction order of 1.61 of catalyst concentration was obtained from the binary 1/nBu4NX catalyst system, indicating the complexity of the copolymerization. The energies of activation determined for cyclic carbonate and copolymer formation in the coupling reaction of CO2 and propylene oxide catalyzed by the binary 1/nBu4NX system are 50.1 and 33.8 kJ/mol, respectively, compared to the corresponding values in the bifunctional catalyst 2 of 77.0 and 29.5 kJ/mol. The big difference in the energies of activation for cyclic carbonate versus copolymer formation accounts for the excellent selectivity for copolymer formation in the bifunctional catalyst systems even at elevated temperatures. In the coupling system of CO2 and cyclohexene oxide, the energy of activation for copolymer (Ea) formation is 47.9 kJ/mol for the binary 1/nBu4NX catalyst system, higher than 31.7 kJ/mol determined in the bifunctional catalyst 2.

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INTRODUCTION It has been over 40 years since the seminal discovery of polycarbonate synthesis from the copolymerization of CO2 and epoxides was reported by Inoue et al. (Scheme 1).1 This process represents an environmentally benign approach for potential large-scale utilization of CO2 in chemical synthesis. These polycarbonates have potential applications as ceramic binders, adhesives, coatings, and packaging materials as well as in the synthesis of engineering thermoplastics and resins.2 A wide variety of catalytic systems,3 including heterogeneous catalysts mainly based on diethylzinc combined with a modifier having at least two labile hydrogen atoms,4 carboxylic zinc,5 double metal cyanide complexes,6 and rare-earth metal coordination ternary catalysts,7 in addition to homogeneous catalysts associated with discrete zinc-based complexes,8 as well as aluminum,9 magnesium,10 chromium,11,12 and cobalt13−15 complexes with a square-pyramidal geometry, have been © 2013 American Chemical Society

developed for this transformation. Prominent among these are the binary or bifunctional catalyst systems based on cobalt− Salen complexes.13−16 These systems provide for efficient CO2/ epoxides coupling reactions even under mild conditions and in some cases allow for the regio- and/or stereoselective polymerization. Notably, this kind of catalyst was shown to be wide substrate applicability during the copolymerization with CO2 in perfectly alternating nature, including aliphatic terminal epoxides, cyclohexene oxide, and epoxides with an electron-withdrawing group.17 Interestingly, in comparison with binary catalyst systems consisted of simple salenCo(III)X and a nucleophilic quaternary ammonium salt, bifunctional Co(III)based catalysts bearing an intramolecular nucleophilic cocatalyst Received: December 16, 2012 Revised: January 24, 2013 Published: February 5, 2013 1343

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Scheme 1. The Coupling Reaction of CO2 and Epoxides

Scheme 2. Binary and Bifunctional Catalysts for CO2/Epoxides Coupling

Figure 1. Three-dimensional stack plot of the IR spectra collected every 30 s during the reaction of CO2 and propylene oxide with (a) binary catalyst system of complex 1 and equivalent nBu4NX (X = 2,4-dinitrophenoxide) and (b) bifunctional catalyst 2. (c) Time profile of the absorbance at 1751 cm−1 corresponding to the poly(propylene carbonate). Reaction conditions: epoxide/catalyst = 5000/1 (malor ratio), 20 bar of CO2, 25 °C. manner, a single 256-scan background spectrum was collected. The catalyst dissolved in neat epoxide of 20 mL was then injected into the reactor via the injection port. The reactor was pressurized to 20 bar of CO2 as the IR probe began collecting scans. The infrared spectrometer was set up to collect one spectrum every 30 s over a certain period. Profiles of the absorbance at ∼1750 cm−1 v(CO) corresponding to the copolymer and around 1800 cm−1 v(CO) corresponding to cyclic carbonate with time were recorded and used to provide initial reaction rates for analysis. (Note: catalyst loading varied with the experiment as described in Results and Discussion section.)

exhibited higher activities, particularly at high temperatures or/ and extremely low catalyst loadings.16 The present study focuses on comparative kinetic studies of the coupling reaction of CO2 and epoxides using both binary and bifunctional Co(III)-based catalyst systems (Scheme 2) by in situ FTIR spectroscopy. The investigation provided herein also involves a temperature-dependent kinetic study of the relative propensity of the two catalyst systems for producing copolymer versus cyclic carbonate as a function of the nature of the epoxide (propylene oxide and cyclohexene oxide).

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RESULTS AND DISCUSSION Previously, Jacobsen and co-workers have present an elegantly mechanistic study of the salenCo(III)OAc-catalyzed hydrolytic kinetic resolution of terminal epoxides.19 In this system, a second-order dependence on the catalyst concentration was observed, suggesting a bimetallic mechanism that one salenCo(III) molecule was proposed to serve as Lewis acid for epoxide activation and another as counterion for the nucleophile. Being distinct from the bimetallic mechanism for the hydrolytic kinetic resolution process, a monometallic mechanism was proposed in the salenCo(III)X-mediated CO2/epoxide coupling in the presence of a nucleophilic cocatalyst.14 ESI-MS study demonstrated that the dissociation of the propagating carboxylate from the metal center is a much faster process than

EXPERIMENTAL SECTION

Methods. All manipulations involving water-sensitive compounds were carried out using standard Schlenk techniques under dry nitrogen. Propylene oxide (PO) and cyclohexene oxide (CHO) were purchased from Acros company and distilled under a nitrogen atmosphere from CaH2 prior to use. Carbon dioxide (99.995%) was purchased from Dalian Institute of Speical Gases and used as received. The complexes 1 and 2 were synthesized according to the previous publication.18 Copolymerization Reactions Monitored by IR Spectroscopy. In a typical experiment, a 100 mL stainless steel Parr autoclave reactor, modified with a ZnSeW AR window to allow for the use of an ASI ReactIR 45 system equipped with a MCT detector and 30 bounce DiCOMP in situ probe, is heated to the desired temperature. In this 1344

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Table 1. Effects of Catalyst Loading on Initial Rate and Induction Perioda run 1 2 3 4 5 6 7 8 9 10 11 12 13

catal./cocatal. (molar ratio) n

1/ Bu4NX 1/nBu4NX 1/nBu4NX 1/nBu4NX 1/nBu4NX 1/nBu4NX 1/nBu4NX 1/nBu4NX 1/nBu4NX 1/nBu4NX 2 2 2

(1/1) (1/1) (1/1) (1/1) (1/1) (1/1) (1/2) (1/4) (2/1) (4/1)

epoxide/catal. (molar ratio)

catalyst concnb (mol/L)

initial ratec (abs × 106/s)

induction periodd (s)

1000 1500 2000 3000 5000 10000 2000 2000 1000 500 2000 5000 10000

0.0142 0.0095 0.0071 0.0048 0.0028 0.0014 0.0071 0.0071 0.0142 0.0284 0.0071 0.0028 0.0014

346.7 151.6 98.6 58.7 28.8 7.3 126.7 142.3 162.3 174.5 102.6 40.2 19.8

0 0 300 1200 2200 3600 300 240 300 0 0 0 0

All reactions were performed in 20 mL neat propylene oxide at 25 °C and 20 bar CO2 pressure. bBased on cobalt complex concentration. cSlope of absorption vs time curves calculated for the linear part. dTime for initial rate at zero, corresponding to the intersection of the linear part at the time axis. a

Figure 2. Logarithmic plots of initial rate versus catalyst concentration: (a) binary catalyst system of complex 1 and equivalent nBu4NX (X = 2,4dinitrophenoxide) and (b) bifunctional catalyst 2.

Usually, an induction period was readily found in the binary catalyst system of complex 1 and equivalent nBu4NX (X = 2,4dinitrophenoxide), and its length significantly depends on catalyst loading. The induction period becomes shorter with increasing complex 1 or/and nBu4NX concentration (Table 1, runs 1−10). Contrarily, no induction period was observed in the bifunctional catalyst 2 at the same catalyst loadings. The initial rate is directly proportional to catalyst concentration for both binary and bifunctional catalyst systems. However, the effect of catalyst concentration on the initial rate is significantly different for the two catalyst systems. For example, a change in catalyst concentration from 0.0071 to 0.0014 mol/L resulted in the initial rate from 98.6 × 10−6 decreasing to 7.7 × 10−6 Abs/s for binary catalyst system, while a decrease from 102.6 × 10−6 to 19.8 × 10−6 Abs/s was observed in the bifunctional catalyst system under the same change in catalyst concentration. Moreover, a reaction order of 1.61 of catalyst concentration was obtained from the binary catalyst system consisted of complex 1 and equivalent nBu4NX (Figure 2a), indicating the complexity of the copolymerization system. This is very different from the bifunctional catalyst 2, wherein the overall reaction pathway is consistent with the firstorder dependence on catalyst concentration (Figure 2b). For salenCo(III)X-mediated CO2/epoxide copolymerization, it is generally known that the central metal ion plays an activating role and the nucleophilic cocatalyst or/and nucleophilic X

propagation, and the free propagating carboxylate can also act as a nucleophile for attack at a cobalt-coordinated epoxide during the copolymerization.14b Further investigation found that the copolymerization rate increased with the concentration of both salenCo(III)X and the nucleophilic cocatalyst. To compare the kinetic difference of binary and bifunctional Co(III)-based catalyst systems for the coupling reaction of CO2 and epoxides, we first performed the kinetic study of this process as a function of catalyst loading by in situ FTIR spectroscopy with a probe fitted to a modified stainless steel Parr reactor. As is easily observed, the intense absorbance for the asymmetric v(CO) vibration of polycarbonates appears at ∼1750 cm−1, while the absorption around 1800 cm−1 corresponds to cyclic carbonates. The reactions were carried out in 20 mL of neat propylene oxide at 25 °C and under 20 bar of CO2 pressure and monitored in the v(CO) region with changing amounts of catalyst. Figure 1 displays typical reaction profiles, where a strong absorption at 1751 cm−1 corresponding to poly(propylene carbonate) is observed for both binary and bifunctional catalyst systems. Peak traces of v(CO) infrared band at 1751 cm−1 clearly give the difference in initial stage (Figure 1c). The initial reaction rates and induction periods resulted from these experiments are listed in Table 1 along with their corresponding catalyst loadings. 1345

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Scheme 3. Difference in Interaction Mode of the Activating Epoxide and the Nucleophilic Cocatalyst for Binary (A) and Bifunctional (B) Catalyst Systems

Figure 3. Three-dimensional stack plots of the coupling of CO2 and propylene oxide at 55 °C catalyzed by (A) binary 1/nBu4NX catalyst system and (B) bifunctional catalyst 2.

Figure 4. Arrhenius plots for the formation of poly(propylene carbonate) and cyclic propylene carbonate from the coupling of CO2 and propylene oxide using different catalyst systems: (A) binary 1/nBu4NX catalyst and (B) bifunctional catalyst 2.

anion plays the initiator role (Scheme 3).3i Since the countercation in the binary system is not linked intramolecularly with the cobalt complex, the ion pair of the countercation and the dissociated anion can be far away from the cobalt center, particularly under highly diluted solution. Therefore, we can reasonably assume that the nucleophilic attack of cocatalyst or the propagating carboxylate species at the activated epoxide by its coordination to the metal center of complex 1 is similar to a bimolecular process. As a result, the rate is proportional to the concentration of the propagating species consistent with cocatalyst and the activated epoxide with regard to the concentration of complex 1. With regard to the bifunctional catalyst 2 containing a Lewis acidic Co(III) ion and a quaternary ammonium salt unit in a molecule, the interaction between the quaternary ammonium cation anchored on the ligand framework and the propagating carbonate anion leads to the dissociated carboxylate species always hanging around the metal center (Scheme 3). This characteristic results in an intramolecular process, wherein the CO2/ epoxide copolymerization involves the activating epoxide with regard to the central metal ion and the dissociated carboxylate

species around the metal center stabilized by the cation of the appended quaternary ammonium salt unit. The copolymerization process of CO2 and epoxide using binary salenCo(III)X/nBu4NX catalyst system is assumed to be a bimolecular process, so the reaction order should approach 2. However, the decomposition of a certain amount of Co(III) to inactive Co(II) species and the adventurous water will be the possible parameters for changing the reaction order. Moreover, the kinetic data of our previously reported binary catalyst system consisting of complex 1 and MTBD (7-methyl-1,5,7triazabicyclo[4.4.0]dec-5-ene, a sterically hindered organic base) as well as the thermally stable Co(III) catalyst with an appended TBD on the ligand framework were obtained by means of in situ infrared spectroscopy (see Supporting Information, Table S1 and Figure S1). A reaction order of 1.55 was found for the binary 1/MTBD catalyst system, while the value of TBD appended Co(III) catalyst is 0.98, close to that observed in the bifunctional catalyst 2. Furthermore, the effect of reaction temperature on the rates of polycarbonate versus cyclic carbonate production was investigated at identical reaction conditions in 20 mL of neat propylene oxide with a catalyst concentration of 0.0028 mol/L 1346

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Figure 5. Reaction coordinate diagram of the coupling reaction of CO2 and propylene oxide using different catalyst systems: (A) binary 1/nBu4NX catalyst and (B) bifunctional catalyst 2. PPC = poly(propylene carbonate), CPC = cyclic propylene carbonate.

Figure 6. Arrhenius plots for the formation of poly(cyclohexene carbonate) from the coupling of CO2 and cyclohexene oxide using different catalyst systems: (A) binary 1/nBu4NX catalyst and (B) bifunctional catalyst 2.

that the activation barrier for copolymer formation is only 29.5 kJ/mol, lower than that in the binary 1/nBu4NX catalyst system. These results give a reasonable explanation on the high catalytic activity and excellent selectivity for copolymer formation in the bifunctional catalyst systems even at elevated temperatures. In previous study, we demonstrated that the bifunctional catalyst 2 also exhibited high activity in the alternating copolymerization of alicyclic cyclohexene oxide (CHO) and CO2, with turnover frequencies (TOFs) of 170 h−1 at 25 °C and 4509 h−1 at 90 °C under 25 bar of CO2 pressure.21 With the use of binary 1/nBu 4NX catalyst system for this copolymerization, the catalytic activity at 25 °C is 87 h−1, while nearly complete loss in catalytic activity was observed at 90 °C under the same CO2 pressure. In order to ascertain this distinction, the kinetic experiments for the copolymerization of cyclohexene oxide and CO2 were also conducted using the two catalyst systems. According to the kinetic data (Figure 6), the resultant coordinate diagrams for the reaction process are shown in Figure 7. From the reaction profiles, the energies of activation for copolymer (Ea) formation are 47.9 kJ/mol for the binary 1/nBu4NX catalyst system and 31.7 kJ/mol for the bifunctional catalyst 2. The difference in activation energy for copolymer formation in the two catalyst systems is 16.2 kJ/mol. It is worthwhile noting here parenthetically that in the present studies of Co(III)-based catalyst systems for the copolymerization of CO2 and cyclohexene oxide linear poly(cyclohexene carbonate) was the sole product. This might be attributed to

(epoxide/catalyst, 5000/1, molar ratio) under 20 bar of CO2 pressure. Figure 3 illustrates the typical three-dimensional stack plots of the coupling of CO2 and propylene oxide at 55 °C catalyzed by binary 1/nBu4NX catalyst system and bifunctional catalyst 2. As is readily observed from the reaction profile, the formation of a certain proportion of cyclic propylene carbonate corresponding to 1802 cm−1 was found in the binary 1/nBu4NX catalyst system, while the use of bifunctional catalyst 2 predominately provided the corresponding polycarbonate at the same conditions. It is of note that a short induction period for propylene carbonate formation was observed in the binary catalyst system in comparison with polycarbonate formation (Figure 3A). This result indicates that the concomitant cyclic carbonate originated from the depolymerization of the resultant copolymer to a great extent.20 The energies of activation for cyclic carbonate and copolymer formation could be obtained from the kinetic data at various temperatures as illustrated in Figure 4. Based on these activation parameters, the reaction coordinate diagrams of the two catalyst systems for the coupling of CO2 and propylene oxide were obtained and are shown in Figure 5. In the binary 1/nBu4NX catalyst system, the activation barrier for cyclic carbonate formation is 50.1 kJ/mol, only higher of 16.3 kJ/mol in energy than that of the formation of the corresponding polycarbonate. Contrarily, up to 47.5 kJ/mol of the difference in the energies of activation for cyclic carbonate versus copolymer formation was observed in the bifunctional catalyst system with regard to complex 2. It is worthwhile noting here 1347

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for its higher activity in the selectively alternating copolymerization of CO2 with both aliphatic propylene oxide and cyclohexene oxide.

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ASSOCIATED CONTENT

* Supporting Information S

Kinetic data of binary 1/MTBD catalyst system and the TBDappended Co(III) catalyst for CO2/epoxide copolymerization. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: (W.-M.R.) [email protected]; (X.-B.L.) Lxb-1999@ 163.com. Notes

Figure 7. Reaction coordinate diagram of the coupling reaction of CO2 and cyclohexene oxide using different catalyst systems: (A) binary 1/nBu4NX catalyst and (B) bifunctional catalyst 2. PCHC = poly(cyclohexene carbonate).

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (NSFC, Grant 21134002, 21104007) and National Basic Research Program of China (973 Program: 2009CB825300).

the increased strain in forming cyclic carbonate imposed by the conformation of the alicyclic group.

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CONCLUSIONS Salicylaldimine complexes of trivalent cobalt in the presence of an inter- or intramolecular nucleophilic onium salt cocatalyst showed excellent activity in the selectively alternating copolymerization of CO2 with both aliphatic propylene oxide and cyclohexene oxide. Kinetic studies of the coupling reaction of CO2 and propylene oxide by means of in situ infrared spectroscopy revealed a significant difference in the relationship of the initial rate and catalyst concentration for two different catalyst systems: binary catalyst system of salenCo(III)X complex 1 with equivalent nBu4NX (X = 2,4-dinitrophenoxide) and bifunctional catalyst 2 bearing an appended quaternary ammonium salt. The overall reaction pathway is consistent with the first-order dependence on catalyst concentration for bifunctional catalyst 2, while a reaction order of 1.61 of catalyst concentration was obtained from the binary catalyst system consisted of complex 1 and equivalent nBu4NX, indicating the complexity of the copolymerization system. Additionally, being distinct from bifunctional catalyst, an induction period was readily found in the binary catalyst system, and its length significantly depends on catalyst loading. The difference also suggests a bimolecularly synergistic effect in the carbonate chain growth process occurred in the binary catalyst system, whereas an intramolecularly cooperative process took place in the system of bifunctional catalyst 2. The energies of activation determined for cyclic propylene carbonate and copolymer formation in the binary 1/nBu4NX catalyst system are 50.1 and 33.8 kJ/mol, respectively, compared to the corresponding values in the bifunctional catalyst 2 of 77.0 and 29.5 kJ/mol. The big difference in the energies of activation for cyclic carbonate versus copolymer formation accounts for the excellent selectivity for copolymer formation in the bifunctional catalyst systems even at elevated temperatures. In the coupling system of CO2 and cyclohexene oxide, the energy of activation for copolymer formation is 47.9 kJ/mol for the binary 1/nBu4NX catalyst system, higher than 31.7 kJ/mol determined in the bifunctional catalyst 2. In comparison with binary catalyst system, the lower activation energy for copolymer formation observed in the system regarding bifunctional catalyst answers

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dx.doi.org/10.1021/ma302580s | Macromolecules 2013, 46, 1343−1349