ISSN 0023-1584, Kinetics and Catalysis, 2017, Vol. 58, No. 4, pp. 397–401. © Pleiades Publishing, Ltd., 2017. Original Russian Text © O.M. Chukanova, G.P. Belov, 2017, published in Kinetika i Kataliz, 2017, Vol. 58, No. 4, pp. 415–419.
Reaction between Carbon Dioxide and Propylene Oxide Catalyzed by Cobalt and Chromium Porphyrin Complexes: The Effect of Reaction Conditions on the Reaction Rate O. M. Chukanova* and G. P. Belov Institute of Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka, Moscow oblast, 142432 Russia *e-mail:
[email protected] Received December 21, 2016
Abstract⎯The effect of the conditions (time, temperature, pressure, and cocatalyst/catalyst ratio) on the rate and selectivity of the reaction between СО2 and propylene oxide catalyzed by TPPCrCl and TPPCoCl (TPP is 5,10,15,20-tetraphenylporphyrin) has been studied. The time variation of the reaction rate has been analyzed by measuring the СО2 uptake during the reaction. The observed dependences of the reaction rate on the temperature and cocatalyst/catalyst ratio are similar for TPPCrCl and TPPCoCl. In the presence of TPPCoCl, the reaction yields a mixture of poly(propylene carbonate) and a cyclic carbonate, while when TPPCrCl is used, only the cyclic carbonate is synthesized. Keywords: carbon dioxide, catalysis, cobalt and chromium complexes, epoxides DOI: 10.1134/S0023158417040048
INTRODUCTION Carbon dioxide is a cheap, renewable carbon source; because of this fact, reactions involving this compound are attracting great attention from researches all over the world. Since the СО2 molecule is quite stable, a catalyst is required to involve it into a reaction. The vigorous growth of interest in reactions of СО2 with epoxides in the past decade is associated with development of active catalytic systems for these processes [1, 2]. Catalysts based on Сo or Cr complexes with the salen (N2O2) type ligands are most thoroughly studied and described in the literature. The reaction of СО2 with epoxides in the presence of porphyrin (N4) complexes can proceed under mild conditions at low temperatures [3]. For example, the reaction between СО2 and propylene oxide (PO) was carried out at 25°С using the TPPCoCl catalyst (TPP = 5,10,15,20-tetraphenylporphyrin) and dimethylaminopyridine (DMAP) cocatalyst [4, 5]. The pressure was 1.5 MPa [4] or 0.7 MPa [5]. Despite the small difference between the reaction conditions, the copolymer poly(propylene carbonate) (PPC) was the main product in the former case, while cyclic propylene carbonate (PC) was the dominant product in the latter case. In addition, different results were obtained for the same catalytic system in a high-temperature region: it was reported [6] that the reaction between СО2 and PO yielded a mixture of products at 80°С and a pressure of 5 MPa, while only propylene carbonate was obtained at 120°С and a pressure of 1.7 MPa [7]. When a PPNCl salt
(bis(triphenylphosphine)iminium chloride) is used as the cocatalyst at 25°С and a pressure of 2–3 MPa, poly(propylene carbonate) is the main product [8, 9]. Therefore, it is quite difficult to understand the regularities of the reaction from these sources because reported selectivity and reaction rate data differ for similar reaction conditions. Only a few works have been devoted to the reaction of СО2 with epoxides over Cr porphyrin complexes. The synthesis of the cyclic carbonate via the reaction between СО2 and propylene oxide in the presence of the TPPCrCl/DMAP catalytic system (80°С, 5 MPa) is described in Ref. [10]. The aim of this work is to compare the characteristic features of the reactions of СО2 with propylene oxide over the Co and Cr porphyrin complexes. To compare the catalytic behaviors of the different systems, reaction conditions were varied, and the reaction rate and product composition were investigated. Earlier [11], we studied the characteristic features of the catalytic behavior of the (salen)CoCl and TPPCoCl complexes under the same conditions at a pressure of 0.6 MPa. The data on the rate of the reaction between СО2 and PO were obtained by measuring the СО2 uptake during the reaction. Based on the rate values at various temperatures, we calculated the effective activation energy of copolymerization in the presence of (salen)CoCl. It was shown that, when TPPCoCl is used under the same conditions, the reaction yields a mixture of products. Here, we report
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0.1-L reactor in a СО2 atmosphere. The pressure in the reactor was increased by introducing СО2, and the reactor was connected to a thermostat heated to the preset temperature. Constant pressure was maintained during the reaction. The reaction kinetics was studied as CO2 uptake in a measuring calibrated vessel. The reaction was stopped by cooling the reactor to room temperature and depressurizing it. A small portion of the solution was taken to record 1Н NMR spectra. 1 H NMR (CDCl3, 500 MHz), ppm: poly(propylene carbonate) (PPC) 1.33, 1.35 (d, 3H, CH3); 4.11–4.30 (m, 2H, CH2); 5.01 (m, 1H, CH); propylene carbonate (PC) 1.49, 1.51 (d, 3Н, СН3); 4.02, 4.03, 4.05 (dd, 1Н, СН2); 4.55, 4.56, 4.58 (dd, 1Н, СН2); 4.86 (m, 1Н, CH). To analyze the selectivity of the reaction using the spectra, the parameter f equal to the ratio of the number of moles of propylene oxide in PPC to the total amount of propylene oxide in the two products (PC and PPC) was calculated.
data on the effect of the reaction conditions on the reaction rate and selectivity in the presence of the Co and Cr porphyrin complexes. EXPERIMENTAL Chemicals and Methods of Investigation The Co(III) and Cr(III) complexes were prepared by oxidizing TPPMt(II) (Aldrich) according to procedures described in Refs. [9] and [12], respectively. The cocatalyst—bis(triphenylphosphine)iminium chloride (PPNCl), 97%—was received from Strem Chemicals. rac-Propylene oxide (Sigma-Aldrich) was dried via distillation from CaH2. CO2 (99.8%) was received from Linde Gas. 1 H NMR spectra were recorded on an AVANCE III spectrometer (Bruker, Germany); the samples were dissolved in CDCl3. Attenuated total reflectance (ATR) IR spectra were recorded on an ALPHA spectrometer (Bruker Optics, Germany). ATR-IR spectra, cm–1: TPPCoCl 704 s, 755 s, 794 s, 836 m, 1005 s, 1074 s, 1179 m, 1352 s, 1441 m, 1490 m, 1599 m, 3026 w, 3055 w; TPPCrCl 705 s, 760 s, 808 s, 841 m, 981 m, 1001 m, 1233 m, 1440 m, 1488 s, 2751 w, 2880 w, 3029 w.
f = nPOPPC/nPO(PPC + PC). PPC was precipitated by МеОН (30 mL) with 5 mL of 5% HCl in МеОН. Next, the polymer product was dried in a vacuum at 120–140°С for 6 h and weighed.
Reaction between СО2 and Propylene Oxide The reaction procedure is similar to the earlier described procedure [11]. The complex (0.025– 0.035 mmol, 1 eq.) was dissolved in rac-PO (3.5– 4.3 mL, 3500–4000 eq.), PPNCl was then added in an argon atmosphere, and the mixture was stirred for 15 min. The solution was transferred into a predried
RESULTS AND DISCUSSION As was noted above, the interaction of СО2 with propylene oxide can yield two reaction products, namely, poly(propylene carbonate) and the cyclic carbonate.
Cl N N Mt N N
O O
O
PPC n
O CO2 +
+ [Ph3P=N=PPh3]+Cl−
O O
The reaction kinetics was studied by measuring the СО2 uptake during the reaction. The reaction rate was measured at PO conversions of <20%, so the change in the concentration did not affect the kinetic data. In the presence of either TPPCoCl or TPPCrCl, the reaction proceeded at a constant rate for a long time. By way of example, Fig. 1 presents the time dependences of the СО2 uptake in the presence of TPPCrCl/PPNCl.
O
PC
The reaction rate in the steady-state stage of the kinetic curve was calculated in terms of TOF (h–1) as −1 mol CO2 mol Mt h–1. Below, we present data on the effect of the conditions of the reaction between СО2 and PO on its rate for the two catalysts—TPPCoCl and TPPCrCl—in the presence of the PPNCl cocatalyst. Figure 2 shows how the reaction rate changes with the temperature.
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mol CO 2 / mol Cr 2 150 1 100
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0
40
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60 Time, min
Fig. 1. Time dependence of the СО2 uptake. The pressure is (1) 1.4 and (2) 1.6 MPa; [PPNCl]/[TPPCrCl] = 1; Т = 70°С.
The Co porphyrin complex demonstrates the maximum activity at a low temperature of 35°С (Fig. 2а), while in the case of the Cr complex, the highest reaction rate is observed at 70°С (Fig. 2b). In both cases, the temperature dependence has an extremum. Currently, it is rather difficult to explain why the reaction rate decreases with an increasing temperature. It was reported [9] that the PO conversion decreases above 40°С in the presence of the TPPCoCl/PPNCl catalytic system at a pressure of 3 MPa in 18 h. The authors assumed that the active Co(III) complexes transform into inactive Co(II) complexes as the temperature increases. At the same time, porphyrin complexes are known to be active at 80–120°С when DMAP is used as the cocatalyst [6, 7, 10]. The fact that the PPC yield decreases as the temperature grows from 25 to 60°С is reported for the same catalytic system in Ref. [8]; here,
the authors do not discuss possible causes of this effect. It can only be noted that investigation of the stability of the porphyrin complexes in reactions conducted at high temperatures may contribute to the understanding of the observed regularities. Conducting the reaction at elevated temperatures in the presence of the Cr complex requires use of higher pressures because of the low boiling point of PO. Figure 3 shows how the reaction rate changes with pressure for the Co- and Cr-containing catalytic systems. In case TPPCoCl is used, the reaction rate changes insignificantly when the pressure is varied within the 0.4–1.0 MPa range (Fig. 3а). With TPPCrCl as the catalyst, the process rate depends on pressure (Fig. 3b). A slight change in the pressure from 1.4 to 1.6 MPa leads to an almost 1.5-fold growth in the rate, while when the pressure is increased above 1.7 MPa, the rate decreases. These data show how different the behaviors of such similar catalytic systems can be. The change in the PPC yield (25°С, 18 h, TPPCoCl/PPNCl catalyst) caused by an increase in pressure from 1 to 5 MPa was described in Ref. [9]. The dependence shows an extremum. The authors explained the decrease in the PPC yield at a pressure of >3 MPa by the increase in the liquid phase volume due to the condensation of СО2 at high pressures. However, similar results were also obtained in the present work at lower pressures. Possibly, the coordination of СО2 to the active site prevents the coordination of epoxide, but additional studies are required in order to confirm this assumption. It is known that the reaction rate and selectivity can be substantially affected by the cocatalyst/catalyst ratio [3]. The change in the process rate in the case of variation of this ratio is presented in Fig. 4. As can be seen from the data presented in Fig. 4, similar dependences of the rate on the cocatalyst/catalyst ratio are observed for the two systems. In both cases, the maximum reaction rate is observed at a
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Fig. 2. Temperature dependence of the reaction rate for the (a) TPPCoCl (Р = 0.6 MPa) and (b) TPPCrCl (Р = 1.6 MPa) catalysts. [PPNCl]/[TPPMtCl] = 1. KINETICS AND CATALYSIS
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Fig. 3. Pressure dependence of the reaction rate for the (a) TPPCoCl (Т = 35°С) and (b) TPPCrCl (Т = 70°С) catalysts. [PPNCl]/[TPPMtCl] = 1.
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1.0 1.5 2.0 [PPNCl]/[TPPCoCl]
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Fig. 4. Reaction rate as a functoion of the [PPNCl]/[TPPMtCl] ratio for the (a) TPPCoCl (Т = 35°С, Р = 0.6 MPa) and (b) TPPCrCl (Т = 70°С, Р = 1.6 MPa) catalysts.
cocatalyst/catalyst ratio of 1; however, the product compositions are different. When the reaction is conducted in the presence of TPPCrCl, only the cyclic f 0.8
0.6
0.4 5
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15 PO conversion, %
Fig. 5. Poly(propylene carbonate) selectivity of the reaction as a function of propylene oxide conversion for the TPPCoCl catalyst.
carbonate forms (the propylene oxide conversion does not exceed 20%). The product composition in the TPPCoCl-catalyzed reaction changes with PO conversion. Figure 5 shows the dependence of the selectivity of the process on the conversion. At low PO conversions, the poly(propylene carbonate) selectivity of the reaction is 0.5, and the fraction of the polymer product increases with an increasing conversion. The change in the PO conversion in poly(propylene carbonate) formation (25°С, 2 MPa, 5 h, TPPCoCl/PPNCl catalyst) at various cocatalyst concentrations is described in Ref. [8]. The PPC yield decreases with an increasing cocatalyst/catalyst ratio. The authors do not explain the observed regularities. The effect of the cocatalyst on the formation of the copolymer of СО2 with PO was studied in detail for Co salen complexes [13]. For the salen systems, an increase in the PPNCl cocatalyst concentration leads to a growth of the copolymerization rate, which, according to the authors, is caused by the stabilization of active sites. However, for porphyrin complexes, the dependences of the process rate on the cocatalyst conKINETICS AND CATALYSIS
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centration are of a different type. To explain the observed differences between the behaviors of the salen and porphyrin complexes, additional studies are required. Possibly, a strong effect on the reaction rate is exerted by the competition between the two comonomers for coordination to active sites. CONCLUSIONS New data on the effect of the conditions of the reaction between СО2 and propylene oxide on its rate and selectivity in the presence of Сo and Сr porphyrin complexes have been obtained. In both cases, the reaction proceeds at a constant rate until a PO conversion of 20% is reached. The maximum TOF values for TPPCoCl and TPPCrCl are similar and are ~130 h–1. In both cases, the temperature dependence of the rate shows an extremum, although the reaction proceeds at higher temperatures and pressures in the presence of TPPCrCl. Interestingly, varying the pressure within the 0.4–1.0 MPa range has no significant effect on the rate of the TPPCoCl-catalyzed process. When TPPCrCl is used, the reaction rate is pressure-dependent. As the cocatalyst/catalyst ratio is varied, the maximum reaction rate is observed at equimolar amounts of the components for both catalysts; however, the selectivities of the reaction are different. The TPPCrCl-catalyzed reaction yields only the cyclic carbonate (over 99%), while a mixture of products forms in the presence of TPPCoCl. The poly(propylene carbonate) content of the products of the TPPCoCl-catalyzed reaction increases with an increasing PO conversion. When the porphyrin complexes are used in the reaction of СО2 with PO, the dependences of the reaction rate on the temperature, pressure, and cocatalyst/catalyst ratio show an extremum, which distinguishes these complexes from the better explored salen complexes. To explain the differences between the behaviors of these two systems correctly, more detailed studies are required, e.g., a study of the stability of the porphyrin catalysts during the reaction.
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ACKNOWLEDGMENTS The work was carried out in the framework of National Science Foundation program no. 01201055317 and was supported by the Presidium of the Russian Academy of Sciences, basic research program no. 25: “Fundamental Aspects of Carbon Energetics Chemistry” (state registration number 0089-2015-0271). REFERENCES 1. Kielland, N., Whiteoak, C.J., and Kleij, A.W., Adv. Synth. Catal., 2013, vol. 355, p. 2115. 2. Childers, M.I., Longo, J.M., Van Zee, N.J., LaPointe, A.M., and Coats, G., Chem. Rev., 2014, vol. 114, no. 16, p. 8129. 3. Klaus, S., Lehenmeier, M.W., Anderson, C.E., and Rieger, B., Coord. Chem. Rev., 2011, vol. 255, p. 1460. 4. Jiang, X., Gou, F., and Jing, H., J. Catal., 2014, vol. 313, p. 159. 5. Jin, L., Jing, H., Chang, T., Bu, X., Wang, L., and Liu, Z., J. Mol. Catal. A: Chem., 2007, vol. 261, p. 262. 6. Sugimoto, H. and Kuroda, K., Macromolecules, 2008, vol. 41, p. 312. 7. Paddock, R.L., Hiyama, Y., McKay, J.M., and Nguyen, S.T., Tetrahedron Lett., 2004, vol. 45, p. 2023. 8. Qin, Y., Wang, X., Zhang, S., Zhao, X., and Wang, F., J. Polym. Sci., Part A: Polym. Chem., 2008, vol. 46, p. 5959. 9. Anderson, C.E., Vagin, S.I., Xia, W., Jin, H., and Rieger, B., Macromolecules, 2012, vol. 45, no. 17, p. 6840. 10. Kruper, W. and Dellar, D.V., J. Org. Chem., 1995, vol. 60, p. 725. 11. Chukanova, O.M. and Belov, G.P., Kinet. Catal., 2016, vol. 57, no. 6, p. 821. 12. Summerville, D.A., Jones, R.D., Hoffman, B.M., and Basolo, F., J. Am. Chem. Soc., 1977, vol. 99, p. 8195. 13. Ren, W.-M., Liu, Z.-W., Wen, Y.-Q., Zhang, R., and Lu, X.-B., J. Am. Chem. Soc., 2009, vol. 131, p. 11509.
Translated by E. Boltukhina
