The Kinetics of the Reaction of Carbon Dioxide with Propylene Oxide Catalyzed by a Chromium–Salen Complex O. M. Chukanovaa, * and G. P. Belova aInstitute
of Problems of Chemical Physics, Russian Academy of Science, Chernogolovka, Moscow oblast, 142432 Russia *e-mail: [email protected] Received August 10, 2017
Abstract⎯The kinetics of the reaction of CO2 with propylene oxide utilizing a salenCrCl/PPNCl active catalytic system is studied with varying reaction conditions (temperature, pressure, and cocatalyst/catalyst ratio). The reaction proceeds selectively to form cyclic propylene carbonate (PC) at [PPNCl]/[salenCrCl] ratios above two. The value of the effective activation energy of PC formation is found. Keywords: carbon dioxide, catalysis, chromium complex, propylene oxide DOI: 10.1134/S0023158418030059
INTRODUCTION The reaction of CO2 with epoxides can proceed in the presence of catalysts to form aliphatic polycarbonates or cyclic carbonates [1, 2]. Aliphatic polycarbonates are referred to as modern biocompatible polymeric materials , while cyclic carbonates are referred to as green solvents which are characterized by high boiling points and low vapor pressure . In recent years, the efforts of researchers have been mainly aimed at finding active and selective catalytic systems for the synthesis of copolymers or cyclic carbonates with the involvement of various epoxides, among which propylene oxide (PO) or cyclohexene oxide (CHO) are used most often. Despite the growing number of publications in this field, there is little information about the kinetics of the reaction of CO2 with epoxides in the literature. The review  presents the results of the investigation of the kinetics of the reaction of CO2 with CHO obtained using IR spectroscopy based on the analysis of the spectra of the reaction mixture in the presence of a salenCrCl catalyst (salen is (1R,2R)-N,N'bis-(3,5-di-tert-butyl-salicylidene)-1,2-cyclohexanediamine). The effective activation energy of the process was estimated by the change in the accumulation rate of the copolymer or cyclic product with temperature. The information about the kinetics of the reaction of CO2 with PO in the presence of a salenCrCl complex was obtained using the same method . This work lacks data on the product yields; only the values of the formation rate of poly(propylene carbonate) (PPC) and propylene carbonate (PC) are presented in the units of the change in the absorption intensity of the corresponding bands with time. In works [7, 8], based
on the data on the product yields, the PC formation rate in the presence of a salenCrCl/dimethylaminopyridine (DMAP) two-component catalytic system was −1 estimated in TOF units (molPO molCr h–1); thus, at –1 for works  and , 75°C, it was 162 and 279 h respectively. The pressure was 0.7 and 1.3 MPa in works  and , respectively. In the first case, the exclusive formation of a cyclic carbonate, PC, was observed at an equimolar ratio of the cocatalyst and catalyst. It was shown in Ref.  that decreasing the cocatalyst/catalyst ratio to 0.5 is accompanied by a change in the selectivity of the process; thus, PPC appears in significant quantities in addition to PC. The selectivity and rate of the reaction of CO2 with epoxides is affected by the composition of the catalytic system (the nature of the catalyst and cocatalyst and their molar ratio) and the reaction conditions (time, temperature, and pressure). In works [9, 10], we studied the effect of the nature of the metal and ligand on the kinetics of the reaction of CO2 with propylene oxide by the example of Co and Cr complexes with known lagans such as salen and TPP (TPP is 5,10,15,20-tetraphenylporphyrin), namely, salenCoCl, TPPCoCl, and TPPCrCl. In the case of the selective copolymer formation in the presence of salenCoCl, the effective activation energy of copolymerization was found to be 42 kJ/mol . As opposed to the published data, we studied the kinetics by measuring the CO2 absorption during the reaction. This work is a continuation of the investigation of the effect of the nature of the metal and ligand on the kinetics of the reaction of CO2 with PO. In the literature, there are no publications devoted to the investigation of the kinetics of the reaction of CO2 with epoxides with the
THE KINETICS OF THE REACTION
use of an active salenCrCl/PPNCl two-component catalytic system; hence, this catalytic system is used in this work in order to compare the obtained data on the reaction rate and selectivity in a series of isomorphic complexes with different ligands and metals. EXPERIMENTAL Chemicals and Study Methods (1R,2R)-N,N'-bis(3,5-di-tert-butyl-salicylidene)1,2-diaminocyclohexane chromium chloride (salenCrCl) was prepared by oxidizing salenCr(II) (Aldrich) according to the procedure . The cocatalyst, bis(triphenylphosphine)iminium chloride (PPNCl) (97%) was provided by Strem Chemicals. Rac-propylene oxide (Sigma-Aldrich) was dried via distillation over CaH2. CO2 (99.8%) was obtained from Linde Gaz. 1H
NMR spectra were registered on an AVANCE III instrument (Bruker, Germany); the samples were dissolved in CDCl3. The attenuated total reflection (ATR) infrared (IR) spectra were registered on an ALPHA instrument (Bruker Optics, Germany). The ATR IR spectra of salenCrCl, cm–1: 748 m, 785 w, 836 m, 1008 m, 1171 m, 1201 w, 1255 m, 1318 m, 1360 m, 1409 w, 1435 m, 1461 w, 1535 m, 1622 s, 2867 w, and 2951 m. The Reaction of CO2 with Propylene Oxide
oxide in two products (PC and PPC) was calculated by the spectra: f = nPOPPC/nPO(PPC + PC). The CO2 consumption in the reaction was compared to the product amount determined by weighing after the removal of PO residues in vacuum and by the 1 H NMR data with respect to the ratio of the signals of the products and PPNCl (ppm: 7.41–7.46 (m, 24H), 7.62–7.66 (m, 6H) ). RESULTS AND DISCUSSION The reaction of CO2 with PO was carried out in the presence of a salenCrCl catalyst and a PPNCl cocatalyst. To study the effect of the reaction conditions on the process rate and selectivity, the kinetics were analyzed by varying the temperature, pressure, and cocatalyst/catalyst ratio. The data on the kinetic process were obtained by measuring the CO2 absorption with time. The composition of the products was studied by analyzing the 1H NMR spectra of the reaction mixture. The reaction can proceed to form PPC or cyclic PC (Scheme 1). O O CO2 +
The reaction procedure is similar to the procedure described earlier in Refs. [9, 10]. SalenCrCl and PPNCl were dissolved in rac-PO in an argon atmosphere and stirred for 15 min. The [salenCrCl] concentration was varied in the range of 7 to 8.5 × 10–3 mol/L. The solution was poured over a preliminarily evacuated 0.1-L metallic reactor equipped with a magnetic stirrer in a CO2 atmosphere. The pressure in the reactor was elevated by feeding CO2, and the reactor was connected to a thermostat heated to the required temperature. Constant pressure was maintained during the reaction. The reaction’s kinetics were studied by measuring the change in the CO2 pressure in a calibrated measuring tank connected to the reactor. The amount of CO2 was calculated by a calibrating curve expressing the dependence of the CO2 weight on the pressure in the measuring tank. The reaction was stopped by cooling the reactor to room temperature and depressurizing. A small part of the solution was taken for the registration of 1H NMR. 1H NMR (CDCl3, 500 MHz) ppm: PPC 1.33, 1.35 (d, 3H, CH3), 4.11–4.30 (m, 2H, CH2), 5.01 (m, 1H, CH); PC 1.49, 1.51 (d, 3H, CH3) 4.02, 4.03, 4.05 (dd, 1H, CH2), 4.55, 4.56, 4.58 (dd, 1H, CH2), 4.86 (m, 1H, CH). To analyze the reaction’s selectivity, an f parameter equal to the ratio of the number of moles of propylene oxide in PPC to the total amount of propylene KINETICS AND CATALYSIS
Scheme 1. General scheme of reaction of CO2 with propylene oxide.
The observed shape of the kinetic curve changes with the process temperature. At temperatures of 50°C and below, two segments with a constant rate can be distinguished in the curve of the dependence of the CO2 absorption on time (Fig. 1, curve 1). At higher temperatures, the kinetic curve is S-shaped (Fig. 1, curves 2 and 3). A segment, where the rate remains constant for 30 and more minutes depending on the process conditions, can be distinguished in the curve. A similar character of the kinetics of accumulation of a copolymer of CO2 with CHO or PO is noted in Ref.  in the case of the study of the reaction via IR spectroscopy. The low reaction rate at the initial section is indicative of the presence of the initiation stage of the process. The duration of the initial period grows with the decrease in the catalyst concentration. It is assumed in Ref.  that two chromium complex molecules participate in the activation of the reaction of CO2 with epoxides. According to Refs. [5, 6], the first order with respect to the catalyst concentration was observed during the next step proceeding at a constant rate. Hereinafter, the data on the reaction rate at the stationary segment of the kinetic curve are presented in TOF units (h–1), which was calculated as the number of moles of CO2 (equal to the
in the case of the use of PPNCl as the cocatalyst. The change in the reaction rate in TOF units depending on the cocatalyst/catalyst ratio is presented in Fig. 2a.
MolCO2 /molCr 3 800
With an increase in the cocatalyst concentration, the reaction rate at the stationary section increases, and the selectivity of the process changes. The relative fraction of propylene oxide consumed for the PPC formation (the f parameter) decreases from 0.62 to 0.03 with the growth in [PPNCl]/[salenCrCl] from 0.5 to 3.3 (Fig. 2b). At a cocatalyst/catalyst ratio of >2, a cyclic carbonate, PC, is mainly formed in the reaction.
40 60 Time, min
Fig. 1. Dependence of CO2 absorption on time at (1) 50, (2) 60, and (3) 70°C. P = 0.6 MPa and [PPNCl]/[salenCrCl] = 2.
number of moles of PO) consumed in the reaction per mole of chromium over one hour. The cocatalyst/catalyst ratio has a considerable effect on the rate and selectivity of the process. According to the data of Ref. , when the [DMAP]/[salenCrCl] ratio was changed from 0.5 to 2 (75°C, 1.3 MPa), an increase in the PC yield was observed and the TOF values changed from 199 to 308 h–1, while at a ratio of 0.5, a copolymer, PPC, was formed in addition to PC. In this work, higher TOF values were obtained under similar reaction conditions O P O
O M Nu
It is known that the selectivity of the process is affected by many factors such as the catalytic system composition, types of the catalyst and cocatalyst being used, their ratio, and the reaction conditions (time, temperature, pressure, medium, etc.) [1, 2, 5]. Generally, the fraction of cyclic carbonate (CC) in the reaction products increases with the growth in the cocatalyst concentration and temperature, although the character of the dependences of the process selectivity on its conditions can change in the case of the use of different catalytic systems. There are different points of view concerning the mechanism of the CC formation in the literature. One of the most common points of view is the CC formation according to the so-called backbiting mechanism  (Scheme 2). According to this scheme, CC is released in the case of the dissociation of the end of the polymeric chain from the catalytic center. Such a process is accelerated in the presence of excess cocatalyst (Nu) ions, which, according to the authors of Ref. , decreases the energy barrier during the CC formation. O P
P O M + Nu
Scheme 2. Formation of cyclic carbonate according to backbiting reaction.
Another point of view is based on the assumption that the CC and PC formation reactions proceed in parallel ; here, the presence of free Nu in the system stimulates the replacement of the carbonate coordinated to the catalyst and cyclic carbonate formation. To prove the validity of any of these points of view, we need to perform additional studies. The character of the change in the reaction rate depending on temperature was studied under the conditions of the selective course of the reaction of CO2 with propylene oxide to form PC. The reaction rate
was calculated in the stationary segment of the kinetic curve by determining the CO2 absorption with time. The kinetic equation for the reaction rate can be presented in the general form w = k[Cat]x[PO]y[CO2]z. To calculate the rate constant, it is necessary to take into account the order of the reaction with respect to all the reagents. In all the cases, the reaction was carried out in the medium of propylene oxide, because of which corrections taking into account only the change in the catalyst and CO2 concentrations were KINETICS AND CATALYSIS
THE KINETICS OF THE REACTION
TOF, h–1 3000
(b) f 0.6
0.2 0 0
1 2 3 [PPNCl]/[salenCrCl]
1 2 3 [PPNCl]/[salenCrCl]
Fig. 2. Effect of cocatalyst/catalyst ratio on reaction (a) rate and (b) selectivity. T = 75°C and P = 0.8 MPa.
introduced. The catalyst content in different experiments was varied in a range of 7 to 8.5 × 10–3 mol/L. It was noted in works [5, 6] that the first order is present with respect to the salenCrCl concentration during the copolymerization of CO2 with CHO. The first order with respect to the catalyst concentration was also observed by us in the copolymerization process of CO2 with PO in the presence of salenCoCl/PPNCl . The change in the reaction rate depending on the CO2 pressure is described below. It is shown that, with an increase in pressure to 0.65 MPa, the reaction rate grows. Similar data were also obtained in the case of the use of a salenCoCl/PPNCl catalytic system . Rate constants at different temperatures were determined at 0.6 MPa. Under these conditions, there is a first order with respect to the CO2 concentration. Taking into account the first order with respect to the salenCrCl and CO2 concentrations, a corrected reaction rate
] ([CO ][salenCrCl]) (d[CO dt ) 2
composition of the catalytic systems and effect of the cocatalyst on the reaction energy barrier. Earlier, during the investigation of the reaction of CO2 with PO using Co–porphyrin and Cr–porphyrin complexes, as well as a Co–salen complex, we observed the extreme character of the dependence of the reaction rate on the CO2 pressure [10, 12]. It was also shown in the work  that the maximum yield of cyclic PC is reached in the case of the CO2 pressure of 0.4 MPa and use of a 1,2-diphenyl-ethylenediaminebis(3,5-di-tert-butyl-salicylidene) chromium(III) chloride at 75°C in the presence of DMAP as the cocatalyst. With the increase in the pressure to 1.4 MPa, the product yield was halved. Therefore, it can be noted that the extreme character of the dependence of the rate on the CO2 pressure is quite characteristic for the processes of copolymerization of CO2 with epoxides in the case of the use of catalysts based ln w1
was then calculated to determine the effective activation energy (Ea) of the process.
Because the CO2 concentration changes with temperature, it was taken into account in the calculations of the activation energy. The data on the CO2’s solubility in propylene oxide at different temperatures were obtained by us earlier . The change in w1 depending on temperature in the Arrhenius equation coordinates is presented in Fig. 3.
The Ea value calculated by these data is 87.4 ± 1.5 kJ/mol. These results can be compared to the activation energy value of 100.5 kJ/mol, which was determined using IR spectroscopy data in works [5, 6] for the synthesis of PC in the presence of salenCrCl without a cocatalyst. Such a difference in the activation energy values can be associated with a difference in the KINETICS AND CATALYSIS
T–1 × 103, K–1 Fig. 3. Dependence of reaction rate on temperature in Arrhenius equation coordinates. P = 0.6 MPa and [PPNCl]/[salenCrCl] = 2.
300 2 200 1 100 0.6
0.7 0.8 P, MPa
the reaction rate is slow, which can be associated with the bimolecular reaction activation process. At a cocatalyst/catalyst ratio of >2, the reaction proceeds selectively to form cyclic PC, while at a ratio of <1, a mixture of products, PPC and PC, is obtained. The reaction rate in the stationary segment significantly changes with pressure, reaching the maximum value at 0.65–0.7 MPa. The value of the effective activation energy of PC formation was found to be 87.4 kJ/mol. The activation energy of the PC formation process exceeds the activation energy value of 42 kJ/mol in the case of selective PPC formation (the catalyst is salenCoCl/PPNCl ).
Fig. 4. Change in reaction rate in (1) initial and (2) second segment of kinetics with pressure. T = 50°C and [PPNCl]/[salenCrCl] = 2.
on Co or Cr complexes. The significant decrease in the rate at high pressures is difficult to explain. Possibly, the CO2 coordination to the active center prevents the coordination of the epoxide. To determine the reasons for the phenomenon being observed, additional research is needed. Both the nature of the metal and the ligand composition can have a substantial influence on the dependence of the reaction rate on the CO2 pressure. Figure 4 shows the character of the change in the reaction rate with pressure in the presence of salenCrCl/PPNCl. As was noted above, two segments with a constant rate can be distinguished in the kinetic curve at the reaction temperature of 50°C. Figure 4 specifies the rate values in both sections. There is quite a significant scattering of the experimental data for the dependence of the rate on temperature in the second segment of the curve. Nevertheless, it can be concluded that the maximum rate is observed at a CO2 pressure of 0.65–0.7 MPa. In the case of a cobalt–salen complex, the maximum yield of the products was also obtained at this pressure , although, at higher pressure values, the reaction rate changes to a lesser extent than in the case of the use of a chromium complex. CONCLUSIONS The new data on the kinetics of the reaction of CO2 with propylene oxide using a salenCrCl/PPNCl active catalytic system were obtained. In the initial section,
ACKNOWLEDGMENTS This work was performed according to a state task, state registration no. 01201055317, and was financially supported by Program no. 25 for Basic Scientific Research of the Presidium of the Russian Academy of Sciences “Fundamental Aspects of Carbon Energy Chemistry,” state task 0089-2015-0271. REFERENCES 1. Taherimehr, M. and Pescarmona, P.P., J. Appl. Polym. Sci., 2014, vol. 131, no. 21. 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. Luinstra, G.A., Polym. Rev., 2008, vol. 48, p. 192. 4. Schaffner, B., Schaffner, F., Verevkin, S.P., and Borner, A., Chem. Rev., 2010, vol. 110, p. 4554. 5. Darensbourg, D.J., Chem. Rev., 2007, vol. 107, p. 2388. 6. Darensbourg, D.C., Yarbrough, J.C., Ortiz, C., and Fang, C.C., J. Am. Chem. Soc., 2003, vol. 125, p. 7586. 7. Paddock, R.L. and Nguyen, S.T., J. Am. Chem. Soc., 2001, vol. 123, p. 11498. 8. Eberhardt, R., Allmendinger, M., and Rieger, B., Macromol. Rapid Commun., 2003, vol. 24, p. 194. 9. Chukanova, O.M. and Belov, G.P., Kinet. Catal., 2016, vol. 57, no. 6, p. 827. 10. Chukanova, O.M. and Belov, G.P., Kinet. Catal., 2017, vol. 58, no. 4, p. 415. 11. Wu, G.P., Wei, S.-H., Lu, X.-B., Ren, W.-M., and Darensbourg, D.J., Macromol., 2010, vol. 43, p. 9202. 12. Chukanova, O.M., Bukhovets, E.V., Perepelitsina, E.O., and Belov, G.P., Polym. Sci. B, 2015, vol. 57, no. 3, p. 224.