Kinetics and Mechanistic Investigation of Epoxide/CO2 Cycloaddition by a Synergistic Catalytic Effect of Pyrrolidinopyridinium Iodide and Zinc Halides

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Kinetics and mechanistic investigation of epoxide/CO2 cycloaddition by a synergistic catalytic effect of pyrrolidinopyridinium iodide and zinc halides

Abdul Rehman a, b,*, Valentine C. Eze a, M.F.M. Gunam Resula, Adam Harveya

a School of Engineering, Merz Court, Newcastle University, Newcastle Upon Tyne,

NE1 7RU, UK

b Department of Chemical and Polymer Engineering, University of Engineering and Technology Lahore, Faisalabad Campus, Pakistan

*[email protected] Abstract

Formation of styrene carbonate (SC) by the cycloaddition of CO2 to styrene oxide (SO) catalysed by pyrrolidinopyridinium iodide (PPI) in combination with zinc halides (ZnCl2,

ZnBr2 and ZnI2) was investigated. Complete conversion of the SO to SC was achieved in 3 h with 100% selectivity using 1:0.5 molar (PPI/ZnI2) catalyst ratio under mild reaction conditions i.e. 100 °C and 10 bar CO2 pressure. The synergistic effect of ZnI2 and PPI resulted in more than 7-fold increase in reaction rate than using PPI alone. The cycloaddition reaction demonstrated the first-order dependence with respect to the epoxide, CO2 and catalyst concentrations. Moreover, the kinetic and thermodynamic activation parameters of SC formation were determined using the Arrhenius and Eyring equations. The positive values of ΔH‡ (42.8 kJ mol–1) and ΔG‡ (102.3 kJ mol–1) revealed endergonic and chemically controlled nature of the reaction, whereas the large negative values of ΔS‡ (–159.4 J mol–1 K–1) indicate a highly ordered activated complex at the transition state. The activation energy for SC formation catalyzed by PPI alone was found to be 73.2 kJ mol–1 over a temperature range of 100–140 °C, –1 which was reduced to 46.1 kJ mol when using PPI in combination with ZnI2 as a binary catalyst. Based on the kinetic study, a synergistic acid-based reaction mechanism was proposed.

Keywords: Cycloaddition, CO2 utilization, Cyclic Carbonates, Homogeneous catalysis, Activation energy.

1. Introduction

In recent years, considerable attention has been paid to reducing the emission of greenhouse gases, especially carbon dioxide (CO2). An effective approach of CO2 capture and utilization

(CCU) is the conversion of CO2 into value-added products. As CO2 is abundant, nontoxic and nonflammable, it is considered to be a potential replacement for other toxic and corrosive chemicals such as phosgene, isocyanates or carbon monoxide [1-4]. Therefore, utilization of

CO2 as alternative C1 feedstock for organic synthesis has attracted the attention of the scientific community. However, due to low reactivity of CO2 attributed to its high kinetic and –1 thermodynamic stability (i.e. low standard heat of formation ΔHf = –394 kJ mol [5]), alternative approaches of CO2 utilization may result in the exhaust of more CO2 than consumed by the reaction. Therefore, in order to provide a thermodynamically feasible process, the reaction of CO2 should be with those compounds which have relatively high free energy. Amongst these processes, the formation of five-membered cyclic carbonates by the cycloaddition of CO2 to the epoxides (Scheme 1) is 100% atom-economical and highly desirable reaction which has been already commercialized [6, 7].

Scheme 1: Synthesis of cyclic carbonates from epoxides and CO2

Conventionally, the formation of cyclic carbonates was carried out by the reaction of diols with phosgene [8-10]. However, this reaction was not considered to be eco-friendly due to the toxic and corrosive nature of phosgene. Therefore, various alternative methods for cyclic carbonates synthesis have been proposed using dimethyl carbonates [11, 12], urea [13, 14] or CO [15, 16] as indirect sources of carbon. Although these processes have the advantages of less toxicity than phosgene route, however, these are still not economically feasible due to relatively expensive feedstocks and additional steps involved in downstream processing. Organic cyclic carbonates have a broad range of applications such as polar aprotic green solvents [17-20], monomer for the production of polycarbonates [21] and polyurethanes [22, 23], electrolytes in Li-ion batteries [24, 25] and intermediate for the production of many other valuable chemicals such as glycol, pyrimidines and carbamates [17, 26]. Although cyclic carbonates formation was –1 observed to be highly exothermic (i.e. ΔHr = –140 kJ mol for ethylene carbonate formation [27]), this reaction does not occur spontaneously due to the high activation energy of the 2 uncatalyzed reaction i.e. (209–251 kJ mol-1 subject to the epoxide type) [28, 29]. Therefore, a series of homogeneous and heterogeneous catalysts have been developed to reduce this activation energy barrier. These mainly include alkali metal salts [30-32], organocatalysts such as quaternary ammonium salts [33-35], quaternary phosphonium salts [36-38], ionic liquids [39-43] and transition-metal (salen) complexes in combination with nucleophilic co-catalysts

[44-48]. The general mechanism of CO2 cycloaddition to epoxides in the presence of an acid- base catalyst has already been well established (Scheme 2) [29, 35]. The catalyst should provide a Lewis acidic site to activate the epoxide by interaction with an oxygen atom, and a basic site to ring-open the activated epoxide via nucleophilic attack on a more accessible carbon atom of the epoxide (Step 1). This results in the formation of an alkoxide intermediate and the stability to this ring-opened epoxide is provided by the counter cation (Step 2). The stability of the alkoxide is considered to be a key factor for rapid CO2 insertion at mild reaction conditions.

The alkoxide intermediates further undergo CO2 insertion to form a carbonate intermediate

(Step 3). Although CO2 is a linear apolar molecule with two polar C=O bonds resulting in a partial positive and partial negative charge on carbon and oxygen atoms respectively. Therefore activation of CO2 can be carried out by both nucleophilic and electrophilic attack. The resulting open-chain carbonate undergoes intramolecular displacement of the halide anion by an SN2 mechanism (Step 4). Finally, a five-membered cyclic carbonate is formed catalyst is restored (Step 5). The nucleophile (anion) provided by the catalyst should have good nucleophilicity and leaving group abilities [49-51].

Scheme 2: General reaction mechanism of cyclic carbonate formation in the presence of an acid-base catalyst.

In this study, the formation of SC by CO2 cycloaddition to SO was carried out using PPI in conjunction with ZnI2 as a highly efficient acid-base binary homogeneous catalyst. The activation of the epoxide was carried out by the Lewis acidic site provided by the Zn salt through metal-oxygen coordination. Whereas high nucleophilicity of iodide anion (I‾) opens the ring of activated epoxide, and the stability to the ring-opened epoxide was expected to be provided by the large delocalized pyrrolidinopyridinium cations (PP+). Therefore, this synergistic effect between PPI and ZnI2 would result in a significantly higher reaction rate under mild reaction conditions. Moreover, a detailed study of reaction kinetics was carried out to investigate the synergistic reaction mechanism. The effect of reaction temperature was also studied to determine the kinetics and thermodynamic activation parameters.

2. Experimental 2.1 Reagents

All chemicals were purchased from Sigma Aldrich and used without further purification. These include: styrene oxide (97%), zinc iodide (98%) zinc bromide (99.9%) and zinc chloride (99.9%), 3-iodopropyltrimethoxysilane (95%), 4-pyrrolidinopyridine (98%) and propylene carbonate (99%). The CO2 (99.9% pure) was supplied by BOC, UK.

2.2 Catalyst preparation

The pyrrolidinopyridinium iodide (PPI) catalyst was synthesized according to the reported procedure [52]. In this study, 2.5 mmol of 4-pyrrolidinopyridine and 2.5 mmol of 3- iodopropyltrimethoxysilane were mixed in a round bottom flask, and the mixture was heated at 120 °C for 24 h. The structure of the catalyst was confirmed by a 13C-NMR spectrum of the catalyst sample (Fig. S1).

Scheme 3: Preparation of pyrrolidinopyridinium iodide (PPI) catalyst.

2.3 General procedure for cyclic carbonate synthesis

The formation of SC by the cycloaddition of CO2 to SO was achieved in a 100 ml high-pressure stainless steel Parr reactor (Model 4750) as a semi-batch process. The CO2 gas was continuously charged into the reactor from a gas cylinder. The pressure inside the reactor was measured by a pressure gauge installed on the top of the reactor and heat to the reactor was supplied by an electric band heater. The temperature inside the reactor was controlled by the Elmatic temperature controller, and stirring was provided by a magnetic stirrer (900 rpm). For a typical experiment, 25 ml 3.5 M SO, 76 mM PPI catalyst and 38 mM ZnX2 (X = Cl, Br, I) co-catalyst were added into the reactor and heated up to the required temperature (100–140

°C). The CO2 (2–40 bar) was introduced into the reactor after achieving the temperature set point. The progress of the reaction was also monitored through collections of samples via the sampling unit at regular time intervals and analyzed by FTIR spectroscopy. The conversion of SO to SC was determined by monitoring the intensity of epoxide peak at 876 cm–1 which decreased as a result of cyclic carbonate formation i.e. carbonate peak at 1800 cm–1 (Fig. S2) [53, 54]. The FTIR intensity data points were also validated using GC chromatography. The detailed procedures are given in the supporting information.

3. Results and Discussion

3.1 Effect of zinc halide anions

Fig. 1 shows the relative catalytic activity of PPI with and without Zn halides (i.e. ZnI2, ZnBr2 and ZnCl2). These preliminary results indicate that the synergistic effect between PPI and ZnI2 has resulted in a more than 7-fold increase in reaction rate than using PPI as catalyst alone. This significant enhancement in reaction rate was attributed to epoxide activation by the Zn site (Lewis acid) sites provided by the catalyst. From the results obtained, the order of Zn halides activity in combination with PPI was found to be ZnI2 > ZnBr2 > ZnCl2. A similar order of Zn halide anions activity in combination with ionic liquids for CO2 cycloaddition to terminal epoxides was also reported previously [55, 56]. The difference in the activity was presumably due to zinc halides with different anions have different activation ability of epoxides which can be explained due to the difference in the electronegativity of halide anions that varies in the order Cl‾ > Br‾ > I‾. The greater the electronegativity, greater would be the strength of interaction between Lewis-acidic zinc site and the oxygen atom of epoxide. This will make the regeneration of the Lewis-acidic zinc site more difficult during the intramolecular ring-closure

5 step to form cyclic carbonate due to a slower rate of cleavage of Zn–O bond. Therefore, Zn

Lewis acidic site in ZnI2 has less strong interaction with the oxygen atom of the epoxide, resulting in higher co-catalytic activity, as compared to the ZnBr2 and ZnCl2. Based on the relative co-catalytic activities of these zinc halides, the ZnI2 was selected in combination with PPI for the detailed kinetic studies.

Fig. 1 Relative catalytic activity of PPI catalyst for SC formation with and without ZnI2. Reaction conditions 3.5

M [SO], [PPI] 76 mM, [ZnI2] 38 mM, 100 °C and 10 bar CO2.

Table 1. Relative catalytic activity in terms of the initial reaction rate.

Catalyst Initial Reaction ratea × 10–3 (mol L–1 min–1) PPI 3.2

PPI/ZnCl2 22.1

PPI/ZnBr2 24.5

PPI/ZnI2 24.8

Reaction conditions: 3.5 M [SO], [PPI] 76 mM, [ZnX2] 38 mM, 100 °C and 10 bar CO2. a Determined from the gradient of plot between [SO] M vs time (min).

Table 2 Comparison of the PPI/ZnI2 with earlier reported catalytic systems for CO2 cycloaddition to SO.

Entry Catalyst Co-catalyst Reaction Yield TOF Ref. (mol %) (mol %) conditions (%) (h–1)

1 Cr(III) salphen Tetrabutylammonium 25 °C, 65 4.3 [46] complex bromide (TBAB) 1 bar p (CO2), (2.5) (5) 6 h 2 Cr(III) salen complex N,N- 85 °C, 99 14 [44] (0.075) dimethylaminopyridine 3.5 bar p (CO2), (DMAP) (0.075) 7 h 3 Al(III) salen TBAB 25 °C, 28 3.7 [57] complex (2.5) 1 bar p (CO2), (2.5) 3 h 4 Bis-Al(III) salphen TBAB 25 °C, 96 1.6 [58] complex (5) 1 bar p (CO2), (2.5) 24 h 5 Heteroscorpionate TBAB 25 °C, 69 0.6 [59] Al(III) complex (5) 1 bar p (CO2), (5) 24 h 6 Zn(II) salphen - 80 °C, 88 9.7 [60] complex 10 bar p (CO2), (0.5) 18 h 7 Silanediols Tetrabutylammonium 23 °C, 93 0.52 [61] (10) iodide (TBAI) 1 bar p (CO2), (10) 18 h 8 1-octyl-3- ZnBr2 100 °C, 100 88 [62] methylimidazolium (1.14) 140 bar p (CO2), Chloride (BMImCl) 2 h (2.28) 9 TBAI ZnBr2 80 °C, 92 646 [63] (0.6) (0.3) 80 bar p (CO2), 0.5 h 10 1-butyl-3- ZnCl2 100 °C, 86 3015 [64] methylimidazolium (0.02) 15 bar p (CO2), bromide (BMImBr) 1 h (0.11) 11 Tetraphenylphosphoni ZnBr2 120 °C, 90 3031 [65] um iodide [Ph4P]I (0.015) 25 bar p (CO2), (0.09) 1 h

12 Dicationic ionic liquid ZnI2 110 °C, 94 108 [55] (0.14) (0.3) 15 bar p (CO2), 3 h 13 Methylimidazole ZnBr2 140 °C, 93 356 [66] (MIm) based (0.2) 25 bar p (CO2), heterocyclic 6 h compounds (1) 14 Pyrrolidinopyridinium ZnI2 100 °C, 98 75.6 This iodide (PPI) (0.43) 10 bar p (CO2), study (0.86) 3 h

3.2 Kinetic study for CO2 cycloaddition to SO

A detailed study of kinetics was carried out to establish the general rate law to understand the effect of reaction parameters on the reaction rate and eventually, to investigate the reaction mechanism in CO2 cycloaddition to epoxide in the presence of PPI and ZnI2 as a binary homogeneous catalyst. A series of experiments were performed to determine the order of the reaction in SO, CO2 and catalyst concentrations. Moreover, the temperature dependence of the reaction was studied to determine the thermodynamic and activation parameters.

The general rate equation for the cycloaddition reaction can be written as Eq. 1, where k is the rate constant and [SO], [CO2], [CAT] are the concentrations of SO, CO2 and PPI/ZnI2 (1:1) respectively; a, b and c are the orders of reaction. The concentration of the catalytic species

(PPI/ZnI2) remained constant during each experiment. Similarly, the CO2 was present in large excess due to continuous supply during a semi-batch operation. Due to this, CO2 concentration remained steady throughout the reaction and can be assumed to be constant. Therefore, Eq (1) was simplified as Eq (2). By taking natural logarithm on both sides of Eq (3), we get Eq (4) which can be used to determine the order of reaction with respect to CO2 and catalyst concentrations from the gradient of double logarithmic plots. Assuming pseudo-first-order dependence of the reaction on epoxide concentration i.e. a=1, and by combining Eq (2) and (5), the rate of reaction can be given as Eq (6). By integrating Eq (6), we get Eq (7) which can be used to determine the order of reaction with respect to [SO].

푎 푏 푐 푟푎푡푒 = 푘 [푆푂] [퐶푂2] [퐶퐴푇 ] (1)

푎 푟푎푡푒 = 푘표푏푠 [푆푂] (2)

Where

푏 푐 푘표푏푠 = 푘 [퐶푂2] [퐶퐴푇] (3)

푙푛 푘표푏푠 = ln 푘 + 푏 푙푛 [퐶푂2] + 푐 ln[[퐶퐴푇] (4)

푑[푆푂] 푟푎푡푒 = − (5) 푑푡

푑[푆푂] − = 푘 [푆푂] (6) 푑푡

– ln[푆푂] = 푘표푏푠.t (7)

3.2.1 Reaction order in [SO]

To determine the order of reaction in [SO], a series of experiments were performed by varying the concentration of SO over the range of 2.5–5.5 M using propylene carbonate (PC) as the reaction solvent [67], while other reaction conditions were kept constant i.e. 76 mM [PPI], 38 mM [ZnI2] at 100 °C and 10 bar CO2 pressure. PC has been increasingly used as a green solvent and a potential replacement of traditional solvents for cycloaddition reaction due to its high 3 boiling point and better CO2 solubility [68-70]. Moreover, the density of the PC (1.2 g/cm ) has also a close similarity to that of SO (1.05 g/cm3). Therefore, a reaction medium could be established that did not vary considerably from solvent-free reaction conditions in terms of mass transfer properties and miscibility. The decrease in the SO concentration was monitored by taking aliquots from the reaction mixture and subsequent FTIR analysis (Fig. 2a). Form the results obtained, the rate of reaction increases with the increase in epoxide concentration and the graph between initial SO concentrations and initial reaction rate (mol. L–1. min–1) has shown a linear dependence (Fig. 2b), suggesting a first-order reaction in SO concentration. The rate of the reaction in [SO] was further confirmed by integrated law method [71]. Thus, an experiment was carried out using 1.5 M SO in PC in the presence of 76 mM [PPI], 38 mM

[ZnI2] at 100 °C and 10 bar CO2 pressure. All the data points obtained were found to be fit into first-order kinetics i.e. ln [SO] vs time (min) (Fig. 3), clearly exhibiting a first-order dependence of the reaction in [SO] as determined by the initial rate law method. These results suggest the participation of one molecule of epoxide per molecule of the catalyst in the reaction mechanism [67, 72, 73].

Fig. 2 (a) Decrease in SO concentration as a function of time (min) (b) Plot showing a linear fit between the initial rate of cycloaddition (mol L–1 min–1) vs. initial SO concentration (Reaction conditions: 2.5–5.5 M SO, 76 mM

PPI, 38 mM ZnI2, 100 °C and 10 bar CO2).

Fig. 3 Plot showing a linear fit of all the data points to first-order kinetics plot (Reaction conditions: 1.5 M SO,

76 mM PPI, 38 mM ZnI2, 100 °C and 10 bar CO2).

3.2.2 Reaction order in [CAT]

Initially, the experiments were performed using PPI as catalyst alone by varying the concentration over the range of 76–304 mM. The results have exhibited a first-order dependence of the reaction with respect to PPI determined from the gradient of a double logarithmic plot i.e. 0.93 ~ 1 (Fig. 4a). These results suggest the involvement of one PPI molecule in the reaction mechanism [74]. Similarly, the second series of experiments were performed using PPI in combination with ZnI2 at a fixed concentration of PPI (76 mM) and varying the concentration of ZnI2 over the range of 38–152 mM. The results have showed a fractional order of reaction (i.e. 0.55) (Fig. 4b), which was attributed to a ‘complex reaction network’[75]. Therefore, another series of experiments were performed using a fixed molar ratio i.e. PPI/ZnI2 (1:1 molar ratio) and varying the concentration of binary catalyst from 19– 76 mM. As expected, the rate of the reaction was increased with the increase in catalyst concentration. All the data points exhibit good fit to first-order kinetics (Fig. 4c). Moreover, the order of the reaction was found to be unity from the gradient of the double logarithmic plot i.e. 1.04 (Fig. 4d). These results suggest that active complex in the catalytic cycle was formed + – by the interaction of PPI with ZnI2 to form PP ZnI3 which activates the epoxide by its Lewis acidic Zn site and a nucleophilic attack by the iodide anion (I‾) opens the ring of activated epoxide. The stability to the ring-opened intermediate was provided by the large delocalized + pyrrolidinopyridinium cations (PP ), a key factor to rapid CO2 insertion.

Fig. 4 a) Double logarithmic plot between k obs and PPI concentration (76–304 mM) to determine the order in PPI concentration b) Double logarithmic plot between k obs and ZnI2 concentration (38–152) mM to determine the order in ZnI2 concentration c) Plots showing a linear fit of all data points into first-order kinetics plot (19–76) mM d) Double logarithmic plot between k obs and catalyst concentration to determine the order w.r.t CAT i.e. PPI/ZnI2

(1:1) (Reaction conditions: 3.5 M SO, 100 °C and 10 bar CO2).

3.2.3 Reaction order in [CO2]

To determine the reaction order with respect to [CO2], the experiments were performed by changing the CO2 pressure over the range of 2.5–40 bar and keeping the other reaction parameters constant. The experiments were performed using propylene carbonate (PC) as a reaction solvent. The rate at which CO2 dissolves in PC is much faster than the rate of cyclic carbonate formation [20, 76]. The order of the reaction w.r.t CO2 concentration was determined over two regimes i.e. 2.5–10 and 10–40 bar. The results obtained from the experiments performed over the range of 2.5–10 bar exhibit a linear increase in reaction rate with increase in CO2 pressure and all data points are found to be fit into first-order kinetics (Fig. S7). Moreover, the order of reaction was determined to be unity from the gradient of the double logarithmic plot i.e. 1.203 (Fig. 5a). However, no significant increase in reaction rate was observed at higher CO2 pressure (i.e.10–40 bar) and the order of reaction tends to decrease i.e.

0.36 (Fig. 5b). This decrease in the order of the reaction at higher CO2 pressure was presumably due to dilution effect (i.e. CO2 induced expansion of reaction mixture), which decreases epoxide and catalyst concentrations as reported elsewhere [77, 78].

Fig. 5 a) Double logarithmic plot between k obs and CO2 pressure over 2.5–10 bar (b) Double logarithmic plot between k obs and CO2 pressure over 10–40 bar (Reaction conditions: 3.5 M SO, 76 mM PPI, 38 mM ZnI2, and 100 °C).

3.3 Thermodynamic and kinetic activation parameters

The effect of temperature on the reaction rate was investigated by performing experiments at the range of 100–140 °C. As expected, the rate of reaction increases with the increase in temperature due to an increase in catalytic activity at higher pressure. From the results obtained, all the data points exhibited a good fit to first order kinetics (Fig. S8). Furthermore, the activation energies (Ea) for SC formation using PPI as a catalyst with and without ZnI2 were calculated from linear Arrhenius plot between ln (k obs) vs. 1/T (Fig. 6a). The synergistic effect –1 of PPI and ZnI2 resulted in a significant decrease in Ea from 73.2 to 46.1 kJ mol . Furthermore, the values of thermodynamic activation parameters such as enthalpy (ΔH‡), entropy (ΔS‡) and Gibb’s free energy (ΔG‡) were determined using Eyring equation (Eq 15, Fig. 6 b) [79]. The summary of the corresponding results is given in Table 2. The positive values of ΔH‡ and ΔG‡ shows endergonic and chemically controlled nature of the cycloaddition reaction. Whereas, the large negative values of ΔS‡ indicate a highly ordered activated complex at the transition state.

k ΔH‡ k ΔS‡ ln ( obs) = + ln ( B) + (8) T RT h R

Fig. 6 (a) Arrhenius plots for SC formation using PPI catalyst with and without ZnI2 (b) Eyring plots for SC formation using PPI catalyst with and without ZnI2. (Reaction conditions 3.5 M [SO], [PPI] 76 mM, [ZnI2] 38 mM, 100–140 °C and 10 bar CO2.)

Table 2. Activation parameters for SC formation using PPI catalyst with and without zinc halides as co- catalysts.

‡ ‡ ‡ (a) Catalyst Ea ΔH ΔS ΔG

(kJ mol-1) (kJ mol–1) (J mol–1 K–1) (kJ mol–1) PPI 73.22 ± 0.559 69.96 ± 0.56 –77.86 ± 1.424 99.01± 0.451 PPI+ZnI2 46.06 ± 1.407 42.80± 1.38 –159.44± 1.741 102.30±1.074

Note: (a) ΔG‡ = ΔH‡ – T. ΔS‡ at 373 K

3.4 Proposed reaction mechanism

Based on the detailed study of reaction kinetics, the reaction mechanism was proposed (Scheme

4). The combination of ZnI2 with PPI resulted in an acid-base zinc-pyrrolidinopyridinium + – iodide complex (PP ZnI3 ) as an active catalyst species (I). The Lewis acidic Zn site helps to activate the epoxide (SO) by the interaction with the oxygen to form Zn-adduct (II), which at the same time undergoes a nucleophilic attack by the iodo-anion (I‾) on the least sterically hindered carbon atom to form an iodo-alkoxide intermediate (III). This further undergoes CO2- insertion into the ring-opened epoxide to form a carbonate species (IV) which eventually leads to a five-membered ring of cyclic carbonate (SC) by the intramolecular elimination of the iodo- anion (I‾), and the catalyst is regenerated. A similar reaction mechanism for cyclic carbonate synthesis catalyzed by other binary acid-base catalytic systems was reported previously [63, 64, 80].

Scheme 4. Proposed reaction mechanism for SC formation catalyzed by PPI and ZnI2.

4. Conclusions

The reaction kinetics of styrene carbonate (“SC”) formation from styrene oxide (“SO”) and

CO2 were investigated in the presence of PPI in combination with Zn halides (ZnCl2, ZnBr2 and ZnI2). The synergistic effect of ZnI2 and PPI resulted in more than 7-fold increase in the reaction rate than PPI alone. The detailed study of reaction kinetics was carried out to give mechanistic insight and to determine the rate law equation for SC formation. As a result, the reaction has shown first-order dependence on epoxide, CO2 and catalyst concentrations. These + – results revealed that the active species consists of a PP ZnI3 as an acid-base complex in the catalytic cycle leading to cyclic carbonate formation. The activation energy for SC formation catalysed by PPI alone was determined to be 77.3 kJ mol–1, which was reduced to 46.1 kJ mol– 1 using ZnI2 in combination with PPI. The catalytic activity of zinc halides in combination with

PPI was determined to be ZnI2 > ZnBr2 > ZnCl2. Based on the kinetic study, a synergistic acid- based reaction mechanism was proposed.

Acknowledgments

This work is supported by the Engineering and Physical Science Research Council (EPSRC) funding for Sustainable Polymers (Project reference EP/L017393/1). We are also grateful to the support of the University of Engineering and Technology, Lahore in providing the scholarship to one of the authors.

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