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Efficient Alcoholysis of Polycarbonate Catalyzed by Recyclable Lewis Acidic Ionic Liquids Jiao Guo,†,∥ Mengshuai Liu,†,∥ Yongqiang Gu,† Yuchen Wang,† Jun Gao,‡ and Fusheng Liu*,† †

College of Chemical Engineering, Qingdao University of Science and Technology, Qingdao 266042, PR China College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266590, PR China

Ind. Eng. Chem. Res. Downloaded from pubs.acs.org by DURHAM UNIV on 08/04/18. For personal use only.

‡

ABSTRACT: In this contribution, an efficient and green protocol for alcoholysis of waste polycarbonate (PC) using Lewis acidic ionic liquids (ILs) was first developed under mild and solvent-free conditions. The influence of Lewis acidity and reaction parameters on the catalytic activity were thoroughly studied. It showed that with only 5.0 mol % [Bmim]Cl·2.0FeCl3 loading, 100% PC conversion with 97.2% bisphenol A (BPA) yield was efficiently obtained at 120 °C and n(CH3OH)/n(PC) = 6:1 for 3.0 h. The alcohol scopes for depolymerization of waste PC into BPA monomer were examined, and the moderate to excellent product yields were obtained under the optimized conditions. Also the [Bmim]Cl·2.0FeCl3 catalyst could be reused five successive times without a significant decrease of the catalytic activity. Combining the kinetic study and the in situ FT-IR analysis, a feasible reaction mechanism was proposed.

1. INTRODUCTION Polycarbonate (PC), as one of the most important plastics, has wide applications in construction materials, packaging, automotive industry, and electrical and electronic engineering.1 Nowadays the production of PC is rapidly increased, which simultaneously gives rise to excessive accumulation of waste PC. The recycling of waste PC has drawn much attention from the viewpoints of sustainable development and reducing waste of resources.2 Both physical and chemical methods can realize recycling of waste PC, while the physical recycling of waste PC into plastic products is always with a drop in mechanical and physical properties.3 Alternatively, chemical depolymerization of waste PC, including thermal pyrolysis,4 hydrolysis,5−9 aminolysis,10,11 and alcoholysis12−14 into its essential monomer, bisphenol A (BPA), or other valuable products represents a potential strategy. However, most of the methods are typically performed in the presence of concentrated acid or base catalyst.15,16 Also the hydrolysis of PC can only obtain valuable BPA and afford byproduct of equimolar greenhouse gases (CO2). The thermal pyrolysis and aminolysis usually need harsh reaction conditions, such as high pressure and temperatures.4,10 By contrast, the alcoholysis of PC waste can obtain monomer BPA, which can be used as raw material for new PC production; also the alcoholysis can afford other valuable chemicals (Scheme 1). Hence, the alcoholysis method © XXXX American Chemical Society

for chemical recycling of waste PC shows great necessity for further development. At present, few studies concerning with alcoholysis of PC have been reported. Quaranta et al.17 developed the organic superbase (DBU) catalyzing route for PC alcoholysis under mild and solvent-free conditions. The DBU showed excellent catalytic activity, but it was difficult to realize catalyst separation and product purification, as BPA-DBU adduct was formed in the reaction system. Nacci et al.10 reported binary system ZnO-NPs/NBu4Cl for depolymerization of waste PC using 1,2-propanediol as reactant; the acid/basic catalyst could catalyze the alcoholysis reaction with high conversion and selectivity, while with low catalytic efficiency, the process needed a long reaction time and the presence of a volatile organic solvent (THF). In our previous work, the eco-friendly basic ILs [Bmim][Ac] and [Bmim][Cl] were first used for PC methanolysis to recover BPA and dimethyl carbonate (DMC), respectively.12,13 The basic IL played the roles of solvent and catalyst to promote swelling or dissolution of PC, and then further to realize its conversion. The strategy required a large Received: May 18, 2018 Revised: July 12, 2018 Accepted: July 20, 2018

A

DOI: 10.1021/acs.iecr.8b02201 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research Scheme 1. Alcoholysis of PC over Lewis Acidic ILs

Figure 1. (A) FT-IR spectra and (B) TGA curve of [Bmim]Cl·2.0FeCl3.

ratios were reacted at 50 °C for 3.0 h to obtain the singlecomponent [Bmim]Cl·xFeCl3, where x represents the molar ratio of FeCl3 to [Bmim]Cl. The typical characterization results are as follows. FT-IR spectra were collected by using a PerkinElmer Spectrum 100 FTIR Spectrometer with anhydrous KBr pellets. NMR spectra were acquired by using a Bruker AV-400 MHz spectrometer. TGA was performed by using a NETZSCH STA449F3 simultaneous thermogravimetric analyzer under N2 in the range from room temperature to 600 °C with a heating rate of 10 °C/min. 2.2. Catalytic Alcoholysis of PC. Pure PC pellets (3 mm length and 2.5 mm diameter with MW about 20 000) were used as model plastics. Typically, the reaction proceeded in a 50 mL stainless-steel reactor equipped with a magnetic stirring bar and a thermometer. Initially, W1 g of PC, 6 equiv of CH3OH (Sinopharm Chemical Reagent Co.), and catalyst [Bmim]Cl·FeCl3 (5 mol % PC) were added into the reactor at room temperature. Then the reactor was heated to 120 °C and kept for 3 h. After the reaction completed, the reactor was cooled to 0 °C in an ice−water bath statically. The mixture was filtered to remove the unreacted PC (W2 g). The filtrate was evaporated under vacuum to remove surplus CH3OH, the residue was dissolved in ethyl acetate, then the solution was washed with deionized H2O three times. The ethyl acetate phase was evaporated to obtain the BPA product (W3 g). The H2O phase was evaporated to recycle the [Bmim]Cl·FeCl3. The as-recycled [Bmim]Cl·FeCl3 was dried under a vacuum (60 °C, 8 h) and reused directly for the next run. Similarly, other alcohols such as ethanol, n-propanol, 2-propanol, n-butyl alcohol, and isobutyl alcohol were also tested. The PC conversion and BPA yield were calculated according to the following formulas:

amount of IL (1−1.5 equiv of PC), which increased the recycling cost of waste PC. Hence, the development of efficient and green catalysts for depolymerization of waste PC into BPA monomer and other valuable chemicals remains a challenge and is highly desirable. Herein, a series of Lewis acidic ILs with different acid strengths were facilely synthesized, and the catalytic activities for alcoholysis of PC to recover the monomer BPA were studied. The influence of catalyst acidity, reaction conditions (such as temperature, alcohol dosage, catalyst concentration and time), and alcohol scopes on the catalytic activity were studied in detail. Moreover, the recyclability of the present Lewis acidic IL and catalytic kinetics were examined for the model methanolysis of PC. On the basis of the experimental results, an insight into the feasible reaction mechanism was proposed. The developed protocol here is low-cost, facilely synthesized, and environmentally benign, showing great potential for reducing the environmental issues of PC waste.

2. EXPERIMENTAL SECTION 2.1. Synthesis and Characterization of Lewis Acidic ILs. The Lewis acidic ILs were synthesized according to the reported method.18 Typically, 2.0 g (24.4 mmol) of purified 1methylimidazole (99%, Aladdin Chemical Co.) was mixed with 20 mL of ethyl acetate (99.8%, Sinopharm Chemical Reagent) at room temperature. Subsequently, 2.5 g (27.0 mmol) of nbutyl chloride (Adamas Reagent Co.) was slowly dripped with stirring, and the reaction mixture was stirred under reflux overnight. Upon completion of the reaction, the solvent was removed by decantation. The product was washed repeatedly with ethyl acetate (3 × 30 mL) to obtain [Bmim]Cl. 1HNMR (400 MHz, CDCl3): 8.80 (1H, s, NCHN), 7.55 (1H, m, CH3NCHCHN), 7.51 (1H, m, CH3NCHCHN), 4.25 (2H, t, NCH2(CH2)2CH3), 3.96 (3H, s, NCH3), 1.89 (2H, m, NCH2CH2CH2CH3), 1.35 (2H, m, N(CH2)2CH2CH3). Under a N2 atmosphere, the [Bmim]Cl and anhydrous FeCl3 (98%, Alfa Aesar Chemical Co.) with different mole

PC conversion = B

W1 − W2 × 100% W1

(1)

DOI: 10.1021/acs.iecr.8b02201 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research BPA yield =

W3 M nBPA = × PC nPC MBPA W1

belongs to transesterification, and acidic or basic capacity plays an important role in catalyzing the reaction.14 FeCl3 could afford 94.6% PC conversion without BPA product (entry 2). And the [Bmim]Cl also showed unsatisfied activities due to the weaker acidity (entry 3). As to our presented Lewis acidic ILs herein, [Bmim]Cl·0.5FeCl3 showed an unobvious increase of the catalytic activity (entry 4). To our delight, a significant improvement of the catalytic activity was observed with the increase of the Lewis acidity (entries 5−7), and the [Bmim]Cl· 2.0FeCl3 could effectively catalyze the methanolysis of PC with 100% PC conversion and 97.1% BPA yields at 120 °C and CH3OH/PC molar ratio of 6:1 for 3.0 h using just 5.0 mol % catalyst loading (entry 7). The result was attributed to the stronger Lewis acidity of [Bmim]Cl·2.0FeCl3 . It was speculated that the catalyst with enhanced Lewis acidity could activate the carbanyl group of PC, which resulted in the methanol being more inclined to nucleophilic attack of the carbonyl, facilitating the methanolysis of PC.19 More work has been done to clarify the exact mechanism of the Lewis acidic ILs mediated reaction shown later in Scheme 2. Further improving the Lewis acidity did not obviously increase the BPA yield under the employed reaction conditions (entry 8). Compared with FeCl3 catalyst, the Lewis acidic [Bmim]Cl· 2.0FeCl3 showed great advantages for both catalytic performance and product separation. In this regard, [Bmim]Cl· 2.0FeCl3 with an effective activity was chosen as the catalyst for further investigation in methanolysis of PC. 3.2. Effects of Reaction Parameters. Given the potential of [Bmim]Cl·2.0FeCl3 as catalyst, we studied the effects of reaction parameters on catalytic activity toward the methanolysis of PC, as shown in Table 2. With the increase of reaction temperature, a significant improvement of PC conversion and BPA yield were observed in the range 100− 120 °C (entries 1−3). The results indicated that a higher temperature was favorable to obtain better PC conversion and BPA yield. At the higher temperature it is easy to realize dissolution or swelling of PC in the reaction medium, which is generally recognized as the rate-determining step for the methanolysis of PC.12,13 Further raising the temperature to 130 °C was insignificant, and a comparable 98.1% BPA yield was obtained (entry 4). In regards to practical energy-saving, the optimal reaction temperature was 120 °C.

(2)

where MPC represents the molar mass of PC unit and MBPA represents the molar mass of BPA. In our experiment, the following factors may result in the calculation errors of PC conversion and BPA yield, and the error analysis was presented here. Inherent error and uncertainties include weighing error and the heat transfer error of the thermocouple and the stainless-steel reactor. The separation loss of BPA and PC samples and artificial control of the operation temperature through the thermometer may bring the random error. To minimize the calculation error, the averages and standard deviations of three repetitions were computed for the reported data within the text.

3. RESULTS AND DISCUSSION 3.1. Catalyst Screening. Several Lewis acidic ILs with different acid strengths were successfully synthesized and characterized (Figure 1). As shown in Table 1, catalytic Table 1. Catalysts Screening for PC Methanolysisa entry

catalyst

1 2 3 4 5 6 7 8

blank FeCl3 [Bmim]Cl [Bmim]Cl·0.5FeCl3 [Bmim]Cl·1.0FeCl3 [Bmim]Cl·1.5FeCl3 [Bmim]Cl·2.0FeCl3 [Bmim]Cl·2.5FeCl3

PC conversion (%) 94.6 20.3 27.8 85.4 97.1 100 100

± ± ± ± ±

1.4 1.1 0.8 1.3 1.0

BPA yield (%)b c 16.3 25.2 81.8 92.4 97.2 98.3

± ± ± ± ± ±

1.2 0.9 1.3 1.5 1.1 0.9

a Reaction conditions: PC 15.7 mmol (4.0 g), n(CH3OH)/n(PC) = 6:1, catalyst 5.0 mol % PC, 120 °C, 3.0 h. bIsolated yield. cViscous oligomers.

activities of the ILs as-synthesized were tested toward the model methanolysis of PC. The reactions were performed at 120 °C for 3.0 h under which the catalyst could keep adequate thermal stability. No PC conversion and BPA yield were detected in the absence of catalyst, demonstrating the critical role of catalyst in methanolysis of PC (entry 1). To our knowledge, the alcoholysis of PC to yield BPA and DMC

Table 2. Effects of Reaction Parameters on PC Methanolysis over [Bmim]Cl·2.0FeCl3 Catalyst entry

T (°C)

n(CH3OH)/n(PC)

cat. loading (mol %)a

time (h)

1 2 3 4 5 6 7 8 9 10 11 12 13 14

100 110 120 130 120 120 120 120 120 120 120 120 120 120

6:1 6:1 6:1 6:1 2:1 4:1 8:1 6:1 6:1 6:1 6:1 6:1 6:1 6:1

5.0 5.0 5.0 5.0 5.0 5.0 5.0 1.0 2.5 7.5 5.0 5.0 5.0 5.0

3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 1.0 2.0 4.0 5.0

PC conversion (%) 15.2 57.8 100 100 90.6 95.9 93.1 76.1 87.3 100 66.8 90.2 100 100

± 0.9 ± 1.2

± ± ± ± ±

1.5 0.9 1.1 1.5 0.9

± 1.2 ± 0.8

BPA yield (%)b 11.3 55.6 97.2 98.1 86.2 91.4 88.4 68.3 79.2 98.8 58.7 86.9 98.3 99.1

± ± ± ± ± ± ± ± ± ± ± ± ± ±

1.3 1.4 1.1 0.9 0.8 0.9 1.4 1.3 0.8 0.7 1.2 0.7 0.6 0.9

a

Amount of catalyst to PC. bIsolated yield. C

DOI: 10.1021/acs.iecr.8b02201 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 2. 1H NMR spectra and TGA curves of BPA product obtained under optimized conditions.

Then the effect of the CH3OH/PC molar ratio on methanolysis of PC was studied. At a theoretical molar ratio of 2:1, the [Bmim]Cl·2.0FeCl3 could afford 90.6% PC conversion and 86.2% BPA yield at 120 °C for 3.0 h with only 5.0 mol % catalyst loading (entry 5). When the methanol dosage was increased, the dynamics was favorable for the alcoholysis reaction. And the BPA yield smoothly improved to 97.2% with an increase in methanol dosage from n(CH3OH)/ n(PC) = 2:1 to 6:1 (entries 3, 5, and 6). With a further increase of the molar ratio to 8:1, a slight decrease of BPA yield was observed due to the dilution effect of excess methanol to the catalyst (entry 7). Hence, the molar ratio of n(CH3OH)/ n(PC) = 6:1 was optimal for PC methanolysis. The catalyst loading played an important role in improving the product yield. With only 1.0 mol % [Bmim]Cl·2.0FeCl3, a moderate 76.1% PC conversion with 68.3% BPA yield could be obtained, indicating excellent catalytic activity of [Bmim]Cl· 2.0FeCl3 (entry 8). When the catalyst loading was raised from 1.0 to 5.0 mol %, the PC conversion was increased from 76.1% to 100%, and accordingly, the BPA yield was increased from 68.3% to 97.2% (entries 3, 8, and 9). The results were ascribed to the improvement of Lewis acidity with increasing of [Bmim]Cl·2.0FeCl3 in the reaction medium. No obvious change in the BPA yield was observed with a further increase of catalyst loading (entry 10). In this regard, the catalyst loading of 5.0 mol % should be a good choice. Moreover, the effect of reaction time on PC conversion and BPA yield was examined under the other optimized reaction conditions. The methanolysis of PC went gradually with time and the reaction was almost completed within 3.0 h, the yield of BPA was over 97.2%, and further prolonging the reaction time resulted in an uncompetitive product yield. Figure 2 showed the 1H NMR and TGA characterized results of BPA product obtained under the optimized conditions, which was identical with the pure BPA sample. 3.3. Alcohol Scopes for PC Degradation. In order to study the alcohol scopes for PC depolymerization, the alcohols bearing different substituents were examined over [Bmim]Cl· 2.0FeCl3 under the optimized conditions (Table 3). The alcohol structures showed a significant effect on the catalysis reaction. For the n-alkanol (entries 1−3 and 5), the alcoholysis rate was gradually decreased with the increase of the alkyl chain, and the BPA yield was decreased from 97.2% to 59.1% when methanol was replaced by n-butyl alcohol. For the isomerous alcohols, the steric hindrance played a more

Table 3. Alcohol Scopes for PC Degradation over [Bmim]Cl·2.0FeCl3 Catalysta entry

alcohol

T (°C)

time (h)

1 2 3 4 5 6

methanol ethanol n-propanol 2-propanol n-butyl alcohol isobutyl alcohol

120 120 120 120 120 120

3.0 3.0 3.0 3.0 3.0 3.0

PC conversion (%) 100 95.1 70.8 43.1 65.3 38.9

± ± ± ± ±

1.5 1.4 1.2 1.1 1.3

BPA yield (%)b 97.2 87.3 64.4 36.8 59.1 32.3

± ± ± ± ± ±

1.1 1.3 1.6 1.4 1.3 1.5

a Reaction conditions: PC 15.7 mmol (4.0 g), n(alcohol)/n(PC) = 6:1, [Bmim]Cl·2.0FeCl3 5.0 mol % PC. bIsolated yield.

obviously negative effect on catalytic activity (entries 4 and 6). The alcoholysis reaction could hardly proceed to yield the corresponding BPA, and only 38.9% PC conversion and 32.3% BPA yield were obtained when isobutyl alcohol was used as substrate under the same conditions. Because the alcoholysis reaction radically occurred via alcohol nucleophilic attacking at the activated carbonyl group of PC, we speculated the alcohol with larger steric hindrance did not facilitate the nucleophilic attack or did not keep stabilization of new formed species after nucleophilic attack. The new formed species would be described later in Scheme 2. 3.4. Reusability of [Bmim]Cl·2.0FeCl3 in PC Methanolysis. Besides the catalytic activity, the recycling potential of the [Bmim]Cl·2.0FeCl3 was evaluated using the model methanolysis of PC. The repeated reaction was carried out under the optimized conditions of 120 °C, n(CH3OH)/n(PC) = 6:1 for 3.0 h using 5.0 mol % [Bmim]Cl·2.0FeCl3. After each run, the catalyst could be easily recycled from the residues by extraction using ethyl acetate/H2O. As can be seen in Figure 3, the PC conversion remained 100%, and the BPA yield was almost constant over six consecutive runs. The results indicated that [Bmim]Cl·2.0FeCl3 has excellent stability and reusability without any loss of activity in the system. 3.5. Catalytic Kinetics and Plausible Reaction Mechanism. A kinetic model was developed for PC methanolysis using [Bmim]Cl·2.0FeCl3 catalyst. According to previous reports,13,20,21 we assumed that the reaction was first order. Then the alcoholysis reaction was controlled by the following kinetic eq 3: D

DOI: 10.1021/acs.iecr.8b02201 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Moreover, the activation energy for the PC methanolysis was calculated using the Arrhenius equations (eq 7 and eq 8), where A is pre-exponential factor (h−1), Ea is the activation energy (kJ·mol−1), T is the absolute temperature (K), and R is the universal gas constant (8.314 J·mol−1·K−1):

dC PC = kC PC dt

(3)

where k represents the rate constant for PC conversion and CPC represents the PC concentration at reaction time of t Defining

C PC = C0(1 − x)

(4)

where x represents the PC conversion, then gave eq 5: dx = k(1 − x) dt

(5)

Integrating eq 5 against time yielded eq 6: ln[1/(1 − x)] = kt

(7)

ln k = ln A − Ea /(RT )

(8)

Based on the relationship of rate constant k and temperature, Figure 4B gave a linear correlation of ln k to the reciprocal of temperature (1/T). Hence, the activation energy Ea for PC methanolysis catalyzed by [Bmim]Cl·2.0FeCl3 was calculated as 98.9 kJ·mol−1. The activation energy was lower than the reported [Bmim][Ac] catalyst with 167 kJ·mol−1 for methanolysis of PC.13 To further provide an insight into the plausible alcoholysis mechanism, in situ FT-IR was used to monitor the PC residue and crude BPA product at different reaction levels over [Bmim]Cl·2.0FeCl3 catalyst. As shown in Figure 5A, the PC residues showed identical characteristic peaks with a pure PC pellet at low conversion of 26.3% and 43.2%, and only a slightly weak hydroxyl peak was observed at about 3400−3500 cm−1 gradually with increasing of PC conversion. This phenomenon resulted from swelling or fragmentation of insoluble PC via the attack of the alcohol molecules in the reaction system. The crude BPA obtained at low yield showed an obvious difference at about 1736 cm−1, which was assigned to the carbonyl group, indicating a few smaller oligomers (as shown in Scheme 2) existed in the crude BPA product. With the increase of BPA yield, the carbonyl peak gradually disappeared, and the structure of the as-obtained BPA at 86.7% yield was almost identical with the pure BPA sample (Figure 5B). It evidenced that the dissolved PC or oligomers was rapidly depolymerized into BPA and DMC monomers, and no additional oligomers existed in the product under the optimized conditions. As shown in Scheme 2, the plausible alcoholysis mechanism over [Bmim]Cl·2.0FeCl3 catalyst was provided. The alcoholysis reaction was initiated by interacting the O atom of the carboxyl groups in PC with the Lewis acidic Fe sites through the formation of a Fe−O adduct; the cation [Bmim]+ could simultaneously undergo an electrostatic interaction to further

Figure 3. Catalytic recyclability of [Bmim]Cl·2.0FeCl3 catalyst. Conditions: PC 15.7 mmol (4.0 g), n(CH3OH)/n(PC) = 6:1, [Bmim]Cl·2.0FeCl3 5.0 mol % PC, 120 °C, 3.0 h. Analytical measurement error is ±1.0−1.8% around mean values.

−

k = Ae−Ea /(RT )

(6)

Based on the PC conversion at four different temperatures, the plot of ln[1/(1 − x)] versus time obtained was linear, as shown in Figure 4A. According to the linear fitting results in Table 4, the correlation coefficients R were all approximately equal to 1, indicating that the methanolysis rate was proportional to the PC concentration at different temperatures, and the methanolysis of PC was first order here.

Figure 4. (A) Plot of ln[1/(1 − x)] versus time at different temperatures and (B) Arrhenius plot for the determination of activation energy using [Bmim]Cl·2.0FeCl3 catalyst. Conditions: PC 15.7 mmol (4.0 g), n(CH3OH)/n(PC) = 6:1, [Bmim]Cl·2.0FeCl3 5.0 mol % PC. The data were based on averages from triplicate measurements. E

DOI: 10.1021/acs.iecr.8b02201 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Table 4. Linear Fitting Results, Correlation Coefficient R, and Reaction Rate Constant k at Different Temperatures T (°C) 100 110 120 130

kinetics equation y y y y

= = = =

0.1602x 0.3275x 0.8793x 1.6062x

− − − −

0.2588 0.2455 0.2526 0.1799

correlation coefficient R

rate constant k (h−1)

1000/T (K−1)

ln k

0.9992 0.9935 0.9830 0.9957

0.1602 0.3275 0.8793 1.6062

2.6810 2.6110 2.5445 2.4814

−1.8316 −1.1162 −0.1287 0.4739

Figure 5. In situ FT-IR spectra of (A) residual PC at different conversions: (a) pure PC, (b) 26.3%, (c) 43.2%, (d) 57.8%, (e) 76.1%, and (f) 95.9%. (B) BPA product at different yield: (a) 24.8%, (b) 40.6%, (c) 86.7%, and (d) pure BPA.

Scheme 2. Plausible Alcoholysis Mechanism of PC over [Bmim]Cl·2.0FeCl3 Catalyst

4. CONCLUSIONS The selective alcoholysis of waste PC to recover monomer BPA catalyzed by [Bmim]Cl·2.0FeCl3 was studied. The catalyst acidity showed a significant impact on the catalytic activity, which was improved with the increase of the Lewis acidity. With the chosen [Bmim]Cl·2.0FeCl3 (5.0 mol %) catalyst, an excellent BPA yield of 97.2% could be obtained under mild reaction conditions without solvent participation. The catalyst exhibited superior stability and could keep durable

activate the carboxyl groups. The activation step afforded an oxyanion intermediate with a positive charge, which was unstable to further formation of carbocation intermediate through electron transfer. Thereafter, the reaction was promoted by nucleophilic attack of the alcohol on the carbocation intermediate in PC, resulting in fragmentation of PC into oligomers and smaller oligomeric species. In a similar way, the oligomers were further attacked by the alcohol to generate the final products of BPA and carbonate. F

DOI: 10.1021/acs.iecr.8b02201 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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(9) Grause, G.; Karrbrant, R.; Kameda, T.; Yoshioka, T. Steam Hydrolysis of poly(bisphenol A carbonate) in a fluidized bed reactor. Ind. Eng. Chem. Res. 2014, 53, 4215−4223. (10) Iannone, F.; Casiello, M.; Monopoli, A.; Cotugno, P.; Sportelli, M. C.; Picca, R. A.; Cioffi, N.; Dell’Anna, M. M.; Nacci, A. Ionic liquids/ZnO nanoparticles as recyclable catalyst for polycarbonate depolymerization. J. Mol. Catal. A: Chem. 2017, 426, 107−116. (11) Hoekstra, E. J.; Simoneau, C. Release of bisphenol A from polycarbonate−A review. Crit. Rev. Food Sci. Nutr. 2013, 53, 386− 402. (12) Liu, F. S.; Li, Z.; Yu, S. T.; Cui, X.; Ge, X. P. Environmentally benign methanolysis of polycarbonate to recover bisphenol A and dimethyl carbonate in ionic liquids. J. Hazard. Mater. 2010, 174, 872− 875. (13) Liu, F. S.; Li, L.; Yu, S. T.; Lv, Z. G.; Ge, X. P. Methanolysis of polycarbonate catalysed by ionic liquid [Bmim][Ac]. J. Hazard. Mater. 2011, 189, 249−254. (14) Zhao, Y. J.; Zhang, X.; Song, X. Y.; Liu, F. S. Highly active and recyclable mesoporous molecular sieves CaO(SrO,BaO)/SBA-15 with base sites as heterogeneous catalysts for methanolysis of polycarbonate. Catal. Lett. 2017, 147, 2940−2949. (15) Piñero, R.; García, J.; Cocero, M. J. Chemical recycling of polycarbonate in a semi-continuous lab-plant. Green Chem. 2005, 7, 380−387. (16) Hata, S.; Goto, H.; Yamada, E.; Oku, A. Chemical conversion of poly(carbonate) to 1,3-dimethyl-2-imidazolidinone (DMI) and bisphenol A: a practical approach to the chemical recycling of plastic wastes. Polymer 2002, 43, 2109−2116. (17) Quaranta, E.; Sgherza, D.; Tartaro, G. Depolymerization of poly(bisphenol A carbonate) under mild conditions by solvent-free alcoholysis catalyzed by 1,8-diazabicyclo[5.4.0]undec-7-ene as a recyclable organocatalyst: a route to chemical recycling of waste polycarbonate. Green Chem. 2017, 19, 5422−5434. (18) Xun, S. H.; Zhu, W. S.; Zheng, D.; Zhang, L.; Liu, H.; Yin, S.; Zhang, M.; Li, H. M. Synthesis of metal-based ionic liquid supported catalyst and its application in catalytic oxidative desulfurization of fuels. Fuel 2014, 136, 358−365. (19) Song, X. Y.; Liu, F. S.; Wang, H.; Wang, C.; Yu, S. T.; Liu, S. W. Methanolysis of microbial polyester poly(3-hydroxybutyrate) catalyzed by Brønsted-Lewis acidic ionic liquids as a new method towards sustainable development. Polym. Degrad. Stab. 2018, 147, 215−221. (20) Wang, H.; Li, Z. X.; Liu, Y. Q.; Zhang, X. P.; Zhang, S. J. Degradation of poly(ethylene terephthalate) using ionic liquids. Green Chem. 2009, 11, 1568−1575. (21) Kim, D.; Kim, B.; Cho, Y.; Han, M.; Kim, B. S. Kinetics of polycarbonate glycolysis in ethylene glycol. Ind. Eng. Chem. Res. 2009, 48, 685−691.

high activity in the catalytic system after six cycles. By the kinetics studies, it was proved that the methanolysis of PC using [Bmim]Cl·2.0FeCl3 catalyst was first order and the activation energy was 98.9 kJ·mol−1, which was lower than the reported [Bmim][Ac] catalyst. According to the reaction mechanism, the hgih activity of [Bmim]Cl·2.0FeCl3 was attributed to the synergistic effect deriving from the Lewis acidity of the anion and the electrostatic interaction of the cation. The novel protocol herein exhibits notable advantages, such as facile synthesis, environmental benigness, high activity, and excellent reusability, which arouses great potential applications for chemical recycling of plastics.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (F. S. Liu). ORCID

Jun Gao: 0000-0003-1145-9565 Fusheng Liu: 0000-0002-4909-1252 Author Contributions ∥

These authors contributed equally to this work

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (No.51673106), the Science and Technology Research Project of Shandong Province (No. 2016GSF116005), the Natural Science Foundation of Shandong Province (No. ZR2018BB009), and the Scientific Research Foundation of Qingdao University of Science and Technology (No. 0100229019).

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REFERENCES

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DOI: 10.1021/acs.iecr.8b02201 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX