Pushing the Limits in Alcoholysis of Waste Polycarbonate with DBU

Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

pubs.acs.org/journal/ascecg

Pushing the Limits in Alcoholysis of Waste Polycarbonate with DBUBased Ionic Liquids under Metal- and Solvent-Free Conditions Mengshuai Liu,†,∥ Jiao Guo,†,∥ Yongqiang Gu,† Jun Gao,‡ Fusheng Liu,*,† and Shitao Yu† †

College of Chemical Engineering, Qingdao University of Science and Technology, Qingdao 266042, P. R. China College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266590, P. R. China

‡

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S Supporting Information *

ABSTRACT: The development of efficient and green protocol for recycling of waste plastics is of great significance in terms of “sustainable society” and “green chemistry” concepts. Several ionic liquids (ILs) with different acidity−basicity and structures were one-step synthesized and characterized. They were applied to alcoholysis of polycarbonate (PC) without metal and solvent. The IL structures and reaction conditions were optimized. Moreover, the alcohol scopes and catalyst reusability were evaluated. The results showed that 100% PC conversion and 99% bisphenol A (BPA) yield were obtained by using [HDBU][LAc] catalyst, and the [HDBU][LAc] could keep high-activity after using for six times under the optimized conditions. Finally, the possible reaction mechanism was proposed via the FT-IR and NMR analysis technique. KEYWORDS: Methanolysis, polycarbonate, DBU-based ionic liquids, homogeneous catalysis, reusability

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INTRODUCTION Recently, the global production of plastics including polyolefins, polyamides, and polyesters, utilized in construction, packaging, automotive, and agricultural industries, has gradually increased, which resulted in recycling of waste plastics being gained widespread attention from both “sustainable society” and “green chemistry” points of view.1 The PC has numerous potential applications ranging from electronic components to medical fields due to its excellent durability, transparency, and mechanical properties.2 However, PC recycling is often difficult, and the mechanical and physical properties are always dropped by using the traditional physical method to manufacture new plastic products.3 Then chemical recycling as an alternative method has been well developed by which the PC waste can be converted into its starting monomer (bisphenol A: BPA) for production of new virgin plastic, and simultaneously, it can afford other valuable chemicals through the chemical process. A broad variety of chemical recycling strategies, such as pyrolysis,4 hydrolysis,5,6 aminolysis,7 alcoholysis,8−10 and glycolysis11−13 has been developed for chemical depolymerization of PC. While the pyrolysis suffers from the release of greenhouse gases and a low monomer selectivity with large amounts of byproducts.14 The other strategies usually need a high pressure and temperature; they also perform with a large quantity of concentrated acid or base catalyst for a long-time processing, which results in equipment corrosion and environmental problems.15 The ILs as novel reaction medium have been widely applied in various chemical processes, because they have unique features of thermal and chemical stabilities, © XXXX American Chemical Society

selective solubility to macromoleclar polymers, and good reusability,16 which may make them efficient catalysts for depolymerization of PC, such as methanolysis of PC to yield BPA and dimethylcarbonate (DMC). In our previous works,8,9 the methanolysis of PC catalyzed by [Bmim][Cl] (11.2 mol %) and [Bmim][Ac] (13.7 mol %) were first reported under mild conditions, while the processes needed a large amount of catalyst together acting as a solvent (Scheme 1). Quaranta et al.17 reported the alcoholysis of PC over a highly active 1,8diazabicyclo[5.4.0]undec-7-ene (DBU) organocatalyst. However, the use of DBU had some problems, for example, it was difficult to reuse and had unpleasant flavor during the operation, which limited its industrial application. Also the use of solid base CaO(SrO,BaO)-modified SBA-15 for heterogenetic methanolysis of PC were reported;18 the process still suffered from harsh reaction conditions (130 °C, 3 h, n(CH3OH)/n(PC) = 8:1) and an auxiliary solvent (THF) participation. Hence, it is of great significance to develop the efficient and green protocol for selective alcoholysis of PC waste. In order to overcome the above-mentioned problems, herein we provided a novel Lewis basic IL-based platform for catalytic depolymerization of PC to monomer BPA (Scheme 2). It showed interesting developments on the facile synthesis of catalysts, simple operation, and environmental friendliness (excluding the use of auxiliary metals and cosolvents). In the Received: June 6, 2018 Revised: August 19, 2018

A

DOI: 10.1021/acssuschemeng.8b02650 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering Scheme 1. Methanolysis of PC with Basic Ionic Liquids

thermometer. Typically, the autoclave was charged with PC (W1 g), CH3OH (5 equiv), and [HDBU][LAc] (0.80 mol %) successively at RT. Then it was heated to the required temperature and kept for 1.0 h. After that, the autoclave was cooled statically in an ice−water bath. The obtained mixture was filtered and washed with methanol to obtain the residual PC (W2 g). The filtrate was concentrated, and the residuum was extracted with ethyl acetate and deionized H2O. The BPA product (W3 g) and [HDBU][LAc] could be respectively obtained by rotary evaporation. The BPA structure was verified by FT-IR, TGA, and NMR technique. The separated [HDBU][LAc] catalyst was reused for the next run after drying under a vacuum (60 °C, 8 h). To support the reaction mechanism, the unreacted PC and BPA obtained at different reaction levels were analyzed and compared via FT-IR characterization. In addition to methanol, other alcohols were also similarly tested for PC degradation to recover monomer BPA. The conversion of PC and yield of BPA were calculated using the following equations:

Scheme 2. Structures of the Present ILs

present work, a series of Lewis basic ILs were one-step synthesized at room temperature and used for the depolymerization of PC waste to obtain the starting monomer BPA and alkyl carbonate. The catalyst structures were first screened, and the influence of reaction conditions on the activity was detailedly studied. The alcohol scopes and catalyst recyclability were also examined. Moreover, a possible mechanism for methanolysis of PC catalyzed by DBU-derived IL was provided based on the FT-IR and NMR analysis technique. The novel, highly efficient and environmentally benign catalysts show a great potential for recycling of PC waste to reduce the environmental impact.

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PC conversion =

BPA yield =

W1 − W2 × 100% W1

(1)

W3 MPC nBPA = nPC MBPA W1

(2)

where MPC and MBPA are the molar mass of PC unit and BPA.

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RESULTS AND DISCUSSION Catalyst Screening. With facile synthesis and adequate thermal stability (Figure S2), the different ILs were used for the methanolysis of PC, and the results obtained were summarized in Table 1. The blank experiment indicated a

EXPERIMENTAL SECTION

Synthesis of Lewis Basic ILs. According to the reported procedures,19 the [HTEA][LAc], [HMIM][LAc], [HMP][LAc], [HDBU][LAc], [HDBU][OAc], and [HDBU][PAc] were synthesized at room temperature via the one-step neutralization of corresponding Lewis bases and proton donors, respectively. In a typical experiment, superbase DBU (3.8 g, 25.0 mmol) was added to a 150 mL round-bottom flask with an ice−water bath, and lactic acid (2.6 g, 25.0 mmol) was slowly dripped with stirring. Then the neutralization reaction was kept at room temperature for 8.0 h. Finally, the product was dried under vacuum overnight to give a paleyellow oil. Yield: ≥98%. The typical characterization (see Supporting Information) of the ILs as-synthesized is described as follows. [HDBU][LAc]. 1H NMR (400 MHz, D2O), δ = 4.05 (m, 1H), 3.53− 3.47 (m, 2H), 3.45 (d, J = 5.9 Hz, 2H), 3.25 (t, J = 5.7 Hz, 2H), 2.60−2.51 (m, 2H), 2.00−1.87 (m, 2H), 1.74−1.53 (m, 7H), 1.26 (d, J = 7.0 Hz, 3H). FT-IR (KBr), γmax/cm−1: 2928−2859, 1729, 1640, 1595, 1448, 1320, 1199, 1104, 1079, 1028, 836, 766, 689. [HDBU][OAc]. 1H NMR (400 MHz, D2O), δ = 3.50−3.18 (m, 6H), 2.50 (d, J = 31.6 Hz, 2H), 1.92 (d, J = 5.1 Hz, 2H), 1.85−1.79 (m, 3H), 1.73−1.46 (m, 7H). FT-IR (KBr), γmax/cm−1: 2928−2859, 1640, 1557, 1448, 1384, 1320, 1199, 1104, 1079, 912, 689. [HDBU][PAc]. 1H NMR (400 MHz, D2O), δ = 3.38−3.07 (m, 6H), 2.38 (m, 2H), 2.02−1.90 (m, 2H), 1.83−1.67 (m, 2H), 1.47 (m, 7H), 0.84 (t, J = 7.7 Hz, 3H). FT-IR (KBr), γmax/cm−1: 2928−2859, 1640, 1550, 1448, 1384, 1320, 1199, 1104, 1079, 862, 689. [HMP][LAc]. 1H NMR (400 MHz, D2O), δ = 4.15−4.05 (m, 1H), 3.86 (m, 4H), 3.32−3.14 (m, 4H), 1.43−1.15 (m, 5H). FT-IR (KBr), γmax/cm−1: 2979−2865, 2482, 1448, 1729, 1104, 1028, 881, 836, 766, 651. Catalytic Alcoholysis of PC. The alcoholysis reaction was allowed to proceed in a stainless−steel autoclave (50 mL). The autoclave was equipped with a magnetic stirring bar and a

Table 1. Catalyst Screening for Methanolysis of PC under Solvent-Free Conditionsa entry

catalyst

1 2c 3e

blank DBU [Bmim] [OAc] [HMP][LAc] [HTEA] [LAc] [HMIM] [LAc] [HDBU] [LAc] [HDBU] [OAc] [HDBU] [PAc] [HTEA] [LAc] [HMIM] [LAc]

4 5 6 7 8 9 10 11

temp (°C)

time (h)

PC conversion (%)

BPA yield (%)b

120 120 120

1.0 1.0 1.0

100 35

d 34

120 120

1.0 1.0

7 14

10

120

1.0

95

90

120

1.0

100

99

120

1.0

100

98

120

1.0

100

98

130

3.0

100

95

120

2.0

100

96

a

Reaction conditions: n(PC) = 15.7 mmol, n(CH3OH)/n(PC) = 5:1, catalyst 0.80 mol % (of reactants). bIsolated yield. cSee ref 17. d Colloidal viscous products. eSee ref 9; [Bmim][OAc] represents 1butyl-3-methylimidazolium acetate. B


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ACS Sustainable Chemistry & Engineering Scheme 3. Proposed Reaction Mechanism for the [HDBU][LAc] Catalyzed Methanolysis of PC

the IL to form oxyanion intermediate, which became easier to react with PC through a nucleophilic attack of the ester groups in PC. Detailed illustration was shown in Scheme 3. It was worth mentioning that [HTEA][LAc] and [HMIM][LAc] could also afford satisfied results but at the expense of a higher temperature or a longer reaction time (entries 10 and 11). Effects of Reaction Parameters. The [HDBU][LAc] with super thermostability was selected as the catalyst to study the influence rules of different reaction parameters. Figure 1A showed the influence of temperature on the catalytic results. The PC conversion was increased with elevated temperature in the range of 100−120 °C, and the product yields were increased with the similar trend. Because PC dissolving or swelling is the rate-determining step for PC depolymerization, and the rigid PC is easier to dissolve or swell at the higher temperature.9 With a further increase of the temperature to 120−140 °C, comparable high BPA yields were obtained. From the practical standpoint of energy-saving, the temperature of 120 °C was desirable. Figure 1B depicted the influence of n(CH3OH)/n(PC) on the catalytic results. When the molar ratio of CH3OH to PC was 2:1, a moderate 85% BPA yield could be obtained with [HDBU][LAc] catalyst at 120 °C for 1.0 h. It smoothly reached to 98% when n(CH3OH)/n(PC) was increased to 5:1. However, no enhancement of the BPA yield was observed with further increasing of n(CH3OH)/n(PC) to 6:1. To our delight, the PC could be completely converted when the n(CH3OH)/ n(PC) was 3:1. To obtain a satisfied BPA yield, the suitable molar ratio of CH3OH/PC was 5:1 for the PC depolymerization.

critical role of the catalyst, and no conversion of PC was detected without a catalyst (entry 1). For comparison, previous reported superbase DBU and [Bmim][OAc] catalysts were examined under the same conditions (entries 2 and 3). The PC could be completely depolymerized over the DBU catalyst, while the separation of desired product was difficult and catalyst recycling was unattainable as the formation of BPA− DBU adduct in the system.17 The [Bmim][OAc] showed an unsatisfactory catalytic activity, it was ascribed to the inferior dissolving or swelling capacity to PC using only 0.80 mol % of catalyst amount, and previous work has suggested that the dissolving or swelling of PC was the rate-determining step.9 As to our present ILs herein, we first measured the acidity− basicity of the reaction system, as given in Table S1. The [HMP][LAc] showed the weakest activity due to the almost neutral medium (entry 4). The [HTEA][LAc] and [HMIM][LAc] with similarly weak acidities showed entirely different catalytic activities (entries 5 and 6), probably because a larger steric hindrance of [HTEA][LAc] affected the methanolysis of PC. The catalytic reaction could proceed smoothly with [HMIM][LAc] as catalyst, a satisfied yield to the desired product was obtained (entry 6). Notably, the DBU-based ILs exhibited more effective activities for the methanolysis of PC. The PC was completely depolymerized and excellent BPA yields were obtained under the chosen reaction conditions (entries 7−9), which could be attributed to the strong basicities of DBU-based ILs. As the methanolysis of PC is transesterification, and the basic medium may be more favorable to catalyze the reaction.9 It was speculated that the proton of methanol could be attracted due to the basicity of C


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ACS Sustainable Chemistry & Engineering

Figure 1. Influence rules of reaction parameters of (A) temperature, (B) n(CH3OH)/n(PC), (C) catalyst loading, and (D) reaction time on the methanolysis of PC. Standard conditions: PC 15.7 mmol, n(CH3OH)/n(PC) = 5:1, [HDBU][LAc] 0.79 mmol (0.8 mol % of reactants), 120 °C, 1.0 h.

Table 2. Comparison of Activity among Various Catalysts in the Methanolysis of PC catalyst

temp (°C)

n(CH3OH)/n(PC)

cat. loading (mol %)a

time (h)

PC conversion (%)

BPA yield (%)

[Bmim][Cl] [Amim][Cl]b [Bmim][OAc] [Bmim][OAc] DBU CaO/SBA-15 NaOHe [HDBU][LAc] [HDBU][LAc]

105 120 90 120 100 130 150 90 120

12:1 5:1 6:1 5:1 38:1 8:1

11.2 0.8 13.7 0.8 5.2 33.3d

5:1 5:1

13.7 0.8

2.5 1.0 2.5 1.0 1.6 3.0 1.0 2.5 1.0

100 23 100 35 100 100 100 100 100

95 21 95 34 c 96 90 98 99

ref 8 9 17 18 20 this work this work

a

Amount of catalyst to reactants. b[Amim][Cl] represents 1-allyl-3-methylimidazolium chloride. cPure BPA was not obtained; the product was the BPA−DBU adduct. dMass ratio of CaO/SBA-15 to PC. eThe chemical recycling of PC performed in a semicontinuous lab-plant under supercritical or near critical condition.

The influence of catalyst loading was also explored on the methanolysis of PC (Figure 1C). With only 0.15 mol % of [HDBU][LAc] catalyst, it could afford a moderate 63% PC conversion under selected conditions, indicating excellent catalytic activity of [HDBU][LAc]. When the catalyst loading was increased to 0.80 mol %, the PC could be completely depolymerized with 98% BPA yield. The result was ascribed to the facile dissolving or swelling of PC in the system with increasing the amount of [HDBU][LAc]. When the [HDBU][LAc] loading was further increased, there was no obvious change for the BPA yield. Taking account of process economy, a 0.8 mol % catalyst loading was selected. Furthermore, the influence of reaction time on the PC methanolysis was

examined, as shown in Figure 1D. The process of PC depolymerization went remarkably with time, the PC could efficiently complete its conversion to give a 98% BPA yield within 1.0 h, and there was no great significance to further prolong the reaction time. The above-mentioned results demonstrated that [HDBU][LAc] could realize the efficient methanolysis of PC under moderate conditions. Table 2 compared the results of PC methanolysis with the reported catalysts. It noted that the present [HDBU][LAc] exhibited the superior or comparable catalytic performance to the previously reported ILs ([Bmim][Cl], [Bmim][Ac], and [Amim][Cl]),8,9 organic superbase (DBU),17 and solid base (CaO/SBA-15)18 catalysts under the D


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product was obtained under the comparable conditions, and only oligomers could be obtained (entries 4 and 6), as the isomerous alcohols with the larger steric hindrance could hardly carry out the nucleophilic attack at the CO groups of PC, indicating that the steric hindrance of alcohols showed an obvious impact on the alcoholysis of PC. Reusability of [HDBU][LAc] in Methanolysis of PC. To evaluate the reusability of the [HDBU][LAc] catalyst, the methanolysis of PC was repeated. No appreciable loss of catalytic activity was observed even after six cycles (Figure 2A). Compared with the organic base DBU catalyst reported by Quaranta,17 [HDBU][LAc] showed great advances in catalyst separation and product purification with the comparable catalytic activity. To better support the experimental results, the recycled [HDBU][LAc] was characterized thoroughly by using TGA, FT-IR, and 1HNMR techniques (Figure 2B−D). Compared with the fresh [HDBU][LAc], the recycled catalyst retained the super thermostability as well as all the characteristic peaks, indicating the pristine form and excellent reusability of [HDBU][LAc]. Plausible Reaction Mechanism. To explore the reaction mechanism, the structures of separated PC residues at different conversions and obtained BPA product at different yields were characterized by FT-IR technique, respectively (Figure 3). The structure of PC residue obtained at a low conversion of 17% was identical with the pure PC pellets, and there were scarcely any oligomers in the residue. With an increase of PC conversion, a hydroxyl peak at 3400−3500 cm−1 gradually appeared, and the carbonyl peak at about 1760 cm−1 obviously

similar conditions. Moreover, the protocol avoided the use of metals, halides, and solvents in the reaction system. The exceptional advantages of our catalyst are of great significance in the practical methanolysis of PC. Alcohol Scopes. The PC depolymerization with a series of substituted alcohols were examined by using [HDBU][LAc] catalyst, the results were listed in Table 3. For the n-alkanol, Table 3. Alcohol Scopes for PC Degradation Catalyzed by [HDBU][LAc]a entry 1 2 3 4 5 6

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

temp (°C)

time (h)

PC conversion (%)

BPA yield (%)b

120 120 120 120 120

1.0 3.0 3.0 3.0 3.0

100 79 55 30 43

99 65 46 c 29

120

3.0

21

c

a Conditions: n(PC) = 15.7 mmol, n(alcohol)/n(PC) = 5:1, [HDBU][LAc] 0.80 mol %. bIsolated yield. cOligomers.

the alcoholysis rates of PC were gradually decreased when the alkyl chains were increased (entries 1−3 and 5), as which the diaryl-carbonate moieties of PC became more difficult to cleave. The results were consistent with the previously reported DBU catalyst for PC alcoholysis.17 For the isomerous alcohols, it was observed that only oligomers without pure BPA

Figure 2. (A) Catalytic recyclability, (B) TGA curve, (C) FT-IR, and (D) 1H NMR spectra of [HDBU][LAc] before and after six reaction runs. Conditions: n(PC) = 15.7 mmol, n(CH3OH)/n(PC) = 5:1, [HDBU][LAc] 0.8 mol %, 120 °C, 1.0 h. E


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Figure 3. FT-IR spectra of (A) PC residues and (B) BPA product. (C) TGA curves of BPA product at different yields.

Figure 4. (A) FT-IR and (B) 1H NMR spectra of the interaction between CH3OH and [HDBU][LAc] with various concentration at 298 K.

shown in Scheme 3. Initially, the PC is activated by dissolving or swelling in the reaction system. Simultaneously, the anions of basic [HDBU][LAc] undergo the electrostatic interactions with hydrogen proton of methanol according to the conjugated acid−base theory; with the additional hydrogen bonds formed between O atom of [HDBU][LAc] and −OH group in methanol, the methanol molecules are also activated to generate oxyanion intermediates. To determine the interaction between the methanol and catalyst, the FT-IR and 1HNMR studies were carried out (Figure 4).21,22 With an increase of the catalyst concentration in methanol, the −OH vibration band became broader and shifted from 3330 to 3236 cm−1 (Figure 4A). This frequency shift (94 cm−1) demonstrated the activation of methanol by [HDBU][LAc]. Moreover, the

enhanced (Figure 3A), which indicates the concomitant formation of oligomers in the reaction progress (Scheme 3). The monitored oligomers in PC residue were attributed to the fragmentation of partial insoluble PC through the attack of methanol molecules. By analyzing the BPA products obtained at different yields (Figure 3B−C), it was observed that they showed the same structure with the pure BPA sample at either a moderate 75% or a high 98% BPA yield. On the basis of our previous works,8,9,18 herein it demonstrated that there were no oligomers within the separated BPA product when a minor amount of catalyst was used. This further evidenced the high activity of [HDBU][LAc]. According to the obtained results and previous literature,8−12 a plausible [HDBU][LAc] catalyzed mechanism is F


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(J18KA065), and the Scientific Research Foundation of Qingdao University of Science and Technology (0100229019).

interaction between the methanol and [HDBU][LAc] was also proved by 1H NMR spectra (Figure 4B). A downfield shift of the methanol −OH proton signal appeared after mixing with [HDBU][LAc], and with the increase of [HDBU][LAc] concentration in CH3OH, the −OH proton signal shifted from 2.66 to 2.80 ppm. It further indicated the activation of methanol by [HDBU][LAc], and the result was consistent with our above studies that the catalytic activity was improved with an increase of catalyst concentration. Next, the activated methanol moleculars (oxyanion intermediates) can bond the ester groups in PC through nucleophilic attack to form the soluble oligomers and smaller oligomeric species. The oligomers further react with the activated methanol, and in this way, it can generate the monomer BPA and DMC.

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(1) Ignatyev, I. A.; Thielemans, W.; Vander Beke, B. Recycling of polymers: A review. ChemSusChem 2014, 7, 1579−1593. (2) Wilks, E. S., Ed. Industrial Polymer Handbook, Polymerization Processes Synthetic Polymers; Wiley-VCH, 2001, Vol. 1. (3) Chandrasekaran, S. R.; Avasarala, S.; Murali, D.; Rajagopalan, N.; Sharma, B. K. Materials and energy recovery from e-waste plastics. ACS Sustainable Chem. Eng. 2018, 6, 4594−4602. (4) Achilias, D. S.; Antonakou, E. V.; Koutsokosta, E.; Lappas, A. A. Chemical recycling of polymers from waste electric and electronic equipment. J. Appl. Polym. Sci. 2009, 114, 212−221. (5) Song, X. Y.; Liu, F. S.; Li, L.; Yang, X. Q.; Yu, S. T.; Ge, X. P. Hydrolysis of polycarbonate catalyzed by ionic liquid [Bmim][Ac]. J. Hazard. Mater. 2013, 244−245, 204−208. (6) Pellis, A.; Gamerith, C.; Ghazaryan, G.; Ortner, A.; Acero, E. H.; Guebitz, G. M. Ultrasound- enhanced enzymatic hydrolysis of poly(ethylene terephthalate). Bioresour. Technol. 2016, 218, 1298− 1302. (7) 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. (8) 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. (9) 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. (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) Sun, J.; Liu, D. J.; Young, R. P.; Cruz, A. G.; Isern, N. G.; Schuerg, T.; Cort, J. R.; Simmons, B. A.; Singh, S. Solubilization and upgrading of high polyethylene terephthalate loadings in a low-costing bifunctional ionic liquid. ChemSusChem 2018, 11, 781−792. (12) Wang, Q.; Geng, Y. R.; Lu, X. M.; Zhang, S. J. First-row transition metal-containing ionic liquids as highly active catalysts for the glycolysis of poly(ethylene terephthalate) (PET). ACS Sustainable Chem. Eng. 2015, 3, 340−348. (13) Bartolome, L.; Imran, M.; Lee, K. G.; Sangalang, A.; Ahn, J. K.; Kim, D. H. Superparamagnetic γ-Fe2O3 nanoparticles as an easily recoverable catalyst for the chemical recycling of PET. Green Chem. 2014, 16, 279−286. (14) Quaranta, E. Rare Earth metal triflates M(O3SCF3)3 (M = Sc, Yb, La) as Lewis acid catalysts of depolymerization of poly-(bisphenol A carbonate) via hydrolytic cleavage of carbonate moiety: Catalytic activity of La(O3SCF3)3. Appl. Catal., B 2017, 206, 233−241. (15) Hong, M.; Chen, E. Y. Chemically recyclable polymers: a circular economy approach to sustainability. Green Chem. 2017, 19, 3692−3706. (16) Olivier-Bourbigou, H.; Magna, L.; Morvan, D. Ionic liquids and catalysis: Recent progress from knowledge to applications. Appl. Catal., A 2010, 373, 1−56. (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) 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. (19) Wu, C. L.; Zhang, H. Y.; Yu, B.; Chen, Y.; Ke, Z. G.; Guo, S. E.; Liu, Z. M. Lactate-based ionic liquid catalyzed reductive amination/

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CONCLUSIONS Several ionic liquids with different Lewis acidity−basicity are facilely synthesized via one-step neutralization and used for efficient catalyzing the alcoholysis of polycarbonate. The catalysis reactions can proceed under mild conditions without transition metals and volatile solvents. The methanolysis of polycarbonate with 99% BPA yield can be achieved using only 0.8 mol % [HDBU][LAc] catalyst, which shows excellent stability and can be reused at least six cycles. We further evidenced the activation of the substrate (CH3OH molecular) by the DBU-based IL and proposed the feasible Lewis base catalytic mechanism via the FT-IR and NMR analysis. The ionic liquid-based platform exhibits notable advantages of environmental friendliness, facile synthesis, and high-activity of the catalyst. The work reported here also will enhance the understanding of the design of materials for efficient alcoholysis of waste polyester.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b02650. FT-IR spectra, TGA curves, and NMR spectra of assynthesized DBU-based ionic liquids and BPA product; determination of ILs basicity in CH3OH (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 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 (51673106), the Science and Technology Research Project of Shandong Province (2016GSF116005), the Taishan Scholars Projects of Shandong (ts201511033), the Natural Science Foundation of Shandong Province (ZR2018BB009), a Project of Shandong Province Higher Educational Science and Technology Program G


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ACS Sustainable Chemistry & Engineering cyclization of keto acids under mild conditions: a metal-free route to synthesize lactams. ACS Catal. 2017, 7, 7772−7776. (20) Piñero, R.; García, J.; Cocero, M. J. Chemical recycling of polycarbonate in a semi-continuous lab-plant. A green route with methanol and methanol−water mixtures. Green Chem. 2005, 7, 380− 387. (21) Anthofer, M. H.; Wilhelm, M. E.; Cokoja, M.; Markovits, I. I. E.; Pöthig, A.; Mink, J.; Herrmann, W. A.; Kühn, F. E. Cycloaddition of CO2 and epoxides catalyzed by imidazolium bromides under mild conditions: influence of the cation on catalyst activity. Catal. Sci. Technol. 2014, 4, 1749−1758. (22) Wang, J. Q.; Zhang, Y. G. Boronic acids as hydrogen bond donor catalysts for efficient conversion of CO2 into organic carbonate in water. ACS Catal. 2016, 6, 4871−4876.

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Pushing the Limits in Alcoholysis of Waste Polycarbonate with DBU

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