Versatile Imidazole-Anion-Derived Ionic Liquids ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

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

Versatile Imidazole-Anion-Derived Ionic Liquids with Unparalleled Activity for Alcoholysis of Polyester Wastes under Mild and Green Conditions Mengshuai Liu,† Jiao Guo,† Yongqiang Gu,† 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

‡

ACS Sustainable Chem. Eng. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 10/06/18. For personal use only.

S Supporting Information *

ABSTRACT: Polyester wastes have caused a series of environmental issues, and it has became imperative to promote recycling of polyester wastes. Herein, a novel protocol for polyester depolymerization to recover the corresponding monomers or chemicals catalyzed by imidazole-anion-derived ionic liquids (ILs) was developed. The catalytic behavior, catalyst recyclability, and versatility for alcoholysis of polyester were studied in detail under mild and green conditions. A comparison of the activity with the reported catalysts was provided, which indicated the unparalleled activity of [HDBU][Im] for the polyester alcoholysis. An in-depth study of the feasible alcoholysis mechanism was given assisting with in situ analysis technique. The developed protocol realized the highly efficient chemical recycling of polyesters via alcoholysis method with unparalleled activity, avoided the pollution caused by toxic transition metal ions and organic solvents, indicating a green and promising alternative for practical recycling of polyester wastes. KEYWORDS: Alcoholysis, Polyester, Ionic liquids, Lewis base catalysis, Mechanism

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corrosion and environmental problems.8−10 Glycolysis was mostly aimed at depolymerization of PET to obtain bis(hydroxyethyl) terephthalate (BHET), while the reported catalysts have deficiencies of transition metal containing or low activity.19 By contrast, the alcoholysis of polyester wastes represents great potential for development, since it can not only obtain the corresponding monomer as raw material for polyester reproduction, but also obtain valuable carbonates or other chemicals, as shown in Scheme 1. With respect to alcoholysis of polyester, some catalysts have been reported. Kim20 and Quaranta et al.21 recently reported the alcoholysis of PC by using organic-bases 1,5,7-triazabicyclo[4.4.0]-dec-5-ene (TBD) and 1,8-diazabicyclo[5.4.0] undec-7-ene (DBU). The organic bases can show excellent activity under mild conditions; however, organic solvents (such as DMC) are usually required. In addition, the superbase has inherent drawbacks including unpleasant flavor, equipment corrosion, and the difficulty of separation and recovery, which limit their industrial application. The state-of-the art technology for alcoholysis of polyester wastes concentrates on ionic liquid (IL) catalysis. Liu et al.14,15 first reported methanolysis of PC over [Bmim][Cl] and [Bmim][Ac], the process could perform under moderate

INTRODUCTION Polyester materials such as poly(bisphenol A) carbonate (PC), poly(lactic acid) (PLA), and poly(ethylene terephthalate) (PET) derived from fossil resources have been widely used, ranging from construction industry to electronics and electrical appliances due to their excellent durability, transparency, and mechanical properties.1 However, these uses have made millions tons of waste polyester plastics generated each year, which results in recycling of waste plastics being of the utmost importance from viewpoints of both “sustainable society” and “green chemistry”.2,3 The traditional methods for processing waste polyester materials were landfill disposal, physical recycling, and energy recovery.4 These gave rise to environmental pollution and caused the waste of resources; however, a drop in mechanical and physical properties is always observed.5 Alternatively, chemical depolymerization of waste polyester materials raised great attention, as it could convert waste polyester into their starting monomers or other valuable chemicals for virgin plastic reproduction.6 So far the chemical recycling methods for depolymerization of polyester materials included pyrolysis,7,8 hydrolysis,9−11 aminolysis,12 alcoholysis,13−16 and glycolysis.17,18 Among them, pyrolysis suffered from low monomer selectivity and the release of greenhouse gases; hydrolysis and aminolysis are usually performed under harsh reaction conditions (high pressure and temperature) and in the presence of large quantities of concentrated acids or bases, which caused equipment © XXXX American Chemical Society

Received: July 24, 2018 Revised: September 9, 2018 Published: September 21, 2018 A

DOI: 10.1021/acssuschemeng.8b03591 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering Scheme 1. Alcoholysis of Polyester Wastes (PC, PLA, PHB, and PET)

conditions (105 or 90 °C) and needed large amounts of IL together acting as solvent. Also, Casiello22 and Song et al.16 respectively studied the bifunctional acid/basic IL/ZnO (NBu4Cl/ZnO) nanoparticles and Brønsted−Lewis acidic 1-(3-sulfonic acid)-propyl-3-methylimidazole ferric chloride as recyclable catalysts for PC and PHB depolymerization with alcohols, while the reactions suffered from harsh reaction conditions and transition metal participation. Hence, it is still a great necessity to develop efficient and green protocols for alcoholysis of polyester wastes under the milder solvent- and metal-free conditions. Taking these into account, herein, this work described for the first time the use of ILs with various imidazole-anions and DBU cation (Scheme 2), as versatile and attractive catalysts in

imidazole anions modified with different substitutions were one-step synthesized under ambient conditions. Typically, 20.0 mmol of DBU was added to a 50 mL round-bottomed flask; equimolar imidazole was slowly introduced with stirring. Then, the neutralization reaction was allowed to proceed at ambient temperature for 5.0 h to obtain the pale-yellow [HDBU][Im]. Yield: ≥98%. Similarly, the imidazoleanion-derived ILs [HDBU][2-MeIm], [HDBU][4-MeIm], [HDBU][2-iPrIm], and [HDBU][2-PhIm] were synthesized. The FT-IR, 1H NMR, and HRMS spectra, and TGA curves of the synthesized ILs were recorded (see Figures S1−S7), and the characterization results were listed as follows. [HDBU][Im]. 1H NMR (400 MHz, CDCl3), δ = 7.62 (s, 1H), 7.04 (d, J = 6.3 Hz, 2H), 3.25 (t, J = 5.5 Hz, 2H), 3.22−3.11 (m, 4H), 2.37 (s, 2H), 1.84−1.68 (m, 2H), 1.61 (dd, J = 14.5, 4.8 Hz, 4H), 1.55 (s, 2H). FT-IR (KBr), γmax/cm−1: 3107−3019, 2925−2849, 1610, 1490−1440, 1364−1314, 1239−1188, 1101, 1062, 818, 742, 660. HRMS (ESI, pos. mode): m/z calcd 153.1386 (for [HDBU]+), found 153.1384 [M − Im−]. C12H20N4 (220.1688). [HDBU][2-MeIm]. 1H NMR (400 MHz, CDCl3), δ = 6.88 (d, J = 4.3 Hz, 2H), 3.32−3.07 (m, 6H), 2.41−2.23 (m, 5H), 1.85−1.69 (m, 2H), 1.60 (dd, J = 13.4, 5.0 Hz, 4H), 1.57−1.45 (m, 2H). FT-IR (KBr), γmax/cm−1: 3151−3051, 2925−2849, 1610, 1572, 1490−1440, 1364−1314, 1239−1188, 1107, 1062, 987−956, 736, 673. HRMS (ESI, pos. mode): m/z calcd 153.1386 (for [HDBU]+), found 153.1383 [M − 2-MeIm−]. C13H22N4 (234.1844). [HDBU][4-MeIm]. 1H NMR (400 MHz, CDCl3), δ = 7.47 (s, 1H), 6.79−6.57 (m, 1H), 3.27−3.21 (m, 2H), 3.21−3.10 (m, 4H), 2.40− 2.28 (m, 2H), 2.22 (s, 3H), 1.81−1.72 (m, 2H), 1.61 (t, J = 5.8 Hz, 4H), 1.54 (s, 2H). FT-IR (KBr), γmax/cm−1: 3082, 2925−2849, 1610, 1490−1440, 1364−1314, 1239−1188, 1107, 1062, 987−918, 818, 749−666. HRMS (ESI, pos. mode): m/z calcd 153.1386 (for [HDBU]+), found 153.1383 [M − 4-MeIm−]. C13H22N4 (234.1844). [HDBU][2-iPrIm]. 1H NMR (400 MHz, CDCl3), δ = 6.88 (d, J = 4.1 Hz, 2H), 3.27−3.11 (m, 6H), 3.10−2.99 (m, 1H), 2.35 (d, J = 1.9 Hz, 2H), 1.82−1.69 (m, 2H), 1.61 (d, J = 2.8 Hz, 4H), 1.57−1.49 (m, 2H), 1.33−1.24 (m, 6H). FT-IR (KBr), γmax/cm−1: 3138−3038, 2925−2849, 1610, 1566, 1490−1440, 1364−1314, 1239−1188, 1107, 1062, 962, 723. HRMS (ESI, pos. mode): m/z calcd 153.1386 (for [HDBU]+), found 153.1384 [M − 2-iPrIm−]. C15H26N4 (262.2157). [HDBU][2-PhIm]. 1H NMR (500 MHz, CDCl3), δ = 8.12−7.81 (m, 2H), 7.35−7.27 (m, 2H), 7.25−7.20 (m, 1H), 7.16−6.99 (m, 2H), 3.31−3.02 (m, 6H), 2.43−2.26 (m, 2H), 1.87−1.68 (m, 2H), 1.64− 1.51 (m, 6H). FT-IR (KBr), γmax/cm−1: 3051, 2925−2849, 1610, 1560, 1490−1440, 1364−1314, 1239−1188, 1107, 1062, 987−918, 704. HRMS (ESI, pos. mode): m/z calcd 153.1386 (for [HDBU]+), found 153.1383 [M − 2-PhIm−]. C18H24N4 (296.2001).

Scheme 2. Structure of Imidazole-Anion-Derived ILs

the alcoholysis of polyester wastes. The relation between catalyst structure and catalytic activity, the effects of reaction conditions, and alcohol scopes on the activity were thoroughly investigated and compared. The versatility for methanolysis of different polyester materials and the reusability for the model methanolysis of PC catalyzed by [HDBU][Im] were also examined. To further enrich the understanding about the behavior of imidazole-anion-derived ILs on alcoholysis of polyester, an in-depth study of the feasible alcoholysis mechanism was given by in situ FTIR and NMR technique. Compared with the reported catalysts, the imidazole-anion-derived ILs are facilely synthesized, and they present the higherefficiency and greater advance for polyester depolymerization under mild and green conditions, which make them an interesting candidate for practically reducing the environmental impact of polyester wastes.

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EXPERIMENTAL SECTION

Synthesis of Imidazole-Anion-Derived ILs. According to the reported procedure,23 several ILs composed of [HDBU] cation and B


Research Article

ACS Sustainable Chemistry & Engineering Table 1. Model Methanolysis of PC Catalyzed by Different IL Catalystsa entry

catalyst

temp. (°C)

cat. loading (mol %)b

time (h)

PC depolymerization (%)c

BPA yield (%)d

1 2e 3g 4h 5 6 7 8 9 10 11

blank DBU [Bmim]Cl [Bmim][OAc] [HDBU][OAc] [HDBU][Im] [HDBU][Im] [HDBU][2-MeIm] [HDBU][4-MeIm] [HDBU][2-iPrIm] [HDBU][2-PhIm]

70 100 105 90 100 70 70 70 70 70 70

none 5.2 11.2 13.7 1.6 1.6 1.6 1.6 1.6 1.6 1.6

1.0 1.6 2.5 2.5 2.0 1.0 2.0 1.0 1.0 1.0 1.0

trace 100 100 100 59 100 100 95 96 90 99

90f 95 95 46 90 96 87 88 83 89

a Reaction conditions: PC 15.7 mmol, n(CH3OH)/n(PC) = 5:1. bAmount of catalyst to reactants (PC and methanol). cCalculated according to the formula as shown in Supporting Information. dIsolated yield. eSee ref 21. fCrude product with 4-cumylphenol. gSee ref 14; [Bmim]Cl represents 1-butyl-3-methylimidazolium chloride. hSee ref 15; [Bmim][OAc] represents 1-butyl-3-methylimidazolium acetate.

Chemical Recycling of Plastic Wastes. The experimental method for chemical recycling of plastic wastes herein was via the alcoholysis strategy. The detailed descriptions for catalytic alcoholysis of polycarbonate (PC), poly(lactic acid) (PLA), poly(β-hydroxybutytrate) (PHB), and poly(ethylene terephthalate) (PET) were provided in the Supporting Information.

[HDBU][Im] could obtain the comparable activity under the milder and greener reaction conditions (70 °C, 1.6 mol % for 1.0 h); [HDBU][Im] showed the better stability than that of DBU and could be easily separated from the product. The other imidazole-anion-based ILs with derived structures also showed satisfied activities under the same conditions; this trend was caused by the approximate basicity of ILs as shown in Table S2. In this regard, the developed novel catalysts show great potential for further investigation. Effects of Reaction Parameters. Using [HDBU][Im] as catalyst, the influence of different reaction parameters on the activity was evaluated. As shown in Figure 1A, it was favorable for PC degradation to obtain the higher BPA yield at a higher temperature. With increasing temperature from 50 to 70 °C, the PC depolymerization and BPA yield were sharply improved due to the favorable dissolution or swelling of PC. When the temperature was further raised, it was insignificant, and comparable high BPA yields of 95−96% were obtained at 80−90 °C. To realize the energy-saving production, the temperature of 70 °C was optimal for this reaction. Figure 1B depicted the effect of n(CH3OH)/n(PC) on the catalysis reaction. [HDBU][Im] could afford 94% PC depolymerization and 77% BPA yield at 70 °C with a theoretical n(CH3OH)/n(PC) of 2:1. When the CH3OH amount was increased from n(CH3OH)/n(PC) = 2:1 to 5:1, it was favorable from the viewpoint of dynamics to obtain 96% BPA yield. While further increasing the CH3OH/PC molar ratio was unfavorable, there appeared to be an obvious decrease for BPA yield, which was attributed to the dilution effect of CH3OH to [HDBU][Im]. To obtain higher BPA yield, the CH3OH/PC molar ratio of 5:1 was optimal for this reaction. The catalyst loading gave an unusual change in PC degradation results (Figure 1C). Initially, a moderate 84% PC depolymerization with 77% BPA yield was obtained with only 0.4 mol % of [HDBU][Im], and the PC depolymerization and BPA yield were gradually improved with increasing of [HDBU][Im] amount in the range of 0.4−1.6 mol %, indicating the superior activity of [HDBU][Im]. While unexpected results were obtained when we further increased the catalyst loading higher than 1.6 mol %, and a significant decrease of BPA yield was observed. According to a previous report,21 a possible explanation was that the excess [HDBU][Im] as super Lewis base interacted with partial acidic BPA product to form [HDBU][Im]−BPA adduct, which was leached in the catalyst

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RESULTS AND DISCUSSION Catalyst Screening. The catalytic activities of various imidazole-anion-derived ILs were investigated for PC methanolysis to produce monomer BPA as a model reaction, and the results were given in Table 1. No PC depolymerization and BPA yield were observed without a catalyst, it proved the necessity of the catalyst in this reaction (entry 1). The reported catalysts, such as superbase DBU, [Bmim]Cl, and [Bmim][OAc] could catalyze the PC depolymerization, while they needed a large amount of catalyst loading and high reaction temperature (entries 2−4). Notably, DBU could depolymerize the PC efficiently, but the separation of BPA monomer was difficult and catalyst recycling unattainable due to the formation of BPA−DBU adduct in the reaction mixture.21 The [HDBU][OAc] with CH3COO− anion showed poor activity compared to that of others due to the weaker basicity (entry 5).24 To our delight, the presented novel ILs with imidazole anions herein afford excellent activities under the milder conditions (entries 6−11), and especially for [HDBU][Im], 100% PC depolymerization with 90% desired BPA yield was obtained using only 1.6 mol % of catalyst loading at 70 °C, n(CH3OH)/ n(PC) = 5:1 for 1.0 h (entry 6). The reaction conditions were distinctly improved on the premise of comparable catalytic effect. It could be explained that with the stronger basicity of the imidazole-anion-derived ILs which could attract the proton of alcohol to form highly active oxygen anions the oxygen anions became more inclined to attack at the ester groups of PC facilitating the depolymerization of PC. More interesting works have demonstrated the mechanism for the imidazoleanion-derived ILs catalyzed reaction (Scheme 3). The as-obtained BPA product was monitored by TLC and melting point (158−160 °C, pure BPA: 158−159 °C), then further characterized by FT-IR, TGA, and 1H NMR, as shown in Figures S8 and 3. All the results verified the high-purity of the BPA, and no oligomers existed in the product. Detailed conditions and comparison for the activity were given in Table S1. It demonstrated the superior catalytic performance of [HDBU][Im] compared to that of the reported catalysts for methanolysis of PC. Compared with the volatile DBU catalyst, C


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ACS Sustainable Chemistry & Engineering Scheme 3. Proposed Mechanism for Alcoholysis of PC

attack of the oxyanion, or because the new species (intermediate) formed by nucleophilic attack could hardly keep stable. The detailed illustration also has been provided later in Scheme 3. Reusability of [HDBU][Im]. The catalyst reusability was examined at 70 °C for 2.0 h with the recycled [HDBU][Im]. As shown in Figure 2A, there was no significant loss of activity even after six runs, indicating the durable high-activity of [HDBU][Im] for the depolymerization of PC. Moreover, the [HDBU][Im] was characterized via FT-IR to contrast the structural changes before and after reuse (Figure 2B). The structure of recycled [HDBU][Im] was essentially similar to its pristine form, only a slight absorption band at 1715 cm−1 was observed, which might correspond to the asymmetric CO vibration of residual oligomer, while it did not obviously affect the catalytic activity. Compared with the organic base DBU reported by Quaranta,21 the developed [HDBU][Im] showed great advance in term of activity, catalyst separation and product purification under mild conditions. Plausible Reaction Mechanism. To support the reaction mechanism, an in situ FT-IR method was adopted to monitor the reaction process (Figure 3A,B). All the structures of PC residue were identical with pure PC pellets, and no hydroxyl peak appeared at 3400−3500 cm−1, indicating that no oligomer existed in the PC residue. Similarly, the structure of the as-obtained BPA was almost the same as pure BPA at a high 94.4% or a moderate 67.6% BPA yield (Figure 3B−D). The

separation process. On the basis of the results, 1.6 mol % of catalyst loading was suitable for the catalysis reaction. Furthermore, the changes of the PC depolymerization and BPA yield with time were examined over [HDBU][Im] catalyst (Figure 1D). The degradation of PC went smoothly with time, and it was completed within 2.0 h, a 100% PC depolymerization with over 96% BPA yield was obtained, respectively. Further prolonging the reaction time was insignificant. The obtained BPA was characterized by using 1H NMR and TGA technique, as shown in Figure 3, which showed the same structure with the pure BPA. Alcohol Scopes. Different substituted alcohols were examined for PC depolymerization over [HDBU][Im] catalyst. As shown in Table 2, the alcohol structures could obviously affect the depolymerization of PC. As to the n-alkanol, the depolymerization rate could be hindered with increasing the alkyl chain of alcohol (entries 1−3 and 5). For the isomerous alcohols, a more negative effect on catalytic activity was observed due to the larger steric hindrance. And only moderate PC depolymerization and BPA yield were obtained under the same conditions (entries 4 and 6). To our knowledge, the alcoholysis reaction was essentially occurred through nucleophilic attack, which occurred between the oxyanion from the activated alcohol and the CO group in PC under alkaline environment. The lower product yield might because that the alcohol with larger steric hindrance impeded the nucleophilic D


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ACS Sustainable Chemistry & Engineering

Figure 1. Effects of different reaction parameters on PC methanolysis: (A) temperature, (B) n(CH3OH)/n(PC), (C) catalyst loading, (D) reaction time. Conditions: PC 15.7 mmol, n(CH3OH)/n(PC) = 5:1, [HDBU][Im] 1.6 mol %, 70 °C, 2.0 h.

distinct 210 cm−1 blueshift from 3315 to 3105 cm−1. Even more interesting, the −OH vibration band was nearly disappeared (Figure 4A). These indicated that the CH3OH could be wellactivated by [HDBU][Im] to form highly active oxyanion, enhancing the ability to interact with the ester group in PC. Furthermore, the activation of CH3OH was proved by 1H NMR, as shown in Figure 4B. The −OH proton signal in CH3OH showed a clear downfield shift from 2.67 to 4.37 ppm after mixing with [HDBU][Im]; this further evidenced the interaction between CH3OH and [HDBU][Im], which could promote the nucleophilic attack of activated CH3OH to PC and played an important role in the PC alcoholysis process. On the basis of the above analysis, a plausible alcoholysis mechanism over [HDBU][Im] catalyst was provided, as shown in Scheme 3. Initially, the PC was partially dissolved or swelled in the reaction medium, and the alcohol was activated by forming hydrogen bond with [HDBU][Im] as mentioned earlier in Figure 4. Simultaneously, the cation [HDBU]+ underwent an electrostatic interaction with the oxygen of carboxyl groups in PC, which was inclined to form the carbonyl carbocation through electron transfer. Then the activated alcohol (oxyanion) that stripped hydrogen proton became more favorable to conduct the nucleophilic attack at the carbonyl carbocation of PC. This process resulted in fragmentation or disconnection of the long molecule chain of PC into the smaller oligomers. Similarly, the oligomers were further attacked by the activated alcohol to afford the BPA monomer

Table 2. Alcohol Scopes for PC Depolymerization Catalyzed by [HDBU][Im]a

entry

alcohol

temp. (°C)

time (h)

PC depolymerization (%)

BPA yield (%)b

1

methanol

70

2.0

100

96

2

ethanol

70

2.0

98

85

3

n-propanol

70

2.0

92

78

4

iso-propanol

70

2.0

65

53c

5

n-butyl alcohol

70

2.0

91

73c

6

isobutyl alcohol

70

2.0

62

41c

a Conditions: PC 15.7 mmol, n(alcohol)/n(PC) = 5:1, [HDBU][Im] 1.6 mol %. bIsolated yield. cThe yield was obtained after recrystallization with toluene.

dissolved PC could be rapidly depolymerized into the final BPA monomer and DMC under the selected conditions, and no oligomeric intermediates were observed in the product when using highly active [HDBU][Im] as catalyst. The above results further evidenced that the dissolving or swelling of PC was the rate-determining step which was consistent with the previous reports.14,25 Besides, the interaction between the reactant (e.g., CH3OH) and [HDBU][Im] was further studied by in situ FT-IR and NMR techniques (Figure 4). When the [HDBU][Im] concentration in CH3OH increased, FT-IR showed that the −OH vibration of CH3OH became broader; there appeared a E


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Figure 2. (A) Catalytic recyclability and (B) FT-IR characterization of [HDBU][Im] catalyst. Conditions: PC 15.7 mmol, n(CH3OH)/n(PC) = 5:1, [HDBU][Im] 1.6 mol %, 70 °C, 2.0 h.

Figure 3. In situ FT-IR spectra of (A) PC residue and (B) BPA product obtained at different extent of reaction; (C) TGA curves and (D) 1H NMR spectra of BPA product at 94.4% yields.

depolymerization (entry 1 vs 2, and 3 vs 4). With respect to PHB, the alcoholysis might become more easier in the acidmediated system (entry 5 vs 6), and the relative lower activity was observed when using the developed Lewis basic catalyst under mild conditions.

and dialkyl carbonate. The synergistic effect between the cation and anion endowed the higher activity of the catalyst. Depolymerization of Other Polyester Materials. Given the potential of [HDBU][Im] as catalyst, the versatility to methanolysis of other polyester wastes, such as PLA, PET, and PHB were examined and compared. The corresponding products were verified by FT-IR and 1H NMR (see the Supporting Information). As shown in Table 3, [HDBU][Im] could show comparable activities to reported ILs while under the lower temperature and less time conditions for PLA and PET

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CONCLUSIONS A series of imidazole-anion-derived ILs were facilely synthesized and used for efficient catalyzing alcoholysis of polyester wastes. Compared with the reported solid base and organic F


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Figure 4. (A) In situ FT-IR spectra and (B) 1H NMR spectra for [HDBU][Im] activating CH3OH at 298 K.

Table 3. Methanolysis of Other Polyester Materials and Comparison with Reported IL Catalystsa entry

additive

catalyst

temperature (°C)

time (h)

depolymerization (%)

yield (%)b

1 2 3 4 5 6

PLA PLAc PET PETd PHB PHBe

[HDBU][Im] [Bmim][OAc] [HDBU][Im] [Bmim]2[CoCl4] [HDBU][Im] [MIMPS]FeCl4

70 120 140 170 100 140

1.0 3.0 3.0 4.0 2.0 3.0

100 96 100 100 100 99

87 90 75 78 33 87

a Conditions: n(additive)/n(CH3OH)/n(Cat.) = 1:5:0.1. bIsolated yield. cSee ref 26; n(additive)/n(CH3OH) = 1:6. dSee ref 19; glycolysis of PET, PET particle size: 2.0 mm × 2.5 mm × 2.7 mm. eSee ref 16; [MIMPS]FeCl4 represents 1-(3-sulfonic acid)-propyl-3-methylimidazole ferric chloride.

superbase catalysts, the present protocol was performed under the milder and greener conditions and was versatile to different polyesters depolymerization, such as PC, PLA, PHB, and PET. For the model methanolysis of PC, 100% PC depolymerization with 96% BPA yield could be obtained at 70 °C for 2.0 h using only 1.6 mol % [HDBU][Im]. The catalysts were easy to reuse for at least six cycles without deactivation. Combining the obtained results with in situ activation analysis, a synergistic catalysis mechanism resulting from cation−anion ions was proposed, which would provide insights into the rational design of robust catalysts for practical depolymerization of polyesters into monomers or other chemicals.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We sincerely acknowledge financial support from the National Natural Science Foundation of China (51673106, 21805154), the Science and Technology Research Project of Shandong Province (2016GSF116005), the Natural Science Foundation of Shandong Province (ZR2018BB009), a Project of Shandong Province Higher Educational Science and Technology Program (J18KA065), and the Scientific Research Foundation of Qingdao University of Science and Technology (0100229019).

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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.8b03591. General information, general procedure for alcoholysis of polyester, characterization and basicity measurement of ILs, comparison of the activity, and products characterization (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

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

M.L. and J.G. contributed equally to this work G


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