Ind. Eng. Chem. Res. 2002, 41, 3993-3998
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APPLIED CHEMISTRY Chemical Recycling of Poly(ethylene terephthalate). 2. Preparation of Terephthalohydroxamic Acid and Terephthalohydrazide Makoto Yamaye,*,† Toshiki Hashime,‡ Koji Yamamoto,† Yoshio Kosugi,‡ Namiko Cho,† Tomoyuki Ichiki,† and Taketoshi Kito† Faculty of Engineering, Kyushu Kyoritsu University, 1-8 Jiyugaoka, Yahatanishi, Kitakyushu 807-8585, Japan, and Department of Material Science, Interdisciplinary Faculty of Science and Engineering, Shimane University, 1060 Nishikawatsu-cho, Matsue 690-8504, Japan
A convenient one-pot-two-step process for chemical recycling of waste PET to produce terephthalohydroxamic acid (TPHA) and terephthalohydrazide (TPHD) was proposed. Decomposition of PET pellets to the corresponding oligomeric mixture in boiling ethylene glycol, followed by its treatment with hydroxylamine and hydrazine at room temperature in 1 h, produced >90% overall yields of TPHA and TPHD, respectively. Introduction Recent global concerns are focused on development of recycling technologies of waste plastics.1,2 Among a variety of waste plastics, poly(ethylene terephthalate) (PET) has been one of the most pressing environmental concerns in recent years, because of its growing demand and supply all over the world.3-5 Plastic recycling, except incineration and landfill, is generally classified into material, thermal, and chemical recyclings. A major part of waste PET, including waste PET bottles, has been treated by material recycling.3,5 However, the repellets from recovered PET are not generally suitable for manufacturing PET bottles, due to lowering of its viscosity. Therefore, they are used for making textiles, carpets, detergent bottles, and other household products, all of which do not always claim severe requirements for their properties. Thermal recycling of PET was proved to have several significant drawbacks: PET produces a comparatively smaller heat of combustion, roughly half the amount produced by polyolefins, and terephthalic acid which is formed during combustion sublimes to stick to the flue pipe and so forth. Among various chemical recycling methods of waste PET,3 depolymerization of PET into its starting materials, terephthalic acid (and its derivatives) and ethylene glycol (EG), has commercially been conducted worldwide. However, this process should usually require further purification for their use in the PET manufacture. In connection with this type of recycling, many basic studies and developmental research have been conducted.6 Other chemical recycling methods involve transformation of waste PET into other polymeric materials, †
Kyushu Kyoritsu University. E-mail:
[email protected]. Work: 093-603-3056. Fax: 093-603-8186. ‡ Shimane University.
including coating materials3 and polyamides,7 and some fine chemicals, including plasticizers.3,8 Ammonolysis and aminolysis of PET wastes have scarcely been studied so far. Transformations of waste PET into such fine chemicals as terephthalamide,4 terephthalohydrazide (TPHD),9 and N,N′-diallyl terephthalamide4,10 have been reported. Partial aminolysis for surface modification of PET fibers found some applications for industrial use.11 Most of these chemical recycling processes described above require rather severe reaction conditions, namely at elevated temperatures and pressures (in an autoclave), and so forth and are not likely to be of commercial significance, except for depolymerization of PET into its starting materials. We here propose a milder and more convenient onepot-two-step process for depolymerization of waste PET, producing terephthalohydroxamic acid (TPHA) and TPHD. The first step involves glycolysis of PET in EG into a mixture comprising bis(2-hydroxyethyl) terephthalate with a small amount of the corresponding lowmolecular-weight oligomers. In the second step, aminolysis of the mixture is performed in the same flask at room temperature to yield TPHA and TPHD. Experimental Section General Methods. Melting points were determined on a Yanaco MP-500D and are uncorrected. 1H and 13C NMR spectra were obtained on JEOL JNM-A500 and JNM-AL400 spectrometers. Chemical shifts are expressed in ppm relative to Me4Si as internal standard. IR spectra were recorded on a Jasco FT/IR-230 spectrophotometer in KBr pellets. Mass spectra (MS) were obtained on a JEOL JMS-SX102A. Microanalyses were carried out on a Yanaco CHN corder MT-5. Commercially available PET pellets (d ) 1.375, av Mv ∼ 18 000, intrinsic viscosity ) 0.59, Tg ) 81 °C, size ∼ 3 × 3.5 × 1 mm3) from Aldrich Chemical Co. were used.
10.1021/ie010894b CCC: $22.00 © 2002 American Chemical Society Published on Web 07/16/2002
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Other reagents (guaranteed grade) were used as obtained from Wako Pure Chemical Industries, Ltd., Kanto Chemical Co., Inc., and Tokyo Kasei Kogyo Co., Ltd. Representative procedures for preparation of TPHA and TPHD are described below. Preparation of TPHA from PET (One-Pot-TwoStep Reaction). In a three-necked 200-mL flask equipped with a reflux condenser, a thermometer, and a nitrogen inlet, a mixture of PET pellets (2.00 g, 1.1 × 10-4 mol), EG (7.5 mL), and (CH3COO)2Zn‚2H2O (6.0 × 10-3 g, 3.0 × 10-5 mol) was flushed with nitrogen and heated to the boiling point of EG (198 °C). The surface of the PET pellets lost transparency at approximately 120 °C. Boiling started in 25-30 min, and dissolution of PET pellets completed in ∼1 h. The total period of 1-2 h was required for dissolution of PET pellets after heating started. At this stage the reaction was temporarily suspended and the reaction mixture (the resulting PET oligomers) was cooled to room temperature. To the flask were dropwise added successively EG (20 mL), hydroxylamine hydrochloride (2.09 g, 3.00 × 10-2 mol in 10 mL of H2O), and NaOH (2.40 g, 6.00 × 10-2 mol in 10 mL of H2O). The resulting mixture was stirred at room temperature, until it became a pale yellow solution and the PET oligomers disappeared, which was confirmed by thin-layer chromatography (TLC). It took 300 °C. IR (KBr): ν3300, 1650, 1560 cm-1. 1H NMR (500 MHz, DMSO-d ): δ 7.81 (s, 4H, ArH), 9.14 6 (s, 2H, NHOH), 11.33 ppm (s, 2H, NHOH). 13C NMR (125.65 MHz, DMSO-d6): δ 126.8 (d, ArC-H), 135.0 (s, ArC-CO), 163.0 ppm (CO). MS (EI): m/z 196 (M+, 26), 164 (100), 148 (75), 146 (84), 104 (100), 90 (85). A solid sample was directly introduced into the mass spectrometer, because a normal FAB procedure gave no molecular ion. Anal. Calcd for C8H8N2O4: C, 48.98; H, 4.11; N, 14.28. Found: C, 48.85; H, 4.17; N, 14.18. Preparation of TPHD from PET (One-Pot-TwoStep Reaction). The first step was the same as that stated above for TPHA. An oligomeric mixture obtained in the first step using PET pellets (2.00 g, 1.1 × 10-4 mol) was cooled to room temperature and mixed with hydrazine monohydrate (1.30 mL, 2.62 × 10-2 mol) and chlorobenzene (20 mL). The resulting mixture was stirred at room temperature until the PET oligomers were not detected on TLC. It took 1 h. Precipitates deposited were collected by suction and recrystallized with hot water to give pure TPHD (1.93 g, 95%): mp > 300 °C. IR (KBr): ν 3330, 1630, 1540 cm-1. 1H NMR (500 MHz, DMSO-d6): δ 4.54 (s, 4H, NHNH2), 7.85 (s, 4H, ArH), 9.85 ppm (s, 2H, NHNH2). 13C NMR (125.65 MHz, DMSO-d6): δ 126.9 (d, ArC-H), 135.4 (s, ArCCO), 165.4 ppm (s, CO). MS (EI): m/z 194 (M+, 59), 163 (100), 104 (98), 76 (59). Anal. Calcd for C8H10N4O2: C, 49.48; H, 5.19; N, 28.85. Found: C, 49.28; H, 5.15; N, 28.42. Preparation of TPHD from PET (One-Step Reaction). A mixture of PET pellets (2.00 g, 1.1 × 10-4 mol), tetrahydrofuran (THF) (30 mL), and hydrazine
monohydrate (1.50 mL, 3.02 × 10-2 mol) was stirred under reflux (65 °C). After 5 h the PET oligomers disappeared on TLC and precipitates deposited from the reaction mixture were filtered by suction to give crude TPHD (1.91 g, 95%). Results and Discussion We believe that recycling processes of waste plastics for industrial applications should be simple and easy to conduct. Thus, we have devised a convenient onepot-two-step process for depolymerization of PET to produce TPHA and TPHD. The process is based on our previous study on alcoholysis of PET. Alcoholysis of PET was carried out with 1,3-propanediol and benzyl alcohol, indicating that a rapid decrease in the molecular weight of PET takes place in a short period.12 For example, a mixture of PET pellets and 1,3-propanediol was heated to reflux (213.5 °C) in the presence of zinc acetate. Dissolution of the pellets completed in 1-2 h. Immediately after the dissolution, the reaction mixture was analyzed by gel permeation chromatography. The product composition consisted of 80-90% monomeric bis(3-hydroxypropyl) terephthalate, and this ratio remained virtually unchanged for monitoring every 1 h, even after heating was continued for 8 h. Another experiment on a quantitative product analysis by high performance liquid chromatography using a calibration curve was carried out for depolymerization of PET with benzyl alcohol in the presence of titanium tetrabutoxide or calcium acetate. The results also showed the presence of 85100% dibenzyl terephthalate, the final depolymerization product, during the same period as described above. In other words, a complete alcoholysis of PET into its starting materials was found to be extremely difficult; even after prolonged heating, a small amount (10-20%) of the corresponding low-molecular-weight oligomers still remained in the reaction mixture.12,13 This may reflect the equilibrium nature of ester solvolysis. The observation indicates that recovery of pure terephthalic acid and EG is not easy from a simple chemical recycling of waste PET and that their further purification should be indispensable for use in PET manufacture. On the other hand, such readiness of PET alcoholysis to form its corresponding PET oligomers prompted us to investigate their transformation into various derivatives of terephthalic acid. It would be advantageous for the chemical recycling to utilize this oligomeric mixture without further purification or any other processings. Among such transformations, we examined aminolysis using the oligomeric mixture as obtained above. There have been very few reports on aminolysis of waste PET. Transformation of waste PET into TPHD in 90% was reported by treatment with hydrazine in boiling water for 5 h.9 Spychaj and Paszun4 reported degradation of PET with ammonia in an autoclave at approximately 20 atm and in the temperature range 120-180 °C for 1-7 h to obtain terephthalamide in >90% yields. They also carried out decomposition of PET with allylamine at
