Article pubs.acs.org/IECR
Recovery of Terephthalic Acid and Ethylene Glycol from Poly(ethylene terephthalate) under Hydrothermal Conditions of Aqueous Trimethylamine Solution Natsumi Wakabayashi, Tomoharu Kojima, and Toshitaka Funazukuri* Department of Applied Chemistry, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan ABSTRACT: Poly(ethylene terephthalate) pellets were subjected to various amine solutions under hydrothermal conditions in a semibatch reactor. Methylamine, ethylamine, dimethylamine, trimethylamine, ammonia, and sodium hydroxide were examined at 473 K and 10 MPa at concentrations of 0.6 mol/kg. The effects of the amine species on reaction rates and monomer yields were compared. For all solvents examined in the present study, the reactions were expressed by a surface reaction model, i.e., the 2/3rd-order reaction kinetics with respect to unreacted polymer mass. The reaction rates with the four amines were not very different and were similar to that with NaOH at the same concentration; but, they were faster than the rate with ammonia. When the reaction was conducted with trimethylamine, the yields of monomers and total yields of terephthalic acid and ethylene glycol were very close to the theoretical values. However, those with dimethylamine, ethylamine, and methylamine were slightly lower with intermediate products found in the product solutions.
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INTRODUCTION Poly(ethylene terephthalate) (PET) is employed in numerous consumer goods such as beverage or food bottles, packaging, containers, and textiles, as well as in various industrial applications. In 2010, about one million tons of PET were produced in Japan,1 and the chemical recycling of postconsumer/waste PET has become increasingly important and is in high demand. Paszun and Spychaj2 conducted an extensive review on the state of PET chemical recycling and classified the methods into six categories: methanolysis, glycolysis, hydrolysis, aminolysis, ammonolysis, and others. The advantages and disadvantages of each method were discussed in the review, as well as the utilities of products from the various recycling processes. Efforts are still continuously being made to develop new or modified methods for the chemical recycling of PET to produce raw materials and ingredients for various chemical products. In addition to the conventional systems, postconsumer PET has been treated by sub- and supercritical methanol3−8 and water9−11 to recover the monomers of terephthalic acid (TPA) (or dimethylterephthalic ester in methanolysis) and ethylene glycol (EG). While all of these methods have advantages, the disadvantages include harsh reaction conditions of high temperature and high pressure, the use of complex mixtures of degrading reagents or additives, difficulty in the separation/purification of solvent recovery or products, and low yields of the desired monomers. Among the various chemical recycling methods, ammonolysis and aminolysis are attractive for degrading PET due to the high reactivities of ammonia and amines toward polyesters. In ammonolysis/aminolysis methods, the main objective is to obtain amide compounds from PET for use as coating materials, plasticizers, and ingredients. However, these methods have not yet been employed in industrial processes,2 and studies on them are limited despite the fact that they have been investigated since the 1960s.12−16 In fact, most initial © 2012 American Chemical Society
aminolysis studies focused on the surface morphology or modification and the strength of PET fibers,12−16 and data on reaction rates and monomer yields are scarce. Therefore, ammonolysis/aminolysis methods are not considered to be suitable for recovering TPA and EG monomers from PET. Mormann et al.17 studied the ammonolysis of various polymers including PET with sub- and supercritical ammonia (critical temperature of 405.6 K and critical pressure of 11.3 MPa) containing water, which converted majority of the PET into terephthalamide and EG. Recently, the authors reported the depolymerization of PET with dilute aqueous ammonia under hydrothermal conditions.18−21 This method has some advantages, such as reaction conditions requiring a relatively milder temperature and dilute alkaline conditions, and the production of monomers with minimal amounts of oligomers due to their low solubilities in aqueous solutions. The reaction is dominated not by alkali hydrolysis but by nucleophilic reaction with ammonia. Thus, a high concentration of ammonia is not required, and ammonia can be recovered by hydrolytic decomposition of the intermediate products under hydrothermal conditions. The products were mainly TPA and EG, and the terephthalamide produced decomposed readily. Since ammonia was found to be effective in the depolymerization of PET18−21 and polycarbonate (PC),19,21 amines were also expected to be effective. Kinetic data and monomer yields on the ammonolysis/aminolysis of PET are limited in the literature. Thus, the objective in the present study is to demonstrate the effectiveness of amines in the depolymerization of PET by comparing reaction rates with various aqueous amines. Received: Revised: Accepted: Published: 5699
December 9, 2011 April 1, 2012 April 5, 2012 April 5, 2012 dx.doi.org/10.1021/ie202885u | Ind. Eng. Chem. Res. 2012, 51, 5699−5704
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Article
EXPERIMENTAL SECTION Materials. PET pellets (cylindrical, 3.3 mm × 3.2 mm × 2.4 mm, DP = 18 000) were obtained from Sigma Aldrich Japan (Tokyo, Japan). The dilute aqueous solutions were prepared from the aqueous solutions of methylamine (40 wt %, Kanto Chemical Co., Inc., Tokyo, Japan), ethylamine (70 wt %, Tokyo Chemical Industry Co., Ltd., Tokyo, Japan), dimethylamine (50 wt %, Tokyo Chemical Industry Co., Ltd.), trimethylamine (30 wt %, Kanto Chemical Co., Inc.), and ammonia (25 wt %, Wako Pure Chemical Industries, Ltd., Tokyo, Japan). The sodium hydroxide (min. 97.0 wt %), TPA (min. 98.0 wt %), and EG (min. 99.5 wt %) solutions were obtained from Kanto Chemical Co., Inc. Apparatus and Procedures. A schematic diagram of the experimental setup is shown in Figure 1. The apparatus was
reactor was opened, and the solid residual was recovered. In most runs, however, no residual solid was found. The densities of the aqueous alkaline solutions were assumed to be equal to that of pure water22 under the current reaction conditions. In this study, the yields of TPA and EG were based on carbon weight as defined by eqs 1 and 2, and the conversion, Y, is defined by eq 3: TPA yield (%) = 100 ×
organic carbon in TPA recovered (g) carbon in PET loaded initially (g) (1)
EG yield (%) = 100 ×
organic carbon in EG recovered (g) carbon in PET loaded initially (g) (2)
Y=
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TPA yield (%) + EG yield (%) 100
(3)
RESULTS AND DISCUSSION Figure 2 shows the TPA, EG, and TPA + EG cumulative yields over time at 473 K and 10 MPa in a 0.6 mol/kg trimethylamine
Figure 1. Schematic diagram of the experimental apparatus.
similar to that used in previous studies.18−21 The reactor was made from a 3.6 mL 1/2 in. O.D. stainless steel tubing with the exit connected to a 1/8 in. O.D. stainless steel tubing via Swagelok reducing unions, and a frit disk (pore diameter of 2 μm) was installed at the exit of the reactor. The pellets (ca. 90 mg) were loaded in the reactor at room temperature. Prior to a run, an aqueous solution (amines, ammonia or sodium hydroxide) was filled throughout the line and reactor. At time zero, the reactor and the preheating column were immersed in a molten salt bath, whose temperature was maintained at the prescribed value within a fluctuation of ±2 K, and the solvent was supplied. The temperature of the solvent in a different reactor, whose size was identical to the reactor used for the depolymerization reaction, reached the prescribed temperature within one minute by measuring the value with a thermocouple inserted in the reactor. The solvent flow rate was set to 3 mL/ min at room temperature and 10 MPa. At flow rates higher than 2−3 mL/min, the residence times of the solvent in the reactor were almost negligible, and the residence times from the reactor inlet to the outlet of the back pressure regulator were less than 30 s.20,21 The solvents eluted from the reactor were quenched in a cooler jacket, a heat exchanger made of two 60 cm long coaxis cylinder tubes, by the flow of water. The product solutions were collected from the outlet of the back pressure regulator every 2 min for shorter reaction times and at 4−15 min intervals for longer reaction times. The solutions were analyzed by HPLC equipped with a UV−vis and a differential refractive index detector, which are connected in series. A 6 mM aqueous ammonia solution flowed through the GS-320HQ column (7.5 mm I.D. × 30 cm, Shodex, Tokyo, Japan) at a flow rate of 0.5 mL/min and 30 °C. The solvent supply was terminated when the reaction was complete. The
Figure 2. Yields of TPA (△), EG (□), and TPA + EG (○) over time at 473 K, 10 MPa, and 0.6 mol/kg trimethylamine concentration.
concentration at a solvent flow rate of 3.0 mL/min. The TPA and EG yields reached 77.6 and 18.6%, respectively, at 50 min, slightly lower than the theoretical values of 80 and 20%, respectively. The cumulative TPA + EG yields in methylamine, dimethylamine, and ethylamine were lower than those in trimethylamine, as shown in Figure 3. The final yields increased in the order of methylamine < dimethylamine < ethylamine < trimethylamine. The yields in trimethylamine and methylamine increased at almost the same rates with time up to 30 min, but those in methylamine were suppressed thereafter. This was caused by the intermediate products from the reactions between the amine and PET. Figure 4 shows the HPLC chromatograms of the product solutions with various amines, ammonia, and NaOH measured at 254 nm by the UV−vis detector at 473 K, 10 MPa, and each concentration of 0.6 mol/kg for 30 min. The highest peak in each chromatogram was TPA and a smaller peak was observed at a retention time of about 11 min with methylamine, ethylamine, dimethylamine, and ammonia, while no peak appeared with NaOH and trimethylamine. It can be speculated that the highest monomer yields with trimethylamine result 5700
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Figure 3. Cumulative TPA + EG yields with methylamine (○), dimethylamine (■), trimethylamine (▼), and ethylamine (△) over time at amine concentrations of 0.6 mol/kg, a solvent flow rate of 3 mL/min, 473 K, and 10 MPa.
Figure 5. Reaction schemes in aqueous methylamine and trimethylamine solutions.
Figure 4. HPLC chromatograms of product solutions with various amines, ammonia, and NaOH at 473 K and concentrations of 0.6 mol/ kg for 30 min.
reaction may take place on the pellet surface. Thus, the overall reaction was expressed by the following surface reaction model:
from higher hydrolytic reactivities of the tertiary amide compounds, compared to secondary and primary amides. Figure 5 shows possible reaction schemes for aqueous methylamine and trimethylamine solutions. When aqueous ammonia, and aqueous primary and secondary amine solutions are used, their nitrogen atoms attack carbonyl carbons in PET polymers and their amides will be produced. While the amides could be mainly decomposed, some of them remain as byproduct. In contrast, tertiary amine also attacks carbonyl carbons in PET polymers as ammonia, and primary and secondary amines do, but the amides produced are easily decomposed to terephthalic acid because of the unstable intermediate having positive charge. The apparent depolymerization reaction is assumed to take place on the surface of the PET pellets in an aqueous amine solution, as seen for PET and PC in ammonia and sodium hydroxide18−21 and poly(lactic acid) in water and sodium hydroxide.23 The overall reaction may be controlled by mass transfer and/or chemical reaction on the surface, probably due to the limitation of the solubility of the oligomers and monomers in the aqueous solution, or reaction between hydroxide ion and the amide intermediates which may be formed in the vicinity of the pellet surface. Although the further studies are required to explore the reaction mechanism, the
dY = k(1 − Y )2/3 dt
(4)
Y=0
(5)
at
t = ti
where Y (−) is the conversion based on the TPA + EG yield, t and ti are the reaction and induction times, respectively, and k (1/min) is the overall rate constant. Figure 6 shows [(1 − Y)1/3 − 1] as a function of reaction time in four aqueous amine solutions, together with aqueous sodium hydroxide and ammonia solutions. The data for each amine are represented by straight lines, and the slopes, equal to −k/3, were very similar. However, the plateau values, i.e., the maximum cumulative TPA + EG yields, were 84.2, 90.3, 93.8, and 96.9% with methylamine, dimethylamine, ethylamine, and trimethylamine, respectively (see Figure 3). The values are directly related to the peak areas for the intermediates shown in Figure 4. The rates with ammonia were so slow, and those with sodium hydroxide were as fast as those with amines. Corresponding to the intermediates found as shown in Figure 4, the conversion with NaOH based on TPA yield was close to that with trimethylamine due to no intermediates left. Figure 7 plots [(1 − Y)1/3 − 1] over time at 0.6 mol/kg trimethylamine concentration and various temperatures. As depicted, the plot of each temperature is well represented by a straight line over a wide conversion range Y, up to those higher 5701
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Table 1. Pre-exponential Factors A and Activation Energies E for Overall Rate Constants with Trimethylamine and Ammonia at 0.6 mol/kg Concentrations solvent trimethylamine ammonia
temperature (K)
A (1/min)
E (kJ/mol)
443−473 483−503 453−473 483−503
× × × ×
61.7 157 25.7 191
3.34 5.42 1.09 1.36
5
10 1015 101 1019
in ammonia. Above 480 K, the slopes in both solutions become steeper, and then plateau above 503 K. The values in trimethylamine were 1.3−2 times higher than those in ammonia at 483 to 503 K. The reason of the change in slopes is not clarified. Figures 9a and b show the plots based on random scission and pseudo-first-order reaction kinetics, respectively, for
Figure 6. [(1 − Y)1/3 − 1] over time at 473 K and 10 MPa for methylamine (○), dimethylamine (■), trimethylamine (▼), and ethylamine (△), together with sodium hydroxide (⧫) and ammonia (▲).
Figure 7. Plots of [(1 − Y)1/3 − 1] over time for trimethylamine at 10 MPa, 0.6 mol/kg trimethylamine concentration, and various temperatures of 443 (○), 453 (●), 463 (□), 473 (■), 483 (◇), 493 (◆), 503 (△), and 513 K (▲).
than 0.90. The induction time decreased with increasing reaction temperature, and the slope increased with increasing reaction temperature. Figure 8 shows an Arrhenius plot for overall rate constants determined from a 0.6 mol/kg trimethylamine concentration, compared with those from a 0.6 mol/kg ammonia concentration. The pre-exponential factors and activation energies with trimethylamine and ammonia at 0.6 mol/kg concentrations were also listed in Table 1. The slopes of the rates in both solutions were not steep below 473 K, and the rate constants in trimethylamine were 2−3 times higher than those
Figure 9. (a) (1/Mn − 1/Mn0) and (b) Mn/Mn0 over time based on random scission and pseudo-first-order reaction kinetics, respectively, for the data by Collins et al.:16 40% methanolamine at 293 K (○), 70% ethylamine at 294 K (□), neat n-butylamine at 294 K (▼), and 10% ethanolamine at 373 K (▲).
degrading an average number of molecular weights for PET fibers in the various aqueous amines measured at 293−294 K by Collins et al.,16 who did not determine the rates. Both the random scission and pseudo-first-order reaction kinetics represented the molecular weight data well except at longer reaction times. The induction times are expressed by eqs 6 and 7 for the random scission and pseudo-first-order reaction kinetics, respectively: ti =
Figure 8. Arrhenius plots for rate constants for trimethylamine (▲) and ammonia (○) at concentrations of 0.6 mol/kg. 5702
⎡ E i,a ⎤⎛ 1 ⎡ E i,a ⎤ 1 1 ⎞ exp⎢ − ⎟M1 ∝ exp⎢ ⎥ ⎥⎜ ⎣ RT ⎦ k 0,a ⎣ RT ⎦⎝ Mn M n0 ⎠
(6)
dx.doi.org/10.1021/ie202885u | Ind. Eng. Chem. Res. 2012, 51, 5699−5704
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⎡ E i,b ⎤ ⎛ M i ⎞ ⎡ E i,b ⎤ 1 exp⎢ ⎟ ∝ exp⎢ ⎥ ⎥ln⎜ ⎣ ⎦ ⎣ RT ⎦ k 0,b RT ⎝ M 0 ⎠
Article
of 0.766. This suggested that the overall reaction could be attributed to the nucleophilic reaction with amine, not to alkaline hydrolysis. Note that the pKa values for methylamine, dimethylamine, trimethylamine, and ammonia in aqueous solutions at 473 K are estimated to be lower than 8.24 The induction times decreased with increasing amine concentrations, but the effect of this is not clear.
(7)
where k0 and Ei are the pre-exponential factor and activation energy for degradation reaction of PET, respectively, the subscripts a and b are the random scission and first-order reaction kinetics, respectively, Mi and M0 are the average molecular weights of the soluble components and initial PET sample, respectively. The induction times in both solutions were well represented by straight lines, as both eqs 6 and 7 suggested. Those with trimethylamine were much shorter, and the temperature effects were weaker than those with ammonia. Figure 10 plots the induction times as a function of 1000/T in 0.6 mol/kg trimethylamine concentration and 0.6 mol/kg
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CONCLUSIONS PET pellets were converted into TPA and EG monomers in dilute aqueous amine under hydrothermal conditions in a small semibatch reactor. The total yield of both monomers with trimethylamine was the highest at 96.2% on a carbon weight basis after 50 min at 473 K. Those with methylamine, dimethylamine, and ethylamine were lower due to the presence of intermediate amide products. The yields of TPA + EG were well-represented by the surface reaction model, i.e., 2/3rdreaction order with respect to unreacted mass of PET. The overall reaction rate constants were proportional to the trimethylamine concentration to the power of 0.77.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are grateful to the Ministry of Education, Sports, Culture, Science and Technology of Japan for the financial support through Grant-in-Aid No. 23651075 and to the Japan Society for the Promotion of Science through the A-step grant.
Figure 10. Induction time as a function of 1000/T with trimethylamine (▲) and ammonia (○) at concentrations of 0.6 mol/kg.
ammonia concentration. Since the solvents were aqueous, unless high pKa, the solubility of TPA was not as high compared with polar organic solvents. The induction times are considered to be the times required for the degradation of products to molecules, small enough to be soluble in aqueous solutions. According to Figure 10, the activation energy with 0.6 mol/kg trimethylamine was 33.9 kJ/mol at temperatures of 443 to 493 K, and those with 0.6 mol/kg ammonia were 75.3 and 41.8 kJ/mol at temperatures of 453−483 and 483−513 K, respectively. Figure 11 shows the effects of the trimethylamine concentration on the overall rate constants and induction times at 473 K and 10 MPa. The rate constants were proportional to the trimethylamine concentration to the power
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REFERENCES
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