Ind. Eng. Chem. Res. 2001, 40, 75-79
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Kinetics of Hydrolysis of Poly(ethylene terephthalate) Powder in Sulfuric Acid by a Modified Shrinking-Core Model Toshiaki Yoshioka,* Tsutomu Motoki, and Akitsugu Okuwaki Department of Applied Chemistry, Graduate School of Engineering, Tohoku University, Aramaki Aza Aoba 07, Aoba-ku, Sendai 980-8579, Japan
Poly(ethylene terephthalate) (PET) powder from waste bottles was degradated at atmospheric pressure in 3-9 M sulfuric acid below 150-190 °C for 12 h to clarify the mechanism for a feedstock recycle process. Terephthalic acid (TPA) and ethylene glycol (EG) were produced by the acid-catalyzed heterogeneous hydrolysis of PET in sulfuric acid. The TPA yield agreed with the degree of hydrolysis of PET, but the EG yield decreased with increasing sulfuric acid concentration because of carbonization of EG. The kinetics of hydrolysis of PET in sulfuric acid could be explained by a modified shrinking-core model for the chemical reaction control, in which the effective surface area is proportional to the degree of hydrolyzed PET, x, affected by formation and growth of pore and crack on PET powder. The apparent rate constant was proportional to the reciprocal of the particle size and the activity of sulfuric acid, and the activation energy was 88.7 kJ/mol. 1. Introduction There are many chemical recycling processes for poly(ethylene terephthalate) (PET), which are as follows: (1) methanolysis, (2) glycolysis, (3) hydrolysis, (4) aminolysis, and (5) others. Methanolysis and glycolysis have been mainly applied on a commercial scale, because dimethyl terephthalate is a raw material for polymerization of PET. However, terephthalic acid (TPA) has been used as a raw chemical for the manufacturing of PET. A growing interest in the hydrolysis to TPA and EG is connected with the development of PET synthesis directly from EG and TPA, which eliminates methanol from the technological cycle. Hydrolysis of esters in a neutral, an acidic, and an alkaline medium is wellknown. Many workers reported the kinetics of hydrolysis for PET. Paszun and Spychaj reviewed the chemical recycling of PET including the hydrolysis.1 Some studies of the hydrolysis of PET by acid catalysis, including hydrochloric acid2,3 and sulfuric acid,4,5 were reported. Ravens2 investigated the effect of hydrochloric acid concentration on the rate of hydrolysis of PET fiber at 70 °C and explained that the reaction rate is principally determined by the solubility of hydrochloric acid in the amorphous regions of PET. PET dissolved in concentrated sulfuric acid was quickly hydrolyzed when added to water.5 Appreciable amounts of TPA and EG were produced by this method. The melting point of PET is around 245-265 °C. The hydrolysis of PET is clearly a heterogeneous reaction at temperatures below its melting point and is thought to be a homogeneous reaction at higher temperatures. Because the reaction temperatures below the melting point proceed at the solid-liquid interface, the effects of changing the effective surface area of PET on the reaction should be considered. Authors have reported that the kinetics of the hydrolysis of PET in nitric acid could be explained by a modified shrinking-core model of chemical reaction control, in which the effective surface area is propor-
tional to the degree of unreacted PET, affected by the deposition of the product TPA.6 On the other hand, we have confirmed that the hydrolysis was accelerated extensively above 5-7 M because of an increase in the specific surface area of PET by the formation and growth of cracks.7 The purpose of this paper is to describe the kinetics of the hydrolysis of PET powder in sulfuric acid using the modified shrinking-core model. 2. Experimental Section 2.1. Materials. PET powder, free from any additive, was prepared from the crush powder by a cutter-type mill using clear PET bottles, from which the crystallized caps and bottom parts had been removed. The powder was sifted through four screens of increasing diameter (75, 106, 125, and 150 µm). The average molecular weight of the PET powder was 30 000. The PET powder was washed in water under irradiation of ultrasonic waves. All chemicals were reagent grade. 2.2. Hydrolysis in Sulfuric Acid Solutions. A mixture of 0.2 g of PET powder sifted to between 75 and 106 µm was placed in a poly(tetrafluoroethylene) (PTFE)-lined 0.035 dm3 SUS-316 microautoclave with 0.025 dm3 of 3-9 M H2SO4. The autoclave was dipped in a shaking thermostable silicon oil bath maintained at a prescribed temperature between 150 and 190 °C for 15 min to 12 h. After the reaction, the autoclave was cooled rapidly to room temperature with cold water. 2.3. Analysis and Definition. The reaction mixture was filtered through a 1G4 glass filter to separate the unreacted PET and TPA powder produced from the mother liquor and washed with 0.05 dm3 of cold water. Then, TPA in the mixture was separated from the unreacted PET by washing with a 6 M NH4OH solution. TPA in the ammonia hydroxide solution was precipitated by addition of sulfuric acid, removed with a 1G5 glass filter, and washed with cold water. The unreacted PET and TPA produced were both dried in an oven for 2 h at 105 °C and weighed. The percent degradation of
10.1021/ie000592u CCC: $20.00 © 2001 American Chemical Society Published on Web 11/23/2000
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Ind. Eng. Chem. Res., Vol. 40, No. 1, 2001
Figure 1. Degradation of PET and the yields of TPA and EG curves in 4 M H2SO4 at 150 °C: (b) degree of PET degraded; (O) TPA; (0) EG.
Figure 2. Effect of the H2SO4 concentration (M) on the degree of hydrolysis of PET 150 °C: (O) 3, (b) 4, (4) 5, (2) 6, (0) 7, (9) 8, (×) 9.
PET and the yield of TPA were determined by gravimetry and defined in the following way:
degradation of PET (%) ) {(WPET,i - WPET,R)/WPET,i} × 100 (1) yield of TPA (%) ) mTPA,O/mPET,i × 100
(2)
yield of EG (%) ) mEG,O/mPET,i × 100
(3)
where WPET,i is the initial weight of PET, WPET,R is the weight of unreacted PET, mTPA,O and mEG,O are the number of moles of TPA and EG, and mPET,i is the initial number of moles of the PET repeating unit. The surface of the PET powder was observed by scanning electron spectroscopy (TOPCPN ABT-32). Also, the PET surface area was measured by BET (Shimazu Asappu 2010).
Figure 3. Effect of the particle size (µm) on the hydrolysis of PET in H2SO4 at 150 °C: (O)
