Catalytic property of poly(ethylene terephthalate-co-isophthalate) synthesized with a novel Sb/Al bimetallic compound catalyst

Abstract

In this paper, the hydrolysis process of aluminum subacetate (AlSA) was studied by Fourier transform infrared (FT-IR), ²⁷Al nuclear magnetic resonance (NMR), X-ray diffraction (XRD), and particle size distribution analysis, and poly(ethylene terephthalate-co-isophthalate) (PETI) was synthesized by using a compound catalyst made of its hydrolysate, γ-AlOOH, and ethylene glycol stibium (EGSb). Meanwhile, the effects of the γ-AlOOH addition stage on the catalytic effect and the relative properties of the polyester were investigated. The results showed that the Sb and Al compound catalyst had obvious synergistic effects on the process of the polyester polycondensation. With the same dosage of catalyst, the usage of EGSb and the polycondensation time were reduced by 60% and 18.6%, respectively, and the diethylene glycol content in the polyester also decreased by using the compound catalyst, compared with that obtained when only using the EGSb catalyst. The use of γ-AlOOH could play a catalytic role and also shows a unique crosslinking center, which could improve the melting point and glass transition temperature.

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1. Introduction

Currently, poly(ethylene terephthalate-co-isophthalate) (PETI), a commercial copolyester, has a low melting point, good crystal and optical performances, and excellent air tightness,^1,2 compared with poly(ethylene terephthalate) (PET). Therefore, PETI has been widely applied in the fields of bottle blow moulding and biaxial oriented films. To date, researchers have carried out much exploration in the field of polyester synthesis catalysts, such as stibium, titanium, and zirconium series catalysts, and it has been validated that these catalysts have good catalytic effects. However, there are many disadvantages in using these catalysts such as easy hydrolysis, poor thermal stability, and the chroma of the synthesized polyester.^3–5 However, it is easy for stibium series catalysts to be released out of some of the polyester final products, such as food packaging or soft-drink bottles, and stibium is a potential human carcinogen, which poses a threat to human health and environmental safety.^6,7 Thus far, ethylene glycol stibium (EGSb) has been used as a catalyst in most polyester manufacturing industries due to its good catalytic efficiency and thermal stability;⁸ however, the synthesized polyester presents a gray chroma and poor environmental friendliness.^9,10 Since the awareness of green chemistry is progressively increasing, new types of catalysts that can substitute or reduce the dosage of EDSb have gained considerable attention as promising methods for green processes. Therefore, it is necessary to search for new types of catalysts that can substitute or reduce the dosage of EDSb. New series of catalysts, such as titanium dioxide/silicon dioxide sols, chelated complexes of titanium, attapulgite-supported aluminum oxide hydroxide, etc., have already been proven to have excellent condensation catalytic performances.^11–13 One very effective approach to reduce the usage of stibium series catalysts is to combine them with other catalysts.¹⁴

Hydrated alumina (Al₂O₃·nH₂O) is a large family, which can be divided into Al(OH)₃, AlOOH, and gel (ρ-Al₂O₃·nH₂O) according to the number of water molecules in the chemical and crystal phase structures. With changes in the preparation formulation and process, a complicated reciprocal transformation is generated among these crystal phase structures.¹⁵ Among the family of hydrate alumina, AlOOH, which can be used as a catalyst and a catalyst support in many reactions, has been researched and applied the most.^16,17 AlOOH can be prepared by the hydrolysis of an aluminum inorganic salt, carboxylic acid salt, and aluminum alkoxide.^18–20 Aluminum subacetate (AlSA), which possesses definite antibacterial and antifungal properties, can be used as a disinfectant in the field of medical treatment. Otitis media and otitis externa are treated using Burow's solution, which is prepared from acetic acid and AlSA.²¹ There are no detailed reports about the preparation of AlOOH via the hydrolysis of AlSA.

In this paper, the coordination state of the AlSA ionization products and the composition of the AlSA hydrolysate are investigated and PETI is synthesized by using a compound catalyst made of the AlSA hydrolysate, (γ-AlOOH), and EGSb. The influence of the addition ratio and addition time of the compound catalyst on the catalytic effect and thermal performances of the polyester (PETI) are studied and the catalytic mechanism of γ-AlOOH has been analyzed in the reaction process.

2. Experimental section

2.1 Materials

Pure terephthalic acid (PTA, 99.9%) was provided by Yizheng Ge Lin Man Chemical Co., Ltd. Isophthalic acid (IPA, 99.9%) was purchased from AGIC International Chemical Company (Japan). EGSb (Sb content of 56–57%) was supplied by Shanghai Qi Zhi Chemical Co., Ltd. Ethylene glycol (EG, 99.8%) and chloroform (CHCl₃) were obtained from Jiangsu Yong Feng Chemical Co., Ltd. AlSA, trimethyl phosphate (TEP), phenol, and acetylene tetrachloride were purchased from Aladdin Industrial Corporation (Shanghai, China). The chemicals without marked purity were of analytical grade.

2.2 General methods

2.2.1 Preparation of the γ-AlOOH sol. First, AlSA was cleaned with deionized water (DIW). Then, it was heated and refluxed for 3 h in DIW. After standing for 10 min, the sediment was removed and the DIW was displaced with EG. Finally, a γ-AlOOH sol dispersed in EG was obtained.

2.2.2 Synthesis of PETI. The PETI was synthesized through a direct process. A certain proportion of PTA, IPA, EG, and EDSb were added into a 5 L polyester synthesis device equipped with a process tower, cooling tower, and a vacuum system. The esterification reaction was operated at 230–265 °C under a purified nitrogen atmosphere. According to the experimental design, γ-AlOOH was added in the esterification stage and before the start of the prepolycondensation, respectively. The catalyst amount accounted for 0.04% of the total acid mass and the molar ratio of PTA and IPA was 1/0.005. Water formed during the reaction as a by-product was collected for the estimation of the reaction conversion.

When the temperature at the top of the esterification process tower was below 100 °C, the actual water yield of the esterification process reached 92–96% of the theoretical water yield and the reaction would undergo the polycondensation stage. In this stage, the TEP stabilizer, which accounted for 0.005 wt% of the diacid, was added and the reactor was evacuated to low pressure (30 kPa) within 1 h while maintaining the temperature at 270 °C. The prepolycondensation and polycondensation reactions were carried out at 270 °C. When the power of the mixing motor reached 62 W, the rotation speed was decreased to 25 Hz and the final polycondensation was conducted under a vacuum degree of less than 20 Pa. The reaction endpoint was judged by a motor power of 68 W. The synthesized polyester (PETI) was cut into grains and stored in a cool dry place.

2.2.3 Characterization. FT-IR spectra (Thermo Scientific, USA) were recorded on a Nicolet iS5 spectrometer with KBr pellets in a wavelength range of 4000–400 cm². ²⁷Al NMR (Avance III HD 400, Germany) was carried out with Al(NO₃)₃ as the internal standard. XRD analysis was carried out with an X-ray diffractometer (D8 Advance, Germany) using a Cu K_α radiation source. Particle size distribution was measured with a laser particle size instrument with water (MS 2000, UK) using water as the medium. The chromaticity value was determined with a chromatic meter (Labscan XE, USA). The melting point (T_m) and the glass transition temperature (T_g) of PETI were measured using a differential scanning calorimeter (DSC 200 F3, Germany). The operating conditions were as follows: first, the sample was heated from 25 °C to 280 °C at a heating rate of 10 °C min⁻¹, maintained for 3 min at a constant temperature of 280 °C and then cooled to 20 °C at a cooling rate of 10 °C min⁻¹. Finally, the sample was reheated to 280 °C at a heating rate of 10 °C min⁻¹. A thermo-gravimetric test (TG 209 F3, Germany) was carried out from 25 °C to 800 °C at a heating rate of 10 °C min⁻¹ to determine the decomposition temperature of PETI. The carboxy end group value was measured by titrating a chloroform/phenol solution (volume ratio, 3 : 2) dissolving a PETI sample using a 0.05 mol L⁻¹ KOH–EtOH solution in an automatic potentiometric titrator (916 Ti-Touch, Switzerland) equipped with a photometric electrode. The liquid product of PETI with methanol alcoholysis was analyzed using a gas chromatograph equipped with a mass selective detector (GC-MS 6800, China) to determine the content of diethylene glycol (DEG). Characteristic viscosity was obtained by dissolving a PETI sample in a phenol/acetylene tetrachloride solution (mass ratio, 50 : 50) in a Ubbelohde viscometer at room temperature.

3. Results and discussion

3.1 Analysis of AlSA hydrolysis to form γ-AlOOH

Fig. 1 shows the FT-IR spectra before and after AlSA hydrolysis. The FT-IR spectrum of AlSA (Fig. 1a) exhibited the characteristic bands of ν_OH (3699–3000 cm⁻¹), ν_COO (1580, 1477 cm⁻¹), ν_COO (1427, 1408 cm⁻¹), δ_OH (1062, 978 cm⁻¹), δ_Al–OH (697, 571 cm⁻¹), and ν_Al–O (508 cm⁻¹), respectively.²² As shown in Fig. 1b, it was observed that the main characteristic bands of AlSA disappeared after hydrolysis and additional peaks appeared. The peaks at 1382 and 638 cm⁻¹ were the stretching vibration of the –OH groups in AlOOH and the peak at 1083 cm⁻¹ was the symmetrical deformation vibration of AlO–H in AlOOH, which indicated that γ-AlOOH was generated.²³

Fig. 1 The FT-IR spectra of AlSA (a) and γ-AlOOH (b).

Fig. 2 presents the XRD patterns of AlSA (a) and its hydrolysate (b). As can be seen in Fig. 2, it was indicated that γ-AlOOH (JCPDS #74-1895) was generated after AlSA (JCPDS #74-0319) hydrolysis. In addition, some polymorphous Al(OH)₃ with bayerite (JCPDS #20-0011) and doyleite (JCPDS #89-4333) was also generated. Therefore, it was necessary to remove the Al(OH)₃ precipitate after the hydrolysis reaction.

Fig. 2 The XRD patterns of AlSA (a) and its hydrolysate (b).

²⁷Al NMR spectroscopy was used to obtain information about the Al species in liquid media. With the chemical shift of Al(H₂O)₆³⁺ ionized from Al(NO₃)₃ for reference, the chemical shifts of [Al(H₂O)₆]³⁺, [Al₂(OH)₂(H₂O)₈]⁴⁺, and Al₃₀([Al₃₀O₈(OH)₅₆(H₂O)₂₄]¹⁸⁺) in six-fold coordinated Al species were 0–0.4 ppm, 0–0.77 ppm, and 70 ppm, respectively.^24,25 Fig. 3a shows that AlSA could slowly hydrolyze two coordination structures at the normal temperature of the water. The chemical shift at 4.02 ppm shows that a small amount of Al(H₂O)₆³⁺ was generated from AlSA hydrolysis and the chemical shift at 70.22 ppm indicates that mostly the Al₃₀([Al₃₀O₈(OH)₅₆(H₂O)₂₄]¹⁸⁺) coordination ion was produced. Fig. 3b shows that the chemical shift of [Al(H₂O)₆]³⁺ disappeared after 3 h hydrolysis at 100 °C and the chemical shift at 0.77 ppm indicates that [(H₂O)₄Al(μ-OH)(H₂O)₄]⁴⁺ was formed. After displacement of water in the solution by EG, only the Al₃₀([Al₃₀O₈(OH)₅₆(H₂O)₂₄]¹⁸⁺) coordination ion existed in the solution, as Fig. 3c shows.

Fig. 3 The ²⁷Al NMR spectroscopy of AlSA after treatment with water and EG: (a) AlSA was stirred for 3 h in DIW at room temperature; (b) AlSA was stirred for 3 h in DIW at 100 °C; (c) AlSA was stirred for 3 h in DIW at 100 °C and then the water was displaced with EG.

The coordination structure of Al and the carboxylic acid could be distinguished by infrared spectroscopy. The absorption peaks of ν_COO in the single coordination structure appeared at 1370 cm⁻¹ and 1650 cm⁻¹, while the absorption peaks of ν_COO in the dual coordination structure appeared at 1450 cm⁻¹ and 1580 cm⁻¹.^26,27 It could be speculated that the coordination structure of the single and dual coordination structures simultaneously existed in the coordination structures of AlSA from its FT-IR spectrum. Therefore, the change in the hypothetical coordination structure model in the process of AlSA hydrolysis is shown in Fig. 4. The hydrolyzate γ-AlOOH was made up of AlO₆ layered octahedrons, which were linked by hydrogen bonds.²⁸

Fig. 4 The change in the coordination structure model in the process of AlSA hydrolysis.

As seen in Fig. 5, AlSA could not dissolve in water and glycol acetate and the particle size distribution of the untreated AlSA was very large, with an average particle diameter d_(0.5) of 8.829 μm. The average particle diameter of the γ-AlOOH sol generated from AlSA hydrolysis could reduce to 0.478 μm. After displacement of water with EG, its average particle diameter could reach up to 1.342 μm. This was conducive to improving its surface area by reducing the particle size of the catalyst, which could improve the reaction activity and catalytic property of the catalyst.

Fig. 5 The particle size distributions of AlSA dispersed in H₂O, γ-AlOOH dispersed in H₂O, and γ-AlOOH dispersed in EG.

3.2 The catalytic effect of the AlSA/EGSb compound catalyst

The balance of the chain growth and thermal degradation reaction existed in the process of polyester polycondensation. According to the catalytic activity theory of metal catalysts proposed by Tomita,²⁹ the rate parameters p of the chain extension reaction of Al and Sb metal catalysts in the process of the catalytic condensation reaction were 47 and 76, respectively, and the rate parameters of the thermal degradation reaction were 5.0 and 5.5, respectively, which indicated that the catalytic activity of the metal Al catalyst was lower than that of the metal Sb catalyst and thus the thermal degradation ability of the metal Al catalyst was weaker than that of the metal Sb catalyst. Fig. 6 shows that the polycondensation time decreased firstly with the improvement of the γ-AlOOH content and then increased with a γ-AlOOH content of more than 60 wt% in the compound catalyst when the γ-AlOOH was added in the esterification stage, which indicated that an obvious synergistic effect existed between the two types of catalysts. Ramadugu, Ilgen, et al.^30,31 proved through theoretical calculations and experimental results that Al and Sb catalysts could form complexes. The complexes could enhance the catalytic activity of the compound catalyst and also might adsorb Sb compounds to enhance its catalytic activity due to the good adsorption property of γ-AlOOH.³² However, when γ-AlOOH was added in the prepolycondensation stage, the synergistic effect of the Al and Sb catalysts became quite weak. Therefore, the improvement in the polycondensation time was much lower than that of adding γ-AlOOH in the esterification stage, which might be because γ-AlOOH dehydration proceeded at high temperatures of 265–270 °C to produce Al(OH)₃ and Al₂O₃ in the prepolycondensation stage and its catalytic activity was lost.

Fig. 6 The effect of the additive amount of γ-AlOOH on polycondensation time in the compound catalyst.

As the viscosity of the melt polyester is proportional to the stirring power, it will be possible to determine the viscosity of the melt polyester by the stirring power change, which could characterize the catalytic efficiency of the catalyst. Fig. 7 shows that when γ-AlOOH was added in the stage of polycondensation, the maximum rate of the polycondensation reaction was observed when the mass proportion of EDSb and γ-AlOOH was 60 : 40. According to the results, the catalytic efficiency of the compound catalyst is higher than that obtained from using EDSb alone. In addition, its catalytic efficiency is lowest when using γ-AlOOH alone. This may prove that EDSb and γ-AlOOH can indeed produce a synergistic catalytic effect.

Fig. 7 Stirring motor power changes in the stage of final polycondensation. (This stirring motor power in the stage of the final polycondensation refers to the adjustment of the motor speed from 50 Hz to 25 Hz. The purpose is to ensure that the stirring motor will not be damaged by too much resistance in the mixing of high viscosity materials.)

The PETI polymerization process includes three stages: esterification, precondensation, and condensation. Without the addition of other acids, the esterification reaction could be conducted spontaneously with the catalysis of H⁺ generated from the reaction system under high temperature and high pressure. In the esterification and prepolycondensation stages, excessive amounts of EG existed in the reaction system. With the reaction of H⁺, it was easy to produce ether due to the occurring hydroxyl dehydration condensation reaction; meanwhile, the polyester chains containing hydroxyls would also conduct the etherification reaction, which increases the flexibility of the polyester chain segment and reduces the thermal stability of the polyester. Therefore, it is usual to reduce the formation of ether bond structure units with sodium acetate and metal acetate.^33,34 These ether bond structure units could be separated into DEG through alcoholysis of PETI, the content of which could indirectly show the occurrence degree of side reactions in the esterification and prepolycondensation stages. Fig. 8 shows that the increase of the γ-AlOOH content in the compound catalyst could significantly reduce the DEG content of PETI, and it was also seen that the inhibition effect of the DEG formation with the addition of γ-AlOOH in the prepolycondensation stage was better than that with the addition of γ-AlOOH in the esterification stage, which might be because Al(OH)₃ produced from parts of the dehydration of γ-AlOOH at high temperatures could capture H⁺ in the reaction system, and thus the generation probability of the ether bond structure unit decreased. The DEG could be produced in the esterification and prepolycondensation stages, so adding γ-AlOOH in the esterification stage could continue to inhibit the generation of DEG in the two stages.

Fig. 8 The effect of the content of γ-AlOOH in the compound catalyst on the content of diethylene glycol.

In order to study the chemical composition changes of γ-AlOOH in the catalytic reaction process, γ-AlOOH dispersed in EG was heated at 198 °C for 3 h and the FT-IR spectrum of the obtained product was contrasted with that of ethylene glycol aluminum (EGAl) prepared by the method described elsewhere.⁴ As shown in Fig. 9, the FT-IR spectrum showed that the methylene absorption peaks appeared at 2947 cm⁻¹ and 2866 cm⁻¹, respectively, indicating that EGAl was produced. In addition, the gas chromatography spectra show that 12.4% of γ-AlOOH was converted to EGAl. Fig. 10 shows that the formation of EGAl prepared by the two methods was accompanied by the generation of Al(OH)₃, which could be due to the hydrolysis of EGAl. When Al(OH)₃ and EGAl were heated at 198 °C for 3 h, the reaction product was EGAl, as shown in Fig. 9c. These results indicate that part of γ-AlOOH was associated with EGSb in the catalytic reaction process to produce a synergistic catalytic effect, and the rest of γ-AlOOH could react with EG to form EGAl, which could participate in the condensation reaction. Under the same experimental conditions, we compared the polycondensation time of EGSb and EGAl. The results showed that their polycondensation times are 315 min and 240 min, respectively, suggesting that the polycondensation catalytic efficiency of EGAl is lower than that of EGSb. Thus, it is further proven that the synergistic effect of γ-AlOOH and EGSb played an important role in increasing the catalyst efficiency.

Fig. 9 The FT-IR spectrum of EG reacting with γ-AlOOH (a), aluminium isopropoxide (b) and Al(OH)₃ (c) at 198 °C, respectively.

Fig. 10 The XRD patterns of the product obtained from EG reacting with γ-AlOOH (a) and aluminium isopropoxide (b) at 198 °C, respectively.

3.3 Properties

In the field of polyester synthesis, besides the polycondensation time, other properties were used to measure the catalytic activity of the catalyst such as thermal performance, characteristic viscosity, and chromaticity. The experiments showed that change in the γ-AlOOH content in the compound catalyst had little impact on the characteristic viscosity of the polyester, which was maintained at about 0.75 dL g⁻¹. However, its content has an obvious impact on the carboxyl end group value and chromaticity of PETI, as shown in Table 1, which could be used to measure the occurrence degree of side reactions in the process of the polyester synthesis. Usually, the more side reactions that take place, the greater the values of the carboxyl end group and chromaticity will be. As the thermal degradation ability and catalytic activity of the Al catalyst were lower than those of the Sb catalyst, the polycondensation time was prolonged and the side effects increased at a certain proportion of the compound catalyst. However, when γ-AlOOH was added in the prepolycondensation stage and the additive amount of the Sb and Al compound catalyst was 60 : 40, the values of the carboxyl end group and chromaticity were better than those obtained when only using the EGSb catalyst.

Table 1 The effect of the Sb and Al catalyst proportions on the properties of PETI

Proportions^a (Sb/Al)	Adding stage of γ-AlOOH	IV (dL g^−1)	Mn^b	–COOH (mol t^−1)	Chromaticity
					L	a	b
a Sb and Al stand for ethylene glycol stibium (EGSb) and γ-AlOOH, respectively.b The Mn value could be approximately calculated according to the formula [η] = KM^α, where K = 2.1 × 10^−4 and α = 0.82.
100/0	Esterification	0.79	22 772	13.63	71.65	−0.91	5.79
80/20	Esterification	0.75	21 525	16.07	72.64	−1.02	6.17
60/40	Esterification	0.74	20 832	13.16	74.12	−0.78	6.96
40/60	Esterification	0.75	21 520	16.07	70.07	−0.46	11.81
20/80	Esterification	0.75	21 647	19.54	72.03	−0.20	11.08
80/20	Prepolycondensation	0.75	21 456	14.63	71.14	−1.28	8.03
60/40	Prepolycondensation	0.75	23 435	13.13	69.73	−0.63	4.65
40/60	Prepolycondensation	0.75	21 508	16.35	67.63	−0.80	10.64
20/80	Prepolycondensation	0.76	21 989	18.29	70.91	−0.87	7.07
0/100	Esterification	0.69	19 432	20.70	72.07	−1.01	9.94

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Although the content of γ-AlOOH in the compound catalyst had little impact on the characteristic viscosity of PETI, the melting point (M_m) and the glass transition temperature (T_g) increased with the increase in γ-AlOOH content, as shown in Fig. 11 and 12. When γ-AlOOH was added in the esterification stage, the M_m increased from 243.2 °C to 249.2 °C and the T_g increased from 74.6 °C to 78.6 °C with the increase in the γ-AlOOH addition amount, which might be due to the formation of macromolecular aluminum alkoxide from the catalytic reaction residues reacting with the end hydroxy of PETI or the formation of the coordination structure from the reaction product and end carboxyl of PETI, affecting the thermal motion of the chain segment. The aluminium alkoxide or coordination structure had been verified through experimental results by McMahon and Li et al.^35,36 The hypothesis design model of the polymer is shown in Fig. 13. However, when γ-AlOOH was added in the prepolycondensation stage, the M_m of PETI rose only 0.8 °C, which might be due to the deactivation of γ-AlOOH at high temperatures.

Fig. 11 The change plots of the glass transition temperature (T_g) of PETI with the addition of γ-AlOOH in the esterification stage.

Fig. 12 The effect of the addition proportion of EGSb and γ-AlOOH on the melting point of PETI.

Fig. 13 The structural model of the crosslink center from the catalytic reaction residue of γ-AlOOH reacting with the end hydroxyl (a) and end carboxyl (b) of PETI.

Fig. 14 shows the TG curve of the γ-AlOOH added in the esterification and prepolycondensation stages, respectively. It can be seen that the addition stage and the proportion of the compound catalyst had no obvious effects on the thermal decomposition temperature of PETI and there was only a small difference in the residual mass, indicating that the usage of compound catalyst did not affect the thermal stability of PETI.

Fig. 14 The TG curve of the γ-AlOOH added in the esterification stage (a) and the prepolycondensation stage (b).

4. Conclusions

AlSA could ionize the Al(H₂O)₆³⁺ and Al₃₀([Al₃₀O₈(OH)₅₆(H₂O)₂₄]¹⁸⁺) coordination ions in water slowly and its main hydrolysates were γ-AlOOH and some polymorphous Al(OH)₃ with bayerite and doyleite. The compound catalyst made of γ-AlOOH and EGSb showed obvious synergetic effects in the process of the catalytic polycondensation of PETI. When the γ-AlOOH and EGSb were added in the esterification stage and the content of γ-AlOOH in the compound catalyst was less than 60 wt%, the polycondensation time decreased with increasing γ-AlOOH content. However, when its content was more than 60 wt%, the polycondensation time was prolonged because of the synergetic effect dropping off. When the mass ratio of γ-AlOOH and EGSb was 60 : 40, polycondensation time decreased by 18.6% compared with that obtained only using the EGSb catalyst. Meanwhile, the use of γ-AlOOH could play a catalytic role and also reduced the DEG content in the polyester effectively.

The improvement of the melting point and glass transition temperature show that the catalytic reaction residues of γ-AlOOH could play a crosslink center role in the PETI structure. The catalytic effect of the added γ-AlOOH in the esterification stage for the polyester synthesis was better than that of the added γ-AlOOH in the prepolycondensation stage and EGAl from the γ-AlOOH reacting with EG participated in the polycondensation reaction in the process of the catalytic reaction.

The above results showed that the use of a compound catalyst made of γ-AlOOH and EGSb could improve the catalytic efficiency and thermal properties of PETI. It has industrial application prospects in reducing the usage of Sb series catalysts for polyester synthesis.

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