Characterization of Solid Acid Catalysts and Their Reactivity in the

Article pubs.acs.org/IECR

Characterization of Solid Acid Catalysts and Their Reactivity in the Glycolysis of Poly(ethylene terephthalate) Minli Zhu,† Zengxi Li,†,* Qian Wang,†,‡ Xueyuan Zhou,†,‡ and Xingmei Lu‡,* †

College of Chemistry and Chemical Engineering, Graduate University of Chinese Academy of Sciences, Beijing 100049, PR China Beijing Key Laboratory of Ionic Liquids Clean Process, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR China

‡

ABSTRACT: Glycolysis of poly(ethylene terephthalate) with ethylene glycol was carried out with the catalysts of sulfated cobalt oxide (SO42−/Co3O4) and zinc-modified sulfated cobalt oxide (SO42−/Co−Zn−O). The catalysts prepared at different calcination temperatures have been characterized by various techniques. These techniques include X-ray diffraction, Raman spectrum, gas adsorption analysis, NH3-temperature programmed desorption, and IR spectroscopy of pyridine adsorption. The performances of these catalysts on the depolymerization of PET under mild conditions were studied, and relationships between the catalysts’ textural properties, the surface acidity, and the catalytic activity have been investigated. The experimental results showed that the catalytic activity on glycolysis of PET was significantly improved by the catalysts of binary sulfur oxides of SO42−/Co−Zn−O. The conversion of PET obtained on SO42−/Co−Zn−O-300 °C was 96%, and the selectivity of BHET was 75%, after 3 h at 180 °C under atmospheric pressure.

1. INTRODUCTION Poly(ethylene terephthalate) (PET) is a semicrystalline thermoplastic polyester, which is widely used in the manufacture of soft-drink bottles, synthetic fibers, films, and textile.1−4 With the widespread use and increasing consumption of PET, the amount of waste PET is growing rapidly,5 and accompanied environmental problems are becoming more and more serious.6 There has been a growing need for the recycling of postconsumer PET goods to increase resource conservation and improve the environment. Chemical recycling through depolymerization of PET into its monomer is an ideal recycling method because, in a theoretical sense, PET could be recycled permanently. Therefore, it is an effective way to preserve the resources and to protect the environment by material recycling.7,8 Glycolysis of PET with ethylene glycol as solvent is an effective method to produce the high purity and significant yield of monomer bis(2-hydroxyethyl terephthalate) (BHET).2,9−11 Traditionally, glycolysis of PET is catalyzed by metal acetate,11−13 alkaline carbonate, or organic metal oxide,10,14,15 etc. These catalysts show high catalytic activity with good yield of the BHET monomer. However, the separation and reutilization of these catalysts from the reaction products are difficult. Many attempts have been made to exploit new kinds of catalysts. Sulfated metal oxides with both Brönsted and Lewis acid sites have been studied as suitable replacement for the liquid acid catalysts. As catalyst, sulfated metal oxides have some additional advantages, such as ease of separation from a liquid reaction mixture, low corrosion of the reaction equipment, and free from environmental pollution.16−18 This work is a continuation of our early research which talked about the properties of sulfated metal oxides of ZnOTiO2/SO42− catalyst.19 Compared with the early research, this work focuses on the Zn and Co-based solid catalysts. Cobalt © 2012 American Chemical Society

oxide catalysts have been investigated extensively and are widely applied as catalysts;20−22 however, less effort has been made on the investigation of the composite transition metal oxides of ZnO−Co3O4. The combination of two oxides leads to the creation of new systems with novel physicochemical properties as compared to a single component system. In this study, SO42−/Co3O4 and SO42−/Co−Zn−O catalysts were prepared and calcined at different temperatures. The effect of the calcination temperature on textural properties was obtained by XRD, Raman spectrum, and the adsorption of N 2 techniques. The nature of acid sites of catalysts was investigated by NH3-TPD and FT-IR of adsorbed pyridine. The catalytic activity of these two series of catalysts was tested by the reaction of glycolysis of PET, and the relationships between textural properties, surface acidity, and catalytic activity were extensively discussed. Further, the reaction process of heterogeneous solid (PET)−solid (catalyst) reaction and the catalytic reaction mechanism of the glycolysis of PET have been investigated.

2. EXPERIMENTAL SECTION PET pellets (2.0 × 2.5 × 2.7 mm3) were pure and supplied by Jindong Commercial Co. Ltd., Jiangsu Province, China. The viscosity-average molecular weight of PET was 2.63 × 104 g mol−1 measured by a viscosity method in the previous study23 of our laboratory. Analytical grade of zinc acetate (≥99.0%), ethylene glycol, ammonia (28%), ammonium sulfate (≥99.0%), and cobalt(II) sulfate heptahydrate (>99.5%,) were obtained from Sinopharm Received: Revised: Accepted: Published: 11659

February 26, 2012 August 15, 2012 August 17, 2012 August 17, 2012 dx.doi.org/10.1021/ie300493w | Ind. Eng. Chem. Res. 2012, 51, 11659−11666

Industrial & Engineering Chemistry Research

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supporting plate (10−15 mg, 13 mm diameter), placed in an IR cell and treated at 150 °C under vacuum (0.0013 Pa) for 1 h. After the IR cell was cooled to room temperature, pyridine vapor was introduced into the cell and adsorbed for 10 min and took 30 min for equilibrium. Then it was scanned after being vacuumed for 15 min and recorded at room temperature, 100 °C, 150 °C, 250 °C under vacuum. 2.3. Catalytic Activity Tests. Catalytic performance was evaluated in a three-neck flask equipped with a reflux condenser, a thermometer, and a magnetic stirrer. When 5 g of PET and 25 mL of ethylene glycol were heated to 180 °C, 0.015 g of catalyst (0.014 mols catalyst/mol of PET) was added, and the reaction was carried out at 180 °C, for 3 h, under atmospheric pressure. When the reaction was finished, the flask was cooled to 60 °C within 1 min in an ice bath. Then the undepolymerized PET was separated, washed, dried, and weighed. The washed water of undepolymerized PET was mixed with the product mixture. The product mixture was diluted with about 200 mL of water and was stirred for half an hour at room temperature, which would dissolve the remaining EG and the BHET monomer. The oligomer was insoluble and separated by filtering and then was dried and weighed. The filtrate was concentrated and stored in a refrigerator at 5 °C for overnight. White crystalline flakes of BHET monomers were formed in the filtrate, and then separated, dried, and weighed.1 In this study, the catalytic activity of the catalyst on the glycolysis reaction was measured by testing the conversion of PET (%) and the selectivity of BHET (%), which were calculated in eqs 1 and 2. In eq 1, WPET,i is the initial weight of PET, and WPET,u is weight of unreacted PET.

Chemical Reagent Beijing Co., Ltd., China. The materials were used without any further treatment. 2.1. Catalyst Preparation. Catalysts of SO42−/Co3O4 (designated as SC) were prepared as follows: A solution of CoSO4·7H2O was used to obtain hydroxide cobalt by precipitation upon aqueous ammonia at room temperature. The aqueous ammonia was added dropwise with vigorous stirring, and the final pH of the solution was adjusted to 9. The solution including precipitation was kept in a water bath at 70 °C for 4 h followed by filtration with suction, washing with deionized water, and drying at 110 °C for 12 h. The powder was pulverized to 140−200 mesh and impregnated with 0.5 M (NH4)2SO4 solution (15 mL g−1) under continuous stirring at room temperature for 4 h followed by filtering, drying, and calcining at 200, 300, 400, 500, and 600 °C for 3 h, respectively. The powder was stored in a sealed glass ampule for each until use. The Co−Zn hydroxides were prepared by the coprecipitation method. A mixture of aqueous solution of CoSO4 and Zn(Ac)2 (Co:Zn = 1:1 mol ratio) were prepared and aqueous ammonia was used as the precipitator to maintain a pH of 9. Subsequently, the catalysts of SO42−/Co−Zn−O (designated as SCZ) were obtained according to the processes with the same sequence above. Catalysts of SC and SCZ calcined at different temperatures are denoted as SC-°C and SCZ-°C in the following cases. 2.2. Catalyst Characterization. X-ray diffraction patterns of catalysts were recorded by X’Pert PRO MPD diffractometer operated at an accelerating voltage of 40 kV and an emission current of 40 mA with Cu Kα radiation. The angle (2θ) was measured in steps of 0.418° s−1 between 20° and 80°. Raman spectroscopy were recorded by a LabRam HR 800 spectrometer (Jobin Yvon-Horiba) with 514.53 nm of an Ar−Kr 2018 RM laser (Spectra Physics) as the excitation source. Weight loss and temperature associated with phase transformation were determined by thermogravimetry and differential thermal analysis (TG/DTA) on a Seiko TG/DTA SSC 5000 analyzer. The fresh samples of SC and SCZ before calcination were heated from room temperature to 600 °C at rate of 5 °C min−1 in nitrogen flow (30 mL min−1). The BET specific surface area and the pore size were obtained from nitrogen absorption and desorption isotherm method at liquid nitrogen temperature on a Quanta Chrome Instrument NOVA 2000. Prior to analysis, samples calcined above 300 °C were degassed at 300 °C for 6 h under 10−3 Torr and samples calcined below 300 °C were degassed at 150 °C for 10 h. TPD of ammonia was performed on an Autochem II 2920 apparatus from Micromeritics. In a typical experiment, 20 mg of catalyst was pretreated in a helium flow at 150 °C at a heating rate of 5 °C min−1 and the sample was kept at this temperature for 1 h. Subsequently the sample was treated with a 10% NH3− He flow for 30 min at room temperature, then the sample was purging in a helium flow for 1 h at 100 °C, until the baseline was stable. The desorption profile was measured by the thermal conductivity detector in flowing helium at a heating rate of 5 °C min−1 to 600 °C. The surface acidity of the catalysts was investigated by means of studying pyridine adsorption via Fourier transform infrared (FTIR) spectroscopy. Pyridine FTIR spectroscopy was recorded on a Nicolet 6700 spectrometer equipped with an in situ quartz cell. The sample was pressed into a self-

conversion of PET (%) =

WPET,i − WPET,u WPET,i

× 100

selectivity of BHET (%) moles of BHET = × 100 moles of depolymerized PET units

(1)

(2)

3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization. 3.1.1. XRD Results. The structural changes occurring with increasing calcination temperatures are shown in Figures 1 and 2 for the series of SC and SCZ catalysts, respectively. For SC in Figure 1, some of the diffraction lines of CoSO3·2.5H2O (JCPD-ICDD 00-0330438) were observed in SC-200. For the samples of SC-300− 400, the peaks were weak and broad and the catalysts were found to be amorphous and poorly crystallized. When the calcination temperature increased beyond 500 °C, the diffraction peaks of crystalline Co3O4 (JCPD-ICDD 01-0781969) were observed, and the intensity of the peaks increased with increasing temperature. A few extra lines due to CoSO4 (JCPD-ICDD 00-028-0386) could also be observed in SC-600. For SCZ in Figure 2, a few lines of Zn2Co3(OH)10·2H2O (JCPD-ICDD 00-021-1477) were found in SCZ-200. With increasing calcination temperature from 300 to 400 °C, the diffraction peaks of ZnO with hexagonal phase (JCPD-ICDD 01-089-7102) appeared distinctly. However, there were no Co related crystalline lines observed clearly, which was consistent with the result of SC-300−400. Further, the diffraction peaks of ZnCo2O4 (JCPD-ICDD 01-081-2299) with cubic phase appeared in the sample of SCZ-500, and the intensity of the diffraction peaks increased with increasing calcination temper11660

dx.doi.org/10.1021/ie300493w | Ind. Eng. Chem. Res. 2012, 51, 11659−11666


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Figure 3. Raman spectra of the SC with different calcination temperatures.

Figure 1. XRD patterns of the SC with differen calcination temperatures: (#) Co3O4, cubic; (o) CoSO4, orthorhombic; (∗) CoSO3·2.5H2O, tetragonal.

Figure 4. Raman spectra of the SCZ with different calcination temperatures. Figure 2. XRD patterns of the SCZ with differen calcination temperatures: (+)ZnCo2O4, cubic; (∗) Zn3O(SO4)2, monoclinic; (o) CoSO4, orthorhombic; (#) ZnO, hexagonal; (v) Zn2Co3(OH)10·2H2O, monoclinic.

observed in the case of XRD result. The Raman spectra exhibited three peaks at around 175−372, 515, and 660 cm−1, which were related to an element of cobalt. However, the intensity of peaks decreased distinctly, and the peaks became much broader than those peaks of Co3O4 in Figure 3. There was a broad band at 1077 cm−1 which attributed to an element of zinc.28,29 The broad band and absent peaks of Co3O4 and ZnO indicate that ZnO and Co3O4 species were highly dispersed in the samples of SCZ-200−400, and most of the ZnO was actually fixed in the framework of Co3O4 to distort the cubic structure of Co3O4 in the samples of SCZ-500−600. Thus, the Raman results support the conclusion made by XRD. 3.1.3. Thermal Analysis (TG/DTA). The thermal analysis of the catalysts SC and SCZ before calcination is depicted in Figure 5. For hydrate materials, there are two or three steps which can be observed due to (1) dehydration, (2) layer dehydroxylation, and (3) anion decomposition.30 Similar results were obtained in this work. For both SC and SCZ, the first weight loss occurred between 100 and 200 °C mainly ascribed to the loss of interlayer and adsorbed water molecules. The second weight loss was between 200 and 400 °C accompanied with the first big endothermic DTA peak at around 300−350 °C, which was attributed to dehydroxylation of the hydrate layers and decomposition of interlayer ammonium and/or acetate. The weight loss between 400 and

ature from 500 to 600 °C. The absent lines of crystalline ZnO in the samples of SCZ-500−600 indicate that ZnO has strongly interacted with Co3O4. The ZnCo2O4 phase occurred as a solid−solid reaction between ZnO and Co3O4.24 In the samples of SCZ-500−600, in addition to the lines of ZnCo2O4 compound, a few extra lines due to the formation of Zn3O(SO4)2 (JCPD-ICDD 00-032-1475) and the formation of CoSO 4 (JCPD-ICDD 01-072-1455) could also be observed.25 3.1.2. Raman Spectroscopy. Raman spectroscopy can provide complementary structural information of these catalysts. The Raman spectra of the series of SC and SCZ catalysts are shown in Figures 3 and 4, respectively. For the samples of SC with different calcination temperatures (Figure 3), the Raman spectra exhibited five main lines at 188, 464, 502, (1) 597, and 660 cm−1, which corresponded to the F(3) 2g , Eg, F2g , F(2) and A symmetry of Co O cubic structures, respec2g 1g 3 4 tively.26,27 The Raman spectra of SCZ with different calcination temperatures (Figure 4) became more complicated due to the overlapping contribution between the Co and Zn oxides as 11661



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catalysts have been performed without ammonia admission under the same experimental conditions. The final NH3-TPD signals were based on blank signals. The NH3-TPD profiles for each series of SC and SCZ catalysts are shown in Figures 6 and 7, respectively. The peaks

Figure 5. TGA/DTA curves of the SC (a) and SCZ (b).

600 °C was little and no visible endothermic DTA peak was observed as well. This indicates no further decomposition of compounds in the catalyst. 3.1.4. BET Specific Surface Area Analysis. The BET specific surface area, pore volume, and mean pore diameter of the catalysts SC and SCZ are summarized in Tables 1 and 2,

Figure 6. NH3-TPD curves for the SC with different calcination temperatures.

Table 1. Textural and Acid Properties of SC Catalysts Calcined at Different Temperatures SC°C

SBET (m2/g)

pore volume (cm3/g)

mean pore diameter (nm)

total acid amount (mmol g−1)

S (wt %)

200 300 400 500 600

23.7 27.5 67.1 14.8 10.2

0.111 0.100 0.145 0.338 0.220

18.9 14.4 9.8 22.3 20.9

2.76 2.81 2.42 0.63 0.12

8.0 8.2 8.2 8.3 8.1

Table 2. Textural and Acid Properties of SCZ Catalysts with Different Calcination Temperatures SCZ°C

SBET (m2/g)

pore volume (cm3/g)

mean pore diameter (nm)

total acid amount (mmol g−1)

S (wt %)

200 300 400 500 600

6.2 10.6 13.7 9.5 7.5

0.043 0.064 0.114 0.335 0.068

28.2 23.6 32.7 37.1 36.4

3.63 3.58 2.04 0.82 0.25

7.2 7.7 7.7 7.8 7.2

Figure 7. NH3-TPD curves for the SCZ with different calcination temperatures.

shown in each of the profiles correspond to the desorption of NH3 which was bound to the acid sites of the oxide surface. The desorption temperature indicates the acid strength of the catalysts. The higher temperature of desorption is, the stronger is the acid strength. Generally the peaks between 150 and 400 °C are corresponding to weak and middle strength acid sites. The peaks between 400 and 600 °C represent the strong acid sites. It was found that SC-200−400 presented middle strength acid sites with desorption peaks at around 340 °C. In the case of binary oxides SCZ-200−300, there were two big desorption peaks at around 250 and 330 °C which corresponded to weak and middle strength acid sites for each. The weak acid sites faded away in the samples of SCZ-400−500, while a few of the strong acid sites around 450 °C were presented. Compared with these two series catalysts, the acid strength on SCZ was more diverse than that on SC. The acid amount corresponding to the amount of adsorbed NH3 was estimated by integrating the area under the peaks. The total acid amount for each series of SC and SCZ catalysts are presented in Tables 1 and 2, respectively. For both catalysts of SC and SCZ, the total amount of acid sites decreased gradually with increasing calcination temperature. Generally

respectively. Primarily, catalysts with mesopores were preferred. It was found that the BET specific surface area and pore volume of the binary oxides SCZ were slightly lower than those of SC. However, the mean pore diameter of catalysts was bigger in the binary oxides SCZ. For each series of catalysts, with increasing calcination temperature, the specific surface area first increased and had a peak at 400 °C and then decreased slightly. Taking the thermal analysis into consideration, there was decomposition of sample between 200 and 400 °C and more pores were produced. The decreased surface area at higher temperature may be primarily due to agglomeration of catalysts. 3.1.5. Acidic Analysis. For NH3-TPD analysis, the peak temperature is a measure of the strength of acid sites while the area under the peak represents the total amount of acid sites present on the catalyst.31 From the thermogravimetry studies, there was the decomposition of ammonium and/or acetate which can be also detected by conductivity detector (TCD). To distinguish the desorbed NH3, blank experiments for all 11662



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pyridine adsorbed at room temperature, 100, 150, and 250 °C on SC-300 and SCZ-300, respectively. Several bands were observed in the region from 1700 to 1300 cm−1. The bands at 1654 and 1540 cm−1 were assigned to the pyridine adsorbed on Brönsted acid, the band at 1605 cm−1 was assigned to pyridine adsorbed with hydrogen bonding, the band at 1450 cm−1 was attributed to pyridine adsorbed on Lewis acid, and the IR band at 1490 cm−1 was attributed to both Brönsted and Lewis acids.34 For both SC-300 and SCZ-300 there were no obvious bands at 1654 and 1540 cm−1 which indicates that less Brönsted acid was on the surface of the catalysts. However, the bands at 1450 cm−1 were distinctly strong and decreased slightly with increasing the vacuum temperature to 250 °C. This means that the Lewis acid predominated over both SC300 and SCZ-300 catalysts and the strength of the Lewis acid was relatively strong. This occurred because pyridine that was adsorbed on the weak acid desorbed at lower temperature, and that which was adsorbed on a strong acid desorbed at higher temperature. 3.2. PET Depolymerise Reaction. 3.2.1. Effect of Calcination Temperature. Calcination temperature is a key factor in synthesizing a solid acid catalyst since it largely affects the textural and catalytic properties of the catalyst. The effect of calcination temperature on the catalytic activity of the series of SC catalysts is shown in the Figure 10. The results showed that

more acid sites were generated on the surface of SCZ calcined at different temperatures than that on the corresponding ones of SC. This may be due to the interaction between the zinc oxide and the cobalt oxide. It is supposed that the generation of acid site is caused by an excess of a negative or positive charge in the structure of mixed oxides.32 Lewis acid sites are electronically deficient of Zn and Co ion centers as a result of the electron-withdrawing nature of the sulfate group. In the case of Co3O4−ZnO binary oxides Zn−O−Co linkages are expected to be present. Therefore, the sulfate ion may be coordinate to the surface of the Co3O4−ZnO binary oxides and form the support of sulfate surface species.25,33 In this case more surface acid sites could be expected on the binary oxides than on the single one. To investigate the decrease of acid sites on catalysts at high calcination temperature, the sulfur content of the sulphated catalysts was quantitative determined by the XRF spectrometry method. From the results in Tables 1 and 2, the sulfur content of both series of SC and SCZ catalysts has been maintained stable and no obvious loss of sulfur was observed with increasing calcination temperature from 200 to 600 °C. From XRD results, the new compounds of CoSO4 and Zn3O(SO4)2 were observed in SC-600 and SCZ −600, respectively. Therefore the decrease of acid sites may be due to part of the sulfate group transformed into sulfate compounds. Infrared spectroscopic studies of pyridine adsorbed on solid surface have made it possible to distinguish Brönsted acid from Lewis acid. Figures 8 and 9 showed the FT-IR spectra of the

Figure 10. Effect of the calcination temperature of the SC on the conversion of PET and the selectivity of BHET. Figure 8. FTIR spectra of pyridine adsorbed on the SC-300 °C.

the conversion of PET was around 65% on the catalysts calcined at low temperature, and then decreased gradually at calcination temperature beyond 500 °C. Take the textural analysis into consideration, the catalysts calcined at low temperature with amorphous and poor crystalline Co3O4 structure showed higher activity than the catalysts calcined at high temperature with cubic Co3O4 structure in glycolysis of PET. Furthermore, from the acid analysis, the conversion of PET on SC-200−600 followed a similar sequence with the total acid amount. The catalysts calcined at low temperature with more middle strength acid sites showed higher activity than the catalysts calcined at high temperature with few acid sites. Although the PET conversion clearly changed according to calcination temperature, their selectivity to monomer BHET remained stable at around 68%. The results of the catalytic activity over SCZ catalysts are shown in the Figure 11. The catalysts of SCZ-200−400 showed the conversion of PET at around 95%, and decreased slightly from 88% to 81% over the SCZ-500−600 catalysts. The

Figure 9. FTIR spectra of pyridine adsorbed on the SCZ-300 °C.

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higher than that of SC-500−600, which indicates that the compound of ZnCo2O4 showed higher catalytic activity than Co3O4. 3.2.2. Thermal Analysis of the Products. The DSC thermograms of BHET, oligomers, and PET are shown in Figure 12, which are represented by curve 1, 2, and 3,

Figure 11. Effect of the calcination temperature of the SCZ on the conversion of PET and the selectivity of BHET.

selectivity of monomer BHET remained stable at around 74% on the catalysts of SCZ-200−600 °C. From the textural analysis, crystalline ZnO with hexagonal phase and Co3O4 with amorphous structure showed higher activity than that of cubic ZnCo2O4 structure in glycolysis of PET. Furthermore, taking the acid property into account, the conversion of PET followed the similar sequence with the total amount of acid sites. The weak and middle strength acid sites on SCZ-200−400 were more than that on SCZ-500−600. The SCZ-600 showed relatively high catalytic activity while its overall acidity was relatively low. From the analysis of NH3-TPD, there were still some acid sites generated on SCZ-600. From XRD results, the compounds of ZnCo2O4 and Zn3O(SO4)2 presented on SCZ600. The electron withdrawing group of SO42− made the Zn3O(SO4)2 show some Lewis acidity, and the electrons transferred between Zn and Co also made the ZnCo2O4 show week Lewis acidity. These Lewis acid sites contributed to the relatively high catalytic activity of SCZ-600. Three catalysts (SCZ-200, -300, and -400) gave similar catalytic results, which perhaps indicates that the equilibrium was attained under the conditions of at 180 °C for 3 h. To analyze the nature of the examined catalysts, experiments with shorter reaction time (2 h) were done. From the results in Figure 11, after 2 h of reaction, SCZ-300 showed the highest catalytic activity, SCZ-400 showed second highest catalytic activity, and SCZ-200 showed relatively low catalytic activity. Considering that the compounds and acid properties are similar on both SCZ-200 and SCZ-300 catalysts, the reason for the decrease of catalytic activity of SCZ-200 may be the relatively small BET surface area. SCZ-400 has the highest surface area, however, on which the overall acid sites began to decrease, which may influence the catalytic activity. Compared with two series catalysts, the binary sulfur oxides of SCZ showed higher catalytic activity and BHET selectivity than single sulfur cobalt oxide SC. The conversion of PET over the catalysts SCZ increased about 20% to 30% more than that over the SC. Taking the catalysts’ structure and acid properties into consideration, the Lewis acid sites were promoted by the interaction between zinc and cobalt oxides and more acid sites were generated on the complex oxides SCZ calcined at low temperatures than the corresponding ones on SC. Therefore the total acid sites may contribute to improve the catalytic activity. Further, when SCZ and SC calcined at high temperature, there were few acid sites on the surface of the catalysts, but the catalytic activity of SCZ-500−600 still was

Figure 12. DSC scans of BHET (curve 1), oligomer (curve 2), and PET (curve 3).

respectively. For curve 1, the only one sharp endothermic peak at 109.6 °C agreed very well with the melting points of BHET,1 which indicate that the experimental procedure (section 2.3) is sufficient to separate the oligomers from BHET. On the curve 2, there is one endothermic peak at 168.9 °C which represented the melting temperature of the dimer.35 There is also a much weaker and broader peak near 216 °C, which can be related to the presence of a small amount of mixture of higher oligomers. For the unreacted PET (curve 3), the melting temperature is at 246.1 °C. 3.2.3. Catalyst Stability and Reusability. The stability of catalysts was tested by recycling the SCZ-300 °C for four times. The reaction conditions were the same as described in section 2.3. To determine whether the catalyst lost activity or not, the reaction time of 100% conversion of PET was recorded. When the PET reached 100% conversion, the PET pellets were all degraded and dissolved into ethylene glycol, and a light yellow transparent liquid was obtained. After the reaction, the catalyst could be separated by centrifugation of the hot liquid reaction product and reused without any further purification. Owing to the viscosity of the liquid mixture, the yield of recovery catalyst was about 80% by the method of washing, drying, and weighing. Therefore before every recycle experiment 20% of fresh catalyst was complemented. The conversion of PET and selectivity of BHET were calculated. The results of the recycling study are given in Table 3. For the fresh and recycled catalysts, it took about 200 min to achieve 100% conversion. Therefore, the catalytic activity of the recycled catalysts remained the same as that of the fresh catalyst, and the selectivity of BHET remained around 70−80%, which showed high stability of the catalyst in the reaction environment. 3.2.4. Mechanism of the Glycolysis of PET. Initially the glycolysis of PET is a heterogeneous solid (PET)−solid (catalyst) reaction where activity will be governed by the external area of both solids. As the reaction proceeds, the PET is further degraded into soluble oligomers at the longer reaction time. To investigate at which extent the internal BET surface 11664



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Table 3. Effect of the Reuse Time of SCZ-300 °C Catalyst on the Glycolysis of PET production distributing (wt %)

cycle

time of 100% conversion (min)

PET conversion (%)

BHET selectivity (%)

BHET

oligomers

fresh 1st 2nd 3rd 4th

202 210 200 198 205

100 100 100 100 100

72.4 72.6 74.4 74.2 76.3

91.4 91.8 91.0 91.3 92.1

8.6 8.2 9.0 8.7 7.9

area is catalytically relevant, the effect of reaction time on the conversion of PET and the selectivity of BHET was investigated using the catalyst SCZ-300 °C. The reaction conditions were the same as described in section 2.3. As seen from the experimental results (Figure 13), the conversion of

Figure 13. Effect of reaction time on the conversion of PET and the selectivity of BHET.

PET increased directly with increasing reaction time. Especially after 1 h of reaction, the reaction speed rapidly increased. It slowed down after 3 h of reaction, and the conversion of PET reached 96%. At the beginning of the reaction, the selectivity of BHET was as low as 14% at 0.25 h. With increasing reaction time, the selectivity increased distinctly and reached 70% at 1 h. Further, with increasing reaction time, the selectivity of BHET remained stable at around 70%, until at a reaction time of 5 h there was no obvious increase of BHET selectivity. The experimental results indicate that at the beginning of the reaction (0.25 h), the reaction is governed by the interaction between external area of both solid PET and solid catalyst. With the reaction going on, the PET is further degraded into oligomers which are soluble in the ethylene glycol, and at this time the internal BET surface area is catalytically relevant. Considering the oligomers are straight chains and the mean pore diameter of catalysts is more than 10 nm, the pores of catalysts are large enough for the oligomers to get into. According to the results of the experiments, the mechanism of glycolysis of PET catalyzed by solid acid catalysts is illustrated in Figure 14. When the catalyst of SC or SCZ is added, this process is a Lewis acid catalytic reaction. The Lewis acid sites on the surface of the catalyst interact with the carbonyl oxygen (CO) in the ester, and then the oxygen in the hydroxyl of ethylene glycol attacks the carbon cation of the ester group, forming a tetrahedral intermediate. Afterward, the

Figure 14. Mechanism of the glycolysis of PET catalyzed by solid acid catalyst.

hydrogen in ethylene glycol leaves and combines with the O of C−O in the ester to form HOCH2CH2−. Then the catalyst leaves, forming CO again. The acyl-oxygen cleaves, and the −HOCH2CH2− group leaves. These transfer processes are repeated, and BHET monomer is formed.

4. CONCLUSIONS Catalysts of SC and SCZ were prepared at different calcination temperatures. The effect of calcination temperature on the textural properties, surface acidity, and catalytic activity of the catalysts has been studied, and the relationships between the catalysts’ textural properties, the surface acidity, and the catalytic activity have been investigated. The low calcination temperature contributed to amorphous and poor crystallization structure and improved the total amount of Lewis acid sites on the surface of both SC and SCZ catalysts. More Lewis acid sites were produced on the surface of SCZ than on SC. Binary oxides of SCZ performed better catalytic activity on glycolysis of PET than the single oxide of SC. Under the conditions of 180 °C, atmospheric pressure, and a reaction time of 3 h, the 11665



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conversion of PET obtained on SCZ-300 was 96% and the selectivity of BHET was 75%.

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AUTHOR INFORMATION

Corresponding Author

*(Z.L.) E-mail: [email protected]. Tel.: +86-10-88256322. Fax: +86-10-88256322. (X.L.) E-mail: [email protected]. Tel.: +86-10-82544800. Fax: +86-10-82627080. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by General program of National Natural Science Foundation of China (No. 21076221; No. 20976174; No. 21036007).

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REFERENCES

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