Metal-Oxide-Doped Silica Nanoparticles for the Catalytic Glycolysis of

Copyright © 2011 American Scientific Publishers All rights reserved Printed in the United States of America

Journal of Nanoscience and Nanotechnology Vol. 11, 824–828, 2011

Metal-Oxide-Doped Silica Nanoparticles for the Catalytic Glycolysis of Polyethylene Terephthalate Muhammad Imran1 , Kyoung G. Lee1 , Qasim Imtiaz1 , Bo-kyung Kim2 , Myungwan Han2 , Bong Gyoo Cho3 , and Do Hyun Kim1 ∗ 1

RESEARCH ARTICLE

Department of Chemical and Biomolecular Engineering and Center for Ultramicrochemical Process Systems, Korea Advanced Institute of Science and Technology (KAIST), 373-1 Gusong-dong, Yuseong-gu, Daejeon 305-701, Korea 2 Department of Chemical Engineering, Chungnam National University, 220 Gung-dong, Yuseong-gu, Daejeon 305-764, Korea 3 Mineral Processing Department, Korea Institute of Geoscience and Mineral Resources, Gwahang-no 92, Yuseong-gu, Daejeon 305-350, Korea Polyethylene terephthalate (PET) was depolymerized to monomer bis(2-hydroxyethyl) terephthalate (BHET) using excess ethylene glycol (EG) in the presence of metal oxides that were impregnated on different forms of silica support [silica nanoparticles (SNPs) or silica microparticles (SMPs)] as glycolysis catalysts. The reactions were carried out at 300 C and 1.1 MPa at an EG-to-PET molar ratio of 11:1 and a catalyst-to-PET-weight ratio of 1.0% for 40–80 min. Among the four prepared catalysts (Mn3 O4 /SNPs, ZnO/SNPs, Mn3 O4 /SMPs, and ZnO/SMPs), the Mn3 O4 /SNPs nanocomposite had the highest monomer yield (>90%). This high yield may be explained by the high surface area, amorphous and porous structure, and existence of numerous active sites on the nanocomposite Delivered by Publishing Technology to: ? catalyst. The BHET yield increased with time and reached the highest level where equilibrium was IP: 93.91.26.12 On: Fri, 17 Jul 2015 06:16:08 established between BHET and its dimer. The catalysts were characterized by their SEM, TEM, Copyright: American Scientific Publishers and BET surface areas, and via XRD, whereas the monomer BHET was characterized by HPLC and FT-IR. The glycolysis with the Mn3 O4 /SNPs nanocomposite as the glycolysis catalyst produced a maximum BHET in a short reaction time.

Keywords: PET Recycling, BHET, Depolymerization, Glycolysis, Nanocomposite.

1. INTRODUCTION Polyethylene terephthalate (PET) is a widely used resin for synthetic fibers, audio and video taps, X-ray films, and softdrink bottles due to its excellent thermal, mechanical, and chemical properties. Industrial and societal concern with the recovery and disposal of huge volumes of post-consumer plastic waste is growing. Severe legal restrictions are being imposed on landfills and incineration, which are the most practiced waste management methods so far, but the trend is changing rapidly towards the evolving recycling technologies.1 PET is one of the most extensively recycled polymeric materials. There are many processes by which post-consumer plastics can be recycled, but the one that is acceptable for sustainable development is the chemical recycling process because it leads to the formation of monomers from which the polymer is made.2 Among various chemical recycling processes, ∗

Author to whom correspondence should be addressed.

824

J. Nanosci. Nanotechnol. 2011, Vol. 11, No. 1

glycolysis using EG as a depolymerization solvent is the oldest, simplest, and least capital intensive process.3 Moreover, the recovered monomer (BHET) from glycolysis can be blended with fresh BHET, and the mixture can be used again for virgin PET production. There have been numerous studies on glycolysis that used different catalysts and different reaction conditions. Baliga and Wong4 reported the use of metal acetates (zinc, manganese, cobalt, and lead) as glycolysis catalysts, and they found that the zinc acetate had the best monomer yield. Shukla et al.5–7 studied several catalysts (sodium carbonate, sodium bicarbonate, glacial acetic acid, lithium hydroxide, sodium sulfate, potassium sulfate, and zeolites) and showed that their monomer yield is comparable with that of conventionally used catalysts such as zinc acetate and lead acetate. Wang et al.8 recently reported the use of ionic liquids for the depolymerization of PET. Most glycolysis processes are conducted at a wide range of temperatures, from 160 to 220 C, and with a reaction time of 0.5–8 h. Because of the long reaction time and the low 1533-4880/2011/11/824/005

doi:10.1166/jnn.2011.3201

Imran et al.

Metal-Oxide-Doped Silica Nanoparticles for the Catalytic Glycolysis of Polyethylene Terephthalate

J. Nanosci. Nanotechnol. 11, 824–828, 2011

825

RESEARCH ARTICLE

monomer yield of existing catalysts, a new process with 2.2. Catalyst Synthesis nano-sized silica supporting metal oxide nanocomposites The silica nanoparticles were synthesized via the Stöber as glycolysis catalysts is proposed and compared with method,14 with a few modifications. 8.0 ml of ammonium microcomposites of conventional micro-sized silica suphydroxide (28 wt%) and 6.0 ml of deionized water were porting metal oxide catalysts. added to 100 ml of ethanol in a sealed round bottom flask Interest in nanomaterials for various applications is and mixed for 15 min. 4.7 ml of TEOS was added to this growing due to their unique optical, electrical, mechansolution and stirred with a magnetic stirrer at room temical, and catalytic properties. Over the past decade, the perature for 3 h. The resulting precipitates were separated exceptional and novel activity of nanomaterials, in combivia centrifugation and washed several times with water and nation with recent advances in materials science, has led 9–11 ethanol, after which they were dried in an oven at 70 C to an explosive development in nanocatalysis. Since all for 8 h. The dried powder was finally calcined at 500 C the elementary reactions occur at the atomic or molecfor 12 h. ular scale, catalysis also takes place at the nanoscale The silica (SNPs or SMPs)-supported manganese or sub-nanoscale. Nanometer-sized catalysts have specific oxide or zinc oxide was prepared by adding a prephysical properties that differ from those of their bulk determined amount of silica support to the 1.0 M precounterpart and lead towards higher catalytic activity, 12 cursor [Mn(NO selectivity, and stability. 3 2 · xH2 O or Zn(NO3 2 · 6H2 O] solution. The metal oxide loading was kept at 15 wt%. The soluDue to the unique features of nanocomposites at a tion was sonicated for 45 min using a horn-type sonicator reduced length scale, such as a high surface area and uni(Branson Sonifier). At the beginning of the sonication, a form dispersion, silica nanoparticles (SNPs) were synthe0.1 M ammonia solution was added to maintain the pH sized and used for nanocatalyst support in this study. Their of the solution at around 9.5. The mixture was separated catalytic performance significantly depends on their prepavia centrifugation and washed with water and ethanol. The ration method, the dispersion of active catalytic species, catalyst samples were then dried at 100 C for 8 h and and the type of the catalyst support. Oxides of manganese calcined at 350 C for 3 h. and zinc (Mn3 O4 , ZnO) were impregnated on the silica nanoparticles (SNPs) through ultrasound irradiation, and on the silica microparticles (SMPs), to compare their catDelivered by Publishing Technology 2.3. Glycolysisto: of ?PET alytic activities. The catalysts thatIP: were prepared using 93.91.26.12 On: Fri, 17 Jul 2015 06:16:08 sonochemical reactions showed higher catalytic American activity Scientific Copyright: Publishers The glycolysis reactions were carried out in a stainless than those that were prepared using conventional impregsteel (SS 316) batch-type reactor (10 ml) at 300 C nation methods.13 PET was glycolized with EG at 300 C and 1.1 MPa. A mixture of 0.3 g of PET, 1.1 g of and 1.1 MPa in the presence of the catalysts that were EG, and 0.003 g of a catalyst (Mn3 O4 /SNPs, ZnO/SNPs, prepared as previously described. The effect of the reacMn3 O4 /SMPs, or ZnO/SMPs) were loaded into the reactor, tion time on the monomer yield was investigated to study and the air inside the reactor was purged with argon the catalytic performance (in terms of the monomer yield) to avoid oxidation of the PET and the glycolysis prodof the nanocomposites (Mn3 O4 /SNPs and ZnO/SNPs) and ucts. The reactor was placed in a furnace at 300 C for the microcomposites (Mn3 O4 /SMPs and ZnO/SMPs). 40–80 min. After the given reaction time, the reactor was quenched in cold water to immediately stop the reaction. To quantitatively determine the monomer BHET, the reac2. EXPERIMENTAL DETAILS tion products were dissolved in THF and analyzed via 2.1. Materials HPLC. The molar yield of BHET was calculated based on the following relationship: Virgin PET pellets (3-mm × 3-mm) were obtained from SK Chemicals from Korea. These pellets were mixed moles of BHET produced Yield of BHET (mol%) = ×100 with dry ice and ground to fine powder with a particle moles of PET units size of 202 m in a common grinder. The number averTo determine the functional groups of the monomer age (MWn ) and weight average (MWw ) molecular weights BHET qualitatively, the glycolysis reaction products were of the virgin PET were 11,053 and 38,378, respectively. dissolved in boiling water, and the resulting hot solution Tetraethyl orthosilicate (TEOS), ammonium hydroxide, was immediately filtered. The BHET was considerably solmanganese (II) nitrate hydrate (Mn(NO3 2 · xH2 O), zinc uble in boiling water, and the filter cake was washed sevnitrate hexahydrate (Zn(NO3 2 · 6H2 O), anhydrous EG, eral times to extract almost all the BHET. The filtrate was and standard BHET were purchased from Sigma-Aldrich. stored in a refrigerator at 4 C for 24 h. White BHET crysTetrahydrofuran (THF), ethanol, and SMPs (1–20 m) tals were formed, which were filtered and dried at 70 C were purchased from J. T. Baker, Merck, and Junsei Chemfor 12 h. The dried crystals were then used for the FT-IR icals, respectively. All the reagents were used as purchased, without further purification. analysis.


Imran et al.

of the support material. The morphologies of the nanocomposites (Mn3 O4 /SNPs and ZnO/SNPs) and microcomposThe catalyst particles were characterized via FE-SEM ites (Mn3 O4 /SMPs and ZnO/SMPs) are shown in Figure 1. (S-4800, Hitachi), FE-TEM (JEM-2100F, Jeol Ltd.), and The SEM (Figs. 1(a and b)) and TEM (Figs. 1(e and f)) XRD [D/Max-RB (12 KW), Rigaku], and with a BET surimages of the nanocomposites showed uniform disperface area analyzer (Tristar-3000, Micromeritics). sion of the metal oxides around the silica nanoparticles. The monomer BHET was analyzed via HPLC (PU-980, In the case of the microcomposites, their SEM images UV-975, Jasco) and FT-IR (IFS 66/S and Hyperion 3000, (Figs. 1(c and d)) showed aggregates of the metal oxide Bruker Optiks). For the HPLC, a reverse-phase Zorbax-C8 nanoparticles around the micro-sized silica. This difference column and an ultraviolet (UV) detector that was set at in the metal oxide distribution was due to the size and shape 254 nm were used. A 50:50 (v/v) THF/H2 O solution was of the support material, as well as the interaction between used for the mobile phase at a flow rate of 1.0 ml/min. the active metal oxide and the support material. The SNPs were amorphous and the SMPs were polycrystalline, as shown by the XRD patterns in Figure 2. Crys3. RESULTS AND DISCUSSION talline ZnO was observed on the polycrystalline SMPs and the amorphous SNPs, as shown in Figures 2(a) and (b), 3.1. Catalyst Morphology and Characterization respectively. Crystalline Mn3 O4 was observed on the polycrystalline SMPs, and the crystalline Mn3 O4 had no difMetal oxide is an important catalyst in a wide range of feraction peaks on the amorphous SNPs, as shown in applications. However, pure metal oxides easily agglomFigures 2(c) and (d), respectively. These show that the erate during their synthesis, which reduces their active sonochemically impregnated Mn3 O4 was homogenously surface area for the catalysis. For better dispersion with dispersed on the amorphous SNP support and exhibited less agglomeration, manganese and zinc oxides were an amorphous state. Sonochemical reactions could produce doped separately on two different forms of support: SNPs amorphous materials, depending on the temperature in the (150±10 nm) and SMPs (1–20 m), with the use of ultraring region of the collapsing bubbles.15 After the collapse sound irradiation. The principle of sonochemical reactions of the bubbles, the high cooling rate of several hundred K/s is based on cavity explosions that produce local high temhinders the organization and crystallization of the reaction Delivered by Publishing Technology to: ? perature and pressure. This local high temperature and products. IP: 93.91.26.12 On: Fri, 17 Jul 2015 06:16:08 pressure assist the growth of the metal oxide on the surface For catalytic applications, high-surface-area materials, Copyright: American Scientific Publishers whether polycrystalline or amorphous, are generally (b) : Zincite-ZnO : Hausmannite-Mn3O4 (a)

Intensity (a.u.)

(a)

: ZnO (b)

20 30 40 50 60 70 80 2 Theta

(d)

Intensity (a.u.)

(c)

Intensity (a.u.)

RESEARCH ARTICLE

2.4. Characterization

(c)

(d)

20 30 40 50 60 70 80 2 Theta

(e)

(f)

20

30

40

50

60

70

80

2 Theta

Fig. 1. Morphologies of the nanocomposites and the microcomposites: SEM images of (a) Mn3 O4 /SNPs; (b) ZnO/SNPs; (c) Mn3 O4 /SMPs; and (d) ZnO/SMPs, and TEM images of (e) Mn3 O4 /SNPs and (f) ZnO/SNPs.

826

Fig. 2. XRD patterns of the synthesized catalysts: (a) ZnO/SMPs; (b) ZnO/SNPs; (c) Mn3 O4 /SMPs; and (d) Mn3 O4 /SNPs. (All the diffraction peaks in (a) and (c), other than the ZnO and Mn3 O4 peaks, corresponded to that of the polycrystalline silica).



needed. The use of supported oxides, which consist of active metal oxides dispersed on an inert support with a high surface area, is one of the ways of obtaining highsurface-area catalysts. The surface area, pore volume, and average pore diameter of the supported catalysts that were measured using the BET method are shown in Table I. High surface areas were obtained in the case of the nanocomposites, because of the small support size and the high pore volume. 3.2. Glycolytic Depolymerization of PET

100 90 80

Monomer (BHET) yield

Imran et al.

Mn3O4/SNPs ZnO/SNPs Mn3O4/SMPs ZnO/SMPs No catalyst

70 60 50 40 30 20

% Transmittance

(a)

Table I. BET surface area, pore volume, and average pore diameter of the supported catalysts.

Catalyst Mn3 O4 /SNPs ZnO/SNPs Mn3 O4 /SMPs ZnO/SMPs

BET surface area (m2 /g) 45.09 22.44 3.38 2.49

Pore volume (cm3 /g) 0.214 0.154 0.020 0.014


Average pore diameter (nm) 18.9 30.2 24.4 21.4

(c) (a) (b)

(d)

O HOCH2CH2O C

4000

3500

C OCH2CH2OH

(c) 3000

2500

2000

Wavenumber

1500

(a) 1000

500

(cm–1)

Fig. 4. Comparison of the FT-IR spectra of the standard BHET and the BHET that was produced in this study with Mn3 O4 /SNPs as the glycolysis catalyst.

827

RESEARCH ARTICLE

PET glycolysis involves the incorporation of diols into 10 PET chains to produce BHET. A mechanism with three 0 steps has been proposed: (i) glycol diffusion into the 40 50 60 70 80 polymer, (ii) polymer swelling, which increases the difReaction time (min) fusion rate, and (iii) the reaction of glycol with an ester bond in the PET chain.16 Polymer degrades to oligomers, Fig. 3. Effect of the type of catalyst on the BHET yield at 300 C and 1.1 MPa. dimers, and ultimately, to monomer BHET (PET + EG → oligomers → dimer ↔ BHET). nanocomposite.17 18 PET glycolysis is a heterogeneous A mixture of 0.3 g of PET, 1.1 g of EG (EG/PET: reaction, and the interface between the solid catalyst and 11/1, mole ratio), and 0.003 g of a catalyst (catalyst/PET = the liquid reaction mixture controls the reaction. The large 1 wt%) were used for the glycolysis reaction (40–80 min) number of active sites and the high surface area of the at 300 C and 1.1 MPa. A relatively high pressure of catalyst could weaken the polymer network and help in 1.1 MPa was needed to keep the EG in the liquid phase. the depolymerization of PET in a short reaction time.7 The effect of the type of catalyst on the BHET yield is Figure 3 illustrates shown in Figure 3. The BHET yield significantly Delivered increased by Publishing Technology to: ?the order of occurrence of the catalytic activity in terms of the monomer yield (Mn3 O4 /SNPs > IP: 93.91.26.12 On: Fri, 17 Jul 2015 06:16:08 with time in the case of the catalytic glycolysis, contrary Copyright: American Scientific Publishers ZnO/SNPs > Mn O4 /SMPs > ZnO/SMPs), which shows 3 to the slow increase without a catalyst. This shows that the reasonable agreement with the order of the catalyst surglycolysis without a catalyst is a slow process under these face area and the pore volume in Table I. It is thus conoperating conditions, and that a suitable catalyst is indisO /SNPs catalyst could provide the cluded that the Mn 3 4 pensable to the enhancement of the reaction rate. Figure 3 most active surfaces for catalysis among the four preshows that the BHET yields are higher with nanocompared catalysts, which enhances the reaction rate and maxposites than with microcomposites. This is because of the imizes the monomer yield. From these observations, it is uniform dispersion of the active metal oxide on the SNP support with a high surface area, as shown in the BET surface areas in Table I. The BHET yield increased with time and reached the highest level (>90%) after an 80-min Aldrich BHET reaction time with the Mn3 O4 /SNPs catalyst, during which an equilibrium was established between the monomer and its dimer. A longer reaction time may increase the dimer amount at the expense of the BHET yield.8 The high Our BHET monomer yield in the short reaction time can be explained by the amorphous structure, high surface area, and high pore volume of the catalyst, as well as by the existence of (b) (d) numerous active sites on the surface of the Mn3 O4 /SNPs


RESEARCH ARTICLE

concluded that the SNPs support is more beneficial to the preparation of active metal oxide nanocomposites than the SMPs support. The FT-IR spectra of the BHET crystals and the standard BHET are shown in Figure 4. The agreement of the FT-IR spectra proved that the monomer produced by the nanocomposite (Mn3 O4 /SNPs) as the glycolysis catalyst was almost pure BHET. The spectrum showed an –OH band at 3,424 and 1,128 cm−1 , an aromatic C–H at 1,456–1,502 cm−1 , C O at 1,712 cm−1 , and alkyl C–H at 2,879 and 2,964 cm−1 .

Imran et al.

Center for Ultramicrochemical Process Systems sponsored by KOSEF.

References and Notes

1. L. Lundquist, Y. Leterrier, P. Sunderland, and J. A. E. Manson, Life Cycle Engineering of Plastics: Technology, Economy, and the Environment, Elsevier Science Ltd., UK (2000), pp. 39–40. 2. D. S. Achilias and G. P. Karayannidis, Water, Air, & Soil Pollution 4, 385 (2004). 3. J. Scheirs and T. E. Long, Modern Polyesters: Chemistry and Technology of Polyesters and Copolyesters, John Wiley & Sons Ltd., England (2003), p. 579. 4. S. Baliga and W. T. Wong, J. Polym. Sci., Part A: Polym. Chem. 27, 2071 (1989). 4. CONCLUSION 5. S. R. Shukla and K. S. Kulkarni, J. Appl. Polym. Sci. 85, 1765 (2002). The glycolysis of PET with anhydrous EG was carried 6. S. R. Shukla and A. J. Harad, J. Appl. Polym. Sci. 97, 513 (2005). out at 300 C and 1.1 MPa in the presence of sono7. S. R. Shukla, V. Palekar, and N. Pingale, J. Appl. Polym. Sci. chemically produced supported metal oxides as glycoly110, 501 (2008). sis catalysts. The nanocomposites performed better as as 8. H. Wang, Y. Liu, Z. Li, X. Zhang, S. Zhang, and Y. Zhang, Eur. Polym. J. 45, 1535 (2009). catalysts than the microcomposites. Among the nanocom9. T. Sanders, P. Papas, and G. Veser, Chem. Eng. J. 142, 122 (2008). posites, the highest monomer yield >90%) was obtained 10. J. J. Bong, K. Y. Jung, K. Wooyoung, K. Pil, and Y. Jongheop, with Mn3 O4 /SNPs. This high yield may be attributed to J. Nanosci. Nanotechnol. 8, 5130 (2008). the high surface area, amorphous and porous structure, and 11. R. Ji, X. Lu, and J. Zhang, J. Nanosci. Nanotechnol. 9, 5134 existence of numerous active sites on the surface of the (2009). 12. U. Heiz and U. Landman, Nanocatalysis, Springer-Verlag, New York nanocomposite catalyst, which weakened the polymer net(2007), p. 245. work and helped depolymerize the PET in a short time. 13. W. R. Moser, Advanced Catalysts and Nanostructured Materials: The monomer BHET was analyzed via HPLC and FT-IR, Modern Synthetic Methods, Academic Press, USA (1996), p. 202. Delivered by Publishing Technology to: ? and the FT-IR spectrum of the produced BHET was com14. W. Stober and A. Fink, J. Colloid Interface Sci. 26, 62 (1968). IP: 93.91.26.12 On: Jul 2015 06:16:08 pared with that of the standard BHET, which verified theFri, 17 15. S. D. Jackson and S. J. Hargreaves, Metal Oxide Catalysis, WileyCopyright: American Scientific Publishers VCH, Germany (2009), p. 631. high level of purity of the produced BHET. 16. J. Aguado and D.P. Serrano, Feedstock Recycling of Plastic Wastes, The Royal Society of Chemistry, UK (1999), p. 33. Acknowledgments: The authors acknowledge the 17. H. Einaga and A. Ogata, J. Hazard. Mater. 164, 1236 (2009). financial support of the Resource Recycling R&D Center 18. C. Reed, Y. K. Lee, and S. T. Oyama, J. Phys. Chem. B 110, 4207 sponsored by the 21C Frontier R&D Program and of the (2006).

Received: 20 July 2009. Accepted: 8 December 2009.

828
