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Boron nitride nanosheets decorated with Fe3O4 nanoparticles as a magnetic bifunctional catalyst for post-consumer PET wastes recycling
Polymer Degradation and Stability 169 (2019) 108962
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Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Boron nitride nanosheets decorated with Fe3O4 nanoparticles as a magnetic bifunctional catalyst for post-consumer PET wastes recycling Mohammad Reza Nabid*, Yasamin Bide**, Mahsa Jafari Department of Polymer & Material Chemistry, Faculty of Chemistry & Petroleum Science, Shahid Beheshti University, G.C., P.O. Box 1983969411, Tehran, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 July 2019 Received in revised form 1 September 2019 Accepted 10 September 2019 Available online 11 September 2019
Due to its enormous consumption in various fields and also the non-degradability, recycling of polyethylene terephthalate is critical for waste management causing reduction in disposal, oil use, and carbon dioxide emissions. Among various recycling methods, glycolysis of PET was carried out because of milder reaction conditions and yielding bis(hydroxyethyl) terephthalate (BHET) as valuable versatile monomer. To conduct an efficient PET glycolysis, a recyclable bifunctional nanomaterial including highly dispersible Fe3O4 nanoparticles immobilized on hexagonal boron nitride nanosheets (h-BNNS) as important two-dimensional nanomaterials are chosen. A simple solvothermal method was used for decoration of h-BNNS with Fe3O4 nanoparticles. After characterization of prepared material, investigation of catalytic activity for glycolysis of recycled PET bottles under different conditions have been conducted which shows a rapid complete reaction with high BHET yield in the presence of very low amount of the catalyst. After reaction completion, the catalyst was easily recycled by an external magnet for several runs without considerable loss of catalytic activity. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Boron nitride nanosheets Magnetic nanoparticles Catalysis Glycolysis Polyethylene terephthalate
1. Introduction Poly(ethylene terephthalate) (PET) as a thermoplastic nonbiodegradable polyester has been extremely used in various fields in our daily life such as fibers, plastic films, insulating materials, food packages, and bottles [1]. Based on a report by the US National Park Service, PET bottles need around 450 years to be decomposed [2]. On the other hand, the annual world consumption of PET has exceeded 50 million tons with still an exponential increase which lead to a serious white pollution and resource waste. Several chemical methods have been employed for PET recycling, such as hydrolysis, aminolysis, ammonolysis, methanolysis, and glycolysis [3e5]. The glycolysis process, which involves transesterification of PET chains with diols to form commercially valuable product, BHET monomers, is the most important recycling process of PET [6,7]. Numerous catalytic researches have been presented for PET glycolysis, but there are still some limitations such as slow reaction rate, low selectivity and harsh reaction conditions and consequently high cost and high energy consumption of PET recycling
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (M.R. Nabid), [email protected] (Y. Bide). https://doi.org/10.1016/j.polymdegradstab.2019.108962 0141-3910/© 2019 Elsevier Ltd. All rights reserved.
which leads to comprehensive investigation on the development of efficient catalysts [6,8e10]. Due to the increased number of active sites and modification of the intrinsic properties in nanoscale, using nanocatalysts instead of conventional metal salts for PET glycolysis has several benefits like higher efficiency and selectivity toward BHET. But, recyclability of the nanocatalyst for further runs is a critical issue that should be considered for less environmental problems. Metal oxides nanoparticles capable of magnetic recycling can be a suitable option for environmental sustainability, easy isolation and reusing of catalyst for further runs as well as their catalytic activity [11,12]. Recently, our group developed the synthesis of g-Fe2O3/nitrogen-doped graphene hybrid material as the first report of bifunctional catalyst for PET recycling which g-Fe2O3 acts as acidic sites and N-doped graphene as basic site [4]. The successful use of graphene as a two-dimensional (2D) carbon nanomaterial has suggested excellent opportunities for other 2D nanomaterials, especially boron nitride nanosheets with the same catalytic activity as well as high thermal and physicochemical stability, mechanical strength, high thermal conductivity, high resistance to oxidation and being structurally robust [13e15]. Moreover, compared with graphene, B-N bonds exhibit partially ionic character making h-BNNS a prominent electronic-tuning support for various applications such as gas adsorbent, water cleaning, hydrogen storage, drug delivery, and catalysts [16e19]. To the best
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of our knowledge, there is no report of using h-BNNS to embrace an interfacial electronic effect on Fe3O4 induced by (2D) h-BNNS with nitrogen and boron vacancies for PET glycolysis. In this paper, Fe3O4 nanoparticles were deposited on h-BNNS as an efficient catalyst for PET glycolysis which possess the advantages of being magnetically recyclable and also a good interaction between PET and boron atoms due to their electron deficient nature.
Veregue et al. [21] After the reaction, the temperature of the solution was reached to 120 C using a bath for separation of PET not depolymerized. The obtained solid material was separated from the liquid phase by filtration. The remaining solution containing the solubilized BHET was refrigerated for 24 h, and then filtered to obtain BHET. The recovered BHET crystals were dried, and characterized by 1H NMR.
2. Experimental
3. Results and discussion
2.1. Materials
3.1. Synthesis and characterization of the catalyst
h-BN powder (5-10 mm in diameter) was purchased from Alfa Aesar. Commercially clear PET soft-drink bottles were recycled, washed, dried, and cut into 4 2 mm pellets for glycolysis experiments. Iron(III) chloride (FeCl3), trisodium citrate (Na3Cit), anhydrous ethylene glycol and sodium acetate (NaOAc) were obtained from Merck.
A gas exfoliation method was employed to prepare h-BNNS according to the recent report by Zhu et al. [20]. Fe3O4 nanoparticles immobilized on h-BNNS were synthesized through a simple solvothermal method during which FeCl3$6H2O was used as the iron source. Owing to the nitrogen groups as Lewis base sites, hBNNS absorb and anchor Fe3þ which prevent the Fe3O4 nanoparticles from spontaneous homogeneous nucleation and formation of independent Fe3O4 nanoparticles separated from h-BNNS. Ethylene glycol was used as both solvent and reductant. Sodium acetate and trisodium citrate performed as alkali source and electrostatic stabilizer, respectively [22]. Fig. 1 shows SEM images of a sample of Fe3O4 nanoparticles immobilized on h-BNNS. The presence of small nanoparticles on boron nitride nanosheets has been confirmed by SEM. Most of the nanosheets settled facially on the target substrate because of vacuum filtration-induced alignment. Well dispersed sphere-like Fe3O4 nanoparticles on the surfaces of h-BNNS are observed from SEM images. The TEM images of Fe3O4 NPs@h-BNNS have been shown in Fig. 2 which clearly reveal the coating of h-BNNS with spherical Fe3O4 nanoparticles having size range of 3e10 nm and a quite uniform size distribution. According to TEM observations, Fe3O4 nanoparticles were successfully immobilized onto the h-BNNS surface. The interconnected structure or branched network morphology can be related to interparticle van der Waals interactions between the h-BNNS. The Fe3O4 nanoparticles obtained by solvothermal method show typical X-ray diffraction (XRD) patterns of magnetite (JCPDS no. 19- 629; Fig. 3A). The broad diffraction peaks further propose the nanocrystalline structure of the Fe3O4 magnetite particles. The XRD pattern of h-BNNS synthesized by gas exfoliation method was presented in Fig. 3B. The (002) diffraction peak is observed confirming the synthesis of h-BNNS. The main peak of h-BNNS at 2q of 20.01 can be observed in the XRD pattern of Fe3O4 NPs@h-BNNS with slightly shift probably due to the interaction of h-BNNS with Fe3O4 NPs. Moreover, the Fe3O4 peaks are observed in the XRD pattern of Fe3O4 NPs@h-BNNS (Fig. 3C). FT-IR spectra of Fe3O4 NPs and Fe3O4 NPs@h-BNNS are displayed in Fig. 4. The FeeO vibration at 578 cm1 is observed in FT-IR spectrum of Fe3O4 NPs. The broad band appeared in the range 3200e3600 cm1 is related to the OH stretching vibration. Moreover, the absorption bands at 1620 and 1388 cm1 are ascribed to carboxylate demonstrating the existence of carboxyl groups of citrate (Fig. 4A). In FT-IR spectrum of Fe3O4 NPs@h-BNNS, two dominant peaks at 1388 and 879 cm1 were observed according to the inplane BeN transverse optical modes of the sp2-bonded h-BN, and the BeNeB out-of-plane bending vibration, respectively [23]. Moreover, the absorption peak of Fe-O can be observed (Fig. 4B). To study the chemical composition of Fe3O4 NPs@h-BNNS, XPS as a more sensitive analysis was used (Fig. 5). The binding energies of B1s and N1s are observed at 190.6 and 398.0 eV, respectively. The values are in good agreement with the reported values. The B/N ratio from our XPS survey is 1.08. The peaks of Fe2p1/2 and Fe2p3/2,
2.2. Instruments and characterization Fourier transform infrared (FT-IR) spectra were recorded on a Bomem MB-Series FT-IR spectrophotometer. X-ray powder diffraction (XRD) data were collected on an XD-3A diffractometer using Cu Ka radiation. Field emission scanning electron microscope (FE-SEM) was performed on a Zeiss Sigma VP instrument equipped with an oxford instrument Energy Dispersive X-ray (EDX) detector. High-resolution transmission electron microscopy (HR-TEM) was performed by FEI Tecnai F20 system (methanol was used as a solvent for HR-TEM). X-ray photoelectron spectroscopy (XPS) was performed using a Thermofisher Scientific K-Alpha in an ultrahigh vacuum. 1H NMR spectra were recorded with a BRUKER DRX-300 AVANCE spectrometer, and CDCl3 was used as solvent. 2.3. Synthesis of hexagonal boron nitride nanosheets The commercial h-BN powder (1 g) was heated to 800 C in a muffle furnace, maintained at this temperature for 5 min and then instantly transferred into a dewar bottle containing liquid nitrogen until it gasified totally. These steps were repeated to obtain h-BNNS which then dispersed in isopropanol and sonicating for 30 min. The dispersion was centrifuged at 800 rpm to remove any remaining bulk crystals. The supernatant was collected and dried in vacuum oven overnight [20]. 2.4. Synthesis of Fe3O4 nanoparticles immobilized on hexagonal boron nitride nanosheets Firstly, h-BNNS (0.3 g), Iron(III) chloride (1 g, 6.2 mmol) and trisodium citrate (0.20 g, 0.68 mmol) were dissolved in ethylene glycol (20 mL). Afterwards, sodium acetate (1.20 g) was added with stirring. The mixture was stirred vigorously for 30 min and then immersed in a Teflon-lined stainless-steel autoclave. The autoclave was heated to 200 C and kept for 10 h, and then allowed to cool to room temperature. The obtained suspension was washed with deionized water three times and then dried in a vacuum oven for 12 h at room temperature. 2.5. General procedure for the glycolysis of PET Typically, 0.3 g of PET, 10 mL of EG, and a certain amount of catalyst were immersed in a Teflon-lined stainless-steel autoclave. The autoclave was heated to 200 C and kept for 5 h, and then allowed to cool to room temperature. The BHET precipitation without water was accomplished according to the recent report by
M.R. Nabid et al. / Polymer Degradation and Stability 169 (2019) 108962
3
Fig. 1. SEM images of Fe3O4 nanoparticles immobilized on h-BNNS.
Fig. 2. TEM images of Fe3O4 NPs@h-BNNS
located at 711 and 724.9 eV, further validated the successful formation of Fe3O4 on h-BN surfaces. 3.2. Catalytic activity To investigate the catalytic activity of as-obtained Fe3O4 NPs@h-
BNNS for glycolysis of PET, several reaction conditions including various temperature, catalyst amount, time, EG content, reflux or autoclave conditions was tested. The PET conversion and BHET monomer yield as the functions of these factors were calculated according to equations (1) and (2), respectively.
4
Conversion of PET ð%Þ ¼
M.R. Nabid et al. / Polymer Degradation and Stability 169 (2019) 108962
initial weight of PET weight of undepolymerized PET 100 initial weight of PET
moles of BHET produced 100 Yield of BHET ð%Þ ¼ moles of PET units
(2)
Fig. 3. XRD patterns of Fe3O4 nanoparticles (A) h-BNNS (B) and Fe3O4 nanoparticles immobilized on h-BNNS (C).
(1)
The results have been summarized in Fig. 6. As an optimum condition, the glycolysis with 0.003 g catalyst, and 10 mL EG at 200 C in the autoclave for 300 min was chosen. To study the influence of temperature as one of the most important factor in the kinetics of a chemical reaction and the catalytic efficiency, the PET glycolysis was conducted at 200, 220 and 240 C in the Teflon-line autoclave and also under refluxing at 198 C in the presence of 0.002 g catalyst with 10 mL EG. The reaction was completed only in 30 min at 240 C in the stainless steel batch-type reactor with 100% BHET yield and selectivity. Decreasing the reaction temperature to 220 and 200 C causes the reaction to complete within 180 and 300 min, respectively. In the next step, to evaluate the effect of catalyst content, the reaction was conducted with 0.001, 0.002 and 0.003 g Fe3O4 NPs@h-BNNS for 0.3 g PET. According to the results, the glycolysis was completely done with 0.002 g catalyst yielding a maximum value of 100% BHET remaining constant also with 0.003 g catalyst. Using very low amount of catalyst (0.6 wt%) is one the most amazing feature of the presented catalyst. It is notable that the glycolysis reaction did not proceed without Fe3O4 NPs@h-BNNS showing the necessity of the catalyst. After optimizing the temperature and catalyst amount, the BHET yield and conversion were monitoring at different reaction times at 200 C with 0.002 g catalyst. As shown in Fig. 6, PET conversion into BHET was increased with increasing the reaction time. Optimal time is the time at which PET conversion is maximum. Afterwards, the yield reduces because of the polymerization of BHET as a side reaction. Based on the results, the PET recycling in terms of BHET conversion and yield complete after 300 min. When the amount of EG increases from 4 to 10 mL, the BHET yield increases. The optimal amount of EG is 10 mL giving 100% yield under the above mentioned conditions. According to the literatures, as the particles size of PET increases, the yield of PET glycolysis decreases. Considering that in this work a relatively large pellets from recycled PET soft drink bottles have been used, the catalytic efficiency of Fe3O4 NPs@hBNNS is excellent.
Fig. 4. FT-IR spectra of Fe3O4 nanoparticles (A) and Fe3O4 nanoparticles immobilized on h-BNNS (B).
M.R. Nabid et al. / Polymer Degradation and Stability 169 (2019) 108962
5
Fig. 5. High resolution XPS spectra of B 1s, C 1s, Fe 2p, and N 1s.
Several previously reported catalysts have been compared with this work in terms of reaction temperature, amount of catalyst, reaction time, PET size, and EG amount and the data have been given in Table 1. As seen, the as-synthesized catalyst shows relatively good results compared to most of the reported catalysts. To investigate the effect of catalyst components and further prove the catalytic performance of as obtained Fe3O4 NPs@h-BNNS for PET glycolysis, Fe3O4 NPs and h-BNNS were also examined under the above mentioned optical conditions. The yield of BHET that was catalyzed with Fe3O4 NPs was found to be 61% in the presence of 0.002 g catalyst for 0.3 g PET, 10 mL EG at 200 C for 5 h. Under similar reaction conditions to have a correct comparison, BHET yield with h-BNNS reached 82.3%. Although the BHET yields for two components of catalyst are relatively high, the yield with Fe3O4 NPs@h-BNNS reached to 100% that suggest the synergetic effect of both Fe3O4 NPs and h-BNNS in the catalytic glycolysis of postconsumer PET waste. Some possible properties of h-BNNS are used to explain the catalytic activity of Fe3O4 NPs@h-BNNS. The unique polarity of boron nitride bonds and high surface area of hexagonal boron nitride related nanostructures result in good catalytic activity. In
addition, the coexistence of boron atoms as Lewis acidic sites and nitrogen atoms as basic sites can afford various types of chemical interaction between h-BNNS and PET. Nitrogen atoms as basic sites can enhance the nucleophilic attack of ethylene glycol through the hydrogen abstraction from hydroxyl groups. Boron atoms as acidic sites promote the electrophilic character of carbonyl groups of PET which make them more vulnerable to the attack of nucleophile EG. In addition, as a graphene-like material, similar pp interaction may exist between h-BNNS and aromatic PET. So, the coexistence of boron and nitrogen atoms as acidic and basic sites as bifunctional catalyst for PET glycolysis and also pp interaction between hBNNS and aromatic PET cause the efficient glycolysis of PET chain under the relatively mild reaction conditions [30]. Fig. 7 gives 1H NMR of as-obtained BHET monomer wherein it may be observed the peaks at 4.03 and 4.53 related to methylene protons of CH2-OH and COO-CH2, respectively. The signal at 4.74 ppm corresponds to eOH groups. The peak of aromatic ring protons has been appeared at 8.15 ppm. From these observations, the successful glycolysis of post-consumer PET to BHET monomer has been proven.
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M.R. Nabid et al. / Polymer Degradation and Stability 169 (2019) 108962
Fig. 6. (A) Effect of the amount of Fe3O4 NPs@h-BNNS on the PET glycolysis. Reaction conditions: PET (0.3 g), EG (10 mL), 200 C, 300 min. (B) Effect of glycolysis time. Reaction conditions: PET (0.3 g), EG (10 mL), Fe3O4 NPs@h-BNNS (0.002 g), 200 C. (C) Effect of the amount of EG. Reaction conditions: PET (0.3 g), Fe3O4 NPs@h-BNNS (0.003 g), 200 C, 300 min. (D) Effect of different catalysts. Reaction conditions: PET (0.3 g), EG (10 mL), catalyst amount (0.003 g), 200 C, 300 min.
Table 1 Comparison of several previously reported catalysts with Fe3O4 NPs@h-BNNS Entry Catalyst
Temp. (oC)
Amount of catalyst/amount Time of PET (min)
PET size and type (mm)
Amount of EG/amount of PET
BHET yield
BHET selectivity
Ref
1
[bmim]FeCl4
178
1/5
240
2.0 2.5 2.7
20/5
100
59.2
[24]
2 3 4 5 6 7
g-Fe2O3
255 190 190 260 300 190
0.01 1/2 20/2 1.0 0.003/0.3 0.018%
60 180 120 60 80 40
0.15 fine powder 4 2.8 3.2 4 2.8 3.2 <0.2 3 2 2.5 2.7
20/2 20/2 17.2 1.1/0.3 1/4
>90 100 100 >90 >96 >84.5
75 100
[11] [25] [8] [12] [26] [27]
8 9
(MgeZn)eAl LDH Fe3O4- MWCNT ZnMn2O4 GO-Mn3O4 POMsa Na12[WZnM2(H2O)2(ZnW9O34)2] Zinc acetate [bmim]FeCl4
195 178
0.0008 1/5
60 240
3 3 0.03 2.0 2.5 2.7
0.75 20/5
31.3 100
59.2
[28] [29]
10
Fe3O4 NPs@h-BNNS
200
0.002/0.3
300
42
10/0.3
100
100
This work
3.3. Catalyst recycling
4. Conclusion
Among the nanocatalysts, magnetic recyclable nanocatalysts can present an important platform for sustainable chemistry due to the high activity and easy separation and recycling. The recyclability and reusability of as-obtained Fe3O4 NPs@h-BNNS catalyst were investigated which shows the successful reusability for four runs in the glycolysis of post-consumer PET without major loss of activity (Fig. 8). The slight decrease in activity can be attributed to the incomplete magnetic separation of the catalyst or leaching of catalyst during the reaction. The significant versatility of the presented material in terms of tune ability for various applications and design based on the process including combination of boron nitride nanosheets on the magnetite supports will be able to provide significant advances for the as obtained material in the near future.
Recycling post-consumer PET by Fe3O4 magnetic nanoparticles on h-BNNS through glycolysis reaction were reported. The influence of each component was investigated which indicates the catalytic activity of Fe3O4 due to Lewis acid character and h-BNNS as graphene-like nanosheets with boron and nitrogen groups acting as Lewis acid and base, respectively. Despite using postconsumer PET with large size, the excellent results were obtained. Relatively fast and complete conversion of PET to BHET virgin monomer at 200 C, using very low amount of the catalyst, magnetic recyclability of the catalyst, and BHET precipitation without water are some advantages of as-obtained catalyst. Combining the unique properties of boron nitride nanosheets with magnetic and catalytic features of Fe3O4 nanoparticles make the presented material suitable for various applications.
M.R. Nabid et al. / Polymer Degradation and Stability 169 (2019) 108962
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Fig. 7. 1H NMR spectrum of BHET.
[8]
[9]
[10]
[11]
[12]
[13] Fig. 8. Effect of recycling on the catalytic efficiency of Fe3O4 NPs@h-BNNS. [14]
Acknowledgements The financial support provided by the National Elites Foundation is gratefully acknowledged. References
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Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Boron nitride nanosheets decorated with Fe3O4 nanoparticles as a magnetic bifunctional catalyst for post-consumer PET wastes recycling Mohammad Reza Nabid*, Yasamin Bide**, Mahsa Jafari Department of Polymer & Material Chemistry, Faculty of Chemistry & Petroleum Science, Shahid Beheshti University, G.C., P.O. Box 1983969411, Tehran, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 July 2019 Received in revised form 1 September 2019 Accepted 10 September 2019 Available online 11 September 2019
Due to its enormous consumption in various fields and also the non-degradability, recycling of polyethylene terephthalate is critical for waste management causing reduction in disposal, oil use, and carbon dioxide emissions. Among various recycling methods, glycolysis of PET was carried out because of milder reaction conditions and yielding bis(hydroxyethyl) terephthalate (BHET) as valuable versatile monomer. To conduct an efficient PET glycolysis, a recyclable bifunctional nanomaterial including highly dispersible Fe3O4 nanoparticles immobilized on hexagonal boron nitride nanosheets (h-BNNS) as important two-dimensional nanomaterials are chosen. A simple solvothermal method was used for decoration of h-BNNS with Fe3O4 nanoparticles. After characterization of prepared material, investigation of catalytic activity for glycolysis of recycled PET bottles under different conditions have been conducted which shows a rapid complete reaction with high BHET yield in the presence of very low amount of the catalyst. After reaction completion, the catalyst was easily recycled by an external magnet for several runs without considerable loss of catalytic activity. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Boron nitride nanosheets Magnetic nanoparticles Catalysis Glycolysis Polyethylene terephthalate
1. Introduction Poly(ethylene terephthalate) (PET) as a thermoplastic nonbiodegradable polyester has been extremely used in various fields in our daily life such as fibers, plastic films, insulating materials, food packages, and bottles [1]. Based on a report by the US National Park Service, PET bottles need around 450 years to be decomposed [2]. On the other hand, the annual world consumption of PET has exceeded 50 million tons with still an exponential increase which lead to a serious white pollution and resource waste. Several chemical methods have been employed for PET recycling, such as hydrolysis, aminolysis, ammonolysis, methanolysis, and glycolysis [3e5]. The glycolysis process, which involves transesterification of PET chains with diols to form commercially valuable product, BHET monomers, is the most important recycling process of PET [6,7]. Numerous catalytic researches have been presented for PET glycolysis, but there are still some limitations such as slow reaction rate, low selectivity and harsh reaction conditions and consequently high cost and high energy consumption of PET recycling
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (M.R. Nabid), [email protected] (Y. Bide). https://doi.org/10.1016/j.polymdegradstab.2019.108962 0141-3910/© 2019 Elsevier Ltd. All rights reserved.
which leads to comprehensive investigation on the development of efficient catalysts [6,8e10]. Due to the increased number of active sites and modification of the intrinsic properties in nanoscale, using nanocatalysts instead of conventional metal salts for PET glycolysis has several benefits like higher efficiency and selectivity toward BHET. But, recyclability of the nanocatalyst for further runs is a critical issue that should be considered for less environmental problems. Metal oxides nanoparticles capable of magnetic recycling can be a suitable option for environmental sustainability, easy isolation and reusing of catalyst for further runs as well as their catalytic activity [11,12]. Recently, our group developed the synthesis of g-Fe2O3/nitrogen-doped graphene hybrid material as the first report of bifunctional catalyst for PET recycling which g-Fe2O3 acts as acidic sites and N-doped graphene as basic site [4]. The successful use of graphene as a two-dimensional (2D) carbon nanomaterial has suggested excellent opportunities for other 2D nanomaterials, especially boron nitride nanosheets with the same catalytic activity as well as high thermal and physicochemical stability, mechanical strength, high thermal conductivity, high resistance to oxidation and being structurally robust [13e15]. Moreover, compared with graphene, B-N bonds exhibit partially ionic character making h-BNNS a prominent electronic-tuning support for various applications such as gas adsorbent, water cleaning, hydrogen storage, drug delivery, and catalysts [16e19]. To the best
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of our knowledge, there is no report of using h-BNNS to embrace an interfacial electronic effect on Fe3O4 induced by (2D) h-BNNS with nitrogen and boron vacancies for PET glycolysis. In this paper, Fe3O4 nanoparticles were deposited on h-BNNS as an efficient catalyst for PET glycolysis which possess the advantages of being magnetically recyclable and also a good interaction between PET and boron atoms due to their electron deficient nature.
Veregue et al. [21] After the reaction, the temperature of the solution was reached to 120 C using a bath for separation of PET not depolymerized. The obtained solid material was separated from the liquid phase by filtration. The remaining solution containing the solubilized BHET was refrigerated for 24 h, and then filtered to obtain BHET. The recovered BHET crystals were dried, and characterized by 1H NMR.
2. Experimental
3. Results and discussion
2.1. Materials
3.1. Synthesis and characterization of the catalyst
h-BN powder (5-10 mm in diameter) was purchased from Alfa Aesar. Commercially clear PET soft-drink bottles were recycled, washed, dried, and cut into 4 2 mm pellets for glycolysis experiments. Iron(III) chloride (FeCl3), trisodium citrate (Na3Cit), anhydrous ethylene glycol and sodium acetate (NaOAc) were obtained from Merck.
A gas exfoliation method was employed to prepare h-BNNS according to the recent report by Zhu et al. [20]. Fe3O4 nanoparticles immobilized on h-BNNS were synthesized through a simple solvothermal method during which FeCl3$6H2O was used as the iron source. Owing to the nitrogen groups as Lewis base sites, hBNNS absorb and anchor Fe3þ which prevent the Fe3O4 nanoparticles from spontaneous homogeneous nucleation and formation of independent Fe3O4 nanoparticles separated from h-BNNS. Ethylene glycol was used as both solvent and reductant. Sodium acetate and trisodium citrate performed as alkali source and electrostatic stabilizer, respectively [22]. Fig. 1 shows SEM images of a sample of Fe3O4 nanoparticles immobilized on h-BNNS. The presence of small nanoparticles on boron nitride nanosheets has been confirmed by SEM. Most of the nanosheets settled facially on the target substrate because of vacuum filtration-induced alignment. Well dispersed sphere-like Fe3O4 nanoparticles on the surfaces of h-BNNS are observed from SEM images. The TEM images of Fe3O4 NPs@h-BNNS have been shown in Fig. 2 which clearly reveal the coating of h-BNNS with spherical Fe3O4 nanoparticles having size range of 3e10 nm and a quite uniform size distribution. According to TEM observations, Fe3O4 nanoparticles were successfully immobilized onto the h-BNNS surface. The interconnected structure or branched network morphology can be related to interparticle van der Waals interactions between the h-BNNS. The Fe3O4 nanoparticles obtained by solvothermal method show typical X-ray diffraction (XRD) patterns of magnetite (JCPDS no. 19- 629; Fig. 3A). The broad diffraction peaks further propose the nanocrystalline structure of the Fe3O4 magnetite particles. The XRD pattern of h-BNNS synthesized by gas exfoliation method was presented in Fig. 3B. The (002) diffraction peak is observed confirming the synthesis of h-BNNS. The main peak of h-BNNS at 2q of 20.01 can be observed in the XRD pattern of Fe3O4 NPs@h-BNNS with slightly shift probably due to the interaction of h-BNNS with Fe3O4 NPs. Moreover, the Fe3O4 peaks are observed in the XRD pattern of Fe3O4 NPs@h-BNNS (Fig. 3C). FT-IR spectra of Fe3O4 NPs and Fe3O4 NPs@h-BNNS are displayed in Fig. 4. The FeeO vibration at 578 cm1 is observed in FT-IR spectrum of Fe3O4 NPs. The broad band appeared in the range 3200e3600 cm1 is related to the OH stretching vibration. Moreover, the absorption bands at 1620 and 1388 cm1 are ascribed to carboxylate demonstrating the existence of carboxyl groups of citrate (Fig. 4A). In FT-IR spectrum of Fe3O4 NPs@h-BNNS, two dominant peaks at 1388 and 879 cm1 were observed according to the inplane BeN transverse optical modes of the sp2-bonded h-BN, and the BeNeB out-of-plane bending vibration, respectively [23]. Moreover, the absorption peak of Fe-O can be observed (Fig. 4B). To study the chemical composition of Fe3O4 NPs@h-BNNS, XPS as a more sensitive analysis was used (Fig. 5). The binding energies of B1s and N1s are observed at 190.6 and 398.0 eV, respectively. The values are in good agreement with the reported values. The B/N ratio from our XPS survey is 1.08. The peaks of Fe2p1/2 and Fe2p3/2,
2.2. Instruments and characterization Fourier transform infrared (FT-IR) spectra were recorded on a Bomem MB-Series FT-IR spectrophotometer. X-ray powder diffraction (XRD) data were collected on an XD-3A diffractometer using Cu Ka radiation. Field emission scanning electron microscope (FE-SEM) was performed on a Zeiss Sigma VP instrument equipped with an oxford instrument Energy Dispersive X-ray (EDX) detector. High-resolution transmission electron microscopy (HR-TEM) was performed by FEI Tecnai F20 system (methanol was used as a solvent for HR-TEM). X-ray photoelectron spectroscopy (XPS) was performed using a Thermofisher Scientific K-Alpha in an ultrahigh vacuum. 1H NMR spectra were recorded with a BRUKER DRX-300 AVANCE spectrometer, and CDCl3 was used as solvent. 2.3. Synthesis of hexagonal boron nitride nanosheets The commercial h-BN powder (1 g) was heated to 800 C in a muffle furnace, maintained at this temperature for 5 min and then instantly transferred into a dewar bottle containing liquid nitrogen until it gasified totally. These steps were repeated to obtain h-BNNS which then dispersed in isopropanol and sonicating for 30 min. The dispersion was centrifuged at 800 rpm to remove any remaining bulk crystals. The supernatant was collected and dried in vacuum oven overnight [20]. 2.4. Synthesis of Fe3O4 nanoparticles immobilized on hexagonal boron nitride nanosheets Firstly, h-BNNS (0.3 g), Iron(III) chloride (1 g, 6.2 mmol) and trisodium citrate (0.20 g, 0.68 mmol) were dissolved in ethylene glycol (20 mL). Afterwards, sodium acetate (1.20 g) was added with stirring. The mixture was stirred vigorously for 30 min and then immersed in a Teflon-lined stainless-steel autoclave. The autoclave was heated to 200 C and kept for 10 h, and then allowed to cool to room temperature. The obtained suspension was washed with deionized water three times and then dried in a vacuum oven for 12 h at room temperature. 2.5. General procedure for the glycolysis of PET Typically, 0.3 g of PET, 10 mL of EG, and a certain amount of catalyst were immersed in a Teflon-lined stainless-steel autoclave. The autoclave was heated to 200 C and kept for 5 h, and then allowed to cool to room temperature. The BHET precipitation without water was accomplished according to the recent report by
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3
Fig. 1. SEM images of Fe3O4 nanoparticles immobilized on h-BNNS.
Fig. 2. TEM images of Fe3O4 NPs@h-BNNS
located at 711 and 724.9 eV, further validated the successful formation of Fe3O4 on h-BN surfaces. 3.2. Catalytic activity To investigate the catalytic activity of as-obtained Fe3O4 NPs@h-
BNNS for glycolysis of PET, several reaction conditions including various temperature, catalyst amount, time, EG content, reflux or autoclave conditions was tested. The PET conversion and BHET monomer yield as the functions of these factors were calculated according to equations (1) and (2), respectively.
4
Conversion of PET ð%Þ ¼
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initial weight of PET weight of undepolymerized PET 100 initial weight of PET
moles of BHET produced 100 Yield of BHET ð%Þ ¼ moles of PET units
(2)
Fig. 3. XRD patterns of Fe3O4 nanoparticles (A) h-BNNS (B) and Fe3O4 nanoparticles immobilized on h-BNNS (C).
(1)
The results have been summarized in Fig. 6. As an optimum condition, the glycolysis with 0.003 g catalyst, and 10 mL EG at 200 C in the autoclave for 300 min was chosen. To study the influence of temperature as one of the most important factor in the kinetics of a chemical reaction and the catalytic efficiency, the PET glycolysis was conducted at 200, 220 and 240 C in the Teflon-line autoclave and also under refluxing at 198 C in the presence of 0.002 g catalyst with 10 mL EG. The reaction was completed only in 30 min at 240 C in the stainless steel batch-type reactor with 100% BHET yield and selectivity. Decreasing the reaction temperature to 220 and 200 C causes the reaction to complete within 180 and 300 min, respectively. In the next step, to evaluate the effect of catalyst content, the reaction was conducted with 0.001, 0.002 and 0.003 g Fe3O4 NPs@h-BNNS for 0.3 g PET. According to the results, the glycolysis was completely done with 0.002 g catalyst yielding a maximum value of 100% BHET remaining constant also with 0.003 g catalyst. Using very low amount of catalyst (0.6 wt%) is one the most amazing feature of the presented catalyst. It is notable that the glycolysis reaction did not proceed without Fe3O4 NPs@h-BNNS showing the necessity of the catalyst. After optimizing the temperature and catalyst amount, the BHET yield and conversion were monitoring at different reaction times at 200 C with 0.002 g catalyst. As shown in Fig. 6, PET conversion into BHET was increased with increasing the reaction time. Optimal time is the time at which PET conversion is maximum. Afterwards, the yield reduces because of the polymerization of BHET as a side reaction. Based on the results, the PET recycling in terms of BHET conversion and yield complete after 300 min. When the amount of EG increases from 4 to 10 mL, the BHET yield increases. The optimal amount of EG is 10 mL giving 100% yield under the above mentioned conditions. According to the literatures, as the particles size of PET increases, the yield of PET glycolysis decreases. Considering that in this work a relatively large pellets from recycled PET soft drink bottles have been used, the catalytic efficiency of Fe3O4 NPs@hBNNS is excellent.
Fig. 4. FT-IR spectra of Fe3O4 nanoparticles (A) and Fe3O4 nanoparticles immobilized on h-BNNS (B).
M.R. Nabid et al. / Polymer Degradation and Stability 169 (2019) 108962
5
Fig. 5. High resolution XPS spectra of B 1s, C 1s, Fe 2p, and N 1s.
Several previously reported catalysts have been compared with this work in terms of reaction temperature, amount of catalyst, reaction time, PET size, and EG amount and the data have been given in Table 1. As seen, the as-synthesized catalyst shows relatively good results compared to most of the reported catalysts. To investigate the effect of catalyst components and further prove the catalytic performance of as obtained Fe3O4 NPs@h-BNNS for PET glycolysis, Fe3O4 NPs and h-BNNS were also examined under the above mentioned optical conditions. The yield of BHET that was catalyzed with Fe3O4 NPs was found to be 61% in the presence of 0.002 g catalyst for 0.3 g PET, 10 mL EG at 200 C for 5 h. Under similar reaction conditions to have a correct comparison, BHET yield with h-BNNS reached 82.3%. Although the BHET yields for two components of catalyst are relatively high, the yield with Fe3O4 NPs@h-BNNS reached to 100% that suggest the synergetic effect of both Fe3O4 NPs and h-BNNS in the catalytic glycolysis of postconsumer PET waste. Some possible properties of h-BNNS are used to explain the catalytic activity of Fe3O4 NPs@h-BNNS. The unique polarity of boron nitride bonds and high surface area of hexagonal boron nitride related nanostructures result in good catalytic activity. In
addition, the coexistence of boron atoms as Lewis acidic sites and nitrogen atoms as basic sites can afford various types of chemical interaction between h-BNNS and PET. Nitrogen atoms as basic sites can enhance the nucleophilic attack of ethylene glycol through the hydrogen abstraction from hydroxyl groups. Boron atoms as acidic sites promote the electrophilic character of carbonyl groups of PET which make them more vulnerable to the attack of nucleophile EG. In addition, as a graphene-like material, similar pp interaction may exist between h-BNNS and aromatic PET. So, the coexistence of boron and nitrogen atoms as acidic and basic sites as bifunctional catalyst for PET glycolysis and also pp interaction between hBNNS and aromatic PET cause the efficient glycolysis of PET chain under the relatively mild reaction conditions [30]. Fig. 7 gives 1H NMR of as-obtained BHET monomer wherein it may be observed the peaks at 4.03 and 4.53 related to methylene protons of CH2-OH and COO-CH2, respectively. The signal at 4.74 ppm corresponds to eOH groups. The peak of aromatic ring protons has been appeared at 8.15 ppm. From these observations, the successful glycolysis of post-consumer PET to BHET monomer has been proven.
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Fig. 6. (A) Effect of the amount of Fe3O4 NPs@h-BNNS on the PET glycolysis. Reaction conditions: PET (0.3 g), EG (10 mL), 200 C, 300 min. (B) Effect of glycolysis time. Reaction conditions: PET (0.3 g), EG (10 mL), Fe3O4 NPs@h-BNNS (0.002 g), 200 C. (C) Effect of the amount of EG. Reaction conditions: PET (0.3 g), Fe3O4 NPs@h-BNNS (0.003 g), 200 C, 300 min. (D) Effect of different catalysts. Reaction conditions: PET (0.3 g), EG (10 mL), catalyst amount (0.003 g), 200 C, 300 min.
Table 1 Comparison of several previously reported catalysts with Fe3O4 NPs@h-BNNS Entry Catalyst
Temp. (oC)
Amount of catalyst/amount Time of PET (min)
PET size and type (mm)
Amount of EG/amount of PET
BHET yield
BHET selectivity
Ref
1
[bmim]FeCl4
178
1/5
240
2.0 2.5 2.7
20/5
100
59.2
[24]
2 3 4 5 6 7
g-Fe2O3
255 190 190 260 300 190
0.01 1/2 20/2 1.0 0.003/0.3 0.018%
60 180 120 60 80 40
0.15 fine powder 4 2.8 3.2 4 2.8 3.2 <0.2 3 2 2.5 2.7
20/2 20/2 17.2 1.1/0.3 1/4
>90 100 100 >90 >96 >84.5
75 100
[11] [25] [8] [12] [26] [27]
8 9
(MgeZn)eAl LDH Fe3O4- MWCNT ZnMn2O4 GO-Mn3O4 POMsa Na12[WZnM2(H2O)2(ZnW9O34)2] Zinc acetate [bmim]FeCl4
195 178
0.0008 1/5
60 240
3 3 0.03 2.0 2.5 2.7
0.75 20/5
31.3 100
59.2
[28] [29]
10
Fe3O4 NPs@h-BNNS
200
0.002/0.3
300
42
10/0.3
100
100
This work
3.3. Catalyst recycling
4. Conclusion
Among the nanocatalysts, magnetic recyclable nanocatalysts can present an important platform for sustainable chemistry due to the high activity and easy separation and recycling. The recyclability and reusability of as-obtained Fe3O4 NPs@h-BNNS catalyst were investigated which shows the successful reusability for four runs in the glycolysis of post-consumer PET without major loss of activity (Fig. 8). The slight decrease in activity can be attributed to the incomplete magnetic separation of the catalyst or leaching of catalyst during the reaction. The significant versatility of the presented material in terms of tune ability for various applications and design based on the process including combination of boron nitride nanosheets on the magnetite supports will be able to provide significant advances for the as obtained material in the near future.
Recycling post-consumer PET by Fe3O4 magnetic nanoparticles on h-BNNS through glycolysis reaction were reported. The influence of each component was investigated which indicates the catalytic activity of Fe3O4 due to Lewis acid character and h-BNNS as graphene-like nanosheets with boron and nitrogen groups acting as Lewis acid and base, respectively. Despite using postconsumer PET with large size, the excellent results were obtained. Relatively fast and complete conversion of PET to BHET virgin monomer at 200 C, using very low amount of the catalyst, magnetic recyclability of the catalyst, and BHET precipitation without water are some advantages of as-obtained catalyst. Combining the unique properties of boron nitride nanosheets with magnetic and catalytic features of Fe3O4 nanoparticles make the presented material suitable for various applications.
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Fig. 7. 1H NMR spectrum of BHET.
[8]
[9]
[10]
[11]
[12]
[13] Fig. 8. Effect of recycling on the catalytic efficiency of Fe3O4 NPs@h-BNNS. [14]
Acknowledgements The financial support provided by the National Elites Foundation is gratefully acknowledged. References
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