Aminolytic Depolymerisation of Polyethylene Terephthalate Wastes using Sn doped ZnO Nanoparticles


 Poly(ethylene terephthalate) (PET) is one of the most consumed polymers because of its excellent thermal and mechanical properties. By increasing in PET production and since the disposal of PET waste has growing to be a major global environmental issue each year. Chemical recycling is a most successful method to achieve circular economy in the PET utilizing industries. Current research work aims to complete depolymerization of waste PET from soft drink bottles by the aminolysis method to produce bis (2-hydroxy ethylene) terephthalamide (BHETA) in the presence of Sn doped ZnO. To evaluate catalytic activity, pure and Sn2+ doped ZnO nanoparticles prepared using different Sn2+ molar ratios at 0.5, 1.0 and 2.0 mol% and calcined at 500 0C for 1h. The synthesized catalysts characterised using FT-IR, XRD, and UV-vis spectroscopy. The surface morphology and percentage doping obtained from SEM and SEM-EDS, respectively. We have observed a reduction in optical band gap and crystallite size of ZnO due to tin doping. Aminolytic depolymerization of PET waste using ethanolamine promoted by Sn doped ZnO effectively under conventional thermal method. Increase in the yield of the BHETA observed with respect to increasing doping percentage of Sn and 1-2 mol% Sn doped ZnO nanoparticles afforded over 90% of BHETA. Structure and purity of BHETA, depolymerised product characterized by FT-IR, 1HNMR, 13C NMR, and MS.


Introduction
Zinc oxide (ZnO) is a unique material when compared to other metal oxides,because of its cost-effectiveness and tunable catalytic properties. The catalytic e ciency of ZnO inherited from the tailor made electronic properties and nanostructure. Advancements in synthetic procedure as developed widen ZnO in various applications [1][2][3][4]. The crystal structure of ZnO comprises zinc blende and wurtzite, a hexagonal structure which can doped with metals and non-metals [1]. Zinc oxide is a binary semiconducting material having pyroelectric and piezoelectric properties (60 meV), ZnO has a high redox potential, superior physical and chemical stability, and non-toxicity [2]. The major advantage of ZnO nanoparticles over others is their large surface area combined with reduced size, which makes it applicable in many areas like biological study including (antimicrobial activity), the photocatalytic and semiconducting properties which make it admit the use of ZnO nanoparticles as a potential candidate for many catalytic transformations [4,5]. The catalytic activity of ZnO nanoparticles can be tuned by doping process and it is seems to be one of the economical solutions [6]. Doping is mainly of two types: one is cationic and the other is of anionic doping. Introducing cationic impurities into the ZnO structure known as cationic doping, most prominently transition metals used as cationic dopants, eg: Sn, Al, Ga, In, Cd, Cu, Mn, Ni. The doping of an anion into ZnO called anionic doping, eg; As, N, and S are anionic dopants [1,3].
In the current investigation, we aim to depolymerise post-consumer PET wastes completely into pure terephathalamide monomer using aminolysis. Forasmuch PET is one of the most important engineering polymers and considered being an excellent material for many applications, including clothing applications. It has excellent tensile and impact strength, chemical resistance, clarity, processability, colorability, and reasonable thermal stability [7]. PET is an aromatic polyester mainly utilised for packaging and textile industries and the post-consumer waste recycling of PET using chemical methods rather than physical and land lling method is a most economical method to achieve circular economy in polyester industries because of its widespread commercial applications [8,9]. The total consumption of plastics in India is about 4 million tons and the waste generated is about 2 million tons. Plastics contributing to about 20% of solid municipal waste in India. The overall world consumption of PET currently amounts to about 13 million tons, of which 9.5 million tons processed by the textiles industry, 2 million tons used in the manufacture of audio and videotapes, and 1.5 million tons used in the manufacture of various types of packaging mainly bottles and jars [10].
PET is a noxious material because of its high resistance to atmospheric and biological agents. Ecological and economic considerations advocate the introduction of wide-scale PET recycling, similar to the recycling of traditional materials such as glass, paper, or metals [11].
Aminolytic depolymerization of PET waste PET recycling is one of the most successful and best examples of polymer recycling. There are many processes by which post-consumer PET waste recycled, but the sustainable method is the chemical recycling process because it leads to the formation of monomers from which the polymer made [8,12]. In contrary, the conventional disposal methods such as land lling become sereve threat terrastial and aquatic lives due to microplastics pollution [13,14].
Chemical recycling process of PET divided: (i) glycolysis, (ii) methanolysis, (iii) hydrolysis, (iv) aminolysis, and (v) ammonolysis. Among the chemical recycling techniques, glycolysis and aminolysis depolymerization of the PET wastes produced bis (2-hydroxy ethyl) terephthalate (BHET) and bis (2-hydroxymethyl) terephthalamide catalysed by Lewis acidic heterogeneous catalysts [15,16]. Aminolysis is any chemical reaction in which post-consumer PET waste completely depolymerized into monomers by reacting with a molecule of an amine. Aminolysis is a method for PET chemolysis which has been relatively lesser explored when compared to other processes. There are only a few literature references regarding the chemolysis of the waste PET using different amines such as ethanolamine, triethanol amine, allylamine, and polyamines [17][18][19]. The catalytic systems reported for glycolysis and aminolysis recycling of PET wastes summarised in Table 1.  [29] In the current study, the chemical recycling of post-consumer PET polymer wastes, especially wastewater bottles, using the aminolysis process. The aminolysis of the PET wastes using the Sn(II) substituted ZnO nanoparticles as catalyst.
Sn/ZnO catalysts showed enhancement in photocatalytic, electron transport behaviourand sensitive to organic molecule containing carbonyl group such as formaldehyde [30][31][32][33]. The doping of Sn on ZnO reduced the band gap and promote catalytic behaviour. Sn(II) substituted ZnO nanoparticles prepared by the sol-gel method at different Sn 2+ to Zn 2+ molar ratio and characteriz using FTIR, XRD, UV-Vis and SEM-EDS analysis. Depolymerization of PET wastes using aminolysis under various parameters such as temperature, PET-to-catalyst ratio, and PET-to-aminolytic reagents.
The nal product of the aminolysis process characterised using 1 H, 13 C NMR and MS.

Materials And Methods
Common chemicals such as zinc nitrate hexahydrate, tin(II) chloride dihydrate, diethylamine, sodium hydroxide, ethanolamine, acetone, ethyl acetate, and n-hexane, all chemicals purchased from SRL and Loba Pvt, Ltd India and used without further puri cation. Post-consumer PET waste obtained from wastewater bottles collected, cleaned PET bottles dried at 80°C for 6 hours and cut into smaller pieces. All glassware and Quick ts we employed in the experimental work made up of corning/borosil glass. These glasswares washed thoroughly and dried in a hot air oven before use.

Synthesis of catalyst
Tin-doped ZnO nanoparticles were synthesized by the sol-gel method. Zinc nitrate hexahydrate and tin(II) chloride dihydrate dissolved in 50 ml of distilled water. NaOH used as the base and diethylamine was used as a stabilizer to get the desired pH value of 11. The mixture kept under constant magnetic stirring for 5 h. After stirring the solution with re ux at 70-75 ˚C for 4 hours and ltered by vacuum. During the ltration, the solution washed with distilled water many times to avoid impurities. After ltration, the ltered sample heated to 80°C in the oven for 24 hours. The heated sample was ground and calcined at 500 0 C for 1 hour. ZnO nanoparticles substituted with Sn 2+ in different Sn 2+ molar ratios of 0.0, 0.05, 0.1 and 0.2 prepared by a sol-gel process [32].
For depolymerization, the aminolysis of Post-consumer PET bottles was collected. After removing the cap and labels they were cleaned thoroughly with soap water followed by distilled water. 100 ml round-bottom glass equipped with a condenser, thermometer and magnetic stirrer. PET akes (500 mg) were treated with 20 ml of ethanolamine and heated to 150-160°C using magnetic stirring. After reaching the desired temperature, the reaction mixture stirred for 1 hour. The PET was dissolved in ethanolamine. And then 50 mg of Sn 2+ -substituted ZnO nanoparticles catalyst was added. The temperature of the reaction was maintained using a temperature controller. After 1 h, 1 ml of the reaction mixture was diluted with acetone and the progress of the reaction was monitored using precoated aluminium TLC plates (E-Merck, UK, Silicagel G60 F254 indicator) and the spots identi ed using 356 nm UV detector. The reaction progress was monitored by the TLC method using 70:30 v/v mixtures of n-hexane and ethyl acetate solvents and spot of BHETA compared with refernce BHETA compound. The reaction mixture was stirred for 3 h. At the end of the reaction, 50 ml of distilled water was added in excess to the reaction mixture with vigorous agitation and followed by ltration to remove the catalyst. After removal of catalysts, the ltrate heated to 70°C for 15 min and kept in 15-20 ˚C for 12-16 hrs. BHETA obtained as white needle crystals, which ltred and dried at 70 ˚C for 2 h at vaccum. Finally, the yield of the reaction is calculated from the weight of the nal product. Further, the nal product subjected to FT-IR, 1 H, 13 C NMR and MS to con rm the structure and purity of BHETA. Likewise, the same procedure used for Sn 2+ 103) and (112), respectively, indicative of the hexagonal structure of wurtzite of ZnO [30,31].

FT-IR spectra of pure and Sn-doped ZnO nanoparticles
FT-IR transmittance spectra of pure and Sn(II) doped zinc oxide nanoparticles of different molar ratios shown in Figure   3, respectively. The spectrum was recorded from 500 to 3500 cm −1 . In the pure and different molar ratio of 0.5%, 1.0%, and 2.0% Sn 2+ , no signi cant change in vibrational stretching of ZnO observed in the SZO samples compared to the pure synthesized ZnO. For all the samples, major and broad peaks were found between 500 cm −1 and 800 cm −1 is assigned due to the stretching vibration of Zn-O. These vibrational frequencies change to 800 cm −1 with an increase in Sn doping. The signals were found in the region 1400 cm −1 -1700 cm −1 and may be due to C=C and C=O vibrations.
Other signals near between 2000 cm −1 and 2400 cm −1 were also observed. Finally, signi cant peaks were found between 3200 cm −1 and 3500 cm −1 . These peaks were attributed to the H 2 O stretching vibration in the ZnO lattice [34].
3.4 UV-vis absorbance spectra of pure and Sn-doped ZnO nanoparticles: UV absorption spectra are analyzed for all samples from 200 nm to 800 nm. The optical properties of prepared Sndoped ZnO nanoparticles were examined by UV Vis spectroscopy (UV Vis 3600 PLUS -SHIMADZU). The UV-visible spectra of the sample Sn (II) doped ZnO nanoparticles showed in Figure 4. Different molar ratios of 0.5%, 1.0%, and 2.0% of the Sn 2+ doped ZnO samples exhibit a well-de ned absorption peak that corresponds to the hexagonal wurtzite phase [30,31].
The peaks of the the excitonic absorption was between 380 and 400 nm, which are typical characteristic ZnO NPs peaks, thus, con rm their presence. The bandgap of Sn doped ZnO (molar ratio 1%) is decrease compared to the other molar ratios are 0.5 and 2%. The shifting in absorption peak to the higher wavelength results in decreasing the band gap for Sn-ZnO nanoparticles from 3.23 to 3.12 eV. The incorporation of Sn(II) enhanced photocatalytic activity. The optical direct band gap calculated from the formula Eg = hc/l is 3.24 eV [31].

HR SEM-EDS:
SEM-EDS study was carried out to examine the morphological changes, particle size, elemental composition of the  Table S1 and Table S2). The ZnO doped with 1 mol % Sn appeared to be well de ned nanoparticles cluster at 1 µm magni cation. Elemental mapping of 1 mol % Sn/ZnO showed the presence of both Zn as well as Sn atoms (Figure 6c and 6d) [31]. In addition, the EDS spectrum for 1 mole Sn doped ZnO showed the presence of Zn as well as Sn and elemental composition showed as shown in Figure 5e and 5f determined Zn 2+ at 80.13% and Sn 2+ and at 0.74%, which is very close to 1 mol% Sn(II) doping on ZnO [35,36].
Depolymerization of PET polymer using ethanolamine: The deoplymerisation of PET wastes using ethanolamine as aminolyting agent which yield BHETA as a single product [37,38]. The aminolysis reaction activated by Lewis acidic nature of ZnO, in which carbonyl group of terephthalic ester carbonyl group attracted by ZnO [24]. Further, the incorporation of Sn(II) enhances its catalytic acitivity to give BHETA as a single product. Figure 6a -d showed the aminolysis of PET wastes using ethanolamine (EA) carried out to study the effect of Sn(II) loadings (Fig. 6a), PET-to-EA ratio (Fig. 6b), catalyst-to-PET ratio (Fig. 6c), and e ciency of catalyst with respect to reaction cycle (Fig. 6d). increasing the amount of Sn 2+ doping reported that increasing the amount of Sn 2+ doping from 3 to 5% as prepared using the coprecipitation method increased the photocatalytic e ciency of ZnO. According to Fig. 6a, the doping of 1 mol % and 2 mol % Sn loaded ZnO showed BHETA yield more than 95% the single product. The BHETA is hot-water soluble, which have been isolated and crystallised as needle shaped pale yellow-coloured crystals without any further puri cation. As observed in Fig. 6a, the pristine ZnO showed almost 85% BHETA without Sn doping. The analysis of BHETA showed the formation of single depolymerised product using the eluent 40:60% of EA:Hex. In order to con rm the structure, FT-IR, 1 H NMR, 13 C NMR and MS analysis have been carried out. Both 1 H and 13 C NMR spectra showed pure form of BHETA. Figure 6b showed the effect of PET to EA ratio has been studied at various weight % of PET:EA ratios. In this study, the amount of PET was kept constant at 1g, with respect to the volume of ethanolamine that varied from 1 ml, 5 ml, 10 ml,  Figure 6c summarises the effect of the catalyst-to-PET ratio, which is crucial for the effective aminolysis of PET wastes. In this study, the amount of catalyst (1 mol % Sn doped ZnO) has been xed at 50 mg throughout the study with respect to catalyst weight, the amount of PET wastes has been varied from 100 mg to 500 mg to achieve 1:2, 1:5, 1:10, 1:15, and 1:20 ratio. The study clearly indicated that the increasing amount of catalyst, that is, the catalyst-to-PET ratio at 1:2, 1:5, 1:20, afforded the yield of BHETA above 90%. However, the ratio at 1:15 almost gives an 85% yield of BHETA. The study revealed that the ratio of catalyst: 1: 5 to 1: 5 to PET could be optimal to achieve more than 85% of the aminolysed product, BHETA in pure and single product. showed the catalyst could be reusable upto 7 cycles without losing its activity.

Spectral characterisaion of BHETA
The complete depolymerisation of PET wastes generate BHETA as a single product. However, there could be possibility of forming dimer and oligomeric products due to incomplete depolymerisation. Thus, spectral characterisation such as 1 H and 13 C NMR, and MS and FT-IR analysis have been carried out. The structural characterisation of depolymerised product, BHETA have been carried out to con rm the purity and structural con rmation. 1 H and 13 C NMR of BHETA is given Fig. 7a and 7b respectively (Full spectrum given in supplementrary information Figure S3 and S4), and chemical structure of Fig. 7e. The ESI-MS and FT-IR spectrum of BHETA given in Fig. 7c and 7d respectively. attached to amide -NH group gives quartet at 3.33-3.37 ppm with J value of 6Hz. The presence of eight aliphatic protons attached to two ethylene group (-CH 2 -CH 2 -) con rms the attachment of two 2-hydroxy ethyl group to terephthaloylamido group (-NH-CO-C 6 H 4 -CO-NH-) of BHETA. Fig. 7b is 13  ppm. Both 1 H and 13 C NMR con rmed the formation of BHETA as a single pure product. The product is isolated in pure form without the need for puri cation.
The mass spectrum of aminolysed product, bis (hydroxy ethyl) terephthalamide is given Fig. 7c (Full spectrum given in supplementary information, Figure S5). The molecular formula of BHETA is C 12 H 16 N 2 O 4 with molecular weight is 252.27 g/mol. The spectrum recorded using the electron spray ionisation method revealed the molecular ion peak at 253.15 (M + H) in the protonated form. Another peak at m/z 275.10 indicates the formation of Na+ form of BHETA due to electron spray ionisation.
Furthermore, the FT-IR spectrum (Fig. 7d) of BHETA has been carried out as a KBr disc, and vibrational frequencies have been recorded from 400 to 4000 cm −1 (Full spectrum given in supplementery information, Figure S6). In 3250 -3400 cm −1 two sharp peaks observed due to the presence of -NH and -OH groups connected methylene carbon C9 and

Conclusion
In conclusion, compared with ZnO, Sn-doped ZnO showed excellent aminolysis yield of BHETA. The Sn-doped ZnO was synthesised by the Sol-gel method and was successfully characterised by XRD, UV-vis, FT-IR, and SEM-EDX techniques. Diffraction patterns of the nanopowders suggest that ZnO has sharp peaks, which con rm the good crystalline structure with a hexagonal wurtzite structure. FT-IR spectroscopy shows the pure and different molar ratio of 0.5%, 0.1%, and 0.2% Sn 2+ no signi cant change was observed in these samples as compared to the pure 95% of BHETA at 155-160 0 C. The product is isolated in pure form without requiring further puri cation. Hence, from out investigation, we proposed Sn doped ZnO nanoparticles could be effective catalyst for aminolytic depolymerisation of PET wastes. We also observed is reusable up to 7 cycles without a a major loss in the BHETA yield. The study provide that the use of Sn doped ZnO could be economical reusable catalyst for aminolysis of PET wastes. Currently, we are investigating ZnO doped with Ag, In and Cd towards catalytic depolymerisation various kinds of PET wastes including dyed polyester fabrics. Figure 1 Yield of Sn 2+ doped ZnO nanoparticles prepared by sol-gel method     SupplementaryInformation.docx