Chemical Recycling of PET Polyester Textile Wastes Using Ag-Doped ZnO Nanoparticles: An Economical Solution for Circular Economy

doi:10.21203/rs.3.rs-2481237/v1

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Research Article

Chemical Recycling of PET Polyester Textile Wastes Using Ag-Doped ZnO Nanoparticles: An Economical Solution for Circular Economy

https://doi.org/10.21203/rs.3.rs-2481237/v1

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published 23 May, 2023

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Polyethylene terephthalate (PET) is a nonbiodegradable and multi-functional plastic commodity that produces a significant amount of polyester (PES) textile waste. The management of PET solid waste is a global concern in many developing countries. Chemical recycling and material recovery from PES wastes can rebuild a circular economy in the textile sectors. The main objective of the present research is to enhance the chemical depolymerization of PES's textile wastes through microwave-induced catalytic aminolysis and glycolysis catalysed by Ag-doped ZnO nanoparticles. Catalysts are synthesised by the sol-gel method and characterised using X-ray diffraction (XRD), Fourier transforms infrared spectroscopy (FTIR), ultraviolet spectroscopy (UV-Vis), field emission scanning microscope (SEM-EDX), and transmission electron microscope (TEM). These parameters included the PET to catalyst ratio, microwave power, time, temperature, and catalyst recycling studies. The catalyst was discovered to be more stable and could be recycled up to six times without losing its activity. Microwave heating is typically faster than conventional heating techniques in terms of reaction time, PET conversion, product yield, and purity. In the aminolysis of PET, the effects of several factors on the conversion of PET and the yield of bis (2-hydroxy ethylene) terephthalamide (BHETA) and the glycolysis process to produce bis (2-hydroxy ethylene) terephthalate (BHET) were examined. The study's findings show that higher temperatures (180 °C) are beneficial for PES textile waste conversion and BHETA yield, which can reach nearly 95% and 94%, respectively. Finally, FT-IR, 1H NMR, and mass spectroscopy (MS) were used to characterise the depolymerized products. The study showed that 2 mol% Ag-doped ZnO showed better catalytic activity.

Aminolysis

Circular economy

Depolymerisation

Glycolysis

Nano catalyst

Waste management

One of society’s challenges today is recycling natural and syntheticfibers in the textile and clothing industries. In the report to Japan Chemical Fibers Association (JCFA), polyesters (PES) alone provide 25.4 metric tons (MT) and play a very important role in the synthetic fiber trade (JCFA 2017; Kawamura et al. 2020). Due to the high production of PES synthetic fibers and its extensive consumption, there has been a huge disposal of PET waste. Recovering materials from post-consumer PES textile waste through physical, chemical and biological methods will contribute to the conservation of raw materials of petrochemical origin (Sun et al. 2018). The discarded textile fabrics act as a source of microplastic pollution in the air (Gasperi et al. 2018; Dris et al. 2016; Dris et al. 2017). On the other hand, achieving a circular economy through a sustainable technique and reducing carbon and water consumption are emerging among the textile and garment industries to meet legislative and consumer needs (Stanescu et al. 2021).

PES textile recycling is the technique of reclaiming PES plastic wastes and recycling them into new, multipurpose materials for various applications. The circular economy uses the 3R principle of reduction, reuse, and recycling to create a closed loop by reducing the utilization of raw materials and achieving social benefits for economic growth, environmental friendliness, and the plastic problem. Recycled plastic consumes 88% less energy than plastic products made using fresh raw materials (PET Recycling, 2020). Chemical recycling yields pure raw materials (derivatives of terephthalic acid monomer) when compared to mechanical or physical recycling techniques (Patti et al. 2020). PES textile waste can be depolymerized by enzymes, which is an environmentally friendly process (Zimmermann et al. 2020). The recycling of PES post-consumer textiles not only helps the economy but also reduces microplastic pollution (Prata et al. 2018). One of the common advantages of applying recycled PES is that it significantly reduces CO2 emissions and the use of petroleum-based feeds and solid waste. In comparison, recycled PES derived from pre-consumer PES fibers are much more economical in contrast to post-consumer PET fibers (Shirvanimoghaddam et al. 2020). However, the need for effective cleaning and removal of dye contaminants is very important (Pensupa et al. 2017). In this chemical recycling progress, consumer textile waste depolymerized as monomers and charcoal-based absorbers can remove contaminants (Piribauer et al. 2019). According to the Intergovernmental Panel on Climate Change (IPCC), traditional PES production uses crude oil-based raw materials, which may hurt Green House Gas (GHG) emissions and global warming. According to the Worldwide Responsible Accredited Production (WRAP) claim using recycled PES decreases CO2 emissions by up to 32% compared to using of virgin PES (Navone et al. 2020; Piribauer et al. 2021).

Recycling of polyester waste provides a secondary source of raw material to produce homogeneous or different materials, thus the need for virgin raw materials is greatly avoided (Cura et al. 2021). However, the existence of additives, mordants, dyes, and coatings limits the potential for fabric recycling (Echeverria et al. 2019). If the dyes in PET fabrics are not appropriately removed prior to recycling, it will put a stop to the use of the recycled materials (Dissanayake et al. 2021). Some fiber-combined compounds are challenging to separate from the recycling capacity of fabric waste. Even though polyester is the major fiber used in the textile industry, the recycling of polyester fabrics is limited, and it is a thrust area of research in the current scenario (Fei et al. 2020). The chemical recycling process is the best way to reduce PET waste economically. Textile waste is classified into three categories based on production: pre-consumer, post-consumer, and industrial textile waste (Cuc et al. 2011). Textile waste is frequently recycled at the end of its life and is used to create new products or recover energy through tertiary recycling, biodegradable processing, and incineration processes (Boustead et al. 2005; Woolridge et al. 2006). In recent times, chemical recycling of polyester thread has been explored as a modernization solution for PET management (Delle Chiaie et al. 2020; Zhang et al. 2017; Al-Sabagh et al. 2016; Jeya et al. 2017; Jeya et al. 2020; Jeya et al, 2022a; 2022b; c). Chemical recycling has great potential in the circular economy of plastic wastes, this can close the loop by creating starting monomers from recycled polymers to produce high-value-added chemicals (Garcia, 2017).

Zinc oxide (ZnO) is an economic and unique material when compared with other metal oxides because of its advantages such as cost-effectiveness, large specific surface area, higher porosity, tunable catalytic and nanostructured properties, as well as excellent optical, electrical, antibacterial, and magnetic properties (Vinitha et al. 2021; Nikolova et al. 2020; Pal et al. 2020; Fazio et al. 2021; Serban et al. 2020). Doped ZnO NPs have applications in many different areas, such as the rubber industry, biomedical field, photocatalysis, antimicrobial agent, energy cells, sensors, and metal surface treatment (Vinitha et al. 2021; Bharat et al. 2019; Beitollahi et al. 2020; Li et al. 2020; Yi et al. 2020; Borysiewicz et al. 2019).

The altered properties of ZnO with noble metals (Ag, Au, Pt, Pd, and so on) are obtainable in studies of Ag-doped ZnO NPs in the literature, and thus there is still space for research on Ag-doped ZnO NPs to explore its effect on ZnO NPs properties and applications. Silver (Ag) is an excellent candidate for forming shallow acceptor levels in ZnO, making it suitable for a variety of optical, band gap reduction, and photocatalytic applications (Gusmao et al. 2021; Zhou et al. 2017)

To generate valuable intermediates, the aminolysis and glycolysis of PET waste are catalyzed by Ag-doped ZnO nanoparticles in our investigation. The conversion of PET waste into monomers was achieved with doped ZnO as a catalyst through microwave irradiation as an environmentally friendly, clean approach. Table 1 summarizes the catalytic systems reported for glycolysis and aminolysis recycling of PET wastes.

S.No	Process	Reactants	Catalyst Used	Temperature (˚C)	Time	Yield %	References
1	Glycolysis	Ethylene glycol (EG)	Al³⁺, Fe³⁺ and Zn²⁺ impregnated bentonite catalysts	200	3h	90	Jeya et al. 2020
2	Glycolysis	EG	CeO₂	196	15 mins	90.3	Ling et al. 2022
3	Glycolysis	EG	Metal azolate framework-6 (MAF-6)	180	4h	81.7	Yang et al. 2021
4	Aminolysis	Ethanolamine (EA)	Sn doped ZnO	160	1h	95	Vinitha et al. 2022
5	Glycolysis	(EG)	Sb₂O₃	200	1h	97.5	Somayeh et al. 2022
6	Glycolysis	EG	(rGO\[TESPMI]2CoCl4)	190	3h	95.22	Najafi et al. 2021
7	Glycolysis	EG	Cu(OAc)2-[Bmim][OAc]	190	3h	53.97	Al-Sabagh et al. 2014
8	Aminolysis	Pentaethylenehexamine (PEHA)	Sodium acetate	210	5h	72	Atta et al. 2020
9	Glycolysis	EG	CoFe₂O₄ magnetic nanoparticle	195	2.5h	95.4	Wang et al. 2021
10	Glycolysis	EG	γ-Fe2O3 nanoparticles	300	1h	90	Bartolome et al. 2014
11	Aminolysis	Aminoethylethanolamine (AEEA)	Zinc Acetate	180	4h	48.05	Jamdar et al. 2017
12	Glycolysis	EG	Nano ZnO	190	80 mins	90	Alzuhairi et al. 2017
13	Glycolysis	EG	Mixed [Hmim]ZnCl₃ and [Hmim]CoCl₃ complexes	190	2h	87.1	Shuangjun et al. 2020
14	Aminolysis	Diethylenetriamine (DETA), triethylenetetramine (TETA),	Zinc Acetate	190	30 mins	-	Chan et al. 2021
15	Glycolysis	EG	Calcium oxide (CaO)	192	2h	76.41	Yunita et al. 2019
16	Glycolysis	EG	Fe3O4@SiO2@(mim)[FeCl4]	180	24h	100	Cano et al. 2020
17	Glycolysis	EG	Alkali metal (Na or K) acetate	197	2h	86	Ngan et al. 2022
18	Glycolysis	EG	Tropine-zinc acetate	190	-	82.3	Deng et al. 2021

In our earlier investigation, the complete conversion of PET bottle wastes was achieved by aminolysis using Sn-doped ZnO at 155–160°C (Vinitha et al. 2022). We hypothesized that Sn-doped ZnO nanoparticles may serve as a cost-effective and efficient catalyst for the aminolytic depolymerization of PET wastes with a high BHETA yield. In this article, we present the synthesis of Ag-doped ZnO nanoparticle catalysts by the sol-gel method at different mol%s of 0.5, 1.0, 2.0, and 3.0 characterized by XRD, FTIR, UV- Vis, SEM-EDX, and TEM analysis. Further, the depolymerization of PET wastes using glycolysis and aminolysis under various reaction parameters such as temperature, microwave power, time, PET to catalyst ratio, and PET to aminolytic reagents such as ethanolamine, diethylene amine, and glycolytic reagents such as ethylene glycol, and diethylene glycol. FT-IR, ¹H, ¹³C NMR, and MS were used to analyze the final product of the aminolysis and glycolysis processes.

PET textile wastes of red, blue, and green threads were collected and thoroughly washed with water. The cleaned PET textile waste was dried at 80 ˚C for 16 hours. After drying, the PET waste was cut into small threads (0.5 cm in size). All common chemicals were zinc nitrate hexahydrate, silver nitrate, diethylamine, sodium hydroxide, ethanolamine, diethanolamine, ethylene glycol, diethylene glycol, acetone, ethyl acetate, and n-hexane, all of which were purchased from SRL and Loba Pvt. Ltd. in India and used as-is. We used corning or borosilicate glass for all of the glassware and Quickfits in the experiment. Before use, these glasswares were thoroughly washed and dried in a hot air oven. The purification of solvents in the present study is detailed below.

Synthesis of Silver doped ZnO nanoparticles

ZnO nanoparticles were synthesised using the sol-gel method, a simple, cost-effective, and commonly used technique. Initially, Zn(NO₃)₂.6H₂O (0.122 M) and AgNO₃ (0.5, 1.0, 2.0, and 3.0 mol%) were dissolved in 50 mL of distilled water for 15 minutes at room temperature with constant magnetic stirring. Diethylamine (30 mL) and 2 M NaOH were added dropwise to maintain pH at 11. After that, the solution was kept under constant magnetic stirring for 5 hours at RT. A greyish-white to greyish precipitate was observed (intensity of color increases with an increase in Ag mol%). The resulting residue (Ag-ZnO) was then filtered and cleaned several times with deionized water. The final product was kept in the oven at 80°C for 24 hours for drying. Finally, the product kept for 1 hour in a static air furnace at 550°C. Bimetallic Ag-ZnO NPs with various molar ratios of Ag are prepared by following the same procedure.

Characterization of catalysts

The synthesized pure ZnO and the different molar ratios of Ag-doped ZnO nanoparticles were characterized using XRD, FT-IR, UV–Vis, SEM-EDX and TEM.

XRD

The crystal size and structure of pure ZnO and Ag-doped ZnO nanoparticles were determined by the X-ray diffraction (XRD) method. The powder was analysed using the Model PANalytical Empyrean, Netherlands X'pert PRO with monochromatic CuKα radiation, and samples were scanned in the range of 20–80°. Crystalline size calculation using the Scherrer equation and the William-Hall equation

FT-IR

The characteristic vibration peaks of Zn-O, Ag-O, and OH functional groups of the nanoparticles are monitored by Fourier transform infrared spectroscopy. The IR spectra of Ag-doped ZnO NPs catalysts and depolymerized BHETA and BHET products were recorded as KBr pellets on an FTIR (Model: IRTRACER 100, SHIMADZU, Japan).FTIR absorption spectra in the 400 to 4000 cm^− 1 range.

UV- Vis

Pure and Ag-ZnO nanoparticle synthesized synthesised using the sol-gel method and their optical properties were measured using ultraviolet spectroscopy (UV-Vis).The optical bandgap and electronic spectrum of Ag-ZnO as catalysts were measured using a UV-Vis spectrophotometer (Model: UV3600 PLUS, Shimadzu, Japan). The UV-Vis absorption spectrum was recorded in the range of 200–800 nm.

SEM-EDX

Field emission scanning microscope was recorded by an FE-SEM (Model: Apreo S, Thermofisher Scientific, Netherlands) combined with energy dispersive spectroscopy (EDX) used for morphological and composite characteristics. ImageJ software (version 1.53K) was used to determine the average particle size of pure and different mol% Ag doped ZnO nanoparticles.

TEM

A transmission electron microscope (TEM) was used to examine Ag-doped ZnO NPs (Model: JEM-2100, PlusJEOL, Japan).

NMR

¹H-NMR and ¹³C-NMR spectra were monitored by an AVANCE III (Bruker, Switzerland) spectrometer to investigate the chemical structure of depolymerization products (500 MHz). DMSO and TMS was used as a deuterated solvent and as internal reference.

MS

Mass spectroscopy (Model: LCMS-2020, Shimadzu, Japan) was used to obtain the mass spectrums of BHET and BHETA.

Catalytic Activity

Scheme 1 represent the depolymerization of PET polyester textile waste threads of red, blue, and green colours using ethanolamine amine (EA) and diethanolamine (DEA) as aminolytic agents and ethylene glycol (EG) and diethylene glycol (DEG) as glycolytic agents under microwave irradiation. PET treated with amine such as EA to produces the amides bis (2-hydroxy ethylene) terephthalamide (BHETA) and the glycolysis process PET reacted with EG to produce bis (2-hydroxy ethylene) terephthalate (BHET), respectively.

General procedure for Glycolysis of PET polyester textile wastes

Polyester thread's red, blue, and green depolymerization reactions are carried out in a microwave oven equipped with a microwave power control system, and temperature is measured after MW irradiation. The coarse polyester powder was used for the glycolysis as well as the aminolysis reactions. The glycolysis of PET polymer was carried out using ethylene glycol and diethylene glycol as glycolytic agents in the presence of bimetallic ZnO as a catalyst.

In a typical glycolysis procedure, 50 ml of the round bottom flask with a reflux condition, 500 mg of PES textile waste, and 50 mg of Ag-doped ZnO were treated with 20 ml of EG or DEG exposed under 300 watts of power for 30 minutes with regular time intervals. When the set temperature (180–200 ˚C) was reached and PES was dissolved in a glycol medium, the reaction time was calculated. Thin Layer Chromatography (TLC) analysis plates showed the reaction's progress. At the end of the glycolysis reaction, 50 ml of hot distilled water was added to the reaction mixture, followed by filtration to remove the catalyst. The filtrate contains EG and BHET. After the removal of the catalyst, the filtrate was allowed to be heated at 70 ˚C for 20 min. The product, BHET, obtained from the reactions is a white crystal. The structure of the glycolysis process was confirmed using FT-IR, ¹H NMR, and MS. The same reaction occurred with 0.5, 1.0, 2.0, and 3.0 mol % of Ag-doped ZnO nanoparticles. The conversion of PES textile waste and the yield of the BHET product was calculated using the following equation: The percentage degradation of PES textile waste was calculated using the following equation:

\(\%{ Conversion}_{PET }= \frac{{W}_{1}-{W}_{2}}{{W}_{2}}X 100\%\) -----------◊ 1

W₁- Weight of PES textile wastes (g); W₂- Weight of residue after filtration (g)

The Yield of the BHET was calculated using,

\(\%{Yield}_{BHET}= \frac{{W}_{E}}{{W}_{T}} X 100\%\) -----------◊ 2

W_E – Experimental Weight (g); W_T – Theoretical weight (g)

General procedure for Aminolysis of PET polyester textile wastes

Similar to the glycolysis procedure, 500 mg of the polyester thread was treated with 50 mg of Ag-doped ZnO as a catalyst, 20 ml of EA or DEA was added, and to achieve the reaction temperature of 150–180 ˚C, a constant power of 180 W was used for 15 min. At the end of the reaction, 50 ml of hot DI water was added in excess to the reaction mixture with vigorous agitation, followed by filtration to remove the catalyst. After the removal of catalysts, the final product was obtained as white to pale yellowish crystals. After that, the same steps were repeated for DEA, as an aminolytic agent, followed by the same procedure. To characterise the end product of the aminolysis process using FT-IR, ¹H-NMR, and MS.

The sol-gel method has been adopted successfully for the synthesis of Ag-doped ZnO nanoparticles. ZnO doped with 0.5, 1.0, 2.0, and 3.0 mol% was synthesised using the reported procedure (Vinitha et al. 2022). Depending on the Ag loading, Ag-doped ZnO was obtained as a light grey to grey powder. The crystallinity, crystallite size, optical bandgap, particle size, and elemental composition are determined using XRD, UV-Vis, SEM-EDX, and TEM-SAED techniques.

Diffraction and crystallite size analysis

Figure 1 shows the X-ray diffraction pattern of pure and Ag-doped ZnO nanoparticles recorded between 20 and 80˚. Strong and sharp peaks indicate the ordered wurtzite structure of ZnO nanoparticles. In addition, diffraction peaks related to AgO appeared to confirm the doping of Ag II (Fig. 1a). Diffraction angles shown at 31.70˚, 34.50˚, 36.30˚, 47.75˚, 56.65˚, 63.10˚, 66.35˚, 66.50˚, 67.90˚ and 69.95˚ accounted for planes (100), (002), (101), (102), (110), (103), (200), (112), (201) and (004) respectively according to the ZnO standard (JCPDS: 036-1451). The diffraction pattern of Ag-doped ZnO is depicted in Figs. 1a and 1b. The characteristic peaks observed at 38.15˚, 44.30˚ and 64.60 reflect the hkl values of planes (111), (200), and (220) corresponding to crystalline AgO-doped ZnO nanoparticles.

Based on Fig. 1a, a gradual increment in the intensity of AgO diffraction peaks indicated an increasing doping level of Ag (II) ions. Three major planes (100), (002) and (101) have not shown any change in diffraction angles (Fig. 1b). Therefore, the diffraction pattern of ZnO is not altered by Ag dopant in the ZnO framework (Fig. 1b). Based on the diffraction analysis, the crystallite size of Ag doped ZnO was calculated using the Debye-Scherrer equation (Eq. 1).

\(D=\frac{\text{K}{\lambda }}{\beta \text{cos}\theta }\) ……………… (Eq. 1)

D is crystallite size in nm, K is structural constant (value taken as 0.9), l is the wave length of X-ray source used for the diffraction studies (l = 1.5406 Å), is diffraction angles (in radians) and \(\beta\) is full width half maximum (FWHM) of diffraction peaks. Alteration in crystallite size, cell parameters and structural strain due to Ag doping calculated from Williamson-Hall method (Eq. 2).

\(\beta \text{cos}\theta =\frac{\text{K}{\lambda }}{D}+4ϵsin\theta\) ………….. (Eq. 2)

D is crystallite size (in nm), is diffraction angles (in radians) and e is lattice strain. Eq. 2 is a linear equation and Sin\(\theta\) is plotted against \(\beta\)Cos\(\theta\). From the plot, crystallite size D is calculated by rearranging intercept to D = K\({\lambda }\)/c and lattice strain (e) obtained from slope divided by 4.

The crystallite size calculated using the Debye-Scherrer equation revealed that crystallite size increased with increasing Ag dopant concentration (Table 2). In contrast, the W-H calculation method showed decreasing the crystallite size while increasing the concentration of Ag dopant, and the results are consistent with the earlier reports (Nigussie et al. 2018; Thakur et al. 2020; Rahman et al. 2019). Furthermore, the lattice strain (ε) increased as the Ag(I) concentration increased from 0.5 to 3.0 mol%. The decrease in crystallite size and increase in lattice strain indicates the incorporation of a large ionic radius (1.55 A°) in Ag(I) as opposed to a smaller ionic radius (0.74 A°) in Zn(II). The particle size of Ag-doped ZnO was determined from SEM images of respective nanoparticles using the software imageJ (version 1.53) to support the crystallite size (Fig. 4). The agglomeration of smaller Ag-doped ZnO results in the formation of nanoparticles with particle sizes ranging from 118 to 135 nm, with a gradual increase in particle size, observed as the concentration of Ag(I) doping increases.

Table 2

Lattice parameter, crystallite size and lattice strain of ZnO and Ag doped ZnO
Material	Lattice parameters			volume (nm³)	Crystallite size (nm) (Scherrer equation)	Crystallite size (nm) (W-H equation)	Lattice strain \({\epsilon }\) X 10^− 3	SEM Particle size (nm)
Material	a (Å)	c (Å)	c/a	volume (nm³)	Crystallite size (nm) (Scherrer equation)	Crystallite size (nm) (W-H equation)	Lattice strain \({\epsilon }\) X 10^− 3	SEM Particle size (nm)
ZnO	1.69	5.19	3.05	12.98	37.65	80.40	1.40	156.92
0.5mol% Ag-ZnO	1.69	5.19	3.06	12.96	50.48	60.34	0.275	118.39
1.0mol% Ag-ZnO	1.90	5.20	2.74	16.18	49.54	51.72	0.375	118.01
2.0mol% Ag-ZnO	1.90	5.19	2.74	16.17	57.42	43.88	0.475	125.47
3.0mol% Ag-ZnO	1.93	5.19	2.68	16.82	61.90	36.24	0.525	134.48

UV-Vis studies and Optical bandgap calculation

Figure 2a shows the UV-Vis absorption spectra of pure and Ag-doped ZnO nanoparticles, recorded in the range of 200–800 nm. The outer shell electrons of ZnO and Ag₂O are excited to a higher energy state by absorbing light to give an absorption peak. The absorption spectra give valuable information on bandgap alteration by the dopants in any kind of semiconducting metal oxides, such as TiO₂ and ZnO. The optical bandgap of Ag-doped ZnO NPs is calculated from the onset of the absorption peak in the higher wavelength region. Ag/ZnO NPs showed an absorption maximum in the range of 350–360 nm, and a change in absorption maxima was not observed due to doping. However, 2 and 3 mol% Ag(I) doped ZnO showed second absorption maxima at 554 and 556 nm, respectively, due to the presence of Ag₂O, and visually, these two samples appeared pale grey and dark grey, respectively.

Table 3

UV-Vis absorption maxima, optical band gap, FT-IR and elemental composition
Material	Absorption maxima (λ, nm)		Optical band gap (eV)	Wave number (cm^− 1)	FT-IR assignments	Weight (%)	Atomic (%)
Material	λ₁	λ₂	Optical band gap (eV)	Wave number (cm^− 1)	FT-IR assignments	Weight (%)	Atomic (%)
ZnO	350	-	3.26	440	Zn-O	-	-
0.5 mol% Ag-ZnO	351	-	3.21	440 500 990–1010 1090–1110	Zn-O Ag-O Zn-O sym Zn-O sym	Zn – 80.24 O – 19.92 Ag – 0.63	Zn − 50.54 O – 49.24 Ag – 0.24
1.0 mol% Ag-ZnO	350	-	3.21			Zn – 75.20 O – 24.80 Ag – 0.65	Zn – 42.20 O – 57.40 Ag – 0.25
2.0 mol% Ag-ZnO	350	554	3.19, 1.95	440 500 990–1010 1090–1110	Zn-O Ag-O Zn-O sym Zn-O sym	Zn – 79.20 O – 19.79 Ag – 1.01	Zn – 49.29 O – 50.33 Ag – 0.38
3.0 mol% Ag-ZnO	350	556	3.21	440 500 990–1010 1090–1110	Zn-O Ag-O Zn-O sym Zn-O sym	Zn – 79.71 O – 19.65 Ag – 1.95	Zn – 49.69 O – 50.07 Ag – 0.89

The bandgap values were obtained from Tauc’s plot by plotting (αhϑ)^−1/2cm^−1/2 against photon energy in eV. Figures 2b and 2c show Tauc’s plot of pure and Ag doped ZnO NPs. The onset of Tauc's plot curves directly gives an optical bandgap in eV. Change in optical band gap is observed due to doping (Table 2). According to Tauc's plot, pure ZnO had an optical band of 3.26 eV, while Ag(I)-doped ZnO had optical bands of 3.21 and 3.19 eV for 2 and 3 mol% doped ZnO NPs, respectively. A decrease in optical bandgap was observed, and in addition, another bandgap was observed at 1.95 eV due to 2 and 3 mol% Ag doping (Bagha et al. 2021; Yousefi et al. 2012).

FT-IR analysis

FTIR spectroscopy is used to determine the molecular structure and various functional groups present in the pure and Ag-doped ZnO NPs. Figure 3a shows the FTIR spectra of pure and 0.5, 1.0, 2.0, and 3.0 mol% of Ag-doped ZnO nanoparticles synthesised by the sol-gel method and calcined at 500 ˚C. Figure 3b depicts the expanded position of the first absorption band. FT-IR spectra are recorded in the range from 400 to 4000 cm^− 1. The position and number of absorption bands depend on the crystal morphology, crystal structure, and chemical composition. All the samples exhibited broad and sharp absorption bands in the range of 420 to 540 cm-¹ attributed to the metal oxygen stretching vibration modes and clearly confirmed the formation of Zn-O and Ag-O stretching bond. This indicates the successful incorporation of Ag ions into the crystal lattice of ZnO. Furthermore, the two broad peaks with approximate intensities ranging from 980 to 1140 cm^− 1 are associated with symmetric and asymmetric stretching vibrational states, respectively, whereas the Zn-O mode, which extends peak intensities at 510 cm^− 1, is more common in undoped ZnO wurtzite (Rajendran, 2020; Sagadevan, 2017; Ramesan, 2018).

SEM morphological analysis

The morphological structures of the prepared nanoparticles were examined by SEM analysis, as shown in Figs. 4a and 4b for 0.5, 1.0, 2.0, and 3.0 mol% Ag doped ZnO, respectively. The SEM-EDX images of pure and different mol% of Ag-doped ZnO NPs composite at two different magnifications. EDX analysis is shown in Fig. 4c Doped and undoped ZnO NPS were magnified at the scale of 1µm and 500 nm is shown in 4a and 4b SEM. The images clearly show the spherically shaped Ag-doped ZnO NPs at all magnifications (Figs. 4a (1) and 4b (1)). The microstructure revealed a uniform and compact structure, interconnected by grains of 100–120 nm size on average. Also, the morphology of the particles is approximately “spherical” in shape. The morphology of integrated Ag-ZnO NP was not affected by the incorporation of Ag ions into the ZnO lattice.

Energy-dispersive X-ray spectroscopy (EDX) (Fig. 4c) is used to determine the basic elemental composition of the synthesised Ag-doped ZnO nanoparticles. The EDX spectra of 0.5, 1, 2, and 3 mol% Ag-doped ZnO NPs are shown in Fig. 4c. EDX analysis confirms the presence of the dopant Ag in the Ag-ZnO nanoparticles. In the observed EDX spectra, well-defined peaks for zinc (Zn), oxygen (O), and silver (Ag) are found. No impurity or peak other than Zn, Ag, and O atoms were observed in the EDX spectrum, which revealed that the integrated NPs were composed of Zn, Ag, and O. The observed EDX results are well-matched with the diffraction pattern of XRD. The composition of 0.5 mol% Ag doped ZnO contains 0.65 wt% and 0.25 atomic% of Ag, 2 mol% Ag doped ZnO contains 1.01 wt% and 0.38 atomic% of Ag, and 3 mol% Ag doped ZnO contains 0.63 wt% and 0.24 atomic% of Ag, which is consistent with ZnO doped with different mol% Ag. (Dhatshanamurthi, 2016; Gnanaprakasam, 2016).

TEM

High-resolution TEM (HRTEM) was used to examine the detailed morphological and structural properties and composition of as prepared 2 mol% Ag-doped ZnO NPs (Kumar, 2015). Figure 5a and b show the TEM images of 2 mol% of Ag-doped ZnO nanoparticles reached at scales of 50 and 100 nm, respectively. Figure 5c and d show the SAED and EDX spectra and elemental composition of 2 mol % of Ag-ZnO nanoparticles. The TEM images, it is clearly shown that the Ag-doped ZnO has a hexagonal structure and a spherical shape. This HRTEM result agrees with the XRD result, indicating that Ag-doped nanoparticles have a higher crystalline nature and that the Ag nanoparticles are uniformly distributed over the ZnO lattice. Both images show the typical hexagonal wurtzite structure that is consistent with their XRD results. Figure 5c shows an image of selected-area electron diffraction (SAED) confirming the patterns of Ag-doped atoms with circular rings as planes (111), (200), and (220), corresponding to polycrystalline in nature (Ali, 2018). The elemental mapping and composition of Ag-doped ZnO NPs are determined using EDX spectra. The EDS spectra of 2 mol % Ag-ZnO NPs are shown in Fig. 5e. The peaks of elements Zn, O, and Ag have been identified in the EDX spectra. The EDX spectra reveal that Ag-doped ZnO NPs are composed mainly of Zn, O, and Ag elements. The composition of 2 mol% Ag doped ZnO is 9.96 wt% and 4.75 atomic% Ag, which is consistent with ZnO doped with 2 mol% Ag (Tsai, 2018; Suresh Kumar, 2015; Ramesh Kumar, 2015).

Depolymerization Of Polyester Threads Using Aminolysis And Glycolysis

Figure 6 summarised glycolytic and aminolytic depolymerisation of red, blue and green polyester threads using 2 mol% Ag doped ZnO NPs and compared them with pure ZnO NPs. All the reactions carried out in the presence of 5wt% of catalyst to PES and, ethanol amine or diethanol amine was used as aminolyting agents, similarly, ethylene glycol(EG) and diethylene glycol(DEG) (PES: EG, 1:20) used as a glycolytic agent (Scheme 1). The aminolysis of PES threads using ethanol amine catalysed by ZnO and 2 mol% Ag doped ZnO afforded more than 94% of bis(2-hydroxy ethyl) terephthalamide (BHETA). The nucleophilic centers in diethanolamine (DEA) and ethanolamine (EA) are hydroxyl and amine. Because nitrogen is a good nucleophile than oxygen, the amine group of DEA and EA attacks the ester linkage of the PET during aminolytic depolymerization.

Optimisation Of Reaction Parameters On The Aminolysis Reaction Of PES Textile Waste

Effect of Ag doping

To study the effect of different mol% of Ag-doped ZnO nanoparticle catalyst on BHETA yield, we used ethanolamine as the aminolytic agent and a constant ratio of 1:20 PET to EA at 180 W under microwave irradiation for 30 h, as shown in Fig. 7a. It can be observed that increasing the mol % ratio of Ag doping waste results in a significant increase in BHETA yield. Among all the catalysts, 2 mol% of Ag-doped ZnO NPs produced the highest BHETA yield, which was greater than the ZnO-catalyzed reaction yield.

Effect of microwave power

Figure 7b showed the PES textile waste decomposition was carried out at different MW of radiation power.The reaction time was kept constant for 30 min with irradiation at 50, 100, 150, 180, 200, and 220 W. Even after 30 minutes of irradiation, the PET degradation is only 50% for the lowest power used (50 W), and when 180 W is used, almost complete dissolution occurs within 2 minutes. After 20 minutes of irradiation at 75 W, complete PET depolymerization occurs. The results indicate an increase in the yield of BHETA, increasing microwave power from 150 to 200 W. This indicates that 180 W of microwave power is optimal for irradiation, as demonstrated by the subsequent reactions.

Effect of irradiation time

To investigate the effect of reaction time on BHETA yield, we performed aminolysis of PES textile waste at 180 W with ethanolamine as the aminolytic agent. The resulting yield is shown in Fig. 7c. The complete depolymerization of PES textile waste under 180 W microwave irradiation was observed in the reaction time range of 2 to 15 min. In the time range of 10 to 15 minutes, an increase in PES conversion from 60 to 90 percent is observed. PES textile waste was completely (100%) converted after 15 min, and the BHETA yield was 96%. Finally, the results indicated that the yield of BHETA and the conversion of PES textile waste increased with increasing irradiation time. This showed that time has been found to be a major factor in the depolymerization of PES textile wastes. In order to find the minimum reaction time required for complete depolymerization of PES, 100 mg of 2 mol% Ag-ZnO and a 1:20 PES:EA ratio were used for the aminolysis reaction.

Effect of temperature

Figure 7d showed the effect of reaction temperature on the depolymerization of PES textile waste under microwave irradiation, temperature is a crucial factor in aminolysis experiments. Temperatures of 100, 120, 150, 180, and 200 ˚C were used at 10- to 15-minute intervals, along with 100 mg of 2% Ag doped ZnO catalyst, 20 ml of ethanolamine, and 500 mg of PES textile wastes. As predicted, increasing the reaction temperature causes more PES textile waste depolymerization and BHETA formation. Even after 60 minutes of irradiation at low temperatures of 100 and 120 ˚C, PES textile waste decomposition was only 55 and 70 percent, respectively. When the temperature is raised to 150–180 ˚C, the dissolution of PES textile waste takes between 5 and 15 minutes to achieve more than 90% depolymerization. It can be assumed that using microwave irradiation significantly reduces the time required for PES textile waste conversion and increases the yield of BHETA.

Recycling of catalyst

2 mol% Ag-doped ZnO nanoparticle catalyst was investigated for catalyst recycling studies shown in Fig. 7e. For economic and environmental reasons, most chemists are focusing on determining the viability of catalyst recycling. Therefore, the utilisation and recovery of the catalyst under optimised conditions were explored for further amionolysis of PES textile waste. Following filtration, the catalyst was washed with methanol and calcined at 550 ˚C for 3 hours. In this method, the Lewis acid catalyst Ag-doped ZnO can be recycled up to six times with negligible activity loss. However, the decrease in BHETA yield observed is due to a stronger interaction of ZnO Lewis acidic sites with ethanol amine, which in turn affects the structure of ZnO.

Aminolysis of PES threads using ethanol amine (EA) and diethanol amine (DEA)

Figure 8a shows PES textile waste recycling through the aminolysis process. PES textile waste included red, blue, and green threads, with EA and DEA used for aminolysis reactions. The reaction of each EA and DEA with PES results in the splitting of the ester bonds of PES and the formation of the amides BHETA and BDHETA, respectively. The amide is the main reaction product due to the higher reactivity of primary amines in ester bond cleavage. More than 90% of bis (2-hydroxyethyl) terephthalamide (BHETA) was obtained from aminolysis of PES threads using ethanol amine catalysed by ZnO and 2 mol% Ag doped ZnO. At 180°C and a PET/EA ratio of 1:20, 2 mol% Ag-ZnO NPs were obtained in 15 minutes, with the highest PES thread conversion and BHETA yields of 95, 95, and 96% for red, blue, and green threads, respectively. Furthermore, the DEA's requirements for producing BDHETA were met at a rate of 80–90%.EA is found to be a good aminolyzing agent for PES textile waste degradation because it produces a higher BHETA yield than DEA.

Glycolysis of PES threads using ethylene glycol (EG) and diethylene glycol (DEG)

Several researchers are becoming more interested in PES textile waste recycling via microwave irradiation. The microwave recycling process enables relatives to take a short reaction time while using much more energy for heating than the traditional heating process. Even so, the benefits of microwave irradiation of PES textile waste via the glycolysis process are still prohibitively expensive due to energy consumption. Glycolysis is the most cost-effective and commercially viable method of chemically recycling PES textile waste. Glycolysis is a type of solvolysis reaction in which glycol is used as the reactive solvent. The glycolysis reaction is chemically defined as the "molecular degradation of PET polymer by glycols in the presence of trans-esterification catalysts, where the ester linkages break and are replaced with hydroxyl terminals."

Figure 8b depicts a comparison of the glycols performance of pure and Ag-doped ZnO NPs catalysts in the glycolysis reaction of PES textile waste. The reactor was placed in a microwave, and the reaction microwave power was maintained at 300 watts for 30 minutes. PES/EG and PES/DEG reaction temperatures were in the 180–200 ˚C range at 300 watts of power. Glycolysis commonly uses EG to produce BHET and DEG to produce BHEOET. The polyester threads (red, green, and blue) reacted with EG when Ag-ZnO NPs were used as a catalyst, and the BHET yield increased to 90%, compared to slightly higher yields from pure ZnO. More than 80% of the BHEOET produced was made possible by polyester threads treated with DEG. The reaction resulted in 100% depolymerization of PES textile waste, 80 to 90% yields of BHET and BHEOET, and more than 10% oligomer.

Spectral Characterisation Of BHETA

The complete depolymerization of PET wastes generates BHETA and BHET as a single product. However, there could be a possibility of forming dimer and oligomer products due to incomplete depolymerization. Thus, spectral characterization techniques such as MS and FT-IR analysis have been carried out. To confirm the purity and structural confirmation of the depolymerized products, BHET and BHETA, structural characterization was performed. FT-IR of BHETA and BHET is given in Figs. 9 and 10, respectively (the full spectrum is given in the supplementary information), as is their chemical structure. The 1HNMR-depolymerized PES waste showed up in Figs. 11 and 13. The ESI-MS spectra of BHETA and BHET are given in Figs. 14 and 15, respectively.

FT-IR spectrum of BHETA and BHET

Figures 9a, b, and c show the FTIR spectra of the aminolyzed product of BHETA, which were recorded from 400 to 4000 cm^− 1. The presence of NH and hydroxyl groups was confirmed by a broad absorption band around 3354 cm^− 1 and 3280 cm^− 1. Broad bands at 3089 and 2958 cm^− 1 were assigned to the aromatic C-H stretching vibrations and aliphatic CH₂ vibrations. The presence of amide linkages (-CONH-) in the structure was confirmed by a sharp peak at 1616 cm^− 1. The presence of aromatic C = C stretching vibrations is responsible for the peak at 1548 cm^− 1. The peak at 1047 cm^− 1 is due to aliphatic C–H stretching vibrations. The presence of BHETA was confirmed by the presence of aromatic and aliphatic -OH and -NH stretching vibrations, as well as an amide carbonyl vibration (Jamdar, 2017; Musale, 2016).

As shown in Figs. 10a, b, and C, Ag-doped ZnO NPs catalysed the glycolysis reaction of PES textile waste of red, blue, and green threads, which are BHET products.The main product's FTIR spectrum agrees well with those described in the literature for BHET. The presence of hydroxyl groups is indicated by the peak at 3437.15 cm-1 (OH). The bands at 2960 to 2879 cm-1 correspond to the stretching vibrations of CH2 domains, respectively. The peaks at C = O, C-O ester bond asymmetric vibration, and C-O ester bond symmetric vibration are assigned to the bands at 1705, 1261, and 1066 cm-1, respectively. The weak band at 1408 cm-1 is very typical of aromatic C = C stretching. The presence of aromatic residue in the glycolyzed products is strongly suggested by the bending frequencies at 721 and 489 cm-1. Furthermore, the sharpness of the FTIR peaks suggests that the BHET obtained from the depolymerization process is of high purity (Sert, 2019; Mendibura, 2021; Hoang, 2018).

As shown in Figs. 10a, b, and c, Ag-doped ZnO NPs catalysed the glycolysis reaction of PES textile waste of red, blue, and green threads, which are BHET products. The main product's FTIR spectrum agrees well with those described in the literature for BHET. The presence of hydroxyl groups is indicated by the peak at 3437.15 cm^− 1 (OH). The bands at 2960 to 2879 cm^− 1 correspond to the stretching vibrations of CH₂ domains, respectively. The peaks at C = O, C-O ester bond asymmetric vibration, and C-O ester bond symmetric vibration are assigned to the bands at 1705, 1261, and 1066 cm^− 1, respectively. The weak band at 1408 cm^− 1 is very typical of aromatic C = C stretching. The presence of aromatic residue in the glycolyzed products is strongly suggested by the bending frequencies at 721 and 489 cm^− 1. Furthermore, the sharpness of the FTIR peaks suggests that the BHET obtained from the depolymerization process is of high purity (Sert, 2019; Mendibura, 2021; Hoang, 2018).

NMR spectrum of BHETA and BHET

Proton ¹H NMR is used to get a better insight into the chemical structure of BHET and BHETA. DMSO was used to obtain the ¹H NMR spectrum of the depolymerized product at room temperature using a 500 MHz instrument. The NMR spectra of the final product of BHETA obtained from red (VSAR1), blue (VSAB2), and green (VSAG3) are shown in Figs. 11a, b, and c. The aminolyzed product, bis-(2-hydroxyethyl)terephthalamide (BHETA), is a symmetrical molecule. The ¹H NMR spectra of BHETA revealed two NH protons bonded to amide carbonyl (C7 and C8) (-C = O) and methylene carbons (C9 and C10) (-CH₂) as a triplet at 8.55–8.58 ppm with a J value of 5.5 Hz. The presence of NH protons shows depolymerization of PES and BHETA formation. The position and multiplicity of aromatic protons provide valuable information about the monomer, dimer, and oligomeric nature of depolymerized products. The skeleton of the BHETA molecule is symmetrical, as evidenced by the signal of aromatic protons attached to C1, C2, C4, and C5 appearing as a singlet at (Fig. 11a) 7.92 ppm and (Fig. 11b and c) 7.93 ppm, indicating that BHETA is the pure and single product formed, which indicates that depolymerization of PET wastes is complete. Furthermore, Fig. 11a showed up singlet at 4.77 ppm, and at 4.75–4.78 ppm with a J value of 5.5 Hz (Fig. 13b and c), two -OH protons attached to methylene carbons (C11 and C12) appeared as a triplet. Between 3.50 and 3.54 ppm (Fig. 11b and c), the protons of the methylene group (C11 and C12) attached to -OH appeared as a quartet with a J value of 6 Hz. All ¹H NMR spectra show a peak corresponding to methylene protons (C9 and C10) attached to an amide NH group, which produces a triplet at 2.51 ppm and a J value of 6 Hz. The spectral values of the aminolyzed product of BHETA agree well with the spectral values published previously (-NH-CO-C6H4-CO-NH-) (Vinitha et al. 2022; Jeya et al. 2022a).

Figures 13a, b, and c depict the 1H NMR spectra of the depolymerized BHET final product [red (VSAR1), blue (VSAB2), and green (VSAG3)], with the corresponding peak assignments marked in the structural formula. The BHET is a symmetrical molecule, and the four aromatic (C1, C2, C4, and C5) hydrogens present in BHET have shown up as singlets at 8.14 ppm, confirming the formation of the glycolyzed product as the BHET monomer. The 1H NMR analysis revealed singlet aromatic protons, confirming the complete depolymerization of PES textile wastes. At the peak 4.97–5.01 ppm (Fig. 13a & c), the two hydrogens attached to carbon (C9 and C10) next to the ester carbonyl oxygen (-CH₂-OC = O) (C7 and C8) appeared as a triplet, and the peak 4.97–5.01 ppm (Fig. 13b) appeared as a quartet with a J value of 5.5 Hz. Due to the exchangeable nature of its kind, some other four hydrogens (C11 and C12) connected to CH2 and OH appear as a triplet in the range of 4.31–4.35 ppm and are present in both the spectra and J value at 6 Hz. A hydroxyl group attached to CH₂ appeared as a broad quartet in the 3.70–3.75 ppm range. At 2.50 ppm, the deuterated DMSO solvent appears as a singlet with a J value of 6 Hz. (Jeya et al. 2017, 2020, and 2022)

Mass Spectrum Of BHETA

Figure 14a, b and c represents the mass spectrum of the aminolyzed products, bis (hydroxy ethyl) terephthalamide obtained from red blue and green PES threads. BHETA's molecular formula is C₁₂H₁₆N₂O₄ and its molecular weight is 251.00 g/mol. The Red PES thread (VSAR1) (Fig. 14a) spectrum recorded using the electron spray ionisation method exhibited a protonated molecular ion peak at m/z 251.35 (M + H). The following peak at m/z 268.20 predicts that the Na⁺ Hydrated form of BHETA was formed as a result of electron spray ionisation. Then the mass spectrum of blue PES (VSAB2) (Fig. 14b) revealed the molecular ion peak at 251.05 (M + H) in the protonated form and the next peak at m/z 287.00 indicates the formation of dihydrated form. Finally, the last image of green-coloured PES (VSAG3) (Fig. 14c), which gives a peak at 250.10 (M + H) attributed to the main product of BHETA, one more peak at m/z 283.15 clearly shows that the Na⁺ form of BHETA due to electron spray ionization (Vinitha, 2022).

Mass Spectrum Of BHET

Figures 15a, b and c represent the mass spectrum of the glycolysed product, bis (2-hydroxyethylene) terephthalate obtained from red, blue and green PES wastes. BHET's molecular formula and the molecular weight is C₁₂H₁₄O₆ and 254.00 g/mol, respectively. The mass spectra demonstrate that the main product was obtained with high purity, as no signals from other compounds were detected. The ESI-MS spectrum of color PES (VSGR1) (Fig. 15a) showed a molecular ion peak with m/z 254.25 (M + H) with 100% intensity, which corresponds to BHET ionised by Na + value is 294.40. A common feature in the figures of (15b and c) VSGB2 and VSGR3 showing mass spectrometry results is that the parent mass peak is located at 252.7 (M + H) In the case of molecular ions of Na⁺ form of BHET. Entirely these data indicate that highly pure BHET was obtained after the crystallization process in water was performed. Figure 15 (b) and (c) mass spectrum of BHET obtained from glycolytic depolymerisation of blue and green PES threads. Both the mass spectrum showed molecular ion peaks at m/z 252.75 and 252.70 respectively for BHET generated from blue and green PES wastes (Sce, 2019).

In textile industries A "circular economy" is an economically viable and executable system through which fibre wastes could be reused to the greatest extent possible. By employing chemical recycling, the concept of zero material waste, generation, and efficiency of each process, component, and material could be achieved. In our present investigation, we have successfully depolymerized PES textile wastes using Ag-doped zinc oxide nanoparticles as catalysts through the glycolysis and aminolysis of PES textile wastes under microwave irradiation. Chemical recycling techniques, particularly aminolysis and glycolysis, are the most cost-effective and environmentally friendly solution when compared to all other recycling methods for depolymerizing PES textile wastes. The XRD diffraction patterns indicate that Ag-doped ZnO has a good crystalline structure with a hexagonal wurtzite structure. FT-IR spectroscopy reveals that there is no significant difference between pure and different molar ratios of Ag at 0.5, 1.0, 2.0, and 3.0 mol % when compared to pure synthesised ZnO. Doping ZnO with Ag2 + results in a decrease in the bandgap. Furthermore, as observed by XRD and W-H calculation, doping of Ag (3.0 mol %) reduced the crystallite size of ZnO from 80.40 to 36.24 nm (1–3 mol%). The SEM analysis of Ag-doped ZnO NPs revealed an increase in size from 118 to 135 nm, which was higher than the crystallite size calculated from powder XRD (Scherrer and W-H method). Depolymerization of PES textile wastes is optimized using various reaction parameters such as microwave power, time, and temperature. PES textile wastes are depolymerized using aminolytic reagents such as ethanolamine and diethanolamine and glycolytic agents such as ethylene glycol and diethylene glycol. Ag-doped ZnO nanoparticles were used to successfully aminolyze and glycolyze PES textile wastes. When compared to other analogues of aminolysis and glycolysis, 2 mol% Ag-doped ZnO produced 96 and 94 percent of BHETA, respectively. To characterise the product of the aminolysis and glycolysis processes using FT-IR, NMR, and MS. Both BHET and BAETA are isolated in their purest form, requiring no further purification. We proposed that Ag-doped ZnO nanoparticles could be a cost-effective and efficient catalyst for aminolytic depolymerization of PET wastes to produce high BHETA when compared to glycolytic depolymerization of PET waste to produce BHET as a result of our investigation. We also discovered that this catalyst can be reused up to seven times without significantly lowering the BHETA yield. The disclosed process even has great potential for recycling the PES textile wastes. Following aminolysis, both blue and green-colored PES produced colorless, transparent crystals of BHETA, whereas red-colored PES produced pure, transparent BHETA and the dye remained in solution. On the other hand, glycolysis afforded a coloured depolymerized product.

Acknowledgement

University Grants Commission (UGC) is gratefully acknowledged by Dr. V. Sivamurugan for proving financial assistance in the form of minor research project [File No. F: MRP-6393/16 (SERO/UGC)]. All the authors are very thankful to Centralized Sophisticated Instrument Facilities (CSIF), Interdisciplinary Institute of Indian System of Medicine (IIISM), SRM Institute of Science and Technology, Katankulathur – 603 203 and SCIF:SRM Central Instrumentation Facility, NANOTECHNOLOGY RESEARCH CENTRE for the characterization of catalysts and depolymerized products.

Authors’ Contributions

Investigation and manuscript writing: VV. Resources and methodology: MP. Formal analysis: MA. Supervision: BN. Conceptualisation, formal analysis, manuscript writing and supervision: VS. All authors have read and agreed to the published version of the manuscript.

Samples availability

The sample of catalysts and all the depolymerised samples available from the corresponding author.

Conflict of Interest

The authors have declared that they have no conflict of interest.

Ethical Approval

This research did not involve human participants and/or animals.

Consent to Participate

All authors are informed and provided consent for this submission.

Consent to Publish

All authors have approved the manuscript and declare that this is an original contribution and none of the material in this paper is under consideration for publication elsewhere.

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Scheme 1 is available in the Supplemental Files section.

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Scheme1.png
Scheme 1 Glycolysis and Aminolysis of PES textile wastes

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Chemical Recycling of PET Polyester Textile Wastes Using Ag-Doped ZnO Nanoparticles: An Economical Solution for Circular Economy

Status:

Journal Publication

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Abstract

Figures

Introduction

Materials And Methods

Synthesis of Silver doped ZnO nanoparticles

Characterization of catalysts

XRD

FT-IR

UV- Vis

SEM-EDX

TEM

NMR

MS

Catalytic Activity

General procedure for Glycolysis of PET polyester textile wastes

General procedure for Aminolysis of PET polyester textile wastes

Results And Discussion

Diffraction and crystallite size analysis

UV-Vis studies and Optical bandgap calculation

FT-IR analysis

SEM morphological analysis

TEM

Depolymerization Of Polyester Threads Using Aminolysis And Glycolysis

Optimisation Of Reaction Parameters On The Aminolysis Reaction Of PES Textile Waste

Effect of Ag doping

Effect of microwave power

Effect of irradiation time

Effect of temperature

Recycling of catalyst

Aminolysis of PES threads using ethanol amine (EA) and diethanol amine (DEA)

Glycolysis of PES threads using ethylene glycol (EG) and diethylene glycol (DEG)

Spectral Characterisation Of BHETA

FT-IR spectrum of BHETA and BHET

NMR spectrum of BHETA and BHET

Mass Spectrum Of BHETA

Mass Spectrum Of BHET

Conclusion

Declarations

References

Scheme

Supplementary Files

Status:

Journal Publication

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