PET depolimerization in supercritical ethanol conditions catalysed by nanoparticles of metal oxides
Graphical abstract
Introduction
Polyethylene terephthalate (PET) is widely used in the production of films, fibers, and plastic containers [1,2]. Due to its excellent properties, such as resistance to plastic deformation under pressure, impact resistance, low permeability to gases and vapors, besides excellent thermal stability and transparency, PET is currently the most important polyester used by industries [3].
The first appearance of PET occurred in 1941, due to the British chemists Dickson and Whinfield, being first used as synthetic fibers in textile industries. At the beginning of the '70 s, with the technique of bi-orientation of bottles, PET presented high expression in the market of containers for packaging [4], especially for carbonated soft drinks, due to impermeability to CO2. The first PET bottle recycling occurred in 1977 through mechanical recycling [5].
With the increase in PET consumption, large amount of waste is increasingly generated, and environmental problems become progressively severe [[6], [7], [8]]. Other significant concern is the fact that this polymer is derived from a non-renewable source: petroleum. In this way, recycling of PET is essential and efficient to reduce the consumption of resources and to protect the environment [[9], [10], [11]].
In Brazil, both the production and the recycling rate of PET have gradually increased year-by-year, with the recycling index reaching 51 % according to recent date from Brazilian Association of the PET Industry, ABIPET [12]. The PET recycling can be chemically, mechanically, and energetically made. Mechanical recycling predominates in Brazil being fibers, fabrics, ropes, and packaging, etc. However, the degradation caused by the shear and hydrolysis that occurs during the PET mechanical reprocessing limits the use and lowers the price of the products [13]. With this, chemical recycling has attracted great attention, since it allows value aggregation to the formed products [13] and even the manufacture of the virgin polymer from the recycled material. The chemical recycling of PET occurs through alcoholysis, hydrolysis, glycolysis, and aminolysis. The most applied is glycolysis, using nanostructured materials as catalysts [[14], [15], [16], [17], [18], [19], [20], [21]]. Two different researches pointed out that without use of catalysts, PET glycolysis is not effective, yielding a low amount of BHET monomer [22,23].
Due to environmental concerns arising from conventional depolymerization processes that tend to be very polluting and generate a lot of waste, other depolymerization methods have aroused interest. The depolymerization in supercritical fluids is one that has lately attracted attention lately. The depolymerization of PET using supercritical fluids is attractive because the products generated in many cases are in distinct phases, which facilitates the separation and purification by conventional and inexpensive physical processes and forming almost nothing as effluents. When a compound surpasses its critical point, it becomes a supercritical fluid, with interesting characteristic to reactions catalyzed by heterogeneous catalysts, as low viscosity and higher diffusivities, compared to the liquid state [24]. Ethanol above its critical conditions is an example of a supercritical fluid (SF) that has been used in the PET depolymerization process. Ethanol is produced in large scale in Brazil from a renewable resource, so that ethanolysis can be classified as a green reaction medium since it is readily available, has low cost and low toxicity and can be recycled after its use employing distillation [25,26]. Another possibility of alcoholysis of PET is the use of methanol as SF [[27], [28], [29], [30]]. However, its toxicity relies as an important disadvantage; the ingestion of 10 mL of pure methanol can cause permanent blindness, and 30 mL is potentially fatal for humans [31]. Besides this, it is not produced from renewable resources, but by synthesis gas, in general derived from oil or coal.
As already pointed before, for the aiming lower reaction time, the use of catalysts is essential in depolymerization using supercritical fluid environment [32]. In this case, simple nanosized metal oxides appear as excellent alternatives for depolymerization catalysts, since they can be synthesized by relatively low-cost and straightforward methods. Also, they are stable at high temperatures and can be easily separated from the products at the end of the reaction. ZnO, Co3O4, Mn3O4, TiO2, Fe2O3, among others, are examples of metal oxide catalysts used for the depolymerization reaction of PET.
Zhu [33], Imran [34], and Lee [35] investigated the use of some of these metal oxide catalysts mentioned above in the depolymerization process of PET via glycolysis and found that the metal oxides presented excellent catalytic activities, thus reducing the reaction time. Due to its high surface area and to the chemical characteristics at the surface, they favored the reaction to obtain the BHET monomer. When the depolymerization is carried out in supercritical ethylene glycol (EG), the temperature and pressure must be at least 450 °C and 8 MPa due to Tc and Pc of EG being 446.70 °C and Pc =7.7 MPa, respectively [36].
Despite this, and for the best of our knowledge, the use of simple and cheap metal oxide catalysts, obtained from Pechini method, in the depolymerization of PET through supercritical ethanolysis has not been investigated yet. In this way, our study is an interesting and promising alternative for PET recycling because critical temperature and pressure of ethanol are 250.7 °C and 6.14 MPa, respectively, conditions significantly lower than that necessary to supercritical ethylene glycol.
Section snippets
Synthesis of metal oxide nanoparticles
The nanoparticles used as catalysts were synthesized from salts of cobalt and nickel using the Pechini method (cobalt II nitrate hexahydrate and nickel II nitrate hexahydrate, purchased from Dynamic Co, Brazil). The amount, in mols, of the metal present in 1 g of its salt (nitrate) and the number of mols present in such amount was calculated. After that, using a 1:4:16 M ratio of metal: citric acid: ethylene glycol, the amount of citric acid (g) and ethylene glycol (mL) necessary to obtain such
X-ray diffraction (XRD)
X-ray diffractograms of Co- and Ni-based catalysts were obtained using a Bruker D8 Advance equipment in a range of 2θ from 5° to 80°, with Cu Kα x-ray source operating with 30 mA of current, 40 kV of voltage, with a scanning speed of 2° min−1 and a step of 0.02°.
Scanning Electron Microscopy (SEM)
The morphologies of metal oxide catalysts and DET were obtained using FEI QUANTA 250 equipment operating at 15 kV. The samples were gold-covered by sputtering before the observation at the microscope.
Transmission Electron Microscopy (TEM)
The size and morphology of the metal
X-Ray diffraction (XRD)
The Co and Ni oxide catalysts were characterized by X-ray diffraction aiming to monitor their structural characteristics regarding crystalline phases [37]. Fig. 1 shows the X-ray diffractograms for cobalt oxide calcined at 700 °C. Peaks at 2θ = 18.92°; 31.31°; 37.20°; 38.84°; 44.85°; 55.90°; 59.39° and 65.52° were assigned to cobalt oxide in the Co3O4 form, characteristic of a cubic spinel structure, according to the standard file obtained from the International Center for Diffraction Data
Conclusions
Nanoparticles based on nickel oxide and cobalt oxide were synthesized through Pechini’s method and used as catalysts for PET depolymerization. The catalysis processes showed high efficiency in the conversion of PET to DET, through the technique of ethanolysis in a supercritical fluid. Conversion yields more than 93 % in the best conditions for both catalysts were obtained. According to the statistical analysis the type and size of the catalyst affected significantly the depolymerization
Declaration of Competing Interest
The authors whose names are listed immediately below certify that they have NO affiliations with or involvement in any organization or entity with any financial interest (such as honoraria; educational grants; participation in speakers’ bureaus; membership, employment, consultancies, stock ownership, or other equity interest; and expert testimony or patent-licensing arrangements), or non-financial interest (such as personal or professional relationships, affiliations, knowledge or beliefs) in
Acknowledgments
The authors would like thanks to funding agencies CNPq, CAPES (AUXPE-PROEX-CAPES-Proc. nº 23038.000872/2018-83) and Fundação Araucária, and to COMCAP-UEM for SEM, TEM, XRD, NMR, HPLC, and DSC analyses.
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2023, Process Safety and Environmental ProtectionCitation Excerpt :It indicates that there was no TPA in solid residue. Some peaks were detected in the 3532–3426 cm−1 in Fig. 2(b) and (c), which is in the OH group region (Rodrigues Fernandes et al., 2020). The vibration of this region might be caused by the adhesion of BHET or mono-hydroxyethyl terephthalate (MHET) in the solid residue produced by the depolymerization process ((b) and (c)).