(83a) Advancing Plastic Waste Valorization: Co-Pyrolysis of PET and Low Density Polyethylene (LDPE) Using Ga- and Zn-Modified Catalysts for Enhanced BTEX Production

Plastics are essential in modern applications, from packaging and construction to automotive and electronics, yet their growing demand and inadequate waste management have led to a substantial accumulation. Research has focused on improving recycling technologies, with pyrolysis emerging as a key method, particularly for polyolefins, given their dominant share in municipal solid waste [1]. Polyethylene terephthalate (PET) is the third most used plastic resin, representing about 6 wt.% of global plastic production [2]. However, unlike polyolefins, research on PET pyrolysis is relatively limited. This is partly because sorted PET bottles can be efficiently recycled through mechanical methods, which are simpler and less energy-intensive than chemical recycling processes like pyrolysis [3]. However, applications for PET extend beyond bottles and are often combined with other materials, such as polyolefins [4]. For instance, chemical recycling remains the only viable alternative to handle such complex streams in multilayer packaging waste.

The pyrolysis process yields three primary products: char, pyrolysis oil (C₅-C₂₀), and pyrolysis gas (PyGas). Polyolefin pyrolysis typically produces negligible char, with PyOil in the gasoline and diesel range, heavier waxes (C₂₀₊), and smaller amounts of PyGas rich in C₁–C₄ paraffins and olefins. These products can be further upgraded to valuable light olefins (C₂–C₄) and mono-aromatics through catalytic cracking in a secondary close-coupled reactor [5, 6]. In contrast, PET pyrolysis generates a significant amount of char and pyrolysis oil (PyOil) and PyGas rich in CO and CO₂. Additionally, the PyOil fraction contains high levels of organic acids, primarily terephthalic and benzoic acids, which present practical challenges, such as increasing the oxygen content of PyOil, causing a two-phase product stream, and crystallization at room temperature. [7]. Catalytic upgrading offers a promising solution by converting these acids into valuable aromatics [8]. Given the common co-occurrence of PET and polyolefins in waste [7, 9], it is of particular interest to investigate the catalytic upgrading of vapors from the co-pyrolysis of LDPE and PET to produce valuable compounds like light olefins and BTEX (benzene-toluene-ethylbenzene-xylenes).

Therefore, the co-pyrolysis of PET and LDPE in a two-stage reactor system was investigated, and the role of modified HZSM-5 catalysts in converting the pyrolysis vapors into valuable products was examined. The efficiency of HZSM-5 in polyolefin cracking to produce light olefins has been demonstrated in our previous work [5, 6]. HZSM-5 was further modified with

with gallium (Ga) and zinc (Zn) to introduce basic sites to catalyst, aiming to convert PET-derived organic acids into BTEX aromatics. Pyrolysis experiments using a micro-pyrolyzer unit coupled with comprehensive two-dimensional gas chromatography employing a flame ionization detector and time-of-flight mass spectrometer (GCxGC-FID/TOF-MS) enabled precise characterization of products from PET-LDPE mixtures with varying compositions (PET:LDPE=3:1, 1:1, and 1:3). Ga- and Zn-modified catalysts enhanced BTEX aromatic selectivity in PyOil, resulting in higher oil yields from PET-LDPE co-pyrolysis than those achieved with unmodified HZSM-5, which primarily produced gaseous olefins.

Previous pyrolysis screening using thermogravimetric analysis (TGA) demonstrated that pyrolysis of PET at 600 °C results in the lowest char formation among studied temperatures, accounting for 16.8 wt.% of the total feed [10]. Thus, 600 °C was selected as the pyrolysis temperature for all experiments to maximize the recovery of PyOil and PyGas. Pyrolysis of PET at 600 °C yielded high quantities of CO₂ (24 wt.%), CO (6 wt.%), and benzoic acid (21 wt.%), along with other oxygen-containing compounds that decrease the quality of PyOil [11].Catalytic upgrading of pyrolysis vapors revealed that introducing Ga and Zn enables effective decarboxylation and decarbonylation, enhancing BTEX selectivity while eliminating oxygenated byproducts. Parent HZSM-5, although highly effective for producing light olefins from polyolefins, exhibited limited suitability for PET-rich feeds due to reduced PyOil yield (34 wt.%). In contrast, Ga- and Zn-promoted HZSM-5 exhibited high PyOil yields (50 wt.%) with excellent BTEX selectivity (80-89 %) for PET and PET:LDPE co-pyrolysis, suggesting a strong affinity for aromatic production, regardless of the PET:LDPE ratio. Zn-HZSM-5, in particular, achieved superior BTEX selectivity with increased PET content, indicating its robustness as a catalyst for mixed plastic waste streams.

Ga- and Zn-impregnated HZSM-5 demonstrates flexibility for BTEX-rich PyOil production, demonstrating their applicability for mixed waste streams where PET and polyolefins coexist. Ga- and Zn-modified catalysts not only improve product selectivity for valuable aromatics but also maintain consistently large PyOil quantities with high (80-90 wt.%) BTEX content across diverse PET:LDPE ratios, making them ideal for recycling scenarios where precise waste sorting is not feasible. This catalytic approach offers a promising pathway toward sustainable plastic waste valorization, yielding products suitable for chemical and fuel applications while reducing dependency on petroleum-based resources.

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