(636f) Upcycling of Polyolefins to Constituent Monomers over Microwave-Responsive Metal Carbides Nanotubes

Globally, the production of plastic accounts for the consumption of ~8% of crude oil resources and generates 1.6 gigatonne of CO₂e emissions (5% global carbon budget) [1]. If the current trends persist, these values will be projected to 20% and 15% by 2050, respectively [2, 3]. Annually, only 9% of plastics are recycled, while the rest worth over US$ 120 billion end up in incineration plants and landfills, resulting in significant environmental impact (193 million tonnes of CO₂e) [4]. As a result, the large-scale production of plastic with mere recycling rates (i.e., cradle-to-grave assessment) leads to a considerable depletion of petroleum resources and also contributes to economic and environmental losses. Currently, polymer recycling is leveraged over mechanical approaches as a substitute for landfill disposal and incineration. However, after multiple recycling cycles, the recycled polymers experience a significant decline in their market value and mechanical properties, discouraging the production of high-quality polymers from post-consumer waste [5]. Therefore, it is essential to shift the plastic life cycle towards a circular economy while ensuring the low environmental impact and high economic benefits of the final product over their virgin counterparts.

Within the toolbox of available methods, chemical recycling of plastic waste is an emerging route for the management of plastic waste into valuable products. For instance, advancements in catalytic hydrocracking and hydrogenolysis have shown high selectivity of liquid fuels with low environmental impacts (0.127-0.214 kg CO₂/kg_plastic) [6]. However, these processes are not fully circular owing to the need for additional steps to produce monomers, leading to making the process energy intensive. Similarly, catalytic pyrolysis has been explored as an enticing strategy for direct monomer recovery [7, 8]. Despite this, the low monomer selectivity, use of expensive noble-metal catalysts, and elevated reaction temperatures limit the sustainability of the whole process. In contrast, microwave-assisted pyrolysis of plastic waste has gained interest and played a dominant role in activating the inert C-C (sp³-sp³) bonds in plastics, leading to the production of high selectivity of commodities (i.e., hydrogen, monomers and fuels etc.) over electromagnetic catalysts and at mild reaction conditions [9]. Despite this, microwave-assisted depolymerization of plastics to constituent monomers faces limitations due to the requirements of catalysts with high dielectric properties and active sites (i.e., acidic sites). For instance, Wang et al. [10] studied the microwave-driven recycling of LDPE over zeolite at 300 ^oC for 30 min and achieved low selectivity of monomers (C₂-C₄ olefins= 23%). Similar results were reported by Zhou et al., [11] who studied the continuous microwave-assisted pyrolysis of HDPE over HZSM-5 and achieved a high fraction of lighter oils (≥C₅= 51%). Also, the occurrence of side reactions led to coke formation, resulting in rapid catalyst deactivation during the reaction. Therefore, new frontiers of microwave-responsive catalysts must be explored by tailoring the morphology and surface properties of catalysts for efficient microwave-assisted monomer recovery.

Herein, we synthesized different transition metal carbides (Cr₂C₃, WC, Mo₂C) via carbothermal reduction of metal precursors and a carbon source (i.e., graphitic carbon nitride, gCN). The synthesized metal carbides (M@gCN) were named according to their metal precursors (i.e., Mo@gCN, Cr@gCN, and W@gCN). Further, to tune the morphology of metal carbides, the fine powdered gCN was protonated via acid modification, followed by sonicated assisted one-pot hydrothermal method to develop metal carbide nanotubes (M@pCN). Finally, to modify the surface properties and to introduce the acid functional groups (–SO₃H) on the surface of metal carbide nanotubes, the carbon source (pCN) was treated with sulfuric acid and ammonium persulfate, followed by the carbothermal synthesis of metal carbides as discussed above. The as-synthesized metal-responsive metal carbide nanotubes (M@sCN) were named Mo@sCN, Cr@sCN, and W@sCN. All the synthesized catalysts were characterized by XRD, SEM, and N₂ physisorption to analyze their structural and morphological properties. Similarly, the permittivity of the catalysts was recorded at a temperature range of 25°C-200°C and at a frequency of 2.45 GHz. Further, the pyrolysis experiments were performed over a single-mode cylindrical cavity at the TM₀₁₀ resonant mode (2.45 GHz). In contrast to the previous literature, a solid-state generator and power amplifier system was used in place of a magnetron for microwave generation [11]. This allows for higher power efficiency and control over frequency. Also, it helped us to identify an optimal resonant frequency, which can be tracked to follow changes in permittivity due to chemical and physical changes during heating. Prior to the experiments, a calculated quantity of catalysts and a well-crushed plastic sample (i.e., HDPE) were introduced into the glass reactor and positioned at the central axis of the cavity. The magnitude of the electric field along this axis is maximum, subjected to an approximately uniform electric field across its entire volume. The reactor was then exposed to microwave irradiation (100-300 W) for 30-60 min, and the generated gaseous products were collected and analyzed by GC, along with a cold trap employed for oil collection. Finally, the best-performing microwave-responsive metal carbide nanotubes were further tested for post-consumer plastic waste (i.e., HDPE, LDPE, PP, and PS).

Based on the results, the gCN showed low conversion of HDPE owing to its poor thermal conductivity. However, the use of various metal carbides exhibited significant improvement in the pyrolysis of HDPE, with a ~35% yield of gases (selectivity of C₂=-C₃= = 70%). This could be associated with the high permittivity of the metal carbides as compared to pristine gCN. Overall, the increase in the conversion and selectivity of monomers followed the order: metal carbides (M@gCN) < metal carbides nanotubes (M@pCN) < microwave responsive metal carbides nanotubes (M@sCN). The difference in the catalytic properties of various metal carbides is due to the fact that they showed distinct physiochemical and dielectric absorption properties. For instance, the synthesis of different metal carbide nanotubes improves the textural properties (i.e., surface area) of catalysts, thereby improving the microwave absorption efficiency. Similarly, the modification and sulfonation of graphitic carbon significantly enhanced the permittivity of microwave-responsive catalysts. Therefore, the superheating behaviour of various microwave-responsive metal carbide nanotubes, along with the introduction of acid functional groups led to improve microwave-assisted pyrolysis of plastics. The results will be presented in detail during the conference. Overall, this study will provide a comprehensive understanding of how the morphology and surface functionalization of various metal carbides impacted their microwave properties. Similarly, the trade-off between the interfacial disorder caused by sulfonation and the potential changes in the dielectric properties of metal carbide nanotubes will be discussed.

References:

1. Peplow, M., Can this revolutionary plastics-recycling plant help solve the pollution crisis? Nature, 2025. 638(8049): p. 22-25.
2. Tiseo, I., Annual carbon dioxide (co) emissions worldwide from 1940 to 2023. Statista, 2023.
3. Co-operation, O.f.E. and Development, Global plastics outlook: Economic drivers, environmental impacts and policy options. 2022: OECD Publishing.
4. Azam, M.U., et al., Pore-Structure Engineering of Hierarchical β Zeolites for the Enhanced Hydrocracking of Waste Plastics to Liquid Fuels. ACS Catalysis, 2024. 14: p. 16148-16165.
5. Martínez, L., et al., Fluidized bed pyrolysis of HDPE: A study of the influence of operating variables and the main fluidynamic parameters on the composition and production of gases. Fuel processing technology, 2011. 92(2): p. 221-228.
6. Hernández, B., et al., Techno-economic and life cycle analyses of thermochemical upcycling technologies of low-density polyethylene waste. ACS Sustainable Chemistry & Engineering, 2023. 11(18): p. 7170-7181.
7. Wang, N.M., et al., Chemical recycling of polyethylene by tandem catalytic conversion to propylene. Journal of the American Chemical Society, 2022. 144(40): p. 18526-18531.
8. Selvam, E., et al., Recycling polyolefin plastic waste at short contact times via rapid joule heating. Nature Communications, 2024. 15(1): p. 5662.
9. Jie, X., et al., Microwave-initiated catalytic deconstruction of plastic waste into hydrogen and high-value carbons. Nature catalysis, 2020. 3(11): p. 902-912.
10. Wang, Y., et al., Microwave-driven upcycling of single-use plastics using zeolite catalyst. Chemical Engineering Journal, 2023. 465: p. 142918.
11. Zhou, N., et al., Catalytic pyrolysis of plastic wastes in a continuous microwave assisted pyrolysis system for fuel production. Chemical Engineering Journal, 2021. 418: p. 129412.