Key Takeaways & Executive Findings
- •• • Complete HDPE conversion achieved at ≥260 °C in low-pressure superheated methanol (<0.5 MPa), yielding 77.1% oil with 62.3% alkanes; below 260 °C conversion drops sharply, producing waxy solids—critical threshold for reactor design and energy input. • • At 290 °C, gasoline-range hydrocarbons (C6–C12) surge from 16.6% to 80.9%, demonstrating temperature-driven selectivity toward high-value fuel fractions—enables tuning product slate for market demand. • • Optimal reactant dosage is 0.80 g/mL at 270 °C; deviations reduce conversion due to impaired heat transfer—defines operating window for scale-up to avoid costly inefficiencies. • • Mechanism involves preferential secondary C–C bond scission, radical-mediated secondary reactions (isomerization, cyclization, hydrogenation), with methanol acting as radical source—provides basis for catalyst or process optimization to steer product distribution.
Abstract
Improper disposal of plastic waste represents both the loss of valuable resources and significant environmental threat. This study investigates the thermal liquefaction of high-density polyethylene (HDPE) using low-pressure superheated methanol. It systematically evaluates the effects of reaction temperature and the ratio of reactant to methanol on liquefaction efficiency and product characteristics. Results indicate that complete conversion of HDPE can be achieved in low-pressure superheated methanol (<0.5 MPa) at a minimum external heating temperature of 260 °C. Under this condition, oil yield reached 77.1% with alkanes comprising 62.3% of the product alongside minor oxygenated compounds. As temperature increased, the average carbon number of hydrocarbons gradually decreased. Below 260 °C, HDPE conversion decreased significantly, and products were primarily waxy. At 290 °C, the proportion of gasoline-like fractions (C6–C12) increased markedly from 16.6% to 80.9%. Furthermore, reactant ratio plays a critical regulatory role; extremes in ratio—either too high or too low—diminish heat transfer efficiency and reduce conversion. Mechanistically, liquefaction primarily involved cleavage of secondary C−C bonds, where resulting oligomers further cracked into free radicals to form diverse hydrocarbons through secondary reactions. This work demonstrates that low-pressure superheated methanol liquefaction is a mild, efficient, and pretreatment-free method to upcycle polyethylene into valuable fuels. Optimizing these process parameters can pave the way for industrial application, aiding in both plastic pollution management and sustainable resource recovery.
1. Introduction
Global plastic production has exceeded 8 billion tons, yet waste management remains critically inefficient: only ~9% is recycled and 12% incinerated, leaving the vast majority to accumulate in landfills or the natural environment. Existing treatment technologies—landfilling, mechanical recycling, incineration, pyrolysis, gasification, and hydrothermal methods—each face inherent limitations: landfilling requires extensive land and prolonged cycles; mechanical recycling degrades polymer quality; incineration emits CO2 and toxic pollutants; pyrolysis and gasification demand high temperatures (typically >400 °C) and energy-intensive processes; hydrothermal methods often require high pressures and catalysts. These bottlenecks hinder scalable, economically viable plastic upcycling, particularly for polyolefins like polyethylene, which are notoriously resistant to depolymerization due to stable C–C bonds.
This study introduces a low-pressure superheated methanol liquefaction route that operates under mild conditions (<0.5 MPa, ≥260 °C) without pretreatment, achieving complete HDPE conversion and high oil yield (77.1%). The key innovation lies in using superheated methanol as both a heat transfer medium and a radical source, facilitating efficient depolymerization at significantly lower temperatures than conventional pyrolysis. By systematically mapping the effects of temperature and reactant dosage, this work provides a clear operational envelope—minimum 260 °C and optimal 0.80 g/mL—that directly addresses the energy and scalability bottlenecks of existing methods. The demonstrated shift from waxy products to gasoline-range hydrocarbons (C6–C12) with temperature offers a tunable pathway to produce valuable fuels, positioning this technology as a practical solution for plastic waste management and circular resource recovery.
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ZHAO Peitao, SONG Chengye, FU Binbin, PENG Xiong, WANG Jingyi, FENG Chao, YE Puhai, ZHOU Haiyun (2026). Oil Production from Thermal Liquefaction of Polyethylene in Low-Pressure Superheated Methanol. Journal of Fuel Chemistry and Technology. https://doi.org/10.1016/S1872-5813(26)60678-0
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Frequently Asked Questions
What is the minimum external heating temperature required for complete HDPE conversion, and what is the corresponding oil yield and product composition?
Complete conversion is achieved at a minimum external heating temperature of 260 °C under low-pressure superheated methanol (<0.5 MPa). At this condition, oil yield reaches 77.1%, with alkanes comprising 62.3% of the product, alongside minor oxygenated compounds. Below 260 °C, conversion drops significantly and products are primarily waxy.
How does reaction temperature affect the product distribution, particularly the yield of gasoline-like fractions?
Elevated temperatures favor the formation of lighter hydrocarbons. At 290 °C, the proportion of gasoline-like fractions (C6–C12) increases markedly from 16.6% (at lower temperatures) to 80.9%. This indicates that temperature can be tuned to selectively produce high-value fuel fractions.
What is the optimal reactant dosage (HDPE to methanol ratio) and why do deviations reduce conversion?
The optimal reactant dosage at 270 °C is 0.80 g/mL. Deviations from this concentration—either too high or too low—lead to a decline in conversion, primarily due to reduced heat transfer efficiency. This highlights the importance of maintaining an optimal solid-to-solvent ratio for efficient thermal transfer and reaction kinetics.
What is the underlying liquefaction mechanism, and how does methanol participate in the reaction?
The liquefaction is initiated by preferential scission of secondary C–C bonds in HDPE, leading to depolymerization into oligomers. These oligomers further crack into free radicals, which undergo secondary reactions such as isomerization, cyclization, hydrogenation/dehydrogenation, and radical recombination to form diverse hydrocarbons. Methanol serves as a radical source, interacting with nascent hydrocarbon fragments, thereby influencing product composition.
What are the implications for industrial scale-up, particularly regarding energy input and process economics?
The process operates at low pressure (<0.5 MPa) and moderate temperatures (≥260 °C), which is milder than conventional pyrolysis (typically >400 °C), potentially reducing energy costs. The high oil yield (77.1%) and tunable product slate (e.g., gasoline-range fractions up to 80.9% at 290 °C) enhance economic viability. However, scale-up must carefully control reactant dosage (optimal 0.80 g/mL) and heat transfer to maintain conversion efficiency, as deviations can significantly reduce performance.
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