Research Progress on Selective Depolymerization of Waste Plastics to High-Quality Liquid Fuels

LI Xinze; LUO Zhicheng; XIAO Rui

doi:10.20078/j.eep.20241106

Abstract：This study evaluates catalytic pyrolysis,microwave pyrolysis,and photocatalytic depolymerization for converting waste plastics into liquid fuels,with an emphasis on the efficiency,selectivity,and scalability.Catalytic pyrolysis achieved a 79.08%liquid yield from high-density polyethylene(HDPE)at 550℃using Fe-HZSM-5 catalysts.Hydrocarbon selectivity was governed by catalyst acidity and pore structure.Hierarchical ZSM-5 further enhanced low-density polyethylene(LDPE)conversion(>95%)by mitigating overcracking through optimized pore architecture.The microwave pyrolysis demonstrated rapid heating kinetics,yielding a 98.78%aromatic-rich liquid fuel from polystyrene(PS)at 600 W with 60 g SiC absorbent.Monoaromatic hydrocarbons dominated the liquid fuel(93.9%),meeting aviation fuel standards.However,excessive power(>6 kW)reduced yields by 10%due to secondary decomposition.Photocatalytic depolymerization in 30%H2O2 facilitated the production of acetic acid yield of 1.1 mmol·g-1·h-1 from polyethylene(PE),utilizing hydroxyl radicals(·OH)to cleave C—C bonds,leading to an increase in PE mass loss from 50.1%to 85.4%.The key findings are as follows:(1)Fe doping in HZSM-5 boosted liquid yields by 16%via enhanced dehydrogenation activity;(2)Microwave absorber loading(e.g.,SiC)nonlinearly affected cycloparaffin selectivity(65.6%at 450 W for polypropylene);(3)H2O2 increased photocatalytic PE conversion by 70%compared to pure water,where limited·OH generation restricted CO2-to-fuel pathways(≤47.4 μg·g-1·h-1).Catalytic pyrolysis faces the challenge of rapid catalyst deactivation(resulting in a 30%activity loss after 5 cycles),while microwave systems incur high capital costs.Photocatalysis prioritizes gaseous products(e.g.,H2,CH4)with liquid fuel selectivity below 15%for most polymers.To address these challenges,three actionable pathways are proposed:(1)Pilot-scale optimization:Current studies predominantly use lab-scale feeds(<100 g),necessitating trials with industrial-grade plastics containing pigments and plasticizers.Electrostatic separation pretreatment reduced PVC-derived HCl corrosion by 80%in pilot tests,while anti-fouling membranes(90%recovery)enhanced acetone purity(>98%)in continuous systems.(2)Hybrid energy systems:Integrating microwave heating(200-300℃/min)with photocatalysis may synergize rapid thermal activation and selective bond cleavage.For instance,microwave-enhanced light absorption in TiO2-MoS2 hybrids doubled charge carrier density,potentially reducing energy consumption by 30%-40%.(3)Intelligent reactors:IoT-enabled sensors and machine learning algorithms stabilized multiphase reactions in simulated trials,minimizing yield fluctuations to±5%versus±15%in batch modes.Real-time monitoring of temperature gradients and microwave power enabled dynamic adjustments,improving diesel-range hydrocarbon selectivity by 25%.Economically,catalytic pyrolysis shows near-term viability with a break-even cost of 0.8-1.2$/L for diesel-range fuels,while photocatalysis requires a 50%-70%reduction in catalyst synthesis costs(e.g.,replacing Pt with Fe-Ni sulfides).Environmentally,microwave pyrolysis reduces carbon intensity by 40%-60%versus incineration,aligning with net-zero roadmaps.Lifecycle assessments revealed that hybrid systems could achieve carbon-negative profiles when coupled with renewable energy.Future work should focus on developing multifunctional catalysts(e.g.,acid-base bifunctional sites for tandem cracking-isomerization),modular reactor designs,and standardized testing protocols to expedite industrial implementation.These strategies underscore the potential of tailored energy-input systems to advance plastic valorization,supporting circular economies and global decarbonization efforts.