Photothermal recycling of waste polyolefin plastics into liquid fuels with high selectivity under solvent-free conditions

This study addresses the challenge of low-efficiency, energy-intensive recycling of polyolefin plastics (LDPE, HDPE, PP, UHMWPE), which constitute the majority of global plastic production and often end up in landfills. Conventional pyrolysis requires >500 °C and significant energy input, while hydrogenolysis at ~300 °C still requires external heating. The authors propose using solar-driven photothermal catalysis to enable efficient depolymerization/hydrogenolysis at lower overall energy cost. They hypothesize that a Ru/TiO2 photothermal catalyst can: (1) rapidly heat under UV-Vis-NIR light to melt polymers and improve catalyst–polymer contact, and (2) leverage UV light to activate inert C–C/C–H bonds in polyolefin chains, creating reactive sites for Ru-mediated scission, thereby converting waste polyolefins into liquid fuels with high selectivity under solvent-free conditions.

The paper surveys advances and limitations of plastic recycling: low current recycling rates for LDPE, HDPE, and PP; high energy demands of pyrolysis; hydrogenolysis enabling lower-temperature depolymerization but still requiring external heating. Photocatalytic routes (e.g., TiO2, ZnO, NiAl2O4) can mineralize plastics under UV/visible light but proceed too slowly for practical recycling. Recent works (e.g., Reisner et al.) show photocatalytic reforming/upcycling of hydrolysis products from PET/PLA/PU with concurrent H2 evolution, while others report oxidation to CO2 and subsequent photoreduction to chemicals. Photothermal catalysis has shown promise in Fischer–Tropsch, CO2 conversion, NH3 synthesis, and pollutant degradation, wherein light absorption causes rapid local heating to catalytically relevant temperatures and can synergize with photocatalysis. For polyolefins, surpassing melting points (LDPE 112 °C, HDPE 138 °C, PP 169 °C) enhances chain mobility and catalyst contact, while UV can generate radicals in polyolefins to facilitate bond scission. These insights motivate a photothermal approach combining UV activation and localized heating for efficient polyolefin recycling.

Catalyst preparation: Ru/TiO2 (P25) was synthesized by aqueous impregnation of TiO2 (Degussa P25) with RuCl3, drying (120 °C), and reduction in H2/Ar (10/90) at 500 °C for 2 h. Characterization showed strong UV-Vis-NIR absorption, P25 phase by XRD (anatase/rutile ~6:1), and ~2.5 nm Ru nanoparticles uniformly dispersed by TEM/EDX. Photothermal reactor and operation (ambient pressure): Mixtures of polymer (typically 80 mg; e.g., LDPE Mw=68.7 kDa, Ð=11.4) and Ru/TiO2 catalyst (20 mg, 2 wt% Ru) were loaded into a stainless reactor with quartz lining/windows. After vacuum/Ar purges, the reactor was pressurized to 1 bar with H2/Ar (v/v=30/70). A Xe lamp irradiated the mixture through the window; light intensity controlled the catalyst temperature (200–350 °C). A thermocouple monitored temperature. Reaction times varied (1–40 h). Gases (C1–C4) were sampled and analyzed by GC; liquid/waxy products (C5+) were extracted (cyclohexane or CH2Cl2 as appropriate), quantified by mass balance and gravimetry, and analyzed by high-temperature GC and 1H NMR. Residues were analyzed by GPC (TCB solvent with BHT). Control experiments included thermal-only heating (resistive element) and photolysis at 25 °C (UV-Vis filtered Xe lamp, water cooling). High-pressure photothermal tests: Pulverized LDPE shopping bags (900 mg) and Ru/TiO2 (100 mg) were placed in a high-pressure stainless reactor with a sapphire window. After vacuum/N2 purges, the reactor was pressurized with H2/N2 (v/v=70/30) to 10–40 bar. The bed was irradiated with a Xe lamp (~3.0 W cm−2), with optional auxiliary heating to hold 180–220 °C; stirring at 800 rpm. After 3 h, gases were analyzed by GC; liquids/waxes were extracted, filtered, and quantified/characterized by GC. Mechanistic probes: LDPE degradation under UV-Vis, Vis, and NIR components at 300 °C; ATR-IR to assess crystalline (730 cm−1) vs amorphous (720 cm−1) LDPE domains; UV-Vis diffuse reflectance of LDPE; GPC to track molecular weight/dispersity. Hydrogenolysis benchmarking with n-hexadecane on Ru powder vs TiO2 established Ru as the active site for C–C scission. Contact angle measurements of molten LDPE on Ru/TiO2-coated substrates at 120–300 °C (with/without irradiation) assessed wetting and catalyst–polymer contact. Sunlight-driven tests: Concentrated sunlight was used to photothermally heat Ru/TiO2 and polymers to 200±20, 300±20, and 400±20 °C; LDPE degradation efficiencies were compared with Xe lamp experiments. Technoeconomic analysis: Aspen Plus simulations for polyethylene hydrogenolysis over Ru/TiO2 quantified energy consumption by unit operations; the reactor accounted for ~90% (347.9 kW/h).

- Photothermal catalyst and contact: Ru/TiO2 exhibits broadband absorption and rapid photothermal heating under Xe lamp or concentrated sunlight. Melting of LDPE improves catalyst–polymer contact; contact angle of molten LDPE on Ru/TiO2 decreased from 112° at 120 °C to 60° at 300 °C, further reduced under irradiation. - Degradation performance at 1 bar (H2/Ar 30/70): At 300 °C, photothermal degradation (PD) of LDPE reached ~95% after 20 h, versus 7.8% for thermal degradation (TD) and negligible for photolysis at ambient temperature. GPC showed Mw dropped from 68.7 kDa (pristine) to 12.9 kDa (1 h) and 3.4 kDa (10 h); after 20 h PD, products were liquid/waxy (Mw ~0.49 kDa, dispersity 1.6) centered around roughly C27–C34, rather than polymeric residues. Without catalyst, only ~180 °C was reached at 3.0 W cm−2 with negligible degradation. - Broad polymer scope (300 °C, 1 bar H2/Ar): Photothermal degradation percentages and selectivities (gas vs liquid/wax): LDPE 95.0% (9% gas, 91% liquid/wax), UHMWPE 90.0% (5%, 95%), HDPE 87.8% (3%, 97%), PP 93.9% (5%, 95%), LDPE bags 97.3% (3%, 97%). Corresponding thermal runs gave <8% degradation across substrates. - Gaseous products and methanation: Over time, gaseous products became predominantly CH4, reaching >90% selectivity among gas products after extended reaction (e.g., 40 h). Experiments indicate CH4 mainly arises from hydrogenolysis of light hydrocarbons on Ru sites; Ru is essential for methanation under low H2 partial pressure. - Role of light spectrum/mechanism: At 300 °C with Ru/TiO2 for 20 h: UV-Vis gave 92.5% degradation, Vis 12.4%, NIR 10.4%, implying Vis/NIR mainly provide heating while UV (<365 nm) activates LDPE chains. ATR-IR showed thermal+UV diminished the 730 cm−1 crystalline band, indicating disruption of crystalline domains. Without catalyst, thermal+UV reduced Mw (19.1 kDa) vs thermal only (80.2 kDa), evidencing UV-induced chain scission at internal C–C bonds. n-Hexadecane tests showed ~25x higher conversion on Ru vs TiO2, confirming Ru-driven hydrogenolysis. - High-pressure photothermal recycling (H2/N2 70/30, 3 h): At 220 °C and 30 bar, LDPE bags were completely degraded with all liquid products within C5–C21 gasoline/diesel range; these liquid fuels accounted for 86% of total products (vs 64% for thermal). Pressure tuned selectivity: at 10/20/30/40 bar, representative distributions were ~10/50/40, 15/80/15, 15/90/10, and 15/85/20 (gas/liquid fuels/wax, %). Low H2 partial pressures yielded more wax; 20–30 bar favored liquid fuels. - Universality, stability, and scale-up: Similar product distributions across LDPE, HDPE, UHMWPE, PP; catalyst retained phase/structure after four LDPE cycles at 300 °C. A 5 g-scale run achieved 87% C5–C21 selectivity. Concentrated sunlight delivered comparable degradation efficiencies to Xe lamp irradiation. - Energy/TEA insight: Aspen analysis indicated reactor duty dominates energy demand (~347.9 kW/h, ~90% of process), suggesting substantial cost savings by substituting reactor heating with concentrated sunlight.

The findings validate the hypothesis that combining photothermal local heating with UV activation and Ru-mediated hydrogenolysis enables efficient, solvent-free depolymerization of inert polyolefins. Vis/NIR irradiation rapidly heats Ru/TiO2 to melt polymers, enhancing contact and enabling C–C/C–H bond scission on Ru. Concurrent UV (<365 nm) directly activates polymer chains and disrupts crystalline domains, accelerating chain scission and lowering molecular weight, which shortens reaction pathways to liquid hydrocarbons. Optimization of hydrogen partial pressure and temperature tunes selectivity: at 1 bar and 300 °C, PD yields liquid/waxy hydrocarbons with gaseous CH4 as the primary gas, while at 220 °C and 30 bar, PD selectively produces gasoline/diesel-range liquids (C5–C21) at high yield in only 3 h. The approach is general across LDPE, HDPE, UHMWPE, PP, and real LDPE bags, and maintains catalyst integrity over multiple cycles. Comparable performance under concentrated sunlight underscores the relevance for solar-driven plastic recycling. Collectively, the results establish photothermal catalysis as a promising route to valorize polyolefin waste into fuels with lower external energy input and high selectivity, addressing key barriers of conventional thermal routes.

The study introduces a solvent-free, photothermal catalytic platform using Ru/TiO2 to upcycle diverse polyolefin wastes into fuels with high selectivity. Under ambient pressure at 300 °C, LDPE and related plastics are efficiently converted to liquid/waxy hydrocarbons and CH4-rich gases; under 30 bar H2/N2 at 220 °C, 86% of total products are liquid fuels within the C5–C21 gasoline/diesel range in 3 h. Mechanistic analysis shows UV-driven polymer activation synergizes with photothermal heating and Ru-catalyzed hydrogenolysis. The method operates effectively with concentrated sunlight, offering a pathway to reduce reactor energy consumption and costs. Future work could focus on further scaling and continuous operation, optimizing reactor/illumination design for solar integration, reducing noble metal loadings or identifying alternative catalysts, tuning product distributions via process control, and comprehensive technoeconomic and life-cycle assessments for industrial deployment.

- Selectivity depends on hydrogen partial pressure: low H2 pressures (<~10 bar) yield substantial wax, whereas 20–30 bar favors liquid fuels. - Prolonged reaction under ambient-pressure conditions increases methanation, leading to high CH4 selectivity among gaseous products. - Optimal fuel-range selectivity (C5–C21) required elevated pressure (e.g., 30 bar) and controlled temperature (220 °C), implying pressurized operation for best outcomes. - UV component (<365 nm) is important to maximize degradation rates; without sufficient UV or catalyst, degradation is minimal. - Stability was demonstrated over four LDPE cycles; longer-term durability and performance under mixed, contaminated waste streams were not reported. - Technoeconomic analysis indicates the reactor dominates energy use (~90%), necessitating effective solar concentration or efficient heating strategies to realize cost benefits.