Converting waste PET plastics into automobile fuels and antifreeze components

The study addresses the challenge of persistent PET plastic pollution, including significant marine contamination and the limitations of current chemical depolymerization routes (hydrolysis, glycolysis, ammonolysis, pyrolysis), which often require harsh conditions, yield mixed products, and suffer from purification difficulties. Motivated by the need for low-cost, selective, and practical solutions—especially for islands with limited industrial infrastructure—the research explores whether a hydrogen-free, one-pot catalytic process can directly convert PET into valuable products, specifically p-xylene (PX) as a gasoline-range fuel component and ethylene glycol (EG) as an antifreeze component. The proposed approach uses methanol as both solvent and in-situ hydrogen donor and leverages a Cu/SiO2 catalyst whose activity is tuned by NaCl-induced morphological control to generate a high Cu+/Cu ratio for tandem PET methanolysis and selective hydrodeoxygenation of DMT.

Recent work on PET upcycling has focused on catalytic hydrogenolysis/hydrogenation using noble metals and high-pressure H2 or producing mixed arenes: Ru/Nb2O5 converted PET to arenes with 87.1% yield and 63% PX selectivity at 200 °C and 0.3 MPa H2; Ru/Nb2O5 with aqueous-phase reforming of EG fragments yielded 91.3% monomers with 19% PX selectivity at 220 °C; Co/TiO2 achieved 70% arenes yield at 340 °C and 3 MPa H2. Three-step routes to C7–C8 cycloalkanes and aromatics have been reported. Emerging photocatalytic and electrocatalytic approaches reform PET to H2 and small organics or to potassium diformate, terephthalic acid, and H2. Broader plastic upcycling includes PE conversion to long-chain alkylaromatics (80% yield) over Pt/γ-Al2O3 at 280 °C and hydrogenolysis of HDPE to diesel/lubricant-range alkanes over ordered mesoporous Pt/SiO2 at 300 °C. These methods often rely on precious metals, external hydrogen, high temperatures/pressures, and yield product mixtures, motivating a base-metal, H2-free, selective route to PX and EG.

Catalyst design and synthesis: Cu/SiO2 catalysts were prepared by several methods—hydrothermal (HT), impregnation (IM), deposition-precipitation with urea (DPU), and with ammonia (DPA). NaCl was introduced during hydrothermal synthesis to modulate morphology and oxidation states; Na+/Cu2+ molar ratios of 2.5:1, 5:1, 10:1, and 15:1 were evaluated. The optimized Cu-5Na/SiO2 precursor was dried, calcined (450 °C, air, 4 h), and reduced (450 °C, H2, 4 h). Reaction system: PET conversion was performed in a 60 mL batch autoclave with 0.12 g PET, 0.1 g catalyst, and 30 mL methanol at 210 °C for 6 h, 600 rpm, after N2 purge. Methanol served as solvent for PET methanolysis to DMT and EG and as hydrogen donor via methanol dehydrogenation for subsequent DMT hydrodeoxygenation to PX. Product analysis: Liquid products were analyzed by GC/GC-MS (Rtx-5Sil MS column), gas products by GC (TDX-01 column, TCD). Catalyst characterization: XRD (phase identification: copper silicate Cu2Si2O5(OH)2, Cu, Cu2O), N2 adsorption–desorption (surface area, porosity), H2-TPR (reduction behavior), FTIR (CO adsorption, structural OH bands), TGA (water content and decomposition), XPS and Cu LMM XAES (surface composition and Cu+/Cu0 ratios), TEM/HRTEM and TEM-EDS (morphology, particle sizes and elemental mapping), SEM (morphology). Kinetics and mechanism: Time-resolved product distributions for DMT and intermediates (methyl 4-(methylol)benzoate, methyl 4-methylbenzoate, 4-methylbenzyl alcohol) at 210 °C determined rate constants (k1–k4) via MATLAB fitting. In-situ transmitted FTIR at 120 °C (10–60 min) tracked functional group evolution (aryl C=C, C=O, CH3 stretching, O–H) to corroborate the stepwise hydrogenation–hydrogenolysis pathway. Comparative solvents and substrates: Ethanol and isopropanol were assessed as alternative solvents/hydrogen donors; polybutylene terephthalate (PBT) was also tested under the optimal system. On-site validation: Various island-sourced PET items (bottles, caps, containers, bags, polyester clothing) were cut and converted under the same conditions; separation by simple distillation was considered.

• Methanol alone alcoholyzes PET to DMT and EG at 210 °C without catalyst (100% DMT in 30 min), but hydrodeoxygenation to PX requires catalytic in-situ H2 from methanol dehydrogenation. • Catalyst screening (210 °C, 6 h, methanol): Cu/SiO2 (HT) yielded 73% PX with by-products methyl 4-methylbenzoate (23%) and 4-methylbenzyl alcohol (4%); Co/SiO2, Ni/SiO2, Fe/SiO2, Cu/ZrO2, Cu/CeO2 were inactive for HDO (stopped at DMT). Cu/TiO2 gave 16–17% PX. Cu/SiO2 made by IM and DPU produced mainly Cu0 and were inactive for methanol dehydrogenation. • NaCl addition during hydrothermal synthesis dramatically enhanced performance: CuNa/SiO2 achieved 100% PX yield with gas pressure increment ~3.4 MPa at RT and gas composition ~60% H2, 36% CO, 4% CH4. Other alkali chlorides showed lower PX yields (e.g., K: 97%; Li: 89%; Rb: 60%; Cs: 67%). • Optimal Na+/Cu2+ ratio was 5:1 ("5 NaCl"), which produced the densest, granular copper silicate precursor with lowest surface area (~46.9 m2 g−1), minimal physisorbed (2.41%) and crystal water (6.75%), highest Cu+/Cu0 ratio (~1.86–1.87), and maximum PX yield (100%). Ratios 2.5:1, 10:1, 15:1 gave 78.3%, 92.3%, and 60.7% PX, respectively. • Structural origin: Na+ partially occupies Si–OH during hydrothermal synthesis, inhibiting layered copper silicate growth and yielding granular, low-crystallinity copper silicate with strong Cu–SiO2 interaction, harder to reduce, leading to higher surface Cu+ fraction post-reduction. Excess NaCl (15:1) covers all Si–OH, favors precipitated copper silicate with better crystallinity, smaller interface area, easier reduction, lower Cu+/Cu0, and poorer activity. • Kinetics at 210 °C (Cu-5Na/SiO2): Stepwise pathway DMT (A) → methyl 4-(methylol)benzoate (B) → methyl 4-methylbenzoate (C) → 4-methylbenzyl alcohol (D) → PX. Fitted rate constants: k1=0.0121 min−1, k2=0.0424 min−1, k3=0.0104 min−1 (rate-limiting ester hydrogenation of C), k4=0.0468 min−1. PX reached 100% yield at 6 h. • In-situ FTIR showed decreasing C=O bands and transient O–H formation consistent with hydrogenation to alcohols followed by rapid hydrogenolysis; aryl C=C bands remained unchanged, confirming ring retention. • Solvent dependence: Ethanol and isopropanol alcoholyzed PET but failed to generate sufficient H2 for HDO to PX over CuNa/SiO2. • Substrate scope: PBT in methanol produced 100% PX and 1,4-butanediol at 210 °C, releasing 2.8 MPa gases (~60% H2). • Reusability: Cu-5Na/SiO2 maintained 96.4% PX yield in the second run but dropped to 52.7% by the third; deactivation correlated with Cu particle growth, over-reduction, and Cu+/Cu0 decrease from 1.87 to 0.57 after four runs. • Environmental metrics (relative comparison): High energy economy coefficient ξ≈1.323×10−5 °C−1·min−1; when scaling PET and catalyst together, achieved low Efactor≈13.41 and reported ξ (process energy metric) ≈1,013,605 °C·min, indicating favorable efficiency relative to Ru/Nb2O5-based processes. • On-site assessment: For Phuket Island beach sediments where PET constitutes ~33.1% of plastic sediment, every 1 ton of plastic sediment could yield ~181 kg PX and ~105 kg EG under optimal conditions; PX and EG separable from methanol by simple distillation.

The results demonstrate a practical, hydrogen-free one-pot route to selectively convert PET into PX and EG using methanol as both solvent and in-situ hydrogen source. The key to high activity and selectivity is the generation of a high Cu+/Cu0 surface ratio on Cu/SiO2, achieved by controlling the copper silicate precursor morphology via NaCl-assisted hydrothermal synthesis. The granular, dense copper silicate increases Cu–support interaction, resists over-reduction, and provides the synergistic Cu+/Cu0 ensemble required for methanol dehydrogenation (H2 supply) and selective hydrodeoxygenation of DMT to PX. Kinetic and in-situ FTIR analyses corroborate a hydrogenation–hydrogenolysis cascade that preserves the aromatic ring, explaining the exclusive formation of PX without ring saturation or scrambling. The inability of ethanol and isopropanol to sustain HDO under identical conditions highlights the crucial match between methanol dehydrogenation kinetics and DMT HDO needs on this catalytic surface. The approach is particularly relevant for island environments with high PET accumulation and limited industrial hydrogen access, enabling direct production of fuel and antifreeze components from local waste streams.

This work introduces a low-cost CuNa/SiO2 catalyst and a hydrogen-free, one-pot process that quantitatively converts PET (and PBT) into p-xylene and ethylene glycol in methanol at 210 °C. NaCl-assisted hydrothermal synthesis tunes the copper silicate precursor to a dense, granular morphology that yields a high Cu+/Cu0 ratio upon reduction, which is pivotal for coupling methanol dehydrogenation with selective DMT hydrodeoxygenation. Mechanistic evidence from kinetics and in-situ FTIR supports a stepwise hydrogenation–hydrogenolysis pathway with preserved aromaticity and identifies the ester hydrogenation of methyl 4-methylbenzoate as rate-determining. The method exhibits strong potential for decentralized plastic-to-fuel implementation on islands, with favorable environmental efficiency metrics and straightforward product separation. Future work should focus on enhancing catalyst durability (mitigating Cu particle growth and over-reduction), optimizing reactor and solvent management for scale-up, expanding solvent/hydrogen-donor options, and broadening the plastic feed scope while maintaining selectivity.

Catalyst deactivation occurs upon recycling due to Cu particle growth and decreased Cu+/Cu0 ratio from over-reduction by excess H2, reducing PX yield significantly by the third run. The process critically depends on methanol as the hydrogen donor; ethanol and isopropanol were ineffective for the hydrodeoxygenation step under the same conditions. Activity is sensitive to the Na+/Cu2+ ratio during synthesis; deviation from the optimal 5:1 ratio reduces performance. Current demonstrations are batch-scale; long-term stability, continuous operation, and processing of mixed/contaminated waste streams require further validation.