A two-stage strategy for upcycling chlorine-contaminated plastic waste

Plastic waste is generated in enormous quantities and largely discarded, with mixed plastic streams (polyolefins, PVC, PET, additives) making mechanical recycling difficult. Pyrolysis of polyolefins is energy intensive and yields low-value products. Catalytic hydrogenolysis/hydrocracking of pure polyolefins can produce valuable lubricants, waxes and fuels, but even small amounts of PVC in real waste streams poison Ru and Pt catalysts and evolve corrosive/toxic HCl, rendering products unusable if chloride-contaminated. The research question is how to enable catalytic upcycling of polyolefins in the presence of chlorine-containing contaminants (notably PVC) by removing or immobilizing chlorine and preventing catalyst poisoning and HCl emissions. The authors propose a practical two-stage process: absorptive dechlorination of mixed plastics over magnesium–aluminum mixed oxide under hydrogen, followed by hydrogenolysis or hydrocracking over metal catalysts, to deliver Cl-free products and maintain catalyst activity.

Prior work shows polyolefin hydrogenolysis over Ru and Pt catalysts (e.g., Pt/SrTiO₃, Ru/C, Ru/TiO₂) and hydrocracking over bifunctional Pt/WO₃/ZrO₂ can convert PP and PE to lubricants, waxes and fuels. However, contaminants in real plastic waste, especially PVC (~4% of PW excluding PET), deactivate catalysts; Ru/C can be deactivated by as little as 0.1% PVC. PVC decomposition releases HCl and forms polyenes, chlorinated organics and aromatics that poison noble metals. Cl removal approaches include solvothermal treatment and zeolite-catalyzed pyrolysis, but typically require higher temperatures, may allow HCl emissions, and still risk Cl in liquids. Mixed Mg–Al oxides and other basic oxides (e.g., ZnO, Fe₂O₃) have been reported as HCl traps during PVC pyrolysis, yet full compatibility with low-temperature catalytic hydroconversion and noble-metal tolerance remained challenging.

Overall strategy: a two-stage absorptive dechlorination followed by catalytic hydroconversion. - Stage 1 (dechlorination): Mix PP with PVC and Mg₃AlO₄.₅ (magnesia–alumina mixed oxide) in a batch high-pressure reactor. Typical conditions: 30 bar H₂, 250 °C, 6 h. The Mg-containing basic oxide traps HCl as solid MgCl₂ while PVC decomposes; PP remains intact. Mg₃AlO₄.₅ was used at ~1 g adsorbent per g PVC (excess vs theoretical capacity to account for diffusion/encapsulation limits). Variations tested: lower temperatures, He (instead of H₂), different H₂ pressures (10–50 bar), and alternative Mg-based oxides (MgO, MgAl₂O₄, MgO/SiO₂). Dechlorination was also evaluated for other chlorinated contaminants: PVDC, 1,2,4-trichlorobenzene (TCB), 1,1,2,2-tetrachloroethane (TCE), and higher PVC loadings (20–30 wt%). For comparison, ambient-pressure flow dechlorination in He at 250 °C for 6 h was conducted. - Stage 2 (hydrogenolysis or hydrocracking): Without separating the trap, the dechlorinated solid mixture was reacted with a fresh catalyst under H₂ at 250 °C. Hydrogenolysis used Ru/TiO₂ (pre-reduced) at 30 bar H₂ for 16 h to produce lubricant-range hydrocarbons. Alternative hydrocracking used Pt/WO₃/ZrO₂ at 250 °C, 30 bar H₂ for 2 h to produce C₅–C₁₂ alkanes. - Catalysts/adsorbents preparation: Ru/TiO₂ prepared per prior procedure and pre-reduced (300 °C, 3 h, 50% H₂/He). Mg₃AlO₄.₅ obtained by calcining commercial hydrotalcite (550 °C, 6 h). Other Mg–Al oxides synthesized by nitrate co-precipitation and calcination; MgO/SiO₂ by incipient wetness impregnation and calcination. - Characterization and analysis: FTIR of headspace gases for HCl detection; GPC for molecular weight (THF mobile phase); ¹H and 2D ¹H–¹³C HSQC NMR of liquids; ¹³C MAS NMR of solids; XRD for phase evolution (MgO→MgCl₂); XPS for surface Cl/Mg/Al ratios; SEM-EDX for bulk composition; TGA-DSC for HCl trapping efficiency; gas GC-FID for C₁–C₇; gravimetric yields and mass balance; XRF for Cl content in solids/liquids. Model poisoning studies used 1-chlorooctadecane mixed with PP over Ru/TiO₂. - Regeneration: Ru/TiO₂ activity restored by H₂ reduction at 300 °C. Spent Mg₃AlO₄.₅ regenerated by calcination in air (550 °C) and subsequent steaming at 550 °C to hydrolyze MgCl₂ back to MgO, releasing HCl for capture in a scrubber. - Key operating optima identified: dechlorination at 250 °C, 30 bar H₂, ~6 h; hydrogenolysis at 250 °C, 30 bar H₂, 16 h.

- Single-step hydrogenolysis of PP mixed with 10 wt% PVC over Ru/TiO₂ is nearly completely inhibited, yielding ~99% residual solids; HCl is detected in the gas phase. - Two-stage process (Stage 1: PP+PVC dechlorination over Mg₃AlO₄.₅ at 250 °C, 30 bar H₂, 6 h; Stage 2: hydrogenolysis over Ru/TiO₂ at 250 °C, 30 bar H₂, 16 h) restores high performance: liquid yield ~70% (comparable to pure PP at 66%), gas yield ~2.2% (vs 28% for pure PP), with gases enriched in isobutane/isopentane (~60% selectivity), and negligible HCl in headspace by FTIR. - Liquids are lubricant-range hydrocarbons with weight-averaged molecular weight Mn ~900 Da (pure PP) vs ~610 Da (PP+PVC two-stage). NMR shows microstructural differences (greater CH₂ content and methyl detachment for PP+PVC case) but both are suitable lubricant-range products. - Dechlorination mechanism: XRD shows MgO converts to MgCl₂ starting at ~200 °C, saturating by ~250 °C; ¹³C MAS NMR indicates PVC transforms to polyenes/aromatics (128 ppm) and alkyls (32 ppm). XPS shows surface Cl/Mg ~1 and Al surface depletion at ≥250 °C, consistent with MgCl₂ surface segregation. - Hydrogen aids dechlorination: conducting dechlorination in He (instead of H₂) reduces liquid yield (e.g., to ~40% vs ~69% in H₂) and increases liquid Mn by ~5×, indicating catalyst poisoning from chlorinated organics. Increasing H₂ pressure during dechlorination to 30 bar maximizes liquid yield (~69%) and reduces Mn to ~600 Da; higher pressures slightly increase gas formation. - Chlorine distribution: With optimized dechlorination, most Cl is segregated to the solid residue as MgCl₂; liquids contain <0.1 wt% Cl; no HCl detected in gas. At ambient pressure dechlorination in He, liquid Cl rises to ~2.5 wt% (vs 0.05–0.00% at 30–50 bar H₂). - Scope with different chlorinated species: Two-stage process yields fully dechlorinated liquids for PP mixed with TCE or TCB (40–60% liquid). PVDC contamination prevents activity under these mild conditions due to slow HCl release and stable C–Cl in polyaromatics; higher T or longer times needed (not optimized). - Higher PVC loadings: At 20 wt% PVC, liquid yield drops to ~37% (still Cl-free liquids); at 30 wt% PVC, liquid formation becomes negligible. - Hydrocracking: Pure PP over Pt/WO₃/ZrO₂ yields ~87% liquid (C₅–C₁₀ focus) after 2 h; dechlorinated PP+PVC feed gives ~68% liquid (shifted to C₈–C₁₂) and ~24% solids after 2 h, retaining ~80% of pure-feed liquid yield. - Poisoning mechanism insights: Chlorinated alkanes (1-chlorooctadecane model) reduce liquid yield (66%→40%), suppress methane formation (76%→4% of gas), and favor C₄–C₅ gases (e.g., isobutane ~52%). Spent Ru/TiO₂ shows chloride on TiO₂ support, altered CO-FTIR signatures, and minor Brønsted acidity—consistent with Cl-induced modification changing product selectivity. - Regeneration: Ru/TiO₂ activity fully restored by H₂ reduction. Mg₃AlO₄.₅ can be regenerated via calcination and steaming (MgCl₂ + H₂O → MgO + 2 HCl), removing >90% Cl and restoring HCl capture performance.

The study demonstrates that absorptive dechlorination under high H₂ pressure transforms mixed PP–PVC streams into Cl-free polyolefin feedstocks that can be effectively upcycled by hydrogenolysis or hydrocracking. By converting HCl to inert MgCl₂ and partially hydrogenating unsaturated PVC decomposition products, the dechlorination step prevents catalytic poisoning and eliminates HCl emissions—two key barriers to processing real-world, mixed plastic waste. The two-stage approach maintains high liquid yields comparable to pure PP hydrogenolysis, while also altering gas formation pathways (suppressing methane and favoring isobutane/isopentane) due to Cl-induced modifications of Ru/TiO₂. The method generalizes to other chlorinated contaminants (chloroalkanes, chloroarenes), making catalyst performance more tolerant to realistic impurities and reducing requirements for labor-intensive sorting. Optimized dechlorination (250 °C, 30 bar H₂, ~6 h) is essential; ambient or inert conditions lead to significant Cl carryover and catalyst deactivation. The findings provide a practical route to integrate dechlorination with catalytic upcycling, enhancing process viability, product purity, and equipment longevity by preventing HCl release.

A practical, regenerable, two-stage strategy—absorptive dechlorination over Mg₃AlO₄.₅ in H₂ followed by Ru/TiO₂ hydrogenolysis or Pt/WO₃/ZrO₂ hydrocracking—enables upcycling of PP contaminated with up to ~10 wt% PVC to Cl-free lubricant- and fuel-range hydrocarbons at high yields (≈65–70%), with no detectable HCl emissions and minimal Cl in liquids (<0.1 wt%). The approach is effective for other chlorinated additives (TCE, TCB) and suppresses methane formation, though it modifies product microstructure and gas distribution. Both the Cl trap and catalysts are regenerable, and the process reduces the need for extensive waste sorting. Future work should extend the strategy to earth-abundant catalysts (Ni, Co, ZrO₂), optimize treatment for PVDC and higher PVC loadings, reduce regeneration energy demand (e.g., lower-temperature Cl removal), and integrate with upstream sorting and solvent-based separations to broaden feedstock compatibility.

- PVDC-containing feeds are not effectively dechlorinated at 250 °C under 30 bar H₂; higher temperatures/times or alternative methods are required. - Liquid yields decline markedly at higher PVC fractions (e.g., ~37% at 20 wt% PVC; negligible liquids at ~30 wt%). - Regeneration of the Cl trap requires high-temperature steaming (≈550 °C) to hydrolyze MgCl₂, adding energy and complexity. - Process relies on high H₂ pressure (≈30 bar) for optimal chlorine capture and to avoid chlorinated species carryover; ambient/inert conditions lead to significant Cl in liquids and catalyst poisoning. - Precious metal catalysts (Ru, Pt) increase cost; although regenerable, they remain susceptible to Cl-induced modifications affecting selectivity. - Product microstructure differs from pure PP hydrogenolysis due to Cl-induced catalyst modification, which may require downstream property tuning for lubricant specifications.