Solvent-free chemical recycling of polyolefins and polystyrene under mild conditions via GaCl3-based molten catalysts

doi:10.1016/S1872-2067(26)65067-X

Abstract

Abstract:

The chemical recycling of polyolefins (PE, PP) and polystyrene (PS) remains a major challenge due to their chemical stability of C-C backbones as well as the limited accessibility of polymer chains to catalytic active sites. Here, we report a solvent-free, low-temperature, and synergistic depolymerization strategy using a GaCl₃-based molten Lewis acid catalyst for the efficient conversion of PE, PP, and PS, both individually and in mixed plastics streams. The low melting point of the catalyst ensures liquid-phase mobility and efficient contact between polymer and catalysts, achieving complete PS conversion and over 60% PE/PP conversion within 30 min. In mixed plastics, a synergistic reaction pathway mediated by carbocation-type intermediates enables Friedel-Crafts alkylation between PS-derived aromatics and polyolefin-derived olefins, enhancing overall conversion (~85%) and selectivity toward mono-alkylated aromatics (up to 33.5%). Raman spectra confirmed the formation of [GaCl₄]^- and [Ga₂Cl₇]^- chlorogallate species during the reaction. This study demonstrates a novel, integrated approach for solvent-free, selective, and energy-efficient chemical recycling of mixed plastics, highlighting GaCl₃-based molten salts as versatile catalysts for circular and sustainable plastic recycling.

Key words: Polyolefins, Polystyrene, Molten salt, Lewis acid, Plastic, Depolymerization

Lei Yuan, Zhiwen Gao, Libin Shi, Cuishi Guan, Dongyue Peng, Shuhui Fang, Wei Zhang, Yue Liu. Solvent-free chemical recycling of polyolefins and polystyrene under mild conditions via GaCl₃-based molten catalysts[J]. Chinese Journal of Catalysis, 2026, 86: 137-148.

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URL: https://www.cjcatal.com/EN/10.1016/S1872-2067(26)65067-X

https://www.cjcatal.com/EN/Y2026/V86/I7/137

Figures/Tables 9

Fig. 1. Catalytic depolymerization of PE on different catalysts: phase distribution (a,d), carbon number selectivity (b,e), and product distribution in C4-15 (c,f) at 130 °C (a-c) and 100 °C (d-f). The break of y axis in (c) and (f) represents the selectivity from 50% to 75%. Reaction conditions: 1.0 g PE, 7.5 mmol MCl3 or 1.875 mmol MgCl2-MCl3 molten salts, 600 rpm, 15 min (M refers to Al or Ga).

Fig. 2. Catalytic depolymerization of PS on different catalysts: phase distribution (a,d), carbon number selectivity (b,e), and aromatic product distribution in C6-15 (c,f) at 130 °C (a-c) and 100 °C (d-f). Reaction conditions: 1.0 g PS, 7.5 mmol MCl3 or 1.875 mmol MgCl2-MCl3 molten salts, 600 rpm, 15 min. (M refers to Al or Ga).

Fig. 3. Correlation plot of ΔT1 (reaction temperature - catalyst melting point) and ΔT2 (reaction temperature - plastic melting point) for different conversion ranges in the reaction of PE or PS on AlCl3- and GaCl3-based catalysts. Circles represent the substrate PE (polyethylene), pentagons represent the substrate PS (polystyrene); solid symbols indicate the GaCl3-based catalyst, and semi-hollow symbols indicate the AlCl3-based catalyst.

Fig. 4. Time-resolved conversion of single-component polymer. Phase distribution PE (a), PP (b), and PS (c). Product distribution in PE (d), PP (e), and PS (f). GPC analysis of residual solid of PE (g), PP (h), and PS (i) after reaction. Reaction conditions: 1.0 g substrates, 1.875 mmol Mg-Ga, 130 °C, 600 rpm.

Fig. 5. Time-resolved conversion of PE-PS mixture (a-c) and PP-PS mixture (d-f), in terms of phase distribution (a,d), product distribution (b-c, e-f). Reaction conditions: 1.0 g substrates (mass ratio = 1:1), 1.875 mmol Mg-Ga, 130 °C, 600 rpm.

Scheme 1. Proposed reaction network exemplified by the PP-PS co-conversion over Mg-Ga catalyst.

Fig. 6. Initial conversion rates of PP (a) and PS (c) reaction catalyzed by Mg-Ga in the temperature range of 80-130 °C. Arrhenius plots for the initial conversion rate of PP (b) and PS (d). Ea in (b) and (d) refers to apparent activation energy. Reaction conditions: 1.0 g substrates, 1.875 mmol Mg-Ga, 600 rpm.

Fig. 7. Raman spectra of GaCl3-based catalysts. Reaction conditions: 1.0 g PE, 1.875 mmol Mg-Ga, 130 °C, 600 rpm, reaction time 1 min and 5 min.

Fig. 8. Catalytic depolymerization of post-consumer plastics on Mg-Ga. Reaction conditions: 1.0 g PE materials, 3.75 mmol Mg-Ga, 170 °C, 600 rpm, 12 h; 1.0 g PP/PS materials, 1.875 mmol Mg-Ga, 130 °C, 600 rpm, 12 h.

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Solvent-free chemical recycling of polyolefins and polystyrene under mild conditions via GaCl₃-based molten catalysts

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