Research progress on chemical recycling of waste plastics mediated by ionic liquids

Abstract

The ever-growing production and consumption of plastics have led to significant challenges in waste management and recycling. Plastics, primarily synthesized from small monomers through polymerization or condensation reactions, are integral to modern society due to their durability, design flexibility, and cost-effectiveness. However, the recycling rate of plastics remains low, around 10%, due to complex recycling processes and poor degradability, leading to substantial resource wastage and environmental pollution. The growing global focus on CO2 emission reduction and the promotion of a circular economy has elevated plastic recycling to a pivotal area of research. This shift offers significant economic advantages, mitigates environmental pollution, and preserves petroleum resources. Chemical recycling processes break down polymer chains into monomers or other valuable chemicals, complementing physical recycling methods that struggle with low-value waste plastics such as cross-linked polymers and thermoplastic elastomers. Chemical recycling presents a viable solution for transforming waste plastics into valuable products, both economically and environmentally. By employing methods such as pyrolysis, reforming, gasification, and chemolysis, it is possible to extract monomers, high-energy liquid oils, various carbon materials, hydrogen, syngas, and other valuable chemicals from various plastic waste. These processes typically necessitate high temperatures due to the inherent chemical stability of plastics. Among various chemical recycling methodologies, solvolysis–which encompasses hydrolysis, alcoholysis, and ammonolysis–demonstrates superior selectivity by effectively targeting specific bonds within polymer structures, including ether, ester, amide, and even the more robust C–C bonds. This process is particularly suitable for condensation polymers such as PET, PC, and PLA, and it represents a form of closed-loop recycling. In addition, solvolysis requires significantly lower temperatures compared to plastic depolymerization methods like pyrolysis, which generally demand temperatures exceeding 400°C. ILs, composed of cations and anions with designable structure, exhibit unique properties such as high hydrogen bond disrupting ability, good solubility for natural and synthetic polymers, low vapor pressure, non-flammability, and high chemical stability, making them ideal for plastic solvolysis. The structural tunability of ionic liquids (ILs) allows for precise modulation of their physical and chemical properties, thereby enhancing their capacity to dissolve and depolymerize polymers effectively. Additionally, the incorporation of Lewis or Brönsted acidic sites within ILs can facilitate the depolymerization of plastics under milder reaction conditions. Furthermore, immobilizing ionic liquids on heterogeneous supports presents a promising strategy to improve their practical applicability and broaden their use in large-scale processes. This review consolidates recent advancements in the chemical recycling of plastics mediated by ILs, emphasizing their role in the depolymerization of common plastics such as polyamide (PA), polyethylene terephthalate (PET), polycarbonate (PC), polylactic acid (PLA), polyethylene (PE), and polyvinyl chloride (PVC). ILs have shown considerable promise due to their ability to swell plastics effectively, allowing coordinated activation through cation-anion interactions that provide catalytic sites with Lewis or Brönsted acidity. The mechanisms underlying these processes often involve hydrogen bonding between IL cations and carbonyl oxygen in polymers, which enhances the electrophilicity of the carbonyl carbon. Concurrently, hydrogen bonding between nucleophilic agents (e.g., alcohols, water, amines) and IL anions boosts nucleophilicity, facilitating C-X (X = O or N) bond cleavage through nucleophilic substitution. While summarizing the advances achieved through the use of various ionic liquids for the solvolysis of plastics containing C–O, C–N, and C–C bonds, this review underscores the primary research gaps that require further investigation. It aims to provide valuable insights to guide the development of enhanced plastic upcycling techniques employing ionic liquids.