Ionic-Liquid-Mediated Deconstruction of Polymers for Advanced Recycling and Upcycling

Abstract

Ionic liquids (ILs) are a promising medium to assist in the advanced (chemical and biological) recycling of polymers, owing to their tunable catalytic activity, tailorable chemical functionality, low vapor pressures, and thermal stability. These unique physicochemical properties, combined with ILs' capacity to solubilize plastics waste and biopolymers, offer routes to deconstruct polymers at reduced temperatures (and lower energy inputs) versus conventional bulk and solvent-based methods, while also minimizing unwanted side reactions. In this Viewpoint, we discuss the use of ILs as catalysts and mediators in advanced recycling, with an emphasis on chemical recycling, by examining the interplay between IL chemistry and deconstruction thermodynamics, deconstruction kinetics, IL recovery, and product recovery. We also consider several potential environmental benefits and concerns associated with employing ILs for advanced recycling over bulk-or solvent-mediated deconstruction techniques, such as reduced chemical escape by volatilization, decreased energy demands, toxicity, and environmental persistence. By analyzing IL-mediated polymer deconstruction across a breadth of macromolecular systems, we identify recent innovations, current challenges, and future opportunities in IL application toward circular polymer economies.

Design of depolymerizable polymers toward a circular economy

“…194 New types of solvents offer new opportunities, such as ionic liquids whose chemical design can be tailored to target catalytic performance, decomposition temperatures, and polymer solubilities for specific deconstruction applications. 30 These studies confirmed that solvent effects on depolymerization are an important factor, which requires more in-depth studies with green solvents being prioritized. The high costs and challenges in handling high dilution needs to be addressed.…”

“…24,25 Catalysis and processes such as temperature-gradient thermolysis are paving the way for upcycling of polyolefins. 26–28 Investigations into different solvents, 29,30 and conditions to depolymerize vinyl polymers 31 are also among the forefront of recent progresses. Developing new polymers to replace current ones also have many challenges to be addressed, chief among them is finding materials that have comparable stability and mechanical performances to existing commodity polymers and achieving economic feasibility of scaling up.…”

Progress in the applications of biocompatible ionic liquids: renewable commodity production, catalytic and pharmaceutical approaches – a review

“…107 Strategies for separating IL monomers to achieve recyclability, reduced viscosities of ILs at high temperatures, and chemical arguments about increased thermal stability are crucial points for effective deconstruction of polymers, which resolve pending issues regarding ILs and facilitate their application on a commercial scale. 108 As most Bio-ILs employ highly basic anions (such as deprotonated amino acids), it is likely that their thermal stability will be lower than that of traditional ILs. 109…”

A general strategy for recycling polyester wastes into carboxylic acids and hydrocarbons

“…Ionic liquids (ILs) composed of organic cations and organic/inorganic anions have been emerging as a kind of functional materials and applied in various chemical processes due to their unique properties 25 – 28 . It was reported the basic ILs (e.g., [EMIm][AcO]) could accomplish PHB decomposition into crotonic acid 29 , 30 . Furthermore, the ILs with halide anions have been widely employed as solvents or catalysts in various chemical reactions.…”