Supercritical Fluids for Enhanced Chemical Transformation of Postconsumer Plastics: A Review

Abstract

Various chemical transformation approaches are being actively developed to address the environmental accumulation of plastic waste. However, most postconsumer plastics are heterogeneous, exhibit high melt viscosity, and are insoluble in most conventional solvents. Such properties result in transport-limiting chemical transformations, low conversion rates, and low product selectivity. Although supercritical fluids (SCFs) have been a matter of continuing scientific interest in several mass-transfer processes, the use of SCFs as tunable media for the chemical transformation of postconsumer plastics is still in its early stages, but has rapidly advanced in recent years. Therefore, this review reports on the current state-of-art of chemical transformation of plastics using SCFs. It addresses the effects of sub and supercritical CO₂ (scCO₂) on solvolysis-based technologies. Additionally, it reviews recent advances on the use of supercritical organic solvents (e.g., ethanol, methanol) and supercritical water (SCW) as reaction media for the solvolysis and liquefaction of plastics, respectively, and the latest developments in the simultaneous conversion of CO₂ and waste plastics. Overall, developing technologies that minimize mass transfer limitations during the chemical transformation of plastics is critical to overcoming some of the major bottlenecks hampering product yield and selectively, and ultimately the economic viability of plastics recycling and upcycling.