Polyurethane foam acidolysis with carboxylic acids: acid structure dictates N-containing product distribution and kinetics

Abstract

Obtaining circularity will be essential in managing plastic waste and moving towards sustainable materials. Chemical recycling offers a pathway to obtain valuable molecules from plastic waste, closing the loop on what is currently a linear economy. Here, we report on the chemical recycling of polyurethane foam (PUF) via acidolysis with dicarboxylic acids (DCAs) to release value-added molecules. While previous work has explored recovery of the recycled polyol (repolyol), we focus in this report on elucidating the product distribution and kinetics of the nitrogen-containing products from the acidolysis reaction. Using Nuclear Magnetic Resonance (NMR) spectroscopy and Ultra-High Pressure Liquid Chromatography – Mass Spectroscopy (UPLC-MS), we demonstrate how acid loading and structure influence product distribution of acidolysis. The use of excess acid can eliminate oligomeric content and aromatic amines from the product mixture. With DCAs composed of 2 or 3 carbons between the carboxylic acid groups, we observe the formation of imide products during acidolysis, which has only very recently been reported in the literature. Furthermore, the kinetics of imide formation were investigated and modeled for glutaric acid (GA) and succinic acid (SA), which form 6- and 5-membered cyclic imides, respectively. Di-imide formation with SA proceeds without accumulation of intermediates and is an order of magnitude faster than imide formation with GA, for which amide-imide intermediate is detected and the rate of reaction is sensitive to steric hindrance. This report offers fundamental insights into the N-containing products formed during acidolysis, which will aid scale-up of closed-loop chemical recycling processes.