Synchronous decolorization and depolymerization of colored waste and scrap PA6 textiles containing diverse dyes to recover regenerated ε-caprolactam

Abstract

The chemical recycling of non-biodegradable colored waste and scrap Polyamide 6 (PA6) textiles (CWPA6T) containing diverse dyes into regenerated ɛ-caprolactam (rCPL) through a straightforward, environmentally friendly, and cost-effective approach is a fervent aspiration and a formidable endeavor. In this study, the efficiently synchronous decolorization and solvent-free catalytic depolymerization of CWPA6T containing diverse dyes to rCPL was successfully accomplished by a cost-effective alkali catalyst readily available in the market. This depolymerization process was characterized by its simplicity, affordability, green, ease of handling small quantities, high-yield (∼80.1%), exceptional purity (∼99.96%), and the absence of additional procedures like distillation. Importantly, it is worth highlighting that the intermediates underwent novel examination through pyrolysis gas chromatography-mass spectrometry (PyGC-MS) for the purpose of emulating real-time analysis of the constituents within the depolymerization products of CWPA6T, which encompasses a variety of dyes. Furthermore, employing advanced analytical techniques such as PyGC-MS, liquid chromatography-mass spectrometry (LC-MS), and gas chromatography-mass spectrometry (GC-MS), comprehensive explanations were provided regarding the underlying mechanisms responsible for the generation of 7-butyl-3,4,5,6-tetrahydro-2H-azepine and 2-ω-aminopentylazacyclohept-1-ene, as observed through the aforementioned three detection methods. This exposition surpasses the prior comprehension, as both were formerly acknowledged as depolymerization byproducts of PA6. After verification, it was determined that a novel compound, 1-methylazepan-2-one, had been produced, potentially resulting from the N-methylation of rCPL. Furthermore, a comprehensive investigation was conducted on the constituents and formation mechanisms of the depolymerization products of CWPA6T encompassing a wide range of dyes. Additionally, the decolorization mechanism was thoroughly examined and analyzed. This study presents an environmentally friendly, streamlined, economical, and scalable technique for recycling CWPA6T containing various dyes. The objective is to achieve the simultaneous decolorization and depolymerization of CWPA6T in order to yield small, readily collectible rCPL.