Selective recovery of caprolactam from the thermo-catalytic conversion of textile waste over γ-Al₂O₃ supported metal catalysts

The massive generation of synthetic textile waste has drawn considerable attention. Landfilling/incineration of textile waste has been widely made. To abate the environmental burdensome from the conventional management processes, a thermo-catalytic conversion was used for rapid volume reduction of textile waste and simultaneous valorization by recovering textile monomer in this study. Stockings were chosen as a model feedstock. Because stockings consisted of nylon with other contents, different products (caprolactam (nylon monomer), imines, cyclic dimers, and azepines) were recovered. The yield of caprolactam from the thermal conversion at 500 °C was 53.6 wt%. To selectively enhance the caprolactam yield, catalytic pyrolysis was done using γ-Al₂O₃ supported metal catalysts (Ni, Cu, Fe, or Co). γ-Al₂O₃ itself increased the caprolactam yield up to 69.0 wt% via a based-catalyzed reaction of nylon depolymerization and intramolecular cyclization. Under the presence of metal catalysts, the caprolactam yield increased up to 73.3 wt%. To offer desired feature of green chemistry, CO₂ was adopted as reactive gas. Under the CO₂-mediated catalytic pyrolysis, caprolactam yield was enhanced up to 77.1 wt% over Cu/Al₂O₃ (basis: stocking mass). Based on the net content of nylon in the stockings, the yield of caprolactam was deemed 95.3 wt%. This study proves that textile waste (stocking) and CO₂ are useful resources for recovery of nylon monomer, which can reduce the waste generation with simultaneous recovery of value-added product.