Sulfonated Fe-MOF Solid Acid Catalyst for High-Efficient Depolymerization of Polyamide 6

Upcycling the wasted polyamide 6 (PA6) to its monomer caprolactam via depolymerization is a promising way to build the resource cycling and alleviate the environmental pollution. High efficient catalyst for PA6 depolymerization is desirable and crucial. Herein, a sulfonated Fe-MOF catalyst was designed and performed as solid acid catalyst for the PA6 depolymerization for the first time. Results showed that a 100% conversion of PA6 solid to water-soluble liquid products can be achieved at temperatures of 250°C after only 1-h reaction. LC/MS results indicated that the water-soluble products were mainly composed of caprolactam and their oligomers. Without catalyst, nearly no water-soluble products can be obtained at the same condition. Moreover, a high selectivity of 90% caprolactam production in the water-soluble liquid products was achieved at 260°C, 5 h. The reaction rate constants at 240°C, 250°C, and 260°C were determined to be 0.024, 0.033, and 0.064, respectively, with a Ea value of 101.8 kJ/mol, showing positive temperature dependent relationship. The comparison of catalytic performances over sulfonated Fe-MOFs, FeCl 3 ·6H 2 O and the pristine un-sulfonated Fe-MOF suggested the synergistic contribution of Fe sites (Lewis acid centers) and the sulfonic acid sites to the catalytic performances. Graphical Water soluble liquid products (100 wt.%) with a high selectivity of 90% caprolactam can be achieved by the addition of Fe-MOF catalyst under a mild condition through the synergistic catalytic contribution of Fe sites (Lewis acid centers) and the sulfonic acid sites.