Recovery of Aromatic Diamines from Aramid Fibres via ZSM-5-Assisted Hydrothermal Depolymerization under Subcritical Water Conditions

Aromatic polyamides (aramids) are high-performance polymers whose end-of-life management remains challenging due to their chemical resistance and limited recyclability. Chemical recycling via depolymerization into monomeric building blocks represents a promising route for their valorisation within a circular economy framework. Here, the hydrothermal depolymerization of Technora® aramid fibres was investigated under subcritical water conditions, both in the absence and presence of an acid-activated ZSM-5 zeolite catalyst. Under mild subcritical conditions (255 °C, 3 h), water alone resulted in limited depolymerization, while increasing the temperature to 350 °C improved fibre conversion but promoted secondary degradation and char formation. The introduction of acid-activated ZSM-5 significantly enhanced depolymerization at 255 °C, achieving a maximum liquid yield of 34.4 wt%. GC-MS analysis revealed the formation of the aromatic diamines: p-phenylenediamine (PPD) and 3,4′-diaminodiphenyl ether (3,4′-DAPE). Their distinct distribution between the aqueous and organic phases suggests the potential for facilitating downstream separation. Post-reaction characterization confirmed preservation of the ZSM-5 MFI framework, while thermogravimetric analysis revealed catalyst fouling due to organic deposition. Overall, ZSM-5-assisted hydrothermal depolymerization under subcritical conditions demonstrated the potential of heterogeneous catalysis for the recovery of aromatic diamines from aramid fibres, although only partial fibre conversion was achieved within the investigated operating conditions.