Catalysts for depolymerization of nylon-6 to ε-caprolactam – insights into activity-structure relationships

Licentiatavhandling, 2025

Catalytic recycling of plastic waste using heterogeneous catalysts represents a promising strategy to address plastic pollution and advance a circular economy within the plastics industry. However, this technology remains underdeveloped, particularly for plastics considered least recyclable, such as heteroatom-containing polymers like nylon-6, which are more valuable than conventional polyolefins yet have received limited attention. This thesis focuses on the reductive depolymerization of nylon-6 using supported heterogeneous catalysts under a hydrogen atmosphere. The results of the study are reported in two manuscripts namely Manuscript I and Manuscript II.

In Manuscript I, the depolymerization of nylon-6 over Ru/ZrO2 was systematically investigated with respect to reaction temperature, pressure, time, catalyst loading and gas atmosphere in a batch reactor. Optimal conditions (350 °C, 30 bar H2, 5 h) resulted in a 94 wt% yield of ε-caprolactam and only 3.4 wt% hexamethyleneimine as a byproduct. The depolymerization was found to be sensitive to Ru particle size, with disruption of the semi-crystalline structure identified as a prerequisite for efficient C-N bond cleavage. A catalyst containing 2.3 wt% Ru with an average particle size of 9.5 ± 2.3 nm exhibited the highest activity. Characterization analyses revealed that smaller Ru particles exhibited enhanced reducibility, promoting synergistic interactions between Ru sites and activated H2, which facilitated both crystallinity disruption and bond scission. Catalyst stability was demonstrated in the presence of plasticizers and common polymer additives.

In Manuscript II, catalyst performance was further optimized through the preparation of an improved Ru/ZrO2 catalyst via incipient wetness impregnation. This catalyst enabled efficient depolymerization at 300 °C, achieving caprolactam yields of 68 wt% after 2 h and 78 wt% after 3 h, with >94% selectivity under 30 bar H2.  This catalyst demonstrated a six-fold higher activity compared to Ru/CeO2 and Ru/Al2O3. Extensive characterization attributed the enhanced performance to finely dispersed small Ru nanoparticles, high surface basicity (139 µmol CO2/g), and large pore width (11 nm). In contrast, strong metal–support interactions in Al2O3 and TiO2-based catalysts limited Ru dispersion and activity. Deuterium-labeling studies indicated that depolymerization proceeds via selective C-N bond cleavage through hemiaminal intermediates. This work establishes a more selective and scalable pathway for nylon-6 recycling, emphasizing the crucial role of support properties and catalyst design in advancing sustainable polyamide recycling technologies.