Hydrothermal upcycling of polyoxymethylene waste on heterogeneous Mn catalyst: The effect of reaction medium

Background

Polyoxymethylene (POM), an engineering thermoplastic widely used in various applications, is difficult to recycle due to its high stiffness and excellent stability. Hydrothermal conversion has emerged as a promising method for recycling POM, offering moderate operating conditions, reduced harmful emissions, and selective production of valuable chemicals like methanol and formic acid. Recent studies show that catalytic hydrothermal processes can further enhance efficiency and selectivity, though homogeneous catalysts raise concerns about recovery and environmental safety. In contrast, heterogeneous catalysts offer better recyclability and reduce the risk of hazardous waste generation.

Methods

This study proposes a hydrothermal conversion process as a POM recycling method. A Mn catalyst supported on activated carbon (Mn/AC) and different reaction media (N₂ and CO₂) are applied to the hydrothermal POM conversion as an alternative approach against homogeneous catalytic POM conversion. The effects of reaction temperature (220–260 °C), reaction medium (N₂ vs. CO₂), and the catalyst presence on the hydrothermal POM conversion are investigated.

Significant Findings

The hydrothermal POM conversion leads to formic acid, methanol, and 1,3,5-trioxane as major reaction products. Formic acid reached its highest yield of 9.79 wt% at 240 °C without the catalyst under N₂ atmosphere. Under CO₂ atmosphere, methanol production peaked at 1.54 wt% at 260 °C with the Mn/AC catalyst. The highest yield of 1,3,5-trioxane (1.08 wt%) was observed at 220 °C under N₂ atmosphere with the Mn/AC catalyst. Therefore, due to the optimal conditions required for each target product, process parameters must be strategically controlled to meet specific production goals. The experimental findings open up the possibility of using heterogeneous catalysts for hydrothermal POM conversion to recover its monomeric compounds.