Product Scope and Reactivity in Hydrothermal Conversion of Polyethylene With Solid Catalysts

Abstract

Plastic waste can be converted into smaller hydrocarbons through hydrothermal conversion using supercritical water. It would be advantageous to introduce suitable catalyst materials under hydrothermal processing conditions to steer product selectivity. In this study, a series of commonly used solid catalysts were evaluated to identify those with the highest potential to tune the product portfolio and increase the overall performance of the hydrothermal conversion of polyethylene. Various metal oxides (i.e., TiO2, ZrO2, CeO2, SiO2 and γ-Al2O3) and aluminosilicates (i.e., zeolites H-Y and H-ZSM-5, as well as different amorphous aluminosilicates) improve the polyethylene conversion rate compared to non-catalytic hydrothermal reaction conditions. CeO2 is a promising material because it produces a naphtha-like product stream, thereby achieving full conversion, high liquid yield, high alkane/alkene selectivity, and good hydrothermal stability. To steer the conversion toward aromatics, aluminosilicates were the most effective catalyst materials, with amorphous aluminosilicates outperforming their crystalline counterparts in terms of liquid yield (40 wt%), aromatic selectivity (16 wt%) as well as stability. Crystalline aluminosilicates, such as zeolites H-Y and H-ZSM-5, also enhanced activity, but lost their structural integrity under hydrothermal reaction conditions. However, despite significant structural changes, catalysts could be reused, as reactivity of spent materials only decreased slightly.