Zeolite Catalysts for Hydrogen Harvesting from Polyethylene: A Sustainable Approach to Plastic Waste Upgrading

Abstract

This study aims to produce hydrogen from polyolefin plastics, using nickel zeolite-based catalysts under inert conditions. Low density polyethylene (LDPE) has been chosen as a model for upgrading polyolefin plastic wastes. Zeolites with a large variety of physicochemical properties have been investigated including large pores beta (BEA/BEB), faujasite (FAU), mordenite (MOR), and the intermediate pore ZSM-5 (MFI). All have been impregnated with nickel to enhance catalytic hydrogen production. Their catalytic performance was assessed through LDPE conversion in a fixed bed reactor under nitrogen flow as well as in a combined TGA-MS setup. All catalysts performed better, that is, at lower temperatures than under thermal decomposition. Ni-LZY-210, Ni-Beta, and Ni-CBV712 are active at low temperatures (∼250 °C) with a high hydrogen selectivity (40–50 mol.% of the gas phase). They also exhibit different selectivity toward isobutane and heavier hydrocarbons (C6+). The composition of the condensed products depends also of the zeolite: with Ni-Beta, the primary products are alkyl benzenes (C8-13), while Ni-CBV712 produces approximately 30 wt.% of branched alkanes in the C8-10 range and Ni-LZY-210 predominantly yields aromatic compounds in the C8-10 range. The coke deposit varies also in the range of 2–20 wt%. The presence of nickel is essential for hydrogen production. In the absence of nickel the yield of hydrogen ranges from 1% to 7% for pure zeolite-based catalysts.