Monocyclic aromatic hydrocarbons production from NaOH pretreatment metallized food plastic packaging waste through microwave pyrolysis coupled with ex-situ catalytic reforming

Abstract

Metallized food plastic packaging waste (MFPPW), as found in packaging bags like potato chips and coffee, presents an intricate structural and compositional complexity, rendering its recycling considerably more challenging in comparison to pure plastic packaging. In this work, a process route to produce monocyclic aromatic hydrocarbons (MAHs) from caustic (NaOH) treatment MFPPW through microwave pyrolysis coupled with ex-situ catalytic reforming was proposed. NaOH pretreatment disrupted the chemical bonds between the plastic layers and aluminum layer, reducing the complexity of the subsequent pyrolysis reaction. The removal of the aluminum layer diminished its shielding effect on the microwave, and enhanced the efficient conversion of the microwave energy into heat energy, thus accelerating the pyrolysis of the remaining plastic. After pretreatment, the activation energy for the pyrolysis reaction decreased from 250.55 kJ/mol to 189.70 kJ/mol, and the MAHs content of the liquid oil increased. The highest MAHs content of 80.50% was achieved with an HZSM-5 (Si/Al = 50) zeolite catalyst, pyrolysis temperature of 550°C, and catalysis temperature of 550°C. The unique characteristics of the MFPPW demonstrate its suitability for MAHs production. MFPPW is predominantly composed of PP and PET polymers. PP, as a hydrogen-rich plastic, provides abundant hydrogen-rich free radicals to the PET and promotes hydrogen transfer reactions, thus facilitating the formation of MAHs. Following pretreatment, these two polymers come into closer contact, further enhancing their synergy. This study provides valuable insight into waste management and recovery for metallized food plastic packaging waste.