Catalytic pyrolysis of polyethylene over bimetallic-modified ZSM-5: Role of alkaline sites on structure regulation, thermal behavior and aromatics production

Abstract

Catalytic fast pyrolysis (CFP) using bimetallic modified ZSM-5 enables efficient upcycling of polyethylene (PE). The present study demonstrates that alkaline sites significantly enhanced the mass transfer capacity of catalysts through generating fragmented structures, thus contributing to the selective regulation of thermal decomposition behavior of PE. Specifically, CuZn/ZSM-5 reduced decomposition temperature range by > 100 °C via enhanced chain-cracking and product transport. The synergistic effects of Cu⁰ and alkaline sites derived from Zn species facilitated the cleavage of C–C bonds under a high heating rate, thereby accelerating the formation of olefin intermediates as well as promoting the aromatization within ZSM-5 channels. CuZn/ZSM-5 optimally combined highly dispersed metal sites, abundant strong alkaline sites, moderate acidic sites, and reinforced diffusion capacity, boosting low-temperature aromatics selectivity by 29.7 %, particularly for monocyclic aromatic hydrocarbons (MAHs). Despite the highest alkalinity of CuMg/ZSM-5, the permanent loss of weak alkaline sites at high temperatures resulted in weak capacity for aromatization. In addition, the plausible pathway for fast pyrolysis of PE into aromatics over CuZn/ZSM-5 was proposed.