Maximizing light olefin production in the cracking of polyethylene using hierarchical acidic–basic zeolites†

Abstract

Fast coking, low selectivity to light olefins, and expensive synthesis are the challenges of FAU-type zeolites in the cracking of plastic waste. We addressed these problems by a one-pot seed-assisted synthesis of hierarchical acidic–basic zeolites using silica completely extracted from a highly impure kaolin containing alkali and alkaline earth metals and quartz minerals and investigating their structural stability and catalytic performance in the cracking of polyethylene. The resulting NaY zeolites, which had excellent surface areas of 509–635 m² g⁻¹, inherited morphology, crystallinity, silicon-to-aluminum ratio (3.3 to 10.52), hierarchical structure, and reduced particle size from the seeds and the basicity from the homogeneously distributed alkali and alkaline earth metals, inducing distinctive structural and catalytic properties. For example, calcining the NH₄⁺-exchanged zeolites into their HY form significantly reduced their crystallinity and etched microporosity while preserving morphology, resulting in a high Al content and well-defined mesoporous aluminosilicate materials. We then looked at the causes of susceptibility to protonation and how to stabilize their structure. Compared with the reference acidic HY zeolite, hierarchical zeolites and mesoporous aluminosilicates possessing enhanced basicity yielded a comparable activity, eight times less coke, and up to twice the olefin production in the cracking of polyethylene. When compared to the acidic HY zeolite, the acidic–basic catalysts generated liquid oils of significantly higher quality, with a composition lacking in naphthalene (3.08 vs. 62.44%) and enriched in long-chain olefins (80.5% vs. 1%). The hydrogen transfer coefficients for the hierarchical acidic–basic zeolites were much smaller than that of the reference acidic HY zeolite (0.037–0.17 vs. 1.59), suggesting dominance of the monomolecular over the bimolecular cracking mechanism by the former catalysts. The acidic–basic hierarchical HY zeolites and mesoporous aluminosilicates displayed a stable mesostructure with improved selectivity and activity over regeneration and reuse. These results showed the possibility of turning the drawback of impurities in kaolin into improved basicity and mesoporosity advantages to maximize olefin production and minimize coke formation in the cracking of plastic waste by zeolites.