Boron-Modified Mesoporous ZSM-5 for the Conversion of Pyrolysis Vapors from LDPE and Mixed Polyolefins: Maximizing the C2–C4 Olefin Yield with Minimal Carbon Footprint

Recovering monomers from polyolefins is challenging due to their chemical inertness. Chemical recycling via pyrolysis produces a complex liquid requiring subsequent upgrading and further processing. In the presented work, low-density polyethylene and postconsumer mixed polyolefin (MPO) waste were pyrolyzed at 550 °C and the vapors were passed over a catalyst to maximize the production of monomers, that is, light olefins and aromatics, which could produce new polymers in a circular approach. Hierarchical HZSM-5 zeolites containing both micro- and mesopores and their boron-modified versions with medium and high Si/Al ratios were studied and thoroughly characterized. The effect of catalyst temperature (550–700 °C), catalyst/feed ratio, steam treatment, and catalyst deactivation by coking was investigated using a tandem micropyrolyzer coupled to comprehensive two-dimensional gas chromatography and flame ionization detection/time-of-flight mass spectrometry detectors, allowing a detailed identification and quantification of the products. All catalysts effectively narrowed the product distribution with high selectivity toward C2–C4 olefins, monoaromatics, and C5–C11 aliphatics. HZSM-5 zeolite catalysts with a medium Si/Al ratio showed a higher monomer recovery (∼90%) than high Si/Al (∼80%), which has a higher selectivity to C5–C11 aliphatics. Importantly, the product yield distribution from MPO closely resembled low-density polyethylene. The addition of boron to the zeolite reduced the aromatization activity. Similarly, at comparable conversion levels, mild steam treatment of the catalyst lowered the selectivity to aromatics and increased the recovery of C2–C4 olefins. Hierarchical HZSM-5 zeolite showed improved tolerance against deactivation by coking compared to conventional HZSM-5, demonstrating the potential of boron-modified hierarchical HZSM-5 zeolite to recover monomers from polyolefin waste at high selectivity.