Conversion of Polypropylene to Light Olefins by HMFI Catalysts below Pyrolytic Temperature: Catalytic, Spectroscopic, and Theoretical Studies

Plastic waste is a major environmental issue; converting it directly into valuable chemicals by using catalysts is a promising alternative to plastic recycling. Here, we report the selective catalytic cracking of polypropylene (PP), a typical commodity plastic, to high-value light olefins (C₂–C₅), below pyrolytic temperature (290 °C) and without external hydrogen supply, by using zeolite catalysts. Among the H⁺-form zeolites with different structures, HMFI showed the highest yields of light hydrocarbons (C₂–C₅), of which light olefins (C₂–C₅) were the major products. The HMFI-catalyzed PP conversion was applicable to the upcycling of a model PP waste, resulting in a 61.9% light hydrocarbon yield. The results of catalytic and in situ IR experiments using model HMFI catalysts with a small amount of external Brønsted acid sites suggested that the Brønsted acid sites on the external surface of HMFI are indispensable for the PP conversion and are posited to be the active sites for the cracking of PP into short-chain (oligomeric) hydrocarbon species as intermediate products. Density functional theory analyses were conducted to determine plausible reaction pathways by adopting 2,4-dimethylheptene as the shortest unit of the oligomeric species. The obtained results show that the β-scission of 2,4-dimethylheptene by Brønsted acid sites yields isobutene and propylene (or a propyl alkoxide group) via carbocation intermediates with an activation energy below 118 kJ mol^–1. Operando UV–vis and IR experiments under the reaction conditions, combined with ex situ ¹H NMR and ¹³C NMR analyses of the spent catalyst, show that some of the olefins are further converted to light or heavy aromatics (coke deposit), probably via carbenium ion species.