PP Upcycling Employing FCC Spent Catalyst: The Role of Contaminants, Atmosphere and Pressure

Abstract

Innovative and sustainable approaches are being pursued to convert plastic waste into high-value fuels, particularly without using noble metal catalysts or external hydrogen sources. In this scenario, we explored the utilization of a spent fluid catalytic cracking (FCC) catalyst (ECAT), aiming to enhance the yield and quality of products through catalytic cracking of Polypropylene (PP) under different reaction conditions. Under a 20bar H2 atmosphere, ECAT efficiently cracked PP into alkanes (71.6%) and aromatics (25.5%), with minimal alkenes (2.2%) production. Similarly, under a 20bar N2 atmosphere, ECAT achieved comparable conversion levels, producing mainly saturated hydrocarbons, which was intriguing. However, tests conducted at 1bar N2 pressure showed that PP conversion remains unaffected but presents higher olefins’ yield and lower aromatics than ECAT. Thus, pressure seems to favor aromatization and in situ hydrogen transfer. By comparing the performance of ECAT with commercial CBV 712 zeolite, we propose that PP acts as an internal hydrogen source, eliminating the need for external supply improving efficiency and selectivity. Interestingly, mechanistic insights suggested that dehydroaromatization and hydrogen transfer within zeolite pores were dominant, and pressure (H2 or N2) could have helped it. In addition, contrary to previous studies that claim that metal contaminants in ECAT are essential for the hydrogenation process, our findings suggest that for this particular ECAT composition, we do not observe any boost in alkane yield due to the presence of these metals. Hence, our study comprehensively evaluates different strategies that can be employed to promote PP upcycling promoted by ECAT.