(607h) Role of Confinement Effects for Zn Deposition Via Polymer Additives Onto Acid Catalysts

Significant efforts are underway to develop a circular economy for plastics including mechanical and chemical recycling. Catalytic routes for chemical recycling and upcycling have been proposed as possible solutions for waste management, as they have the potential to transform waste plastic into its original building blocks or higher value products.

The presence of polymer additives may play an important role in the long-term activity of the catalysts used for chemical recycling processes. Zinc stearate (ZnSt) is a commonly used polyethylene additive, which works as an acid scavenger. We have observed that the presence of ZnSt in the polymer melt permanently modifies ZSM-5 used for recycling reactions. The deposited zinc on zeolite has been shown to increase activity of the catalyst, likely by increasing the hydrocarbon activation capabilities of the catalyst via facilitating dehydrogenation.

However, when Zn is deposited onto mesoporous acid catalysts, we observe that there is a decrease in activity, which is an opposite trend when compared to ZSM-5. There is evidence in the literature that zeolites having larger pores (BEA) or big cavities (CHA) do not show enhanced alkane cracking conversions after exchange with zinc. On the other hand, when observing more confined zeolites, there is evidence that Zn-exchanged ZSM-22 (TON) present higher alkane cracking conversion and aromatics selectivity.

In this work, we systematically study the role of confined structures for zinc atoms to form active sites, by exchanging zinc with different zeolite frameworks, both via incipient wetness impregnation and zinc-containing polyethylene decomposition (and subsequent regeneration). We also assess the effect of pre-existing paired Bronsted sites on the Zn exchange by exchanging different Si/Al ratio zeolites within the same framework structure. This work reveals the interesting role that some common polymer additives may play when in contact with catalysts often employed in polymer upcycling reactions.