Tertiary Recycling of Low-Density Polyethylene by Catalytic Cracking over ZSM-11 and BETA Zeolites Modified with Zn2+: Stability Study

The behavior of Zn-ZSM-11 and Zn-BETA zeolites during the decomposition of low-density polyethylene has been studied in a batch, fixed-bed quartz reactor during successive reaction cycles without catalyst regeneration. The study of the reaction products evolved in the catalytic cracking showed that Zn-ZSM-11 zeolite maintained an essentially stable behavior during all the reaction cycles. On the other hand, Zn-BETA zeolite showed a decrease in the gaseous hydrocarbon fraction. The liquid hydrocarbons disappear after 100 min, and the waxes appear in the last three cycles of polymer cracking, which indicates a behavior similar to thermal degradation. The characterization results (TG, FTIR, and BET surface area) are consistent with the catalytic activity exhibited by both zeolites. The yields of accumulated coke increased steadily throughout the cycles up to maximum values of ∼6.5 and ∼21 wt % for Zn-ZSM-11 and Zn-BETA, respectively. These results were confirmed by TG under air flow.