Deconvoluting the roles of polyolefin branching and unsaturation on depolymerization reactions over acid catalysts

Abstract

Developing a circular economy for plastics is a societal priority with a range of stakeholders engaged in developing pathways to mechanical and chemical recycling. Acid catalysis has been proposed as an attractive strategy to chemically recycle or upcycle plastic waste. In principle, the energetics that govern polyolefin decomposition reactions over Brønsted acid sites should be the same ones known for alkane cracking. However, the nature of polymers as feedstock imposes new challenges for these well-known processes, e.g., strong mass transfer limitations. To better understand the role of polymer structure and diffusion on cracking rates, the effect of tertiary carbons, chain unsaturation, and catalyst active sites distribution were studied. Our findings confirm that higher concentrations of tertiary carbons will govern decomposition reactions, regardless of diffusion limitations. Also, we unveil that LLDPE reactions mostly take place on the external surface of ZSM-5 crystals while HDPE conversion takes place within the zeolite pores.