(178g) Understanding the Role of Branching on Thermal and Catalytic Polyethylene Decomposition Reactions

Plastic production and consumption have been growing exponentially in the last decades. As a direct consequence, plastic waste disposal is one of the challenges mankind faces this century. Currently, over half of the discarded polymers are deposited into landfills or leaked into the environment, whereas a minor fraction (20% globally, 10% in the US) is recycled. It is estimated that roughly 8 million metric tons of plastic are leaked into the ocean every year. Chemical recycling and upcycling have been appointed as possible solutions for waste management, and also as green alternatives to fossil resources.

Polyethylene (PE) is one of the most produced plastics worldwide. In this study, samples of high- and low-density polyethylene (HDPE and LDPE) were decomposed both thermally and catalytically at different temperature conditions. HZSM-5 at two different levels of acid density (Si/Al = 40 or 140) were used to assess internal diffusion. Also, acid mesoporous catalysts such as amorphous silica alumina were used. Reactions were carried out under inert atmosphere in a thermogravimetric analysis (TGA) system coupled to a mass spectrometer, a micropyrolyzer unit followed by a gas chromatography – flame ionization/mass spectrometry (GC-FID/MS) system, and a semibatch reactor. Infrared spectroscopy (IR) was used to characterize the polymers degree of branching. In this contribution, we decouple the effects of acid site density, diffusion path, and polymer characteristics (degree of branching and olefin content) influence the initial conversion rate and sequential reactions along the diffusion path. We contrast these results with those of model compounds with the same functionality, revealing the differences to local confinement created by polymer bound systems and sequential reaction paths induce on reaction selectivity upon catalytic polymer degradation.