2025 AIChE Annual Meeting

(348e) Acid Catalytic Conversion of High Density Polyethylene to Nylon Precursors

Authors

John Dorgan, Colorado School of Mines
Scott Calabrese Barton, Michigan State University
Across the globe, plastic waste generation rates are significantly outpacing recycling, leading to billions of tons of slowly degrading plastic in the environment, which have the potential for long lasting deleterious effects on society. Conventional recycling of plastics using mechanical reprocessing leads to undesirable material properties, thereby limiting their use in commercial products. Chemical recycling and upcycling of plastics in recent decades have gained traction as promising alternatives to mechanical reprocessing. However, common approaches utilize high temperatures, pressures, expensive specialized catalysts, and toxic organic solvents that have led to prohibitively uneconomic processes.

Mineral acid treatments present a practical and sustainable solution towards upcycling recalcitrant plastic wastes such as Polyethylene (PE) since they are cheap, readily available, can act as both solvent and oxidizer, and can be readily regenerated using water and oxygen. Previous literature and patents describe processes converting PE to dicarboxylic acids using nitric acid but tend to form less desirable nitro dicarboxylic acid products or obtain low carbon yields.

To address this problem, a new one-pot process was developed, which utilizes a novel mixture of acids and inexpensive catalyst at moderate temperatures and ambient pressures to degrade high density polyethylene (HDPE) to aliphatic dicarboxylic acids. Removal of residual functionalities, such as nitro groups, to form pure dicarboxylic acids was found to be the result of the acid mixture and catalyst participating in a catalytic cycle. Adjustments to reaction parameters enabled fine control of the molecular weight distribution of dicarboxylic acid products with carbon numbers (CN) ranging between 4 and 150. Particularly, a moderate yield of valuable Nylon precursors (4 ≤ CN ≤ 10) around 0.4 g/g of HDPE were readily isolated with most of the remaining carbon recovered as large molecular weight acids (CN ≈ 100, 0.8 g/g of HDPE). This approach led to process intensification, such as four-fold fewer unit operations with a comparable reduction in process time and atom economy compared to processes described in the literature. The novel acid-catalyst system for upcycling PE marks a major step towards the necessary development of elegant plastic recycling processes that enable economic recovery of carbon waste as platform chemicals for sustainable material production.