Bottled Potential: Upcycling Polyethylene Terephthalate into a Moisture-Swing Sorbent for Direct Air Capture

Abstract

In the last century, rising atmospheric carbon dioxide (CO2) levels have demanded carbon removal technologies in addition to simple emissions cuts. Among these negative emission technologies is direct air capture (DAC), which extracts CO2 directly from ambient air. While DAC is a promising negative emission technology, it is limited by energy-intensive sorbent regeneration that typically involves high temperature or pressure swings. This limitation prompts interest in moisture-swing adsorption (MSA), which relies on changes in humidity to regenerate sorbent. In a moisture-swing DAC system, CO2 is adsorbed at low humidity and desorbed at high humidity. Tangentially, polyethylene terephthalate (PET) plastic is one of the most produced and under-recycled plastics globally, single-handedly contributing to both emissions and waste problems. This work proposes a chemical synthesis process to upcycle post-consumer PET bottles into a quaternary ammonium bicarbonate sorbent for moisture-swing DAC. Functionalization occurs via a four-step reaction sequence: aminolysis, Eschweiler-Clarke methylation, Menshutkin quaternization, and bicarbonate ion exchange. Successful conversion of the products is confirmed at each step via 1H NMR spectroscopy. The carbon capture performance of the upcycled PET-derived sorbent is then evaluated in the moisture-swing DAC system. The sorbent demonstrates reversible CO2 capture and release over five adsorption/desorption cycles. CO2 levels correlated directly with humidity swings from 35% relative humidity (dry) to 70% relative humidity (wet), consistent with the MSA mechanism. In addition, cyclical stability was observed with no measurable capacity loss. Thus, this research serves as a proof of concept for a negative emissions process that simultaneously addresses plastic waste and atmospheric CO2 mitigation.