Polyurethane Recycling Using Catalytic Pyrolysis: Insights into Catalyst Stability

Abstract

Polyurethanes are widely used in many applications because of their versatility. However, efficient recycling remains challenging. Recent findings indicate that aniline recovery from rigid polyurethane foams via catalytic pyrolysis with an alumina-based catalyst is an attractive option. Techno-economic analyses showed that catalyst costs are a major factor and that catalyst recycling is essential. Here, we report an extensive study on the recyclability of a selected alumina-based catalyst (Puralox 5/200) for the catalytic pyrolysis of PUR. Experiments with fresh and spent catalysts in a batch-operated staged free-fall reactor at gram scale (3.0 g nPUR, catalyst-to-feed ratio: 1 to 1, 550 °C) using virgin PUR showed differences in activity and chemoselectivity for aniline, indicative of catalyst deactivation. Spent catalyst characterization (TGA–MS, TEM–EDX, N2 physisorption, TPD, and XRD) showed hard coke (14.4 wt %) on the catalyst. Catalyst regeneration was successfully demonstrated using an oxidative treatment at 550 °C, though some irreversible deactivation, including loss in acidity and Brunauer–Emmett–Teller (BET) surface area, as well as inorganic accumulation, was observed after five regeneration/recycles. Additional experiments to assess catalyst performance after multiple regeneration-recycles were carried out on a milligram scale in a tandem microreactor coupled to GC–MS/FID (catalyst-to-feed ratio of 5:1 at 550 °C), using rigid polyurethane (nPUR) and a contaminated end-of-life (EOL) feed. With nPUR, catalyst performance for aniline and toluidine formation declined in the first 4 cycles, then stabilized at half the original activity for 28 cycles, and finally dropped to very low levels. For EOL PUR, catalyst performance over multiple runs was significantly worse, and the catalyst was inactive after 18 cycles. We characterized the spent catalysts from both PUR feeds (a.o TEM–EDX), which showed that inorganic impurity accumulation contributes to catalyst deactivation.