Tim G. W. Engels

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.

Selective hydrogenation is a crucial process in the synthesis of chemical intermediates. Here, we report the efficient hydrogenation of 5-hydroxymethylfurfural (HMF) to 2,5-bishydroxymethylfuran (BHMF) over N-doped C-supported cobalt catalysts (CoNC) derived from ZIF-67 for the enzymatic synthesis of poly(2,5-furandimethylene adipate) (PFAd). By varying the oxidative pretreatment temperature, a series of CoNCX catalysts (where X denotes the oxidation temperature) were obtained, enabling effective tuning of catalytic activity. The optimized catalyst, CoNC200, achieved 93% HMF conversion and 99% BHMF selectivity, with a BHMF yield approximately 30% higher than that of the nonoxidized sample at 100 °C within 20 min. Notably, near-complete conversion and high selectivity can be achieved at room temperature within 8 h, while similar performance is obtained at 120 °C within 30 min, demonstrating high selectivity and efficiency across a wide temperature range. The low apparent activation energy (39.9 kJ mol–1) further confirms the high intrinsic activity of CoNC200. The enhanced low-temperature performance is attributed to the synergistic effect of CoOOH and Co3O4 species with high-valence cobalt sites on the N-doped carbon support, as revealed by comprehensive characterization. Furthermore, the catalytic system is compatible with enzymatic polymerization, enabling direct conversion of crude BHMF mixtures containing residual HMF into polyesters using dimethyl adipate. The presence of HMF allows modulation of the polymer molecular weight without compromising the thermal properties. These findings underscore a robust and versatile strategy for integrating selective hydrogenation with mild enzymatic polycondensation toward bio-based polyesters.