A Robust and Efficient Heterogeneous MoS2/Al2O3 Catalyst for the Hydrogenative Recycling of Polyurethane Waste

Abstract

The chemical recycling of polyurethane (PU) waste into its original monomers remains a significant challenge. Herein, we report a highly efficient catalytic hydrogenation system for the depolymerization of PU using a well-dispersed MoS2/Al2O3 catalyst prepared via a two-step solid-phase exchange and sulfurization method. Evaluation using a PU model compound derived from 4,4'-diphenylmethane diisocyanate (4,4'-MDI) and 1,6-hexanediol showed that MoS2/Al2O3 effectively cleaves C–O bonds, yielding up to 92.5% hexanediol. The addition of K2CO3 as a co-catalyst was found to be essential for promoting C–N bond scission, increasing the yield of 4,4'-methylenedianiline (4,4'-MDA) from 41.4% to 84.2%. Combined with in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) studies, a plausible reaction mechanism involving heterolytic hydrogen activation and K2CO3-assisted elimination is proposed. The catalyst demonstrated excellent stability over ten cycles and remarkable tolerance to additives. Its practical application was demonstrated by the near-complete (>98% conversion) degradation of real thermosetting PU foams (sponge and refrigerator foam) at the gram scale, yielding valuable aromatic amines and polyol products. This work provides an efficient and scalable catalytic strategy for the chemical recycling of complex PU wastes.