Montmorillonite K10-induced decomposition of methyl N-phenylcarbamate to phenylisocyanate and its prospect for recovering isocyanates from polyurethanes

Abstract

The significant growth of the production and use of flexible polyurethane (PU) foam causes increased waste accumulation, thereby creating a demand for the recycling of PU. While upgrading PU materials to recycled polyols has been studied in detail, the recovery of diisocyanates is a longstanding challenge. Montmorillonite K10 (MK10) has been proposed as a catalyst to convert carbamates into isocyanates, but its prospects are unclear. Here, the MK10-catalyzed decomposition of methyl phenylcarbamate (MPC) into phenylisocyanate (PI) and the side product N,N’-diphenylurea (DPU) has been studied as a model reaction for the decomposition of PU foam and the potential recovery of PI. The effects of the amount of catalyst, the temperature, and the solvent were investigated by HPLC analyses. Kinetic analysis revealed that the uncatalyzed rate of the MPC decomposition is much lower than the catalyzed rate, indicating that the thermally driven decomposition is negligible. Our results, involving both experimental kinetic studies and models of the kinetics, show that, while MK10 effectively catalyzes the decomposition of MPC, it also causes the formation of aniline, which reacts with PI to form DPU. As a result, large amounts of MK10 favor DPU and prevent the selective formation of PI, and yields of PI >30 % were never observed. Likely, the OH groups of MK10 form covalent bonds with PI, causing a deficiency in the mole balance. Overall, MK10 is unsuited to provide high yields of isocyanates.