Carbamate-bond breaking on bulk oxides realizes highly efficient polyurethane depolymerization.

Abstract

Polyurethane is a versatile plastic finding applications across diverse sectors ranging from construction to household products. Recently, there is growing interest in the chemical recycling of polyurethane via catalytic hydrogenation to recover anilines and polyols. However, examples of heterogeneous catalysts are lacking despite their practicality for scale-up to a commercially relevant level. Herein, the conversion of model carbamate compounds is investigated using different metal-oxide catalysts, with CeO₂ exhibiting the best activity and achieving the highest yield of aniline products (up to 100% conversion and 92% yield of anilines). A volcanic correlation is found between the acidity of the metal-oxide catalysts and their activity in cleaving the carbamate bond. The high activity of CeO₂ may be primarily attributed to a low oxygen vacancy formation energy and highly redox active Ce³⁺/Ce⁴⁺ pairs. Based on control reactions under different conditions and in situ NMR studies, a mechanism for carbamate bond dissociation on CeO₂ was proposed. Notably, both solvent-free hydrogenation and hydrogen-free transfer hydrogenation approaches may be utilized to depolymerize various commonly encountered polyurethane (thermoplastic and thermoset) products using CeO₂.