Ammonolytic Hydrogenation of Secondary Amides: An Efficient Method for the Recycling of Long-Chain Polyamides

Abstract

Long-chain polyamides (LCPAs) are depolymerized by ammonolysis of the secondary amide bonds into building blocks for new polymer synthesis. Research on the ammonolysis of PA is quite limited and often proposes harsh reaction conditions with homogeneous catalysts. Here, we describe a heterogeneous catalytic system based on Nb2O5. Reactions were performed at a relatively low temperature of 200 °C in cyclopentyl methyl ether (CPME) as a green solvent and with limited addition of NH3 and H2. The ammonolysis of secondary amides was extensively studied, initially with N-hexylhexanamide as a model compound. Since ammonolysis is an equilibrium reaction, it was eventually coupled to a hydrogenation process, with addition of a RuWOx/MgAl2O4 hydrogenation catalyst, in order to achieve complete depolymerization. Industrial LCPA samples were successfully and completely depolymerized to α,ω-diamine monomers and oligomers, resulting in product distributions of 62% primary amines and 36% secondary amines. The catalytic system was proven to be very robust against a variety of contaminants, e.g., fillers, other plastics, and additives. Only sulfur-containing compounds poison the Ru-catalyst and have to be removed completely.