Model-Based Design for Chemical Recycling of Polyamide through Ammonolytic Hydrogenation

Consecutive ammonolysis and hydrogenation are promising chemical recycling routes for the depolymerization of polyamide (PA). A challenge is the design of the reaction conditions, leading to sufficiently fast unzipping with limited side reactions. In the present contribution, a kinetic model is developed, consisting of 15 reactions, with the rate coefficients tuned for a given catalyst combination. This is done considering, in a first step, the isolated ammonolysis and, in a second step, the isolated hydrogenation of the small-molecule compounds N-hexylhexanamide and hexanamide, respectively. In a third step, the mixed small-molecule system is considered, and, in a fourth step, diffusional limitations are taken into account for the ammonolysis with PA chains to facilitate a good description of the balance of scission and recombination reactions. To validate the kinetic model, size exclusion chromatography (SEC) traces of the polymer at various stages of the ammonolysis-hydrogenation reaction are modeled and systematically compared with experimental data. It is demonstrated that the kinetic model provides a first step toward the in silico selection of the most suited ammonolysis-hydrogenation reaction conditions for the efficient recycling of PA in view of scale-up targets. This achievement brings depolymerization technology closer to a wider application in our striving for a circular implementation of this important PA waste stream. Further investigation of catalyst concentration and carrier dependencies, as well as the presence of contaminations, is, however, necessary.