Process for the preparation of a mixture of epsilon-caprolactam and/or epsilon-caprolactam precursors

Process for the preparation of a mixture of ε-caprolactam and/or ε-caprolactam precursors from 5-formylvalerate ester and/or 5-formylvaleric acid, ammonia and hydrogen in the presence of a hydrogenation catalyst, wherein the process is conducted in a reactor of which the internal wall material is an inert material