A very early patent application for the ”production” (actually the chemical separation of uranium fission products) of neptunium and plutonium was filed in 1942. It was not published, probably because of security considerations until 1960: Process for the production of elements 93 (neptunium) and 94 (plutonium).
Since then there have been many patent applications regarding the chemical processing of neptunium for example:
References:
All patent information has been obtained from Espacenet (European Patent Office).