In connection with the first regeneration method mentioned above, oxidizing conditions may be avoided by heating the solution (a) under a vacuum of from 0.2 to 20 cmHg for about 1 to 48 hours, (b) in an inert atmosphere such as nitrogen or argon for about 1 to 72 hours, or (c) in a reducing atmosphere such as hydrogen for from about 1 to 72 hours, with or without the presence of a reduction cataly