Improved process for preparation of 1, 5-pentanediisocyanates from free 1, 5-pentanediamine

The invention provides a method for preparing free PDA by desalting PDA hydrochloride obtained by biological fermentation. The method comprises the following steps: converting lysine hydrochloride into PDA hydrochloride by using lysine decarboxylase; preparing the obtained PDA hydrochloride into an aqueous solution, and adding alkali metal hydroxide into the aqueous solution; evaporating under reduced pressure to remove most water; adding a certain amount of lower aliphatic alcohol; filtering out generated chloride; evaporating under reduced pressure to remove most solvent; optionally, repeating the steps of adding the alcohol, filtering out the chloride and evaporating the solvent under reduced pressure for at least one time; and obtaining a free PDA containing a small amount of the alcohol and a very small amount of water. The invention also provides a method for preparing 1, 5-pentamethylene diisocyanate (PDI) from PDA hydrochloride obtained by biological fermentation, and the method comprises the followin