impromptu vs ad lib

impromptu is a noun but ad lib is not a noun.

Word	Noun	Adjective	Verb	Adverb
impromptu	Yes	Yes	No	Yes
ad lib	No	No	No	Yes

As adverbs, impromptu and ad lib are synonyms defined as:

Other synonyms of impromptu include ad libitum, spontaneously.

impromptu (noun)	ad lib (noun)
an extemporaneous speech or remark
a short musical passage that seems to have been made spontaneously without advance preparation

impromptu (adjective)	ad lib (adjective)
with little or no preparation or forethought

impromptu (adverb)	ad lib (adverb)
without advance preparation	without advance preparation