ad libitum vs impromptu

ad libitum is not a noun while impromptu is a noun.

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

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

Other synonyms of ad libitum include ad lib, spontaneously.

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

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

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