twenty-one vs xxi

twenty-one is a noun but xxi is not a noun.

Word	Noun	Adjective	Verb	Adverb
twenty-one	Yes	Yes	No	No
xxi	No	Yes	No	No

As adjectives, twenty-one and xxi are synonyms defined as:

Other synonyms of twenty-one include 21.

twenty-one (noun)	xxi (noun)
a gambling game using cards; the object is to hold cards having a higher count than those dealt to the banker up to but not exceeding 21
the cardinal number that is the sum of twenty and one

twenty-one (adjective)	xxi (adjective)
being one more than twenty	being one more than twenty