urban center vs Frankfurt on the Main

urban center and Frankfurt on the Main both are nouns.

Word	Noun	Adjective	Verb	Adverb
urban center	Yes	No	No	No
Frankfurt on the Main	Yes	No	No	No

urban center (noun)	Frankfurt on the Main (noun)
a large and densely populated urban area; may include several independent administrative districts	a German city; an industrial and commercial and financial center