Frankfurt on the Main vs urban center

Frankfurt on the Main and urban center both are nouns.

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

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