[ThomPete]

"National banknotes are generally legal tender, meaning that medium of payment is allowed by law or recognized by a legal system to be valid for meeting a financial obligation."

So yes it requires a central bank as it's through them the recognition happens. Philosophically all money requires is trust no matter what not even a legal entity as long as people trust it, but practically to claim that central banks aren't needed for FRB is wrong to the best of my knowledge.