24 or 72 hours transit?

If I buy a ticket from Japan>Beijing>United States (round trip), will I have any trouble if I have a 22 hour layover in Beijing? Its a round trip, coming back its a 3 hour layover.

For the visa free 24 hour transit, am I able to visit the city of Beijing or do I have to sign up for the 72 hour free transit? I am confused and I hope somebody will get back to me on this
thank you!