144 hour Transit Visa. I am arriving at Xi'an Airport

then the following day visiting various sites from there . The day after I travel on the Bullet Train to Beijing where I will spend a further 2 days before boarding a cruise ship . I have been informed that it may be possible to obtain a 144 hour free transit to cover my stay. Is this correct or is the maximum free transit 72 hours. If this is the case please confirm that I will need to apply for a normal Visa?