Transit from Phnom Penh to Beijing and then to Mongolia

Hi, I have a flight from Phnom Penh to Beijing the 20th of july.
I would like to take a bus from Beijing directly to Erlian (Erenhot) which is out of the zone, and cross the border to Mongolia by jeep...
I can't book a bus crossing the border so I'm not sure to be able to have a ticket to go out of China at the immagration office.
Do you think it's still possible for the 144h free transit ??