Transit at Beijin capital Airport

I'll travel to USA from Ulan-bator Mongolia. It is located in Beijin capital airport arrival from Ulan-bator to Beijin by MIAT (Mongolian Airline) at 9.45am and fly to America by UA(United Airline) 12.00 am, and I'm stopping in PEK over for two hours. Will I have enough time? Will I need to re-register our luggage? Thank you