HI, I was wondering if I can use 72h transit time in Beijing.

I am travelling from UK to PEK on one ticket and then to Hongkong on a different ticket and airline. Basically, I am not sure, first, if I can use 72h policy if I have a separate ticket. Also I have not booked my second flight yet but I have an option to fly after 72h of landing. IS it possible to wait at the airport until my flight? (landing in Beijing at 3:30 PM, flight to Hongkong at 5:30PM 3 days after 74h difference).

Thanks for your reply