visa-fee transit

I have some executives who are holding the USA passports, they will depart US to HK for staying Hong Kong for 4 days, then they will travel to Beijing 3-4 days. The will go back to USA directly from Beijing. Can they apply for a 72 hour free transit or 144 hour free transit in due course? How can I calculate the time for this free transit(i.e. is it based on their flight arrival time to BJ and departure flight time leaving from BJ and USA?)