Can I use the 72h free transit if I fly from Beijing to Xian?

Hi, I hold a Spanish passport who plans to visit Xian just for 3days. my Itinerary will be Dubai-Beijing- Xian I will not leave the airport in Beijing. I have told I am entitle of this 72 h free transit as far as I return to Dubai from a third country ,for example Dubai-Beijing-Xian -Hong Kong -Dubai. I want to double check if this information is valid .If not Can I do it stopping in Shanghai instead of Beijing? My only final destination in China will be Xian.
Thank you very much