US Citizen traveling from Thailand to Shanghai for 24 hour stay.

I will be staying in Bangkok, Thailand for one week. I learned that I must attend a meeting in Shanghai. I will be in Shanghai for less than 24 hours. I will then return to Thailand for two days and then return to America. Airline tickets (US to Thailand and Thailand to China) are separate tickets on different airlines. Do I need a visa for this short period? Thank you!