Well, do you mean that you have to go from Tianjin Airport to Beijing Airport, or from Tianjin Airport to Beijing downtown area?

Currently, the Tianjin Airport has several direct buses to downtown Beijing. If you miss the bus, please take the frequent airport shuttle to Tianjin Railway Station where you could catch a bullet train to Beijing South Railway Station in 40 minutes. The train information could be search at https://www.travelchinaguide.com/china-trains/

For the return trip, a little difficult, it is because the first bullet train from Beijing to Tian leaves at 06:35.