transit visa for istanbul-Bejing-Seoul flight

Isa, your itinerary is definitely eligible for 24 hour visa free transit and you do not need any kind of visa. However, if you plan to exit the airport, you have to be aware that you will need to apply for a temporary entry permit upon your landing in Beijing. The issuing of the temporary entry permit is not a passenger's right, but it should be treated more as a bonus that is offered to those with long layovers, especially an overnight ones, those traveling with small children and disabled passengers. In any case, since your layover is a very long one, your chances to be granted a temporary entry permit are very good, but not guaranteed.

Upon your landing in Beijing the first thing that you will do is to approach 24/144 hour visa free transit counter where you will apply for a temporary entry permit by presenting your flight tickets. This application is free of any charge. After a short check, your permit will be granted and after that you will head to the immigration and then to the city/hotel to enjoy your time on your own. Have a nice journey.

Also, please, use correct terminology because this is not 24 hour transit visa, but 24 hour visa free transit, so there is no visa of any kind at all.