It is a very long journey, not possible in a day. There is a night train between Vienna and Split but it only runs on Wednesdays, Fridays and Sundays.

There are engineering works in Cologne which don’t help either.

Here is a possible route :

• ICE Liège-Guillemins - Frankfurt Airport 15:14 - 17:17. You can also leave earlier with a few extra changes
• ICE Frankfurt Airport - Stuttgart Hbf 18:09 - 19:22 4.50€ optional seat reservation for the journey (2 trains, very recommended)
• Nightjet Stuttgart Hbf - Zagreb Glavni Kol 20:29 - 10:39 book ASAP on tickets.oebb.at : add Interrail as a discount and select one-way tickets. Different options from couchettes to private sleepers (a seat is not recommended)
• 16th July : IC Zagreb Glavni Kol - Split 15:19 - 21:40

Or another route :

• 14th July : Nightjet Liège - Vienna 20:33 - 09:46 book ASAP on tickets.oebb.at : couchettes, sleepers available
• updating...

Use bahn.com for planning, very reliable.

Thank you for your answer, but I need the other direction Split-Liège

Ah yes sorry… then it’s easy as it’s the same route the other way around

• Saturday 15th July : Split - Wien Hbf 17:20 - 09:21 55€ seat (not recommended) or 90€ sleeper (couchettes sold out). Book ASAP
• Sunday 16th July : Wien Hbf - Nürnberg Hbf 10:13 - 14:30
• Nürnberg Hbf - Frankfurt (Main) Hbf 15:33 - 18:04
• Frankfurt (Main) Hbf - Liège-Guillemins 18:28 - 21:44

Option 2 :

• Saturday 15th July : Split - Zagreb Glavni Kol 08:14 - 14:55
• Zagreb Glavni Kol - Stuttgart Hbf 19:38 - 08:37. Book ASAP : multiple options avaialble from seats to sleepers
• many connections to Liège : arrival 12:44 (tight) or 14:56 or later

Surely more options, check bahn.com