As you just said, you don't know much about Catalonia. 90% of their exportations are directed toward EU, if EU decides to tax them or close economicals borders, they are fucked. They have litteraly 0 support world-wide except Scotland (LUL). They will also lose all trade agrements fixed by Spain, the whole healthcare system, the military protection, a lot of fundings from the EU institutions all across the country (medical, social, education, etc). They have also huge debts to pay to the EU : with new exchange rate and growing inflation, they will have to pay much more. Without them, they are FUCKED.
EDIT : also as I said, since they use the euro illegally, nobody will make any agreements with them because nobody want to trust a country with such methods, and nobody want to fight the EU over something as insignifiant as Catalonia.
Your example is also wrong : it's not like China making law for EU because the euro is the property of the European Central Bank. They have all the right in the world to vote laws about it, effective in the whole world.
And why would EU support separatism in their own organisation, are you dumb ? They want unity above all.