The constitution from Spain allows the independence of any of its parts.
The state of Spain can not allow Catalonia to leave Spain, because that would force the rest of Spain to leave the European Union (due to economics), thats not my opinion, its the opinion of a PP exminister.
The last time that Catalonia declared independence, which was in the 1930s, the Spanish civil war came (1936) and the dictator Franco allowed the nazis to take the president of the Catalonian republic and they tortured and killed him. I was not there, but apparently Catalonia has been forced to be part of Spain a few times in history.