From Infidel753:
I think each case should be judged not based on some general rule about secession, but on how the individual case would affect freedom and justice. So, for example, seceding from an authoritarian state to create a democracy (Baltics 1990, for example) would be just, but seceding in order to preserve slavery (Confederacy 1861) would clearly be unjust. The US in 1776 was justified by this criterion since it created a representative government for people who had not been allowed democratic rights under the previous imperial rule — the thirteen colonies had no representation in the British Parliament. Secession by Southern states today (something that was muttered about by some during the Obama administration) would be unjust because black people in those states would very likely suffer suppression of their civil and political rights, while gays and non-Christians might well become targets of outright persecution.
Catalonia is a difficult case to judge under that or any other criterion.