From a theoretical standpoint, any group's attempt to break away from a larger political entity is hard to argue with. Ultimately, though, the principle of self-determination, taken to its logical extreme, would mean dissolving the state and replacing it with voluntary associations. Which is why some classical liberals have taken a more limited view of self-determination.
Ludwig von Mises, for instance, argued in Liberalism that the only defensible self-determination was that of individuals, not nations, and since it is impracticable on technical grounds, it is necessary to accept the will of the majority in a given territory.
Whatever one's position, when considering the Catalan issue one should take into account that the region's nationalist movement places a collectivist notion, the "Catalan nation", above individual rights.