There was a large Catalanist movement in Catalonia through the Second Republic, and Lluís Companys, President of the Catalan Generalitat, did declare an "independent Catalonian state within a Spanish Federal Republic" in the midst of the 1934 Asturian revolt. Although this didn't really succeed, the independence movement still managed to stay fairly strong and Catalonia was allowed autonomy under the Second Republic's Popular Front government.

The revolutionaries (mostly CNT, but also from the POUM and UGT) in Catalonia were a lot more powerful than the central government at the outbreak of war, with some 40,000 armed CNT militants in Barcelona compared to only 5,000 police forces under government control. Upon seizing the area, the CNT leaders realised that unity was important for fighting against Franco. Lluís Companys did reportedly offer up the entire Generalitat to the CNT, however the CNT instead opted to negotiate for a joint Central Committee of Anti Fascist Militias, which held all effective power up until the May Days of 1937, and was made up of members of the major parties in the region. Although the Generalitat did remain, it was more of a formality than anything else.

The CNT feared that, even though a large portion of its membership demanded immediate libertarian communism, if they did follow through, which they could given their strength in the region, the economy would be crushed and so would the revolution. This was because they realised that if they did, the Madrid government would be very hostile to them, as well as foreign parties who were concerned with the region, as well as the existing and ever present threat of Franco's advancing forces.

Full Catalonian independence just wasn't a viable option during the war; the bourgeois independence movement, the Catalan Republican Party, didn't hold the power at the outbreak of the war, and the major power holder, the CNT, did not wish to declare independence. They both saw it as a harmful move in the context of the civil war.

Sources:

Beevor, Antony. The Battle for Spain: The Spanish Civil War

Preston, Paul. The Spanish Civil War: Reaction, Revolution and Revenge

Catalonia did declare independence twice before the war, in 1931 and in 1934, but spain inmediately contested those, first by negotiation and then by armed repression. A very important issue is that the CNT, at first, wasn t at all against independence, and one of its most important and influential leaders, Salvador Seguí (also known as "el noi del sucre" or "sugar boy") even publicly declared for it. He was assassinated in the late 1920s by an unknown gunman, and controversy continues even today. Personally i do think that his warmth towards independentism made him just too dangerous to be alive. After his death, the aragonese anarchists took over (durruti and the others) and this relationship moslty ended.

Another point is that before the war, some of the most important leaders of the catalan armed organisations, like the Badia brothers, were assassinated (it happened a lot), sometimes by "anarchists", sometimes by falangists or others. President Macià himself represented the early 20th century catalan independence movement, which was very much in favour of armed struggle, unlike now. President companys was not exactly independentist himself, and in the previous years clashed constantly with Macià over this issue.

When the war started then, the leadership of the main institutions and organisations was in firm control of either "hispanic anarchists" and/or "federalist catalanists", and although independentists were everywere, even to the front (Macia-Companys militia columns fighted in the aragon front alongside the POUM and cnt-fai) they had no real position of power to influence policy.

There was, however, several plans the catalan government studied for becoming a protectorate under france or uk, offering them bases in catalonia, but just at what happened to the basques, the talks werent serious enough and the issue was dropped.

And interestingly, the Estat Català party (independentist party started by president Macià) seemed to have had plans for a coup against companys during the war.

Had the war started with Macià as president and the Badía brothers as commanders of the security forces Catalonia very likely would have declared independence even in 1936