A process for preparing C2 to C4 carboxylic acids from synthesis gas includes introducing a feed stream comprising hydrogen gas and a carbon-containing gas comprising carbon monoxide into a reaction zone of a first reactor, converting the feed stream into an intermediate stream comprising C2 to C4 hydrocarbons in the reaction zone in the presence of a first catalyst, wherein the intermediate stream further comprises carbon dioxide and wherein the first catalyst is a composite catalyst comprising a metal oxide catalyst component and a microporous catalyst component, and converting the intermediate stream into a product stream comprising C2 to C4 carboxylic acids in the presence of a second catalyst in a second reactor. The second reactor can be configured for olefin oxidation or paraffin oxidation.