Catalyst system and process

A catalyst system for a liquid phase carbonylation reaction comprises a homogeneous acid catalyst component and a porous solid component. The homo genius acid catalyst may be selected from sulphuric acid, traffic acid, sulphonic acid, trihyroxysilylethlysulphonic acid, alkyl phosphonic acids, trihydroxysilylethylsulfonic acid, phosphoric acid and formic acid. The solid component may be an un-functionalised porous silica material, or a functionalised porous silica. The solid component may have a surface area of 250-900 m² and a poor volume of 0.2-1 cc/g. The amount of homo genius acid catalyst present in the catalyst system may be from 10 ppm to 25 wt%. A process for the carbonylation of an aldehyde to form carboxylic acid or derivatives thereof comprising the steps of contacting the catalyst with carbon monoxide, water and an aldehyde is also disclosed.