Two commercial routes to IPA are used. The older method is based on the indirect hydration of refinery-grade propylene using sulphuric acid to form isopropyl sulphate which is then hydrolysed with steam to form sulphuric acid and IPA. The crude IPA is distilled to the desired purity.
A more modern route is the direct hydration of chemical grade (90-99%) propylene avoiding the need for sulphuric acid. Propylene and water are heated and the liquid-vapour mixture under pressure passes into a trickle flow reactor containing sulphonated polystyrene cation ion exchange resins. Alternatively, the reaction can be carried out in the gas phase over a phosphoric acid-based fixed bed catalyst. There is also a liquid phase route employing a soluble tungsten catalyst. The IPA is obtained from the aqueous solution by distillation.
A very small amount of IPA is produced by the hydrogenation of acetone in the liquid phase. This process is only suitable where excess acetone is available.