Monsanto and Cativa processes
/ mon-SAN-toh and kuh-TEE-vuh /
Vinegar is dilute acetic acid, and acetic acid is one of the workhorse bulk chemicals of industry — billions of kilograms a year go into making plastics, paints, and adhesives. The most elegant way to make it is to take methanol (the simplest alcohol, just CH3OH) and stitch one molecule of carbon monoxide onto it: CH3OH plus CO gives CH3COOH. That single carbon from cheap CO turns a one-carbon alcohol into a two-carbon acid. Two famous homogeneous catalytic processes do this — the Monsanto process using rhodium, and its successor the Cativa process using iridium.
Both processes are textbook homogeneous catalysis built from the standard organometallic steps, and both need an iodide promoter to work. In outline: methanol first reacts with the iodide to make methyl iodide, CH3I, which is the real substrate. Methyl iodide undergoes oxidative addition to the metal centre, putting a methyl group and an iodide onto the metal; a carbon monoxide ligand then inserts into the metal-methyl bond by migratory insertion, building a metal-acetyl group; reductive elimination releases acetyl iodide, which reacts with water to give acetic acid and hand the iodide back, regenerating the catalyst. The Monsanto process, introduced in the 1970s, ran on a rhodium complex. The later Cativa process, commercialized by BP in the 1990s, swapped rhodium for iridium. Iridium gave a faster, more robust catalyst that tolerates less water (cutting the energy cost of distillation), wastes less feedstock, and uses a cheaper metal than rhodium.
These two processes matter as a vivid case study in incremental, real-world catalyst improvement and in green chemistry economics. The chemistry barely changed — same skeleton of oxidative addition, CO insertion, reductive elimination — yet switching the metal from rhodium to iridium delivered better selectivity, less by-product, lower energy use, and cheaper metal, which at this scale is worth a fortune. It is a clean demonstration that catalysis research is not only about inventing wholly new reactions but about refining the metal and ligands of an existing cycle. One honest caveat: both are excellent on atom economy in principle (almost every atom of methanol and CO ends up in the product), but they rely on a corrosive iodide promoter and demanding engineering, so green here is about waste and energy, not the absence of hazards.
BP's Cativa plants make industrial acetic acid by feeding methanol and carbon monoxide over a dissolved iridium catalyst with an iodide promoter — the same one-carbon-stitching chemistry as the older rhodium Monsanto process, but cheaper and cleaner.
One CO molecule turns one-carbon methanol into two-carbon acetic acid — and changing Rh to Ir made it pay better.
Neither catalyst acts on methanol directly — the iodide promoter first converts it to methyl iodide, the species that actually adds to the metal. Drop the iodide and the cycle stops, a reminder that promoters are not optional extras.