High productivity methanol carbonylation catalysis using iridium - The Cativa (TM) process for the manufacture of acetic acid

Methanol carbonylation to acetic acid is catalysed with high rates at low w ater concentrations using an iridium/iodide based catalyst. The catalyst sy stem exhibits high stability allowing a wide range of process conditions an d compositions to be accessed without catalyst precipitation. Two distinct classes of promoters have been identified for the reaction: simple iodide c omplexes of zinc, cadmium, mercury, indium and gallium and carbonyl complex es of tungsten, rhenium, ruthenium and osmium. The promoters exhibit a uniq ue synergy with iodide salts, such as lithium iodide, under low water condi tions. A rate maximum exists at commercially attractive low water condition s, and optimisation of the process parameters gives acetic acid with a sele ctivity in excess of 99% based upon methanol. The levels of liquid by-produ cts formed are a significant improvement over that achieved with the conven tional high water rhodium based catalyst system used in the Monsanto proces s and the quality of the product obtained under low water concentrations is exceptional. The Cativa(TM) process has now been successfully commercialis ed on three world scale plants. (C) 2000 Elsevier Science B.V. All rights r eserved.