The Cativa process is a method for the production of acetic acid

Acetic acid

Acetic acid is an organic compound with the chemical formula CH3CO2H . It is a colourless liquid that when undiluted is also called glacial acetic acid. Acetic acid is the main component of vinegar , and has a distinctive sour taste and pungent smell...

 by the carbonylation

Carbonylation

Carbonylation refers to reactions that introduce carbon monoxide into organic and inorganic substrates. Carbon monoxide is abundantly available and conveniently reactive, so it is widely used as a reactant in industrial chemistry.-Organic chemistry:...

 of methanol

Methanol

Methanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol...

. The technology, which is similar to the Monsanto process

Monsanto process

The Monsanto process is an important method for the manufacture of acetic acid by catalytic carbonylation of methanol. This process operates at a pressure of 30–60 atm and a temperature of 150–200 °C and gives a selectivity greater than 99%. It was developed 1960 by German BASF and...

, was developed by BP Chemicals and is under license by BP Plc. The process is based on an iridium

Iridium

Iridium is the chemical element with atomic number 77, and is represented by the symbol Ir. A very hard, brittle, silvery-white transition metal of the platinum family, iridium is the second-densest element and is the most corrosion-resistant metal, even at temperatures as high as 2000 °C...

-containing catalyst, such as the complex

Complex (chemistry)

In chemistry, a coordination complex or metal complex, is an atom or ion , bonded to a surrounding array of molecules or anions, that are in turn known as ligands or complexing agents...

 [Ir(CO)₂I₂]⁻ (1).

The Cativa and Monsanto processes are sufficiently similar that they can use the same chemical plant. Initial studies by Monsanto had shown iridium to be less active than rhodium for the carbonylation of methanol. Subsequent research, however, showed that the iridium catalyst could be promoted by ruthenium, and this combination leads to a catalyst that is superior to the rhodium-based systems. The switch from rhodium

Rhodium

Rhodium is a chemical element that is a rare, silvery-white, hard and chemically inert transition metal and a member of the platinum group. It has the chemical symbol Rh and atomic number 45. It is composed of only one isotope, 103Rh. Naturally occurring rhodium is found as the free metal, alloyed...

 to iridium

Iridium

Iridium is the chemical element with atomic number 77, and is represented by the symbol Ir. A very hard, brittle, silvery-white transition metal of the platinum family, iridium is the second-densest element and is the most corrosion-resistant metal, even at temperatures as high as 2000 °C...

 also allows the use of less water in the reaction mixture. This change reduces the number of drying columns necessary, decreases formation of by-products, such as propionic acid

Propionic acid

Propanoic acid is a naturally occurring carboxylic acid with chemical formula CH3CH2COOH. It is a clear liquid with a pungent odor...

, and suppresses the water gas shift reaction

Water gas shift reaction

The water-gas shift reaction is a chemical reaction in which carbon monoxide reacts with water vapor to form carbon dioxide and hydrogen:The water-gas shift reaction is an important industrial reaction. It is often used in conjunction with steam reforming of methane or other hydrocarbons, which is...

.

The catalytic cycle

Catalytic cycle

A catalytic cycle in chemistry is a term for a multistep reaction mechanism that involves a catalyst . The catalytic cycle is the main method for describing the role of catalysts in biochemistry, organometallic chemistry, materials science, etc. Often such cycles show the conversion of a...

 for the Cativa process, shown above, begins with the reaction of methyl iodide with the square planar active catalyst species (1) to form the octahedral iridium(III) species (2), the fac-isomer of [Ir(CO)₂(CH₃)I₃]⁻. This oxidative addition reaction involves the formal insertion

Insertion reaction

An insertion reaction is a chemical reaction where one chemical entity interposes itself into an existing bond of typically a second chemical entity e.g.:...

 of the iridium(I) centre into the carbon-iodine bond of methyl iodide. After ligand exchange (iodide for carbon monoxide

Carbon monoxide

Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...

), the migratory insertion of carbon monoxide into the iridium-carbon bond, step (3) to (4), results in the formation of a square pyramidal

Square pyramidal molecular geometry

In molecular geometry, square based pyramidal geometry describes the shape of certain compounds with the formula ML5 where L is a ligand. If the ligand atoms were connected, the resulting shape would be that of a pyramid with a square base. The geometry is common for certain main group compounds...

 species with a bound acetyl

Acetyl

In organic chemistry, acetyl is a functional group, the acyl with chemical formula COCH3. It is sometimes represented by the symbol Ac . The acetyl group contains a methyl group single-bonded to a carbonyl...

 ligand. The active catalyst species (1) is regenerated by the reductive elimination of acetyl iodide

Acetyl iodide

Acetyl iodide is an organoiodine compound wth the formula CH3CI. It is a colourless liquid. It is formally derived from acetic acid. Although far rarer in the laboratory than the related acetyl bromide and acetyl chloride, acetyl iodide is produced, transiently at least, on a far larger scale...

 from (4), a de-insertion reaction. The acetyl iodide is hydrolysed

Hydrolysis

Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...

 to produce the acetic acid product, in the process generating hydroiodic acid which is in turn used to convert the starting material (methanol

Methanol

Methanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol...

) to the methyl iodide used in the first step.