Hydroformylation

Hydroformylation, often called oxo synthesis or the oxo process, is a vital industrial chemical reaction.

It is used to produce aldehydes from alkenes. An aldehyde is a type of organic compound that is very useful in manufacturing. These molecules serve as building blocks for many secondary products. For example, aldehydes can be hydrogenated into alcohols. These alcohols are then used to create detergents. Hydroformylation is also essential for making specialty chemicals, including fragrances and pharmaceuticals. Because of its massive impact on manufacturing, it is considered a premier achievement of 20th-century industrial chemistry.

The chemical mechanism involves adding a formyl group and a hydrogen atom to a carbon-carbon double bond.

This reaction requires specific conditions to proceed effectively. Typically, the process uses high pressures between 10 and 100 atmospheres. The temperature is usually maintained between 40 and 200 °C. A transition metal catalyst is required to drive the reaction. In most cases, this is a homogeneous catalysis, meaning the catalyst dissolves directly into the reaction medium. One variation of the process uses formaldehyde instead of synthesis gas, which is a mixture of carbon monoxide and hydrogen.

To understand the molecular steps, we can look at the cobalt-catalyzed cycle.

The process begins when a molecule of carbon monoxide dissociates from cobalt tetracarbonyl hydride. This creates a 16-electron species. Next, an alkene binds to this species to form an 18-electron species. The alkene then inserts into the metal-hydrogen bond to create an alkyl tricarbonyl. A new molecule of carbon monoxide coordinates to the metal, forming an alkyl tetracarbonyl. Through migratory insertion, a 16-electron acyl species is formed. Finally, oxidative addition of hydrogen occurs, and the aldehyde is released through reductive elimination.

A major challenge in hydroformylation is controlling selectivity between "normal" and "iso" products.

When hydroformylating a molecule like propylene, two different isomers can form: butyraldehyde (the normal isomer) and isobutyraldehyde (the iso isomer). The normal isomer has a straight carbon chain, while the iso isomer is branched. Industrial users often prefer the straight-chain normal isomer because it is more stable and useful. Scientists manage this through steric effects and electronic effects. Bulky ligands can create steric hindrance, which pushes the reaction toward the anti-Markovnikov addition. This helps ensure the production of straight-chain aldehydes.

History shows how this field has evolved through discovery and refinement. German chemist Otto Roelen discovered the process in 1938. He was investigating the Fischer–Tropsch process when he noticed ethylene produced aldehydes and diethylketone. Roelen identified that cobalt catalysts were excellent for this reaction. In the 1960s, Richard F. Heck and David Breslow elucidated the specific cobalt-catalyzed mechanism. By 1968, researchers reported highly active rhodium-based catalysts. Since the 1970s, most industrial hydroformylation has relied on these rhodium catalysts because they are more active than cobalt.

Various industrial processes have been developed to handle different types of molecules.

The BASF-oxo process uses cobalt carbonyl-based catalysts for higher olefins. It operates at a pressure of about 30 MPa and temperatures between 150 and 170 °C. The Exxon process also uses cobalt for C6–C12 olefins, operating at 30 MPa and 160 to 180 °C. The Shell process uses cobalt complexes with phosphine ligands for C7–C14 olefins. The Union Carbide process, or low-pressure oxo process, uses a rhodium catalyst in thick oil to process propene at 1.8 MPa. Finally, the Ruhrchemie/Rhône-Poulène process uses a water-soluble rhodium catalyst. This modern two-phase system allows for easy separation of the product from the catalyst.

The Ruhrchemie/Rhône-Poulène process is a highly efficient example of modern chemical engineering.

It uses a rhodium catalyst with a water-soluble TPPTS ligand. This ligand makes the catalyst highly soluble in water but not in the organic product phase. The process uses propene and synthesis gas in a 1.1:1 ratio. It produces a mixture that is mostly butyraldehyde, with a ratio of 96:4 compared to isobutyraldehyde. A plant in Oberhausen reached a production capacity of 500,000 tons of butanal per year. This system is so efficient that less than 1 part per billion of rhodium is lost during the lifetime of a catalyst batch.