Hydroformylation is a chemical process that adds a formyl group and a hydrogen atom to an alkene to form an aldehyde. The process is achieved by heating hydrogen gas, carbon monoxide, and an alkene under pressure with a catalyst. The ratio between the different forms of aldehydes produced can be altered by varying the conditions. Cobalt complexes were the dominant catalysts until the 1960s when rhodium complexes began to replace them. Rhodium complexes allow for the large-scale production of aldehydes used in the manufacture of various products.
Hydroformylation, also known as oxosynthesis, is a chemical process that adds a formyl group and a hydrogen atom to an alkene to form an aldehyde. An alkene is a molecule that contains only carbon and hydrogen atoms with at least one double bond between the carbon atoms. The aldehyde that results from the hydroformylation of a specific alkene is a compound in which at least one of the carbon-carbon double bonds has been replaced with a carbon-carbon single bond and a carbon-oxygen double bond.
The aldehydes that are produced by hydroformylation are a mixture of those with straight carbon chains and those with branched carbon chains. Depending on whether the aldehydes are used, one form may be more desirable than another. The ratio between the two forms can be shifted by altering the conditions used in the process.
Hydroformylation is achieved by heating hydrogen gas (H2), carbon monoxide (CO), and an alkene under pressure. The mixture is stable under these conditions until the addition of a catalyst, a substance which causes or accelerates the chemical reaction of two or more compounds without itself being consumed or modified in the process. Varying the pressure and gas ratio, the temperature of the components, the catalyst used, or any combination of factors can affect the ratios between the different forms of the aldehydes produced.
When Otto Roelen discovered hydroformylation in 1938, he was using a cobalt complex which acted as a catalyst in the reaction. For over 30 years, various cobalt complexes have been the dominant catalysts in the industrial use of this process. Cobalt complexes that use phosphines, or hydrides of phosphorus, as the source of electrons in the reaction allow it to occur at lower pressures and higher temperatures. This increased the ability to vary conditions, making it easier to push a reaction towards the desired shape of the aldehyde produced.
In the 1960s, researchers began looking for catalysts that would give them even greater control over the products of hydroformylation. One approach they took was to investigate the use of other elements in the same transition metal group as cobalt, particularly rhodium and iridium. Rhodium complexes using phosphines allow for the use of both lower temperatures and lower pressures while producing a high ratio of linear to branched aldehydes.
In the 1970s, rhodium complexes began to replace cobalt complexes as catalysts in commercial processes. In 2004, 75% of commercial aldehyde production used rhodium catalysts. This widespread use of rhodium complexes in hydroformylation allows for the large-scale production of aldehydes which are then modified to form compounds used in the manufacture of products such as plastics, detergents, solvents and lubricants.
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