Process
In typical industrial process the alcohol soluble catalyst along with soluble alkene prochiral compound is introduced in autoclave under reaction conditions of 50°C and 3 atm H2. Since the products obtained are insoluble, separation is easier. The optical yield is 95%.
Hydroformylation
The hydroformylation reaction involves conversion of alkene to aldehyde in the presence of CO and hydrogen. The reaction is given as
The process is carried out over cobalt or rhodium complex based catalysts. Both linear and branched chain products can be produced. Linear aldehydes are more valuable feed stock for plasticizers and linear alcohols. Hence, the main objective of process is to have high selectivity for more useful linear product.
Cobalt complex catalysts
In cobalt complex catalyzed hydroformylation reaction the catalytically active form is HCo(CO)4 complex. The sequence of hydroformylation process for linear aldehyde formation is shown below.
The first step is the replacement of CO ligand by an alkene. In the next step hydride migration to alkene occurs producing alkyl cobalt complex in which alkyl may be either linear or branched. In the subsequent step one incoming CO occupies the vacant site. It is followed by migration of the alkyl to a co-ordinated CO to give an acyl-complex. In the final step dihydrogen reacts with acyl complex to form aldehyde product and regenerate the starting hydrido-cobalt-carbonyl complex. In cobalt catalyzed hydroformylation the hydrogenation step is rate determining.
Phosphine modified Co catalysts
The performance of the cobalt complex is significantly modified when associated with tertiary alkyl phosphines ligands. The selectivity to linear products is increased significantly. But the reaction becomes slower, and as a result reaction needs to be carried out at a higher temperature. The intermediate carbonyl complex formed is more stable and hence the process can be carried out at lower pressure. In case of tetra cobalt carbonyl complex HCo(CO)4, high pressure is needed to prevent decomposition of carbonyls to metal and CO. The phosphine complexed catalysts can also catalyze hydrogenation and consequently cocurrent hydrogenation of aldehyde to alcohol, which is usually the desired final form.
Process
Hydroformylation of both higher and lower alkenes can be carried out using HCo(CO)3 L complex. Reaction is done in two stages as shown in Fig. 2, to limit the hydrogenation of alkene. In the first reactor, low hydrogen partial pressure is used, and in the second reactor high hydrogen partial pressure is used to ensure hydrogenation of aldehyde to alcohol. The unreacted alkenes and gases are recycled back to the first reactor. Multiple distillation columns are used to separate the unreacted reactants and catalysts. Catalysts remain in the bottom phase of the distillation column and recycled back. The aldehydes and alcohols are collected as overhead products from the distillation tower.
Fig 2. Process flow diagram hydroformylation process.