Ionic Liquids, Catalyst Recycle, Selectivity, and Product Separation

Ionic Liquids, Catalyst Recycle, Selectivity, and Product Separation 

The most used ionic liquids in the hydroformylation are those based on the 1-n-butyl-3-methylimidazolium cation, in particular, associated with hexafluorophosphate anion (BMI PF, see Table 6.1). However, the activity and selectivity in the hydro-formylation of 1-hexene catalyzed by rhodium-TPPTS complexes in BMI BF4 were much higher than those reported in other ionic liquids. Under optimum conditions, the TOF of 1 -hexene and selectivity for aldehyde were 1508 h and 92%, respectively. The high activity of the catalyst was ascribed to the much higher solubility of hydrogen [27] and rhodium-TPPTS complexes in BMI BF4 than in BMI PF ]28]. Other ionic liquids have also been used, such as those containing polyether chains attached to ammonium salts 2 (entries 7 and 8, Table 6.1) ]14], phosphonium salts (entry 3, Table 6.1) [11], tris[oxoethyl(trimethyl)ammonium]triazine derivatives 

11 (entry 18, Table 6.1) [24], l-aIkyl-3-methylimidazolium tosylates 13 (entry 19, Table 6.1) [25], and organostannate melts (entries 20-22, Table 6.1) [26,29]. 

The observed reaction rates for the hydroformylation in ionic liquids are usually quite superior to those performed in aqueous-phase regimes, and the selectivities are similar to those obtained in homogeneous conditions. 

The products - mainly aldehydes and unreacted or isomerized alkene - are simply removed by decantation and filtration, extraction with the aid of an organic solvent immiscible with the ionic liquid or using supercritical carbon dioxide, and distillation. 

---