A Posteriori Separation of Products and Catalysts

As alternative to these methods, several methodologies are known to separate the catalyst and products under the assistance of water after hydroformylation. The hydroformylation itself is conducted under homogeneous conditions. 

For example. Union Carbide claimed the addition of water to the final reaction mixture in Af-methylpyrrolidone (NMP) [28]. Under these conditions, phase separation took place and the Rh(TPPMS) (TPPMS = triphenylphosphine monosulfate) catalyst was completely transferred into the aqueous phase. 

A related principle was established by Eckert and Liotta who advocated the use of organic-aqueous tunable solvents (OATSs) such as water-tetrahydrofurane (THE) mixtures [31]. Upon the effect of COj, the water-THE phase splits into relatively immiscible phases and allows thus the separation of the catalyst and aldehyde [32]. 

---

Carbon dioxide is a ubiquitous and environmentally benign compound. Several attempts have been made to use it as medium or as support for individual steps of the hydroformylation. The acidic properties of CO2 in solution have been advantageously employed to remove homogeneous catalysts with basic properties from the neutral organic reaction products (see Section A Posteriori Separation of Products and Catalysts ). BASF claimed supercritical carbon dioxide (SCCO2) for the extraction of the so-called heavy ends from low-boiling hydroformylation products and rhodium catalyst [55]. In recent years, also implementations of reactions in compressed carbon dioxide in supercritical or near-critical conditions have attracted particular attention [56]. A first review on the application of supercritical fluids (SFCs) in homogeneous catalysis was authored by Jessop, Ikariya, and Noyori in 1999 [57]. Later on, Leitner and Abraham [58,59] provided updates with special focus on the use of compressed carbon dioxide. 

---