Water-Soluble Phosphines and Rhodium Recovery

Rhodium is an expensive metal, and the commercial viability of the rhodium-based hydroformylation process depends crucially on the efficiency of the catalyst recovery process. In the past this has been achieved either by a complicated recycle process or more commonly by energy-requiring distillation. A major advancement in catalyst recovery in recent years has been the introduc- 

The protonated form is extractable with organic solvents between pH 0 and — 1, while at higher pH the sodium salt is soluble in water to the extent of 1100 g/liter. The nontoxicity of the ligand (an oral LD50 5 g/kg) is another feature that makes its large-scale industrial use possible. 

The successful commercial use of TPPTS has triggered much research for other water-soluble phosphines. While some of these certainly give highly active and selective catalytic systems, as yet their cost of manufacturing make them commercially unattractive. For example, the fc-phosphine 5.14 gives a catalytic system that is about ten times more active than the one with TPPTS and also shows a higher selectivity (—98%) towards n-butyraldehyde. Its synthesis, however, is a five-step process and involves expensive reagents. 

Another novel technique for separation of the TPPTS-based catalyst is by immobilizing it on a solid support. This catalyst consists of a thin film of an aqueous solution of rhodium-TPPTS complex supported on high-surface-area 

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