Hydroformylation


The hydroformylation of alkenes, discovered in 1938 by Otto Roelen at Ruhrchemie, Oberhausen, Germany , is one of the largest applications of homogeneous transition metal catalysis in industry. Worldwide production capacities exceeded 8 mio t/year in 1995. Industrial hydroformylation catalysts are based on cobalt and rhodium complexes exclusively, with the rhodium processes operating at much milder conditions (typically 80 °C, 15 bar) than the cobalt processes (typically 200 °C, 300 bar). The aldehydes formed by the addition of hydrogen and carbon monoxide to the double bond are important precursors for e.g. plasticizers and detergent alcohols. Besides the desired linear n-aldehydes, branched iso-aldehydes are obtained in a parallel reaction as depicted in Scheme 1.

Hydroformylation

The selectivity of the catalyst (referred to as n:iso ratio or linearity (in %)) can be influenced by the use of ligands, mainly based on phosphines (TPP 1, PBu3 2, Xanthene 3) and phosphites (P(OPh)3 4, biphephos 5). A selection of ligands is shown in Scheme 2.

Ligand structures: TPP 1, PBu3 2, Xanthene 3, P(OPh)3 4, biphephos 5

In 1984, a biphasic propene hydroformylation process, introduced by Ruhrchemie-Rhône-Poulenc (RCH/RP), made use of the immiscibility between an aqueous phase containing the water-soluble Rh-complex and the organic product phase. Simple product separation and quantitative catalyst recovery were achieved in a decantation unit, thus overcoming the drawbacks of homogeneous catalysis. Ever since, a variety of immobilization concepts for hydroformylation catalysts have been studied, ranging from biphasic fluorous phase over micellar systems to supported catalysis.

First investigations of the rhodium-catalyzed hydroformylation in room temperature liquid liquids were published by Chauvin et al. in 1995. Here reaction of 1-pentene with the catalyst system Rh(acac)(CO)2/triarylphosphine was carried out in a biphasic reaction mode using [BMIM][PF6] as the ionic liquid, according to Scheme 3.

Hydroformylation of Pentene

In this study it was not possible to combine high activity, complete retention of the catalyst in the ionic liquid and high selectivity for the desired linear hydroformylation product, with any of the well-established ligands tested. The use of TPP 1 resulted in significant leaching of the Rh-catalyst into the product phase, thus resulting in catalyst activity in both phases. The catalyst leaching could be suppressed by the application of the same sulfonated triarylphosphine ligands (e.g. TPPMS 6) that are used with great success in aqueous biphasic catalysis (e.g. RCH/RP process).

Ligand structures: TPPMS 6, TPPTS 7, BINAS 8, SX 9, Nixantphos 10

Since these early examples of ionic liquid biphasic catalysis a variety of novel ionic liquids and ligands have been developed and the hydroformylation was carried out successfully. The sulfoxantphos ligand SX 9 depicted in Scheme 4 proved to give high linearity towards the desired n-aldehydes at acceptable rates (TOF) biphasic systems. This ligand is the standard ligand in the current SILP hydroformylation projects.

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