New strategies for the rhodium-catalysed aqueous-biphasic hydroformylation of medium chain alkenes

Aqueous-biphasic organometallic catalysis is, as illustrated by the industrial hydroformylation of propene and butene, one of the most promising ways to overcome the intrinsic problem of catalyst separation in organometallic catalysis. However, for poorly water-soluble substrates, mass transfer limitations bring the reaction rate below any that could be economically viable, greatly limiting the scope of this elegant technology. We have studied three different strategies to overcome this limitation. We developed additives that speed up the reaction whilst retaining fast phase separation and good metal retention. Evidence suggests that those additives affect the reaction by forming emulsions with poor stability under the reaction conditions These emulsions increase the interfacial surface area but break after settling for a short time. We also developed ligands that allow the catalyst to be reversibly transported between an aqueous and an organic phase upon addition and removal of carbon dioxide. This allows the reaction to be carried out under homogeneous conditions, only limited by intrinsic kinetics, and the catalyst to be separated by aqueous extraction triggered by carbon dioxide. The catalyst can be returned to a fresh organic phase by flushing out the carbon dioxide. By applying this methodology for the hydroformylation of medium chain length alkenes, very high reaction rates were obtained and the catalyst could be recycle three times with excellent retention of activity and low metal leaching. This methodology could also be reversed with the reaction being carried out in an aqueous phase in the presence of carbon dioxide and extracting the catalyst into an organic solvent using nitrogen flushing. Finally, we briefly investigated the use of an oscillatory baffled reactor as a mean for mass transfer improvement for aqueous-biphasic hydroformylation. This new type reactor did not improve the performance of the system under the investigated conditions, but may require less energy input for equivalent agitation and mixing

New strategies for the rhodium-catalysed aqueous-biphasic hydroformylation of medium chain alkenes

Aqueous-biphasic organometallic catalysis is, as illustrated by the industrial hydroformylation of propene and butene, one of the most promising ways to overcome the intrinsic problem of catalyst separation in organometallic catalysis. However, for poorly water-soluble substrates, mass transfer limitations bring the reaction rate below any that could be economically viable, greatly limiting the scope of this elegant technology. We have studied three different strategies to overcome this limitation. We developed additives that speed up the reaction whilst retaining fast phase separation and good metal retention. Evidence suggests that those additives affect the reaction by forming emulsions with poor stability under the reaction conditions These emulsions increase the interfacial surface area but break after settling for a short time. We also developed ligands that allow the catalyst to be reversibly transported between an aqueous and an organic phase upon addition and removal of carbon dioxide. This allows the reaction to be carried out under homogeneous conditions, only limited by intrinsic kinetics, and the catalyst to be separated by aqueous extraction triggered by carbon dioxide. The catalyst can be returned to a fresh organic phase by flushing out the carbon dioxide. By applying this methodology for the hydroformylation of medium chain length alkenes, very high reaction rates were obtained and the catalyst could be recycle three times with excellent retention of activity and low metal leaching. This methodology could also be reversed with the reaction being carried out in an aqueous phase in the presence of carbon dioxide and extracting the catalyst into an organic solvent using nitrogen flushing. Finally, we briefly investigated the use of an oscillatory baffled reactor as a mean for mass transfer improvement for aqueous-biphasic hydroformylation. This new type reactor did not improve the performance of the system under the investigated conditions, but may require less energy input for equivalent agitation and mixing.EThOS - Electronic Theses Online ServiceEaStCHEMSasol Technology UK LtdGBUnited Kingdo

Aqueous-biphasic hydroformylation of alkenes promoted by "weak" surfactants

The aqueous-biphasic hydroformylation of higher alkenes catalyzed by Rh/TPPTS has been carried out in the presence of imidazolium, pyridinium and triethylammonium salts. High reaction rates are achieved with imidazolium and triethylammonium salts provided that their alkyl "tail" is &gt;= C-8. Fast and complete phase separation, and good retention of the metal in the aqueous phase could be achieved with an octyl "tail". Imidazolium salts were found to give the highest rate enhancement. The nature of the anion showed a moderate influence on the reaction. Evidence suggests that the additive can act as weak surfactant allowing emulsions to be formed and broken by simply switching the stirring on and off.</p

Biphasic and Flow Systems Involving Water or Supercritical Fluids

Two processes are described for improving reaction rates for relatively hydrophobic substrates in aqueous biphasic systems. In the first, 1-octyl-3-methylimidazolium bromide ([Octmim]Br) increases the rate of hydroformylation of 1-octene from 8% conversion in 24 h to full conversion of 1.5 h. Phase separation is fast and catalyst retention is good. 1-Hexyl-3-methylimidazolium bromide gives little rate enhancement, whilst 1-decyl-3-methylimidazolium bromide gives stable emulsions., The mechanism of action of these additives is discussed. In the second approach, functionalising PPh3 with amidine groups allows the rhodium catalysed hydroformylation of 1-octene in toluene with a very high reaction rate. The catalyst can be switched between toluene and water by bubbling CO2 and back into toluene by bubbling N-2 at 60 A degrees C. This switching has been used to separate the catalyst from hydrophobic (from 1-octene) or hydrophilic (from allyl alcohol) aldehydes obtained from hydroformylation reactions. CO2 expanded liquids have been shown to be effective media for transporting substrates and catalysts over supported ionic liquid phase (SILP) catalysts. The advantages offered over all gas phase and liquid phase catalysts are discussed.</p