Rhoduim phosphine catalysed hydroformylation

[摘要] English: The aim of this study was to investigate hydroformylation reactions using phosphine modified rhodium catalyst systems. Comparisons between the traditional monodentate PPh3 ligand and the bidentate xantphos ligand were performed. Xantphos was chosen due to its capability of producing high normal:isomer (n:iso) ratios and linearities resulting from its configuration (wide bite angle). Another benefit of using xantphos as a ligand of choice is the inhibitor resistance it confers to the rhodium catalyst. Unfortunately xantphos, a bidentate ligand, results in the formation of low activity hydroformylation catalysts. It was therefore decided that PPh3 and xantphos be used together with the aim of harnessing the benefits of both ligands, i.e. high rates from PPh3 as well as high selectivities and inhibitor resistance from xantphos. Both NMR and IR spectroscopic studies were performed for the characterisation of these catalytic species. Due to advancements in spectroscopic technology, HP-NMR and HP-IR experiments could be carried out under actual hydroformylation conditions which allowed the study of the actual catalytic species involved during hydroformylation reactions. High pressure autoclave experiments were conducted to investigate the kinetics and selectivity of the three different catalyst systems, i.e. [RhH(CO)2(PPh3)2], [RhH(CO)2(xantphos)] and the mixed species [RhH(CO)2(PPh3)(xantphos)]. Unless otherwise stated, typical conditions employed were 90 °C and 20 bar syngas (H 2:CO = 1:1 [Rh(acac)(CO)2] was also employed as the catalyst precursor together with the ligand(s) of choice . It was found that in a mixed system, where both PPh3 and xantphos were employed in one reactor, the higher the PPh3 concentration the lower the selectivity towards the linear product and the higher the reaction rate. Conversely, higher xantphos concentrations led to higher selectivities but lower rates. Equally importantly is the stability of the catalyst especially when there are components in the feed that might have a negative impact on the catalyst. For instance, the presence of acids in the feed might lead to heavy products formed at the expense of the intended product. Other components inhibit the catalyst making it inaccessible to hydroformylation either temporarily or permanently. The inhibitory effect of methyl vinyl ketone (MVK) on the selected catalyst system was investigated whilst varying the amount of both PPh3 and xantphos with the aim of shifting the equilibrium either to the right or left hand side following the scheme below: (see PDF). The more xantphos species formed, the less the inhibition time thus the equilibrium is shifted to the right hand side. The more the PPh3 concentration the longer the inhibition time, resulting from the population of the [RhH(CO)2(PPh3)2] species which is sensitive to MVK inhibition.

[发布机构] University of the Free State