Rhoduim phosphine catalysed hydroformylation

Loading...
Thumbnail Image
Date
2005-12
Authors
Mokheseng, K. M.
Journal Title
Journal ISSN
Volume Title
Publisher
University of the Free State
Abstract
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.
Afrikaans: Die doel van die studie was om hidroformilerings-reaksies te ondersoek wat gebruik maak van fosfien gemodifiseerde rodium katalis sisteme. Vergelykings tussen die algemene monodentate PPh3 ligand en die bidentate xantphos ligand is getref. Xantphos is gekies aangesien dit in staat is om hoë normaal:isomeer verhoudings en hoë lineariteit te lewer as gevolg van sy konfigurasie (wye ko-ordinasie hoek). Nog ’n voordeel om xantphos as eerste keuse ligand te gebruik is die inhiberings weerstand wat dit aan die rodium katalis verleen. Ongelukkig lei xantphos, ‘n bidentate ligand, tot die vorming van hidroformilerings-kataliste met ’n lae aktiwiteit. Daar is dus besluit om PPh3 en xantphos te kombineer met die doel om die voordele van beide ligand sisteme te probeer vasvang, i.e. die hoë tempos van PPh3 sowel as die hoë selektiwiteite en inhiberings weerstandigheid van xantphos. Beide KMR en IR spektroskopiese studies is gedoen vir die karakterisering van hierdie katalitiese spesies. As gevolg van vooruitgang in spektroskopie tegnologie, kon hoëdruk-KMR (HD-KMR) en hoëdruk-IR (HD-IR) eksperimente onder werklike hidroformilerings kondisies uitgevoer word wat in staat gestel het om die werklike katalitiese spesies wat betrokke is tydens hidroformilering te ondersoek . Hoëdruk outoklaaf eksperimente is uitgevoer om die kinetika en selektiwiteite van die drie verskillende katalis sisteme, i.e. [RhH(CO)2(PPh3)2], [RhH(CO)2(xantphos)] en die gemengde spesie [RhH(CO)2(PPh3)(xantphos)], te ondersoek. Tensy ander gestel was die tipiese kondisies gebruik 90°C en 20 bar singas (H2:CO = 1:1) [Rh(acac)(CO)2] was altyd gebruik as die katalis uitgangstof tesame met die ligand(e) van voorkeur. Dit is gevind dat in ‘n gemengde sisteem, waar beide PPh3 en xantphos in een reaktor gebruik is, hoe hoër die PPh3 konsentrasie hoe laer die selektiwiteit ten gunste van die lineêre produk en hoe hoër die reaksie tempo. Omgekeerd, hoër xantphos konsentrasies lei tot hoër selektiwiteite maar stadiger reaksie tempos. Ewe belangrik is die stabiliteit van die katalis veral wanneer daar komponente in die voer is wat ’n negatiewe impak op die katalis kan hê. Byvoorbeeld, die teenwoordigheid van sure in die voer kan aanleiding gee tot die vorming van hoë koolstof getal produkte ten koste van die verlangde produk. Ander verbindings inhibeer die katalis wat dit ontoeganklik maak vir hidroformilering, tydelik of permanent. Die inhiberings effek van metiel viniel ketoon (MVK) op die gekose katalis sisteem is ondersoek terwyl beide die hoeveelhede PPh3 en xantphos gevarieer is met die doel om die ewewig na regs of links te skuif volgens die onderstaande skema: (sien PDF) Hoe meer van die xantphos spesie gevorm is, hoe min der die inhiberings tyd en die ewewig is dus na regs verskuif. Hoe meer die PPh3 konsentrasie hoe langer die inhiberings tyd as gevolg van die vermeerdering van die [RhH(CO)2(PPh3)2] spesie wat meer sensitief is vir MVK inhibering.
Description
Keywords
Rhodium, Cobalt, Homogeneous catalysis, Hydroformylation, Triphenylphosphine, Bidentate ligands, Xantphos, Kinetics and selectivity, Inhibition, Methyl vinyl ketone, HP-NMR, HP-IR, Dissertation (M.Sc. (Chemistry))--Universtity of the Free State, 2005
Citation