Homogeneous rhodium catalysed hydrocarbonylation reactions of alkenes
The conversion of ethene, carbon monoxide and water to propanoic acid by the catalytic action of various rhodium species, was investigated by infra-red spectroscopy and autoclave batch reactions. Utilisation of a high pressure infra-red cell and a Fourier Transform infra-red spectrometry enabled study of the spectra of the catalytic systems at temperatures up to 180ºC, and at pressures of 10 -150 bar. The batch autoclave reactions enable rates, yield and selectivities to be determined with the aid of Gas Chromatography and Mass Spectroscopy. Rhodium halides were used as precursors and the effects of solvents, water, iodide initiators, were studied. It was shown that polar solvents favoured hydrocarbonylation as did hydrous conditions. Ethyl iodide and hydrogen iodide were active initiators, however hydrogen iodide was observed to poison the catalyst forming [Rh(CO)(_2)I(_4)](^-) and other rhodium(III) species. At temperatures above 170ºC rhodium(I) species such as [Rh(CO)(_2)I(_2)](^-) were more prominent. A novel catalyst system was produced by employing rhodium(III) chloride, tin(II) chloride and hydrochloric acid. The nature of this system was studied in terms of solvents, the interaction of rhodium and tin, and the role of hydrogen chloride. Infrared studies revealed rhodium-tin species in the reaction media. Studies were conducted to ascertain the role of cations and additives. Further studies determined the activity of other metals as co-catalysts to rhodium, showing tin to be a superior co-catalyst.