Understanding heterogeneously catalysed transformations.
PhD thesis, University of Glasgow.
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Industrial reactions can either be catalysed by acidic, basic or neutral supported catalysts. The work within this thesis includes two different projects of industrial interest, both of which are catalysed by basic and acidic supported catalysts.
(1) Acrylonitrile is one of the top twenty large-volume commodity chemicals in the world. Nearly every person in the modern world owns something that is made of acrylonitrile. Currently acrylonitrile is synthesised industrially by the ammoxidation of propylene. During this process acetonitrile is produced as by product and is used commercially as a solvent. However, the production of acetonitrile is far greater than demand therefore considerable interest lies in the conversion of acetonitrile to acrylonitrile. In our studies the synthesis of acrylonitrile from methanol and acetonitrile was attempted using magnesium oxide and chromium-doped magnesium oxide catalysts. The catalysts were initially prepared by impregnation methods and then subsequently characterised. It was found that an impregnation of the magnesium hydroxide by chromium salt decreased the phase transformation temperature from magnesium hydroxide to magnesium oxide and yielded larger crystallite sizes. Using the chromium-doped magnesium oxide catalyst the reaction between acetonitrile and methanol gave 100% selectivity towards acrylonitrile. It is suggested that CrVI/V species play an important role in this reaction and act as a stabiliser for the acetonitrile carbanion. Further study showed that the main deactivation route was the reduction of the chromium from CrVI/V to lower oxidation states and the deposition of coke. It was found that over the course of a year the Cr/MgO catalyst significantly aged. Because the extent of ageing was so significant, it was decided to cease work on this project as it was of concern that the relationship between structure and activity would be difficult to rely on.
(2) Hydrogen is one of the clean sources of energy which is currently obtained by the steam reforming of non-renewable fossil-fuel resources. However the rapid depletion of fossil-fuel resources has spurred further research into alternative and renewable H2 sources. Among the many different renewable sources available for H2 production, the steam reforming of bioethanol has attracted significant interest in recent years. However, crude bioethanol contains organic impurities which may deactivate the catalyst more rapidly than the pure ethanol. Therefore in the current project we have examined the tolerance of pure Al2O3 and Al2O3 supported noble metal (Rh, Ru and Pt) catalysts to the different impurities present in crude bioethanol. The direct use of crude bioethanol in the steam reforming reaction could result in a huge saving in capital expenditure for an industrial plant, as huge capital costs are associated with the distillation of the crude bioethanol.
In the initial stage of the project, the Al2O3 and the noble metal impregnated Al2O3 catalysts were tested over a range of temperatures, under 20 barg pressures and a 5:1 steam to ethanol ratio. This was to determine the optimum temperature of reaction. A temperature of 500oC was found to be the optimum reaction temperature due to “hard” coke formation at higher temperatures over the Ru and Rh catalyst.
The effect of the different impurities was examined by systematically adding 1mol.% of each impurity separately with respect to ethanol content in the water/ethanol mixture. The different noble metal catalysts showed similar tolerances towards the impurities. The addition of C3 alcohols significantly decreased the conversion of ethanol and increased the rate of catalyst deactivation. This deactivation of the catalyst in the presence of C3 alcohols was attributed to high olefin formation and incomplete decomposition of the C3 alcohols which deposited over the catalysts as coke. Separate propanal, propylamine and acetone addition to the water/ethanol mixture significantly increased the ethanol conversion and the activity of all the noble metal catalysts tested. It was found that the presence of these impurities in the ethanol significantly decreased the C2H4 in effluent mixture as these impurities blocked the acidic sites of the catalysts. The compound C2H4 was found to be the main route towards coke formation.
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