Chapter 2 Experimental
4.5 The influence of iron loading on the activation of n -octane
4.5.1 Iron supported on hydroxyapatite catalysts
104 4.4.4 Summary of the effect of carbon to oxygen ratio
Under an anaerobic environment the 3-COP and the 3-WET catalysts showed low activity at high temperatures giving low conversion. The introduction of oxygen increased the activity of these catalysed reactions by overcoming the thermodynamic limitations associated with these endothermic dehydrogenation reactions. The conversion over the 3-COP catalyst decreased above 400 °C when the oxygen content of the reaction was increased above levels that gave a C:O ratio of 8:2, most likely due to the phase change observed in the in situ PXRD of the 3- COP catalyst (Figures 3.3 and 3.4). The formation of value added C8 products was favoured in low oxygen environments, as proved by the greater selectivity towards octenes, C8 aromatics and C8 oxygenates at a C:O ratio of 8:2. The selectivity towards cracked oxygenates increased with an increase in oxygen content for the 3-COP catalyst. This increase in selectivity corresponded to a decrease in the selectivity towards value added C8 products.
Conversion over the 3-WET catalyst plateaued above 450 °C. An overall increase in selectivity towards cracked oxygenated compounds was observed for the 3-WET catalyst.
This corresponded to a decrease in selectivity towards other ODH products.
105 conversion was attained at an iron loading of 9 wt%, however, this was at the expense of the selectivity towards octenes and C8 aromatics, whose formation was not thermally favorable at higher iron loadings as inferred from the decrease in selectivity, over the 9-WET catalyst, with increased temperature (Figure 4.32). Overall catalytic activity was altered, with an increase in iron loading as observed from the variation in conversion.
Figure 4.28: Conversion as a function of increasing temperature over the iron supported on HAp catalysts with varied iron loadings, tested at a C:O ratio of 8:2 and a GHSV of 6000 h-1 Analyses of the product streams of the HAp, 1-WET, 3-WET and 9-WET catalysts identified a range of octenes, aromatics, cracked products and COx. Cracked products included lower alkanes and alkenes, methanol, ethanol, 2-propanol and acetone, the last four of which were classed as cracked oxygenates. Desirable C8 products included octene isomers, C8 oxygenates (or more specifically 1-octanol) and C8 aromatics, viz. ethylbenzene, styrene, and o-xylene. Other aromatic compounds detected were toluene and benzene and have been classed together as C7 aromatics.
Combustion leading to COx formation was favoured at all temperatures over HAp (Figure 4.29). Cracked products showed the second highest selectivity over the temperature range tested and increased with temperature. This suggested that thermal cracking was somewhat
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Conversion/ mol%
Temperature/ °C
HAp 1-WET 3-WET 9-WET
106 favourable over this support [3]. Selectivity towards octenes was only seen from 450 °C onwards and also increased as a function of temperature. The increase in selectivity of the latter two products with temperature suggested that their formation was somewhat thermally favoured.
Figure 4.29: Selectivity profile over HAp tested at a GHSV of 6000 h-1 and a C:O ratio of 8:2
Figure 4.30: Selectivity profile over the 1-WET catalyst tested at a GHSV of 6000 h-1 and a C:O ratio of 8:2
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Selectivity/ mol%
Temperature/ °C
Octenes Cracked COx
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Selectivity/ mol%
Temperature/ °C
Octenes C8 Aromatics C7 Aromatics Cracked COx Cracked Oxygenates
107 What was immediately noticeable was the significant change in product profile of the iron supported on HAp catalysts when compared to HAp. An iron, 1 % by weight, supported on HAp (1-WET) catalyst showed no significant changes in selectivity towards octenes (Figure 4.30) as a function of increased temperature to warrant a discussion of any observed trend.
Figure 4.31: Selectivity profile over the 3-WET catalyst tested at a GHSV of 6000 h-1 and a C:O ratio of 8:2
However, at higher iron loadings, viz. 9 % (Figures 4.32), a decrease in selectivity towards octenes was observed with an increase in temperature. Once hydrogen abstraction from n- octane occurred [13], the resultant octenes could have either desorbed from the surface of the catalyst or served as reaction intermediates for other ODH product formation. Coincidentally, a decrease in selectivity towards C8 aromatics was observed with an increase in temperature for the 9-WET catalyst. This could have been due to the rapid desorption of octenes, which have been shown to serve as precursors for C8 aromatics formation [3, 4, 9, 11], from the catalyst surface. The basic nature of the catalyst could have influenced desorption of the formed octenes, thus inhibiting their selectivity and the formation of C8 aromatics.
1-Octanol was the only C8 oxygenate detected. This compound was only observed when the 3-WET catalyst was tested and showed a selectivity of 18 mol% at a temperature of 350 °C
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Selectivity/ mol%
Temperature/ °C
Octenes C8 Aromatics C8 Oxygenate C7 Aromatics
Cracked COx Cracked Oxygenates
108 (Figure 4.31). The absence of this product from the product streams of the other catalysts and at different temperatures could have been the result of it undergoing cracking to form cracked oxygenated compounds. It seemed plausible to suggest that 1-octanol formation resulted from an oxygen insertion reaction upon formation of an activated octene isomer, most likely 1- octene. The presence of 1-octanol in the product stream of the 3-WET catalyst was attributed to the -Fe2O3 phase of the catalyst as no oxygenated compounds were observed in the product stream of the HAp catalyst (Figure 4.29). It could be suggested that iron influences the functionalisation of adsorbed octenes by reacting with chemisorbed oxygen species on the surface of the active metal phase (-Fe2O3).
Figure 4.32: Selectivity profile over the 9-WET catalyst tested at a GHSV of 6000 h-1 and a C:O ratio of 8:2
Other aromatic compounds, viz. toluene and benzene ( C7 aromatics), showed a marginal decrease in selectivity with increased temperature for all catalysts. These aromatic compounds could have resulted from the cracking of the alkyl substituents of the C8 aromatics. If this was so, the decrease in both their selectivity with an increase in temperature suggested that the formation of C8 aromatics was favorable or a competitive pathway existed between cyclisation of formed octenes and the cracking of oxygen functionalised C8 products.
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Selectivity/ mol%
Temperature/ °C
Octenes C8 Aromatics C7 Aromatics Cracked COx Cracked Oxygenates
109 Cracked products and COx showed significant selectivity when HAp (Figure 4.29) was tested.
Contributions from COx were far lower for the iron supported on HAp catalysts (Figures 4.30, 4.31 and 4.32). In all cases a decrease in COx selectivity was observed with an increase in temperature. This suggested that the formation of other ODH products were thermally favourable.
Iron seemed to have influenced an increase in the formation of cracked oxygenated products.
An increase in the selectivity towards cracked oxygenates, as a function of temperature, was observed over all the iron supported on HAp catalysts. The selectivity towards cracked oxygenated compounds observed over the 1-WET catalyst remained constant within experimental error, ± 5 mol%. At higher temperatures (450 – 550 °C) cracked oxygenates were the dominant products formed over the 3-WET and 9-WET catalysts. The increase in selectivity and a corresponding decrease in selectivity towards other ODH products suggested a preference for cracked oxygenates formation. Cracked oxygenate formation could have occurred by the reaction of adsorbed octenes or gaseous n-octane radicals with chemisorbed oxygen species, thus forming oxygenated compounds that underwent thermal cracking on the surface of the catalyst influenced by the nature of the support HAp, which showed an increased selectivity towards cracked products with increasing temperature (Figure 4.29). The presence of chemisorbed oxygen species would have been a result of the active iron phase as no cracked oxygenated compounds were detected in the product stream of HAp. There was a pronounced decrease in the selectivity towards octenes observed over the 9-WET catalyst.
Correspondingly the most pronounced increase in cracked oxygenates selectivity was observed over the 9-WET catalyst. This suggested that a competitive pathway existed between octene isomer formation and oxygenated compound formation.