DECLARATION 2 CONFERENCE CONTRIBUTIONS
5. CHAPTER FIVE – RESULTS AND DISCUSSION
5.5. Catalytic results for barium promoted catalysts
The COx yield was found to be higher for the lower fuel-air ratios. It was therefore concluded that 15% octane in air would be chosen as the optimum fuel-air ratio as the production of COx is reduced (compared to the fuel-air ratios < 15% octane in air), while the yields to C8 aromatics and olefins are still substantial.
Figure 5.21: Effect of fuel-air ratios on yields at 500 C for n-octane conversion
The optimum conditions for the Mg-V-HTlc were chosen to be 15% octane in air using a contact time of 0.5s. Under these conditions, reasonably good selectivities and yields to both the total C8 olefins and total C8 aromatic compounds are obtained. Further, the production of COx is not as high as for other reaction conditions tested.
the unpromoted catalyst, it was observed that the Mg-V-HTlc displays higher conversions from 450 C to 550 C.
Figure 5.22: Conversion of n-octane as a function of temperature for Ba promoted catalysts
The C8 olefins were initially produced at 350 C, specifically 2-octene due to the ease with which hydrogen abstraction from the secondary carbon, as compared to the primary carbon, can occur.
The 6% BaMg-V-HTlc, however, did not produce any C8 olefins at this temperature, possibly due to the catalyst composition as determined from XRD. In fact, the 6% BaMg-V-HTlc produces the lowest total C8 olefin selectivity at all temperatures (Figure 5.23). The highest total selectivity (27%) was obtained with the 1.4% BaMg-V-HTlc at 500 C, 9.1% of which was due to the production of 1-octene. It was thought that the addition of a basic promoter would decrease the acidity of the catalyst, therefore possibly increasing the desorption of the primary products i.e. C8 olefins, thereby increasing their selectivity. The 0.2% BaMg-V-HTlc was the only catalyst that showed this at 400 C – the selectivity to total C8 olefins was marginally higher than that of the unpromoted catalyst; 26.6% for 0.2% BaMg-V-HTlc vs. 23.4% for Mg-V-HTlc. It is therefore possible that the lack of improved selectivity to C8 olefins could be due to the change in the structure of the catalyst with increasing barium loading (Section 4.2, Figure 4.16 – Figure 4.19).
Object 285
Figure 5.23: Selectivity to total C8 olefins for Ba promoted catalysts
The total selectivity to the C8 aromatics, which comprised of ethylbenzene, styrene and xylene, is presented in Figure 5.24. The three lower Ba promoted catalysts showed a similar trend with increasing temperature, while the 6% BaMg-V-HTlc was found to produce the lowest selectivity at all temperatures. It was observed that as the barium loading increased, the selectivity to the aromatics decreased. Literature reports, for MgVO catalysts in the ODH of n-butane, suggest that the ability of the catalyst to produce one product vs. another is related to the presence of acid and base sites on the surface of the catalyst [8, 9]. Basic sites favour a fast desorption of olefinic species, while acid sites adsorb oxygen-containing species which may then be transformed further, finally to CO2 [8, 9]. The TPD studies on the barium promoted catalysts showed a decrease in acidity as the barium loading increased. It follows therefore, that the selectivity to aromatics is low since these are the secondary products. We also expect then, a high selectivity to COx for the 6%
BaMg-V-HTlc and this is indeed observed (Figure 5.25). The total COx selectivity is, in fact, found to decrease with decreasing Ba loading and hence increasing acidity. As observed for the Mg-V- HTlc, all barium promoted catalysts produced styrene as the dominant aromatic product, as shown for the 1.4% BaMg-V-HTlc in Figure 5.26. However, the best selectivities were obtained with the unpromoted catalyst at 500 C and 550 C.
Object 287
Figure 5.24: Total selectivity to C8 aromatic compounds for Ba promoted catalysts
Figure 5.25: Total COx selectivity for barium promoted catalysts
Object 289
Object 292
Figure 5.26: Selectivity to the aromatic products for the 1.4% BaMg-V-HTlc
Products with a carbon number < 8, both alkanes and alkenes, were grouped as “cracked” products and their total selectivity for the different barium promoted catalysts is presented in Figure 5.27. At temperatures lower than 450 C, no clear trend could be observed. However, from 450 C to 550 C, there was an increase in the selectivity to cracked products. Further, in this temperature range, it was evident that the selectivity increased with increasing barium loading. Surface area studies on the used catalysts showed a decrease in surface area (Table 5.4). This, together with the grey colour of the used catalyst, suggests that coking mechanisms occurred [2]. As such, the selectivity to the cracked products is likely to increase as the C8 olefins and C8 aromatics are further converted ultimately to smaller chain compounds and COx.
Object 295
Object 297
Figure 5.27: Selectivity to total “cracked” products for barium promoted catalysts
Table 5.4: Surface areas of the used Ba promoted catalysts
CATALYST SURFACE AREA (m2/g) 0.2% BaMg-V-
HTlc
30.2 0.7% BaMg-V-
HTlc
29.8 1.4% BaMg-V-
HTlc
28.4
6% BaMg-V-HTlc 27.2
The catalytic results for the barium containing catalysts were compared at constant conversion of
~ 16% at 450 C (Figure 5.28). The 0.2% and 0.7% BaMg-V-HTlc was found to give marginally higher selectivities to the total octane isomers than the Mg-V-HTlc, but these decreased with an increase in barium loading. The 1.4% barium promoted catalyst gave the highest total selectivity to the C8 aromatics, however, all barium promoted catalysts were found to less selective to total aromatics than the Mg-V-HTlc. The total selectivity to COx was found to be significantly affected by the barium loading, with the lowest selectivity obtained over the unpromoted Mg-V-HTlc and the highest over the 6% BaMg-V-HTlc. This may be tentatively related to the acidity of the catalyst, which was found to decrease with increasing barium loading. The total yield to the cracked products was found to increase with increasing barium loading, while the reverse trend was observed for the total diene selectivity – dienes being mainly 1.3-octadiene and a smaller contribution from cyclooctadiene.
Figure 5.28: Comparison of results at ~ 16% conversion and 450 C