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CHAPTER 8: Oxidative Dehydrogenation of /i-Hexane 8.1 Catalyst testing below the lower explosive limit
8.2 Catalyst testing above the upper explosive limit
16VMgO, 19VMgO and 24VMgO were tested on a 7.8 % n-hexane in air feed in a fixed bed reactor. The experimental details have been previously discussed (Chapter 6). Catalysts were tested between 350 and 550 °C. The volume of the catalyst bed was 1.6 ml (0.4 g). A flow rate of 100 ml/min was used and this corresponded to a gas hourly space velocity of 3750 h"1. The results for w-hexane ODH over 16VMgO, 19VMgO and 24VMgO are shown in Tables 8.9, 8.10 and 8.11.
Catalytic testing of the 3 catalysts below the lower explosive limit (0.6 % #-hexane in air) had previously yielded 4 products i.e. CeH6, CO2, CO and H20. Testing above the upper explosive limit yielded several new products. In addition to C6H6, COx and H20 the following products were also formed: 1-hexene, 2-hexene, propane and propene. Cavani and Trifiro [2] indicate that a high fuel to air ratio results in the surface of the catalyst becoming saturated with adsorbed species. They report that under fuel lean conditions pentane oxidation over vanadyl pyrophosphate yields maleic anhydride, phthalic anhydride and COx.
Under fuel rich conditions other products, which did not appear for low «-pentane concentrations in the feed, were observed. On a saturated catalyst surface, catalytic sites are scarce. The catalytic reaction then becomes nonselective because intermediate olefinic species desorb into the gas-phase. These desorbed species may then undergo further transformations in the gas-phase [2]. Desorbed species may also be readsorbed.
It is well known that the surface of a catalyst is in dynamic interaction with the gas-phase [2,3]. Under fuel lean (below LEL) conditions the gas-phase has high oxidizing power. The converse is also true. In this way the oxidation state of vanadium in VMgO catalysts is influenced. In oxidation catalysis, a more oxidized catalytic surface usually results in more active but less selective catalysts [2]. Catalysts operating above the UEL are thus expected to be less active and more selective. The results shown in Tables 8.9, 8.10 and 8.11 for n- hexane ODH under fuel rich conditions are in full agreement.
Temp (°C) 350 400 425 450 475 500 525 550
C (%) 28.3 41.4 47.9 51.2 50.3 56.2 56.8 59.7
S (C6H6) 0 8.4 12.5 12.9 15.3 18.7 25.0 21.4
Y (QH6) 0 3.5 6.0 6.6 7.7 10.5 14.2 12.8
S (Hexenes)
0 _j
4.4 4.6 4.8 4.1 4.0 3.8 3.7
S (C3H8) 0 0 0 0 1.6 2.9 5.6 7.9 T a b l e 8.9: The effect of temperature on conversion, selectivity and yie
Temp (°C) 350 400 425 450 475 500 525 550
C (%) 29.8 56.3 59.9 56.3 61.7 65.3 67.1 68.8
S (C6H6) 0 8.5 14.1 17.5 19.8 21.4 29.2 26.0
Y (C6H6) 0 4.8 8.4 9.8 12.2 14.0 19.6 17.9
S (Hexenes) 0 4.8 4.6 4.0 3.9 4.0 3.9 3.7
S (C3H8) 0 0.9 0.8 1.1 1.9 3.5 7.3 10.3
S (C3H6) 0 0 0 0 0.8 0.4 3.0 4.0
S (COx) 100 87.2 82.9 82.3 78.1 74.0 62.5 63.0
Y (Hexenes) 0 1.8 2.2 2.5 2.1 2.3 2.2 2.2
Y (C3H8) 0 0 0 0 0.8 1.6 3.2 4.7
Y (C3H6) 0 0.0 0.0 0.0 0.4 0.2 1.8 2.4
Y (COx) 28.3 36.1 39.7 42.1 39.3 41.6 35.5 37.6
TDS 0 12.8 17.1 17.7 20.2 23.2 31.9 29.1 d in «-hexane ODH over 16VMgO
S (C3H6) 0 0 0 0.2 1.1 2.0 3.9 5.1
S (COx) 100 85.8 80.5 77.2 73.2 69.2 55.7 55.1
Y (Hexenes) 0 1.6 2.1 2.6 2.4 2.6 2.6 2.5
Y (C3H8) 0 0.5 0.5 0.6 1.2 2.3 4.9 7.1
Y (C3H6) 0 0 0 0.1 0.7 1.3 2.6 3.5
Y (COx) 29.8 48.3 48.2 43.5 45.2 45.2 37.4 37.9
TDS 0 13.3 18.7 21.6 24.9 27.4 37.0 34.8 Table 8.10: The effect of temperature on conversion, selectivity and yield in «-hexane ODH over 19VMgO
Temp (°C) 350 400 425 450 475 500 525 550
C (%) 27.3 49.4 53.6 56.3 55.7 58.1 59.1 62.0
S (C6H6) 0 6.4 8.5 10.2 15.9 19.3 24.7 20.7
Y (QH6) 0.0 3.2 4.6 5.7 8.9 11.2 14.6 12.9
S (Hexenes) 0 5.8 5.7 5.2 5.0 4.4 4.2 4.2
S (C3H8) 0 0 0.8 1.5 2.0 4.2 5.5 8.5
S (C3H6) 0 0 0 1.0 1.1 2.8 3.8 4.4
S (COx) 100 87.8 85.0 82.1 76.0 69.3 61.8 62.2
Y (Hexenes) 0.0 2.9 3.1 2.9 2.8 2.6 2.5 2.6
Y (C3H8) 0.0 0.0 0.4 0.8 1.1 2.4 3.3 5.3
Y (C3H6) 0.0 0.0 0.0 0.6 0.6 1.6 2.3 2.7
Y (COx) 27.3 43.4 45.6 46.2 42.3 40.2 36.5 38.6
TDS 0 12.2 14.2 16.4 22.0 26.5 32.7 29.3 Table 8.11: The effect of temperature on conversion, selectivity and yield in n-hexane ODH over 24VMgO
350 400 450 500 Temperature (°C)
550
Figure 8.6: Effect of temperature on conversion in «-hexane ODH
In Figure 8.6 above it can be seen that 19VMgO again gives higher «-hexane conversion at a specific temperature. In all 3 catalysts conversion increases rapidly from 350 °C to about 425
°C and then stabilizes over a few degrees Celcius. At 450 °C there is a drop in conversion with 19VMgO. The conversion in 24VMgO and 16VMgO also drops albeit at a slightly higher temperature. After this point conversion increases slowly. This latter increase in conversion is accompanied by an increase in cracking products. Oxygen conversion was difficult to accurately determine with the analytical system used due to the inefficient separation of oxygen and nitrogen. Nevertheless, the absence of oxygen in the analysis of the product stream was conspicuous. Complete oxygen conversion was observed at 475 °C for 16VMgO and 24 VMgO. For 19VMgO complete oxygen conversion was observed at 450
°C.
Figures 8.7, 8.8 and 8.9 show the effect of temperature on selectivity for 16VMgO, 19VMgO and 24VMgO. At 350 °C, COx is formed exclusively over all catalysts. Benzene and hexenes start to form at 400 °C. Propane and propene form at temperatures greater than 425
°C. The selectivity to benzene increases with temperature in all catalysts until a maximum is reached at 525 °C. Above this temperature benzene is unstable and consecutive oxidation of benzene to COx becomes strongly favoured. At 550 °C, selectivity to COx was observed to increase.
I COx • Benzene • Hexenes • Propane • Propene 100
80
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u
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60 40 20
0
tfflffl
350 400 425 450 475 500 525 550 Temperature (°C)
Figure 8.7: Effect of temperature on selectivity in «-hexane ODH over 16VMgO
COx • Benzene • Hexenes • Propane D Propene
350 400
425 450 475 500Temperature (°C)
525 550 Figure 8.8: Effect of temperature on selectivity in n-hexane ODH over 19VMgO
In all three catalysts 1-hexene and 2-hexene are produced at 400 °C. No 3-hexene was observed. The ratio of 2-hexene to 1-hexene was usually close to two. The selectivity to hexenes slowly decreased with increasing temperature. At higher temperatures propane and propene are produced. Typically, the selectivity to propane close to double that of propene and this ratio increased with temperature probably because propene is more reactive than propane. At 550 °C, the selectivity to propane over 19VMgO was over 10 % and the selectivity to propene was close to 5 %.
COx • Benzene • Hexenes • Propane • Propene
350 400 425 450 475 500 525 550 Temperature (°C)
Figure 8.9: Effect of temperature on selectivity in w-hexane ODH over 24VMgO
CO
40 30 20 10
D16VMgO B19VMgO D24VMgO
350 400 425 450 475 500 Temperature (°C)
525 550
Figure 8.10: Effect of temperature on the total dehydrogenation selectivity (TDS) in n- hexane ODH over 16VMgO, 19VMgO and 24VMgO
Figure 8.10 above shows that the overall selectivity to dehydrogenation (benzene, hexenes and propene) increases with temperature and peaks at 525 °C. The highest selectivity at any particular temperature was obtained with 19VMgO. Figure 8.11 shows the highest selectivity to benzene and total dehydrogenation obtained by the three catalysts tested. For 16VMgO the highest selectivity to benzene was 25.0 (± 0.4) % and its highest TDS was 31.9 (± 0.4) %.
For 19VMgO the highest selectivity to benzene was 29.2 (± 0.4) % and its highest TDS was 37.0 (± 0.4) %.
40
-? 30
Selectivity D O
I U
n u
16VMgO
n Benzene BTDS
19VMgO Catalyst
24VMgO
;
!
Figure 8.11: Highest selectivity to benzene and highest selectivity to dehydrogenation products (benzene+hexenes+propene) obtained with 16VMgO, 19VMgO and 24VMgO.
D Benzene D Dehydrogenation products
^ 20 2
16VMgO 19VMgO Catalyst
24VMgO
Figure 8.12: Highest yield obtained for benzene and highest yield obtained for dehydrogenation (benzene+hexenes+propene) with 16VMgO, 19VMgO and 24VMgO.
The highest selectivity to benzene with 24VMgO was 24.7 (± 0.4) % and the highest TDS attained was 32.7 (± 0.4) %. Figure 8.12 shows the highest yields to benzene and dehydrogenation achieved by 16VMgO, 19VMgO and 24VMgO. The overall highest yield to benzene was 19.6 (± 0.4) % and the highest yield to dehydrogenated products was 24.8 (±
0.4) %. These yields were obtained with 19VMgO at 525 °C.