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CHAPTER 7: Results and Discussion
7.1 Introduction, aim and overview
Olefins are commonly derived from the steam cracking of naphtha and fluidized catalytic crackers in oil refineries. They are commonly used as feedstock in the synthesis of chemicals in the chemical industry. Paraffins can be obtained from natural gas, LPG and Fischer- Tropsch processes. Activation of paraffins is considered to be more difficult than the corresponding olefin and consequently there has been limited success in utilizing the more economic paraffin feedstock [1].
Most of the available literature on paraffin oxidation is based on short chain paraffins [2-12].
The aim of this study was to investigate the oxidative dehydrogenation of longer chain paraffins i.e. rt-hexane and «-octane, using a heterogeneous catalytic system. The catalytic transformation of longer chain paraffins into more desired products represents a proactive step to counter their predicted surplus arising from anticipated Fischer-Tropsch expansion (coal to liquids and gas to liquids) and changing environmental laws. The vanadium magnesium oxide (VMgO) catalytic system was selected, based on its intrinsic properties (Chapter 5) as well as its relatively good success with butane oxidative dehydrogenation [6,7,13,14].
Several VMgO catalysts with varied amounts of vanadium were synthesized (Chapter 6), characterized (Chapter 7) and their catalytic activity was investigated for «-hexane ODH (Chapter 8).
The optimized vanadium loaded catalyst was then promoted with small amounts of antimony, tellurium, molybdenum, bismuth, cesium and niobium oxide in an attempt to improve its selectivity and yield towards ODH products. Promoted catalysts were tested on w-hexane (Chapter 8) and «-octane (Chapter 9). Process conditions such as space velocities and fuel to oxidant ratios were also optimised. Air was used as the oxidant.
Lastly, to gain an insight into the reaction mechanism, plausible reaction intermediates were fed into the reactors (Chapter 10). These results were evaluated and a potential reaction pathway towards desired products was hypothesized.
The results of catalyst characterization and an evaluation of catalytic performance are now further discussed.
7.2 Catalyst characterization: V M g O 7.2.1 X-ray diffraction (XRD)
The overlaid XRD patterns of VMgO catalysts synthesized with varied vanadium loadings are shown in Figure 7.1. The x-ray diffraction patterns of all the catalysts are reflective of low crystallinity and/or small average crystallite size. This is consistent with the high surface areas of these catalysts. Broad peaks for MgO are observed in all the catalysts (2d = 50.3, 73.9). No diffraction lines for V2O5 are observed. As the vanadium loading (hence the mol
% of magnesium orthovanadate) increases in the catalysts, diffraction lines for a poorly crystalline magnesium orthovanadate phase can be observed. The diffraction line at 26 = 41.2, in particular, becomes more intense (Figure 7.1). This line is strongly associated with the orthovanadate phase [14]. The orthovanadate phase is not detected in the x-ray diffractograms of those catalysts with low vanadium contents. This is due to the amount of orthovanadate phase, present in these catalysts, being below the detection limit of the x-ray diffraction technique and/or the small crystallite size of this phase. Nevertheless, the orthovanadate phase is observed with other techniques (Raman and infrared). Diffraction lines corresponding to other vanadate phases (MgV206 and Mg2V207) were not detected in any of the catalysts. The XRD patterns of all the VMgO catalysts were similar to those previously reported [14-19]. X-ray diffraction lines for the 19VMgO catalyst are given in Table 7.1 below. Full x-ray diffractograms of all catalysts are given in Appendix 3.
19 VMgO 29a 41.06 43.19 44.96 50.29 52.41 73.76 77.28
d Spacing (A) (relative intensity)a
2.55 (8)c 2.43 (6) 2.34 (5)c 2.10(100) 2.02 (23)c 1.49(54)
1.43(4)
d Spacing (A) (relative intensity) b
2.42 (6)
2.10(100)
1.49(53) 1.269(5) 1.218(12), 1.053(5), 0.966 (3), 0.943 (12)
Assignment Mg3V208
MgO Mg3V208
MgO Mg3V208
MgO MgO MgO
"Synthesized 19VMgO catalyst, b19VMgO according to [14], "Assigned to Mg3(V04)2 according to 54VMgO[14]
Table 7.1: X-ray diffraction lines of synthesized 19VMgO compared to those previously reported [14]
7.2.2 Chemical composition
The chemical compositions, determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES), of MgO, V205 and synthesised VMgO catalysts are shown in Table 7.2 below. Samples were prepared for ICP by completely digesting them in aqua regia. ICP is a bulk characterization technique. The experimentally determined compositions were found to be close to the theoretical values. This is consistent with the synthetic technique used (one pot synthesis). Catalysts were also analysed by energy dispersive x-ray spectroscopy (EDS), which is a localized technique. The atomic ratios of magnesium to vanadium determined by ICP compared well to those determined obtained by EDS.
Catalyst MgO 14 VMgO 16 VMgO 19 VMgO 24VMgO 35VMgO 49VMgO 60VMgO
v
2o
5V2Os (wt %) (ICP)
0 13.9 16.0 18.9 24.2 35.0 48.7 58.6 100
MgO (wt %) (ICP)
100 86.1 84.0 81.1 75.8 65.0 51.2 40.8 0
wt % Mg3(V04)2
(theoretical)3 - 23.1 26.6 31.5 40.3 58.3 81.1 97.6
-
(Mg/V)ICP
(atomic) - 13.9 11.8 9.7 7.1 4.2 2.4 1.6 -
(Mg/V)EDS (atomic)
- 13.2 11.5 9.4 7.0 4.0 2.1 1.5 -
a Calculated on the basis that all V205 reacts with MgO according to V205 + 3MgO -> Mg3(V04)2 Table 7.2: Chemical characteristics of synthesized VMgO catalysts
7.2.3 Average vanadium oxidation state
Average vanadium oxidation states were determined by a redox titration procedure [20]. The experimentally determined average vanadium oxidation states of freshly calcined VMgO catalysts were consistent with the expected oxidation state of 5 (Table 7.3.). The 19VMgO catalyst had a slightly lower average vanadium oxidation state after being used online for 168 hours.
Catalyst 14VMgO 16VMgO 19VMgO 24VMgO 35VMgO 49VMgO 60VMgO
Average vanadium oxidation state 4.8 (±0.3)
4.8 (±0.3) 4.8(±0.3),4.5a(±0.3)
4.7 (± 0.3) 4.8 (± 0.3) 4.8 (±0.3) 4.9 (± 0.3)
a value for aged catalyst
Table 7.3: The average vanadium oxidation state of freshly synthesized VMgO catalysts
7.2.4 Brunauer-Emmet-Teller (BET) surface area
The BET surface areas, pore volume and average pore diameters, measured for calcined VMgO catalysts are shown in Table 7.4 below:
Catalyst MgO 14VMgO 16 VMgO 19 VMgO 24VMgO 35VMgO 49VMgO 60VMgO V205
5 Point BET (m2/g)
137 189 190 189 133 55 23 9 4
Single BET (m2/g)
132 183 189 185 132 53 23 9 4
Pore volume (cm3/g)
0.21 0.60 0.68 0.58 0.57 0.19 0.11 0.08 0.02
Average pore diameter (A)
62 111 142 123 146 139 191 342 207
Table 7.4: BET surface areas, pore volumes and pore diameters for synthesized VMgO catalysts
The 16VMgO catalyst had the highest surface area. Vanadium loadings of up to 24 wt % resulted in catalysts with relatively high surface areas, even higher than that of the MgO precursor. The formation of material with a higher surface area than its precursors is indicative of the formation of a new phase. This is consistent with V2O5 not being merely supported on MgO and further supports the XRD data (Chapter 7.2.1). Pore volumes were observed to decrease with an increase in the vanadium loading except for 16VMgO which
between the average pore diameter and vanadium loading could be made. In Figure 7.2 it can be seen that as the vanadium loading increases beyond 20 wt %, the BET surface area rapidly decreases.
Figure 7.2: Graph showing the effect of vanadium loading on the surface area of VMgO catalysts (0 wt % V205 = MgO)
The BET data suggests that there exists an optimum mole % of magnesium orthovanadate in VMgO catalysts, which would yield a catalyst with the highest surface area (Figure 7.3).
200 T -
23.1 26.6
SS8&!
m Hi HI
31.5 40.3 58.3 wt % Mg3(V04)2
81.1 97.6
Figure 7.3: Graph showing the effect of wt % Mg3(V04)2 on the BET surface areas
The effects of calcination times on the BET surface area were investigated for the 19VMgO catalyst (Table 7.5). It was observed that at 550 °C, longer calcination times result in catalysts with lower surface areas. On the other hand, calcination times, which are too short, results in the presence of mixed phases.
Calcination time
<2hrs 3hrs 4hrs 5hrs
BET surface area Mixed phases
189 184 178
Table 7.5: Effect of calcination times on the BET surface area at 550 °C
7.2.5 Differential scanning calorimetry - thermogravimetric analysis (DSC- TGA)
DSC-TGA analysis was carried out on uncalcined 24VMgO (Appendix 3). Heating the catalyst to 200 °C under nitrogen resulted in a 10 % weight loss. This corresponds to the removal of water and surface carbonates and to the decomposition of NH4VO3 [18,19]. The endothermic peak at 148 °C is attributed to the decomposition of NH4VO3 and the weak intensity endothermic peak at 200 °C is due to the elimination of crystallization water [18,19]:
2NH4V03(s) -> V205(s) + 2NH3(g) + H20(g)
The very strong intensity endothermic peak at 390 °C is attributed to the decomposition of Mg(OH)2 to form MgO [18,19,21]:
Mg(OH)2(s) -» MgO(s) + H2Q(g)
Thermal peaks corresponding to the formation of Magnesium vanadates were not observed.
These thermal peaks are usually observed as low intensity exothermic peaks around 400 °C to 600 °C depending on the preparation procedure and the vanadium loading in the catalyst [18,21,22]. The presence of the strong intensity endothermic peak at 390 °C could have occluded the appearance of any low intensity exothermic peaks in this temperature region.
No thermal peak corresponding to the melting of V205 was observed. This peak normally occurs at 660 °C for pure V205 [23]. This also suggested that V2Os had completely reacted with MgO and V205 crystallites were not dispersed on the surface of MgO.