Temperature (°C)
10.3 Reaction network and mechanism .1 Homogeneous gas-phase reactions
The contribution of gas-phase reactions in this study was previously shown to be small (Chapter 7.4). A gas-phase reaction involves the formation of free radicals. The initial step is considered to be the hydrogen atom abstraction of the paraffin to form an alkyl radical [2].
An oxygen molecule can react directly with the paraffin, forming an alkyl radical and a hydroperoxy radical, or a radical such as a hydroxyl radical could desorb from the catalyst surface and abstract a hydrogen atom from the paraffin (Figure 10.2) [2].
RH RH
+ +
O o u
2
OH-
R- R-
+ +
HOO- H20 Figure 10.2: Alkyl radical formation [2]
Alkyl radicals that form can dehydrogenate oxidatively to form olefins [2]
10.3.2 Heterogeneous reactions
«-hexane »-«-hexenes »-cyclohexane >- cyclohexenes *- benzene I >• c o x •<
Figure 10.3: Proposed reaction network in oxidative dehydrogenation of K-hexane
Figure 10.3 shows the possible reaction pathways during the ODH of «-hexane leading to the formation of benzene. There are three primary steps forming hexenes, cyclohexane and COx.
COx are formed directly from »-hexane, intermediate products and from the final benzene product itself. If the rates of the consecutive reactions to benzene are rapid then its selectivity will be high (presuming that benzene itself is only slowly oxidised to COx). If the rates of the consecutive steps leading to benzene are slow then these steps will favour deep oxidation to COx. Olefins that have desorbed may be subsequently readsorbed on unselective oxidizing sites resulting in the formation of carbon oxides. In VMgO catalysts the basic nature of the surface facilates the desorption of olefins from the catalyst surface. In addition to this, the
basic surface ensures that the interaction between a desorbed olefin and the catalyst surface is low. Furthermore, the basicity of VMgO catalysts has been correlated to the absence of oxygenated organic products [3]. The low residence times of olefins on the catalyst surface does not favour oxygen insertion.
The following 3 different mechanisms have been proposed for ODH of paraffins:
1) A Mars-van Krevelen mechanism were selective reactions take place on partially reduced sites and deep oxidation on highly oxidised sites [4].
2) A dual - site Mars-van Krevelen mechanism in which different sites catalyze the formation of selective and unselective ODH products [5].
3) A Mars-van Krevelen mechanism which operates for selective dehydrogenation and a Langmuir-Hinshelwood mechanism for deep oxidation [6].
According to the Mars-van Krevelen mechanism, the oxidation of paraffins proceeds by two steps (Chapter 2.6):
1) The reactant paraffin molecule initially reduces an oxidized surface site.
2) The reduced surface site is subsequently reoxidized by gas-phase molecular oxygen.
10.3.2.1 Mechanism of 1-hexene and 2-hexene formation
C H3— CH2— CH2— CH2— CH2— CH3
o2-
© *
CH3 CH— C H , — CH2— CH2— CH3
CATALYST
o2- T
CH2 = C H — C H2— C H j — C H2— C H3 ~ CH3—CH = C H — C H2— C H2— C H3
Figure 10.4: Mechanism of 1-hexene and 2-hexene formation [7]
It is generally accepted that that the activation step and rate limiting step in paraffin ODH is the irreversible hydrogen abstraction from a secondary carbon to give an adsorbed alkyl radical [8-11]. The alkyl radical that forms is very reactive. The removal of another hydrogen atom then leads to olefins (Figure 10.4). The adsorbed olefin can undergo further
transformation or it can desorb into the gas-phase. The active centre on VMgO catalysts for C-H bond activation is believed to be a V5 +- O2" couple, which is considered to be an acid - base pair [12]. It is still unclear as to whether it is the nucleophilicity/basicity of the oxygen or the electrophilicity/acidity of the vanadium atom that is responsible for C-H bond cleavage.
Centi and Trifiro [13] believe that a concerted attack of both Lewis acid and base centres on VPO catalysts leads to the contemporaneous removal of two hydrogen atoms from butane resulting in adsorbed butene. At variance with this mechanism, Busca et al. [14] believe that nucleophilic oxygen species are responsible for paraffin activation, leading to the formation of a surface hydroxyl group.
The general consensus is that ODH on VMgO catalysts proceeds via the reduction of tetrahedrally coordinated V5 T ions, although the oxidation state of the reduced vanadium cation is contentious [8]. In this study only V5+ and V4+ cations were observed by XPS analysis of the used 19VMgO catalyst suggesting a V5+ «-> V4+ redox cycle.
10.3.2.2 Oxidative dehydrocyclisation of «-hexane to benzene
Figure 10.5: Proposed mechanisms for the cyclisation of w-hexane [15,16]
Possible cyclisation mechanisms for «-hexane are shown in Figure 10.5 above:
A) 1,6 ring closure to cyclohexane followed by ODH to benzene [16].
B) 1,5 ring closure, followed by ring expansion to cyclohexane and ODH to benzene [16].
C) ODH of hexane to hexenes, hexadienes, hexatrienes, followed by thermal ring closure and ODH to benzene [15].
To assess the feasibility of pathways B and C, selected intermediates were used as feeds. A feed composition of approximately 1 % intermediate in air was fed over 19VMgO at 400 °C.
A gas hourly space velocity of 3750 h"! was used. The major and minor products observed are shown in Table 10.1 below:
Intermediate 1-Hexene 2-hexene
Methylcyclopentane Cyclohexane Cyclohexene
Major products observed COx
COx COx
Benzene, Cyclohexene Benzene
Minor products observed Benzene
COx COx
Table 10.1: Major and minor products observed by feeding intermediates over 19VMgO
The results shown in Table 10.1 suggests that the direct 1,6 ring closure mechanism is most likely the dominant pathway to benzene. When 1-hexene was fed into the reactor only trace amounts of benzene was observed, suggesting that this was not a significant reaction pathway.
When 2-hexene was fed into the reactor only COx was observed. No isomerization products were observed consistent with the weakly acidic character of 19VMgO. C5 to C6 ring expansion is usually catalysed by acid catalysts [16]. The lack of isomerization activity with
19VMgO suggests that ring expansion pathways would not be a favourable route to benzene.