Oxidation of propane and propene

As a consequence of the simplicity of the propyl radical, studies of propane oxidation throughout the temperature range embracing the ntc region present an unrivalled opportunity to explore the extent to which the kinetic mechanisms involving alkylperoxy radical chemistry are consistent with experiment. However, interpretation of data is made difficult because molecular intermediate products can be more reactive than the parent fuel. Thus the experimental results may be complicated by secondary oxidation of the intermediates. For this reason, studies are made which involve only the very earliest stages of reaction [149]. The kinetics discussed in Chapter 1 may be applied to propane oxidation to give a skeleton structure. 

As discussed in Sections 1.6 and 1.7, the initiation reaction and principal propagation reactions are 

Although activation energies and pre-exponential factors can be deduced for a homogeneous initiation, the intervention of surface activity makes the interpretation of the chain initiation rates and even the mechanism of 

The HO2 and RO2 abstraction processes are believed to have rather similar rate parameters, the activation energies for which are 62.5 kJ mol in a primary H atom abstraction, and 52.5 kJ moF in a secondary H atom abstraction from alkanes (Section 1.7.2). By contrast to OH radical propagation, these confer a selectivity on the relative rates of HO2 and RO2 propagation, leading to 1- and 2-propyl radicals in the ratios 0.55, 0.60 and 0.67 at 700, 750 and 800K, respectively. 

Clearly, a fully quantitative interpretation of relative proportions of propyl radicals requires a numerical simulation which takes into account all possible reactions. Such procedures are discussed in Section 6.6 but if we anticipate that OH radical propagation may be dominant at temperatures below 700 K, whereas HO2 propagation may be dominant at temperatures above 750 K, it follows that a predominance of 1- over 2-propyl radicals formed at the lowest temperatures is changed to a 2-fold excess of 2-propyl radicals at the higher temperatures. The overall rate of propagation would also decrease by about a factor of 1000 over the temperature range 700-750 K if there was a switch from an exclusively OH to HO2 radical propagation. 

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The results presented in this paper therefore show that V and Mo species supported on alumina can give rise to a catalyst which has a high selectivity for the oxidation of propane to propene and a reasonable selectivity to acrolein and that both species are essential to give the optimal behaviour. Contrary to our previous observations and what observed for bulk catalysts [5], the presence of Nb and W seem to have little effect, perhaps because the methods used here restrict the active phase to a monolayer whereas previously prepared materials may have contained multilayer oxidic species. 

Most industrially desirahle oxidation processes target products of partial, not total oxidation. Well-investigated examples are the oxidation of propane or propene to acrolein, hutane to maleic acid anhydride, benzene to phenol, or the ammoxidation of propene to acrylonitrile. The mechanism of many reactions of this type is adequately described in terms of the Mars and van Krevelen modeE A molecule is chemisorbed at the surface of the oxide and reacts with one or more oxygen ions, lowering the electrochemical oxidation state of the metal ions in the process. After desorption of the product, the oxide reacts with O2, re-oxidizing the metal ions to their original oxidation state. The selectivity of the process is determined by the relative chances of... 

The oxidation of propane and of propene to acrylic acid has also been investigated 87,106,256-261). Vanadium phosphate catalysts that show good performance for the oxidation of C3 hydrocarbons and vanadium phosphate catalysts that are active and selective for C4 and C5 hydrocarbon oxidation have several differences in their structure and operating conditions. 

Following the discussion from the preceding section, consideration will be given to the oxidation of ethene and propene (when a radical pool already exists) and, since acetylene is a product of this oxidation process, to acetylene as well. These small olefins and acetylene form in the oxidation of a paraffin or any large olefin. Thus, the detailed oxidation mechanisms for ethane, propane, and other paraffins necessarily include the oxidation steps for the olefins [28]. 

Despite the kinetic objections to this scheme, which centred on the reaction sequence for the conversion of HO2 to -OH radicals, considerable support has been furnished for the role of the conjugate alkene as the primary product. Brown and Tipper [51] studied the oxidation of propane and cyclohexane at initial temperatures in the range 240—325 °C. With propane the amount of propene present just prior to the cool flame was considerable (ca. 10 % of the propane introduced) and additions of... 

The yields of propane and propene were found to be linearly dependent on the concentration of tritium. Temperature has no effect on the initial rate of formation of propane or propene. NO inhibits completely the formation of propane, but only partly that of propene. Propane can still be formed by hot labeling, through ring-opening and subsequent abstraction of hydrogen—a process which is inhibited by a radical scavenger. Isobutane formation was independent of tritium concentration and temperature and was completely inhibited by the addition of nitric oxide. In the presence of NO, the yield of... 

Figure 2. XC2H4 vs. p°H p ( ) andp°co2 ( ) for T=463 K, Wcat=0.5 g, Ftot=5 mmol s. being most pronounced at low steam partial pressures. Marecot et al. [21] found inhibition due to steam of propane and propene oxidation over Pt/y-Al203. Bart et al. [22] found inhibition of the propane oxidation over a three-way catalyst for reducing conditions and rate enhancement for oxidizing conditions. The inhibition by steam is in contrast to the oxidation of CO by O2 over the same catalyst, where steam was found to strongly enhance the reaction rate [15]. Carbon dioxide also inhibits the reaction rate, although the inhibition is much smaller.

Westbrook, C. K., and Pitz, W. J., "A Comprehensive Chemical Kinetic Reaction Mechanism for the Oxidation and Pyrolysis of Propane and Propene, submitted for publication, 1983. 

Vemoux P, Gaillard F, Bultel L, Siebert E, Primet M (2002) Electrochemical promotion of propane and propene oxidation on PtA SZ. J Catal 208 412-421... 

In general, mixed-metal oxides are at present the most important catalytic system for the partial oxidation of C2-C5 alkanes into 0-containing partial oxidation products. However, different characteristics should be considered among the several catalysts within this group. For instance, VPO- and MoVO-based catalysts are the most effective systems for -butane (or -butene) oxidation to MA and propane (or propene) oxidation to acrylic acid, respectively. It must be indicated that, although some similarities are observed for the ammoxidation " over MoVO- or SbVO-based catalyts, the latter show very low selectivity during the partial oxidation of propane and no results with other alkanes have been reported. " Therefore, SbVO-based catalysts will not be explicitly included in the following discussion. 

Takenaka S, Tanaka T, Funabiki T, Yoshida S (1998) Effect of alkali-metal ion addition to silica-supported molybdenum oxide on photocatalysis photooxidation of propane and propene, and photo-assisted metathesis of propene. J Chem Soc Faraday Trans 94 695-700... 

V sol in acetic acid, ethanol and w. Prepn is by dehydration of propan-2-ol over Al oxide at 330°. It is also obtd as a pyrolysis product of propane and as a fraction of petr well head gases Propene has a Qc of 460.47kcal/mole the expln limits with air are 2.0 to 11.1% (Ref 2) it has an autoign temp of 927°F. Under unusual conditions, such as 955 atms press and... 

TPSR results are presented in Fig. 4. Propene is produced when the sample temperature is above 350 TC on both samples, which means converting of propane over CNF catalysts could occur without oxygm. The desorption products amounts are 0.35 and 0.26 mmol/g for CNF-RA and CNF-HA respectively while the percentages of propene in llie desorption substances over these two sample are 51.4% and 87.7%. These results imply that the propene selectivity may increase, at least partly, due to restriction of oxidation of propane to COx by heat treatment at the cost of catalytic activity. 

Table 8.6. Kinetic parameters of CO, propane and propene oxidation over M/A1203 and M/Ce02—A1203 catalysts (M = Pt, Pd or Rh)...

A catalytic system Mo-V-Nb-W supported on alumina was prepared by impregnation and investigated for the selective oxidation of propane. The effects of the variation of each metal and of the catalyst preparation were analysed. The results show that Mo and V species supported on alumina can lead to catalysts with high selectivity to propene and reasonable selectivity to acrolein. The presence of Nb and W seems to have little effect. The catalyst can be affected by the method of impregnation. 

Vanadia catalysts exhibit high activity and selectivity for numerous oxidation reactions. The reactions are partial oxidation of methane and methanol to formaldehyde, and oxidative dehydrogenation of propane to propene and ethane to ethcnc.62 62 The catalytic activity and selectivity of... 

Above 300°C. the effective reaction of an alkyl radical with oxygen may be Reaction 3 rather than 2 because of the reversibility of Reaction 2. If it is assumed that Reaction 3 is important at about 450°C., its rate can be estimated from the competition between pyrolysis and oxidation of alkyl radicals. Falconer and Knox (21) observed that the ratio of (pro-pene)/(ethylene) from the oxidation of propane between 435° and 475°C. increased with oxygen concentration and decreased with temperature—the apparent activation energy difference for the two reactions forming the olefins being 27 =t 5 kcal. per mole. They interpreted this result in terms of a competition between Reactions 1 and 3. The observed ratio (propene)/(ethylene) was 3.5 at 435°C. and 10 mm. of Hg pressure. If log ki(propyl) = 13.2 — 30,000/2.30RT, the value for the n-propyl radical (34), then log k3 = 8.0. If the A factor is 109-3, we derive the Arrhenius equation... 

Selective trapping of alkyl radicals from the alkyl halide component during the course of the catalytic disproportionation is the same as the previous observation with silver, and it indicates that the prime source of radicals in the Kharasch reaction lies in the oxidative addition of alkyl halide to reduced iron in Equation 47. Separate pathways for reaction of i-propyl groups derived from the organic halide and the Grignard reagent are also supported by deuterium labelling studies which show that they are not completely equilibrated.(49) Furthermore, the observation of CIDNP (AE multiplet effect) In the labelled propane and propene... 

Figure 4 Selectivity changes of acrylic acid (circle), propene (triangle), and further oxidation products (lozenge) as a function of contact time in the oxidation of propane over Mo-V-Te oxide (open symbol) and Mo-V-Te-Nb oxide (closed symbol) catalysts...

One-step partial oxidation of propane to acrylic acid (an essential chemical widely used for the production of esters, polyesters, amides, anilides, etc.) has been investigated so far on three types of catalysts, namely, vanadium phosphorus oxides, heteropolycompounds and, more successfully, on mixed metal oxides. The active catalysts generally consist of Mo and V elements, which are also found in catalysts used for the oxidation of propene to acrolein and that of acrolein to acrylic acid. 

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