Ethane, Oxidation Products

Figure 9.4 shows the temperature depertdence of the )deld of the sum of oxygenates for the oxidation of methane and ethane, whereas Fig. 9.5 displays the temperature dependence of the composition of ethane oxidation products. 

TABLE 10.1 Pressure Reactor 10. PARTIAL OXIDATION OF METHANE HOMOLOGUES Dependence of the Selectivity of Formation of Ethane Oxidation Products in a Static [22] ... 

FIGURE 10.1 Pressure dependence of the selectivity of formation of the main ethane oxidation products in an 8C2H6 + O2 mixture under static conditions ( ) methanol, ( ) ethanol, (A) formaldehyde, (A) acetaldehyde, and (O) acetic add. According to the data from Table 10.2. 

A topic of current interest is that of methane activation to give ethane or selected oxidation products such as methanol or formaldehyde. Oxide catalysts are used, and there may be mechanistic connections with the Fischer-Tropsch system (see Ref. 285). 

A typical oxidation is conducted at 700°C (113). Methyl radicals generated on the surface are effectively injected into the vapor space before further reaction occurs (114). Under these conditions, methyl radicals are not very reactive with oxygen and tend to dimerize. Ethane and its oxidation product ethylene can be produced in good efficiencies but maximum yield is limited to ca 20%. This limitation is imposed by the susceptibiUty of the intermediates to further oxidation (see Figs. 2 and 3). A conservative estimate of the lower limit of the oxidation rate constant ratio for ethane and ethylene with respect to methane is one, and the ratio for methanol may be at least 20 (115). 

Although ethylene is produced by various methods as follows, only a few are commercially proven thermal cracking of hydrocarbons, catalytic pyrolysis, membrane dehydrogenation of ethane, oxydehydrogenation of ethane, oxidative coupling of methane, methanol to ethylene, dehydration of ethanol, ethylene from coal, disproportionation of propylene, and ethylene as a by-product. 

An appreciation of statistical results can be gained from a study conducted to support the first application of computer control for an ethylene oxide production unit at Union Carbide Corporation in 1958. For the above purpose, twenty years of production experience with many units was correlated by excellent statisticians who had no regard for kinetics or chemistry. In spite of this, they did excellent, although entirely empirical work. One statement they made was ... [ethane has a significant effect on ethylene oxide production.] This was rejected by most technical people because it did not appear to make any sense ethane did not react, did not chemisorb, and went through the reactor unchanged. 

For illustration, we consider a simplified treatment of methane oxidative coupling in which ethane (desired product) and CO, (undesired) are produced (Mims et al., 1995). This is an example of the effort (so far not commercially feasible) to convert CH, to products for use in chemical syntheses (so-called Q chemistry ). In this illustration, both C Hg and CO, are stable primary products (Section 5.6.2). Both arise from a common intermediate, CH, which is produced from CH4 by reaction with an oxidative agent, MO. Here, MO is treated as another gas-phase molecule, although in practice it is a solid. The reaction may be represented by parallel steps as in Figure 7.1(a), but a mechanism for it is better represented as in Figure 7.1(b). 

In this paper selectivity in partial oxidation reactions is related to the manner in which hydrocarbon intermediates (R) are bound to surface metal centers on oxides. When the bonding is through oxygen atoms (M-O-R) selective oxidation products are favored, and when the bonding is directly between metal and hydrocarbon (M-R), total oxidation is preferred. Results are presented for two redox systems ethane oxidation on supported vanadium oxide and propylene oxidation on supported molybdenum oxide. The catalysts and adsorbates are stuped by laser Raman spectroscopy, reaction kinetics, and temperature-programmed reaction. Thermochemical calculations confirm that the M-R intermediates are more stable than the M-O-R intermediates. The longer surface residence time of the M-R complexes, coupled to their lack of ready decomposition pathways, is responsible for their total oxidation. 

In the investigation of hydrocarbon partial oxidation reactions the study of the factors that determine selectivity has been of paramount importance. In the past thirty years considerable work relevant to this topic has been carried out. However, there is yet no unified hypothesis to address this problem. In this paper we suggest that the primary reaction pathway in redox type reactions on oxides is determined by the structure of the adsorbed intermediate. When the hydrocarbon intermediate (R) is bonded through a metal oxygen bond (M-O-R) partial oxidation products are likely, but when the intermediate is bonded through a direct metal-carbon bond (M-R) total oxidation products are favored. Results on two redox systems are presented ethane oxidation on vanadium oxide and propylene oxidation on molybdenum oxide. 

Ethane Oxidation on Supported Vanadium Oxide. Figure 1 shows the rates of production of the major products of ethane oxidation over a series of silica-supported vanadium oxide catalysts. As was described earlier, the structure of the catalyst changed considerably with the active-phase loading (77). The low loading samples (0.3 -1.4%) were shown to consist primarily of 0=V03 monomeric units, while the high loading catalysts (3.5 - 9.8%) were composed of V2O5 crystallites. 

The selective oxidation of alkanes is cuiTently one of the most widely studied classes of catalytic reactions. This work mainly concentrates on the oxidative dehydrogenation of methane, with some attention paid to the partial oxidation of the product of this reaction, ethane. As regards the latter reaction, higher yields of pai tial oxidation products (acetaldehyde and ethylene) were achieved when N2O was used instead of O2 (1-6). 

In this alternative route, the first one-electron oxidized product in Eq. (40) is the kinetically favored as well as the thermodynamically stable electronic isomer, M. The evolution of the oxidation process, is now facilitated by the rapid isomerization to the unstable electronic isomer M7 in Eq. (41), which has again an available Ru11 site for the reaction proceeding to Ox as in Eq. (42). This route has been also found for R= [(NC)5FeIIbpaRuII(NH3)5], containing the non-communicating ligand bpa= Li-l,2-bis(4-pyridyl)ethane (131), and for... 

All these derivations assume that radical R is converted to oxidation products and that it does not reform the hydrocarbon. The relations are summarized in Table II, which also includes the experimental results for methane and ethane. 

Independent studies, including the addition of ethane to Do + 02 mixtures, are planned to evaluate any small occurrence of Reaction 26e, but in the meantime the results for ethane have been interpreted using Equation h which can be derived from Equation g by assuming all C2H5 radicals are converted to oxidation products. 

Methane reacts only slowly with oxygen below 400° C. Ethane oxidation was observed by Bone and Hill (S) at 290° to 323° C. Formaldehyde, a reaction product, was found to increase, reach a maximum, and then decrease. Addition in amounts of 1% to a 3 to 1 ethane-oxygen mixture at 316° C. and 720 mm. eliminated the induction period, but other additives such as nitregen dioxide, acetaldehyde, ethyl alcohol, or water, were also more or less effective. 

A prototype study for this issue was performed for the conversion of ethane to acetic acid [71] and the same group highlighted in an earlier comparative study of C3 oxidation [54] that, although initial propane activation is a difficult step, subsequent reactions associated with either excessive residence times of intermediates or with branching of reaction sequences into total oxidation may interfere with the overall selectivity to partial oxidation products. 

Fig. 8.8 Product yields evaluated by PAS detector on catalytic ethane oxidation over N02-treated catalysts. N02 gas was flowed onto each catalyst before reaction to produce active site (reproduced by permission of Elsevier from [20]).

C.M. Timperley, R.M. Black, M. Bird, I. Holden, J.L. Mundy and R.W. Read, Hydrolysis and oxidation products of the chemical warfare agents l,2-bis[(2-chloroethyl)thio]ethane Q and 2,2-bis(2-chloroethylthio)diethyl ether T, Phosphorus, Sulfur, Silicon Relat. Elem., 178, 1-20 (2003). 

In the slow combustion of ethane, on the other hand, ethyl alcohol has actually been detected amongst the oxidation products,1 and an analogous scheme is suggested 2 to that for methane. Thus ... 

The appeal of an acetic acid process, based on ethane oxidation, lies mostly in the absence of the need for the energy demanding step for syngas production. On the other hand, it has to compete not only with the well established methanol carbonylation (Section 4.2), but also with the current utilization of ethane in steam crackers for ethylene manufacture. In fact, ethane feedstock becomes attractive for acetic acid production if it is locally abundant and can be supplied at minimal cost, e.g., in a petrochemical complex close to a large gas field. The construction of a semi-commercial plant of 30 kt/a in the Persian Gulf region has been announced. 

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