Oxidative dehydrogenation of ethane

Unlike higher alkanes, ethane contains only primary C—H bonds, and the dehydrogenation product ethene contains only vinylic C—H bonds. As shown in Table I, these are strong bonds. Thus one would expect that, compared to other alkanes, the activation of ethane would require the highest temperature, but the reaction might be the most selective in terms of the formation of alkene. Indeed, this appears to be the case. 

It has been demonstrated, however, that the activity of an oxide catalyst for ethane oxidation can be preferentially increased by treating it with chloride or sulfide (14). If a Co-Zr-P-Na-K oxide catalyst was treated with CH3C1, an ethene selectivity of 85% at 55% ethane conversion was obtained at 675°C, compared with 74% selectivity at 32% conversion on the 

It is interesting to note that the catalysts that show good selectivities at the higher temperatures generally do not contain easily reducible metal ions, such as V, Mo, or Sb. Many of the catalysts for the lower-temperatures operation, on the other hand, contain these reducible cations. In a study using a Li-Mg oxide, it was established that gas-phase ethyl radicals could be generated by reaction of ethane with the surface at about 600°C (17). These radicals could be trapped by matrix isolation and identified by electron spin resonance spectroscopy. 

The dependence of ethene selectivity on the conversion of ethane for the better catalysts shown in Fig. 1 shows that the selectivity is high at low conversions and decreases as the conversion increases. This trend is consistent with a reaction pathway that consists of mostly sequential reactions [Eq. (3)]. Depending on the reaction temperature, the reaction network may involve two parallel reaction pathways shown below, which is modified from 

The surface reaction consists of two competitive pathways. Their relative rates determine selectivity. The ethyl species may undergo further dehydrogenation to form ethene [Eq. (6)] or be oxidized to ethoxide and then to acetaldehyde or acetate [Eq. (7)], and possibly to carbon oxides. The formation of ethoxide is favored at lower temperatures and in the presence of water vapor (18, 19). Other surface reactions are also possible. They are discussed later. 

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Wang SB, Murata K, Hayakawa T, Hamakawa S, Suzuki K (1999) Excellent performance of lithium doped sulphated zirconia in oxidative dehydrogenation of ethane. Chem Commun 103-104. 

Oxidative Dehydrogenation of Ethane. The dehydrogenation of alkanes also occurs, but in a catalytic manner, over molybdenum supported on silica (22,23). In addition to the stoichiometric reactions, the role of the 0 ion in this catalytic reaction is further suggested by the observation that N2O is an effective oxidant at temperatures as low as 280°C, but no reaction is observed at these temperatures with O2 as the oxidant (22). It should be noted that at moderate temperatues N2O gives rise to 0 , whereas O2 yields O2 over Mo/Si02. Under steady-state conditions the rates of formation of C2Hi were in the ratio of 7 1 at 375°C and 3.7 1 at 450°C when N2O and O2 were used as the oxidants, respectively (23). ... 

A. Christodoulakis, E. Heracleous, A.A. Lemonidou and S. Boghosian, An operando Raman study of structure and reactivity of alumina-supported molybdenum oxide catalysts for the oxidative dehydrogenation of ethane, J. 

We wish to produce ethylene by the oxidative dehydrogenation of ethane. Over a suitable catalyst the reactions and rates are found to be... 

We have a catalyst that causes the oxidative dehydrogenation of ethane to ethylene... 

For example, the oxidative dehydrogenation of ethane and propane was examined via UV-visible and Raman spectra. The study investigated the catalytic properties vanadia formulations that possessed a range of VO surface species density (1.4—34.2 V/nm ) on an AI2O3 support. The observations showed increased surface densities, greater than 2.3 V/nm , favored two-dimensional polyvanadates. At lower surface densities, ca. 2.3 V/nm , predominately isolated monovanadate species were observed. Further increasing surface densities to more than 7.0 V/nm yielded V2O5 crystallites. ... 

A technology called the Ethoxene process was developed by Union Carbide for the oxidative dehydrogenation of ethane.165 Under the relatively mild conditions applied (300-400°C, 1-40 atm) cracking is minimal and the only byproduct is acetic acid. The combined efficiency to ethylene and acetic acid is about 90% with an ethylene acetic acid ratio of 10. 

Similar improvements of activity and selectivity were reported in the oxidative dehydrogenation by C02 of ethane over Ga2C>3 (18.6% ethylene yield and 94.5% selectivity)391 and that of propane over rare earth vanadates.392 Cr203 shows medium activity in the oxidative dehydrogenation of ethane, but support on Si02 enhances the catalytic performance (55.5% ethylene yield at 61% conversion at 650°C).393... 

Combinatorial methods were also applied in the discovery of new catalysts for the low-temperature oxidation213 and oxidative dehydrogenation of propane.214 A 144-member catalyst library was screened by photothermal deflection spectroscopy and mass spectrometry to find the most active compositions of V-Al-Nb and Cr-Al-Nb oxides for the oxidative dehydrogenation of ethane.215 The ternary combination V(45)-Sn(45)-Mo(10)-O selected by laser-induced fluorescence imaging gave much higher yield than did V205 in the selective oxidation of naphthalene to naphthoquinone.216... 

Fig. 1. Selectivity for oxidative dehydrogenation of ethane to ethene. Data taken from Table 3. ( ) Reaction under or (+) above 600°C. Solid line denotes selectivity-conversion relationship for = kt..

The difference in reactivity of the butadiene precursor toward 02 and NzO is interesting. N20 is known to be active in selective oxidation (16). For example, on molybdenum oxide (16) and cobalt magnesium oxide (17), NzO decomposes at room temperature to form an O adsorbed species which is very active in the oxidative dehydrogenation of ethane. The results presented above suggest that the degradation of butadiene precursor on a-iron oxide requires an 02 and not an 0" species. This implies that the degradation proceeds via a peroxide intermediate. 

In early 1997, Schuurman, et al. reported Ni-based catalysts for the oxidative dehydrogenation of ethane to ethylene [12], but with only a limited ability to explore composition space the results were not compelling, and the real potential for Ni-based systems was missed. Tab. 1.1 shows secondary screening performance data for pure Ni oxide and binary and ternary Ni compositions containing Ta and Nb. The pure Ni catalyst is poor in terms of both conversion (11%) and selectivity (54%). The Ni catalyst containing 12% Ta was essentially the same (12% conversion and 55% selectivity). Increasing the Ta concentration in the binary to 38%... 

Using this set-up, two new catalysts were found for the oxidative dehydrogenation of ethane to ethene. With Cr/Mo-Ox and Co/Cr/Sn/W-O, catalysts an industrially relevant product yield of more than 60% was reached. 

Fig. 6.2 Example architecture of a neural network used to study the dependence of ethylene yield on catalyst composition and contact time in high-throughput experiments for the oxidative dehydrogenation of ethane.

Fig. 8.13 Ethene concentration in the oxidative dehydrogenation of ethane. The concentration estimated by PAS was compared with that by GC (reproduced by permission of Wiley-VCH, Weinheim from [24]).

Micro structured wells (2 mm x 2 mm x 0.2 mm) on the catalyst quartz wafer were manufactured by sandblasting with alumina powder through steel masks [7]. Each well was filled with mg catalyst. This 16 x 16 array of micro reactors was supplied with reagents by a micro fabricated gas distribution wafer, which also acted as a pressure restriction. The products were trapped on an absorbent plate by chemical reaction, condensation or absorption. The absorbent array was removed from the reactor and sprayed with dye solution to obtain a color reaction, which was then used for the detection of active catalysts by a CCD camera. Alternatively, the analysis was also carried out with a scanning mass spectrometer. The above-described reactor configuration was used for the primary screening of the oxidative dehydrogenation of ethane to ethylene, the selective oxidation of ethane to acetic acid, and the selective ammonoxidation of propane to acrylonitrile. 

Figure 3.35 Ethylene produced by oxidative dehydrogenation of ethane over Mo-V-Nb and Ni-Ce-Ta oxide catalyst libraries. The detection of ethylene was performed in a scanning mass spectrometer using a photothermal deflection method. Inactive Mo-V-Nb oxide catalyst (a) active Ni-Ce-Ta oxide library (b) [7] (by courtesy of Kluwer Academic Publishers).

Primary screening can be done on wafer-based ternary mixed metal oxide libraries. For the oxidative dehydrogenation of ethane, two interesting libraries consist of Ni-Ce-Nb and Ni-Ce-Ta oxides. The maximum amount of ethylene produced is 1800 ppm at 400 °C in nickel-rich regions of the catalyst mixture (Figure 3.35b) compared with inactive Mo-V-Nb oxide catalysts (Figure 3.35a) [7]. 

Volpe, A. F., Weinberg, W. H., Woo, L., Zysk, J., Combinatorial heterogeneous catalysis oxidative dehydrogenation of ethane to ethylene, selective oxidation of ethane to acetic acid, and selective ammonoxidation of propane to acrylonitrile, Top. Catal. 2003, 23, 65-79. 

Thorsteinson etal.21 investigated mixed Mo-V-Nb catalysts containing 20 to 30 at% Vfor the oxidative dehydrogenation of ethane. Extensive reduction by ethane at 400 °C showed an increase of the intensity of the Nb peaks relative to those of Mo. Comparison of catalysts with and without V showed that this strongly facilitates the reoxidation of Mo4+ to Mo6+ at the surface. 

As in previous investigations [63, 64], the oxidative dehydrogenation of ethane to ethylene was chosen as a model reaction for an experimental study on the laboratory scale. A VOx/y-AhCti catalyst with 1.4 % V was used, and prepared using standard techniques described in Ref. [65]. The particle size of the support was 1.8 mm. 

For various types of catalyst there are results of kinetic investigations for the oxidative dehydrogenation of ethane available (e.g., for a magnesium oxide catalyst doped with samarium oxide, lithium nitrate and ammonium chloride [64] or a V2O5/Y-AI2O3 catalyst [68]). In another study with a Sn.oLai.oNdi.oOx catalyst, investigations were reported of noncatalytic reactions, which were found to be significant at temperatures above 700 °C [69]. 

Argyle, M.D., Chen, K.D., Bell, A.T. and Iglesia, E. (2002) Effect of catalyst structure on oxidative dehydrogenation of ethane and propane on alumina-supported vanadia. Journal of Catalysis, 208 (1), 139-49. 

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