Ethane conversion

If the production of vinyl chloride could be reduced to a single step, such as dkect chlorine substitution for hydrogen in ethylene or oxychlorination/cracking of ethylene to vinyl chloride, a major improvement over the traditional balanced process would be realized. The Hterature is filled with a variety of catalysts and processes for single-step manufacture of vinyl chloride (136—138). None has been commercialized because of the high temperatures, corrosive environments, and insufficient reaction selectivities so far encountered. Substitution of lower cost ethane or methane for ethylene in the manufacture of vinyl chloride has also been investigated. The Lummus-Transcat process (139), for instance, proposes a molten oxychlorination catalyst at 450—500°C to react ethane with chlorine to make vinyl chloride dkecfly. However, ethane conversion and selectivity to vinyl chloride are too low (30% and less than 40%, respectively) to make this process competitive. Numerous other catalysts and processes have been patented as weU, but none has been commercialized owing to problems with temperature, corrosion, and/or product selectivity (140—144). Because of the potential payback, however, this is a very active area of research. 

Fig. 7. Equihbrium conversion of ethane versus temperature at 210 kPa in a membrane reactor. The effect of hydrogen removal on ethane conversion is...

Fig. 1 compares the activities of vanadium-, cobalt- and nickel- based catalysts in ODH of ethane. Representative catalysts contained between 2.9 and 3.9 wt.% of metal. It is to be pointed out that metal oxide-like species was not present at any of the catalysts, as its presentation is generally the reason in the activity-selectivity decrease. The absence of metal oxide-like species was evidenced by the absence of its characteristic bands in the UV-Vis spectra of hydrated and dehydrated catalysts (not shown in the Figure). The activity of catalysts was compared (i) at 600 °C, (ii) using reaction mixture of 9.0 vol. % ethane and 2.5 vol. % oxygen in helium, and (iii) contact time W/F 0.12 g. i.s.ml 1. These reaction conditions represent the most effective reaction conditions for V-HMS catalysts [4] The ethane conversions, the ethene yields and the selectivity to ethene varied between 13-30 %, 5-16 %, and 37-78 %, respectively, depending on the type of metal species (Co, Ni, V) and support material (A1203, HMS, MFI). 

Figure 1 Ethane conversion (X), ethene yield (Y) and selectivity to ethene (S) in ODH of ethane over Co-, Ni-, and V- loaded -Al203, -HMS, and MFI catalysts (wt. % of Co, Ni, and V are given in an figure for individual catalysts). Reaction conditions 9.0 vol. % ethane, 2.5 vol. % 02 and He, 200 mg catalyst, total flow 100 ml.min1, W/F 0.12 g.s.cm"3, and 600 °C.

The contribution deals with the catalytic performance of V-, Co-, and Ni-based microporous (MFI), mesoporous (HMS) and alumina catalysts in ODH of ethane. Representative catalysts contained between 2.9 and 3.9 wt.% of metal. Ni-, V- and Co-A1203, and V- and Ni-HMS were effective catalysts in ODH of ethane. However, Ni-A1203 had the best selectivity-conversion behavior. The most favorable set up corresponded to 46 % in the ethane conversion, 30 % in the ethene yield 30 %, 65 % in the selectivity to ethene, and 0.91 g(C2=).gca, 1.h 1 in the ethene productivity for Ni-A1203. The activity was stable for 6 hours time-on-stream. 

Plot the selectivity to C4H8 as a function of ethane conversion. Does it behave like a secondary or primary product Consult the paper by Dean (1990), and describe additional reactions which lead to molecular weight growth in hydrocarbon pyrolysis systems. While some higher molecular weight products are valuable, the heavier... 

Such advantageous catalytic properties were not exhibited by the divalent metal molybdates the ethane conversion was low and was not connected with higher selectivity. Ethylene was the main product of oxidative dehydrogenation, but its selectivity and yield were much less than on the above catalysts. Acetaldehyde was produced in only small amounts, with low selectivity. A decrease was observed in the selectivities for C2H4 and CH3CHO with increasing conversion, which is a... 

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... 

Catalysts similar to those claimed by Union Carbide were later studied by Bordes and coworkers [4], and by Burch and coworkers [5]. Merzouki et al. [4a, b] proposed that the Mo/V/Nb/O catalyst is made up of (VNbMo)5014-type microdomains in a M0O3 matrix. At 200 °C, a selectivity of 45% to acetic acid and 45% to ethylene was obtained at 25% ethane conversion an increase of temperature caused a loss in selectivity to acetic acid in favor of that to ethylene. Burch and Swarnakar [5a] compared the reactivity of Mo/V/O and Mo/V/Nb/O systems. The former contained M0O3, Mo6V9O40 and Mo4V6025 crystalline compounds, while the latter also contained Mo3Nb2On, the most intense diffraction line of which occurred at 4.01 A The addition of Nb increased both activity and selectivity, and the formation of Mo3Nb201 i was proposed to account for the increase in performance. The product distribution was independent of the conversion, indicating the absence of consecutive reactions. 

Catalyst composition Ethane conversion (%) Ethylene selectivity (5)... 

Fig. 3.15 Comparison of (a) ethane conversion and (b) ethylene selectivity between Nio.63Nbo.37O, and Mo0.72V0.26Nb0.02Oj, as a function of temperature.

Selectivities and conversions for ethane conversion are summarized in Table IV33 for comparison. 

Blank runs, in which porcelain chips were used to fill the catalyst zone in the quartz reactor, were made at 820 C. Methane at 10/1 CH4/O9 ratio (55 cc/min CH4 + 55 cc/min 107. O2 in He) was 1.57. converted at 1007. selectivity to ethane. No conversion of methane was measured in the absence of Oo. Ethane conversion at 5/1 2%/ 2 total gas rate of 82.5 cc/min over the porcelain was... 

For a given ethane conversion, it was found that a plot of the total coke yield vs. ethylene yield resulted in a straight line and that the sum of ethylene and total coke yields was identical for a given ethane conversion, within experimental accuracy. Figure 1 shows a plot of these two yields at an ethane conversion of 65% in this case, the sum of the yields was about 93%. At lower conversions the sum of yields was higher, approaching 100% at zero conversion. 

The Mo(VI)/HZSM-5 catalyst exhibits excellent activity for cleavage of the C- C bond in C2H5 and thus higher CH4 selectivity can be observed. This also can be served as an evidence that the formation of CH2 carbene over Mo(VI)/HZSM-5 and that the carbene is stabilized by the formation of Mo=CH2 metal carbene. The ethane conversion over Mo(VI)/HZSM-5 thus can be schemed as follows ... 

Sample Metal content (%) Crystallinity (%) Ethane conversion ... 

The variation of the ethane conversion and the selectivity to ethene with the reaction temperature on VAPO-5 (sample A-2), MgVAPO-5 (sample B-2), and on the MgO-supported vanadium sample, are shown in Figure 4. In all cases, ethene, CO and CO2 are the reaction products. Oxygenated products other than carbon oxides have been not obtained. 

From the results of Figure 4a, it can be seen that different contact time are necessary to achieved similar ethane conversion. Thus, it can be concluded that the activity, per gram of catalyst, decreases in the following trend V/MgO > VAPO-5 > MgVAPO-5. However, when the specific activities, calculated per vanadium atom, are considered, it can be seen that the activity of vanadium atoms decrease in the trend VAPO-5 > MgVAPO-5 > V/MgO. 

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