Methane reaction, selectivities

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. 

It is obvious that one can use the basic ideas concerning the effect of alkali promoters on hydrogen and CO chemisorption (section 2.5.1) to explain their effect on the catalytic activity and selectivity of the CO hydrogenation reaction. For typical methanation catalysts, such as Ni, where the selectivity to CH4 can be as high as 95% or higher (at 500 to 550 K), the modification of the catalyst by alkali metals increases the rate of heavier hydrocarbon production and decreases the rate of methane formation.128 Promotion in this way makes the alkali promoted nickel surface to behave like an unpromoted iron surface for this catalytic action. The same behavior has been observed in model studies of the methanation reaction on Ni single crystals.129... 

The di-w-methane reaction is not only regioselective but is also stereo selective, as indicated by the following examples<13,14) ... 

Catalysts were prepared by impregnation using cobalt (ii) nitrate. Co/A1203 was the most the active and selective catalyst. Suppresses the ethanol decomposition and CO methanation reactions... 

Reaction between carbon monoxide and dihydrogen. The catalysts used were the Pd/Si02 samples described earlier in this paper. The steady-state reaction was first studied at atmospheric pressure in a flow system (Table II). Under the conditions of this work, selectivity was 100% to methane with all catalysts. The site time yield for methanation, STY, is defined as the number of CH molecules produced per second per site where the total number of sites is measured by dihydrogen chemisorption at RT before use, assuming H/Pd = 1. The values of STY increased almost three times as the particle size decreased. The data obtained by Vannice et al. (11,12) are included in Table II and we can see that the methanation reaction on palladium is structure-sensitive. It must also be noted that no increase of STY occurred by adding methanol to the feed stream which indicates that methane did not come from methanol. 

Owing to low copper content, copper-ceria catalysts are nonpyrophoric and stable, showing little or no deactivation during the experiments. The Cu0 2Ce0 x02 r catalyst prepared by coprecipitation method showed good catalytic activity for the WGS reaction. The Cu01Cc()9O2, catalyst prepared by sol-gel method was found to be less active, which could be due to lower number of active copper sites, or to different crystallite size and structure of copper-containing species. The copper-ceria catalysts were shown to be selective for the WGS reaction and no methanation reactions were observed over any catalyst under the experimental conditions used. 

Replacing one or several of the hydrogen atoms in methane by one or several other atoms than hydrogen automatically creates secondary or tertiary C-H bonds. Secondary and tertiary C-H bonds are more reactive than a primary C-H bond. During oxidation reactions, this leads to an easier oxidation of the reaction products than methane, and consequently to a low(er) reaction selectivity. Such reactions therefore produce complicated reactant mixmres that require costly... 

Praseodymium chloride pretreated in a helium flow at 750°C for 1 h produced a low conversion of methane and selectivity to C2+ compounds after 0.5 h on-stream both in the absence and presence of TCM (Figure 2 and Table I). When TCM was present, the conversion and selectivity increased to 17.1 and 46.4% after 1.8 h onstream, respectively, and then the values remained almost constant. In the absence of TCM, the conversion and selectivity also increased to 16.0 and 54.5%, respectively, after 1.8 h on-stream while in the latter case the values decreased gradually to 11.7 and 37.2% over 6 h on-stream. Although no TCM was added to the feedstream, methyl chloride was formed in the reaction. After 0.5 h on-stream, the selectivity to methyl chloride was 2.6% but decreased to 0.1% over 6 h onstream. The XRD pattern of the catalyst after the reaction with TCM present in the... 

Pros The choice of targets or objective functions is rather wide, depending on the reaction and application envisioned. Possibly, one easily accessible objective function such as CO conversion can be replaced by another more intrinsic parameter like the OSC for the Selox reaction, subject to a preliminary mechanistic investigation that clearly demonstrates the equivalence of the two parameters for evaluating performance. For the WGS reaction, selectivity is not a problem, except for methane and methanol side formation. For the Selox reaction, selectivity towards CO oxidation rather than towards H2 oxidation becomes a priority objective function. 

To check this assumed isothermal behavior, one first has to examine the temperature rise in a single well due to the chemical reaction [38], As test reaction, the catalytic partial oxidation of methane was selected ... 

As seen from Table VIII, in the gas phase the methanation reaction CO + 3H2 —> CH4 + H20, with AN = -49 kcal/mol, is much more exothermic than is the methanol formation CO + 2H2 - CH3OH, with AN = -22 kcal/mol. Thus, the only chance to selectively produce methanol is to have a catalyst on which hydrogenation of H CO species (x =... 

For the activation of methane, SO4 /Zr02 was found to be active for the reaction of methane with ethylene to form C3, t-C4, 1-C5 [64] In Table 5, the product distribution is compared with that of the other solid superacid, TaFs/AEC [65] For the chlorination of methane, the selectivity of S04 /Zr02 for the formation of methyl chloride exceeds 90% with 30% of chlorine conversion at 473 K and is above 80% with 70-90% conversion at 439 K The selectivity is enhanced by adding Pt, while Fe203-Mn02-S04 /Zr02 exhibits excellent sclectivities (99-100%) at lower conversions [66] The electrophilic insertion is said to involve electron-deficient metal-coordinated chlorine at the methane C H bond... 

Rostrup-Nielsen and Pedersen (209) recently studied sulfur poisoning of supported nickel catalysts in both methanation and Boudouard reactions by means of gravimetric and differential packed-bed reactor experiments. In their gravimetric experiments a synthesis mixture (H2/CO/He = 5/7/3) containing 1-2 ppm H2S was passed over a catalyst pellet of 13% Ni/Al203-MgO at 673 K and 1 atm. The rates of Boudouard and methanation reactions were determined from weight increases and exit methane concentrations respectively. In the presence of 2 ppm H2S a factor of 20 decrease was observed in both methanation and Boudouard rates over a period of 30-60 min. However, the selectivity or ratio of the rates for Boudouard and methanation reactions was constant with time. From these results the authors concluded that the methanation and Boudouard reactions involve the same intermediate, carbon, and that sulfur blocks the sites for the formation of this intermediate. 

The selectivity for C—C recombination and ultimately deactivation relative to chain growth or methanation will depend on the relative rate of C—C bond formafion versus that of C—H bond formation. In the methanation reaction, the M—C bond is replaced by a C—H bond in CH4, whereas the M—C bond remains partially intact upon C—C bond formation. Hence, the relative rate of methanation should decrease with increasing M—C bond energy (83). 

A limited number of uses and some particular reactions of methane have been disclosed in the patent literature, and owing to a relative lack of reactivity in comparison with its functionalized derivatives, it has been shown that the number of sufficiently selective methane reactions is very limited. Besides the useful reactions described above, a set of processes has been described in the patent literature that have, to date, received no practical application. We shall briefly summarize some examples of these reports ... 

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