Ethane product selectivity

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). 

Oxychlorination reactor feed purity can also contribute to by-product formation, although the problem usually is only with low levels of acetylene which are normally present in HCl from the EDC cracking process. Since any acetylene fed to the oxychlorination reactor will be converted to highly chlorinated C2 by-products, selective hydrogenation of this acetylene to ethylene and ethane is widely used as a preventive measure (78,98—102). 

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. 

On-line GC analysis (Shimadzu GC 14A) was used to measure product selectivity and methane conversion. Details on the analysis procedure used for batch and continuous-flow operation are given elsewhere [12]. The molecular sieve trap was found to trap practically all ethylene, COj and HjO produced a significant, and controllable via the adsorbent mass, percentage of ethane and practically no methane, oxygen or CO, for temperatures 50-70 C. The trap was heated to -300°C in order to release all trapped products into the recirculating gas phase (in the case of batch operation), or in a slow He stream (in the case of continuous flow operation). 

Supported metal hydrides of early transition metals catalyze the hydrogenolysis of alkanes at relatively low temperatures (50-150 °C) [29,90-92]. Noteworthy are their differences in product selectivities. For example, the hydrogenolysis of propane in the presence of a large excess of H2 gives a 1 1 mixture of ethane and methane in the case of zirconium hydride, a group 4... 

Fig. ii. Selectivity for ethane production from n-butane hydrogenolysis on iridium as a function of effective particle size. (From R s. HI. 112.) Also shown are data for n-butane hydrogenolysis on supported Ir catalysts The temperature is 47S K in all cases. 

Figure 5. Product selectivity for n-butane cracking over (a) hydrothermally-treated sample H3, and (b) AHFS-treated sample FS9.363. The reaction products are methane (0), ethane and ethene (A), propane (O), isobutane (b), but-l-ene ( ) and but-2-ene (a).

When using Pt/a-alumina, the product selectivity did not change with time (ethane was the primary product), although TCA conversion declined rapidly. The difference between the selectivities for the two Pt catalysts as they deactivate suggests that the ri-alumina support may initiate the reaction. Specifically, HCl elimination to form DCE (Reaction 2) appears to be the initial reaction that occurs primarily on the acidic sites of the ri(5)-alumina support. Hydrogenation of this unsaturated chlorocarbon to DCA then occurs on the Pt. This is consistent with the observation that large concentrations of DCE were only observed on the two catalysts that had acidic sites (Ti8-alumina,Pt/Ti-alumina Table 1). However, Pt has activity for the HCl elimination step since the Pt/a-alumina was active for the hydrodechlorination reaction while the a-alumina was not. In addition, because DCE was observed in the effluent when using the Pt/p-alumina before the conversion decreased, deactivation most likely occurred first by the loss of... 

Experiments were also conducted to assess the impact of compressed N2, ethane, and propane on product selectivity of whole cell biocatalysis (45). Pressurized incubations, with and without a compressed solvent headspace, lead to an increase in the ratio of ethanol to acetate produced by the organism (Fig. 1). These results are consistent with increased dissolution of H2, a product gas which affects the pathways of acetate production (50), in the fermentation media with increasing pressure. Lactate formation was also decreased in the presence of compressed and liquid solvents (but not nitro-... 

Comparison of H-GaMFI zeolite catalysts with and without silica binder (50 wt%) for their product selectivity, dehydrogenation / cracking (D/C) and aromatization / cracking (A/C) activity ratios, and aromatization / (methane + ethane) mass ratio and p-X / m-X ratio in the propane aromatization at isoconversion of propane (x). 

Figure 1. Effect of Sr/La ratio on the ethane and O2 conversion, product selectivity and CO/CO2 ratio in the oxidative dehydrogenation of ethane to ethylene over Sr-La2O3/SA-5205 catalyst [ Reaction condition Temperature = 700°C, C2H5/O2 = 6.0, steam/C2H6 = 1.0, space velocity = 100,104 cm. g. h ].

Figure 1. Product selectivity (methane, ethane, propane, isobutane, butane, isopentane, pentane, hexanes, heptanes) in metatheses of ethane, propane, butane, and isobutane.

Anthony and Singh concluded from a kinetic analysis of the methanol conversion to low molecular weight olefins on chabazite that propylene, methane, and propane are produced by primary reactions and do not participate in any secondary reactions, whereas dimethylether, carbon monoxide, and ethane do. Ethylene and carbon dioxide appear to be produced by secondary reactions. It was also shown that the product selectivities could be correlated to the methanol conversion even though the selectivity and the conversion changed with increasing time on stream due to deactivation by coke formation. 

Exploration of alkaline earth/metal oxide catalysts and other metal/metal oxide catalysts has been continued at Union Carbide. As an example, after over 350 hours of methane coupling with a 5 wt% barium carbonate on titanium oxide (with ethyl chloride in the feed gas), a C2 yield of 22%, a Cj selectivity of 58%, and an ethylene/ethane ratio of 8 1 were obtained. The coupling catalysts were comparable in selectivity, activity, and Cj yield to the better literature catalysts, but provide hundreds of hours of stable operation in the oxidation of methane to Cj s. These catalysts require the presence of a small amount of halides, either as a catalyst component or as a periodic or continuous additive to the catalyst. The chloride appears to serve three distinct roles, resulting in suppression of carbon dioxide formation, increased rates to Cg products, and higher ethylene-to-ethane product ratios. There have been numerous other recent reports. 

Like their Mo-Ir analogs, these clusters were examined for n-butane hydro-genolysis. The selectivity of the [WIr3] MMCD catalyst for ethane production was >70%, compared with <50% for the [W2lr2] catalyst. ... 

It is convenient to examine structure-sensitivity as revealed only by rates, since much of the available information concerns ethane there are however major and important effects on product selectivities in the reaction of n-butane especially. 

The reviewer of the literature faces considerable difficulties. Quite often, those studying the reactions of propane and -butane do not trouble to measure or report product selectivities, and when they are given it is most usually in graphical form, from which numbers of limited accuracy have to be extracted by tedious interpolation. Sometimes only ethane selectivities are quoted. Measurements made in UHV systems seem to be more scattered than those made in conventional equipment, and almost all values have to be converted into the Kempling-Anderson formalism (equations 13.13 and 13.14) to make them comparable. In the accompanying Tables 13.9 and 13.10, most values of F and Tj, are obtained by method (2) above. Although for n-butane products the values of 5i can easily be derived from S2 and 53,they are quoted in the tables to save the reader unnecessary labour. 

The alkane metathesis of highly branched alkanes and product selectivity also follows the same mechanism with catalyst 24. A selective and catalytic conversion of 2-methylpropane into 2,3-dimethylbutane (42%), and ethane (41%) (Scheme 23) [80] was observed when 2-methylpropane was passed over the catalyst 24 at 150°C. Conversion was reached up to 8% and 37 TON was achieved over 43 h (Scheme 23). 

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