Methanation reaction increasing yields

The question of which pathway is preferred was very recently addressed for several diimine-chelated platinum complexes (93). It was convincingly shown for dimethyl complexes chelated by a variety of diimines that the metal is the kinetic site of protonation. In the system under investigation, acetonitrile was used as the trapping ligand L (see Fig. 1) which reacted with the methane complex B to form the elimination product C and also reacted with the five-coordinate alkyl hydride species D to form the stable six-coordinate complex E (93). An increase in the concentration of acetonitrile led to increased yields of the methyl (hydrido)platinum(IV) complex E relative to the platinum(II) product C. It was concluded that the equilibration between the species D and B and the irreversible and associative1 reactions of these species with acetonitrile occur at comparable rates such that the kinetic product of the protonation is more efficiently trapped at higher acetonitrile concentrations. Thus, in these systems protonation occurs preferentially at platinum and, by the principle of microscopic reversibility, this indicates that C-H activation with these systems occurs preferentially via oxidative addition (93). 

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. 

The stepwise electron reduction of C02, whether direct or indirect, catalyzed, or by direct transfer on an apparently inert conductive surface, has been the object of considerable attention since the first concise reports of formate anion production. Since then, the list of possible derivatives has grown from formates to carbon monoxide, methane, ethylene, and short-chain saturated hydrocarbons. As noted in Section 12.1, this area of research has been expanded in recent years [8, 80, 83], with information relating to increased yields, to the effect of electrode materials on selectivity, as well as further speculations on possible reaction mechanisms, having been obtained on a continuous basis. Yet, the key to these synthetic processes-an understanding of the relationship between the surface of the electrode and the synthetic behavior of the system-seems no closer to being identified. 

Figure 4.10 shows temperature influence on the process results formaldehyde yield reaches its maximum (about 40%) with temperature raise to 520 °C and total methane conversion increase. Above 520 °C, CO and C02 are detected in reaction products. Their formation rates noticeably increase with temperature. The occurrence of these compounds in the system is explained by sequential formaldehyde transformation to intense degradation products in the high temperature range. After-oxidation of methanol synthesized in the system also contributes to formation of these products. 

The curve of 02 accumulation shows that short contact times, at which the methane oxidation rate is low, are enough for complete H202 dissociation. However, as observed from shapes of 02 and CH3OH accumulation curves, methanol yield increases synchronously with 02 yield decrease, and from the moment r = 10.2 s both curves stabilize. Such stabilization and synchronization of catalase and monooxygenase reaction product yields is the experimental proof of their interaction, displayed by chemical conjugation. The existence of the stabilization zone of 02 and CH3OH yields is associated with full H202 dissociation. 

The radiolysis of methane in the solid phase has been examined in several studies. ESR studies by Smaller and Matheson and by Wall eta/. have shown that methyl radicals and hydrogen atoms are formed in nearly equal quantities and that Gchj = 0.9. A product analysis in a study by Ausloos et shows that hydrogen and ethane are almost the only products of solid methane radiolysis at 20 or 77 °K. It seems that ethane is formed by both methylene insertion and by methyl radical combination, while hydrogen is formed by direct elimination and by bimolecular processes. A small quantity of ethylene formed at 20 °K is absent at 77 °K this has been attributed to the reaction of hydrogen atoms with ethylene at 77 °K (but not at 20 °K) to give ethyl radicals and finally higher products which are observed in increased yield at 77 °K. 

The yield maximum as a function of temperature is probably due to two effects. At low temperatures the reaction rate of the chemical reaction increases with temperature. At high temperatures the recombination of to with subsequent desorption leads to a lowering of the surface concentration of hydrogen and to a lower methane production rate. 

Alternative technologies to the PSA process for H2 purification include, after the HTS reaction, a low-temperature shift (LTS) reaction followed by C02 scrubbing (e.g., monoethanolamine or hot potash).11 The LTS reaction can increase the H2 yield slightly. However, the product stream, after the HTS, needs to be cooled to about 220 °C. Preferential oxidation (Prox) and/or methanation reaction as shown in Equations 2.6 and 2.7, respectively, removes the traces of CO and C02. The product H2 has a purity of over 97%. 

No measurable amount of CH4 was formed during the reaction of CO + H2 (1 4) pulses over Ru metal at temperatures of 300 and 370 K. While no CH4 was produced from the first pulse at the sample temperatures of 420 and 470 K, successive pulses gave rise to progressively increasing yields of methane and the CO(.d) --> CH4 conversion reached a saturation value of about 45 and 98% for the reaction temperature of 420 and 470 K, respectively. 

Other important endothermic transformations are those involving C02 (reactions 10 and 11). The Boudouard reaction occurs between carbon and carbon dioxide to increase the yield of carbon monoxide (reaction 10). Finally, some methanation reactions by hydrogenation of carbon oxides also take place (reactions 12 and 13), which may lead to a significant decrease in the H2 concentration of the final synthesis gas. The overall energy requirement of gasification can be balanced by a suitable combination of exothermic and endothermic reactions, mainly through the control of the 02/H20 ratio in the reaction medium. 

The influence of COj on the oxidative coupling of methane with LijCOj/MgO catalysts was studied by Korf et al. It was observed that at 800 C, the yield of C -hydrocarbons decreased from 13% to 3% after 40 hours of reaction. The introduction of COj after the reaction increased C2 yield by 12%. Thus, the catalyst had been restored to its initial behavior by the introduction of COj, despite the loss of Li. It was concluded that the active sites created on the Li/MgO catalyst as a direct result of the loss of carbonate species were not stable in the atmosphere of the oxidative coupling reaction. The authors were able to achieve a yield of 18% for a period of 13.5 hours by carefully choosing reaction conditions for Li/MgO. 

Anhydrous hydrogen fluoride and fluorosulfuric acid were found as improved solvents for the nitration of methane with nitronium salts. In these strong acid solvents even under the mild reaction conditions (room ten jerature) at least tenfold increased yields ( %) could be obtained. As no protolytic reactions of methane take place in HF under the reaction conditions, side reactions are not observed. This is not the case in the nitration of isobutane in HF solution, where 90 of the nitroalkanes obtained consisted of l-nitro-2-methylpropane. Since only traces of this iscmer were found in nitronium salt nitration in aprotic CH2Cl2-sulfolane solution, the formation of l-nitro-2-methylpropane must be considered due to formation of isobutylene (either frcm isobutane itself in HF containing PFc or more probably frcm the protolytic cleavage of 2-nltro-2-meuhylpropane). 

Observe that there is no net production of CO2. The percent conversion of methane is slightly greater at 95.9%, with the production of synthesis gas slightly increased to 12.946 + 38.859 = 51.805 kmol/hr. Note that in case (a), the production of CO2 from CO by the water-gas shift reaction gives an additional mole of H2 for every mole of CO2 produced. Thus, by eliminating the net production of CO2, less H2 is produced. The usual benefit of the increased yield of the main product(s) by recycle to extinction is not achieved... 

Tungstophosphoric acid (TPA) supported on zirconia was employed as an efficient, heterogeneous catalyst for the liquid-phase electrophilic substitution reactions of indoles with aldehydes or indolecarboxaldehyde to afford bis(indolyl)methanes or tris(indolyl)methanes in high yields. The catalytic efficiency of TPA was increased by supporting on zirconia, which was expressed in terms of turnover number (TON) and turnover frequency (TOP per hour) (Scheme 2.19) (Satam et al. 2008). 

For the pyrolysis of paraffinic hydrocarbons at 700- 800 C, yields of olefins such as ethylene, propylene, butenes, butadiene and cycloolefins increase during the initial stage of the reaction, pass through their maxima, and later decrease yields of aromatics, hydrogen and methane however increase monotonically throughout the reaction course. Sakai et al. (1 ) reported previously the result of a kinetic study on thermal reactions of ethylene, propylene, butenes, butadiene and these respective olefins with butadiene at the conditions similar to those of paraffin pyrolysis, directing their attention on the rates of formation of cyclic compounds. Kinetic features of the thermal reactions of these olefins are sunnnarized in Table I combined with the results obtained in later investigations for thermal reactions of cycloolefins ( 2) and benzene O). 

The water-gas shift reaction is promoted because of the increase in water availability when steam is used. The water-gas shift can be understood as a pathway through which the H2 CO ratio increases. Note that, if that ratio at some point increases to 3 1, we reach the stoichiometric ratio for the methanation reaction, which in turn produces methane. Therefore, when steam is used as the gasifying agent, one should expect larger yields of H2, CO2, and CH4, and lower yields of CO. 

Although this process offers the advantage of driving a low temperature, carefully controlled oxidation of methane, thereby increasing the yield of methanol, it also utilizes sulfuric acid to produce the intermediate methyl bisulfate. The need for acid resistant containers to perform these reactions may raise costs of the process. And although the sulfuric acid is recovered and recycled into the process, the environmental benefits of this methane conversion are somewhat offset by the need to ship and store hazardous sulfuric acid. The trade-off between safer methane transport versus increased sulfuric acid transport and storage needs to be considered from the perspective of accidental releases. 

You are now in a position to understand the optimum conditions for the methanation reaction. Because the reaction is exothermic, low temperatures should favor high yields of methane that is, the equihbrium constant is large for low temperature. But gaseous reactions are often very slow at room temperature. In practice, the methanation reaction is run at moderately elevated temperatures (230°C-450°C) in the presence of a nickel catalyst. Under these conditions, the rate of reaction is sufficiently fast but the equilibrium constant is not too small. Because the methanation reaction involves a decrease in moles of gas, the yield of methane should increase as the pressure increases. However, the equilibrium constant is large at the usual operating temperatures, so very high pressures are not needed to obtain economical yields of methane. Pressures of 1 atm to 100 atm are usual for this reaction. 

---