Methane homologation reaction

Methane is chemisorbed below 723 K by dehydrocondensation on the metal clusters [Mx] in pores of Ru3/NaY to yield metal carbide species [MxC ]( 1). By 

The reactive Cj carbon is effectively converted with hydrogen at 300-373 K to provide methane and ethane (12-23% selectivity) as the C2+ products. Additionally, the less reactive species gave methane and C2 hydrocarbons including ethane and C3-C5 fraction (2-59% selectivity) at 373 523 The yields of higher 

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Co2(CO)g has been used to obtain encapsulated cobalt clusters in Y-faujasite, which have been used as model catalysts for methane homologation [152]. The gas phase adsorption of Co2(CO)8 under N2 rendered predominately encaged Co4(CO)i2 species whereas Co,s(CO)iis was obtained when the impregnation of Co2(CO)8 was carried out under a CO/H2 atmosphere [152, 155], Samples were oxidized at 80°C, subsequently reduced at 400 °C and then structurally characterized by EXAFS. Clusters of two and three cobalt atoms were formed from encaged Co4(CO)i2 and COis(CO)iis, respectively. Higher methane conversion and selectivity to C2+ products in the CH4 homologation reaction have been obtained for the two atoms-size cluster sample the results were discussed using a DFT model [152]. 

The transformation was called an homologation reaction because essentially it consisted in going from one alcohol to an alcohol containing one carbon atom more than the starting material (Wender, Levine, and Orchin, 14). Tertiary alcohols reacted most rapidly, secondary alcohols less rapidly and primary alcohols only very slowly. It was of considerable importance to ascertain whether the olefin intermediate was essential and for this purpose, methanol and benzyl alcohol, neither of which can dehydrate to an olefin, were used in the reaction. Both compounds, contrary to other primary alcohols, reacted quite rapidly and gave the homologous alcohol of the methanol converted, about 40 mole per cent went to ethanol and with benzyl alcohol, a 30% yield of 2-phenylethanol was secured. In both examples, however, reduction products were also present of the methanol converted, 8 mole per cent went to methane and from benzyl alcohol, a 50 to 60% yield of toluene was secured. The conversion of methanol to methane appears to be the only case in which an appreciable quantity of hydrocarbon is secured from a purely aliphatic alcohol. The behavior of benzyl alcohol and its derivatives will be discussed later. 

The formation of methane may be associated with initiation of the homologation reaction. The mechanism of methane formation is unknown (perhaps of free-radical nature ref. 24). What is clear is that the C-H bonds of methanol are labile at the 250-300°C temperatures at which methanol conversion begins. 

The metal catalysts derived from the zeolite-entrapped metal cluster complexes have been studied because of the interest in a uniform distribution and a high degree of metal dispersion through the zeolite frameworks. Nevertheless, little information is available on the structural and chemical behavior of the entrapped metal cluster complexes, particularly on the retention of the cluster character under the reaction conditions, e.g., CO + H2, alkane hydrogenolysis and methane homologation re-... 

It should be noted at this point that primary and secondary reaction products can be distinguished not only by kinetic data (13) but also by suppression of the secondary reactions. E.g substitution of 2,2,2-trifluoroethanol for p-dioxane as solvent for HCoCCO) suppresses homologation and methane formation addition of a phosphine to give the less acidic catalyst HCo(CO)3PR3 has the same effect, as has the substitution of the less acidic catalyst HMn(CO)5. 

Reduction of C02 past formic acid generates formaldehyde, methanol or methane (Eqs. (16-18)), and ethanol can be produced by homologation of the methanol. The liberation of water makes these reactions thermodynamically favorable but economically less favorable. The reductions typically require much higher temperatures than does the reduction to formic acid, and consequently few homogeneous catalysts are both kinetically capable and able to withstand the operating conditions. 

In the presence of an acyclic alkane, 3 catalyzes at moderate temperature (25-200 °C) the metathesis reaction, leading to the formation of heavier and lower homolog alkanes by simultaneous breaking and formation of C-H and C-C bonds. For example, propane is transformed, even at 25 °C into a quasi-equimolar mixture of ethane and butanes (n- and iso-mixture) as well as methane and pentanes, in lower quantities. Lower and heavier homologs are also obtained starting from... 

Within the reaction parameters used, the nickel catalyst is highly selective towards carbonylation. With the exception of trace a-mounts of methane formed, no other hydrogenation product is found. This is in contrast with cobalt whose carbonylation catalytic activity is enhanced by hydrogen but generally associated with aldehyde formation and homologation. 

A unique example of alkane-alkene reaction is the homologation of olefins with methane in a stepwise manner over transition-metal catalysts.269 First methane is adsorbed dissociatively on rhodium or cobalt at 327-527°C then an alkene... 

Most of the investigations into disproportionation reactions have mainly concentrated on chlorofiuoro derivatives of methane and ethane. When trichlorofluoromethane is refluxed with aluminum trichloride or aluminum tribromide, dichlorodifluoromethane and carbon tetrachloride are obtained. Dichlorofluoromethane yields chlorodifiuoromethane and chloroform chlorofiuoro derivatives of ethane and longer chain homologs exhibit a tendency towards isomerization as well as disproportionation, i.e. intramolecular halogen atom exchange. In this case, both types of reaction take place simultaneously. In other words, disproportionation of l,l,2-triehloro-1.2,2-trifiuoroethane (1) forms l,l,1.2-tetrachloro-2,2-difluoroethane (2) and... 

Initially, we will be concerned with the physical properties of alkanes and how these properties can be correlated by the important concept of homology. This will be followed by a brief survey of the occurrence and uses of hydrocarbons, with special reference to the petroleum industry. Chemical reactions of alkanes then will be discussed, with special emphasis on combustion and substitution reactions. These reactions are employed to illustrate how we can predict and use energy changes — particularly AH, the heat evolved or absorbed by a reacting system, which often can be estimated from bond energies. Then we consider some of the problems involved in predicting reaction rates in the context of a specific reaction, the chlorination of methane. The example is complex, but it has the virtue that we are able to break the overall reaction into quite simple steps. 

As already mentioned, with time the mid-molecule cleavage typical of a bifunctional catalyst decreases over the molybdenum based catalyst and the demethylation reaction becomes dominant. Demethylation also increases with increasing pressure. Amir-Ebrahimi and Rooney proposed that the metallacyclobutane isomerization mechanism should have a significant methanation and homologation contribution.34 Homologation products were not analysed in this study but have been observed in studies of the C4 and C5 reactions 35 however, methane was an important component of the cracking products over the molybdenum catalysts. 

This is unlikely, however, since in methanol homologation studies, methane is generated preferentially, even when there are comparable amounts of methanol and ethanol present in the reaction media (55). Acetic acid decarboxylation has also been suggested as a pathway for methane formation ... 

As in the cobalt system, the reaction generates significant quantities of CH4 as a by-product (in some cases, the methane is the major product). The selectivity of homologated alcohol to hydrocarbon appears to be independent of the partial pressures of either CO or H2 127), and the authors suggest that this could be attributable, at least in the Mn system, to the relative rates of methyl migration to homolytic bond dissociation. In the iron system, methane is generated by simple reductive elimination from the HFe(CH3)(CO)4 intermediate. 

At prolonged reaction times, increasing amounts of high molecular weight condensates are funned. In the gas phase, products like methane, dimethylether and CO2 are found in addition to the syngas components. All reaclion products have been identified by GC/MS measurements and by comparison of CjCproduct composition of a typical methanol homologation run obtained by a cobalt/iudine catalyst is given in Table H. 

Methanol homologation catalyzed by ruthenium has been studied by Braca etal. [86, 89, 90]. Catalyst systems such as Ru(acac)3/Nal and Ru(C0)4lj/NaI have been shown to be active. In contrast to cobalt catalysts, no reaction occurs in the absence of 1" and a proton supplier is needed. As can be taken from Table XI, the reaction is higidy selective to C -products and no higlter products are formed. Due to the high hydrogenation activity of ruthenium, however, methane and ethane arc formed as side products in considerable amounts as well as dimethyl ether. Thus, the overall yield of ethanol is limited. The same catalyst systems have also been shown to be active in the homologation/carbonylation of ethers and esters. 

Unsubstituted cyclic dienes, such as 1,4-cycbhexadiene, which contain both double bonds in the same ring, possess very low extinction coefficients at standard wavelengths. The reaction can be carried out by mercury sensitization or under direct irradiation at 185 nm (equation S). The next higher homolog, i.e. l,4 ycloheptadiene, can be generated in the photolysis of bicyclo[4.1.0]hept-3-ene (5) and further transformed into the expected di-ir-methane product (6) at 185 nm (equation 6). ... 

The behavior of methane is different from the higher homologs in that the hot insertion product, CH4S undergoes extensive, pressure-independent fragmentation, even at X = 2490 A. The principal reaction... 

The insertion reaction with ethane and propane is a few kilocalories per mole more exothermic than with methane. The observed lack of decomposition of the mercaptans produced from these higher homologs demonstrates the marked effect on product lifetime resulting from the availability of additional internal degrees of freedom in the mercaptan molecule and the capacity of the mercaptan for stabilization by equiparti-tion of its excess energy. 

Aldehydes and ketones can be converted to their homologs with diazo-methane. ° Several other reagents are also effective, including MesSil, and then silica gel, or LiCH(B-0CH2CH20-)2- With the diazomethane reaction. 

H-D exchange in HF-SbFs via hypercoordinate isotopic methonium ions [Eq. (6.5)] without any detectable side reactions (see Chapter 5, Section 5.4.1.1). Exchange involving protolytic ionization via CH + HD is improbable in the case of methane, because of the unfavorable, highly energetic nature of the primary methyl cation. However, in higher homologous alkanes protolytic ionization takes place with ease. 

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