Methane, acid strength

Another class of acids of interest in organic chemistry is the group of carbon acids. Here we may discern three kinds of effects on acidity. The first of these is illustrated by the acidity of methane (pKa a 48) compared with that of cyclohexane (pKa a 52) (Table 3.1). It would appear that the trend is in the direction of decreasing acid strength with substitution of hydrogen by alkyl. Note that the tendency here is in the direction opposite to the effect in alcohols if we take Brauman s gas-phase results to be the more accurate indication of intrinsic acid strength. The hydrocarbon data are from solution measurements subject to considerable uncertainty, and the differences are small. It seems risky to interpret the results in terms of intrinsic molecular properties. 

The situation regarding a base strength scale (Lewis and Hr0nsted) is far more complex and less well developed than that regarding Lewis acid strengths. A spectroscopically simple, versatile, and universal probe has not yet been found (although many probe molecules, including CO and methane,... 

For the linear alkanes studied (methane, ethane and propane), the fact that only one type of acid site can be represented with the 3T and 5T clusters should not be a major problem. As shown by the MD studies, because of their sizes, steric effects are of minor importance and these molecules have equal probability of visiting aU the distinct sites of the zeolite. In another words, for these molecules, as far as steric effects are concerned, the acid sites are all alike. Thus, the interaction between any of these molecules and the zeolite will depend mainly on the sites acidic strengths, which do not differ very much from each other. Therefore, for these molecules it is a reasonable approximation to treat aU the acid sites alike. However, for isobutane steric effects are more important and the molecule should be more sensitive to the type of the acid site. It will be easier for the isobutane molecule to approach the acid sites represented by 3T and 5T clusters than the one at the channels intersection, in the real zeolite, where it preferentially adsorbs. Therefore, for isobutane and other branched alkanes (and most probably for the large n-alkanes), the chemical reactions at the 3T and 5T clusters may take place artificially easier than in the real zeolite. 

Fig. 11. Corrections to the calculated activation energies for the cluster acid strength. Activation energies for protolytic cracking of ethane, protolytic dehydrogentation of ethane, methane-methoxy hybrid transfer, and methane deprotonation energies are computed at the MP2/6-31++G //HF/6-31G level with the ZPE corrections [17,113].

The more endothermic Eq. 9.50 is, the weaker the acid will be. Therefore Table 9.5 may readily be used to compare gas-phase acid strengths, and HF is a weaker acid in the gas phase than are the other HX acids, as it is also in aqueous solution. In the same way. acetic acid (PACHlCOO- = + 459kJmol-l)isa weaker acid than trifluoroacetic acid (PACFlCOO- = + 1351 kJ mol- ). Which is the stronger acid, methane or toluene Does Table 9.5 confirm or contradict your memory from organic chemistry ... 

Adsorption complexes of methane at MgO are interesting because they relate to the conversion of methane to ethylene and methanol. In particular, oxidative coupling of methane on metal-oxide catalysts attracted great attention [119]. Usage of methane as a probe to identify and characterize adsorption sites of different acid strength on oxide catalysts is another important aspect. Because CH4 is not easily captured by surfaces of metal oxides, the nature of the interaction of methane with surface sites was little understood until recently. A FTIR spectroscopy investigation of methane on MgO at 173 K revealed adsorbed molecular species preferentially bound at Lewis basic sites CH4 adsorption on a Lewis acid-base pair has also been proposed [120]. 

In recent years, a lot of research effort has been directed towards dehydroaromatisation of methane in which methane is converted to aromatic products such as benzene and naphthalene in addition to hydrogen. Perhaps the most well studied system has been that employing Mo/ZSM-5 based catalysts, where the bifunctional interaction between the zeolite Bronsted acidity and molybdenum species is well recognised. Under reaction conditions, the active molybdenum species are known to be in the form of carbides or oxycarbides, and recently it has been proposed that the a-MoCi-x phase is the most active form. Deactivation, primarily due to coke formation, is well precedented in this reaction and represents a major obstacle to be overcome in the successful application of these catalysts. In this respect, it is interesting to note that Ichikawa and co-workers have published studies indicating that the inclusion of low levels of CO or CO2 in the feed can promote the reaction via the suppression of coke formation in the case of both Mo/HZSM-5 and Re/HZSM-5 catalysts. Other approaches adopted towards this aim have been the inclusion of second metal components and a reduction of the acid strength of the HZSM-5 support. ... 

Transition metal-incorporated zeolites have been shown to be effident catalysts for direct conversion of methane to benzene and toluene under nonoxidative conditions [45,46]. Bao and co-workers revealed that Mo/ H-MCM-22 catalysts are desirable bifiinctional catalysts for methane dehydroaromatization reaction [47]. In terms of catalytic performances of Mo/H-MCM-22 with varied metal loading, catalyst with a Mo loading of ca. 6 wt% was found to exhibit the optimal benzene selectivity, suppressed naphthalene yield, and prolonged catalyst hfe under a moderate methane conversion. Although both Bronsted and Lewis acid sites are capable of catalysing methane conversion reaction, active sites with higher acidic strengths are anticipated to play the dominant role. 

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