Alkanes, preferential branching

In trifluoroacetic acid [0.4 M TBABF4 (tetrabutyl ammonium tetrafluoroborate)] unbranched alkanes are oxidized in fair to good yields to the corresponding triflu-oroacetates (Table 2) [16]. As mechanism, a 2e-oxidation and deprotonation to an intermediate carbenium ion, that undergoes solvolysis is proposed. The isomer distribution points to a fairly unselective CH oxidation at the methylene groups. Branched hydrocarbons are preferentially oxidized at the tertiary CH bond [17]. 

TS-2 belonging to a different structure group exhibits a similar activity and similar high selectivity in oxidation of hexane.191 Branched and cyclic alkanes react much more slowly,186,187,189 indicating that the oxidation occurs preferentially inside the channels of the catalyst structure.189... 

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. 

The protolytic cracking involves the attack of the zeolitic proton to a carbon atom of the alkane molecule and the simultaneous rupture of one its adjacent C-C bond. The carbon atom being attacked and the C-C bond being broken will be preferentially those which produce the most stable carbenium ion. As for the dehydrogenation reaction, the protolytic cracking of linear and branched alkanes also follow different mechanisms, the latter ones producing olefins instead of alkoxides. 

Air oxidation of /i-butane to maleic anhydride is possible over vanadium phos(4tate and, remaiicably, a 60% selectivity is obtained at 85% conversion. In the gas phase oxidation, in conffast to the situation found in the liquid, n-allcanes are oxidized more rapidly than branched chain alkanes. This is because secondary radicals are more readily able to sustain a chain for branched alkanes the relatively stable tertiary radical is preferentially formed but fails to continue the chain process. Vanadium(V)/ manganese(II)/AcOH has been used as a catalyst for the autoxidation of cyclohexane to adipic acid, giving 25-30% yields after only 4 h. ... 

Fluorinated alkenes are able to insert into weak C-H bonds of various compounds branched alkanes, haloforms. alcohols, ethers, aldehydes and their corresponding ketals. These reactions usually involve the use of UV irradiation or radical-initiation catalysts, such as peroxides or azobisisonitriles. Variable amounts of telomcric products are also formed. Under the influence of ) -irradiation ( °Co source), one-to-one adducts are obtained predominantly. Attack of the intermediate radical occurs preferentially on the less-hindered carbon of the fluorinated alkene. 

Alkanes collected over a 12 months period, offshore from Louisiana and Florida, were characterised by Ledet and Laseter (1974) unexpectedly, methyl branched alkanes ranging in chain length from C15 to C3S and cycloalkanes were frequently the dominant components. Possible explanations for this enrichment may be selective removal by autoxidation, preferential oxidation of n-alkanes by bacteria, adsorption onto particles, a contribution from some crude oils particularly rich in methylalkanes or a contribution from certain plants containing large quantities of 3-methyl branched alkanes (Weete et ed., 1971). 

The loading of n-hexane in mixtures is somewhat higher than it is expected to be if it were proportional to its partial pressure (Fig. 12). On the contrary, the 2-methylpentane loading is somewhat lower. This points to preferential adsorption of -hexane over isohexane in their mixtures in H-ZSM-5 than in silicalite-1. In earlier experimental [50] and CBMC simulation studies [44] of n-hexane/isohexane mixtures in silicalite-1, a slight preferential adsorption of the linear alkane over the branched one has been found. The most prominent explanation for this preference is the molecular siting of these two hydrocarbon molecules. Whereas -hexane exhibits no clear preference for a position in the micropore system of MFI zeolite, the branched isomer is preferentially located at the channel intersections due to entropic reasons [44]. Consequently, 2-methylpentane will be pushed out from silicalite-1 by -hexane. These effects are even stronger for H-ZSM-5, most likely due to the stronger... 

Summarizing, we observe that the presence of acid sites causes a decrease in the self-diffusivity of n-hexane and 2-methylpentane. In H-ZSM-5, we find that the diffusivity of n-hexane in mixtures with its branched isomer is determined by two factors (i) the interaction with acid sites, strong for the linear alkane, which decreases the diffusivity and (ii) the presence of 2-methylpentane which has an order of magnitude lower diffusivity. At low 2-methylpentane loadings the influence of the acid sites is dominating. However, at a loading of about 2.7 molecules per unit cell, the effect of pore blocking by the preferential location of the branched alkane in the intersections dominates. The diffusivities are then more or less equal in silicalite-1 and H-ZSM-5. 

We have discussed the adsorption and diffusion of binary mixtures of hnear (n-hexane) and branched (2-methylpentane) alkanes in silicahte-1. It turned out that not only the size but also the siting of the molecules in the particular zeohte plays an important role in the behavior of the mixture components. A shght preference for the adsorption of n-hexane over 2-methyl-pentane was observed because of the higher packing efficiency of the hnear alkane. This is due to the preferential location of the branched alkane in the zeohte intersections. A consequence of this is that the diffusivity of n-hexane... 

A comparison between sihcalite-1 and H-ZSM-5 teaches that acid sites have a profound influence on the self-diffusivity of alkanes. The self-diffusivities of both components decrease strongly, and we observe a significant preferential adsorption of the linear over the branched hexane. This is caused by the relatively stronger interaction of the linear hexane with the acid sites. On the contrary, 2-methylpentane loadings in mixtures in sihcahte-1 and H-ZSM-5 are very close. In H-ZSM-5, the diffusivity of the hnear alkane in mixtures with the branched alkane is influenced by two factors... 

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