Mefa-Xylene

Davey JF, DT Gibson (1974) Bacterial metabolism of para- and mefa-xylene oxidation of a methyl substitnent. J Bacterial 119 923-929. 

Application The MX Sorbex process recovers mefa-xylene (m-xylene) from mixed xylenes. UOP s innovative Sorbex technology uses adsorptive separation for highly efficient and selective recovery, at high purity, of molecular species that cannot be separated by conventional fractionation. 

Analysis of products obtained with para- and mefa-xylene shows that all the catalysts are more active for the acylation than the alkylation reaction. Indeed, indanones 58 and 59 (Figure 4.11) are obtained in 65% and 79% yield. 

More problematic results are obtained with the acetylahon of aromatic compounds in facf, wifh AC, the preferential formation of ketone degradafion producfs is observed, whereas neither AAN nor AAC alone prove to be effective. On the contrary, when an equimolecular mixture of AAC and AAN is refluxed wifh toluene, mefa-xylene, or mesitylene in the presence of Nafion, fhe corresponding APs are isolafed in 3%, 21%, and 72% yield, respectively. 

Figure 3.5 allows us to predict, for example, that hydrocarbons with methyl branches (molecular diameter 0.53 nm) would have too great a diameter to pass through the pores of erionite (internal dimensions 0.36 x 0.51 nm), and that faujasite (internal diameter 0.74 nm) could easily accommodate ortho- and mefa-xylene (molecular diameter 0.63 nm), as well as the slimmer para-xylene. 

Figure 6.20. Examples of arenes and substituted arenes (a) benzene (b) methylbenzene (toluene) (c) 1,2-dime thy Ibenzene (orfho-xylene) (d) 1,3-dimethylbenzene (mefa-xylene) (e) 1,4-dimethylbenzene (para-xylene) (f) naphthalene (g) 1-methylnaphthalene (a-methylnaphthalene) (h) 1,2-dime thy Inaphthalene (a, 3-dimethylnaphthalene) (i) anthracene (j) phenanthrene.

Figure 5 shows the Arrhenius plot of the diffusivities of aromatics. The diffusivities of benzene, toluene and / am-xylene, minimum molecule sizes of which are the same and are close to that of the pore diameter. Whereas, those of mefa-, or/Ao-xylene, which minimum molecule sizes are larger than the size of the pores, are almost 1/10 of the former group. This difference among the diffusivities is the reason why ZSM-5 shows shape selectivity. 

It has been known since the early days that behavior of the aromatic polyamides (aramids) depends critically on the type of isomeric substitutions -para-substimtimis result in crystalline, while mefa-substitutions in amoiphous polymers (Kwolek et al. 1962). Similarly, the two aramids poly(m-xylene adipamide) and poly (hexamethylene isophthalamide), MXD6 and PA-61, respectively, show different miscibility, e.g., with aliphatic polyamides. Clearly, blind application of the segmental interaction strategy to aromatic or semi-aromatic polyamides leads to conflicts. However, the problem can be resolved considering p- and m-substituted phenyl as two different statistical segments (Ellis 1995). This idea is indeed evident in the segmental contributions listed in Table 2.14. 

One example of producf selectivity in a zeolite system is catalysed xylene formation from toluene and methanol. An electrophilic substitution of the aromatic ring occurs within the pores of the zeolite ZSM-5, which gives a crude mixture of the three xylene isomers ortho, meta and para). Due to the narrow, linear channels within this zeolite, it is only the para-isomer that is able to diffuse out of the zeolite and be isolated (Figure 4.4). The other ortho- and mefa-isomers can diffuse through the pores but at much slower rates (14 and 1000 times slower, respectively). Due to these slow rates of diffusion, it is more likely that the undesirable isomers will remain within the zeolite long enough to isomerise to the p-xylene product. 

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