2-Methylnaphthalene disproportionation

The catalytic isomerization of 1-methylnaphthalene and all lation of 2-methylnaphtha-lene with methanol were studied at ambient pressure in a flow-type fixed bed reactor. Acid zeolites with a Spaciousness Index between ca. 2 and 16 were found to be excellent isomerization catalysts which completely suppress the undesired disproportionation into nwhthalene and dimethylnaphthalenes due to transition state shape selectivity. Examples are HZSM-12, H-EU-1 and H-Beta. Optimum catalysts for the shape selective methylation of 2-methylnaphthalene are HZSM-5 and HZSM-li. All experimental finding concerning this reaction can be readily accounted for by conventional product shape selectivity combined with coke selectivation, so there is no need for invoking shape selectivity effects at the external surface or "nest effects", at variance with recent pubhcations from other groups. 

As already stated, isomerization on zeolite HY was always accompanied by disproportionation, even at 180 C. With time on stream, Yo,. Np increases, because these heavy products are most efficiently held by the fresh catalyst. It is an interesting result that, at 180 °C, the yield of naphthalene passed through a maximiun as well. Obviously, under appropriate reaction conditions, the disproportiotuOion of methylnaphthalenes in zeolite HY exhibits an induction period, as does the disproportionation of ethylbenzene in large pore zeolites [39,40]. 

It was concluded at this point that zeolites with a very spacious pore system, such as faujasites or ZSM-20, are inappropriate catalysts for the isomerization of 1-methyl-naphthalene. Subsequently, a zeolite with much narrower pores was tested, viz. HZSM-5. Pertinent results are shown in Fig. 2. At 300 C, the conversion is low and even a temperature increase of 100 °C does not bring about a considerable increase in conversion. We presume that the reaction of 1-methylnaphthalene in HZSM-5 is controlled by diffusioiL There were practically no side reactions such as cracking, dealkylation or transalkylation, in other words XjMHp Y2.M.NP "e identical. This is at variance with the results of Matsuda et al. [21] who did observe some disproportionation on their H2SM-5 sample at 300 °C. More work is needed to elucidate the reasons for this different catalytic behavior of various samples of HZSM-5. As a whole, zeolite ZSM-5 was discarded at this stage due to its too narrow pore system. 

Neither the relative number of benzylic hydrogens nor the base strength accounts for the slow oxidation rate of the methylnaphthalenes. Formation of radicals in the presence of aromatic hydrocarbons can lead to radical attack on the aromatic ring. Addition of phenyl or methyl radical to the ring gives a cyclohexadienyl radical that may disproportionate or dimerize, or undergo hydrogen abstraction by another radical (3, 9,13). 

Disproportionation of 2-methylnaphthalene over ZSM-5 zeolite to dimethylnapthalenes (DMN) has recently been reported to be shape selective by... 

Kikuchi, E., Y. Mogi and T. Matsuda. Shape Selective Disproportionation of Methylnaphthalene on ZSM-5 Catalyst. Collect. Czech. Chem. Comm., 1992, 57, 909-919. 

Lin, S.D., and C. Song. Noble Metal Catalysts for Low-Temperature Naphthalene Hydrogenation in the Presence of Benzothiophene. Catalysis Today, 1996, 31 (1), 93-104. Matsuda, T., K. Yogo, Y. Mogi and E. Kikuchi. Shape Selective Catalysis by ZSM-5 in Disproportionation of 2-Methylnaphthalene. Chemistry Letters, 1990, 1085-1088. 

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