The Effects of Higher-boiling Compounds on the Liquid-phase Isomerization of m-Xylene over a Silica-Alumina Catalyst under Pressure

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The liquid-phase isomerization of m-xylene was studied over a silica-alumina catalyst under pressure on the additions of 2% of 8 higher-boiling compounds and 20–50% of benzene to reactant m-xylene. The catalyst activity in the isomerization was not greatly affected by hexamethylbenzene, naphthalene, α-methylnaphthalene, diphenyl, or s-dibenzyl. On the other hand, the activity was greatly lowered by 1,6-dimethylnaphthalene, diphenylmethane, and anthracene. Moreover, a very rapid exchange reaction occurred between an aromatic ring of diphenylmethane and a reactant xylene molecule, while the content of the isomerization product decreased linearly with an increase in the amount of benzene. From these results, the chemisorption states of xylene were discussed as follows. Most xylene molecules may be deduced to be chemisorbed on acid sites by those benzene rings. On the other hand, the fact that diphenylmethane inhibits the isomerization implies the existence of such a chemisorption state of xylene as a methylbenzylcarbonium ion, which may be an intermediate in the disproportionation reaction of xylene. The acid sites available for the chemisorption of diphenylmethane and for the disproportionation were speculated to be common and to have both acidic characters, protonic and Lewis.
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