Alkylation of toluene with methanol III: Para-selectivity on modified ZSM-5 zeolites

Selectivity factors S have been derived for three xylene isomers obtained by methylation of toluene over ZSM-5 zeolites modified by the introduction of Fe, B and P ions. In the experimental conditions the reaction proceeds with formation of o + p xylene up to onset of isomerization. Our results and previously published independent data show a close correlation of S and the guest electronegativity, indicating a role of the acidity in the reaction mechanism. The para-selectivity is attributed to the selective adsorption of o + m xylenes on the acidic reaction site; the less polar p-isomer is rapidly desorbed.