P-Xylene-selective Disproportionation of Toluene over a Modified Pentasil Type Zeolite.

Abstract

A pentasil type zeolite was modified with vaporized tetramethoxysilane, and disproportionation of toluene was carried out over the modified zeolite catalyst at 500°C for about 200h of time-on-stream. Xylene containing 95% p-xylene was produced at 25% conversion of toluene. The yield of p-xylene was maintained at about 10%, and slightly changed with time-on-stream, except for the initial 40h. It seems that the disproportionation of toluene produces initially 100% p-xylene, and then a small amount of p-xylene isomerizes to meta or ortho isomers, or is dealkylated to toluene. The rates of drop with time-onstream of velocities of disproportionation and side reactions are in the following ascending order; disproportionation<isomerization<dealkylation. This relationship brought about the constant yield of p-xylene, which is maintained for more than 160h, as shown above. The sites for these three reactions are likely to be different in nature from one another, since the velocities of these reactions decreased at different rates during a prolonged run.