Selective formation of p-xylene with disproportionation of toluene over MCM-22 catalysts

Disproportionation of toluene has been studied over dealuminated MCM-22 zeolites. The dealumination of MCM-22 was performed by acid reflux and the combination of calcination or steaming with the subsequent acid reflux. These treatments effectively eliminated Al atoms from the framework site but left most of them in the crystallites as non-acidic extra-framework species. The activity of MCM-22 for the toluene disproportionation measured at relatively low temperatures (473-573 K) was comparable to that of mordenite and much higher than that of ZSM-5. Toluene disproportionation was found to occur mainly within the supercages of MCM-22 but not in the 10-membered ring (10-MR) channels of MCM-22 at these temperatures. The fraction of p-xylene in xylene isomers (para-selectivity) formed on MCM-22 was higher than its equilibrium value, and increased further by the dealumination. p-Xylene formed within the supercages as a primary product would be isomerized to o- and m-xylene by the acid sites not only on the external surface but also within the 10-MR channels. The improvement in para-selectivity by the dealumination resulted from the suppression of isomerization activity probably because the dealumination treatments eliminated framework Al atoms predominantly on the external surface and inside the 10-MR channels compared with those inside the supercages. (C) 1998 Elsevier Science B.V. All rights reserved.