Shape selective properties of MCM-22 catalysts for the disproportionation of ethylbenzene

The disproportionation of ethylbenzene (EBz) has been investigated over MCM -22 and its dealuminated samples at low temperature. Dealumination was cond ucted by steaming followed by acid leaching. The number of acid sites was m easured by using pyridine and 2,6-di-tert-butyl-pyridine as probes. 1,1,1,3 ,3,3-Hexamethyldisilazane (HMDS) was used as a selective poison to external acid sites. Under the reaction conditions of this work, MCM-22 showed unex pectedly high initial activity but deactivated rapidly. This peculiar catal ytic behavior of MCM-22 is attributed to the high activity of external acid sites. After poisoning of the external surface, the catalytic activity dec reased to a very low level but the rate of deactivation decreased significa ntly. p-Diethylbenzene among diethylbenzene (DEBz) isomers was found to for m within the micropore as the primary product. The m- and o-diethylbenzene were mainly produced by the secondary isomerization of p-isomer on the exte rnal surface of MCM-22. The para-selectivity of MCM-22 increased with the d egree of dealumination. The improvement in para-selectivity by dealuminatio n can be attributed to the suppression of the secondary isomerization react ion because the dealumination treatment removes preferentially the Bronsted acid sites on the external surface rather than those within the micropores . When the concentration of external acid sites of MCM-22 was reduced to a very low level by poisoning or dealumination, the induction period typical of large pore zeolites was observed. (C) 2001 Elsevier Science B.V. All rig hts reserved.