CATALYTIC STUDIES WITH DEALUMINATED Y-ZEOLITE .2. DISPROPORTIONATION OF TOLUENE

Toluene disproportionation has been studied on a series of hydrothermally dealuminated Y zeolite catalysts, both in the 'as prepared', unextracted state and following extensive extraction of non-framework aluminium with aqueous Na2H2EDTA (H4EDTA = ethylenediamine tetracetic acid). Extraction had negligible effect on the framework aluminium content, which ranged from 13-25 Al atoms per unit cell, but reduced the non-framework aluminium to an average of five Al atoms per unit cell. The initial activity at 673 K for both unextracted and extracted catalysts, using a microreactor in a continuous flow system, arose from a limited concentration of strong Bronsted-acid sites. These were generated by synergic interaction between framework hydroxy groups and non-framework aluminium species. After 4-20 h on stream a pronounced maximum in the activity was observed with all catalysts. Since considerable coke had formed at this stage, 'catalytically active coke' was considered to be the seat of reaction, with an active site concentration directly related to the total Bronsted acidity of the freshly activated zeolite. Two mechanisms, involving either proton addition to, or hydride ion abstraction from, the reactant toluene molecule, are suggested; the contribution these each make depends on the extent of reaction with respect to complete poisoning by coke.