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

Toluene disproportionation has been studied on a series of hydrotherma lly dealuminated Y zeolite catalysts, both in the 'as prepared', unext racted state and following extensive extraction of non-framework alumi nium 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-f ramework aluminium to an average of five Al atoms per unit cell. The i nitial activity at 673 K for both unextracted and extracted catalysts, using a microreactor in a continuous flow system, arose from a limite d concentration of strong Bronsted-acid sites. These were generated by synergic interaction between framework hydroxy groups and non-framewo rk aluminium species. After 4-20 h on stream a pronounced maximum in t he activity was observed with all catalysts. Since considerable coke h ad formed at this stage, 'catalytically active coke' was considered to be the seat of reaction, with an active site concentration directly r elated to the total Bronsted acidity of the freshly activated zeolite. Two mechanisms, involving either proton addition to, or hydride ion a bstraction from, the reactant toluene molecule, are suggested; the con tribution these each make depends on the extent of reaction with respe ct to complete poisoning by coke.