THE ROLE OF GALLIUM IN THE CATALYTIC ACTIVITY OF ZEOLITE [SI,GA]-ZSM-5 FOR METHANOL CONVERSION

The transformation of methanol over a series of aluminum-free [Si,Ga]-ZSM-5 zeolitic catalysts with Ga/(Ga + Si) ratios of 0.015, 0.019, 0.032, 0.039, and 0.054 (corresponding to Si/Ga ratios of 65.7, 51.6, 30.3, 24.6, and 17.5, respectively) has been studied in the 150-450-degrees-C temperature range. There is strong dependence of selectivity and activity on the concentration of Ga for the formation of dimethyl ether at 150-300-degrees-C and for the conversion of alkanes into aromatics at 300-450-degrees-C. The curves describing the yield of dimethyl ether and aromatics rise steadily with the content of Ga, but the curve of the dependence for the yield of olefins is bell-shaped. Framework gallium in Al-free [Si,Ga]-ZSM-5 does generate Bronsted acidity, and the sequence of steps in the reaction of MeOH is the same over [Si,Ga]-ZSM-5 and [Si,Al]-ZSM-5. The role of gallium in the transformation of olefins into aromatics is discussed.