EFFECTS OF PRETREATMENTS ON STATE OF GALLIUM AND AROMATIZATION ACTIVITY OF GALLIUM ZSM-5 CATALYSTS

Additional evidence is presented for the role of gallium reduction in preparing active Ga/ZSM-5 catalysts for alkane aromatization. The gallium in ion-exchanged ZSM-5, both with and without additional Ga2O3, can be reduced to Ga+, in the former case up to a limit determined by the number of anionic framework sites. An ion-exchanged, reduced catalyst exhibits similar redox behavior, and is as active for propane aromatization, as catalysts prepared from purely physical mixtures of Ga2O3 and H-ZSM-5. Although reduction generates dispersed, active gallium species in zeolite channels, it is shown that for ethane dehydrogenation to aromatics the reduced (with hydrogen) Ga/ZSM-5 catalysts are not as active or selective as are materials which have been substantially reoxidized either by air or by traces of oxygen in inert gases. A combination of thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and X-ray diffraction (XRD) studies has been used to confirm that rapid reoxidation takes place at 550-degrees-C. However, the reoxidation does not regenerate beta-Ga2O3, but rather a higher energy, dispersed Ga3+ which appears to be optimal for production of aromatics from ethane. This form of Ga3+ is stable under reaction conditions typical of ethane dehydrogenation, but can be reduced by hydrogen.