EXPLORING THE EFFECT OF TEMPERATURE ON GA-DOPED ZEOLITE CATALYSTS BY IN SITU SYNCHROTRON X-RAY POWDER DIFFRACTION

Gallium-containing zeolites have been extensively investigated due to their unique catalytic performances in light hydrocarbon aromatization. Newest applications concern the conversion of biomass into biofuels and bio-based chemicals. These Ga-doped catalysts can be prepared by post synthesis methods, including impregnation, ion exchange or Chemical Vapor Deposition (CVD). The complex mechanism of galliation by post synthesis treatment is suitable to introduce gallium into both tetrahedrally coordinated framework and interstitial non-framework positions. Brønsted acidity within zeolites is generated when tetravalent Si4+ is replaced with trivalent Ga3+. Oxo cations GaO+ on exchange positions of the framework behave as Lewis acids. Non-framework gallium is the source of Lewis acidity. Thermal treatments cause the migration of Ga3+ to extraframework positions and its progressive aggregation in form of isolated, dimeric and polymeric species up to oxide nanoparticles. This migration leads to the appearance of a different type of acid sites of Lewis nature. As a consequence, after this treatment Ga-zeolites can possess both Brønsted and Lewis acid sites, working separately or in a synergistic way in acid catalyzed reactions. In spite of the growing interest on the nature of gallium species in Ga-catalysts, there is a lack of in situ studies of Ga-zeolite systems at the high temperature range, where they exhibit their catalytic activity. To predict catalysts behaviour at non-ambient conditions is mandatory to improve their effectiveness and prevent from a possible deactivation, which is still a challenging task. To ensure the long-term stability during catalytic reactions, detailed information about catalyst modifications upon thermal treatment are required. In particular, a better understanding of extraframework metal cations nature and the way they contribute to catalytic reactions is of primary importance to support the design of high-performing Ga-zeolite catalysts, at whose request in the industrial field is continuously growing. In spite of that, the particular role of the framework and non-framework gallium is still very controversial. As a result, the nature of the catalytically active sites in Ga-zeolites as well as the possible cooperative effects (framework and non-framework gallium) and the evaluation of their stability under operating conditions remain debatable.