Abstract
The effect of the thermal treatment of some zeolitic materials was studied on oxidative dehydrogenation (ODH) of n-octane. Gallium containing faujasite catalysts were synthesized using isomorphic substitution, specifically, a galosilicalite (Ga-BaY(Sil)) and an aluminosilicalite substituted with gallium (Ga-BaY(IS)), with constant Si/M ratio. The catalysts were thermally treated at different temperatures (250, 550, and 750 °C) before catalytic testing. The quantification of total and strength of acid sites by FT-IR (O-H region), pyridine-IR, and NH3-temperature-programmed desorption (TPD) confirmed a decrease in the number of Brønsted acid sites and an increase in the number of Lewis acid sites upon increasing the calcination temperature. Isothermal n-octane conversion also decreased with the catalysts’ calcination temperature, whereas octene selectivity showed the opposite trend (also at iso-conversion). The COx selectivity showed a decrease over the catalysts calcined from 250 to 550 °C and then an increase over the 750 °C calcined catalysts, which was due to the strong adsorption of products to strong Lewis acid sites on the catalysts leading to the deep oxidation of the products. Only olefinic-cracked products were observed over the 750 °C calcined catalysts. This suggested that the thermal treatment increases Lewis acid sites, which activate n-octane using a bimolecular mechanism, instead of a monomolecular mechanism.
Highlights
Zeolite materials offer a variety of properties in catalysis, resulting from their ability to be tuned both during synthesis and post synthesis
There were no diffraction peaks observed for any gallium oxide species which suggests good dispersion of the metal within the zeolite framework
Ammonia-temperature-programmed desorption (TPD) analysis was only performed on the catalysts that were calcined at 550 °C and 750 °C
Summary
Zeolite materials offer a variety of properties in catalysis, resulting from their ability to be tuned both during synthesis and post synthesis. Treating the zeolitic materials causes the T atoms to migrate into partial or total extra framework positions (Scheme S2). The monomolecular mechanism is the more pronounced and faster of the two mechanisms, it proceeds via the feed molecule protonation, resulting in a pentacoordinated carbonium ion which can crack to give off a smaller alkane fragment and an adsorbed carbenium ion. This ion subsequently cracks by ß-scission resulting in an olefin and a smaller carbenium ion. The ODH reaction was studied because it offers an interesting and less energy-intensive route for the production of olefins from paraffins by rendering the reaction exothermic as a result of the insertion of an oxygen source into the reaction, suppressing the thermodynamic constraints [12]
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