Effects of Framework Disruption of Ga and Ba Containing Zeolitic Materials by Thermal Treatment

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.

n-Butene Synthesis in the Dimethyl Ether-to-Olefin Reaction over Zeolites

“…In contrast, for 840NHA (MFI) and 940HOA (BEA), both acid sites decreased monotonically with an increase in the calcination temperature. These results could be attributed to a decrease in the Brønsted acid sites because an increase in the temperature promoted dehydration [37,38]. 720NHA (FER) and 840NHA (MFI) showed two relatively clear peaks (Figure 6a,b), while 940HOA (BEA) exhibited three broad peaks (Figure 6c).…”

“…In contrast, for 840NHA (MFI) and 940HOA (BEA), both acid sites decreased monotonically with an increase in the calcination temperature. These results could be attributed to a decrease in the Brønsted acid sites because an increase in the temperature promoted dehydration [37,38]. Ferrierite zeolites contain a two-dimensional pore structure in which 10 MR (member ring, 5.4 × 4.2 Å) channels and 8 MR (4.8 × 3.5 Å) channels intersect.…”