Faujasite silicalites for oxidative dehydrogenation of n-octane: Influence of alkali metals, gallium, and boron on catalyst activity

The sol-gel method was used to synthesize faujasite type silicalites bearing gallium and boron in the framework. Barium and sodium were used as charge balancing cations since isomorphic substitution of Si⁴⁺ by Ga³⁺ or B³⁺ results in a negative excess charge of the framework. The successful synthesis of this type of silicalites (GaBaY-S, BBaY-S, GaBBaY-S(IE), GaNaY-S) was confirmed using powder-XRD. SEM analysis showed that the morphology of the catalysts with respect to particle size depended on the framework metals and the charge balancing cation used. Framework Ga containing catalysts showed smaller particle size compared to B containing catalysts. Sodium also yielded a smaller particle-sized catalyst compared to barium. The catalysts were tested in the continuous flow oxidative dehydrogenation (ODH) of n-octane, and the catalytic results showed dependence on the active metal reducibility and acid-base character of the catalysts. At iso-conversion of 8 ± 1 %, the least acidic BBaY-S gave the highest selectivity to octenes (40 %) and the lowest selectivity to COx (28 %), and the most acidic GaNaY-S showed the opposite results with octenes at 17 % and COx at 56 %. The catalysts (BaBY-S and GaBBaY-S(IE) with least total acidity had the greatest quantity of strong acid sites which were attributed to Lewis acid sites, confirmed by the pyridine IR analysis. The GaNaY-S, with the highest total acidity, had the least strong acid sites.