Effect of the metal promoter on the performances of H-ZSM5 in ethylene aromatization
Résumé
The effect of the metal and the zeolite on the physicochemical properties and catalytic performances in ethylene aromatization was investigated on M/H-ZSM5 (M = Ni, Mo, Zn and Ga). The addition of the metal to the zeolite increases the amount of Lewis acid sites and in parallel decreased the amount of Brønsted ones. The extent of the change depended mainly on the location and dispersion of the metal cation, which varied not only with the nature of the metal but also with the one of the metal precursor. The higher Lewis acidity associated with the lowest Brønsted acidity was obtained with the Zn/H-ZSM5 catalyst. Ethylene aromatization was studied at 500 °C and atmospheric pressure. For the catalyst series, it was demonstrated that the ethylene conversion and yield in aromatic products were proportional to the number of Lewis acid sites, except for Ni/H-ZSM5 that presented a singular behavior with a high initial conversion but rapid deactivation, due to carbon formation. Among the other catalysts, Zn/H-ZSM5 led to the highest ethylene conversion and yield in aromatics. An increase in Zn content increased the amount of Lewis acid sites, decreased the amount of Brønsted acid sites, which favored the initial catalytic performances but also decreased the stability of the Zn/H-ZSM5 catalyst.