Faouzi Ayari

Ammoxidation of ethylene to acetonitrile over Cr-ZSM-5 catalysts

Abstract Cr-ZSM-5 catalysts prepared by solid-state reaction from different chromium precursors (acetate, chloride, nitrate, sulphate and ammonium dichromate) were studied in the selective ammoxidation of ethylene to acetonitrile. Cr-ZSM-5 catalysts were characterized by chemical analysis, X-ray powder diffraction, FTIR (1500-400 cm −1 ), N 2 physisorption (BET), 27 Al MAS NMR, UV-Visible spectroscopy, NH 3 -TPD and H 2 -TPR. For all samples, UV-Visible spectroscopy and H 2 -TPR results confirmed that both Cr(VI) ions and Cr(III) oxide coexist. TPD of ammonia showed that from the chromium incorporation, it results strong Lewis acid sites formation at the detriment of the initial Bronsted acid sites. The catalyst issued from chromium chloride showed higher activity and selectivity toward acetonitrile. This activity can be assigned to the nature of chromium species formed using this precursor. In general, Cr 6+ species seem to play a key role in the ammoxidation reaction but Cr 2 O 3 oxide enhances the deep oxidation.

Physicochemical and catalytic properties of over- and low-exchanged Mo‒ZSM-5 ammoxidation catalysts

A series of Mo/ZSM-5 catalysts prepared by solid-state ion exchange at different Mo/Al molar ratios were characterized and tested in ethane and ethylene ammoxidation into acetonitrile. It has been concluded that the low-exchanged solid (Mo/Al = 0.2) stabilized MoO3, [Mo2O7]2− and [Mo7O24]6− species. However, besides these species, the solids prepared at Mo/Al = 0.5 and 1.5 stabilized [MoO4]2−. Nevertheless, only MoO3 and [Mo2O7]2− species were stabilized at Mo/Al = 1. The study performed by diffuse reflectance spectroscopy allowed the determination of the molar fraction relative to each Mo specie and, therefore, the calculation of the turnover frequency values. The catalytic activities of the various solids have been classified by taking into consideration the inefficiency of Al2(MoO3)4 phase, which inhibits the diffusion of reactants molecules towards the active sites, and amorphous MoO3 which catalyzes the undesired hydrocarbons’ combustion. However, [MoO4]2‒ species are efficient in the oxidative dehydrogenation of C2H6 into C2H4, while dimeric species catalyze the ammoxidation.