Abstract
Cr-ZSM-5 catalysts prepared by solid-state ion exchange with different Cr/Al molar ratios were characterized and tested in ethylene ammoxidation to acetonitrile in the temperature range 425-500 °C. Starting from Cr acetate like precursor and lower Cr load (Cr/Al = 0.5), small amount of chromate species, sited inside the zeolite matrix, exhibited lower catalytic activity in ammoxidation (12.5% of CH3CN yield). Increasing the Cr amount (Cr/Al = 1) does not significantly improve the catalytic properties since agglomerates of amorphous Cr2O3 inhibited the accessibility of chromate species to the reactants. However, at higher Cr amount (Cr/Al = 1.5), crystalline Cr2O3 particles reduced the diffusion of reactants to the internal active sites. Starting from Cr chloride like precursor, the corresponding solids are low-exchanged since CrCl3 evaporates. At Cr/Al molar ratios of 0.5 and 1, chromate species, sited in the exchange sites, exhibited interesting catalytic properties (18.5 and 25% of CH3CN yield at 500 °C, respectively). Nevertheless, at higher metal load (Cr/Al = 1.5), the excess of Cr transforms into crystalline Cr2O3 which inhibits the diffusion of reactants to the active sites and enhances the hydrocarbon oxidation mainly at low temperatures. As ethylene ammoxidation required ammonia activation over (poly)chromate sites, a possible pathway of such a step has been proposed on the basis of NH3-TPD results.