NICKEL MODIFIED H-ZSM-5 CATALYSTS

Nickel containing H-ZSM-5 catalysts were prepared by impregnation with nickel nitrate at pH 1.8 and 4.8. The catalysts were characterized by infrared spectroscopy of adsorbed carbon monoxide and ammonia, by temperature-programmed reduction as well as by chemisorption of hydrogen and oxygen. The conversion of n-hexane served as a catalytic test reaction. Impregnation at pH 1.8 leads to nickel ions exchange of Bronsted sites in the channel system. These are hardly reducible and are not detectable by IR spectroscopy of adsorbed carbon monoxide. Aromatization is increased. This is explained by the presence of ionic nickel Lewis sites, active in dehydrogenation. With impregnation at pH 4.8, tetrameric nickel ions exist, not able to penetrate into the channels, and nickel is mainly located outside the channels as metallic nickel. The selectivity to aromatics is still higher. This should be due to the action of metallic nickel, which promotes the direct dehydrocyclization. The hydrogenolysis activity of metallic nickel is suppressed.