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 tem
perature-programmed reduction as well as by chemisorption of hydrogen
and oxygen. The conversion of n-hexane served as a catalytic test reac
tion. Impregnation at pH 1.8 leads to nickel ions exchange of Bronsted
sites in the channel system. These are hardly reducible and are not d
etectable by IR spectroscopy of adsorbed carbon monoxide. Aromatizatio
n is increased. This is explained by the presence of ionic nickel Lewi
s sites, active in dehydrogenation. With impregnation at pH 4.8, tetra
meric 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 ac
tion of metallic nickel, which promotes the direct dehydrocyclization.
The hydrogenolysis activity of metallic nickel is suppressed.