ChemInform Abstract: Hydrocracking of n-Butane and n-Heptane Over a Sulfided Nickel Erionite Catalyst

A sulfided Ni/H-erionite catalyst was prepared from the natural mineral, and its catalytic characteristics for hydrocracking n-alkanes were studied. Because of its shape selectivity toward straight chain hydrocarbons, branched products were largely absent and the major cracked product was propane. The observed product distribution could not be explained by simple primary cracking of the feed molecule or by secondary cracking or recombination of the primary products. It is proposed that the feed molecule participated in a set of reactions, including hydrogenation, dehydrogenation, hydrogen transfer, oligomerization and cracking. These reactions are also needed to explain the yield of less than two moles of product from each mole of feed converted. A set of six first order differential equations representing the proposed molecular transformations is shown to fit the product distribution data for n-butane very well. A fit of the n-heptane data was not attempted; however, it is proposed that the mechanism is analogous to that proposed for n-butane. The rate of cracking of n-butane is first order in n-butane concentration, while the rate of n-heptane cracking appeared to follow half order kinetics. For n-butane cracking, the apparent activation energy of all the important reactions are roughly equal, resulting in a product distribution which is nearly independent of temperature. For n-heptane, a larger difference between the apparent activation energy of n-heptane cracking and secondary cracking of its primary products, lead to a temperature dependence in product selectivity.