Alkane hydrocracking: shape selectivity or kinetics?

A critical evaluation of published alkane hydrocracking product distributions shows that the kinetic network shifts from predominantly ααγ-trimethylalkane to predominantly αα- and αγ-dimethylalkane hydrocracking when the acid sites are insufficiently covered with alkenes. Since ααγ-trimethylalkane hydrocracking has a higher symmetry than αα- and αγ-dimethylalkane hydrocracking, this alteration in the predominant hydrocracking pathway changes the product distribution from a histogram with a single sharp maximum irrespective of the alkane length to histograms with several maxima depending on the feed alkane length. Thermodynamic, kinetic, and mechanistic considerations are presented to explain both types of histograms in great detail. These largely kinetic explanations supplant earlier attempts at linking the features of the hydrocracking product distributions to features of the topologies of the various (zeolite-based) catalysts employed.