Cracking of light alkanes over high silica zeolites - the role of elementary steps in controlling the selectivity

Protolytic cracking, dehydrogenation and isomerization of n-alkanes over high silica zeolites was studied using conventional kinetic methods, steady state isotope transients and adsorption techniques. The rate of cracking normalized to the concentration of acid protons in the samples was constant for each hydrocarbon and increased from propane to hexane. The apparent energy of activation dropped from 155 (propane) to 121 kJ/mol (n-hexane) while the enthalpies of adsorption of the reactants increased to the same degree. Steady state isotope transient experiments show that the H/D exchange is significantly faster than the rate of cracking excluding the protonization of the hydrocarbon to be the rate determining step. The results suggest a constant difference between the enthalpy of the hydrogen bonded and protonated hydrocarbons. The differences in the activation barriers for dehydrogenation, isomerization and cracking will be used to discuss possibilities and limitations in the zeolite selection to change the selectivity.