MONOMOLECULAR AND BIOMOLECULAR MECHANISMS OF PARAFFIN CRACKING - N-BUTANE CRACKING CATALYZED BY HZSM-5

The cracking of n-butane catalyzed by the zeolite HZSM-5 has been characterized by measurements of the conversion determined with a flow reactor at temperatures of 426-523°C and n-butane partial pressures of 0.01-1.00 atm. The primary products, each formed in a first-order reaction, are H2 + butenes; methane + propylene; and ethane + ethylene. In the limit approaching zero conversion, each compound in each stated pair was formed at approximately the same rate as the other. Propane and a small amount of isobutane were formed as secondary products in second-order reactions. The results are consistent with the occurrence of two simultaneous mechanisms: (1) a monomolecular mechanism proceeding through the pentacoordinated carbonium ion formed by protonation of the n-butane at the two position and (2) a bimolecular hydride transfer proceeding through carbenium ion intermediates. The former proceeds almost to the exclusion of the latter in the limit approaching zero n-butane conversion. The limiting product distribution characterizes the intrinsic selectivity of the collapse of the carbonium ion; at 496°C, the relative rates of decomposition of the carbonium ion to give H2 + butenes, methane + propylene, and ethane + ethylene are 30 ± 6, 36 ± 4, and 34 ± 5, respectively, with the corresponding activation energies all being approximately 140 kJ/mol. These results provide the first demonstration of stoichiometric dehydrogenation accompanying paraffin cracking. © 1992.