On the formation of methane and hydrogen during cracking of alkanes

A qualitative explanation of the low dehydrogenation/cracking ratio observed in reactions with long-chain n-alkanes catalyzed by superacids is attempted via calculation of the MINDO/3 reaction profiles for the complete dehydrogenation ⇆ intermolecular interconversion ⇆ cracking (D.I.C.) set of unimolecular processes that a protonated n-alkane can undergo. It is shown that the three simplest systems can suitably model all possible D.I.C. surfaces that can be found in a long-chain linear alkane. The importance of the primary protonation site in the distribution of products is discussed. Some experimental results from cracking of n-heptane over a set of acid zeolites are used, together with those of this theoretical study, to show that a recent controversy about the presence of methane and hydrogen as primary or secondary products in saturated alkane cracking catalyzed by zeolites can be settled.