Alkanes can crack and dehydrogenate monomolecularly through penta-coordinated carbonium ions over Bronsted acid sites in zeolites. Primary products can undergo further monomolecular cracking or initiate bimolecular reactions enabled by carbenium ions, which can then propagate via hydride transfer. Monomolecular pathways are favored over bimolecular pathways at low acid site densities, in small pores, at high temperatures, low pressures and low conversions. We show that, by limiting Silicalite acid site density to 1 Al/4 unit cells, hydride transfer from feed alkanes to light alkenes can be inhibited to the extent that the range for monomolecular cracking can now be extended to 170 kPa or higher and to 80-90% conversion. Through use of a microkinetic model, we identify key cracking pathways for linear, mono-methyl-branched and cycloalkanes, compute rate constants and activation energies. We compare measured activation energies for disappearance of each of the alkanes with activation energies for key cracking pathways.

Monomolecular cracking of alkanes over 10 and 12-MR zeolites

244th National Meeting (2012)

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