The role of different types of acid site in the cracking of alkanes on zeolite catalysts

Abstract The cracking of n -heptane on a rare-earth (RE)HY zeolite has been studied in the temperature range 400–470 °C. By measuring the initial selectivities with respect to the various reaction products, and carrying out molecular orbital calculations it has been found that cracking can initially take place on Bronsted acid sites via protolytic cracking, or on Lewis acid sites via a classical β-scission mechanism. The activation energy for protolytic cracking is lower than for β-scission both for n -heptane and for the iso-heptanes. From energetic considerations, a scheme of cracking can be suggested where the cracking proceeds by hydride transfer and β-scission mainly via branched heptanes.

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