Reactivity of Propene, n-Butene, and Isobutene in the Hydrogen Transfer Steps of n-Hexane Cracking over Zeolites of Different Structure

The reaction of n-hexane cracking over HZSM-5, HY zeolite and mordenite (HM) was studied in accordance with the procedure of the β-test recently proposed for quantitative characterization of zeolite hydrogen transfer activity. It is shown that this procedure allows one to obtain quantitative data on propene, n-butene, and isobutene reactivities in the hydrogen transfer steps of the reaction. The results demonstrate that in the absence of steric constraints (large pore HY and HM zeolites) isobutene is approximately 5 times more reactive in hydrogen transfer than n-butene. The latter, in turn, is about 1.3 times more reactive than propene. With medium pore HZSM-5, steric inhibition of the hydrogen transfer between n-hexane and isobutene is observed. This results in a sharp decrease in the isobutene reactivity: over HZSM-5 zeolites isobutene is only 1. 2 times more reactive in hydrogen transfer than n-butene. On the basis of these data it is concluded that the β-test measures the "real" hydrogen transfer activity of zeolites, i.e., the activity that summarizes the effects of the acidic and structural properties of zeolites. An attempt is made to estimate the "ideal" zeolite hydrogen transfer activity, i.e., the activity determined by the zeolite acidic properties only. The estimations obtained show that this activity is approximately 1.8 and 1.6 times higher for HM zeolite in comparison with HZSM-5 and HY zeolites, respectively.