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 mordenit e (HM) was studied in accordance with the procedure of the beta-test r ecently proposed for quantitative characterization of zeolite hydrogen transfer activity. It is shown that this procedure allows one to obta in quantitative data on propene, n-butene, and isobutene reactivities in the hydrogen transfer steps of the reaction. The results demonstrat e that in the absence of steric constraints (large pore HY and HM zeol ites) isobutene is approximately 5 times more reactive in hydrogen tra nsfer than n-butene. The latter, in turn, is about 1.3 times more reac tive than propene. With medium pore HZSM-5, steric inhibition of the h ydrogen transfer between n-hexane and isobutene is observed. This resu lts in a sharp decrease in the isobutene reactivity: over HZSM-5 zeoli tes isobutene is only 1. 2 times more reactive in hydrogen transfer th an n-butene. On the basis of these data it is concluded that the beta- test measures the ''real'' hydrogen transfer activity of zeolites, i.e ., the activity that summarizes the effects of the acidic and structur al 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 tha t this activity is approximately 1.8 and 1.6 times higher for HM zeoli te in comparison with HZSM-5 and HY zeolites, respectively. (C) 1994 A cademic Press, Inc.