Influence of acidity-basicity in 13C-labeled hexane isomerization reactions on exchanged Pt-β zeolite:: Mechanistic studies

The acidity-basicity effect on the catalytic behavior of exchanged Pt-beta zeolites in hexane reforming reactions has shown: (i) from X-ray photoelectron spectroscopy, that the platinum binding energy decreases when the basicity increases (therefore there is an electronic shift from zeolite to Pt particles which increases with basicity); (ii) at low hydrogen pressure (P-H2 = 226 or 38 Torr), the apparent activation energy for the 2MP reaction increases when the basicity of the zeolite increases; (iii) the ring enlargement reaction of methylcyclopentane to cyclohexane and benzene is increased when Bronsted acidity is increased; (iv) with acyclic molecules as 2-methylpentane and 4-methyl 1-pentene and n-hexane, the isomerization selectivity is high when the acidity is high but decreases when the basicity increases; (v) from the use of C-13-labeled molecules, no cyclic mechanism was detected but only bond shifts, for which the ethyl bond shift predominates on the acidic catalyst and the methyl bond shifts on the basic catalyst. The combination of these experimental points with kinetic data suggests that the hydrocarbon rearranges at the metal-support interface and that electron donor or electron acceptor effects due to the acidity-basicity of the zeolite lead to changes of the reaction mechanism pathway. (C) 1999 Academic Press.