Cracking and skeletal isomerization of hexenes on acidic MoO3-WO3/alpha-Al2O3 oxide

The correlation between the cracking and skeletal isomerization of some hex enes and heptenes as a function of reduction time and temperature and the d istribution of Mo and W surface species (determined by X-ray photoemission spectroscopy (XPS)), have been carried out on a mixed WO3-MOO3/alpha -Al2O3 sample, prepared by co-impregnation of ammonium metatungstate and ammonium paramolybdate. The amounts of each precursor were calculated in order to d eposit a theoretical monolayer with the same proportion Of WO3(50%) and MoO 3(50%). The use of different kind of hexenes and heptenes as probe molecule s, varying by their structure, and of a saturated reactant, the 2-methylpen tane, allowed us to postulate a monofunctional acidic mechanism, in which b oth cracking and skeletal isomerization occur on acidic sites via surface a lkoxy intermediates. From the correlation with XPS measurements, it can be concluded that Mo(V) species, acting mainly as Bronsted acidic center throu gh surface O-H groups, are responsible for cracking and isomerization react ions. Nevertheless, the presence of acidic Lewis center, like Mo(VI), W(VI) or M(V) coordinately unsaturated sites cannot be excluded. The dual acid-b asic nature of O-H surface groups is pointed out. (C) 2001 Elsevier Science B.V. All rights reserved.